Coconut shell-derived biochar application reduced nitrous oxide emissions in vegetable season but increased methane emissions in late-rice season in a tropical double rice-vegetable rotation system in China: a 2-year field experiment

Context: High nitrogen (N) fertilizer inputs coupled with abundant water and heat in region of rice-vegetable rotation systems lead to low N use efficiency and high greenhouse gas (GHG) emissions. Biochar amendment can mitigate GHG emissions and enhance crop yield. However, its effect on these systems remains poorly understood. Objective and methods: A 2-year field experiment following the coconut shell-derived biochar treatment was conducted to measure GHG emissions and crop yield in a rice-vegetable rotation system with three treatments: conventional N fertilizer (CON), and CON + coconut shell-derived biochar at 20 (B1) and 40 (B2) t ha- 1. Results: The findings revealed that annual methane (CH4) and nitrous oxide (N2O) emissions were predominantly influenced by biochar application in the late-rice and pepper seasons, respectively. Compared with CON, biochar treatments significantly increased mean CH4 emissions by 111-169 % in the late-rice season, which were positively correlated to soil pH and soil organic carbon (SOC) of this season. However, B1 and B2 treatments significantly reduced mean emissions of N2O by 63.1 % and 36.7% in the pepper season, respectively. B1 treatment significantly reduced N2O emissions in both rotations, which had higher ratio of nosZ/(nirK+nirS), whereas B2 treatment reduced N2O emissions in only the first rotation. Moreover, biochar application significantly increased rice and pepper yields and increased the ratio of nosZ/(nirK + nirS) in the early-rice and pepper seasons, indicating mitigation of N2O emissions from bacterial denitrification. The global warming potential (GWP) and greenhouse gas intensity significantly decreased under the B1 treatment by 27.9 % and 60.0%, respectively. In contrast, the GWP of the B2 treatment was comparable with the CON treatment over the 2-year observation period. Conclusions: Biochar amendment at a rate of 20 t ha- 1 proved to be an effective approach to reducing total GWP and increasing yield in rice-vegetable rotation systems. Implications: Biochar application with alternative irrigation strategies in the late-rice season and other nitrate content control measures in the pepper season were recommended to further reduce the total GWP and increase N use efficiency.