This presentation is on the value of reducing emissions and enhancing removals of greenhouse gases related to land use and land cover change in

Transcription

1 This presentation is on the value of reducing emissions and enhancing removals of greenhouse gases related to land use and land cover change in tropical wetland forests. 1

2 The objective of this presentation is to introduce some of the methods available for assessing whether a wetland ecosystem is a net sink or a net source of greenhouse gases. 2

3 Carbon dioxide is one of the greenhouse gases that needs to be monitored. This gas is simultaneously taken up by the ecosystem through photosynthesis and emitted via respiration and decomposition processes. 3

4 Methane can be an important contributor to net greenhouse gas budgets in wetlands. This gas is both produced and consumed by the microorganisms in the soil. Some wetland plants such as rice plants can act as gas exchange channels from the soil to the atmosphere. In such cases it is important to monitor the emissions both at the soil and plant level. Nitrous oxide emissions can be substantial, especially in converted wetlands that are nitrogen fertilized. Nitrous oxide is produced and consumed by soil microorganisms through nitrification and denitrification. The capacity in heating the atmosphere is different for each greenhouse gas. This capacity is called global warming potential. Methane and nitrous oxide are more harmful than carbon dioxide with global warming potentials of 25 and almost 300 times that of carbon dioxide, respectively. 4

5 The net exchange of carbon between a terrestrial ecosystem and the atmosphere can be evaluated by looking at the change over time in either the carbon stocks or the carbon fluxes. 5

6 How do those two methods compare in the specific context of peatlands? Measuring carbon stocks in biomass is relatively straightforward and inexpensive. However measuring peat carbon stocks is more difficult, especially in waterlogged conditions. The peat has to be sampled as far as the mineral subsoil which can be complicated if the peat is deep. The main problem is the high spatial heterogeneity of the peat surface and subsurface which represents a challenge for monitoring the changes in carbon stock over time. In a flux change approach, measuring the net exchanges of carbon from the ecosystem to the atmosphere requires the use of meteorological techniques, which are expensive and sophisticated. Assessing carbon transfers into and out of the peat is feasible. The main carbon inputs to measure are litter fall and root mortality rates. Major carbon outputs are peat mineralization also called heterotrophic respiration and dissolved organic carbon losses. The stock and flux change approaches can be combined for estimating the net carbon exchange. The stock change approach is used for calculating carbon gain or loss in the aboveground biomass, while the flux change approach is used for calculating carbon gain or loss from the peat. 6

7 This slide illustrates the average rates of the carbon fluxes entering and exiting the peat in an intact peat swamp forest of Southeast Asia. The higher inputs than outputs lead to a peat carbon accumulation rate of 1.4 megagrams of carbon per hectare per year. As the vegetation is assumed to be in equilibrium there is no carbon accumulation in the biomass. Therefore the ecosystem carbon accumulation rate is the same as that in the soil. 7

8 After being converted to an oil palm plantation, the peat becomes a high source of carbon to the atmosphere. The vegetation uptakes carbon but at a low rate; therefore the ecosystem is a net carbon source emitting about 26 megagrams of carbon dioxide per hectare per year. 8

9 In addition to the ecosystem net carbon dioxide uptake from or emissions to the atmosphere. the uptake or emission of methane and nitrous oxide need to be accounted for. Both gases are exchanged through the soil but methane, as previously mentioned, may also be 9

10 channeled by some plants. Additional emissions of greenhouse gases arising from fires are important and can be extreme. The net exchange of greenhouse gas fluxes is expressed in CO2 equivalent. The fluxes of nitrous oxide and methane are converted into CO2 equivalent using their respective global warming potential. The net exchange of greenhouse gas sums the emissions of all gases from all of the mentioned pools or activities. 9

11 For instance, an intact peat swamp forest is a net sink of carbon dioxide and a small source of both methane and nitrous oxide. It s a net sink of greenhouse gas that uptakes 1.3 megagrams of CO2 equivalent per hectare per year. 10

