Effects of long-term cropping regimes on soil carbon sequestration and aggregate composition in rainfed farmland of Northeast China

Soybean (Glycine max L.)-maize (Zea mays L.) rotation has been recommended as a good cropping practice for soil quality improvement and crop productivity enhancement. However, its impacts on carbon sequestration in soil are not well documented. The main objective of this study was to learn whether soybean-maize rotation can promote soil organic carbon (SOC) sequestration. Based on a long-term field experiment (started in 1990) in Northeast China, we investigated the differences in soil organic carbon (SOC) and soil aggregate composition between cropping patterns. This experiment included four treatments: continuous maize cropping (CMC), maize-soybean rotation (MSR), continuous soybean cropping (CSC) and farmland fallow (FALL) in a Haplic Phaeozem soil. All treatments showed a sustained trend toward increasing SOC storage since 1990. The contents of SOC in the topsoil and in the profile to the 1 m depth were in the following order of CMC > MSR > CSC >= FALL suggesting a greater potential of C sequestration under cropping with manure application than under the farmland fallow without any fertilizer application. In the 1 m soil profiles, SOC levels decreased with soil depth with a major part (around 60-71%) being distributed in the 0-40 cm layer, whereas different practices led to great differential of SOC distribution. The CMC had the highest SOC levels (47.3 Mg ha(-1)) in the topsoil (0-20 cm) among the four systems. Meanwhile, maize cropping system (e.g. CMC and MSR) promoted more SOC allocation in >40 cm soil layers. Moreover, cropping pattern also differently influenced the formation and transformation of soil aggregates and the distribution of SOC in the aggregates. Macrosized aggregate and the associated C (18.6 Mg ha(-1)) dominated in the FALL while the micro-sized fractions (44.0% and 52.5%, respectively) and corresponding associated C (14.8 Mg ha(-1) and 19.2 Mg ha-1, respectively) were maximized in the MSR and CSC. The CMC had the greatest silt + clay-sized aggregate fraction (42.0%) and associated C in the macro- (13.4 Mg ha(-1)) and silt + clay-sized aggregate (12.9 Mg ha(-1)) fractions when compared with the MSR and CSC. Thus, maize-soybean rotation may be not the best cropping practice for C sequestration in the rainfed farmland Mollisol (Cumulic Hapludoll) in Northeast China, and intensive cropping with manure application can sustain the soil fertility for a long-term with high crop yield. (C) 2011 Elsevier B.V. All rights reserved.