Microorganisms and nutrient stoichiometry as mediators of soil organic matter dynamics

Microbial detritus contributes substantially to the soil organic matter (SOM). Analysis of global literature indicated that microbial detritus carbon (C) contributed 59 and 64% of total soil C in arable agricultural and grassland systems respectively, with a 2.5% greater contribution of bacterial-derived detritus in grasslands and with no difference in the proportional contribution of fungal detritus. Total soil C and nitrogen (N) content was higher in grasslands with an average of 2.8 and 1.6 g N kg⁻¹ soil and 28.8 and 16.8 g C kg⁻¹ soil in grassland and arable systems, respectively. Soil N content explained 11 to 28% of the variance in microbial detritus contribution to soil C. Further, total soil N and C content explained more variance than other factors which are commonly considered to mediate SOM content including precipitation, acidity and clay. Microbial biomass C assimilation and re-metabolism of SOM are affected by nutrient supply and the dissimilarity of the C to N, phosphorus (P) and sulfur (S) ratios between fresh organic matter (FOM), SOM and microorganisms (C:N:P:S 10,000:261:32:48, 10,000:833:200:143, and 10,000:1,494:458:154, respectively). In agricultural systems, stoichiometrically balanced nutrient addition to FOM can increase C transfer to SOM by 6 to 52% and importantly reduce the mineralization of pre-existing SOM by 24 to 50%. Future research to quantify economic and environmental implications is warranted with need for a paradigm shift in thinking to focus on the nutrient requirements of the whole soil–plant system rather than the agronomic requirements of crops alone.