Sedimentation and carbon sequestration at Brynemade, Funen, Denmark

Transcription

1 Sedimentation and carbon sequestration at Brynemade, Funen, Denmark M.Sc. thesis (30 ECTS) by Eva Make Stud.nr ( ) June, 2017 Supervisor: Carl Christian Hoffmann, senior researcher, Aarhus University Master Degree Programme in Agro-Environmental Management Department of Bioscience, Faculty of Science and Technology, Aarhus University

2 Preface This thesis is submitted as a completion of a master degree in Agro-Environmental Management at Aarhus University. The thesis work was performed from February to June, The thesis took place at Department of Bioscience at Silkeborg and was supervised by Carl Christian Hoffmann. Acknowledgments I would like to thank all who have helped me throughout the work of my master thesis. Firstly, I sincerely thank my supervisor Carl Christian Hoffmann for the guidance, valuable and constructive comments, as well as encouragement during the thesis work. Furthermore I wish to thank Jane Rosenstand Poulsen for all the help and knowledge she has provided. And a special thanks to my dear friend Zane Pruse for the productive discussions and her valuable insight into the clay chemistry. At last, thanks to all the friends and family that have supported me during these last months. Aarhus University, 2017 Eva Make 2

3 Abstract Wetlands provide a great variety of ecosystem services. Among them being sediment, organic matter and nutrient retention. The studied site is a restored riparian meadow, which is expected to be flooded during winter months. Sedimentation of phosphorus and suspended solids during flooding events has been studied. Carbon accumulation during a 10 year period post-restoration has also been analyzed. Sedimentation is measured with artificial grass mats, soil cores have been extracted to analyze carbon content in the soil. Particulate phosphorus and suspended solid sedimentation relationship with the flooding events is studied. Results do not show a clear relationship between the flooded days and sedimentation rates. The average phosphorus sedimentation rate is 8,32 g/m 2, but for suspended solids 4263,02 g/m 2. Three different flooding scenarios based on the flooded area are established. Results reveal that the sedimentation of phosphorus and suspended solids increases in quantity as the flooding takes greater area. Carbon sequestration rate is found to be 127,90 g C/m 2 per year, the total accumulated carbon over 10 years is 148 kg. The highest carbon accumulation is observed closest to the stream. The 14 C age of sediments range between 417 and 1347 years. The 14 C age results show that younger sediments lay on top, indicating the carbon accumulation at the site. 3

4 Table of contents Preface... 2 Acknowledgments... 2 Abstract Introduction Problem statement Theoretical background River Odense restoration and the Danish Action Plans Phosphorous forms and dynamics in wetlands Phosphorus pathways and forms Phosphorus retention within the wetland system Carbon sources and cycling in wetlands Materials and methods Study site Sediment sampling and phosphorus analysis Radiocarbon dating and stable isotope analysis Method of calculation and data analysis Phosphorus deposition Carbon sequestration Results Flooding Sedimentation and phosphorus accumulation Carbon sequestration and age Discussion Flooding Suspended solids and phosphorus sedimentation Sedimentation and flooding Results from other studies Phosphorus release from the deposited sediment Carbon sequestration Sediment transport Sediment age Methane production

5 Conclusion Perspectives Bibliography Appendices Appendix Appendix Appendix Appendix Appendix Appendix

6 1. Introduction The area of wetlands have significantly decreased in Denmark over the last century. Since 1900, when the wetland area constituted 7460 km 2, more than 70% of the wetland area in Denmark have been lost. Today there are around 569 km 2 of freshwater wetlands. The main reason for the loss of wetlands is the expansion of agricultural land. Wetlands have been converted to agricultural land and stream channels straightened to improve the drainage and make stream discharge faster. (Hoffmann & Baattrup-Pedersen, 2007) Wetlands are a crucial part of earth s ecosystems, because they provide such important ecosystem services as regulating the global biogeochemical cycles, water storage and filtration, and wildlife support. (Reddy & DeLaune, 2008) Phosphorus and carbon are important elements of the global biogeochemical cycle. Phosphorus plays a vital role in all the energy transfer processes, including photosynthesis. (Schachtman, et al., 1998) Within the expansion of agriculture, the fertilizer usage increases, thus encouraging a possibility for nutrient leaching to aquatic ecosystems. Phosphorus can be a limiting factor in freshwater ecosystems, enrichment of phosphorus can negatively impact such systems and lead to eutrophication. In the landscape wetlands act as buffers by retaining and filtering the incoming material between the upland and adjacent water body. (Hoffmann, et al., 2009) Carbon is the most important element to life, as all the living organisms contain carbon compounds. Wetlands are able to sequester significant amounts of carbon, due to their high productivity and slow decomposition rates. (Nahlik & Fennessy, 2016) As the plants grow they are taking the carbon dioxide from the atmosphere to perform photosynthesis. Slow decomposition rate means that carbon is accumulated at the site. One of the wetland s characteristics is restricted oxygen supply. Decomposition slows down significantly without the presence of oxygen, thus leading to carbon accumulation at a site. With the increased attention to climate change, restoring and preserving wetlands could possibly be one of the tools how to reduce the increasing carbon dioxide emissions in the atmosphere. (Reddy & DeLaune, 2008) 6

7 Several initiatives have already been placed to restore the wetlands in Denmark. Danish action plan II in 1998 is the first initiative that have included wetland restoration as a tool to reduce phosphorus and nitrogen loading to aquatic ecosystems from agricultural activities. Wetland restoration has been addressed in the Danish action plans ever since. (Poulsen, et al., 2013) 1.1. Problem statement Aim of the thesis is to find out phosphorus sedimentation in a restored floodplain during flooding events and carbon sequestration during the 10 year post-restoration period. How efficient is the wetland at retaining phosphorus? Is it possible to determine the carbon sequestration in a flooded riparian wetland using 14 C dating? 2. Theoretical background 2.1. River Odense restoration and the Danish Action Plans River Odense was restored in 2003 as a part of the Danish Action Plan II from One of the targets was to reduce the nutrient emissions to surface water by the restoration of rivers and floodplains. (Grant, et al., 2002) Newly restored floodplains are expected to reduce the nitrogen and phosphorus loading to aquatic ecosystems, as they serve as the interface between the upland and the river channel. The river restoration is aimed to restore the natural state and dimension of the river system. One of the river restoration measures is remeandering of channelized river sections. Streams in Odense river basin are typically lowland steams. Natural lowland stream will never be a straight channel, because the water will always follow the lowest point in topography, creating meanders. (Environment Centre Odense, 2007), (Sand-Jensen, et al., 2006) Re-meandering will bring back this natural flow regime as the natural dimensions of the stream channel are restored. This will lead to occasional overbank flooding creating inundated floodplains during peak flows in winter. (Poulsen, et al., 2013) 7

