Tide-driven soil respiration: Impacts of Tidally-Induced Water Table Fluctuations on Soil Respiration in Riparian Zones

Abstract

As a critical "biogeochemical reactor," the riparian zone regulates greenhouse gas (GHG) emissions under conditions of fluctuating groundwater levels. However, quantifying these emissions remains a formidable challenge due to the complex coupling between continuous soil respiration (CO2) and episodic methane (CH4) pulses. To address this, we propose a "point-event dual-scale framework" designed to integrate and decipher these dynamic processes using annual high-frequency monitoring data collected from the Qiputang riparian zone in Jiangsu Province, China. Our findings reveal a distinct functional zonation and a hierarchical threshold mechanism within the riparian zones of tidal rivers. Regarding CO2, continuous seasonal modeling confirmed that the lag in groundwater level relative to the soil surface (GWlag) significantly enhanced aerobic respiration (coefficient = -0.99, p < 0.001). Regarding CH4, we identified a decoupled response pattern: methanogenesis commenced at a biological activation threshold of -1.3 m, whereas an explosive release of soil methane was triggered when the groundwater level breached a shallow physical threshold of -0.91 m. Finally, we calculated the ebullitive contribution fraction (Efrac) to quantify the environmental magnitude of this mechanism. The net cumulative CH4 budget (+928.13 μmol m-2) comprised two opposing components: a continuous baseline methanotrophic sink of -748.07 μmol m-2 (representing periods of net biological uptake when GW < -0.91 m) and episodic ebullitive emissions of +1676.20 μmol m-2 (triggered by threshold-breaching events). Because the massive ebullitive source (+1676.20) substantially exceeded the overall net cumulative flux (+928.13, where Z=Y−X), these short-lived physical outbursts alone mathematically accounted for 180.6% of the net budget. This striking percentage explicitly means that without these episodic hydraulic compression events, the riparian zone would have functioned entirely as a resilient net CH4 sink. This calculation confirms that physical ebullition overwhelmingly dictates the net "sink-to-source" status of this highly dynamic riparian ecosystem.