2021 年 77 巻 3 号 p. 167-178
Root respiration (Rr) plays a crucial role in the global carbon balance, because Rr accounts for about a half of soil respiration in typical forest ecosystems. Plant roots are different in metabolism and functions according to size. Fine roots, which are typically defined as roots < 2 mm in diameter, perform important ecosystem functions and consequently govern belowground carbon cycles mainly because of their high turnover rates. However, the phenological variation of fine root functions is not well understood yet. To quantitatively examine the fine root functions, we adopted an approach to partition Rr into growth respiration (Rg) and maintenance respiration (Rm) using a modified traditional model, in which Rg was proportional to root production, and Rm was proportional to root biomass and exponentially related to soil temperature. We conducted a field experiment on soil respiration and fine root biomass and production over a year in a larch‑dominated young forest developing on the bare ground after removing surface organic soil to parameterize the model. The model was significantly parameterized using the field data measured in such simplified field conditions, because we could control spatial variation in heterotrophic respiration and contamination from roots other than fine roots. The annual Rr of all roots was 94 g C m‑2 yr‑1 and accounted for 25% of total soil respiration on average. The annual Rr was partitioned into fine root Rg, fine root Rm and coarse root Rm by 30, 44 and 26%, respectively; coarse root Rg was presumed to be negligible. Fine root Rg and Rm varied according to the seasonal variations of fine root production and soil temperature, respectively; the contribution of fine root biomass was minor because of its small seasonality. The contribution of Rg to total fine root respiration was lower in the cold season with low production.