Soil respiration, carbon balance, and economic and environmental benefits of triple intercropping system of fava bean, maize and sweet potato under conservation tillage

Ridge tillage and straw mulching are suitable modes of conservation tillage that increase crop productivity and sequestrate carbon in the purple hilly region of Southwest China. A field experiment was conducted with six treatments-traditional tillage without straw mulching (T), ridge tillage without straw mulching (R), traditional tillage with straw mulching at 3 750 kg·hm^-2 (TS1), ridge tillage with straw mulching at 3 750 kg·hm^-2 (RS1), traditional tillage with straw mulching at 7 500 kg·hm^-2 (TS2) and ridge tillage with straw mulching at 7 500 kg·hm^-2 (RS2). The characteristics of soil respiration, carbon balance, crop yield, and economic and environmental benefits for the triple intercropping system of fava bean, maize and sweet potato were evaluated. The study provided the theoretical basis for quantifying carbon budget of farmland ecosystems in the region. It was noted that mean soil respiration rates during growth period of fava bean, maize and sweet potato were 3.704 μmol·m^-2·s^-1, 4.847 μmol·m^-2·s^-1 and 4.606 μmol·m^-2·s^-1, respectively. Ridge tillage reduced soil respiration rate during growth period of three crops, whereas straw mulching increased it (P < 0.05). Ridge tillage with straw mulching increased cumulative soil respiration and cumulative microbial respiration (P < 0.05). All treatments of conservation tillage improved carbon sequestration of crops in the triple intercropping system. The soil-crop system exhibited carbon sink and the treatments of RS2, TS2, RS1, TS1 and R significantly increased carbon sequestration respectively by 25.41%, 25.37%, 9.84%, 26.74% and 13.26%, compared with treatment T. Ridge tillage and straw mulching treatments increased total crop yield of the triple intercropping system, which was highest under RS2 (17 460.45 kg·hm^-2) and next TS2 (16 498.73 kg·hm^-2). The amount of CO₂ released from soil per 1 kg grain in the triple intercropping system was in the order of:T (1.88 kg·kg^-1) > TS1 (1.83 kg·kg^-1) > R (1.76 kg·kg^-1) > TS2 (1.75 kg·kg^-1) > RS1 (1.69 kg·kg^-1) > RS2 (1.68 kg·kg^-1). This meant that both ridge tillage and straw mulching increased the economic and environmental benefit index. The more straw was added during mulching, the more was the economic and environmental benefits. In conclusion, the RS2 treatment (ridge tillage + straw mulching at 7 500 kg·hm^-2) had the largest carbon sink capacity and the optimal economic and environmental benefits. Thus, it was recommended for adoption as tillage pattern to increase carbon sequestration and reduce carbon release in the study area.