12 The relationships between tropical peat greenhouse gas emissions and easily measurable proxies are being developed. A promising proxy for evaluating all three greenhouse gases is the mineral nitrogen content of the peat, however the relationships is 11

13 still based on a limited number of cases. The relationship between soil respiration and any proxy does not provide an estimate of peat carbon losses. Total soil respiration does not equal to net peat carbon dioxide emissions. First, the contribution of root respiration needs to be subtracted from the soil respiration rate. Second, all carbon inputs as well as other carbon outputs have to be included in the balance for calculating peat net carbon dioxide emissions. 11

14 A relationship published in 2010 indicated that peat net carbon dioxide emissions could be simply calculated from the groundwater table in converted peatlands drained deeper than 0.3 meters. It needs to be clarified that: 12

15 this relationship does not assess peat net carbon dioxide losses but soil respiration; is based on a very limited number of studies 5 only. and, taking into account all available studies from tropical peatlands, is not significant. Given the still limited knowledge on greenhouse gas emission rates in tropical peatlands, the existing proxies may be used to provide the order of magnitude of the transfers but not to reliably calculate the emissions. 12

16 GHG fluxes display high temporal variability and therefore measurements using chambers deployed on the ground need to be carried out at least monthly over a year. Diurnal and nocturnal variations should be evaluated, especially in 13

17 converted wetlands with an opened canopy where temperatures can be expected to vary substantially. Whenever a nitrogen fertilized land use is studied, intensive sampling is required following the application of the fertilizer. 13

18 Greenhouse gas fluxes are also variable in space, therefore a stratified sampling approach is recommended if different spatial positions are clearly suspected to produce different emission rates. For instance, emerged hummocks and immerged hollows during the wet season may produce substantially different methane fluxes. In an oil palm plantation, nitrous oxide or soil respiration can be expected to vary if measured close to or further from the palm. 14

19 The annual budget can be calculated by integration using a linear interpolation between sampling dates. The spatial upscaling is done by weighing the fluxes from each strata by their representative area at the plot scale. 15

20 For instance, in an oil palm plantation two spatial positions are suspected to generate different nitrous oxide emissions. The first one is the fertilized area which is a 1.5 meter radius circle around each palm; the second one the nonfertilized area is the rest of the plantation. Using the palm planting density, these areas are estimated to represent 10 and 90% of the plantation, respectively. Therefore, the average emission rates are weighted before being summed up to calculate the emissions at the plot scale. 15

21 Before any gas sampling, the chambers should be fanned manually as gases may have accumulated in them. Soil respiration is usually measured using a portable infra red gas analyzer, whereas the fluxes of 16

22 nitrous oxide and methane are commonly analyzed by gas chromatography. Gas samples are taken at chamber closure and at 10, 20 and 30 minute intervals after. The gas samples are stored in pre evacuated vials and their nitrous oxide and methane concentrations are analyzed in the laboratory by gas chromatography. The flux is calculated from the concentration change over time. 16

23 Root respiration can be excluded from soil respiration by using different methods. The most simple ones consist of either incubating root free soil cores in the laboratory or in the field and measuring carbon dioxide production or in establishing a regression between respiration and root density. Root trenching is a common method but can disturb the ecosystem. Finally in some cases, it is possible to distinguish root respiration using their isotopic signature. Aboveground litter fall is easily measurable in the field using traps. In situ litter decomposition experiments can also be undertaken for a better understanding of carbon cycling. Root dynamics can be assessed using mini rhizotrons which are clear tubes inserted into the soil where digital images are taken to follow the fate in time of individual roots. Other methods such as sequential coring or ingrowth nets can also be used. For aboveground litter an in situ experiment on decomposition rates of dead roots may provide useful information. 17

24 Greenhouse gas emissions are generally influenced by climate. Therefore it is recommended that rainfall and air temperature are monitored at an hourly or daily time step over the monitoring period. Groundwater table, soil temperature, moisture and water filled pore space have to be monitored at the same time as greenhouse gas sampling. Finally, laboratory experiments on soil nitrogen mineralization and nitrification rates are useful for understanding soil nitrous oxide emission rates. 18

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