Temperature sensitivity of soil microbial respiration in soils with lower substrate availability is enhanced more by labile carbon input

Abstract

The temperature sensitivity (Q10) of soil microbial respiration (Rs) is a critical parameter for predicting soil carbon (C) fluxes under changing climatic conditions, and labile C is continuously input into soil via root exudates or plant litter in field. However, how Q10 responds to labile C input remains uncertain, especially across large geographical regions. We collected eight soils from tropical to cold-temperate forests along a north–south transect in eastern China, and soils were added with either water as a control or glucose as an analogue for labile C input to investigate how Q10 responds to labile C input at a large geographical scale. Then, soils were incubated under varying temperature conditions from 5 to 30 °C for 2-weeks, and Rs was measured at a high frequency (each sample was measured at 20 min intervals within 12 h) after 1 d, 7 d and 14 d incubation. Rs and Q10 were then calculated for both the control and glucose addition treatments. Compared with those in the control, glucose addition significantly increased Rs and Q10 in all forest soils. The increase in Q10 (ΔQ10) between the control and glucose addition treatments differed significantly among the eight forest soils and was the highest in mid-latitude regions. Furthermore, ΔQ10 was significantly and negatively correlated with the soil C availability index (CAI), indicating that the Q10 of soils with lower basal substrate availability (e.g., soils in mid-latitudes of eastern China) are enhanced more by labile C input than those of higher basal substrate availability. A significant negative relationship between Q10 and substrate quality (SQI, measured as basal microbial respiration rate at 0 °C) without glucose addition supported the carbon quality temperature hypothesis; however, this hypothesis was not always valid after adding glucose. Our findings highlight the importance of basal substrate availability in influencing Q10 after labile C addition, suggesting that C-climate models should incorporate different scenarios of labile C input in the future.