Long-term organic fertilization combined with deep ploughing enhances carbon sequestration in a rainfed sorghum-maize rotation system

Organic fertilization and deep ploughing are expected to contribute to carbon (C) sequestration in arable soils. This hypothesis was evaluated over 11 years of chemical and organic fertilization under conventional tillage (20 cm) and deep ploughing (40 cm) in a rainfed sorghum-maize rotation system on the Loess Plateau. Soil was collected and analysed to determine the soil organic C (SOC) and total nitrogen (TN) contents in the soil profiles (0-60 cm) or in the macroaggregates, microaggregates and silt and clay fractions (0-20 cm only). The SOC and TN stocks at the 0-60 cm depth displayed a net decrease in the unfertilized control, a marginal increase with chemical fertilizer, a significant increase of 23-45 % and 19-42 %, respectively, with organic amendments and a significant increase of 54 % and 48 %, respectively, when combined with deep ploughing. A critical input amount of 2064 kg C ha(-1) yr(-1) or 162 kg N ha(-1) yr(-1) was found to be required to maintain the initial SOC or TN stock. The annual N input was positively correlated with the C sequestration efficiency (P < 0.001). This supports the microbial N mining theory. Organic fertilization increased soil aggregation and the specific activity of N-acetyl-glucosaminidase, which was positively correlated with the specific respiration rate in the whole soil profile (P < 0.001) and aggregates (P < 0.001). This appears to support the stoichiometric decomposition theory. However, deep ploughing decreased the specific activity of N-acetyl-glucosaminidase at the 40-60 cm depth compared with conventional tillage. Collectively, long-term organic fertilization combined with deep ploughing enhances SOC sequestration in three ways: directly through the input of exogenous C, indirectly by alleviating microbial N limitations and increasing the amount and stability of subsoil-associated C, and to a lesser extent by improving soil aggregation.