Biosolids amendment dramatically increases sequestration of crop residue-carbon in agricultural soils in western Illinois

• Biosolids amendment increased the sequestration of crop residue-C in agricultural soils by 2-fold.
• Biosolids amendment can relieve microbial stress and improved microbial metabolism in agricultural soils.

In agricultural soils, a large portion of C in crop residues (i.e., non-harvested plant parts left in the field) is annually lost to atmosphere due to the low C use metabolism of soil microorganisms adapting to the environmental stress (moisture stress and substrate C and N imbalance). In this study, we tested the hypothesis that amending soil with biosolids (treated sewage sludge with high stable organic matter and low C:N ratio) can improve the C metabolism of microorganisms in agricultural soils through alleviation of microbial stress, leading to increased sequestration of crop residue-C in agricultural soils. Biosolids were applied at a mean annual rate of 4.2 kg m−2 (dry weight) to eight agricultural fields (biosolids-amended) for 13 years (1972–1984) in western Illinois. Four agricultural fields (unamended) received chemical fertilizer as control. We measured the sequestration rate of crop residue-C in the soils over the span of 34 years (1972–2006) using a 13C technique. We found dramatically greater sequestration rate of crop residue-C in biosolids-amended soil (32.5 ± 1.7% of total crop residue-C) versus unamended soil (11.8 ± 1.6%). Soil microbial metabolic quotient was significantly lower in biosolids-amended than in unamended fields, indicating that biosolid-amendment reduced soil microbial stress and improved microbial C metabolism. The study concludes use of a soil amendment with high stable C and low C:N is a valid approach to transform agricultural soils from current C-neutral status to a C sink. Biosolids represent a good choice of such soil amendments.