Effects of long-term fertilisation on soil organic carbon sequestration after a 34-year rice-wheat rotation in Taihu Lake Basin

To evaluate the long-term effects of fertilisation on soil organic carbon (SOC) sequestration in rice-wheat cropping ecosystems, SOC dynamics, stocks and fractionation were determined. The treatments included no fertiliser, mineral N and P, mineral N, P and K, organic fertiliser (OF), OF plus NP and OF plus NPK. The results showed that the average carbon inputs that derived from crop stubble, root residues and organic fertilisers were between 1.47 and 4.33 t/ha/year over the past 34 years. The average SOC stocks measured in the samples collected in 2011–2013 ranged from 31.20 to 38.52 t/ha. The range of the SOC sequestration rate was 0.11–0.40 t/ha/year with a SOC sequestration efficiency of 6.3%. Overall, organic fertilisation significantly promoted C-input, SOC and the sequestration rate compared to mineral fertilisation. The "active pool" (very labile and labile fractions) and "passive pool" (less labile and recalcitrant fractions) accounted for about 71.0% and 29.0% of the SOC fractions, respectively. Significant positive relationships between C-inputs and SOC fractions indicated that SOC was not saturated in this typical rice-wheat cropping system, and fertilisation, especially organic amendment, is an effective SOC strategy sequestration.