Understanding the balance between soil organic carbon (SOC) accumulation and depletion under different fertilization regimes is important for improving soil quality and crop productivity and for mitigating climate change. A long-term field experiment established in 1989 was used to monitor the influence of organic and inorganic fertilizers on the SOC stock in a soil depth of 0-60 cm under an intensive wheat-maize cropping system in the North China Plain. The study involved seven treatments with four replicates: CM, compost; HCM, half compost nitrogen (N) plus half fertilizer N; NPK, fertilizer N, phosphorus (P), and potassium (K); NP, fertilizer N and P; NK, fertilizer N and K; PK, fertilizer P and K; and CK, control without fertilization. Soil samples were collected and analyzed for SOC content in the 0-20 cm layer each year and in the 20-40 cm and 40-60 cm layers every five years. The SOC stock in the 0-60 cm depth displayed a net decrease over 20 years under treatments without fertilizer P or N, and in contrast, increased by proportions ranging from 3.7% to 31.1% under the addition of compost and fertilizer N and P. The stabilization rate of exogenous organic carbon (C) into SOC was only 1.5% in NPK-treated soil but amounted to 8.7% to 14.1% in compost-amended soils (CM and HCM). The total quantities of sequestered SOC were linearly related (P < 0.01) to cumulative C inputs to the soil, and a critical input amount of 2.04 Mg C ha(-1) yr(-1) was found to be required to maintain the SOC stock level (zero change due to cropping). However, the organic C sequestration rate in the 0-60 cm depth decreased from 0.41 to 0.29 Mg C ha(-1) yr(-1) for HCM and from 0.90 to 0.29 Mg C ha(-1) yr(-1) for CM from the period of 1989-1994 to the period of 2004-.2009, indicating that the SOC stock was getting to saturation after the long-term application of compost. The estimated SOC saturation level in the 0-60 cm depth for CM was 61.31 Mg C ha(-1), which was 1.52 and 1.14 times the levels for NPK and HCM, respectively. These results show that SOC sequestration in the North China Plain may mainly depend on the application of organic fertilizer. Furthermore, the SOC sequestration potential in the 0-20 cm layer accounted for 40.3% to 44.6% of the total amount in the 0-60 cm depth for NPK, HCM, and CM, indicating that the SOC sequestration potential would be underestimated using topsoil only and that improving the depth distribution may be a practical way to achieve C sequestration. (C) 2014 Elsevier B.V. All rights reserved.
