The Nutrient Cycle Atmospheric pool Precipitation Canopy, wood, and root Litter fall SOIL 2 Soil and rock minerals cations 1 2 3 Groundwater level Organic material 5 Soil solution storage 4 4 Channel BEDROCK 1 Cation exchange 4 Leaching 2 H + and exudates 5 Acids, chelates, nutrients 3 Nutrients
Nutrients Distribution in Soils Source: Jobbagy, EG, and RB Jackson. 2001. The distribution of soil nutrients with depth: Global patterns and the inprint of plants. Biogeochemistry 53: 51-77.
Litter Production Above ground production is easy to measure. Below ground production is very difficult to assess because of the: Short life and rapid root turnover (duration of root growth). Most roots live less than one year. Difficulty in quantifying rates of root exudation.
Nutrient Cycling in Soils and Vegetation Biome Total biomass Mineral elements in biomass Net primary production Net mineral uptake Total litter fall Minerals returned in litter (kg ha -1 ) (kg ha -1 y -1 ) Tropical rainforest 517,000 11,081 34,200 2,029 27,500 1,540 (76%) Forest (Central Europe) 370,000 4,196 13,000 492 9,000 352 (71%) Northern taiga 260,000 970 7,000 118 5,000 100 (85%) Dry savanna 26,800 978 7,300 319 7,200 312 (98%) Artic tundra 5,000 159 1,000 38 1,000 37 (97%) Source: Chorley, RJ, SA Schumm, DE Sugden. 1984. Geomorphology. Methuen
Vertical Distribution of Nutrients Source: Jobbagy, EG, and RB Jackson. 2001. The distribution of soil nutrients with depth: Global patterns and the inprint of plants. Biogeochemistry 53: 51-77.
Figure 11.17 Distribution of organic carbon in four soil profiles, two well drained and two poorly drained. Poor drainage results in higher organic carbon content, particularly in the surface horizon.
Soil Organic Matter in Soil Amount in mineral soil (w/w) range in A horizon 0.5-7.0 % for North Jersey 1.5-3.0 % for South Jersey 0.75-2.0 % subsoils have less: 0.25-1.75 % Organic Soils > 20%
Classification of SOM Soil Organic Matter Living Organisms: BIOMASS Indentifiable dead tissue: DETRITUS Non-living, Non-tissue: HUMUS Humic Substances NonHumic substances
Humus Complex organic substances in soil not identifiable as organic tissue Amorphous, colloidal non-humic compounds identifiable biomolecules humic compounds product of decomposition + synthesis (polymerization) relatively stable, resistant to further breakdown
A Model of SOM Source: modified from http://www.soils.wisc.edu/virtual_museum/som/index.html
Functional Groups in SOM Carboxyl groups: -COOH Phenolic groups: -ArOH Proteinaceous material Alcoholic groups: -ROH Saccharides (sugars) Water Source: modified from http://www.soils.wisc.edu/virtual_museum/som/index.html
Humic Substances Random, complex polymers resistant to breakdown Resulting composition C 50-60% N 5% C:N ratio=10:1 P 0.6-1.2% S 0.5% Solubility classification Humin, Humic Acids, and Fulvic Acids
From Schlesinger, W.H. 1997. Biogeochemistry. Academic Press.
Humin and Fulvic and Humic Acids Component Residence time, y C,% N, % Molecular mass Climate Fulvic Acid (FA) 1,800-4,300 43-52 0.5 0.2 low Cool, temperate Humic Acid (HA) 1,900 5,400 50-60 2.4 5.0 high warm Humin 2,900 3,500 >60 5.0 8.0 high The long residence time of FA and HA contributes to the memory of a soil. Fulvic acids are soluble in water and are transported to deeper horizons along plant roots. Humic acids form strong complexes with Al, Ca, Mg, and Fe. Chatsworth, NJ
Simplified Carbon Cycle Photosynthesis: Numbers represent Pg (10 15 g) of carbon stored in the respective pools. Plants + CO 2 = Organic Molecules + O 2 Respiration: Organic Molecules + O 2 = Humus + CO 2
Estimates of Active N Pools Medium Air Form Pg N N 2 N 2 O 3,900,000 1.4 Land Plant Animals SOM 15 0.2 1,500 Sea Plant Animals Solution or suspension Dissolved N 2 0.3 0.2 1,200 22,000
The Nitrogen Cycle Figure 12.1
Carbon and Nitrogen Balances Soil C and N storage (pools) are in general balanced. Exceptions to this are: Peat bogs (accumulation of carbon). Northern peatlands contain ~30% of the global storage of SOC. Extreme deserts (accumulation of nitrogen) The result is that the amounts change rapidly over limited spans of time and then stabilize (steady state) at levels characteristic of climate, topography, etc.
