Effects of biochar on annual greenhouse gas emissions and freeze-thaw effects in saline soil

Inner Mongolia is the main grain-producing region in China and is one of the regions subject to the most significant effects of climate warming. This study investigated the effects of biochar on annual greenhouse gas emissions and the freeze-thaw effect of saline soil in the Hetao Irrigation Area, Inner Mongolia. Three biochar addition treatments biochar rate of 15 t∙hm⁻² (B15), 30 t∙hm⁻² (B30), and control without biochar application (CK) were set up before the freeze–thaw period in 2019, and no biochar was applied thereafter. Soil CO₂, CH₄ and N₂O emission fluxes and physicochemical properties were measured in different treatments during the freeze–thaw period in the first year, sunflower growth period, and freeze–thaw period in the second year in the experiment. The results showed that biochar treatment increased soil pH, electrical conductivity, water content, temperature, and available P and K contents, and this effect persisted after two years of soil freezing-thawing. Soil CO₂ and N₂O emission fluxes were positively correlated with soil temperature and soil moisture during the entire experimental period. However, there was no significant correlation between CH₄ flux and soil temperature, moisture, conductivity, pH, or available P and K. The soil CO₂ flux in different sampling periods showed obvious seasonal variation, with a lower level in the freeze–thaw period and a higher level in the crop growing period. The overall variation pattern of CO₂ flux was similar to that of temperature. The application of biochar significantly reduced the accumulated soil CO₂ emissions during the sunflower growing period, with B15 and B30 reducing CO₂ emissions by 9.86% and 14.37%, respectively, compared to CK, whereas there was no significant effect on CO₂ emissions during the freeze–thaw periods in the first and second years. Soils in the freeze–thaw period in the first year, sunflower growth period and freeze–thaw period in the second year were dominated by CH₄ uptake, where the intensity of CH₄ uptake by soils in the freeze–thaw periods was greater than that in the sunflower growth period. Biochar application reduced cumulative soil CH₄ emissions by 31.15%−55.59%, 18.75%−28.13% and 9.33%−25.36% in the three periods, respectively, and the reduction rate decreased gradually over time. N₂O emissions usually reach their peak when soil moisture is sufficient, and in this experiment, soil N₂O emissions were mainly concentrated in the melting and crop-growing stages. Compared with CK, biochar addition significantly reduced cumulative soil N₂O emissions in three periods, ranging from 42.86% to 54.76% in the freeze–thaw period in the first year, 14.08% to 26.76% in the sunflower growing period, and 24.07% to 59.26% in the freeze–thaw period in the second year. The global warming potential (GWP) was negative in both freeze–thaw periods, indicating that no warming effect was observed in the soil during the freeze–thaw period. The GWP value of biochar treatment was significantly lower than that of CK, and the order of warming potential was CK > B15 > B30. Compared to CK, the GWP values of B15 and B30 decreased by 15.74% and 30.19%, respectively. Compared with CK, B15 and B30 significantly increased the sunflower yield by 6.51% and 9.44%, respectively. Biochar applied to salinized soil underwent two rounds of the freeze–thaw could significantly inhibit N₂O emissions and promote CH₄ uptake without significantly reducing soil CO₂ emissions.