Interactions between nitrification, denitrification and anaerobic ammonium oxidation in N-enriched natural and man-made wetlands: implication for N2O emission
07 / 2005 - 07 / 2009
Diffuse pollution of nutrients as nitrogen and phosphorus from agricultural areas is increasingly recognised as a major problem in water management. In many ecosystems nitrogen is a growth-limiting factor, and an increased nitrogen input through groundwater flow or atmospheric deposition causes a major change in vegetation and microbial communities. Riparian wetlands (wetlands along streams and rivers) are known to have an important role in the reduction of diffuse pollution by removing and modifying pollutants from agricultural runoff. Recently it has been shown that nitrogen transformation in highly nitrate-loaded riparian wetlands results in a significant increase in N2O emissions. Nitrification, anaerobic ammonium oxidation, denitrification and dissimilative nitrate reduction are processes central to the N-cycle. In many wetland systems these processes are linked at the sediment/water interface. This research will unravel the complex interaction of microbial nitrogen transformations in representative wetlands primarily enriched with ammonium and others primarily enriched with nitrate. Microbial process rates in the N cycle are dependent on the supply of substrates. Plants, therefore, significantly influence the N transformation processes. Many wetlands are characterized by strong fluctuations in environmental conditions (redox, substrates, pH) both in space and in time. The variability in environmental conditions and substrate availability results in spatial patches or short periods of time with extremely high reaction rates relative to the surrounding soil matrix or relative to longer time periods (hotspots and hot moments). The objective of this project is to gain insight into the microbial N transformation processes and their interactions in N-enriched wetlands. In the field nitrous oxide emission will be measured with a gas analyser and nitrification, denitrification rates will be measured involving spiking soil samples with 15NH4 and/or 15NO3. The nutrient cycles in the root water interface and in the sediment water interface will be monitored with available (bio)sensors to measure depth profiles (ammonium, nitrate, nitrite, sulphide, pH, oxygen and redox). In this way hotspots of microbial activity in the N-cycle can be localised. Sliced sediment cores will be analysed with molecular and ecophysiological methods. The combined information of the molecular data and the field studies will reveal the identity of the major bacteria involved in the N cycle of wetlands. Results from this research will help to evaluate the environmental risk of greenhouse gas emission associated with water purification in riparian wetlands.