8 2.2. Phosphorous forms and dynamics in wetlands Phosphorus is a crucial nutrient to aquatic life. However excess concentrations of phosphorus can significantly increase algal and periphyton growth in streams and lakes leading to eutrophication in freshwater aquatic systems. Increased plant and algae growth will not only affect aesthetics of a stream and lake, but will also deplete the oxygen levels causing suffocation for aquatic animals. Excessive nutrient concentrations stimulate the algae growth causing algal blooms. When the algae die, the microbial decomposition process is depleting the oxygen of the water body, causing anoxic conditions. (Chislock, et al., 2013), (Sharpley, et al., 2001) In natural conditions the phosphorus concentrations are low because the major part of phosphorus is in a particulate form or adsorbed to soil minerals, therefore the bioavailable phosphorus entering the stream is rapidly being removed by algae or aquatic plants. The major part of the phosphorus contributing to streams are of allochthonous input, meaning that it originates outside the system. Large inputs of phosphorus come from anthropogenic sources. Manure or artificial phosphate fertilizer are widely used in Denmark. (Hoffmann, et al., 2009) Phosphate containing organic (manure, compost) or inorganic fertilizers are commonly used to ensure crop and plant growth. If the phosphorus application is not managed and timed well, phosphorus that is not retained in the soil can be transported during the runoff, erosion and heavy rainfall events towards the stream channel. (Ulén & Snäll, 2007) In this thesis the suspended solid sedimentation is analyzed. Suspended solids are insoluble particles that remain in the suspension and cause the turbidity of water. These insoluble particles can be of organic and inorganic origin. Inorganic materials include fine soil and sediment particles, such as silt and clay. Presence of these fine particles in the suspended solids allow pollutants, such as dissolved phosphorus, become attached to them. During overbank flooding or runoff events sediments are being transported and deposited on the floodplain, particulate phosphorus adsorbed to the these particles can be therefore transported together with the suspended solids. (Udeigwe, 2005) 8

9 2.2.1 Phosphorus pathways and forms Phosphorus enters aquatic ecosystems in dissolved or particulate form and can be represented in both organic and inorganic form. The major part of the phosphorous associated with wetlands is the particulate phosphorus. (Reddy & DeLaune, 2008) Dissolved inorganic phosphorus, for example, orthophosphate (PO4 3- ) is bioavailable to aquatic organisms and it is being incorporated into the living tissue after it reaches the stream. (Søndergaard, et al., 2001) Particulate phosphorus most often include inorganic phosphorus forms sorbed onto clay particles or soil minerals, such as iron compounds and their oxides, aluminium oxides and calcium. (Reddy & DeLaune, 2008) Particulate phosphorus can be directly deposited in the stream sediment, or deposited on the floodplain during overbank flooding. Particulate phosphorus needs to undergo transformations to become bioavailable. (Søndergaard, et al., 2001). Organic phosphorus derives from biological activity, as it is incorporated into the living tissue during growth period. Detrital material from plants, algal and microbial biomass are the major sources of organic phosphorus. In addition, the organic phosphorus can leach from the tissue after the plant death. Organic phosphorus cycle is very dynamic. Microbial community mineralizes the organic phosphorus into inorganic-bioavailable forms, which is then rapidly adsorbed or up-taken by living organisms. (Dunne, et al., 2005), (Sims & Sharpley, 2005) Figure 1. Main pathways of phosphorus (Sharpley, et al., 2001) 9

10 The main pathways for phosphorus are summarized in the Figure 1. The main pathways for phosphorus transport to the stream are runoff and erosion. When water is moving over the soil surface, it interacts with the top soil layer. During this interaction phosphorus can be removed from the soil by dissolving into the rainwater. The moving water or rain drops detaches the soil particles containing phosphorus and carries them in a suspension downhill. (Sharpley, et al., 2001) Dissolved phosphorus is transported with the runoff water, but the particulate phosphorus is generally linked to soil or organic particles. Particulate phosphorus is attached to clay or mineral particles or organic matter. Therefore particulate phosphorus needs to be first detached, before it can be transported with the water. The major part of phosphorus is transported as particulate phosphorus. (Shinohara, et al., 2016) Erosion can be distinguished between stream bank and hillside erosion. (Sharpley, et al., 2001) According to Laubel, et al. (1999) stream bank erosion contributes to more than 50% of the sediment transport to the stream. Eroded material from the river banks can have high content of phosphorus, as the phosphorus is attached to soil particles. Phosphorus in freshwater systems can be transported with fine sediments. Due to the fact that significant part of the eroded material is transported as suspended sediment, bank erosion can be a significant contributor of phosphorus to the stream. (Laubel, et al., 2003) Even though soils have a great capacity to retain phosphorus, however as more and more phosphorus containing fertilizer is added to the soil, it eventually becomes oversaturated as the phosphorus sorption capacity becomes exhausted. During runoff or erosion events, soil particles containing phosphorus are disturbed, they can become detached and transported together with the water downhill eventually entering the stream. (Walling, 1999) Another phosphorus transport pathway is subsurface flow. Subsurface flow can contribute to phosphorus transport directly through tile drains or by percolation through the soil. In general soils, have high phosphorus sorption capacity, therefore phosphorus loss through the subsurface flow is usually minimal. (Sharpley, et al., 2001) 10

11 2.2.2 Phosphorus retention within the wetland system The ultimate goal is to retain phosphorus in the system and avoid its leaching to stream channel, as the excess amounts of phosphorus have a detrimental effect on the aquatic ecosystem. Phosphorus removal depends on the retention capacity of the wetland, retention can occur through physical, chemical, and biological processes. Hence, the major phosphorus retention mechanisms are sedimentation, adsorption and precipitation, and biotic uptake. (Surridge, et al., 2007) One of the most important retention mechanisms on a floodplain is sedimentation. Wetlands act as a buffer for rainwater and floodwater between the terrestrial and aquatic ecosystems. When water is moving from the upland towards the wetland, dry wetland vegetation acts as a physical barrier and reduce the runoff-water velocity coming down the slope, allowing sedimentation of suspended solids carried with water occur. (Craft, 1997) Floodwater is carrying the sediment that can be deposited along the river channel. River restoration will often lead to inundation of the floodplains. (Poulsen, et al., 2013) As it is known large quantities of phosphorus are being transported with the sediment associated with the flooding. During flooding events, the floodplain is being covered with water, when the water slows down, the sedimentation takes place and suspended solids settle on the floodplain. (Brady & Weil, 2010), (Reddy & DeLaune, 2008) Residence time is a crucial factor for sedimentation, which is directly related to flow velocity and volume. With increased time, the more particles can settle. (Hoffmann, et al., 2009) Figure 2. Phosphorus exchange between solid and solution phase (Reddy & DeLaune, 2008) 11

