Potential measures to reduce nitrate and nitrous oxide losses from renovated grasslands

Grassland renovation (grassland renewal or temporary conversion to arable land) is a common practice employed by farmers to reverse declining yields in agricultural grasslands. Renovation practices may lead to substantial nitrate (NO 3 ) leaching and nitrous oxide (N 2 O) emissions. Farmers lean towards renovating grasslands in autumn instead of spring because of better sward establishment and low weed infestation. Limited nitrogen (N) uptake of renewed grassland during autumn may, however, increase the potential risk of NO 3 leaching and N 2 O emission. Potential options to mitigate these N losses are reduction in tillage intensity and N application rate, or the application of nitrification inhibitors. We assessed the effects of these measures as well as timing of renewal and the conversion to grassland to maize on NO 3 leaching and N 2 O emissions after one growing season. Five replicated field experiments were conducted of one year each at five locations in the Netherlands. We measured crop yield, soil mineral N, NO 3 concentrations in groundwater and N 2 O emission. Averaged over all experiments, the measures had an effect on NO 3 concentration in groundwater, although not significant. The highest NO 3 concentrations in groundwater were observed after autumn renewal (17.2 mg NO 3 -N L −1 ), autumn renewal combined with mitigation strategies (12.8–19.4 mg NO 3 -N L −1 ) and conversion to maize (12.5–12.8 mg NO 3 -N L −1 ), with all of these treatments exceeding the European norm of 11.3 mg NO 3 -N L −1 . Concentrations after renewal in spring were lower (3.1 mg NO 3 -N L −1 ) and did not lead to an increase compared to the control (4.4 mg NO 3 -N L −1 ). N 2 O emissions increased significantly directly after grassland renewal relative to the reference grassland and N 2 O fluxes were similar in spring and autumn. We conclude that autumn renewal leads to significant increases in the risk of NO 3 leaching which cannot be mitigated by the technical measures in this study. • Five field experiments to assess timing of renewal and technical measures on N losses. • Renewal in autumn leads to an increased risk of soil mineral N and NO 3 leaching. • Conversion to maize results in increased risk of NO 3 leaching. • N 2 O emissions increased significantly directly after grassland renewal. • Increased risk of NO 3 leaching cannot be mitigated by technical measures.