Effects of biochar and humic acid application on global warming potentials of CH₄ and N₂O in a rice-wheat rotation system

Biochar is one of the most important soil amendment materials. An ever increasingly prominent role can be found in carbon sequestration and greenhouse gas, especially in the methane (CH₄) and nitrous oxide (N₂O) mitigation potential. Among them, humic acid amendments to the cropland have dominated the C, N cycling and yield. In this study, a systematic investigation was implemented to clarify the influencing mechanisms of the biochar and humic after one year of application on global warming potential (GWP) and greenhouse gas intensity (GHGI) of CH₄ and N₂O emissions under a rice-wheat rotation system. A field experiment was designed with the double factors randomized block design in an acidic paddy of Hangzhou, Zhejiang Province, China. Six treatments were conducted in 2020 before the rice seedlings were transplanted as control without biochar or humic acid (B0F0), 0.6 t/hm² humic acid without biochar (B0F1), 1.2 t/hm² humic acid without biochar (B0F2), 20 t/hm² biochar without humic acid (B1F0), 0.6 t/hm₂ humic acid with 20 t/hm² biochar (B1F1), and 1.2 t/hm² humic acid with 20 t/hm² biochar (B1F2). The CH₄ and N₂O emissions were then measured in the different treatments one year later in 2021-2022. An evaluation was also performed on the one-year field-aged biochar and humic acid amendment in a typical rice-wheat rotation system. The results showed that after application of one year 1) the biochar increased the soil organic carbon (SOC) content both in rice and wheat season; Humic acid significantly increased the SOC content in the rice season, whereas, there was no significant effect on the SOC in the wheat season. Meanwhile, there was no effect on rice yield in the application of biochar alone, humic acid alone and application of biochar and humic acid together, compared with the B0F0 treatment. The application of biochar and humic acid together significantly increased the wheat yield by 1.0%-5.0%. 2) The application of biochar and humic acid addition reduced the cumulative CH₄ emissions by 11.1%, 21.8%-25.8% and 24.7%-34.1%, compared with the B0F0 treatment, but the cumulative N₂O emissions were promoted by 33.3%, 10.0%-30.1% and 7.2%-23.7%, respectively. 3) The estimated gross GWP of CH₄ plus N₂O among the treatments was significantly reduced by the application of biochar and humic acid. The reason was attributed to the reduction of cumulative CH₄ emissions. In addition, the biochar and humic acid addition significantly reduced the GHGI during the whole rice and wheat season, in terms of greenhouse gas emissions and yield. The application of biochar and humic acid reduced the GHGI by 10.2%, 17.3%-20.2%, and 27.2%-36.2%, respectively, compared with the B0F0 treatment. The lowest GWP and GHGI were achieved in the B1F1 treatment (biochar and humic acid were added at the rate of 20 and 0.6 t/hm², respectively) among all the treatments. Consequently, outstanding performance can be expected in biochar and humic acid after the application of one year of the GHGs mitigation and carbon sequestration. A beneficial measure can also be utilized to improve the grain yields with less greenhouse gas emission in the rice-wheat rotation system.