Wetlands are the most efficient terrestrial carbon sinks yet remain the largest natural source of methane (CH4). Subtropical wetlands, such as Israel's Hula Nature Reserve (HNR), are highly productive but remain under-sampled in global models. This study examines how temperature, dominant vegetation, and soil organic carbon (SOC) properties regulate anaerobic CH4 and CO2 production.
We conducted anaerobic slurry incubations using sediment from three hydro-ecological zones: open-water (Ceratophyllum submersum dominated), flooded vegetated littoral zone, and drought-affected littoral zone (both Phragmites Australis dominated). Samples were incubated across a thermal gradient (14°C, 23°C, 33°C) representing seasonal means and future projections. Analyses included gas chromatography and Rock-Eval organic carbon characterization.
Results show a positive correlation between temperature and CO2/CH4 production; N2O remained undetectable. The flooded littoral zone exhibited the highest cumulative yields, but the open-water zone showed significantly higher efficiency per gram of SOC. At 33°C, both sites produced nearly identical CH4 when normalized to pyrolyzable carbon (∼1X10-3 mol CH4/gPC). This suggests that while methanogenesis is enhanced thermally, the pyrolyzable carbon pool (probably representing an organic fraction with higher C bioavailability) dictates the maximum methanogenic potential. Interestingly, methanogenesis remained suppressed at the dried site even after 8 weeks of re-wetting, highlighting the lasting impact of sediment oxygenation. The thermal sensitivity coefficient (Q10) reached 4.42 between 14°C and 23°C, suggesting that warming winters may trigger disproportionate emission spikes.
Extrapolating production rates and Q10 values, factoring in average methane oxidation rates, reveals that HNR emissions are less than half the average recorded for subtropical wetlands. This may position HNR as a model site for low-emission subtropical wetlands. The relatively high refractory carbon and low methane emissions observed in the P. Australis sediment suggest that vegetation can be effective in maximizing carbon sequestration and mitigating methane trade-offs in wetland restoration initiatives.