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Biochar has demonstrated potential for stabilising high yields and sequestering carbon in dryland farmland, but it is unclear whether biochar affects the carbon sequestration capacity and carbon balance of annual farmland ecosystems. For this purpose, we conducted a plot control trial in salinised farmland in 2019-2021, where we set three treatments, control, and two biochar rates, 0 (CK), 15 (B15), and 30 t/ha (B30). The results showed that biochar application decreased soil organic carbon stocks in the early part of the experiment (first freeze and freeze period); these increased in the later part, and overall, the biochar treatments increased soil organic carbon storage by 3-6% compared with the control. Compared with the control (CK), biochar inhibited the total soil respiration rate and microbial respiration rate significantly (P < 0.05) during the crop growing period compared with the freeze-thaw period. After two years of freeze-thaw cycling, biochar application increased sunflower plant carbon sequestration and net primary productivity and suppressed total soil microbial respiration, thereby increasing net ecosystem productivity. Therefore, the application of biochar is conducive to carbon sequestration in farmland ecosystems and presents a carbon sink effect, thus being a good choice for improving the soil carbon pool and reducing emissions in the northern dry zone. [ABSTRACT FROM AUTHOR]