Microorganisms and nutrient stoichiometry as mediators of soil organic matter dynamics
2020
Coonan, Elizabeth C. | Kirkby, Clive A. | Kirkegaard, John A. | Amidy, Martin R. | Strong, Craig L. | Richardson, Alan E.
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.
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