Responses of Soil, Heterotrophic, and Autotrophic Respiration to Experimental Open-Field Soil Warming in a Cool-Temperate Deciduous Forest

How global warming will affect soil respiration (R (S)) and its source components is poorly understood despite its importance for accurate prediction of global carbon (C) cycles. We examined the responses of R (S), heterotrophic respiration (R (H)), autotrophic respiration (R (A)), nitrogen (N) availability, and fine-root biomass to increased temperature in an open-field soil warming experiment. The experiment was conducted in a cool-temperate deciduous forest ecosystem in northern Japan. As this forest is subjected to strong temporal variation in temperature, on scales ranging from daily to seasonal, we also investigated the temporal variation in the effects of soil warming on R (S), R (H), and R (A). Soil temperature was continuously elevated by about 4.0A degrees C from 2007 to 2014 using heating wires buried in the soil, and we measured soil respiratory processes in all four seasons from 2012 to 2014. Soil warming increased annual R (S) by 32-45%, but the magnitude of the increase was different between the components: R (H) and R (A) were also stimulated, and increased by 39-41 and 17-18%, respectively. Soil N availability during the growing season and fine-root biomass were not remarkably affected by the warming treatment. We found that the warming effects varied seasonally. R (H) increased significantly throughout the year, but the warming effect showed remarkable seasonal differences, with the maximum stimulation in the spring. This suggests that warmer spring temperature will produce a greater increase in CO2 release than warmer summer temperatures. In addition, we found that soil warming reduced the temperature sensitivity (Q (10)) of R (S). Although the Q (10) of both R (H) and R (A) tended to be reduced, the decrease in the Q (10) of R (S) was caused mainly by a decrease in the response of R (A) to warming. These long-term results indicate that a balance between the rapid and large response of soil microbes and the acclimation of plant roots both play important roles in determining the response of R (S) to soil warming, and must be carefully considered to predict the responses of soil C dynamics under future temperature conditions.