Nitrogen (N) limitation of terrestrial productivity is commonly observed across biomes, and soil N availability is assumed to control terrestrial productivity in many theories of ecosystem ecology. Yet there are conflicting conceptual models to explain how N availability influences decomposition of organic matter by soil microbial communities. Several lines of evidence suggest that N availability limits decomposition: the earliest stages of leaf litter decay are associated with a net import of N from the soil environment, and both observations and models show that litter with low N content organic matter decomposes more slowly. Furthermore, ectomycorrhizal fungi are thought to selectively mine organic N from soils, exacerbating N-limitation of decomposition and increasing soil carbon (C) storage. In direct contrast to these findings, experimental additions of inorganic N to soils broadly show a suppression of microbial activity, which is inconsistent with N-limitation of decomposition. Additionally, N-rich substrates may increase carbon use efficiency (CUE) of microbial growth, thereby increasing retention of C in the microbial biomass and ultimately, enhancing soil C storage by increasing the mineral sorption rate. These examples highlight the divergence in ideas regarding the role of N in regulating the processes of decomposition and soil C storage.
Here we present a mathematical synthesis of these concepts using a simple microbial model, resolving contrasting interpretations of the role of N in decomposition and soil C storage. We believe the discrepancy between patterns observed across gradients of litter stoichiometry vs. N addition experiments reflects differences in the nature of the N applied. Experimental N addition can drive acidification and osmotic stress, which can slow decomposition independent of any positive effect of relieving nutritional limitation of decomposition. Second, N-limitation of decomposition will increase C stored in particulate organic matter, but may decrease transfer rates of C to mineral surfaces due to a negative effect on microbial CUE. Hence, N availability may always limit the rate of decomposition, but the ultimate effect on soil C storage will depend on the relative contributions of particulate vs. mineral associated organic matter in a given soil profile.