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Where X represents the element concentration in soils Y represents the element concentration in microbes. The ratio of elements in soil microbial biomass to those in soil organic matter was used to represent the microbial assimilation of elements, following the similar approach in our previous modeling analysis 15. The objective of this study was to test the hypothesis that there is convergence of microbial assimilation of soil organic carbon across C, N, P and S we further evaluated the Redfield-like stoichiometry of microbial biomass and its potential mechanisms and implications. To explore the microbial assimilation of soil elements, we analyzed a recently compiled global database of elemental concentrations in soils and soil microbial biomass 1. The living organisms, however, may also assimilate individual elements independently, given the various biochemical roles of the different elements 7. Regarding microbial C, N, P and S, it is reasonable to expect that the element concentrations in microbial biomass might resemble those in soil organic matter, the primary source of most of these elements. It is unclear, however, how soil microbes regulate internal concentrations of various elements through microbial assimilation. It is well-known that soil microbes regulate soil N and P cycling and keep their internal concentrations relatively stable compared to the C:N:P:S stoichiometry in soils 7. However, there are large variations of this ratio among terrestrial plants and microbes 13, 14. Recently, a large number of studies have reported similar Redfield-type ratios in terrestrial ecosystems, particularly for plants 12 and microbes 1, 13. Since Redfield reported the well-constrained C:N:P ratio of 106:16:1 in sea water and plankton more than seventy years ago 10, 11, many studies have confirmed nutrient stoichiometry as a backbone of ecological theory 6, 7, 12. One applicable example of the homeostasis is the constrained element ratio in living organisms 9 and the most well-known is the Redfield ratio 10.
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This homeostatic regulation is one of the basic properties of organisms, keeping the state of the organisms (e.g., nutrient contents) less variable compared to external supply variations 7, 8, 9. It is well-accepted that all organisms take up these elements from external environments and keep relatively stable concentrations inside their cells to support metabolism 5, 6, a phenomenon called stoichiometric homeostasis 7. These findings provide a mathematical explanation of element imbalance in soils and soil microbial biomass and offer insights for incorporating microbial contribution to nutrient cycling into Earth system models.Ĭarbon (C), nitrogen (N), phosphorus (P) and sulfur (S) are arguably the four most important elements in global biogeochemical cycling and the C:N:P:S stoichiometry in soils and soil microbes 1 plays an essential role in biogeochemistry-climate feedback 2, 3, 4. Meanwhile, it is estimated that the minimum requirements of soil elements for soil microbes are 0.8 mmol C Kg −1 dry soil, 0.1 mmol N Kg −1 dry soil, 0.1 mmol P Kg −1 dry soil and 0.1 mmol S Kg −1 dry soil, respectively. This correlation explains the well-constrained C:N:P:S stoichiometry with a slightly larger variation in soils than in microbial biomass. The element concentrations in soil microbial biomass follow a homeostatic regulation curve with soil element concentrations across C, N, P and S, implying a unifying mechanism of microbial assimilating soil elements. We found a convergence of the relationships between elements in soils and in soil microbial biomass across C, N, P and S. We compiled a global database of C, N, P and S concentrations in soils and microbes and developed relationships between them by using a power function model. How soil microbes assimilate carbon-C, nitrogen-N, phosphorus-P and sulfur-S is fundamental for understanding nutrient cycling in terrestrial ecosystems.