Microbial use of C and N from organic sources - Insights from isotopic tracer literature data

Soil microorganisms use various sources of organic matter to meet their carbon (C) and energy needs. Additionally, they require nutrients such as nitrogen (N) in appropriate stoichiometric proportions. Organic sources often have higher C/N ratios than microbial biomass, which influences organic matter decomposability and the fate of C and N in the microbe-soil-plant system. In general, microbial carbon-use efficiency (CUE), the ratio of growth to C uptake, is higher for organic sources with lower C/N, promoting C stabilization in soil. While CUE has received increasing attention, less often microbial C and N transformations are jointly studied.
We collect literature data from studies applying ¹³C- and ¹⁵N-enriched organic sources with variable C/N (e.g., plant litter, microbial necromass, and small organic molecules). Isotope tracing allowed quantification of C and N originating from these sources in soil and microbial biomass. We aim to determine across studies how the recovery of C and N in the microbial biomass over time is impacted by the organic source C/N and system-specific conditions. We hypothesize that high source C/N leads to greater loss of C via respiration and thus higher relative recovery of source N than C in the microbial biomass. In contrast, low source C/N likely results in a reduced difference in the relative recovery of C and N in the microbial biomass. These patterns are likely modified by system-specific conditions such as the presence of plants or inorganic fertilization. Our contribution aims to provide insights into the joint microbial use of C and N related to organic source stoichiometry.