Reduction in net greenhouse gas emissions through a combination of pig manure and reduced inorganic fertilizer application in a double-rice cropping system: Three-year results

Manure amendment in croplands is common practice for soil carbon sequestration, and may also reduce greenhouse gas emissions. Few studies focus on the effects of manure application on the net greenhouse gas emissions (NGHGE, the global warming impacts of soil carbon sequestration and CH₄ and N₂O emissions) in double-rice cropping fields. Herein, a field experiment was conducted to analyze the effects of pig manure application in combination with reduced chemical fertilizers on the NGHGE, soil properties, and yields in a double-rice paddy field in 2012–2015. Four treatments were included: 0 N (no nitrogen fertilizer); 1/2 N (chemical nitrogen fertilizer reduced by 50%); N (100% chemical nitrogen fertilizer); and 1/2 N + PM (pig manure complemented with chemical fertilizer application). The average annual CH₄ emissions for 1/2 N + PM were 53%, 50%, and 32% higher than those for 0 N, 1/2 N, and N treatments, respectively (p < 0.05). The soil organic carbon sequestration rates (SOCSR) for 1/2 N + PM were 224%, 208%, and 192% higher than those for 0 N, 1/2 N, and N treatments, respectively (p < 0.05). The average annual N₂O emissions from 1/2 N + PM were 51% lower than those from the N treatment. Compared to 0 N, 1/2 N and N treatments, the average NGHGE for 1/2 N + PM decreased by 41%, 41%, and 52%, and the average greenhouse gas intensity (GHGI, the yield-scaled NGHGE) from 1/2 N + PM reduced by 67%, 52%, and 53%, respectively. The decreases in NGHGE and GHGI were predominantly due to increased SOCSR (contributions of 187–308% and 81–325%, respectively) in 1/2 N + PM. The average soil nitrate, microbial biomass carbon and nitrogen, soil organic carbon contents, and pH value for 1/2 N + PM treatment were higher than those for the 1/2 N and N treatments (p < 0.05). Compared to 0 N and 1/2 N treatments, 1/2 N + PM treatment significantly increased the average rice yield. However, no significant difference in average yield was observed between the 1/2 N + PM and N treatments. Gross margin analysis showed that the economic profit for 1/2 N + PM was higher than that for the other three treatments. Thus, the combined application of reduced chemical fertilizers and pig manure is an effective and economic way to neutralize greenhouse gas emissions and increase soil fertility in double-rice cropping systems.