Influence of surface soil chemistry on nutrient leaching from Japanese cedar plantations and natural forests

The tree species composition has a significant impact on the biogeochemical cycle of forest ecosystems, which in turn affects nutrient leaching and stream water quality. We have formulated a hypothesis that nutrient leaching varies depending on the tree species composition owing to the properties of surface soil chemistry characterized by the tree species composition. To test this hypothesis, we have collected soil water samples below the root zone in Japanese cedar plantations (CF) and natural forests (NF) and compared nutrient leaching. We analyzed major ion chemical characteristics, net N mineralization, and net nitrification in the surface soil (A layer) to determine possible causes for differences in nutrient leaching. Our results revealed that calcium (Ca²⁺) and nitrate (NO₃⁻) concentrations in soil water below the root zone were significantly higher in CF than in NF. Additionally, the Ca²⁺ and NO₃⁻ contents in surface soil were higher in CF than in NF, which may explain the higher concentrations of Ca²⁺ and NO₃⁻ in soil water in CF. In contrast, there was no significant difference in soil net nitrification rates between CF and NF. Correlations between NO₃⁻ and Ca²⁺ concentrations in soil solution suggest that the higher availability of Ca²⁺ from surface soils in CF could partially account for the higher NO₃⁻ leaching from CF. Our study suggests that the properties of surface soil chemistry characterized by the tree species composition are responsible for causing the differences in nutrient leaching.