1. Introduction
It is reported that forests contain soil carbon (C) of 618 Pg in the world (Brown, 1996), which accounts for roughly 41% of global soil C. Soil respiration ( R s ) is the second largest flux in the global C cycle (Raich and Schlesinger, 1992). Therefore, understanding variation in soil CO 2 efflux is critical to accurately predict the global C cycle. In addition, fine root dynamics would be highly involved in CO 2 efflux because of its essential function in root respiration. In this study, fine root dynamics (biomass ( B ) and production ( P )) and soil CO 2 efflux were simultaneously measured in a young larch forest in southern Hokkaido over a 12-month period to partition root respiration ( R t ) into the maintenance ( R m ) and growth ( R x ) respirations of woody fine roots. 2. Methods The study was conducted in Tomakomai, Hokkaido. A forest was naturally established on the bare ground, at which top organic soil was removed after a typhoon attack in 2004. The forest was studded with Japanese larch ( Larix kaempferi ) trees. Eight pairs of control and trenched collars were set around larch trees; five and three pairs were installed at 0.5 m (near) and 1 m (far) from the nearest larch stem, respectively. Trenching was made in July 2017. CO 2 efflux was measured on each collar (0.25 m 2 ) using a chamber system bimonthly in the snow- free period from May 2017. Soil temperature ( T s ) and soil water content (SWC) were continuously measured. B and P of the fine roots of larch trees were measured by the sequential core sampling and ingrowth core methods, respectively, in every control collar at intervals of about 60 days in the snow-free period from July 2017. R , was estimated as the difference between soil CO 2 effluxes in control and trenched collars considering root decomposition ( R d ) in trenched collars, and then R , was partitioned into R m and R g using the following model. (1) R, = Rg + Rm + h = c · P + d · exp(f · Ts) · B + h, where c, d, f and h are parameters determined by curve fitting. 3. Results and Discussion Soil CO 2 efflux showed a significant exponential relationship with T 3 but was not correlated to SWC. Using the exponential relationship, annual soil CO 2 efflux was calculated from continuous T 3 data. The annual values ( n = 5 or 3) from September 2017 were 400 ± 170 ( R s ), 284 ± 98 ( R T in trenched collars) and 7.3 ± 1.4 ( R d ) g C m 2 yr -1 in near collars, and 303 ± 81 ( R s ), 229 ± 35 ( R T ) and 3.4 ± 0.6 ( R d ) g C m -2 yr -1 in far collars. As a result, annual R r was 123 ± 105 and 77 ± 50 g C m 2 yr 1 , respectively, in near and far collars. For the same period, mean B was 63 g C m 2 and annual P was 80 g C m 2 yr 1 . Using the data of soil CO 2 efflux and fine root dynamics, Eq. (1) was parameterized ( P = 0.05), and then annual R g and R m were estimated to be 48 and 31 g C m 2 yr 1 , respectively. The result suggests that root respiration of the fine roots of young larch trees ( R α + R m ) accounted for 66% of total R r , including the respiration of thicker roots.
4. Conclusions
A relatively simple model was parameterized using field data of soil CO 2 efflux and fine root dynamics. Using the model, total root respiration was partitioned into the growth and maintenance respirations of the fine roots of young larch trees.