Seasonal variations in leaf litter substrate-induced respiration and metabolic quotient in a warm temperate broad-leaved forest

Seasonal variations in microbial biomass and metabolic quotient within the litter layer and the microorganism-mediated mechanisms causing seasonal variation in litter heterotrophic respiration in warm temperate forests have yet to be determined. We aimed to elucidate seasonal variations in substrate-induced respiration (SIR) as an index of microbial biomass and the ratio of basal respiration to SIR (BR/SIR) as a relative measure of the metabolic status of a microbial community in the litter layer and to investigate the microorganism-mediated mechanisms causing seasonal variation in litter heterotrophic respiration. We collected samples of unfragmented leaf litter (LL) and <1-cm litter fragments (FL) within the litter layer on a monthly basis over a 1-year period. SIR varied in accordance with seasonal temperature changes. Additionally, SIR showed a negative relationship with the C/N ratio in the early decomposition phase and a positive relationship when the C/N ratio was less than 30. BR/SIR in both LL and FL decreased as decomposition proceeded - possibly resulting from succession in the microbial community. Moreover, seasonal hysteresis in litter heterotrophic respiration at the site was similar for SIR; it was higher in spring than in autumn at the similar temperature, which may have been affected by the depletion of available carbon for microorganisms as decomposition proceeded in the mobilization phase. Our findings suggest that studies such as these should integrate assessments of the microbial community to better understand the microorganism-mediated heterotrophic respiration process.