Effects of understory vegetation loss by deer over-browsing on plant nitrogen uptake, soil nitrogen mineralization, and nitrate leaching in Japanese cool-temperate forests

Large mammals significantly influence biogeochemical cycling in terrestrial ecosystems by altering vegetation structure through browsing. In Japanese forested watersheds, deer overpopulation has led to substantial dwarf bamboo (Sasamorpha borealis) loss from the understory. However, whether this over-browsing by sika deer alters plant nitrogen (N) uptake and triggers cascading effects on soil N cycling processes remains poorly understood. To address these gaps, we conducted empirical research in broad-conifer mixed forests (stands with and without bamboo) in Japan, where deer over-browsing has occurred since the 1980s. In 2023, we compared N cycling processes including plant N uptake, soil N mineralization, inorganic N concentrations and stocks, and nitrate leaching between stands with and without bamboo. N uptake by dwarf bamboo in stands without bamboo was significantly lower than stands with bamboo, while overstory N uptake did not differ significantly. This indicates that deer over-browsing primarily reduces N uptake through the consumption of dwarf bamboo. Additionally, although soil nitrification rates were similar, nitrate concentrations and stocks in surface soil (10-cm) and soil leachate (50-cm) were significantly higher in stands without bamboo than in stands with bamboo. This suggests that deer over-browsing leads to nitrate accumulation in surface soil and subsequent leaching due to the loss of dwarf bamboo N uptake. Our findings indicate deer negatively reshape forest N cycling via mere vegetation consumption, underscoring the importance of deer population management.