Variations in biomass, production and respiration of fine roots in a young larch forest

Root respiration ( R r ) plays a crucial role in the global carbon balance, because R r 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 R r into growth respiration ( R g ) and maintenance respiration ( R m ) using a modified traditional model, in which R g was proportional to root production, and R m 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 R r of all roots was 94 g C m - 2 yr - 1 and accounted for 25 % of total soil respiration on average. The annual R r was partitioned into fine root R g , fine root R m and coarse root R m by 30, 44 and 26 % , respectively; coarse root R g was presumed to be negligible. Fine root R g and R m 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 R g to total fine root respiration was lower in the cold season with low production.