Stagnation of Soil Organic Carbon Equilibrium in the United States Midwest and Transcendence By Biosolids.

Following the cultivation of prairie soil and woodland over a century, soils in the Great Plains of the United States had lost 30 - 50% of soil organic carbon (SOC) in the upper 30 cm depth by the 1950s. With the increased availability of storage for crop residue-C entering soil, it was widely expected that SOC in agricultural soils would increase during the past 50 years under the conservation management. The 30-year monitoring in large fields in Western Illinois showed that agricultural soils with residue return and reduced tillage still lose organic carbon at a rate of 0.003-0.02 kg C m^-2 yr^-1. Our models showed that for the next 100 years, SOC in Mollisols would continue to decline until it reaches a level as that in Alfisols. It is evident from our study, although agricultural soils are potentially ideal CO₂ sink, the stagnant soil organic carbon equilibrium for the past 50 years reflected the ineffectiveness of agricultural soils in sequestering CO₂ without an improved management. We consider that the conversion of systems from native to agricultural soils had caused stress to the microbial community, which forces microbes to have high maintenance respiration and low growth, leading to a reduced microbial carbon use efficiency. Using the ¹³C technique, we found that in conventional corn-soybean cropping systems in Western Illinois nearly 90% of crop residues-C was returned to the atmosphere annually by microbial respiration without being cycled through soil organic matter. Biosolids application resulted in the increase in the rate of crop residue-C sequestered in soil by 2.8-fold, leading to a considerably lower annual return (68%) of crop residues-C to the atmosphere by microbial respiration. The study indicates that with biosolids application, SOC equilibrium in Alfisols can be transcended to that in Mollisols. We suggest a paradigm shift in soil organic matter management to improve soil microbial C use efficiency through the alleviation of microbial stress by soil amendments in addition to the change from intensive tillage to reduced tillage, and so far, biosolids are found to be a good option.