Nitrogen leaching from surface soil in a temperate mixed forest subject to intensive deer grazing

We aimed to determine the grazing impact on soil nitrogen (N) mineralization and N leaching in a temperate mixed forest on Mt. Ohdaigahara, central Japan, where Sika deer (Cervus nippon Temminck; hereafter deer) grazing has decreased biomass of Sasa nipponica Makino et Shibata (hereafter sasa) that dominates the forest understory. Soil N mineralization and N leaching of the top 0.3-m layer were measured in situ from 2006 to 2007 by an ion-exchange resin core method inside and outside a deer exclosure. Deer grazing decreased the carbon to nitrogen (C:N) ratio of sasa litter because of the declining dead culm C:N ratio and the increasing ratio of leaves with a lower C:N ratio than culms in the aboveground biomass. Grazing increased N mineralization and nitrification rates as well as annual N (ammonium-N and nitrate-N) and nitrate-N leaching. In contrast, deer grazing did not increase the total soil C and N content. The positive effect of grazing on the N mineralization rate may have resulted from a qualitative change in sasa litter due to grazing. The higher annual N leaching resulting from deer grazing may have been related to the facilitated N mineralization rate and decreased N uptake in plants, e.g., sasa with less aboveground biomass due to grazing.