Soil mineral N dynamics and N2O emissions following grassland renewal

Managed grasslands are periodically renewed in north-western Europe, primarily in response to a decline in yield and nutritive value or sward damage. Grassland conversion to arable land is also a common agricultural practice on intensively used grassland sites. However, depending on the soil and its management, grassland break-up (i.e. the destruction of the grass sward and soil disturbance) is associated with the mineralisation of soil organic nitrogen (N) and the decomposition of stubbles and roots from the old grass sward, with both leading to enhanced nitrous oxide (N2O) emissions and nitrate (NO3-) leaching. Two sites were set up to investigate the effects of different grassland renewal techniques (keeping and improving the old sward, chemical sward killing and chemical killing of the sward followed by ploughing) with grassland conversion to maize cropping and permanent grassland as the reference treatments. The sites (Histic Gleysol and a Plaggic Anthrosol) differed in their organic matter content and groundwater level. N2O fluxes were measured weekly using static closed chambers for a period of two years. The relationship between N2O fluxes and explanatory/controlling variables was investigated using generalised additive models (GAM). The potential NO3- losses via NO3- leaching were quantified by taking weekly measurements of the soil mineral N (Mak) from the topsoil layer (0-30 cm) and from depth profiles (0-90 cm) in the autumn (pre-winter) and spring (post-winter). The aboveground biomass in the different treatments was also measured. Grassland renewal was not a significant source of direct N2O emissions at either experimental site during the two years of the study. There was only a short two-month period during which there were significantly increased N2O fluxes (up to a maximum of 1.6 kg N2O ha-1 day-1 in the Histic Gleysol) and treatment differences. N fertilisation (as reflected in the N content in soil), soil moisture, and microbial and plant respiratory activity were identified as important drivers of N2O emission. The destruction of the old grass sward (i.e. chemical killing by herbicide application and ploughing and conversion to maize cropping) resulted in an increased net N release of Nmin during the first year, which indicated losses via indirect N2O emission and a higher risk of NO3- leaching, especially on the sandy Plaggic Anthrosol. No yield effects were found after grassland renewal at either site. With respect to N2O mitigation and the prevention of NO3- leaching, it is recommended that the new grass sward should be rapidly established as a sink for Nmin and that the amount of available mineralised N following grassland renewal is taken into account when applying N fertiliser, as mineralisation following sward destruction provides high amounts of plant-available N.