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.