Building up the soil carbon pool via the cultivation of green manure crops in the Loess Plateau of China

The growth of summer legumes as green manures (GM) may influence the soil C pool, thereby affecting soil quality and productivity in a traditional cropping system. However, there is a lack of research on whether the inclusion of GM crops enhances the C economy under semiarid conditions. An ongoing field experiment was conducted on Cumulic Haplustoll in the northwest region of China to (i) quantify the biomass C release patterns of three GM legumes; (ii) evaluate the effect of the GM legumes on the soil organic carbon (SOC) stock (0-20 cm depth), and (iii) estimate the soil C dynamics for the GM systems under semiarid conditions in more detail. The results showed that the percentages of the remaining GM C in the soil after one year of decomposition averaged 26% and 33% for the above-ground and below-ground residues, respectively. Thus, the 5-yr growth of GM legumes continuously significantly improved the SOC and easily oxidized organic carbon (EOOC) concentrations, as well as the corresponding stocks (except for soybean) (P < 0.01), compared with the original soil at the 0-20 cm depth. We estimated the annual decomposition rate of SOC as 1%, and the C added by the crops that remained in the SOC as 23% each year using the one-compartment model. The mean turnover time of the SOC at equilibrium was estimated to be 22 years, indicating that the loess soil was not C saturated and still had the potential for C sequestration. In general, Huai bean performed better on biomass production, C accumulation, and soil C sequestration than mung bean and soybean under the dryland conditions during the 5-yr period of this study. Consequently, the cultivation of leguminous GM during the summer is a better option than bare fallow to improve the soil fertility and environmental sustainability not only in the Loess Plateau of China but also in other similar dryland regions.