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Fluxes of N 2and N 2O and contributing processes in summer after grassland renewal and grassland conversion to maize cropping on a Plaggic Anthrosol and a Histic Gleysol

Grassland renewal and grassland conversion to arable land are common agricultural practices on intensively used grassland sites, especially in north-western Europe. However, grassland ploughing can cause a flush of soil organic nitrogen (N) mineralisation due to soil disturbance during tillage and decomposition of stubble and roots from the old grass sward. This is known to result in enhanced nitrous oxide (N2O) emissions, but information about the underlying microbial processes, especially the role of N2O reduction to N2 via denitrification, is scarce. Therefore we applied the 15N gas flux method in situ to grassland recently ploughed under for maize cropping, renewed grassland and permanent grassland on a Histic Gleysol and a Plaggic Anthrosol which differed in organic matter content and drainage. We used needle injection of 15N-labelled KNO3- at three different depths in the soil to achieve homogeneous label distribution. Fluxes of N2O and N2, mineral N concentration and 15N enrichment of these were measured for 44 days after label addition. Overall, no differences in N2O and N2 emissions were found between grassland conversion/renewal and permanent grassland. N2 emissions increased up to 9115 g N ha-1 day-1 on a single sampling day following grassland conversion to maize cropping on the Histic Gleysol, leading to a great contribution of denitrification when N2O/(N2O þ N2) ratio was low. However, heterogeneity of 15N label distribution proved to be a major difficulty in the water-saturated Histic Gleysol and caused potential uncertainty in identification of various production pathways. Lower gaseous losses and higher nitrification potential were detected in the Plaggic Anthrosol, indicating a higher threat of possible leaching of excess mineral N following grassland conversion/renewal.

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