Objective Wheat/maize/sweet potato intercropping is the main planting model of dryland grain production in the hilly region of central Sichuan, which once accounted for more than 60% of the area of dry triple cropping in this region. We studied the carbon balance situation of farmland under different tillage methods, to provide a foundation for choosing a green and low-carbon production model for the region.

Methods The long-term positioning runoff field experiment was located in Yucheng Comprehensive Experimental Base in the Eastern New District of Sichuan Academy of Agricultural Sciences. Four treatments were setup in wheat season, included 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 lasted 16 years when this research was carried out in 2023. During wheat growing period of the intercropping system, the soil CO₂ emission rate and total emission were monitored, and the carbon sequestration, carbon footprint and carbon balance of farmland were assessed.

Results The soil respiration rate during wheat season in T1, T2, T3 and T4 treatments were 3.69, 4.45, 5.39 and 4.30 μmol/(m²·s), respectively, showing order of T1 < T4 < T2 < T3. The soil respiration rate in the root zone and rootless zone of wheat belt decreased first and then increased with the growth and development of wheat, which were significantly correlated with soil temperature change. T2 treatment decreased the sensitivity of soil and microbial respiration to soil temperature in wheat belt, T4 treatment decreased the sensitivity of microbial respiration to soil temperature in fallow zone. Among all treatments, T3 treatment were recorded the highest soil CO₂ flux and the lowest carbon sequestration. The total soil respiration (5225.78 kg/hm²) in T3 was 43.8%, 19.9% and 23.6% higher than T1, T2 and T4, respectively, and the carbon sequestration (6954.30 kg/hm²) was 0.93%%, 17.77% and 23.77% lower than T1, T2 and T4, respectively. Straw returning treatments (T2, T3, and T4) significantly increased wheat yield, relative to T1, and T4 was recorded the highest yield. In addition, T4 treatment exhibited the lowest, but T3 treatment exhibited the highest carbon footprint, with the carbon footprint per unit output in order of T1>T3>T2>T4. The wheat belt in T1 treatment was carbon emission, while in the other three treatments were carbon sinks, and the fallow belt were all carbon emission. The total carbon sinks in farmland of T2 and T4 treatments were comparable, and were 2.32 and 2.05 times of that in T3, respectively.

Conclusion During the whole growth period of wheat, the wheat belt soil are carbon sink and the fallow belt soil are carbon emission. Straw mulching will increase the carbon emission of farmland, but the significantly improved carbon sequestration offset the emission, and the carbon balance shows net sequestration. No-tillage reduces carbon emission and the balance is carbon sink. Compared with flat planting, ridge planting significantly increases carbon emission and reduces carbon sink, resulting in net carbon emission in farmland system. The tillage package of no-tillage and flat planting plus straw mulching is recommended as the high yield, reduced carbon emission and enhanced carbon sequestration, while the ridge plant should be less used.