Euro-limpacs Deliverables

ABSTRACT - DELIVERABLE 52

Report on the use of climate controlled mesocosms for the incubation of intact cores in static chambers

Wetland ecosystems are currently under ecological stress from land−use change and pollution but may be subjected to additional pressures from climate change. Wetlands perform key ecological and socio−economic functions such as carbon sequestration and water quality amelioration. The IPCC predicts a 1.4 ? 5.8 °C average increase in the global surface temperature over the period 1990 to 2100. Local changes in temperature and flooding may have dramatic impacts on wetland soil processes that may affect their function as regulators of river water quality and as contributors to global warming.

Nitrate (NO3−) is one of the key nutrients responsible for degradation of the quality of surface waters, principally through promotion of the process of eutrophication. Elevated concentrations in groundwater are a significant concern in many parts of the world. In the UK, agriculture is the main source of nitrate − approximately 75 % of nitrates released into most UK rivers and groundwaters. There has been a long−term trend of rising nitrate concentrations in rivers in Central and Southeast England since the 1920s and although nitrate levels in many surface water sources have stabilised in recent years, they still remain high and seasonal variations frequently exceed the EC limit in drinking water.

River marginal wetlands convert nitrate leaching from agricultural land to nitrous oxide (N2O) and dinitrogen (N2) through the microbial process of denitrification. The removal of nitrate in marginal wetlands occurs as a result of both biological denitrification and assimilation by plants. The processes that operate within these ecosystems, due to their unique biogeochemical characteristics, means that they have great potential for removing nitrate from waters draining agricultural land, and preventing it entering the aquatic environment. Reduction of nitrogen in waters passing through wetlands has been reported as being regularly in excess of 80 %. The drawback to the positive effect of denitrification in wetlands on nitrate leaching from agricultural land is the emission of N2O. Nitrous oxide is a potent greenhouse gas with a global warming potential 310 times that of carbon dioxide (CO2) and a residence time of approximately 100 years in the atmosphere. Thus there is potentially a dilemma in that changes in climate (i.e. temperature, flooding) that stimulate denitrification whilst enhancing river water quality may also stimulate N2O emissions to levels that may have a positive feedback on global warming. Management strategies aimed at increasing the area of constructed marginal wetlands or re−establishing riparian zones to reduce river nitrate, may also promote global warming. Changes in temperature and flooding may also affect soil respiration of carbon dioxide (CO2) and methane (CH4) that may affect the carbon sequestration capacity of these wetlands. Thus an important question regarding denitrification in wetland soils is how climate change will affect the N2O/N2 ratio. The work in this task aims to assess the effect of climate change on greenhouse gas emissions and denitrification.

This report describes practical studies that will involve the collection of soil cores from three types of floodplain wetland identified in the Tamar catchment and their incubation in static chambers under controlled conditions in the laboratory. Incubations will be carried out across a range of temperatures (0 ? 25 ºC) and under both flooded and non−flooded conditions for greenhouse gas emissions and denitrification. The acetylene blockage technique will be used to measure denitrification rates and N2O/N2 ratios. Models will be developed (in the form of regression equations) for predicting denitrification rates and greenhouse gas emission rates from floodplain wetlands under future climatic and hydrological scenarios.
The hypotheses to be tested in this investigation are as follows:

Hypothesis 1 ? Climate change in the Southwest of England will result in changes in temperature, rainfall and patterns of flooding.
Hypothesis 2 ? Rates of denitrification, and hence N−transfer from agricultural land to the River Tamar will change as a result of climate change.
Hypothesis 3 ? Fluxes of greenhouse gases from floodplain wetlands to the atmosphere will alter in the Tamar catchment as a result of climate change.

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