Interpreting stoichiometric homeostasis and flexibility of soil microbial biomass carbon, nitrogen, and phosphorus

The soil microbial biomass (SMB) adapts to altered soil resources either by maintaining roughly constant stoichiometry to soil carbon (C), nitrogen (N), and phosphorus (P) availability or by shifting to the elemental balance in the soil. Although a shift from stoichiometric homeostasis to flexibility (or vice versa) may affect terrestrial C and nutrient dynamics, a holistic understanding of the global distribution of microbial stoichiometric homeostasis and flexibility patterns is lacking. We synthesised three existing soil and SMB C:N:P stoichiometric ratio datasets with newly collected data to create a dataset containing 4,363 records. We devised a novel method for interpreting these data, in which the scatter plot representing the SMB C:P/soil C:P (y-axis) to SMB C:N/soil C:N (x-axis) relationship is classified into four distinct patterns: (1) both C:N and C:P are in homeostasis (along the 1:1 line), (2) only C:P exhibits flexibility (line parallel to the x-axis), (3) only C:N exhibits flexibility (line parallel to the y-axis), and (4) both C:N and C:P exhibit flexibility (concentrated at a single point). Applying this model to the large dataset, we found that microbial stoichiometric homeostasis and flexibility exhibit geographical patterns related to biome type, soil type, and precipitation, and more specifically, that natural ecosystems exhibit Pattern 1, whereas agroecosystems exhibit Pattern 3. Our findings also indicate that the SMB C:P/soil C:P and the SMB C:N/soil C:N relationships can be expressed as a simple function and are maintained by different microbial responses to soil nutrient status. These findings improve our understanding of the relationships between terrestrial C and nutrient dynamics and microbial stoichiometric homeostasis and flexibility and will enable improved modelling of these relationships.