12 Adsorption is a physical process that occurs when the phosphorus moves from porewater to soil mineral surfaces. (Sims & Sharpley, 2005) Schematic representation of the process is illustrated in the figure 2. It can be seen from the figure, that phosphorus in porewater and adsorbed to mineral surface tends to be in equilibrium. Presence of clay and soil minerals increases the phosphorus adsorption capacity. If phosphorus concentration increases in porewater (second step in the figure), it will fast adsorb to the mineral surface to reach the equilibrium again. Exchange in the first two steps occur without penetrating the soil mineral structure. It can be seen in the last step that over longer period of time, the adorsbed phosphorus diffuses in to the solid phase absorbs, forming phosphate minerals. Decrease of the phosphorus adsorbed on the mineral surfaces increases the sorption capacity and provides new sites for porewater phosphorus to adsorb. (Dunne, et al., 2005), (Sims & Sharpley, 2005), (Reddy & DeLaune, 2008) If the porewater concentration of phosphorus keeps increasing, the sites for phosphorus adsorption become exhausted. In such case, precipitation of phosphate minerals may occur to retain the phosphorus. (Reddy & DeLaune, 2008) Apart from adsorption, precipitation is a chemical process. Phosphate ions are reacting with metallic cations, such as aluminium, iron and calcium, forming different precipitates (Fig.3). (Sims & Sharpley, 2005) The presence of aluminium oxides and iron compounds in the soil determine the soil s capacity to retain phosphorus by precipitation. (Ulén & Snäll, 2007) In addition, soils high in clay and organic matter provides more surfaces for phosphorus adsorption. (Reddy & DeLaune, 2008) First reaction below (Fig.3) shows a precipitation reaction with aluminium. Aluminium ion reacts with phosphate ion forming insoluble solid called precipitate. Similar reactions occur also with other metallic ions, such as iron and calcium, these reactions occur where high concentration of either phosphate or metal ions are available. Reaction 2 (Fig.3.) shows the anion exchange between the negative phosphate ions and positively charged sites. In acidic conditions, the gain of excess hydrogen ions (H + ) result in positively charged mineral surfaces (Al(OH)2 + ) allowing the negatively charged phosphate ion (PO4 3- ) to adsorb. Reaction 3 (Fig.3) represent the ligand exchange. During this reaction, the hydroxyl group on a surface of a metal, in this case aluminium, is being replaced by a phosphate ion. Reaction 4 (Fig.3) shows the fixation by silicate clays. In this example, the clay kaolinite is broken down and phosphate ion is replacing one of the hydroxyl groups on the broken edge of the clay. Reactions 2, 3 and 12

13 4 are highly ph dependent. ph will determine if the clay, organic matter, aluminium and iron oxide surfaces will lose or gain protons (H + ). In general, the clay and organic matter surface has a negative charge, but changes in ph towards acidic will result in positive charge on the surface, due to the gain of protons, allowing negative phosphate ions to adsorb. (Reddy & DeLaune, 2008), (Sims & Sharpley, 2005) Figure 3. Reactions associated with aluminium and phosphate (Reddy & DeLaune, 2008) These newly formed precipitates are sensitive to ph and redox potential changes. Under reduced conditions, the previously precipitated iron phosphates can be released. Phosphorus bound to aluminium is not affected by changes in a redox potential, as the aluminium is not redox sensitive. (Reddy & DeLaune, 2008), (Dunne, et al., 2005) In alkaline soils calcium minerals play the dominant role in phosphorus precipitation forming such precipitates as calcium phosphate (Ca(H2PO4) and hydroxyapatite (Ca5(PO4)3(OH). In acidic soils phosphate minerals associated with acidic soils are vivianite (Fe3(PO4)2), variscite (AlPO4) and strengite (FePO4). (Dunne, et al., 2005), (Reddy & DeLaune, 2008) Iron phosphates are stable under aerobic and drained soil conditions. However, wetlands experience seasonal changes in water table elevation, leading to changes in oxic and anoxic zones within a wetland, which influence the phosphorus dynamics. Phosphorus is precipitated in the oxic zone, but mobilized in the anoxic zone. (Hoffmann, et al., 2009) 13

14 Fe 3+ + PO4 3- FePO4 FePO4 + H + + e - Fe 2+ + HPO4 2- Iron phosphate solubility increases, as the conditions become reduced during waterlogged periods. (Sims & Sharpley, 2005) First reaction above shows a precipitation reaction between and ferric iron and phosphate forming iron phosphate. Under reduced condition, the ferric iron is reduced to ferrous iron, as a consequence phosphate is released. (Dunne, et al., 2005) Oxygen presence increases the redox potential, as it is the most favorable electron acceptor yielding the most energy. Oxygen can be depleted in a short time after the flooding. (Wodarczyk, et al., 2007) If the redox potential in the wetland decreases, the phosphorus can be released. The redox conditions, ph and microbial communities are sensitive to change between dry and wet periods. Phosphorus in sediments is often bound to iron minerals, iron is redox and ph sensitive, therefore changes in these conditions alter the binding capacity and cause phosphorus desorption and release into the water column. Oxygen depletion induces the iron reduction, thus releasing the bound phosphorus by desorption from iron minerals. (Schönbrunner, et al., 2012) On the contrary drying of the sediment encourages the iron oxidation and phosphorus immobilization by producing ferric phosphates. (Baken, et al., 2015) More phosphorus is being released, if the floodplain experiences long dry periods and then being re-wetted, rather than being inundated all year long. Prolonged sediment drying causes changes in the mineral structure of the iron minerals, decreasing the sorption capacity. (Kinsman-Costello, et al., 2016), (Schönbrunner, et al., 2012) Phosphorus can be incorporated into plant or microbial biomass. Plants during growth season are up-taking the phosphorous and incorporating it into biomass. As wetlands are highly productive, large amounts of nutrients, such as phosphorus, can be stored into the biomass. However, phosphorus is released and mineralized after the plant death, to ensure complete phosphorus removal, vegetation must be harvested. Peat forming wetlands provide a long term phosphorus storage, if the decayed plant material gets incorporated into the peat. (Haygarth & Jarvis, 2002) 14

15 2.3. Carbon sources and cycling in wetlands Wetlands have been broadly valued by their many functions, such as removing nutrients, supporting wildlife and storing floodwater. With the increased attention to the world s changing climate, wetlands have been also recognized as an important part of the global carbon cycle. Even though wetlands constitute a small part of the earth s ecosystems (2-6%), they are able to store and sequester vast amounts of carbon. (Kayranli, et al., 2009) It is estimated that wetlands contain around 20-25% ( Gt of C) of the global terrestrial carbon (Department of Sustainability, Environment, Water, Population and Communities, 2012), (Mitra, et al., 2005), therefore wetlands play a significant role in the global carbon cycling. Due to the high amount of carbon stored within the wetland ecosystems, even small changes in the carbon balance in wetlands can cause a significant impact on the global carbon cycle. (Kayranli, et al., 2009) Apart from phosphorus which usually comes from outside the system, the carbon mainly comes from autochthonous input (produced on site). Plants uptake the carbon dioxide from the air or water, and in return during plant decay and decomposition organic matter is accumulating on the site. However, carbon can also be transported with sediments washed by runoff and floodwater. (McLatchey & Reddy, 1998), (Reddy & DeLaune, 2008) Figure 4. Carbon cycle and reservoirs in the wetland (Reddy & DeLaune, 2008) 15

