Substrate availability controls the temperature sensitivity of soil organic carbon decomposition: Evidence from different soil depths

Background/Question/Methods

Soils contain about twice as much carbon as the atmosphere and the decomposition of soil organic carbon (SOC) is intrinsically sensitive to temperature. However, there is still no scientific consensus on how the temperature sensitivity of SOC decomposition (as often measured in Q₁₀ value) varied with biotic and abiotic factors, one of which is substrate availability. In addition, most previous studies on SOC decomposition tend to focus on surface soils (0-20 cm), and less attention is paid to subsurface soils. The objective of this study was to explore (1) how the Q₁₀ of SOC decomposition changes with increasing soil depth (and decreasing substrate availability); and (2) how increasing substrate availability by adding glucose affects the Q₁₀ of SOC decomposition at different soil depths. We measured short-term soil CO₂ production at four different temperatures (6, 14, 22 and 30°C) using an infrared CO₂ analyzer. Treatments include four soil depths (0-20, 20-40, 40-60 and 60-80 cm), two sites (a grazed and an ungrazed annual grassland in California), and ambient (adding water) and elevated (adding glucose) substrate availability.

Results/Conclusions

We found that Q₁₀ significantly decreased with increasing soil depth (e.g. 2.08±0.07, 1.89±0.05, 1.69±0.04 and 1.53±0.07 in the four soil depths from the top to the bottom of grazed grassland) at ambient substrate availability (without glucose addition). Elevated substrate availability (adding glucose) led to significantly higher Q₁₀ in all soil layers, and the increase in Q₁₀ due to substrate addition is larger in deeper soils than in the surface soil. In addition, we observed a significantly negative relationship between carbon availability index (CAI) and glucose-induced change in Q₁₀ (ΔQ₁₀) (P<0.05), and a significantly positive relationship between microbial biomass carbon and Q₁₀ at ambient substrate availability (P<0.05). These results indicate that higher substrate availability leads to higher temperature sensitivity of SOC decomposition. Subsurface soils are less sensitive to temperature compared to surface soils, but additional substrate input to subsurface soils could increase the temperature sensitivity of SOC decomposition and potentially lead to more carbon loss to the atmosphere.