Characteristics and mechanisms of phosphine production in sulfur-based constructed wetlands

Phosphine (PH₃) is an important contributor to the phosphorus cycle and is widespread in various environments. However, there are few studies on PH₃ in constructed wetlands (CWs). In this study, lab-scale CWs and batch experiments were conducted to explore the characteristics and mechanisms of PH₃ production in sulfur-based CWs. The results showed that the PH₃ release flux of sulfur-based CWs varied from 0.86±0.04 ng·m^-2·h^-1 to 1.88±0.09 ng·m^-2·h^-1. The dissolved PH₃ was the main PH₃ form in CWs and varied from 2.73 μg·L^-1 to 4.08 μg·L^-1. The matrix-bound PH₃ was a staging reservoir for PH₃ and increased with substrate depth. In addition, the sulfur-based substrates had a significant improvement on PH₃ production. Elemental sulfur is more conducive to PH₃ production than pyrite. Moreover, there was a significant positive correlation between PH₃ production, the dsrB gene, and nicotinamide adenine dinucleotide (NADH). NADH might catalyze the phosphate reduction process. And the final stage of the dissimilatory sulfate reduction pathway driven by the dsrB gene might also provide energy for phosphate reduction. The migration and transformation of PH₃ increased the available P (Resin-P and NaHCO₃-P) from 35 % to 56 % in sulfur-based CW, and the P adsorption capacity was improved by 12 %. The higher proportion of available P increased the plant uptake rate of P by 17 %. This study improves the understanding of the phosphorus cycle in sulfur-based CW and provides new insight into the long-term stable operation of CWs.