16 Carbon in wetlands can be found stored in five main reservoirs: plant biomass carbon, particulate organic carbon, dissolved organic carbon, microbial biomass carbon and gaseous end products. Plant biomass carbon is the carbon stored in the living plants. Detrital matter represents the particulate organic carbon. It is being broken down to simpler organic compounds and at the same time undergoing mineralization. Soil microbes are responsible of these processes decomposition and mineralization, hence they are responsible for the carbon cycling in the soil. Soil microbes use the detritus to obtain their energy, and during the breakdown processes some of the carbon is assimilated in the microbial biomass. Dissolved organic carbon is a product of detritus decomposition and leaching from the detrital pool. Dissolved organic carbon serves as an energy source for the microbial biomass. Carbon dioxide and methane are the gaseous end products of the carbon cycle. Carbon dioxide and methane are produced under anaerobic decomposition, but only carbon dioxide is produced during aerobic decomposition. (Reddy & DeLaune, 2008) The carbon pools and cycling is illustrated in the figure 4. Carbon dioxide is taken from the atmosphere by plants and used for photosynthesis. Dead plant material detritus is being buried in the soil eventually forming peat. The oxygen poor environment in the deeper layers of the soil slow down the decomposition rate significantly, hence the carbon is being stored rather than being decomposed and respired back in the atmosphere as carbon dioxide. During anaerobic decomposition methane is being produced and released in the atmosphere. (Reddy & DeLaune, 2008) Wetlands can act as a sink or source of the carbon, depending if the organic matter is decomposed or accumulated in the system. Carbon accumulates in the wetland when primary production is faster than the decomposition rate, thus leading to a net accumulation of carbon. Decomposition rate depends on several factors, some of the most important factors are: oxygen presence, temperature, hydro-period, characteristics of organic matter and microbial communities. Wetlands are generally characterized as highly productive ecosystems. Frequent inundations cause changes of the water table in the wetland. Raised water table leads to waterlogged conditions and restricted oxygen supply. Therefore wetlands provide suitable conditions for reduced decomposition rates, thus encouraging the carbon accumulation at a site. (Reddy & DeLaune, 2008) 16

17 Slow organic matter decomposition rates and waterlogged conditions provide suitable conditions for peat formation. The organic matter forming peat in wetland originates from the plant material. Plants are known to consist of complex carbohydrates, such as lignin and cellulose. Phenol oxidase is one of the enzymes capable to degrade such complex compounds. However, phenol oxidase activity is inhibited under anaerobic conditions, therefore securing the organic matter from decomposition in a wetland. (Freeman, et al., 2001), (McLatchey & Reddy, 1998) Temperature is a controversial factor, as it encourages the plant growth, which promotes the carbon sequestration. However, temperature also increases the decomposition rates, therefore exhausting stored carbon. (Sutfin, et al., 2016), (Pant, et al., 2003) During decomposition the carbon dioxide is produced and released in the atmosphere. Therefore if a wetland is drained, aerobic conditions prevail and carbon is being decomposed at faster rates, simply due to increased enzyme activity. (Freeman, et al., 2001) In such situation wetland would act as carbon source, as the carbon dioxide is being released in the atmosphere. However when wetlands are preserved and restored, the decomposition rates are significantly slowed down and carbon can be stored within the system. (Kayranli, et al., 2009) 17

18 3. Materials and methods This thesis is a theoretical thesis. It is based on literature studies and review. Data sets for analysis have been provided by the Department of Bioscience, Faculty of Science and Technology, Aarhus University 3.1. Study site The study site Brynemade wetland is located on the island of Funen, Denmark. Brynemade is situated at the Odense river basin. The physical location of the study site can be seen in Figure 5. Odense river catchment area is approximately 1050 km 2. The dominating land use is agriculture 68% of the catchment area. The rest of the area is taken by built-up areas (16%), woodlands (10%) and natural/seminatural countryside (6%). The dominating soil type in the catchment area is clayey soils (51%). (Environment Centre Odense, 2007) Figure 5. Location of the river Odense in Denmark (Audet, et al., 2011) 3.2. Sediment sampling and phosphorus analysis As mentioned before River Odense was restored in 2003, 3,6 km long section of the river channel was re-meandered extending it by 1,2 km. As the floodplain connectivity is restored, the Brynemade area is flooded almost every winter. (Poulsen, et al., 2013), (Madsen & Debois, 2006) In order to determine the sedimentation and deposition of phosphorus the sediment samples have been collected at various distances from the stream. Artificial grass mats sized 15 x 15 cm have been used to determine the deposition of the sediment carried by water. These mats are proven to be one of the most representative methods to assess the deposition 18

19 in sites that are repeatedly inundated. Grass mats have numerous advantages. Their flat and rough surface is very convenient for trapping the incoming sediments with water, as well as they are lightweight and easy to install. Mats are secured by metal pins to withstand the force of the water, this allows to fully recover the mats together with the accumulated sediments. (Steiger, et al., 2003) The grass mats have been installed at increasing distances from the stream channel (1 m; 5,6 m; 10,7 m; 16,5 m; 23,8 m; 31,1 m; 40,8 m; 52,3 m; 71,8 m; 101 m). Placement of the artificial grass mats can be seen in Figure 6 and Table 1. Artificial grass mats were installed every winter from Table 1. Artificial grass mat position and distance from the stream Position Distance (m) N ,1 E ,6 1 N ,0 E ,9 5,6 N ,0 E ,0 10,7 N ,0 E ,4 16,5 N ,0 E ,9 23,8 N ,0 E ,7 40,8 N ,2 E ,4 52,3 N ,2 E ,5 71,8 N ,1 E ,1 101 Figure 6. Map showing artificial grass mat sampling distances from stream 19

20 Artificial grass mats are being installed in autumn, before the expected flooding period and harvested at spring. After harvesting the artificial grass mats are being weighed, both wet and dry. The samples are analyzed using method described in Svendsen, et al (1993) paper. After drying at 60 o C the samples are grinded and put in a muffle furnace and heated up to 560 o C. The ash left after heating contains the phosphorus. In order to extract the phosphorus from the ash, they are boiled for one hour in a mild hydrochloric acid (HCl) solution. Afterwards it is being diluted and the phosphorus is determined as phosphate. After such treatment all the phosphorus has been dissolved into phosphate. The phosphate is coloured blue using molybdenum blue method and afterwards measured in a spectrophotometer. (Murphy & Riley, 1962) 3.3. Radiocarbon dating and stable isotope analysis Sediment, plant and leave samples have been used to analyse carbon content and 14 C age at the study site. The sediment samples have been dried and sieved, to get the finest grained fraction (<63 μm). After sieving the samples were acidified with 0,5 M hydrochloric acid (HCl) to remove the carbonates. Afterwards samples were neutralised by rinsing with demineralised water and freeze-dried. In order to determine the carbon age, the sediment samples were weighed into quartz glass tubes containing 200 mg of pre-combusted copper oxide (CuO). Afterwards the quartz tubes were combusted for one hour at 900 C to convert the sample carbon to carbon dioxide (CO2). The CO2 was then reduced to pure carbon (graphite). Afterwards the stable isotope of carbon ( 13 C) can be measured. 14 C atoms are counted by the accelerator mass spectrometer. Stable isotope and radiocarbon dates were measured at the Aarhus AMS centre, Department of Physics and Astronomy, Aarhus University by Bente Philippsen. Principle behind the radiocarbon dating lies on the fact that plants during their growth use carbon dioxide from the atmosphere, part of this carbon will be the radioactive isotope 14 C. After the plant death the 14 C is no longer uptaken from the atmosphere. Since the 14 C isotope decays at a known rate, the 14 C concentration of the sample can be compared to the atmospheric 14 C when the plant was alive to determine the sample age. (Philippsen, 2012) 20