Soil Carbon vs. Climate Soil C increases with Mean Annual Precipitation (MAP) and decreases with Mean Annual Temperature (MAT). Pattern is due to balance of inputs and losses as influenced by climate. Source: Amundson, R. 2001. The Carbon Budget of Soils. Annu. Rev. Earth Planet. Sci. 2001. 29:535 62
US Carbon Budget: Land Use Change Annual net sources and sinks of carbon resulting from different types of land use in the United States. Source: Houghton, RA, JL Hackler, and KT Lawrence. 1999. The U.S. Carbon Budget: Contributions from Land-Use change. Science 285: 574-578.
In the News: April 2008
Carbon Losses Carbon was lost from soils across England and Wales over the period 1978-2003 at a mean rate of 0.6% yr -1 (relative to the existing soil carbon content), reaching 2% yr -1 in soils with a carbon content greater than 10%. Source: Bellamy, P. H., P. J. Loveland, R. I. Bradley, R. M. Lark, and G. J. D. Kirk. 2005. Carbon losses from all soils across England and Wales 1978 2003. Nature 437: doi:10.1038/nature04038.
Carbon Content of Different Soil Groups Soil Group Temperate Forest Temperate Grassland Tropical Forest Tropical Grassland Shallow/saline/arid Wetlands/paddy Histosols Andosols TOTAL C mass virgin (Pg C) 7.3 222 C mass cultivated (Pg C) 5.4 168 Historic loss (1700-2000) ----------------------- Pg C ------------------------ 24.4 49.8 47.3 21.4 17.7 10.6 43.6 18.1 36.9 35.1 15.9 13.1 7.8 35.6 6.3 12.9 12.2 5.5 4.6 2.8 8.0 1.9 54
Anthropogenic Alteration to the N Cycle The main source of alteration to the N cycle derives from the application of fertilizers, which shows the most dramatic rate of increase over the last 40 years. Source: Vitousek, PM et al. 1997. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7: 737-750.
Trends in Fertilizer Use Source: Tilman et al., 2001. Forecasting agriculturally driven global environmental change. Science 292: 281-284.
Consequences of N Cycle Alteration According to Vitousek et al. (1996) human alteration of the N cycle resulted in: Approximately doubled the rate of input to the terrestrial N cycle, Increased the concentration of N x O gases in the atmosphere, Contributed to the acidification of soils, stream and lakes Increased the quantity of organic C.
What is the Effect of Increased N in the Ecosystems? Terrestrial ecosystems: Accumulation in degraded soils. Influences the accumulation (storage) of carbon. Loss in biodiversity In N-saturated systems, N is lost to groundwater (problem in southern NJ), streams, estuaries (eutrophication) and to the atmosphere.
Nitrogen Input/Output Relationship in Large Watersheds
Organic Carbon in Urban Soils B A A. In the presence of non-native earthworms and higher temperatures, the layer of litter (O horizon) is thinner in urban forest soils, but the amount of C in the soil is greater than in suburban/rural forest soils. Without earthworms the O horizon in urban forest soil is thicker than in suburban/rural soils (lower quality litter in urban stands). B. Well maintained laws (low density residential and institutional land) contained SOC densities similar to forest soils. In the city of Baltimore, Organic matter content in urban soils was negatively correlated with bulk density. Caution: these observations should not be generalized before other similar studies in cities located in different climates. Source: Pouyat, R, P Groffman, I Yesilonisc, and L Hernandez. 2002. Soil carbon pools and fluxes in urban ecosystems. Environmental Pollution: S107-S118.
Decomposition Decomposition and mineralization reactions are microbial enzymatic oxidation and reduction reactions. Enzymes are catalyst that aid decomposition or building of organic material. During decomposition the material is broken down into their organic constituents and finally into CO 2 and H 2 O. Decomposition processes can take place in aerobic or anaerobic conditions.
Rate of Decomposition of Organic Materials Organic compound Sugar, starches and single proteins Hemicellulose Rate of decomposition Rapid Cellulose Fats, waxes Lignins and phenolic compounds Slow
An Example A mixture of residues from Pinus nigra, P. sylvestry, and Quercus robur % of total %age lost by decomposition by Original litter 1 st year 2 nd year 5 th year 10 th year Sugars 15 99 100 -- -- Cellulose 20 90 100 -- -- Hemicellulose 15 75 92 100 -- Lignins 40 50 74 97 100 Waxes 5 25 43 77 95 Phenols 5 10 20 43 75
Decomposition in Aerobic Soils In the presence of oxygen, the general reaction is: Organic C + O 2 CO 2 + H 2 O + energy First, the cell constituents (aminoacids, proteins, lipids, etc) are released. Then decomposition of the most resistant material occurs in stages. Final products: NH 4+, SO 2-4, NO 3-, and H 2 0.
Decomposition in Anaerobic Soils Without oxygen, decomposition proceeds very slowly. Under anaerobic conditions organic matter tends to accumulate. The final products are a variety of partially oxidized compounds: organic acids, alcohol, and methane gas.
