A synthetic analysis of livestock manure substitution effects on organic carbon changes in China's arable topsoil

Application of livestock manure has been recognized as an important strategy for enhancing soil organic carbon (SOC) stocks. However, the magnitude of manure application impacts on SOC changes as compared with conventional mineral fertilizers still needs to be better assessed. In this study, we conducted a meta-analysis to address the effects of various fertilization practices on SOC changes in China's arable topsoil. The analysis was based on 148 peer reviewed articles reporting 69 sites and 729 observations with an average experimental duration of 18 years across China. We compared the response of SOC content to application of manure alone (M) or manure plus mineral fertilizers (NPKM) with the SOC response to NPK or no fertilizers (Control). We systematically analyzed how SOC responses were influenced by different environmental and management factors (land use, climate and soil properties, etc.). First, the retention coefficient of manure was used to present the percentage of C input from manure "converted" to SOC. Compared to Control, on average, manure had C-retention coefficients of 31% +/- 12% (95% Confidence Interval, CI) and 18% +/- 2% for the experimental duration of <= 10 years and > 10 years, respectively. When compared to NPK, manure had C-retention coefficients of 29% +/- 13% and 9% +/- 3% for the experimental duration of <= 10 years and > 10 years, respectively. In addition, compared to Control and NPK, application of manure increased SOC content by 0.23-0.26 and 0.18-0.19 g kg(-1) yr(-1), respectively. The results also indicated that the mean differences of SOC change between the treatments with or without manure application were mainly controlled by soil properties (28-43%) and anthropogenic farming practices (21-27%). Our results highlight the significance of improving or maintaining SOC stocks by manure applications, and provide insights for making manure application recommendations or guidelines to improve SOC stocks of China's cropland based on soil properties, climatic conditions and management practices.