Grazing-induced vegetation patchiness controls net N mineralization rate in a semi-natural grassland

Herbivores influence nutrient cycling either through direct effects (e.g. excreta) or through indirect effects such as a modification of plant–soil feedbacks. This work investigated if grazing-enhanced net N mineralization rates were related to (1) enhanced quality of plant litter and/or (2) reduced plant litter inputs. Rates of net N mineralization in soil and both the quantity and quality of litters were characterized in various plant patches occurring within a grazed grassland. Soil incubations were performed in controlled conditions to assess the respective role of litter quantity and quality on N mineralization.In laboratory incubations, the effect of litter quantity on net N mineralization rates was found to depend on litter quality. High inputs of litter produced by grazing-promoted species (C/N 11) stimulated net N mineralization rates, while high inputs of litter produced by grazing-reduced species (C/N 46) decreased rates of net N mineralization. The intensity of either the negative or positive effects of litter was then regulated by litter quantity. In the field, litter quality only varied within a limited range. Litter with the highest quality in the field (C/N 22) increased N mineralization minimally compared to the lowest quality litter (C/N 46). Grazing-induced variations in litter quality monitored in the field thus appeared unlikely to cause measured variations in net N mineralization rates. Litter with C/N ratios of 46 and 22 stimulated N immobilization and reduction of their inputs increased the rate of N mineralization due to decreased microbial N immobilization. Within-grassland variations of litter quantity were large and negatively correlated with net N mineralization rates. Our results support the hypothesis that grazing-induced patchiness modifies net N mineralization rates by controlling microbial N immobilization mainly through changes in the quantity of litter-C supplied to the soil.