Soil organic carbon sequestration as influenced by long-term manuring and fertilization in the rice-wheat cropping system

Fertilization is a feasible approach to increase the soil organic carbon. To investigate the effect of fertilization on crop biomass carbon, dynamics of soil organic carbon and soil carbon sequestration rate in the (Oryza sativa L.)-wheat (Triticum aestivum L.) cropping system under the middle reach of the Yangtze River, central China, a thirty-three years (1981 - 2013) long-term fertilizer experiment was conducted with nine treatments, including no amendment addition treatment (control), nitrogen (N), phosphorus (P), potassium (K) fertilizer treatments (N, NP, NPK), manure (M) and manure combined with chemical fertilizer treatments (MN, MNP, MNPK, hMNPK). The results indicated that average crop biomass carbon was increased by 39.9 - 77.2% compared to unfertilized control (4.43 t ha(-1) yr(-1)) due to fertilizer application, the highest crop biomass carbon was 7.85 t ha(-1) yr(-1) in the hMNPK treatment and the lowest crop biomass carbon was 5.21 t ha(-1) yr(-1) in the N alone treatment. The annual total organic carbon input were 4.14 t ha(-1) yr(-1) in the M treatment and 5.80 t ha(-1) yr(-1) in the hMNPK treatment, which was 1.95 - 2.74 times of those in the NPK treatments (2.12 t ha(-1) yr(-1)). Total organic carbon input of soil were increased by 10.2 - 23.3 kg C ha(-1) yr(-1), and increment rate in the appended manure treatments were much higher than those in the control and inorganic fertilizer treatments. Soil organic carbon retention in the topsoil (0 - 20 cm) decreased by 0.11 - 0.14 t ha(-1) yr(-1) in the control, N and NP treatments; nevertheless, soil organic carbon sequestration rates varied from 0.03 to 0.20 t ha(-1) yr(-1) in the NPK and appended organic manure treatments. These results demonstrate that organic manure use or integrated organic manure with chemical fertilizer application can be important strategies for increasing soil organic carbon sequestration and maintaining soil quality in the rice-wheat cropping system of China.