Response of soil organic C fractions and enzyme activity to integrating N fertilisation with cotton stalk or its biochar in a drip-irrigated cotton field

The objective of this study was to investigate the effect of nitrogen (N) application combined with cotton stalk or its biochar on soil organic carbon (SOC) fractions and enzyme activity. The field experiment was conducted over three cropping seasons with three organic C treatments and three N application rates. Three organic C treatments were set as no organic C amended (CK), cotton stalk (ST), cotton stalk biochar (BC). Three N applied at rates of 0, 300, and 450 kg N ha−1 (N0, N300, and N450, respectively). Results showed that N fertilisation alone reduced dissolved organic C (DOC) and N450 reduced easily oxidisable organic C (EOC) in 2015–2016. Microbial biomass C (MBC) basal soil respiration (BSR), β-glucosidase activity (BG), and N-acetyl-glucosaminidase activity (NAG) activity in N450 were greater than in N0. On average, SOC was 13.4% −36.6% greater in ST and 23.7–59.2% greater in BC than that in CK. ST and BC increased soil organic C fractions, MBC and non-labile organic C (non-LOC) were greater in BC than in ST. BC decreased soil DOC/TOC, EOC/TOC ratios, increased soil non-LOC/TOC ratio and cotton yield, ST increased the ratios of labile C and enzymes activity in soil. Therefore, integrating 300 kg ha−1 N fertilisation with biochar is an optimising management strategy to promote C sequestration and sustain the crop productivity in a drip-irrigated cotton field.