In-situ measurement of the effect of canopy tree fine roots on nitrogen availability in forest soil

Plant fine roots affect the amount of soil inorganic nitrogen (N) via their N uptake and their effects on organic matter decomposition and N mineralization. However, the effects of the fine roots of canopy trees on N availability in forest soil remain largely unknown owing to the limitations of in-situ measurements. Therefore, to reveal the effects of the fine roots of canopy trees on forest soil N availability, we developed a novel in-situ method that combines a modified in-situ resin-core method, using live fine roots, with a tree N demand estimation, using the N-balance method. Furthermore, we measured soil extracellular enzyme activities in the rhizosphere and bulk soils to determine whether fine roots stimulate soil enzyme activities. The results showed that soil enzyme activities were stimulated by the fine roots of canopy trees, especially in the rhizosphere, and this indicates that fine roots stimulate organic matter decomposition and N mineralization. However, fine roots had no significant effect on either the pool size or leaching of inorganic N. The estimated amount of potential N uptake by fine roots was greater than the amount of inorganic N released via fine root-induced N mineralization. Overall, our results indicate that almost all of the fine root-induced increment of mineralized inorganic N is taken up by the fine roots; thus, the fine roots of canopy trees did not affect N availability in forest soil. This study was limited by the fact that the fine roots in the resin cores were less dense than natural roots, and the fact that the actual root N uptake can differ from the estimated N uptake; these limitations should be resolved in future studies. Nevertheless, the proposed method can be used to investigate the effects of fine roots on the N cycle in forest soil under field conditions considering N uptake and priming