Rhizosphere to the atmosphere: contrasting methane pathways, fluxes, and geochemical drivers across the terrestrial–aquatic wetland boundary

Luke C. Jeffrey, Damien T. Maher, Scott G. Johnston, Kylie Maguire, Andrew D. L. Steven, Douglas R. Tait
2019 Biogeosciences  
<p><strong>Abstract.</strong> Although wetlands represent the largest natural source of atmospheric <span class="inline-formula">CH<sub>4</sub></span>, large uncertainties remain regarding the global wetland <span class="inline-formula">CH<sub>4</sub></span> flux. Wetland hydrological oscillations contribute to this uncertainty, dramatically altering wetland area, water table height, soil redox potentials, and <span class="inline-formula">CH<sub>4</sub></span> emissions. This study compares
more » ... study compares both terrestrial and aquatic <span class="inline-formula">CH<sub>4</sub></span> 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 <span class="inline-formula">CH<sub>4</sub></span> diffusion and ebullition rates and plant-mediated <span class="inline-formula">CH<sub>4</sub></span> fluxes from three distinct vegetation communities, thereby examining diel and intra-habitat variability. <span class="inline-formula">CH<sub>4</sub></span> emission rates were related to underlying sediment geochemistry. For example, distinct negative relationships between <span class="inline-formula">CH<sub>4</sub></span> fluxes and both Fe(III) and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="06a1e144313624090049b6627390d3e8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-1799-2019-ie00001.svg" width="29pt" height="17pt" src="bg-16-1799-2019-ie00001.png"/></svg:svg></span></span> were observed. Where sediment Fe(III) and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="8c898138530c760447165fe6cdc920bb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-1799-2019-ie00002.svg" width="29pt" height="17pt" src="bg-16-1799-2019-ie00002.png"/></svg:svg></span></span> were depleted, distinct positive trends occurred between <span class="inline-formula">CH<sub>4</sub></span> emissions and Fe(II)&amp;thinsp;<span class="inline-formula">∕</span>&amp;thinsp;acid volatile sulfur (AVS). Significantly higher <span class="inline-formula">CH<sub>4</sub></span> emissions (<span class="inline-formula"><i>p</i></span>&amp;thinsp;&amp;lt;&amp;thinsp;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 <span class="inline-formula">CH<sub>4</sub></span> flux. The highest <span class="inline-formula">CH<sub>4</sub></span> fluxes were consistently emitted from the permanent wetland (1.5 to 10.5&amp;thinsp;mmol&amp;thinsp;m<span class="inline-formula"><sup>−2</sup></span>&amp;thinsp;d<span class="inline-formula"><sup>−1</sup></span>), followed by the <i>Phragmites australis</i> community within the seasonal wetland (0.8 to 2.3&amp;thinsp;mmol&amp;thinsp;m<span class="inline-formula"><sup>−2</sup></span>&amp;thinsp;d<span class="inline-formula"><sup>−1</sup></span>), whilst the lowest <span class="inline-formula">CH<sub>4</sub></span> fluxes came from a region of forested <i>Juncus</i> spp. (<span class="inline-formula">−0.01</span> to 0.1&amp;thinsp;mmol&amp;thinsp;m<span class="inline-formula"><sup>−2</sup></span>&amp;thinsp;d<span class="inline-formula"><sup>−1</sup></span>), which also corresponded to the highest sedimentary Fe(III) and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="66431dc7c7d5e1df65243409a551c241"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-1799-2019-ie00003.svg" width="29pt" height="17pt" src="bg-16-1799-2019-ie00003.png"/></svg:svg></span></span>. 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.</p>
doi:10.5194/bg-16-1799-2019 fatcat:cvmmpcxzgjgchim2phdnl5npqq