Different responses of soil, heterotrophic and autotrophic respirations to a 4-year soil warming experiment in a cool-temperate deciduous broadleaved forest in central Japan

The responses of soil respiratory components to global warming are poorly understood particularly in high altitude forest ecosystems. To examine the different responses of soil respiration (R-s) components, i.e., autotrophic (R-A) and heterotrophic (R-H) respirations, in a mountainous temperate forest,to elevated temperature, we conducted an open-field soil warming experiment combined with trenching treatments in a 60-year-old cool temperate deciduous broadleaved forest in central Japan. Here we particularly focused on the temporal variations of respirations under remarkable seasonal change of air and soil temperatures in the region. Rs was divided into R-A and R-H by trenching method in both warmed and control plots (three subplots for each treatment). We installed heating cables at 5 cm below the soil surface to increase soil temperature by 3.0 degrees C during snow-free seasons in the four experimental years from 2012 to 2015. The soil warming significantly increased R-s, however, the soil warming did not alter fine root biomass, carbon concentrations nor nitrogen availability in the soils, which were positively related to R-s. The magnitude of the warming effects on R-H was negatively correlated to both soil temperature and soil moisture content. In this cool-temperate forest characterized by monsoon climate in summer, the high soil moisture level may not be a limiting factor for the soil warming effect on R-H. The effect of soil warming on R-s did not vary throughout the daytime, but the effect on R-s varied seasonally because R-H showed more sensitive response to soil warming in the late-growing season when compared to other seasons. We demonstrated that the season-specific Q(10) model is able to quantify the warming impacts on annual respiration rates more accurately than apparent annual Q(10) model. The soil warming increased annual R-s, R-H, and R-A for the second year by 18.1%, 25.6%, and 5.3%, respectively, but the magnitude of those increments declined in the fourth year to 9.9%, 19.5%, and -2.8%, respectively. Our results suggest that soil warming, did not alter the temperature sensitivity of R-H in this forest soil, but might have induced thermal acclimation of R-A. This 4-year soil warming study in a mountainous forest contributes to the better understanding of the different responses of R-H and R-A to changing soil temperature conditions, and we highlight that the seasonal variations in the warming effects on R-H and R-A to future climate conditions should be taken into consideration in projecting the future soil carbon cycle.