Abstract
Abstract. Although wetlands represent the largest natural source of atmospheric CH4, large uncertainties remain regarding the global wetland CH4 flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials, and CH4 emissions. This study compares both terrestrial and aquatic CH4 fluxes in permanent and seasonal remediated freshwater wetlands in subtropical Australia over two field campaigns, representing differing hydrological and climatic conditions. We account for aquatic CH4 diffusion and ebullition rates and plant-mediated CH4 fluxes from three distinct vegetation communities, thereby examining diel and intra-habitat variability. CH4 emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between CH4 fluxes and both Fe(III) and SO42- were observed. Where sediment Fe(III) and SO42- were depleted, distinct positive trends occurred between CH4 emissions and Fe(II) ∕ acid volatile sulfur (AVS). Significantly higher CH4 emissions (p < 0.01) in the seasonal wetland were measured during flooded conditions and always during daylight hours, which is consistent with soil redox potential and temperature being important co-drivers of CH4 flux. The highest CH4 fluxes were consistently emitted from the permanent wetland (1.5 to 10.5 mmol m−2 d−1), followed by the Phragmites australis community within the seasonal wetland (0.8 to 2.3 mmol m−2 d−1), whilst the lowest CH4 fluxes came from a region of forested Juncus spp. (−0.01 to 0.1 mmol m−2 d−1), which also corresponded to the highest sedimentary Fe(III) and SO42-. We suggest that wetland remediation strategies should consider geochemical profiles to help to mitigate excessive and unwanted methane emissions, especially during early system remediation periods.
Highlights
Wetlands are considered one of the most valuable ecosystems on Earth (Costanza et al, 2014) and may be classified as both permanently inundated and seasonally inundated biomes
This campaign in April 2017 (C1) deployment was categorised as the “post-dry–flooded” period, during which air temperatures ranged from 13.3 to 22.8 ◦C and the average water column temperature in the permanent wetland was 20.4±0.5 ◦C
Our CH4 emissions rates were at the low end of the scale of measurements made in Southern Hemisphere subtropical systems but within the www.biogeosciences.net/16/1799/2019/
Summary
Wetlands are considered one of the most valuable ecosystems on Earth (Costanza et al, 2014) and may be classified as both permanently inundated (i.e. lakes and shallow waters) and seasonally inundated (i.e. vegetated) biomes They are biodiversity hotspots that provide ecosystem services such as water filtration, sediment trapping, floodwater retention, and carbon (C) storage (Bianchi, 2007). Wetlands account for ∼ 5.5 % of terrestrial surfaces (Melton et al, 2013) and have been estimated to store ∼ 4% (Bridgham et al, 2014) to ∼ 30 % (Mitsch et al, 2013) of Earth’s estimated 2500 Pg soil C pool (Lal, 2008). Jeffrey et al.: Rhizosphere to the atmosphere lands are recognised as Earth’s largest natural source of CH4 to the atmosphere (185±21 Tg C yr−1) (Saunois et al, 2016)
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