Denitrification rates in relation to groundwater level in a peat soil under grassland

In this study spatial and temporal relations between denitrification rates and groundwater levels were assessed for intensively managed grassland on peat soil where groundwater levels fluctuated between 0 and I m below the soil surface. Denitrification rates were measured every 3-4 weeks using the C2H2 inhibition technique for 2 years (2000-2002). Soil samples were taken every 10 cm until the groundwater level was reached. Annual N losses through denitrification averaged 87 kg N ha(-1) of which almost 70% originated from soil layers deeper than 20 cm below the soil surface. N losses through denitrification accounted for 16% of the N surplus at farm-level (including mineralization of peat), making it a key-process for the N efficiency of the present dairy farm. Potential denitrification rates exceeded actual denitrification rates at all depths, indicating that organic C was not limiting actual denitrification rates in this soil. The groundwater level appeared to determine the distribution of denitrification rates with depth. Our results were explained by the ample availability of an energy source (degradable C) throughout the soil profile of the peat soil.