Redox condition and litter quantity co-determine soil carbon mineralization: Implications for carbon sequestration in restored wetlands

Soil carbon (C) sequestration is a central objective of wetland restoration, however, the influence of either flooding (redox condition) or plant litter input on soil carbon sequestration during restoration remains unclear. Here, we focused on the theory (the enzyme latch theory) that oxygen limitation suppresses phenol oxidase (POX) activity, allowing phenolic compounds to accumulate and inhibit carbon mineralization rates. We tested how phenolic dynamics and redox status interact to shape soil carbon mineralization rates using a gradient of litter additions (no, low, intermediate and high litter treatments) under controlled anoxic and aerobic conditions. Litter inputs significantly increased soil phenolic concentrations in both redox environments. Under anoxic conditions, intermediate and high litter treatments significantly lowered C mineralization rates compared to no and low litter treatments. Phenolic accumulation strongly inhibited carbon mineralization rates. Furthermore, decreased pH suppressed C mineralization rates under high litter input conditions both directly and by increasing phenolic solubility indirectly. Under aerobic conditions, litter input significantly increased C mineralization relative to the no-litter treatment, but C mineralization rates and concentrations of phenolics were not different between mediate and high litter treatments. POX significantly decreased in high litter treatments compared with intermediate litter addition treatments. The concentration of phenolic compounds was found to be strongly positively correlated with C mineralization rates, likely due to enhanced microbial phenolic degradation capacity. Increasingly, POX activity not only promoted carbon mineralization directly, but also appeared to promote carbon mineralization by indirectly ‘priming’ additional release of phenolics from litter in a form of positive feedback. Overall, results demonstrate that redox conditions and litter abundance jointly determine soil carbon dynamics during wetland restoration, advancing the understanding effect of enzyme latch on C mineralization in restored wetlands. Management strategies that maintain prolonged anoxia and incorporate phenolic-rich plant material may enhance the functional recovery of carbon storage in restored wetlands.