Are variations in heterotrophic soil respiration related to changes in substrate availability and microbial biomass carbon in the subtropical forests?

Soil temperature and moisture are widely-recognized controlling factors on heterotrophic soil respiration (R(h)), although they often explain only a portion of R(h) variability. How other soil physicochemical and microbial properties may contribute to R(h) variability has been less studied. We conducted field measurements on R(h) half-monthly and associated soil properties monthly for two years in four subtropical forests of southern China to assess influences of carbon availability and microbial properties on R(h). R(h) in coniferous forest was significantly lower than that in the other three broadleaf species-dominated forests and exhibited obvious seasonal variations in the four forests (P < 0.05). Temperature was the primary factor influencing the seasonal variability of R(h) while moisture was not in these humid subtropical forests. The quantity and decomposability of dissolved organic carbon (DOC) were significantly important to R(h) variations, but the effect of DOC content on R(h) was confounded with temperature, as revealed by partial mantel test. Microbial biomass carbon (MBC) was significantly related to R(h) variations across forests during the warm season (P = 0.043). Our results suggest that DOC and MBC may be important when predicting R(h) under some conditions, and highlight the complexity by mutual effects of them with environmental factors on R(h) variations.