21 3.4. Method of calculation and data analysis Phosphorus deposition Results from (excluding winters , and ) with the total sedimentation (g) and phosphorus (mg P/ g DW) deposited from sediment on each grass mat in transect have been provided. The goal is to find the phosphorus sedimentation in the transect area. Provided results are thought to represent the grass mat (15x15 cm) and the area around it. Figure 7. Illustrative representation of the grass mats and the transect It is known that each grass mat represents the area around the midpoints between the neighboring grass mats. The area around the mat is marked with the red circle in the Figure 7. In order to find the area around the mat, the midpoints between the mats have to be found. The first mat is placed 1 m from the river bank therefore that is the midpoint between the river and the first mat. The midpoint between the mat placed at 1 m and 5,6 m is calculated by subtracting the distances (5,6 m 1 m = 4,6m) and finding the half of that distance ( 4,6 m / 2 = 2,3 m). Adding both midpoints (1 m + 2,3 m = 3,3 m), the length of the area around the first mat is obtained. Width of the grass mats is known (0,15 m). Therefore multiplying the length between the midpoints and the width of the grass mat, the area around the grass mat at 1 m can be found (0,15 m * 3,3 m = 0,495 m 2 ). Same principle applies for the rest of the grass mats. 21

22 After finding out the area, the provided data of sediment and phosphorus deposition on each mat can be used to find the deposition around the area of each grass mat. Summing all the areas around the mats, the total deposition of suspended solids and phosphorus at the transect area (115,6 m * 0,15 m = 17,34 m 2 ) can be calculated. (Transect length is 101 m (placement of the last grass mat) plus the length of the half point at the last grass mat, which is 14,6 m, hence the 115,6 m in the calculation previously). Afterwards this sum is divided with the calculated deposition of suspended solids and phosphorus in the area around the grass mats. This value represents a rate, which is then assumed to be the same anywhere in the study site. The same is applied for each year, results can be seen in App.1. Knowing the suspended solid sedimentation and phosphorus deposition at the transect area, the results can be then extrapolated to the different flooded areas according to each flooding scenario minimum, small or medium flooding. Each of these scenarios are determined by the changes in discharge and water stage, resulting in difference in the flooded area. (Poulsen, et al., 2013) Minimum flooding occurs when only the area at transect is covered (1403 m 2 ). Minimum flooding represents the initiation of the flooding. Small flooding is covering the area of 1871,58 m 2 (water stage at 24,42 meters above sea level (m.a.s.l)), but medium flooding covers 43787,35 m 2 (water stage at 24,69 m.a.s.l) (Poulsen, et al., 2013) Discharge data from has also been provided. It is estimated that the transect area is flooded when discharge in river Odense exceeds 5200 l/s, which corresponds to the water stage of 24,42 m.a.s.l. (Poulsen, et al., 2013), (Riis, et al., 2014) With this information and discharge data, the days of flooding for each year can be found. Flooding days are counted for the period that grass mats have been placed Carbon sequestration Soil cores (approx. 30 cm depth) from the following distances 10,7 m, 23,8 m, 40,8 m, 71,8 m and 101 m have been extracted. Afterwards the extracted soil is sliced in 3,5 cm intervals and analyzed for the carbon content (%) (see App.4). Sediment sampling has been carried out since 2003, when the river was re-meandered giving the results with the total accretion of 22

23 the sediments at each distance during the period The total sediment accretion height at each distance is known (see App.3). The sediment accretion was calculated using the volume weight of 1 g/cm 3, because it proved to be the best fit in this situation. Several distances have multiple carbon content results available measured at 3,5 cm depth intervals. Therefore it is necessary to calculate how much sediment in total has been accumulated in the 3,5 cm layer. The sample depth (3,5 cm) and the sediment volume weight (1 g/cm 3 ) is known. As the volume weight is 1 g/cm 3, the width and length is assumed to be 100 cm. It is assumed that the 15x15cm grass mat is representing 1 m 2, therefore transect area changes from 17,34 m 2 to 115,6 m 2 (115,6 m length x 1 m width). The visual representation of the 3,5 cm layer can be seen below. Multiplying the depth, width and length with the volume weight, the total amount of sediments in the 3,5 cm layer is found ( g). Carbon content is not available for all the distances from the river bank. Values for distances 1 m and 5,6 m are taken from the closest available distance with measurements, which is 10,7 m. As the other missing values ( 16,5 m, 31,1 m and 52,3 m) fall in-between the known points, the carbon content value can be estimated more precisely using linear interpolation. Example of estimating the carbon content at the distance of 16,5 m, depth 0-3,5 cm: 23

24 In the example above it can be seen, that the carbon contents for 10,7 m and 23,8 m distances are known. In order to find out the value for 16,5 m, the total distance between the known points (13,1 m) and the distance between the 16,5 m and 10,7 m (5,8 m) has been found. 1. 5,5% + 10,6 % = 16,1 % 2. 16,1% / 13,1 m = 1, ,229 * 5,8 m = 7,13% 4. C% at 0-3,5 cm = 35000g / 100 * 7,13 % = 2496 g of C Above the steps for calculating the carbon content are illustrated. In the first step the both known carbon concentrations are summed up, to see the carbon content in the whole layer between 10,7 m and 23,8 m. Afterwards the value is divided with the total distance between the two known points, thus obtaining the value (1,229) that shows how much the carbon increasing with the each unit (in this case meters). In the third step, this value is multiplied with the distance between the known and unknown point (5,8 m), thus providing the estimated carbon content at the 16,5 m distance. Knowing the total amount of sediments in the 3,5 cm layer (35 000g) and the calculated carbon content 7,13%, the amount of carbon in this layer can be found. If, for example, the accretion of sediment is deeper than 3,5 cm, (distances 1 m, 5,6 m, 10,7 m and 16,5 m) then the carbon content is being calculated for each of the 3,5 cm interval until the accretion depth (see App.5) is reached. Then it is summed up to get the total carbon content for all the accumulated layer at the specific distance. The same principle has been applied to other missing values and can be seen in Appendix 6. 24

