北海道東部の森林において林冠木の細根が土壌窒素動態に与える影響

　Plants release mixtures of labile organic matter from their fine roots (root exudates) into the surrounding soil (rhizosphere). Partly due to the release of root exudates, microbial community structures and their activities within the rhizosphere differ significantly from those in other soil areas (bulk soil). Consequently, nutrient cycling processes, including nitrogen mineralization, are accelerated in the rhizosphere, facilitating nutrient acquisition by plants. This phenomenon, known as the rhizosphere effect, has been repeatedly reported in studies of herbaceous plants; however, the impact of canopy tree fine roots on soil nitrogen dynamics through the effect in forest ecosystems remains largely unknown. Here, I introduce our research investigating the root exudates and rhizosphere effects of the fine roots of canopy trees, Quercus crispula, and how these fine roots affect soil nitrogen dynamics. The quantity of root exudates varied daily rather than seasonally, with solar radiation having a strong and positive effect on the amounts. However, even after leaf fall, root exudation was observed. In the rhizosphere, specific bacterial communities were present regardless of season, while ectomycorrhizal fungal populations were higher than in the bulk soil only in summer. Extracellular enzymatic activity relating to nitrogen cycling was higher in the rhizosphere than in the bulk soil across seasons. Nitrogen uptake by the tree was likely lower in winter and spring, leading to labile nitrogen accumulation in the rhizosphere during these periods. On an annual basis, however, the impact of fine roots on apparent inorganic nitrogen dynamics was minor. These results suggest that the canopy tree, Q. crispula, accelerates soil nitrogen cycling through root exudation and rhizosphere effects, regardless of season, while the acceleration of the cycle and the utilization of available nitrogen are well-balanced annually, thereby avoiding unnecessary carbon investment.

研究紹介 (Research Report)