Sequence of Redox Reactions
Factors Controlling Decomposition and Mineralization Environmental conditions promoting mineralization and decompositions are: Temperature: 25-35º C is optimum. Water content: extreme (dry and waterlogged) conditions reduce plant growth and microbial activity (~60% of porosity filled with water is optimum). Soil texture: other conditions being equal, clay tend to retain more humus. Near-neutral ph.
The Priming Effect Fresh residue addition fuels microbial activity. Microbial population and metabolic capacity increases Native, stable soil humus is attacked. Slow pool of SOM is depleted, while total SOM has increased.
Figure 11.3 Diagram of the general changes that take place when fresh plant residues are added to a soil. The arrows indicate transfers of carbon among compartments. The time required for the process will depend on the nature of the residues and the soil. Most of the carbon released during the initial rapid breakdown of the residues is converted to carbon dioxide, but the smaller amounts of carbon converted into microbially synthesized compounds (biomass) and, eventually, into soil humus should not be overlooked. Although the peak of microbial activity appears to accelerate the decay of the original humus, a phenomenon known as the priming effect, the humus level is increased by the end of the process. Where vegetation, environment, and management remain stable for a long time, the soil humus content will reach an equilibrium level in which, the carbon added to the humus pool through the decomposition of plant residues each year is balanced by carbon lost through the decomposition of existing soil humus.
Typical C/N Ratios for Soil-Related Organic Material Young legumes 12-20:1 Young grasses 20-40:1 Manure 20-50:1 Corn stalks 60:1 Oat/wheat straw 80-90:1 Tree leaves 60-100:1 Pine needles 200-250:1 Woody material 250-400:1 A C/N ratio depends on the biochemical composition of a tissue, i.e. relative amounts of protein, cellulose, lignin, etc.
Figure 11.4 Changes in microbial activity, in soluble nitrogen level, and in residual C/N ratio following the addition of either high (a) or low (b) C/N ratio organic materials. Where the C/N ratio of added residues is above 25, microbes digesting the residues must supplement the nitrogen contained in the residues with soluble nitrogen from the soil. During the resulting nitrate depression period, competition between higher plants and microbes would be severe enough to cause nitrogen deficiency in the plants. Note that in both cases soluble N in the soil ultimately increases from its original level once the decomposition process has run its course. The trends shown are for soils without growing plants, which, if present, would continually remove a portion of the soluble nitrogen as soon as it is released.
Estimates of P Pools Medium Air Land Top 0.5 m Sea Animals Form NOT APPLICABLE Plant SOIL-inorganic P SOIL-organic P Surface (top 300m) Deep Plant (sea and land) Pg P 0.5-0.55 35-40 5-10 8 85 0.07-0.075 0.03-0.05 Source: Smil, V. 2000. Phosphorous in the environment: Natural flows and human interferences. Annu. Rev. Energy Environ. 25: 53-88.
Distinctive Features of the P Cycle Much less attention than the C or N cycles: Between 1979 and 1990 >1,000 papers on the N cycle and <100 on the P cycle. P is not present in the atmosphere: Land- and water-based P cycles are disconnected. It takes longer to notice human interference. There no biotic mechanism to fix the element: Plants rely only on P present in the soil. Litter returns to the soil a large share of the assimilated nutrient.
Figure 13.6 The phosphorus cycle in soils. The boxes represent pools of the various forms of phosphorus in the cycle, while the arrows represent translocations and transformations among these pools. The three largest white boxes indicate the principal groups of phosphorus-containing compounds found in soils. Within each of these groups, the less soluble, less available forms tend to dominate.
Anthropogenic Alteration of the P Cycle (Smil, 2000) Fluxes Wind Erosion Water Erosion River Transport (particulate) River Transport (dissolved) Biomass combustion Crop Uptake Animal Waste Human Waste Organic Recycling Inorganic Fertilizers 1 Tg: Tera (10 12 ) grams Natural Preindustrial (1800) Recent (2000) ---------------------------------Tg 1 P/y------------------------------ --- <2 <3 >3 >8 >12 >27 >6 >8 >20 >1 <2 >2 <0.1 <0.2 <0.3 1 12 >1 >15 0.5 3 <0.5 >6 15
Soil P Dynamics The chemical forms of P in soils: organic (20-80%): turned over rapidly (from few days to one year). inorganic: calcium-bound, and iron- or aluminum-bound compounds. The amount of P in solution is very low: Available P 0.01% soil P tot Soil P is immobile: Diffusion to plant root Root extension/mycorrhizae
Figure 13.10 Inorganic fixation of added phosphates at various soil ph values. Average conditions are postulated, and it is not to be inferred that any particular soil would have exactly this distribution. The actual proportion of the phosphorus remaining in an available form will depend upon contact with the soil, time for reaction, and other factors. It should be kept in mind that some of the added phosphorus may be changed to an organic form in which it would be temporarily unavailable.