Mechanisms of the effects of the biochar application rate and time on farmland water, heat and gas environments during soil thawing in seasonally frozen soil areas: A two-year field trial

Biochar has shown great potential in improving the soil environment and alleviating greenhouse gas (GHG) emissions. However, the biochar effect in seasonally frozen soil areas remains unclear due to the complexity of the freeze-thaw cycle processes. To effectively regulate the soil water-heat-gas environment, three treatments, a single high-dose application (BL), two low-dose applications (BS), and no biochar application (CK), were established in a two-year field experiment. The characteristics of soil GHG emissions under the different treatments and their response relationships to soil water, heat and air permeability were explored. The results indicated that the average water content under BL and BS treatments increased by 10.78 % and 7.05 %, respectively, relative to the CK treatment in 2021, while the BS treatment exhibited the best performance in 2022. The temperature regulation effect under the different treatments showed similar trends, with the average temperature under the BS treatment in 2022 was 0.62 degrees C and 1.38 degrees C higher, respectively, than that under the BL and CK treatments. The two-year cumulative CO(2 )emission flux under the BL and BS treatments was 9.36 % and 2.98 % higher, respectively, than that under the CK treatment. Biochar increased soil macropores (>75 mu m) and promoted soil gas circulation. Resulting in increased pmoA and nosZ gene abundance and inhibited CH4 and N2 O fluxes in soil. The two-year global warming potentials (GWPs) under the BL and BS treatments were 65.7 and 113.17 kg CO2 hm(-2) lower, respectively, than that under the CK treatment. Overall, multiple low-dose biochar applications could comprise an efficient and sustainable application mode.