Environmental risk and cost restraint mechanism for incorporating large quantities of vegetable residues into fields in semi-arid area of the Loess Plateau

The incorporation of vegetable residues can fertilize low-quality loess, but it is still unclear whether large quantity of vegetable residues incorporation will cause secondary environmental pollution and increase processing costs. In this study, a plot test was designed in the semi-arid area of the Loess Plateau in Yuzhong, which was randomly combined with three thicknesses of incorporated vegetable residues (20, 40, and 60 cm), three thicknesses of surface soil covering (10, 20, and 30 cm). Meantime, two medium-scale site tests were conducted, in which the thickness of incorporated vegetable residue was up to 350 cm, and the surface soil covering thickness was 30 cm. The degradation rate of vegetable residues, emission rate of NH₃ and H₂S on the soil surface, residue of heavy metals and pesticides in the soil, salt ions contents, and processing cost were investigated. The cumulative degradation rate of vegetable residues in all treatments showed a logarithmic growth curve, which was first fast and then slow. On the 20th and 35th days, the degradation rate of vegetable residues reached 70.0% in the plot and the medium-scale test sites, respectively, and subsequently slowed down. When the thickness of the incorporated vegetable residues was 60 cm and the depth of covering soil was 10–30 cm, the emission of NH₃ was reduced by 71.0%–86.0%, and the emission of H₂S was reduced by 84.9%−87.9%, compared with QC (vegetable residue thickness of 60 cm and no soil cover). The time series changes of the NH₃ emission rate on the soil surface showed a single narrow peak curve, and the peak value of emission rate and cumulative emission were significantly positively correlated with the thickness of incorporated vegetable residues, and significantly negatively correlated with the depth of the covering soil. There was no significant difference in H₂S emissions from the soil surface of the plot test and QT (no incorporation of vegetable residues), and the H₂S emissions from the medium-sized test increased significantly. The larger the amount of vegetable residues into the field, the smaller the emission intensity of pollutants was. There was no significant difference in the contents of heavy metals, pesticide residues, and Ca²⁺ in the vegetable residue layer and the upper and lower soil layers of the medium-sized test sites compared with those in QT (no incorporation of vegetable residues), whereas Na⁺ leached into the deep soil layer. There is a power-law negative correlation between the thickness of incorporated vegetable residues in the field and the processing cost in a medium-sized test field. The larger the incorporating capacity of vegetable residues, the lower the processing cost, and the lowest cost was 25.0 ¥∙t⁻¹ (fresh weight). Therefore, in the semi-arid area of the Loess Plateau, using the method of covering soil and burying pressure to incorporate vegetable residues into the field in high quantities is a low-cost, simple, eco-friendly, and efficient processing scheme for utilizing vegetable residues.