Effect of ploughing and reseeding of permanent grassland on soil N, N leaching and nitrous oxide emissions from a clay-loam soil

This study evaluated the effect of grassland renovation on soil total N, soluble N, N leaching to groundwater and nitrous oxide (N₂O) fluxes from poorly drained clay loam soils on a dairy farm in Ireland. The experimental area was divided into six blocks. In June 2008, one paddock in each block was ploughed and reseeded, and subsequently compared with a permanent grassland paddock. Nitrogen in groundwater was determined in a dense network of piezometers. Soil total N along with soluble N assessed in KCl extracts were determined at three depths to 0.9 m. The N₂O fluxes were measured using the static chamber technique. Renovation decreased topsoil total N and soluble organic N (P < 0.05) resulting in an average loss of 3.1 t and 13.4 kg N ha^-1 in 1.7 years following renovation, respectively. Renovation increased nitrate and nitrite concentrations in groundwater (P < 0.0001), and N₂O fluxes (P < 0.0001) for more than a year; the residual effect decreased with time. Although the N losses due to net mineralisation were high (>3 t N ha^-1), the proportion lost via N leaching and N₂O emissions was unsubstantial (27.11 kg N ha^-1y^-1) and represented only 4.8 and 0.49 % of the N input to renovated grassland, respectively. This was likely a result of soil inherent properties and anoxic status of the soils, which likely promoted complete denitrification. Since this study was a single site study and the measurements started only months after the renovation and were relatively short term, there is some uncertainty in the results.