Similar Microbial Carbon Limitation with Soil Depth despite Decreasing Carbon Availability

Carbon availability regulates the rate of microbial soil organic matter decay and respiration. However, it remains unclear how much carbon is available to microbes, and if soil organic carbon content influences microbial carbon limitation. Here, the Michaelis-Menten parameters (V_max and k_m) were employed to estimate available carbon and the degree of carbon limitation along soil depth profiles (0-60 cm) at two study sites with varying soil organic carbon content. Available carbon as well as soil organic carbon decreased along soil depth profiles at both sites. Yet, the relative proportion of available carbon to the soil organic carbon did not change across soil depth intervals and sites. Overall, 0.11% (w/w) of soil organic carbon was available to microbes. In addition, the ratio of available carbon to K_m, a proxy for relative carbon limitation, was invariant among soils (0.22 ± 0.03), indicating common carbon limitation. Our results highlight that the Michaelis-Menten model is useful in estimating microbial carbon availability and limitation, that microbes use only a small portion of soil organic carbon at our study sites, and that microbial communities experience similar degrees of carbon limitation regardless of soil organic carbon content.