Crop yields and soil organic carbon fractions as influenced by straw incorporation in a rice-wheat cropping system in southeastern China

Field experiments were conducted to assess the effects of straw incorporation (SI) on total organic carbon (TOC) and labile organic C fractions at 0-0.4 m depths and on crop yields in a rice-wheat-cropping system at two sites [Jiangyan (JY) and Guangde (GD)] from 2012 to 2016. Two treatments were established at each site: straw removal (SR) and SI. Soil samples were collected from depths of 0-0.1, 0.1-0.2 and 0.2-0.4 m. Dissolved organic C (DOC), microbial biomass C (MBC), easily oxidizable C (EOC) and particulate organic C (POC) were measured. A C pool management index (CPMI) was also calculated. Straw incorporation increased average rice and wheat grain yields by 11.6 and 11.1%, respectively, at the JY site. However, at the GD site, no significant differences were found in average crop yields between treatments. The TOC at 0-0.2 m was 13.8-22.3% higher under SI than under SR at the JY site, whereas 32.2% higher value was found only at 0-0.1 m at the GD site. DOC, MBC and EOC at 0-0.1 m, and POC at 0-0.2 m were all greater under SI than under SR at both sites. Thus, POC is a more sensitive indicator of changes in TOC induced by SI in topsoil. Straw incorporation led to a 1.4-1.6 times higher CPMI at 0-0.1 m at both sites. We conclude that SI is an effective practice for improving crop productivity and TOC sequestration in a rice-wheat-cropping system, although the site specificity of this practice should be considered before it is adopted.