Differences in aboveground biomass alonga slope position are often overserved because of varyinglevels of nutrient availability. Such differences can affect the spatial variation in soil respiration (Rsoil) via changes in biological factors (e.g., fine root biomass and litter mass), in addition to environmental factors. This study clarified the differences in Rsoil and the factors affecting Rsoil, between the upper and lower slope positions with contrastingaboveground biomass, within a small watershed covered by a Japanese cypress forest. The soil water content (SWC) was lower, whereas the soil temperature (Tsoil) and fine root biomass were higher in the upper plot (UP) than in the lower plot (LP). Rsoil was negatively correlated with SWC, but positively correlated with Tsoil and fine root biomass. These results gave rise to a positive effect of Rsoil on the UP. However, Rsoil was comparable between the plots. The results from a multiple linear regression model indicated that factors other than SWC, Tsoil, and fine root biomass increased Rsoil in the LP. We speculate that high litterfall could enhance Rsoil in the LP, as litterfall is an important source of decomposed respiration. The higher aboveground net primary production and lower fine root biomass in the LP suggest that more carbon was allocated aboveground and less carbon was allocated belowground, resultingin comparable Rsoil but different contribution of aboveground and belowground sources on Rsoil between the plots. It is considered that differences in phosphorus availability between the plots caused the different carbon allocation patterns, even at a small spatial scale of less than 100 m.
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