The effect of grassland renovation on soil mineral nitrogen and on nitrate leaching during winter

Renovation of grassland may increase the mineralization of organic material and leads to a high amount of mineral N in soil which can be leached in the winter period. Soil mineral N (SMN) in autumn and calculated nitrate leaching during winter were measured after the renewal of 8 y–old cut grassland on a sandy soil in NW Germany in 1999 to 2002. Several factors, which may influence the intensity of N mineralization, were investigated in the 2 years following renewal: the season of renovation (spring or late summer/early autumn), the technique (rotary cultivator or direct drilling), and the amount of N fertilization (0 or 320 kg N ha–1 y–1 in the 7 years before the renovation). Calculated nitrate-N leaching losses during winter were significantly higher following renewal in early autumn (36–64 kg N ha–1) compared to renewal in spring (1–7 kg N ha–1). This effect was only significant in the first, not in the second winter after renovation. The renovation technique had a significant effect on the nitrate-N leaching losses only in the first year after the renovation. Direct drilling led to higher leaching losses (35 kg N ha–1) than the use of a rotary cultivator (30 kg N ha–1) in the same year. Calculated nitrate losses (on average over 60 kg N ha–1) were highest after renewal of N-fertilized grassland in late summer/early autumn. To minimize N leaching losses, it would be more effective to plan grassland renewal in spring rather than in late summer/autumn. Another, however, less effective option is to reduce N fertilization before a renovation in autumn.