Optimization of removal and recycling ratio of cover crop biomass using carbon balance to sustain soil organic carbon stocks in a mono-rice paddy system

The cultivation of a winter cover crop as green manure is strongly recommended to improve soil quality in mono-rice paddy systems; however, the biomass is largely removed to feed cattle in many Asian regions. To determine the minimum recycling ratio of the biomass that can sustain soil organic carbon (SOC) levels and produce more fodder for cattle, the SOC balance, which is the difference between OC input and output during rice cultivation, was evaluated with the various levels of biomass addition. The sources of OC input included cover crop biomass and fertilizer, and the OC outputs were estimated by the losses from the mineralization of C (emissions of CH4 and CO2). A mixture of barley (75% of the recommended dose, RD) and hairy vetch (hereafter, vetch, 25% of the RD) seeds were broadcast after rice harvests in 2011 and 2012, and the aboveground biomass (11.5-12 Mg ha−1, based on dry weight) harvested in the following years was incorporated at different ratios (0-100%) into soils one week before transplantation of rice with the same chemical fertilization. The incorporated OC was lost primarily through emissions of CO2 (73-85% of the OC output). However, the proportion of CH4 loss increased significantly with an increase in the rate of aboveground biomass application, which was caused by the development of anaerobic soils. A negative SOC balance, which implied soil fertility was at risk from a decreasing stock of SOC, was observed with total aboveground biomass removal. However, the balance of SOC increased significantly with an increase in level of biomass recycling and reached a sustainable level at approximately 28-30% recycling of aboveground biomass; thus, the current levels of SOC could be sustained. In conclusion, more than 30% of the aboveground biomass of the cover crop (3.4-3.6 Mg ha−1 dry weight) should be incorporated as a green manure to sustain levels of SOC in mono-rice cultivation systems with chemical fertilization.