Effects of organic amendments on soil carbon sequestration in paddy fields of subtropical China

Although organic amendments have been recommended as one of the practices for crop production and soil carbon sequestration, little has been done to evaluate soil organic carbon (SOC) dynamics following long-term application of organic amendments. The objective of this research were to (1) assess the effect of long-term organic amendments on SOC dynamics in rice-based systems; (2) evaluate the relationship between soil carbon sequestration and carbon input based on various mineral and organic fertilization treatments. A multi-sites analysis was conducted on four long-term experiments with double-rice (three sites) and rice-wheat (one site) cropping systems which started in the 1980s in Southern China. We selected three groups of treatments in common at each site: (1) control (no fertilizer), (2) mineral nitrogen-phosphorus with and without potassium (NPK/NP), and (3) the combined treatments of mineral NP/NPK with pig manure (M), green manure (G, Astragalus sinicus L.), rice straw (S), and/or their combinations. Harvestable crop biomass was annually recorded for all plots. SOC in topsoil was determined in 1-5 yearly intervals after rice harvest. Analysis showed that organic amendments sustained or significantly increased carbon biomass, but had little effects on the coefficient of variance (CV) of the carbon biomass production compared with the mineral NPK/NP treatments. With additional carbon input, organic amendments increased SOC significantly by 7-45% after 25-28 years of fertilization compared with the mineral treatments. These combined treatments sequestered carbon at a rate from 0.20 to 0.48 t ha(-1) year(-1) under the double-rice and 0.70 to 0.88 t ha(-1) year(-1) under rice-wheat cropping system. The estimated annual SOC decomposition rate ranged from 0.15 to 0.82 t ha(-1) at these studied sites. Our analyses revealed strong positive correlations between soil carbon sequestration and carbon input for all sites. We concluded that organic amendments applied as substitution and extra nutrients had significant effect on soil carbon sequestration and served as a carbon sink for the duration of the experiments. Paddy soil high in clay content had the potential to sequester more carbon. Soil carbon sequestration efficiency-declined with carbon input at some sites with loam soil texture, suggesting applying a large amount of organic amendments is not recommended as a sustainable agricultural management practice because of the high risk of non-point environment pollution.