25 4. Results This chapter starts with the presentation of the calculated flooded days at the site and the given areas for three different scenarios of flooding. Later in the chapter the sedimentation rates and total amounts for both suspended solids and total phosphorus and their relation to flooding are being illustrated. Chapter ends with the results of carbon sequestration and 14 C age of the sediments Flooding Table 2. Number of days of flooding each year Year Days flooded Table 2 summarizes the flooded days over the 10 year period at the study site. It can be seen from the table that the number of flooded days varies each year significantly. There have been no flooded days observed on winter of The longest period of flooding 105 days has occurred in winter of Table 3. Different scenarios of flooding Minimum Flooding Small Flooding Medium Flooding Area (m 2 ) , ,35 Area (ha) 0,14 0,19 4,38 The area flooded during three different flooding scenarios based on water stage have been summarized in the table 3 above. (Poulsen, et al., 2013) As it can be seen there is not a large difference in area between minimum and small flooding (1403 m 2 and 1871,58 m 2 ), however 25

26 there is a great difference in area between small and medium flooding 1871,58 m 2 to 43787,35 m 2, 4.2. Sedimentation and phosphorus accumulation Figure 8 reveals the sedimentation rate for each year of the suspended solids and total phosphorus. The rate has been calculated based on the results from the grass mats retrieved from transect. It can be observed that the sedimentation rate is much higher in the first few years. There cannot be detected a consistent trend of suspended solid sedimentation over the years. Figure 8 also reveals that the phosphorus sedimentation rate is fluctuating over the years. Average, standard deviation and minimum and maximum values for the sedimentation rates over the 10 years are presented in the table 4. Maximum value is from year , which is thought to be false, therefore in the brackets there is included the next maximum value from year Table 4. Average, minimum and maximum values for sedimentation rates for years Total Phosphorus (g/m2) Suspended Solids (g/m2) Average 8, ,02 Standard deviation 5, ,19 Minimum 2, ,51 Maximum 21,43 (12,38) 11026,14 26

27 Figure 8. Sedimentation rate of suspended solids and total phosphorus for years Note: Total phosphorus value from is false. It is being investigated, if there is a fault in analysis. It is suggested that there is an error in standard concentration. Sample have not yet been re-analyzed. The relationship between suspended solid and total phosphorus sedimentation and the days of flooding has been showed in the figure 9A and B. Both flooded days and sedimentation is varying significantly throughout the years. It can be noted from the figure that there is no clear relationship between the number of flooded days and sedimentation rates for both suspended solids and phosphorus. 27

28 Figure 9. Relationship between suspended solids (A) and phosphorus (B) sedimentation and flooded days Figure 10A, B and C is summarizing the total sedimentation of phosphorus and suspended solids for each year for all three flooding scenarios. Note that axis have different units. It can be observed that the amount of suspended solids and phosphorus sedimentation increases as the flooded area increases in size. It can be noted from the axis that phosphorus sedimentation increases in quantity, as the flooding increases in size. 28

29 Figure 10. The total sedimentation of suspended solids and total phosphorus for years at minimum, small and medium flooding events. Note: axis have different units. 29

30 C (kg) Table 5. Total sedimentation of suspended solids and phosphorus during Minimum Flooding Small Flooding Medium Flooding Total Phosphorus (kg) 116,72 155, ,87 Suspended Solids (t) 59,81 79, ,66 Table 5 summarizes the total amount of sedimentation of suspended solids and total phosphorus for all three flooding scenarios during years It can be observed from the table that sedimentation for both suspended solids and phosphorus increases with the increased area of the flooding, being the highest during medium flooding Carbon sequestration and age The total carbon accumulation for the years can be seen in the table 6 and figure 11. It can be noted that highest carbon content is detected closest to the river, which is also were the highest sediment accretion during years was observed (see App.3). In addition, it can be observed that there is a slight increase in the carbon concentration at the last two distances (71,8 m and 101 m). Table 6. Carbon accumulation at different distances form the river for the period Distance from river bank (m) Carbon accumulation (kg) 1 39,13 5,6 34,93 10,7 17,26 16,5 10,51 23,8 3,71 31,1 5,43 40,8 8,96 52,3 7,04 71,8 10, ,85 Total Carbon accumulation ( ) 1 5,6 10,7 16,5 23,8 31,1 40,8 52,3 71,8 101 Distance from river bank (m) Figure 11. Visual representation of carbon accumulation at different distances from the river. Average, standard deviation and minimum and maximum values for the carbon accumulation during the years are presented in the table 7. 30

31 Table 7. Average, minimum and maximum values for carbon accumulation for years Carbon accumulation (kg) Average 14,79 Standard deviation 12,32 Minimum 3,71 Maximum 39,13 The sediment 14C age has been illustrated in the Figure 12. Sediment age has been measured at different depths at 10,7 m distance from the stream. The reference measurement has been done in year 2003, before the restoration was carried out. Sediment age increases with depth, reference sample being the oldest. 14 C age of the sediments is years old. The reference sample is C years old. Figure 12. Stable isotope results at A3 transect, 10,7 m from the stream (Philippsen, 2016) 31

32 5. Discussion 5.1 Flooding Days of flooding are varying throughout the years (Table 2). It has been established that flooding at the site occurs when discharge exceeds 5200 l/s, corresponding to water stage at 24,42 m.a.sl. Therefore it can be said that the discharge over 5200 l/s will make the stream overflow its banks and cause flooding in the Brynemade area. As the discharge is not the same for each year, also the days of flooding are varying throughout the years (Table 2). Discharge depends on different physical and anthropogenic factors. The most influential physical factor is the climate at the given area. (Milliman, et al., 2008) The weather conditions have a direct effect on the stream discharge, for example, heavier the rainfall, the higher the discharge of the stream, thus increasing the stream discharge eventually to a point where flooding occurs. (Allan & Castillo, 2007) Other physical factors that can affect the discharge include a variety of factors concerning geology (textural composition of the material) and hydromorphology (size of the drainage basin). Anthropogenic factors include land use changes and river management practices, such as, urbanization, deforestation and channelization. (Vörösmarty & Sahagian, 2000) Human activities in the past century have altered the global climate by increasing the temperature and carbon dioxide levels in the atmosphere. (Gerten, et al., 2008) As a consequence the precipitation pattern is being changed, due to the general increase of evaporation caused by rising temperature (Huntington, 2006), which leads to the increase in precipitation. Such outline is predicted in the northern-hemisphere temperate zones, such as Denmark is placed. (Zwiers, et al., 2007) As the precipitation is the main factor causing changes in the annual discharge pattern, changes will strongly influence the river systems, in this case by increasing the discharge. (Vecchia, et al., 2005) With the increase in discharge caused by climate change, more floods can be expected in the future. (European Environment Agency, 2013) Amongst other functions, an important ecosystem service wetland can provide is flood protection. Wetlands are able to store large amounts of water, therefore providing protection from floods. In Denmark, the flood protection is a side benefit, due to the fact, that wetlands were initially restored with the purpose to decrease the nitrogen load to the aquatic systems. (Hoffmann & Baattrup- 32

