Limited stable soil carbon accrual and increased CH4 emission by green manure in a temperate rice paddy

Abstract

Cover cropping and its biomass incorporation into the soil as green manure are recommended to increase soil organic carbon (SOC) stock and soil fertility under paddy rice cropping. However, the capacity of the soil to accumulate SOC from biomass might be limited by soil carbon saturation or priming effects. To investigate the influence of green manure on SOC accumulation in long-term rice cropping, we applied varying rates of green manure over nine years under temperate mono-rice cropping. The cover crop (a mixture of barley and hairy vetch) was incorporated at 0, 25, 50, and 100 % of the cover-crop aboveground biomass (average 0, 0.9, 1.7, and 3.4 Mg C ha-1 year-1 on dry weight, respectively) prior to the rice cropping season. The SOC stock increased proportionally with increasing green manure addition up to 50 % of biomass (3.75 Mg ha-1 y-1). However, there was no further increase in SOC at greater application rate. The contribution of mineral-associated (MAOC) and particulate OC (POC) fractions to the total SOC did not vary with green manure application rate (approximately 66-73 and 27-34 % of SOC, respectively). However, the contribution of biomass-derived C to the POC, quantified by natural abundance C isotope ratios (delta 13C), increased significantly with increasing green manure application. Conversely, the native C fraction of MAOC increased with increasing biomass addition, peaking at around 1.7 Mg C ha-1 year-1 , but declined beyond this point. The mineralized C loss, particularly as methane (CH4) and its contribution to the total loss, increased significantly with increasing biomass application. In conclusion, green manure addition can stimulate the degradation of the native SOC stock, priming effect, even in stable form (MAOC), while soil C saturation limits the ability of green manure to increase the SOC stock. More recalcitrant types of organic amendment addition (e.g. biochar) should therefore be considered to increase SOC stock and mitigate CH4 emission in rice paddy agriculture.