Methane emissions and soil carbon in floodplain wetlands of the Macquarie Marshes

Methane (CH₄) emission in wetlands is critical because CH₄ has 25 times the global warming potential of carbon dioxide (CO₂), and wetlands play a critical role in global carbon cycling. Using flux chambers this research investigated CH4 emissions in three wetland zones in the Macquarie Marshes: reed bed, dry floodplain and dryland. Methane emissions were highly heterogenous and CH₄ production only occurred in the reed bed (1.73E+01 kg ha⁻¹ d⁻¹). In contrast, CH₄ was oxidised in the dry floodplain (-1.03E-03 kg ha⁻¹ d⁻¹) and there was no production or oxidation of CH₄ in the dryland. Methane flux was strongly correlated with in situ moisture content. The median isotopic signature of CH₄ (δ¹³C;-56.3±2.36 ‰) agrees with wetlands globally and can be used in regional mixing models. The reed bed (809.h ha) has the potential to release 4.97E-03 Tg yr⁻¹, and an equivalent area of dry floodplain could oxidise -2.97E-07 Tg yr⁻¹, yielding a net CH₄ flux of 4.97E-03 Tg yr¹. The results demonstrate for the first time that freshwater floodplain wetlands in dry landscapres can both sequester and emit CH₄, and that where freshwater floodplainwetlands emit CH₄ the rate is comparable to coastal mangroves.