Balancing nutrient stoichiometry facilitates the fate of wheat residue‑carbon in physically defined soil organic matter fractions

Preserving and enhancing soil organic carbon (SOC) stocks is one of the major objectives for sustainable agriculture. The exogenous nutrient supply along with returning crop residues, i.e., integrated residue-nutrient management, may increase carbon (C) cycling and residue-derived microbial biomass, and therefore to affect SOC stocks. However, there is a lack of knowledge about how the integrated residue-nutrient management, that balances the resource nutrient stoichiometry, facilitates the fate (or partitioning) of residue-C in physically defined SOC fractions. Hence, through a laboratory study, we quantified the fate of wheat residue (δ¹³C-enriched, 494‰) into sequentially separated physical SOC fractions, under the interaction of different residue rates (6.7 and 20.0 g kg⁻¹ soil) and nutrient inputs (nil, low and high supplies of nitrogen, phosphorus, and sulfur) in two contrasting soils (Luvisol and Vertisol). The results showed that after 245 days, 42.7–54.2% of the newly-added residue-¹³C remained in organic matter (OM) fractions in the soils, with 22.1–40.8% in the light fraction [LF; defined as free particulate organic matter (f-POM)] and 13.9–19.5% in the heavy fraction [HF; defined as aggregate- & mineral-protected OM, which included silt-clay OM and occluded POM (o-POM)]. Following the sequential separation of HF, 8.3–15.3% of residue-¹³C was distributed to silt-clay OM and 4.2–6.1% to o-POM after 245 days. The high-residue rate (cf. low-residue) increased the amount of residue-C in SOC fractions. Narrowing the C-nutrient stoichiometric ratio in the residue treated soils via the exogenous nutrient input affected the proportional distribution of residue-C in SOC fractions at the high-residue rate only. With the high-residue rate in both soils, nutrient input (cf. no-nutrient) at both rates increased “new” residue-derived stable C formation in the HF by 17% or silt-clay associated OM by 27%, while decreased the distribution of residue-¹³C in the f-POM (LF) by 26% or o-POM by 18%. In the current study, soil type also affected the incorporation of residue-C in the organo-mineral fractions, i.e., 20% higher residue-C was incorporated in the silt-clay OM in the Vertisol than Luvisol. This study improved our knowledge on the distribution of residue-C in SOC fractions in response to integrated residue-nutrient management, which could be used to refine conceptual and mechanistic models for predicting changes in SOC storage.