Nitrogen addition to soil affects microbial carbon use efficiency: Meta-analysis of similarities and differences in ¹³ C and ¹⁸ O approaches

The carbon use efficiency (CUE) of soil microorganisms is a critical parameter for the first step of organic carbon (C) transformation by and incorporation into microbial biomass and shapes C cycling in terrestrial ecosystems. As C and nitrogen (N) cycles interact closely and N availability affects microbial metabolism, N addition to soil may shift the microbial CUE. We conducted a meta-analysis (100 data pairs) to generalize information about the microbial CUE response to N addition in soil based on the two most common CUE estimation approaches: (i) ¹³ C-labelled substrate addition (¹³ C-substrate) and (ii) ¹⁸ O-labelled water addition (¹⁸ O-H₂ O). The mean microbial CUE in soils across all biomes and approaches was 0.37. The effects of N addition on CUE, however, were depended on the approach: CUE decreased by 12% if measured by the ¹³ C-substrate approach, while CUE increased by 11% if measured by the ¹⁸ O-H₂ O approach. These differences in the microbial CUE response depending on the estimation approach are explained by the divergent reactions of microbial growth to N addition: N addition decreases the ¹³ C incorporation into microbial biomass (this parameter is in the numerator by CUE calculation based on the ¹³ C-substrate approach). In contrast, N addition slightly increases (although statistically insignificant) the microbial growth rate (in the numerator of the CUE calculation when assessed by the ¹⁸ O-H₂ O approach), significantly raising the CUE. We explained these N addition effects based on CUE regulation mechanisms at the metabolic, cell, community, and ecosystem levels. Consequently, the differences in the microbial responses (microbial growth, respiration, C incorporation, community composition, and dormant or active states) between the ¹³ C-substrate and ¹⁸ O-H₂ O approaches need to be considered. Thus, these two CUE estimation approaches should be compared to understand microbially mediated C and nutrient dynamics under increasing anthropogenic N input and other global change effects.