Nitrogen transformation and retention in riparian buffer zones

Abstract

Diffuse pollution of nutrients and pesticides from agricultural areas is increasingly recognised as a major problem in water management. Ecotechnological measures as constructed wetlands and riparian buffer zones clearly have an important role in the reduction of diffuse pollution by removing and modifying pollutants from agricultural runoff. However the processes that account for the pollution retention capacity are diverse and the performance of buffer zones along climatic gradients and under varying hydrological regimes is largely unknown. This study was conducted to determine the influence of N-loading rate, vegetation and hydrologic regime on the mechanisms of nitrogen removal in riparian zones along a climatic gradient.The research was performed in several locations across Europe within the framework of a joint research project (NItrogen COntrol by LAndscape Structures in agricultural environments). Partners in this project were researchers from The Netherlands, France, England, Spain, Switserland, Romania and Poland. In the European buffer zones, denitrification was identified as the dominant process of N removal, denitrification is however also considered as a major source of the greenhouse gas nitrous oxide (N2O). Higher rates of N2O emissions found in the Dutch forested buffer zone were associated with higher nitrate concentrations in the groundwater. We conclude that N transformation by N-loaded buffer zones results in a significant increase of greenhouse gas emission. Until now, only the beneficial function of wetlands on water quality improvement has received a lot of attention. To perform a full assessment, however, we have to evaluate the precise consequences of both forms of environmental pollution to determine the environmental risks. Overall, no significant effect of climate has been observed in measurements of N removal efficiency in a range of European sites. However, N transformation proceses rates were strongly related to water table level. Three consistent water table thresholds were identified. When water table levels are within -10 cm of the soil surface, ammonification prevailed and ammonium accumulated in the topsoil. Average groundwater tables between -10 and -30 cm favor denitrification and therefore reduce the nitrogen availability in soils. At sites with water table levels below -30 cm, nitrate is the main end product as a result of high net nitrification. Tracing the groundwater flow paths in the Dutch riparian zones revealed that dilution of agricultural runoff with groundwater from a deeper aquifer caused a significant decrease in nitrate concentrations which could cause an over-estimation of the N-removal capacity of upto 60%. Besides the dilution both Dutch riparian zones were capable of reducing nitrate in subsurface runoff by biological N removal, the grassland riparian zone as a whole removed about 63% of the incoming nitrate load whereas in the more heavily loaded forested zone clear symptoms of saturation were visible and only 38% of the incoming nitrate load was removed. Riparian zones are highly valuable landscape elements from the perspective of water quality improvement and landscape connectivity, however in N-loaded systems a certain risk of N2O emission remains inevitable, still we support the general belief that riparian buffer need to be protected, restored or re-established.