The effect on the carbon footprint of the rice-wheat system of substituting chemical fertilizers by pig manure: The results of a field experiment

Livestock manure is widely recycled to enhance intensive agricultural production. Nevertheless, the environmental and agronomic benefits of liquid manure, such as biogas slurry, have rarely been comprehensively evaluated. To boost food production while maintaining environmental sustainability, a field experiment was performed to assess the effects of solid and liquid manure substitution for chemical fertilizer on direct greenhouse gas (GHG) emissions, soil organic carbon (SOC), grain yield, and carbon footprints (CF) in a rice-wheat system. The treatments involved only chemical fertilizer (NPK), 50 % NPK + 50 % pig manure (50 % PM), 100 % pig manure (100%PM), 50 % NPK + 50 % biogas slurry from PM (50 % PS), and 100 % biogas slurry from PM (100 % PS) in the circumstances of equal nitrogen fertilization. The results showed that rice CH4 emissions dominated the direct GHG emissions, and followed the order of 100%PM > 50%PM > 50%PS = NPK = 100%PS on average of three years. The 100%PM had the highest area-scaled CF with 11.84 Mg CO2-eq ha−1 due to high CH4 emissions and carbon emissions from pig manure production and compost indirectly without considering the changes in SOC stock. Meanwhile, 100%PM significantly increased SOC stock at 0–30 cm depth by 49.1 %, 11.8 %, 44.8 %, and 56.5 % compared with NPK, 50%PM, 50%PS, and 100%PS, respectively (P < 0.05). Notably, 100 % PM had the lowest area-scaled CF with 0.99 Mg CO2-eq ha−1 when considering the changes in SOC stock, followed by 50%PM. The 50%PM significantly increased the annual grain yield of rice and wheat from 2010 to 2018, with an average of 4.9 %, 4.5 %, and 5.7 % compared with NPK, 100%PM, and 100%PS, respectively (P < 0.05). Overall, although there was a trade-off between SOC sequestration and CH4 emissions under pig manure, pig manure substitution for chemical fertilizer significantly decreased yield-scaled CF, followed by biogas slurry substitution relative to NPK (P < 0.05). Thus, pig manure substitution for chemical fertilizer is recommended for environmental and yield benefits simultaneously considering the whole chain of production process in a rice-wheat system. In future, strategic fertilizer management based on PM and PS could potentially offer a sustainable solution to mitigate the trade-off and sustain the respective benefits of PM (i.e. SOC and grain yield) and PS (i.e. CH4 emissions).