Microbial Ecophysiology Explains Agricultural Soil Carbon Responses to Changes in Plant Communities.

Soil carbon sequestration in agricultural systems is vital to mitigating atmospheric increases in CO2 and promoting agroecosystem sustainability, yet the underlying mechanisms remain contentious. The relationship between soil C input characteristics and soil C accumulation is particularly enigmatic. Models employing first-order kinetics assume that soil C is linearly related to C inputs; however multiple studies have shown that this may not always be true, suggesting that variations in the diversity or chemistry of plant residue inputs, and their processing by microbial communities, may influence the stabilization efficiency of plant residue inputs. For example, non-linearity exists at the KBS LTER, where soil C is higher in organic than conventional systems despite receiving fewer C inputs and more intensive tillage. Previous studies have shown that changes in aggregation and particulate organic matter dynamics do not explain these differences. Here we explored a novel mechanism based on microbial ecophysiology for increases in soil C associated with organic production. We hypothesized that microbes in the organic production system would have higher growth rates and growth efficiencies and that this would result in greater biomass production and turnover over time. Using isotopically labeled substrates, we show that new C inputs are more rapidly incorporated into biomass in organic systems, which is related to 20% higher microbial growth rate, 15% higher biomass, and 20% higher growth efficiency. These differences in microbial physiology are related to greater stabilization of new C inputs in mineral-associated fractions.  Microbial communities in diverse organic cropping systems have microbial physiological characteristics that enhance the efficiency of new C input conversion to soil C.