33 Pedersen, 2007) However, the flood protection plays a more dominant role in other parts of the world, for example, in Central Europe (Danube river basin). (Ramsar Convention Secretariat, 2011) The increased flooding will change the wetland hydrological regime towards a more natural regime, as the wetland will be flooded for extended period than previously. (Erwin, 2009) In addition, more areas can become flooded, as the increase in discharge can cause the river to overflow. Taking into account that many Danish streams have been restored to regain the natural dynamics between the stream and floodplain (Hoffmann & Baattrup-Pedersen, 2007), (Environment Centre Odense, 2007), the wetlands will become the recipients of the flood water. Therefore, it is important to study the nutrient and sediment dynamics of inundated floodplains, since flooding is expected to increase due to the climate change. 5.2 Suspended solids and phosphorus sedimentation Results (Fig 8.) clearly reveal that the sedimentation occurs in the studied floodplain. The focus of this thesis is the particulate phosphorus, which is associated with the suspended solids. Agriculture is the dominating land use in the catchment (Environment Centre Odense, 2007). It is known that phosphorus fertilizers are commonly used in the agricultural practices. In addition, the main soil type in the catchment is clay (51%), (Environment Centre Odense, 2007) Phosphorus is known to bind to the soil particles. Clay particles in the soil are characterized by their fine texture and large surface area. These characteristics encourage phosphorus adsorption to the soil particles. During runoff or erosion events, the phosphorus-containing soil particles can be detached and transported with water. Besides the flooding, the hillslope runoff and erosion are also sources of phosphorus transport over the years to the wetland. Due to the fact that soil in the area has high content of clay, there is a high possibility, that eroded soil will contain phosphorus particles attached to soil minerals, such as aluminum or iron. (Hoffmann, et al., 2009) In addition, as the floodplain is connected to the stream channel, during high discharge events, 33

34 the stream will overflow its banks. Suspended solids and phosphorus is transported with the flood water to the adjacent floodplain. (Noe & Hupp, 2009) Wetland vegetation traps the particles, as it slows down the water velocity and increases the residence time. If vegetation is not removed, phosphorus that are assimilated by the vegetation during growth season, can be released after their death. (Hoffmann, et al., 2009) Sedimentation occurs during the periods of inundation, growing wetland vegetation are reducing the water velocity and let the particles settle. (Kronvang, et al., 2007), (Kronvang, et al., 2009) Typically a greater rate of phosphorus sedimentation are observed with increased quantities of suspended solids. (Brunet & Astin, 1998) A greater sediment load transport provides a possibility for higher phosphorus transport as well. It can be witnessed (Fig.8) that the years with higher sedimentation rate reveal higher phosphorus sedimentation rate and the same applies for lower sedimentation rate Sedimentation and flooding It is expected that with the higher number of flooded days, the sedimentation rates would increase. However, suspended solid and phosphorus sedimentation and the number of flooded days does not show a clear correlation, if the whole 10 year period is taken into account. (Fig.9). The last two winters and have the highest number of flooded days, however the suspended solid sedimentation rate is low (Fig.9). The high water flow, as the flooded days are based on discharge, could cause lower particle setting and shorter residence time. Also the phosphorus sedimentation is low in (Fig.9). On the contrary, another reason behind this could be, that there have been few flooding events, but long lasting, hence the high flooded day number. Long flooding periods can result in lower sedimentation accumulation, since new sediment transport is restricted. More flooding events allow more sediments to be carried with the water to be deposited on the floodplain. First two winters after the restoration have the highest suspended solid sedimentation rates. First years after the restoration, the banks and vegetation cover is not yet fully developed. 34

35 Larger amount of floodwater can be carried over the floodplain allowing more sediment to be transported. In addition, the new banks of the re-meandered river are more susceptible to erosion because the bank vegetation has not yet fully been developed. Eroded material ends up in the stream and can be transported with the floodwater to the floodplain, hence the high sedimentation rated during first two years. The amount of sedimentation increases as the flooding takes larger area. (Fig.10) A significant part of particulate phosphorus is being transported with the floodwater, therefore with the increase in the flooded area, the more sedimentation can occur, as the increase in stream discharge means that more sediment can be transported Results from other studies Results from this thesis for the 10 year period are 4,26 kg DW/m 2 for the sediment deposition and 8,32 g P/m 2 for the total phosphorus sedimentation. Summary of the comparison between different results can be seen in the table 8 below. Table 8. Results comparison with other studies Sediment deposition (kg DW/m 2 ) Results from this thesis 4,26 8,32 Denmark (Poulsen, et al., 2013) 4,72 11,4 Denmark (Kronvang, et al., 2007) 3,0 6,57 1,2 7,3 Denmark (Kronvang, et al., 2009) 0 12,6 0 8,83 England (Walling, 1999) - 5,6 The United States (Johnston, et al., 1984) - 3,6 The United States (Noe & Hupp, 2009) - 3,48 Total phosphorus deposition (g P/m 2 ) Results obtained in this thesis are comparable to the results determined by Poulsen et al. (2013), where deposition of sediment and total phosphorus during winter of was measured to be 4,72 kg DW/m 2 and 11,4 g P/m 2. Both studies are investigating the same floodplain of river Odense. In the study by Poulsen, et al. (2013) the results are obtained using a Kriging model. 35

36 Sedimentation results are also in compliance with another study in Denmark done by Kronvang, et al. (2007), where sedimentation were measured in one natural and three restored river floodplains. Results for sedimentation are in the range of 3,0 6,57 kg DW/m 2. However the phosphorus sedimentation ranges from 1,2 7,3 g P/m 2, meaning that results from this thesis show in general higher phosphorus deposition. Difference could be that the Kronvang, et al. (2007) studied sites have different catchments and land use, thus representing more soil types, including coarse-sandy type soil, which are in general not rich in phosphorus. (Kronvang, et al., 2007) As for this thesis, there is only Odense catchment represented, which is known to be rich in clay soil type. Meaning, that more phosphorus-rich particles can be deposited in clayey catchments, such as river Odense is placed. Another two year study in Denmark done by Kronvang, et al. (2009) for winters and from river Odense has obtained the average deposition of sediment to be 0-12,6 kg/m 2, but phosphorus 0-8,83 g P/m 2. Phosphorus sedimentation rate is in the same range as results from this thesis. However the sedimentation rate is higher in Kronvang, et al. (2009) paper, due to the fact that in this thesis the average of 10 years in being used, but Kronvang, et al. (2009) have studied the sedimentation for 2 years. If only the winters and are compared (11,03 kg/m 2 and 8,48 kg/m 2 ) and not the average for the whole period, then the results lie within the same range. Looking outside of Denmark, the phosphorus deposition results within same range has been found in 20 inundated floodplains in England i.e. 5,6 g P/m 2 (Walling, 1999) and in the United States floodplains i.e. 3,6 g P/m 2 (Johnston, et al., 1984) and 3,48 g P/m 2 (Noe & Hupp, 2009) Phosphorus release from the deposited sediment This thesis studies the phosphorus sedimentation during the winter months, when the inundation of the floodplain is most likely to take place. However, the floodplain is not inundated all year (Table.2). One of the wetland characteristics is the water level fluctuations, even though the deeper horizons stay water-saturated, the shallow zones of the floodplain can experience both dry and wet periods. (Schönbrunner, et al., 2012) During these dry and re-wetting period, phosphorus dynamics can change between wetland being a sink or source of phosphorus. 36

