Abstract: Aims Soil respiration (Rs) is a major process controlling soil carbon loss in forest ecosystems. However, the underlying mechanisms leading to variation in Rs along forest successional gradients are not well understood. In this study, we investigated the effects of biotic and abiotic factors on Rs along a forest successional gradient in southeast China.
Methods We selected 16 plots stratified by forest age, ranging from 20 to 120 years. In each plot, six shallow collars and six deep collars were permanently inserted into the soil. Shallow and deep collars were used to measure Rs and heterotrophic respiration (Rh), respectively. Autotrophic soil respiration (Ra) was estimated as the difference between Rs and Rh. Litter layer respiration (R L) was calculated by subtracting soil respiration measured in collars without leaf litter layer (R NL) from Rs. Rs was measured every 2 months, and soil temperature (ST) and soil volumetric water content (SVWC) were recorded every hour for 19 months. We calculated daily Rs using an exponential model dependent on ST. Daily Rs was summed to obtain cumulative annual Rs estimates. Structural equation modelling (SEM) was applied to identify the drivers of Rs during forest succession.
Important findings Rs showed significant differences among three successive stages, and it was the highest in the young stage. Ra was higher in the young stage than in the medium stage. Cumulative annual Rs and Ra peaked in the young and old stages, respectively. Cumulative annual Rh and respiration measured from soil organic matter (R SOM) decreased, whereas R L increased with forest age. The SEM revealed that cumulative annual Rs was influenced by fine root biomass and SVWC. Our results indicated that the dominant force regulating Rs on a seasonal scale is ST; however, on a successional scale, belowground carbon emerges as the dominant influential factor.

Key words: forest succession, soil respiration, fine root biomass, soil temperature