Biogeochemical Trade-Off in a Dual-Coupling Oxidative Model: How Hydroxyl Radicals Govern Greenhouse Gas Emission in Coastal Wetlands across a Salinity Gradient

Coastal wetlands are major sources and/or sinks of greenhouse gases (GHGs), yet the role of abiotic oxidants like hydroxyl radicals (^·OH) in regulating these fluxes, especially across salinity gradients, remains poorly understood. Here, we reveal that ^·OH is a pivotal biogeochemical agent whose function is governed by a "dual-coupling oxidative model". External coupling is demonstrated by salinity acting as a master variable, driving a significant increase in ^·OH production potential. This salinity-driven gradient of oxidative pressure then interacts with the internal coupling─the functional duality of ^·OH─to produce divergent GHG dynamics. For CO₂, net promotion (contributing an average of 9.38 ± 3.92%) resulted from a trade-off where direct abiotic mineralization overcame the suppression of C-degradation enzymes. In contrast, ^·OH acted as a net suppressant of CH₄ emission, an effect predominantly driven by biotic responses. For N₂O, an apparent promotion (contributing 4.36 ± 4.39%) resulted from a powerful indirect effect via suppression of its sink enzyme, which overwhelmed a direct chemical inhibitory path. Ultimately, the magnitude of the ^·OH effect on GHG fluxes was codetermined by initial abiotic (23-41%) and biotic (14-29%) conditions. Our model provides a new framework for understanding abiotic-biotic interactions and predicting blue carbon ecosystem responses to global change like seawater intrusion.