Effects of biochar and straw on greenhouse gas emission and its response mechanism in seasonally frozen farmland ecosystems

Abstract Freeze-thaw cycle promotes the decomposition of soil organic matter in cold regions, causing carbon and nitrogen to be emitted in the forms of CO2, CH4 and N2O, resulting in positive feedback to climate warming. To effectively regulate greenhouse gas emissions, four different regulation modes, namely, biochar addition (BA), straw addition (SA), combined biochar and straw (CBS) and a natural control (BL), were established. The characteristics of soil greenhouse gas emissions under different treatments and their response relationships to soil water, heat, carbon and nitrogen were explored. The results revealed that the SA and CBS treatments effectively inhibited the substantial reduction in soil temperature, moisture content, inorganic nitrogen and dissolved organic carbon during the freezing period; among them, the average soil inorganic nitrogen under the SA and CBS treatments increased by 15.36 and 11.62 mg·kg−1 compared to that in the BL treatment, respectively. Simultaneously, both N2O and CO2 emission fluxes were low, and the difference was small under the various treatments. However, the soil showed an absorption trend with respect to CH4, and the BA and CBS treatments promoted this effect; furthermore, the response relationships between CH4 and soil water, heat and carbon were enhanced. During the thawing period, the CBS treatment most effectively promoted the increase in soil water, heat, carbon and nitrogen, while it inhibited the flux of CH4 and N2O in soils, and the average CH4 emission flux under the CBS treatment decreased by 8.25 ~ 30.75 μg∙kg−1 relative to that under the other treatments. Concurrently, the responses of CH4 and N2O emission fluxes to soil water, heat, carbon and nitrogen were weakened under this treatment. Although the CBS treatment increased the CO2 emissions flux during this period, in view of the overall effect of the entire freeze–thaw period, the CBS treatment most effectively reduced the global warming potential (GWP) of the soil. Therefore, it is suggested that the joint application of biochar and straw is the most effective strategy for greenhouse gas budget management and soil nutrient restoration in seasonally frozen areas.