Effect of chemical and mechanical grassland conversion to cropland on soil mineral N dynamics and N2O emission

Grassland conversion to cropland bears a risk of increased nitrate (NO3-) leaching and nitrous oxide (N2O) emission due to enhanced nitrogen (N) mineralization. This study investigates the dynamics of mineral N and N2O emissions following chemical and mechanical conversion from permanent grassland to cropland (maize) at two sites with different texture (clayey loam and sandy loam) and fertilization regime (with and without mineral N-fertilization) over a two-year period. Soil mineral N levels increased shortly after conversion and remained elevated in converted plots compared to permanent grassland or long-term cropland in the second year of investigation. Fluxes of N2O were higher from converted plots than permanent grassland or cropland. However, soil mineral N contents and cumulative N2O emissions did not differ between conversion types. Only the distribution of N2O losses over the two years differed: while losses were of similar magnitude in both years in mechanically converted plots, the major part of N2O loss in chemically converted plots occurred in the first year after conversion while emissions approximated grassland level in the second year. N2O fluxes were mainly controlled by water-filled pore space and soil NO3- levels. Despite differences in N levels at the two sites, these key findings are similar on both study sites. They indicate strongly accelerated mineralization after conversion, an effect that still lasted in the converted plots at the end of the two-year investigation irrespective of the conversion type used.