Seasonal and annual variability of soil warming effects on belowground carbon cycles in a cool-temperate deciduous forest
Global warming would alter carbon cycle processes in terrestrial ecosystems. Although a number of studies have demonstrated the influence of mean annual temperature rise on soil respiration (Rs), we need to explore the responses of those processes to changing environments throughout seasons and years by considering the phenology and inter-annual variation in climate conditions to achieve accurate estimate of future changes in carbon cycle. In this study, to clarify the warming effects on belowground carbon cycle and their temporal changes under natural conditions, we conducted an open-field soil warming experiment in a cool-temperate deciduous broad-leaved forest at Takayama site (TKY for AsiaFlux and JaLTER, 36°08′N, 137°25′E, 1420 m a.s.l.) in Japan characterized by monsoon climate in summer. Soil temperature was increased by +3°C during snow-free seasons from 2012 to 2014, by heating cables under the soil surface. We investigated seasonal and annual variations in Rs, heterotrophic (Rh) and autotrophic (root, Ra) respirations with the changes in carbon and nitrogen concentrations in litter, roots and soils. Separation of Rh and Ra was made by a trenching method.
Q10 of temperature response of Rs showed remarkable seasonal changes both in warmed and control plots for the three experimental years: higher in early and late growing seasons than in mid-season (summer). Q10 in warmed plots was lower than in control plots throughout the seasons, suggesting thermal acclimation in the former. Magnitude of warming effect was variable among components and seasons: warming effect on Rs was higher in early and late growing seasons, effect on Rh was highest in late growing season, while effect on Ra was often negative in mid-season. Overall, soil warming stimulated Rs, Rh and Ra by 19%, 24% and 10%, respectively, in 2013. However, soil warming did not apparently alter the carbon and nitrogen contents. Warming effect on Rs gradually declined over the experimental years from +24% (2012) to +12% (2014). The decline in the warming effect was larger in Ra than in Rh. Our results demonstrate that (1) thermal acclimation in temperature-response of Rs, (2) seasonality of temperature response of Rs, and (3) possible influence of plant phenology must be considered for the accurate prediction in changing environments. We suggest further research on soil carbon dynamics should be conducted in couple with aboveground processes as they are responsible for belowground processes.