Contrasting impacts of manure and inorganic fertilizer applications for nine years on soil organic carbon and its labile fractions in bulk soil and soil aggregates

Careful nutrient management to increase soil organic carbon (SOC) content is important in increasing agricultural productivity and maintaining ecosystem health. A field experiment was conducted for nine years to investigate the effects of manure (M) and inorganic fertilizer (NPK) on SOC and its labile fractions within the bulk soil and in soil aggregates in a wheat-maize rotation on the North China Plain. Nine treatments were included: control (CK) with no fertilization, cattle manure (M, applied at rates of 3000, 6000, 9000, and 12,000 kg ha−1 crop−1), and inorganic NPK fertilizer applied to give equivalent rates of N, P and K. Straw was returned to all plots. Results showed that fertilization significantly increased SOC sequestration and the concentrations of SOC and labile SOC fractions (cold water - extractable SOC, hot water-extractable SOC, microbial biomass C, and dissolved organic C within the bulk soil and soil aggregates). The values increased with increasing application rate of manure but not with increasing NPK fertilizer rate. The differences between manure and NPK fertilizer were apparent at rates equivalent to 9000 and 12,000 kg manure ha−1. Labile SOC fractions were linearly correlated with SOC within the bulk soil and aggregates and hot water-extractable C was the labile C fraction most sensitive to changes in SOC content. Aggregate stability was significantly positively correlated with SOC content and its labile fractions in both bulk soil and aggregates. The results indicate that straw return and/or combined application of fertilizers and manures may be feasible in achieving the ‘4 per 1000’ initiative, with manure application particularly effective. Manure application at a rate of 9000 kg ha−1 crop−1 may be the optimum strategy to sequester C and maintain high crop productivity. Periodic high application rates of manures should be integrated with appropriate inorganic fertilizer application rates to optimize nutrient management strategies on calcareous soils.