Response of soil organic carbon fractions to increasing rates of crop residue return in a wheat–maize cropping system in north-central China

Labile organic carbon (C) in soil can act as a sensitive indicator of its quality, and understanding its response to crop residue incorporation rates is critical to increase soil C storage by residue return in conjunction with chemical fertilisation. A 30-year field experiment was carried out to study the effects of various rates of maize residue return on soil organic C fractions in the presence of chemical fertilisers in a wheat–maize cropping system in north-central China. Studies included a no-fertiliser and no-residues control (CK) and maize residue return at rates of 0 (S0), 2250 (S1), 4500 (S2), and 9000 kg ha-1 (S3) using chemical fertilisers. Soil total organic C (TOC) and labile organic C fractions were determined. The S0 treatment increased soil microbial biomass C (MBC), KMnO4-oxidisable C (KMnO4-C), and TOC, but did not change water-soluble organic C (WSOC), light fraction organic C (LFOC), and particulate organic C (POC), relative to CK. All organic C fractions did not differ between S0 and S1; however, S2–S3 increased MBC, WSOC, LFOC, POC, KMnO4-C, and TOC by 31.8–41.0%, 17.7–28.6%, 33.9–81.3%, 35.3–82.4%, 19.3–42.8%, and 9.7–20.4% compared with S0 respectively. The KMnO4-C had the highest correlation with TOC, with LFOC and POC showing higher sensitivity to different residue-return rates. Redundancy analysis showed that LFOC, POC, and KMnO4-C were mainly affected by residue-C and root-C, while MBC was closely correlated with rhizodeposition-C levels. Overall, low rates of residue return did not affect soil labile organic C and TOC, with they only started to increase significantly when annual residue return exceeded 4500 kg ha-1 under chemical fertilisation; and the rate of increase for labile organic C was found to be higher than for non-labile C as residue inputs were increased.