Global patterns of nutrient limitation in soil microorganisms

The nitrogen (N) and phosphorus (P) limitations in soil microorganisms have profound implications for key soil functions such as organic matter decomposition and soil carbon (C) sequestration. However, the extent and magnitude of microbial N and P limitation in soils worldwide remain largely unknown compared to N and P limitation in plants. Moreover, the spatial variability of microbial N and P limitation may lead to disproportionate responses of microbially driven soil processes and functions to global change factors along environmental gradients. Thus, better understanding of global patterns and drivers of microbial N and P limitation is urgently needed for predicting changes in soil functions and their consequences for terrestrial ecosystem functioning. Herein, we evaluated global patterns of microbial N and P limitation by combining profiles of extracellular enzymes (i.e. ecoenzymes; 5,259 observations) with multiple sets of observational and experimental data from natural (i.e. outside of agricultural and urban areas) terrestrial ecosystems. Our analyses reveal widespread indications of microbial P and N limitation (65 and 40% of observations, respectively) in soils worldwide, with unexpectedly frequent N and P co-limitation in the tropics. This co-limitation could be attributable to elevated microbial N demand for the synthesis of P-acquiring enzymes under P limitation, and thus likely as a secondary N limitation resulting from the inherent P deficiency in tropical soils. Upscaling prediction (0.1 × 0.1° spatial resolution) further indicated certain regions such as the Amazon Basin, Tibetan Plateau, and Siberian regions, which harbor substantial soil organic C, showed signs of strong N and P limitation in soil microorganisms, suggesting a high sensitivity of soil C cycling in these regions to nutrient perturbations. As the first global assessment of spatial variation in microbial N and P limitation, these findings provide clues to explain the long-standing “Tropical N Paradox” (i.e. the apparent up-regulation of ecosystem N cycling processes, such as biological N fixation, despite primary P limitation and high soil N levels in tropical ecosystems) and could be useful for understanding and predicting soil biogeochemical cycles in a changing world. [This study is a work that will be published in PNAS (revised stage)].