Trade-off between soil carbon sequestration and net ecosystem economic benefits for paddy fields under long-term application of biochar

The application of biochar and nitrogen fertilizer can increase rice yield, soil organic carbon (SOC) storage and reduce greenhouse gas (GHG) emissions. However, few studies have systematically evaluated the carbon footprint (CF) and net ecosystem economic benefits (NEEB) of paddy ecosystems under long-term application of biochar and nitrogen fertilizer. Here, the life cycle assessment method was used to quantify the CF and NEEB of paddy fields under different biochar and nitrogen fertilizer application rates in 7 years. Three biochar rates of 0 (B0), 4.5 (B1) and 13.5 t ha⁻¹ year⁻¹ (B2) and two nitrogen fertilizer rates of 0 (N0) and 300 kg ha⁻¹ year⁻¹ (N) were set. The results showed that B2 significantly increased methane (CH₄) emission by 38%, decreased nitrous oxide (N₂O) emission by 29%, and significantly increased global warming potential by 27% compared with B0. Besides that, biochar application significantly increased ΔC_SOC by 87%–173% and reduced CF by 1.6–1.8 Mg CO₂ eq ha⁻¹. Among them, CH₄ and N₂O emissions contributed 46%–95% of total GHG emissions, and the production and transportation of nitrogen fertilizer and biochar contributed 17%–52% of total GHG emissions. Nitrogen fertilizer application can significantly increase rice yield by 85% compared to the N0, which could bring the largest NEEB. Biochar application had a negative influence on the NEEB regardless of N application. This might be attributable to the fact that the economic gains from increased rice production and SOC caused by biochar cannot outweigh the high cost of biochar. These results suggest that the biochar application can significantly improve the SOC sequestration and reduce the CF, but also had negative effect on NEEB in paddy filed.