For decades, relationships among climate, soil texture, litter chemistry, and soil organic carbon (SOC) have been incorporated into biogeochemical models. However, these models cannot accurately reproduce the spatial distribution of SOC, nor the response of this critical C pool to environmental change. These limitations have led to an emphasis on the role of microbial substrate use efficiency (SUE, or the partitioning of assimilated carbon (C) between new microbial biomass versus respired CO2) in mediating soil C losses and the formation of stable SOC. However, relationships among environmental drivers (e.g. temperature or litter quality), SUE, and soil C storage remain poorly understood.
Here, we created ‘synthetic soils’ to evaluate how substrate recalcitrance, temperature, and soil mineralogy interactively affect SUE and the formation of SOC. Three soils varying in clay content (Mollisols, Entisols, and Vertisols) were chemically treated to remove organic C, allowing us to manipulate the chemistry of C inputs independently from mineral characteristics. The C-free soils were then amended with one of three C substrates (glucose, cellulose, and lignin), inoculated with microbes, and incubated along a temperature gradient (18, 28, and 38°C). We then examined carbon cycling in relation to our full-factorial manipulation of soil mineralogy, substrate type, and temperature.
Results/Conclusions
Mean respiration rates varied four-fold across substrate treatments, with the highest respiration rates occurring on lignin, the most chemically complex C source (p < 0.001). Additionally, the temperature sensitivity of respiration varied by substrate type (p < 0.001): respiration rates were positively correlated with temperature in the glucose (Q10 = 1.6) and lignin (Q10 = 3.4) treatments, but not the cellulose treatment. Surprisingly, respiration rates were also strongly dependent on the interaction between substrate quality and soil type (p < 0.001). In particular, glucose respiration rates were 4-6 times higher in the clay-rich Vertisols vs. the other two soil types, whereas lignin respiration rates were 50% lower in Vertisols. Our results partially support kinetic theory that the temperature sensitivity of decomposition is correlated with substrate complexity (with cellulose being the exception). However, our research also highlights how soil mineralogy can modify the relationship between substrate quality and respiration. Ultimately, these results emphasize that climate, litter quality, and soil properties mediate microbial activity in complex, non-linear ways that are not captured by current biogeochemical models.