Intracellular carbon storage of microorganisms and resulting C sequestration in biosolids-amended agricultural soil

To advance our understanding of how biosolids drive soil microorganisms to contribute to carbon sequestration in agricultural soil, we quantified microbial intracellular C storage using neutral lipid fatty acids (NLFA), microbial structural biomass using phospholipid fatty acids (PLFA), and soil C sequestration by analyzing soil organic C (SOC) changes. Soil samples were collected in 2024 and several other years since 2008 in Fulton County, western Illinois, in six agricultural fields treated with biosolids (1972–1984) at 550 Mg ha⁻¹ cumulatively, five control fields received agronomic rates of chemical fertilizer, and one reference field (the highest SOC in the region). Biosolids-amended soil showed a higher NLFA per unit of PLFA (NLFA/PLFA ratio) than conventional fertilizer soil, especially for bacteria, indicating higher microbial intracellular C storage as NLFA in biosolids-amended soil. In particular, the NLFA/PLFA ratio was almost fourfold higher for Actinomycetes markers, threefold higher for Gram+ bacteria, and twofold higher for Gram- bacteria in biosolids-amended soil than in conventional fertilizer soil. Bacteria in biosolids-amended soil were evidently better supported with higher active SOC and water-holding capacity than in conventional fertilizer soil. The new SOC equilibrium 40 years after the last biosolids application was 3.18 ± 0.08 % in biosolids-amended soil, 1.26 ± 0.02 % in conventional fertilizer soil, and 2.24 ± 0.07 % in reference soil. Resulting mean net soil C sequestration in response to biosolids application was 37.1 ± 4.2 Mg ha⁻¹ compared to 3.9 ± 0.2 Mg ha⁻¹ in conventional fertilizer soil. Greenhouse gas emission reduction by biosolids application was estimated to be on average 0.59 Mg CO₂-equivalent Mg biosolids⁻¹. Microbes associated with biosolids addition have the potential to improve SOC levels for agricultural soil C sequestration via their superior NLFA intracellular storage potential.