Contrasting temporal drivers of methane flux across land uses in freshwater wetlands

Abstract

Wetlands are collectively the largest natural source of methane (CH₄), which complicates the climate mitigation potential of wetland restoration. Land-use change alters wetland soils, vegetation, and hydrology, but the impact of this disturbance legacy on CH₄ fluxes from restored wetlands is uncertain. To better understand CH₄ dynamics across a gradient of human disturbance, we examined 5 yr of continuous CH₄ flux data from a drained, restored, and least-disturbed (“natural”) freshwater mineral-soil wetland. We quantified temporal relationships between CH₄ flux and three key biophysical drivers: soil temperature, gross primary productivity, and hydrology. Using information theoretic and nonlinear state-space reconstruction approaches, we assessed how these relationships varied across different time scales and evaluated evidence for causality. All sites had highest diel CH₄ fluxes at night, primarily driven by gross primary productivity in the natural wetland and soil temperature in the restored and drained wetland. The seasonal scale accounted for most variability in CH₄ fluxes at the natural and restored wetland, with fluxes influenced by the hysteretic causal effects of soil temperature and water level. The effects of seasonal drivers were less evident at the drained site, where fluxes varied more on shorter timescales. Overall, CH₄ fluxes in the natural and restored wetland were comparable in magnitude, had similar temporal patterns, and responded similarly to drivers, indicating convergence in underlying processes. Our work furthers understanding of how land-use history modulates wetland ecosystem processes, which is essential for accurate climate projections and effective management amid global change.