Consequence of grazing pattern and vegetation structure on the spatial variations of net N mineralisation in a wet grassland

The aim of this study was to determine whether the spatial heterogeneity of grassland vegetation structure would lead to spatial heterogeneity in the net nitrogen mineralisation process in the soil and therefore in the quantity of mineral nitrogen available for the plants. The net nitrogen mineralisation in the soil was compared between different vegetation patches generated by grazing, on two different types of plant communities: mesophilous and meso-hygrophilous.In ungrazed conditions, the net soil nitrogen mineralisation rates did not vary significantly between the two plant communities and remained relatively constant with time. Grazing by cattle or horses appeared to have two effects on the process of net soil nitrogen mineralisation. Firstly, it significantly stimulated net nitrogen mineralisation compared to ungrazed conditions and secondly, it led to spatial heterogeneity in mineralisation rates in the grazed enclosures. This spatial heterogeneity of nitrogen available for plants occurred both between and within plant communities.In the meso-hygrophilous plant community, net nitrogen mineralisation increased with grazing pressure. We suggest that a decrease of C inputs to the soil, concomitant with increasing grazing pressure, could decrease microbial nitrogen immobilisation.By contrast, in the mesophilous plant community net nitrogen mineralisation did not vary with grazing pressure. These differences in the functional responsiveness to grazing and biomass between the two plant communities could be related to the differences in the functional traits characterizing their dominant species along the grazing gradient. In the meso-hygrophilous community, the species composition switch with grazing intensity gradient led to the replacement of the perennial plant species by annual plant species which could lead to an improvement in the litter nitrogen content and an acceleration in the litter decomposition rate. By contrast, in the mesophilous plant community, the perennial species remained dominant along the grazing intensity gradient and could explain the absence of effect on the net nitrogen mineralisation rates.We suggest that at the scale of the vegetation patch, the decrease in plant biomass linked to grazing could regulate soil microorganism activity, in relation with shift in plant functional traits which improve litter decomposability.