37 Several studies (Audet, et al., 2011), (Kronvang, et al., 2009) have tried to find out the possible phosphorus release from the sediment during the alternation of wet and dry periods. Study site for the both experiments is the same as discussed in this thesis. In the Audet, et al. (2011) paper, the release of the phosphorus from the sediment corresponded to 0,021 0,065% of the total phosphorus deposition. However, experiment by Kronvang, et al. (2009) showed that the maximum release of the deposited phosphorus can range between 11-25%. The total deposition of phosphorus for the three flooding scenarios (minimum, small and medium) are 116,72 kg, 155,71 kg and 3642,87 kg (Table.5). These are the sedimentation results for the 10 years. If we assume, the worst case scenario, that 25% of the total deposited phosphorus during 10 years is released, then 29,8 kg, 38,93 kg and 910,72 kg, accordingly to the flooding scenarios, could be potentially released in the water column. Evidently as more phosphorus is deposited in the sediment, the more phosphorus can be in theory released. Even though 910,72 kg appears as a large amount of phosphorus released, the given floodplain is able to deposit significant quantities of phosphorus each year (See App.2). In addition, some of the released phosphorus can be used for plant uptake or become resorbed back to the oxidized sediments. (Noe & Hupp, 2009), (Audet, et al., 2011) It also has to be kept in mind that this is the maximum release rate found during a laboratory experiment. Nevertheless the worst case scenario, 75% of the phosphorus would still be retained within the floodplain. This statement is true if it is assumed that laboratory experiments cover all the influencing factors. Both studies are done under aerobic conditions. However, floodplains can have both aerobic and anaerobic zones, which influence the phosphorus dynamics in the floodplain. 5.3 Carbon sequestration Wetlands sequester carbon when carbon dioxide fixation exceeds the decomposition rate or it is being deposited with the sediments. Decomposition rate depends on the oxygen availability, water level, temperature, substrate and nutrient availability and microbial population. (Reddy & DeLaune, 2008) It is known, that the studied wetland is inundated 37

38 during winter months (Table 2). Carbon decomposition slows down significantly during waterlogged conditions, thus enhancing the carbon accumulation at the site. Carbon content increases with the depth at the distance of 10,7 m from the stream(app. 4), this indicates that the organic material is buried into the deeper layers of the soil, encouraging the peat formation. The total carbon accumulation at the site over the 10 years is 148 kg, the carbon content has been calculated for a transect area of 115,6 m 2, giving the rate 127,90 g C/m 2 per year. Extensive study done by Mitsch, et al. (2013) have found out the average carbon sequestration rate to be between 42 and 306 g C/m 2 /year. However, the study by Mitsch, et al. (2013) includes also tropical region wetlands in Costa Rica and Botswana, which are known to sequester carbon at higher rates. Temperate zone wetlands in Ohio (The United States), from this study expressed average carbon sequestration rates to be g C/m 2 /year. (Mitsch, et al., 2013) The general range for carbon sequestration in wetlands according to Mitra, et al. (2005) is g C/m 2 /year. This range is based on the estimated total area of wetlands globally and carbon stock they are holding. Wetland studied in this thesis falls at the top of the range. The most studied wetland types in the literature are coastal and forested wetlands or tropical region wetlands and in general zones outside temperate regions. Relatively few studies have been done to study carbon sequestration in temperate climate zone riparian wetlands. One of the few studies on temperate region riparian wetlands is done by Bernal & Mitsch (2012). Their 3 studied temperate zone riparian wetlands in Northern Ohio (The United States) show a carbon sequestration rate to be g C/m 2 /year. Wetland type can also affect the carbon sequestration rates. Different wetland types will have different hydrological settings and vegetation communities. The study by Bernal & Mitsch (2012) compared the carbon sequestration between forested wetlands receiving water mainly from groundwater and precipitation with riparian wetlands that receive water also from the adjacent river. Studied wetlands are located in Ohio, the United States. On average the forested wetlands accumulated more carbon (317 g C/m 2 /year) than riparian sites (140 g C/m 2 /year). Riparian wetlands receive allochthonous carbon input from flooding, runoff and erosion events, but forested wetlands get additional organic matter input from leaf litter and 38

39 precipitation that passes through the tree canopy. Woody plant debris contain highly complex carbohydrates, such as lignin and cellulose, which are hard for microbial community to degrade. Shading in the forest lowers the temperature in forested wetlands which also contributes to decreased decomposition rates. These conditions encourage the carbon sequestration. However, both types of wetlands are able to sequester significant amounts of carbon. Even though both wetlands are located in the temperate zone, they showed different carbon sequestration rates. (Bernal & Mitsch, 2012) Therefore it can be said, that hydrological settings, organic matter that enter the wetland and vegetation play a crucial role in carbon sequestration rates and these factors have to be taken into consideration, when studying wetland ability to be a carbon sink Sediment transport Highest carbon accumulation is observed closest to the stream, with a tendency to decrease as the distance from the stream increases. Transect A3 is dominated by small inundations (Poulsen, et al., 2013) During this overbank flooding, large amounts of sediments are being transported and deposited on the floodplain. It can also be seen from the sedimentation height (App. 3 ), that it is the highest closest to the stream and rapidly decreasing towards the furthest points of the transect. There is a slight increase in the carbon content at 40,8 m, 71,8 m and 101 m distances. This can be explained by the shift in the flowpath during medium inundations (Fig 13.). The number of medium inundation events is not known, but based on the fact that small inundations are the most frequently observed at A3 transect (Poulsen, et al., 2013), it can be assumed that medium inundation events are relatively rare. Flowpath during small inundation events is marked as FLA in the Figure 13. It can be noted that the flowpath during small inundation originated from the meander located next to the transect. During medium inundation it can be seen that the flowpath (marked FLB) is also approaching the farthest sampling points of the A3 transect. Organic matter floats on top of the water, therefore it might not be evenly distributed over the floodplain. Medium inundation events can possibly bring more organic matter together with the floodwater, hence the slight increase in the carbon content farthest from the stream. (Poulsen, et al., 2013) 39

40 Figure 13. Flowpaths during small (A) and medium (B) inundations (Poulsen, et al., 2013) Sediment age Results reveal that 14 C age increases with the depth. 14 C age indicates that the sediments are between 417 and 1347 years old. Age is increasing with the depth until it reaches the reference layer from year Younger sediment on top indicates that carbon is accumulating on the site. According to the 14 C age measurements, the sequestered carbon is rather old. Which could mean that this wetland is recycling old carbon. Therefore it cannot be concluded that the studied wetland is sequestering carbon dioxide from the atmosphere, instead it deposits old organic matter, possibly transported during inundation events. Results do not clearly show that the studied wetland is exclusively using the atmospheric carbon, instead results indicate that carbon from other sources might be accumulated on the site. 40