Objectives Wheat-maize-sweet potato intercropping is a primary dryland grain production model in the hilly region of central Sichuan, once accounting for more than 60% of the dry triple cropping area in this region. We studied the carbon balance of farmland under different tillage methods, to provide a foundation for developing a green and low-carbon production model for the region.

Methods A long-term field experiment was conducted in the Yucheng Comprehensive Experimental Base in the Eastern New District of Sichuan Academy of Agricultural Sciences. Four treatments were setup during wheat season: plough+flat planting (T1), plough+flat planting+straw mulch (T2), plough+ridging+straw mulch (T3), no-tillage + flat planting+straw mulch (T4). The experiment had been ongoing for 16 years by 2023. During wheat growing period of the intercropping system, soil CO₂ emission rate and total emission were monitored, and carbon sequestration, carbon footprint and carbon balance of farmland were assessed.

Results The average soil respiration rates of T1, T2, T3 and T4 treatments were 3.69, 4.45, 5.39 and 4.30 μmol/(m²·s), respectively, following the order T1<T4<T2<T3. Soil respiration rate in both the root zone of wheat and non-root zone of wheat decreased initially and then increased with wheat growth and development. Correlation analysis revealed a significant relationship between soil respiration rate and soil temperature. T2 treatment reduced the sensitivity of soil respiration and microbial respiration to soil temperature in wheat root zone, and T4 treatment reduced the sensitivity of microbial respiration to soil temperature in fallow zone. Among the four treatments, T3 treatment had the highest soil respiration rate and the lowest carbon sequestration in the root zone, non-root zone and fallow zone of wheat. The total soil respiration (C 5225.78 kg/hm²) was significantly increased by 43.8 % compared with T1, and there was no significant difference with the other treatments. There was no significant difference in carbon sequestration among the treatments. From the perspective of carbon footprint, T4 treatment had the lowest carbon footprint, and T1 treatment had the highest carbon footprint. From the yield analysis, T4 treatment was significantly greater than T1 treatment, and there was no significant difference with the other treatments. The order of carbon footprint per unit yield was T1>T3>T2>T4. From the perspective of carbon balance, T1 treatment of wheat belt soil was carbon emission, and the other treatments were carbon sinks; the soil in the idle zone was carbon emission. The net carbon sequestration in T3 was the lowest, and the difference between T2 and T4 was not significant, which was 2.83 and 3.25 times that of T3, respectively.

Conclusions During the whole growth period of wheat, soil in wheat belt under straw returning acted as carbon sink, and the idle belt exhibited carbon emission. Although straw mulching increased farmland ecosystem carbon emission in the wheat season, it increased the carbon sink capacity and belonged to a net carbon sink. Compared with tillage, no-tillage reduced carbon emissions and increased carbon sinks. Ridge tillage increased carbon emissions and reduced carbon sinks compared to flat tillage. Under the intercropping mode of wheat-maize-sweet potato in the hilly area of central Sichuan, wheat production can achieve emission reduction and carbon sequestration while increasing yield by using no-tillage flat tillage with straw mulching and returning to the field, and try to avoid the use of tillage + ridge tillage + straw mulching and returning to the field.