Soil greenhouse gas fluxes from tropical coastal wetlands and alternative agricultural land uses

• Main Authors: N. Iram, E. Kavehei, D. T. Maher, S. E. Bunn, M. Rezaei Rashti, B. S. Farahani, M. F. Adame
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-09-01
• Series: Biogeosciences
• Online Access: https://bg.copernicus.org/articles/18/5085/2021/bg-18-5085-2021.pdf
• ISSN: 1726-4170; 1726-4189

<p>Coastal wetlands are essential for regulating the global carbon budget through soil carbon sequestration and greenhouse gas (GHG – <span class="inline-formula">CO₂</span>, <span class="inline-formula">CH₄</span>, and <span class="inline-formula">N₂O</span>) fluxes. The conversion of coastal wetlands to agricultural land alters these fluxes' magnitude and direction (uptake/release). However, the extent and drivers of change of GHG fluxes are still unknown for many tropical regions. We measured soil GHG fluxes from three natural coastal wetlands – mangroves, salt marsh, and freshwater tidal forests – and two alternative agricultural land uses – sugarcane farming and pastures for cattle grazing (ponded and dry conditions). We assessed variations throughout different climatic conditions (dry–cool, dry–hot, and wet–hot) within 2 years of measurements (2018–2020) in tropical Australia. The wet pasture had by far the highest <span class="inline-formula">CH₄</span> emissions with <span class="inline-formula">1231±386</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mg</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9a351d59159677021c73cdc4349d131e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00001.svg" width="56pt" height="15pt" src="bg-18-5085-2021-ie00001.png"/></svg:svg></span></span>, which were 200-fold higher than any other site. Dry pastures and sugarcane were the highest emitters of <span class="inline-formula">N₂O</span> with <span class="inline-formula">55±9</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mg</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9e5722e5bcab6aa6a1a092c5b6ff7b79"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00002.svg" width="56pt" height="15pt" src="bg-18-5085-2021-ie00002.png"/></svg:svg></span></span> (wet–hot period) and <span class="inline-formula">11±3</span> m<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">g</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="5c097377f94715c27c133a5814e4070b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00003.svg" width="47pt" height="15pt" src="bg-18-5085-2021-ie00003.png"/></svg:svg></span></span> (hot-dry period, coinciding with fertilisation), respectively. Dry pastures were also the highest emitters of <span class="inline-formula">CO₂</span> with <span class="inline-formula">20±1</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">g</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="c0aab21059d0ee7939623231dab627b6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00004.svg" width="47pt" height="15pt" src="bg-18-5085-2021-ie00004.png"/></svg:svg></span></span> (wet–hot period). The three coastal wetlands measured had lower emissions, with salt marsh uptake of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">0.55</mn><mo>±</mo><mn mathvariant="normal">0.23</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="6441b7abd15f03ffdd21a117acd67400"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00005.svg" width="64pt" height="10pt" src="bg-18-5085-2021-ie00005.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">1.19</mn><mo>±</mo><mn mathvariant="normal">0.08</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="e1975a3ba25cfd0f9036d25f0ca90dc4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00006.svg" width="64pt" height="10pt" src="bg-18-5085-2021-ie00006.png"/></svg:svg></span></span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">g</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8521700b0b05a966a960bc5a0514c3d0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00007.svg" width="47pt" height="15pt" src="bg-18-5085-2021-ie00007.png"/></svg:svg></span></span> of <span class="inline-formula">N₂O</span> and <span class="inline-formula">CO₂</span>, respectively, during the dry–hot period. During the sampled period, sugarcane and pastures had higher total cumulative soil GHG emissions (<span class="inline-formula">CH₄+N₂O</span>) of 7142 and 56 124 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><msub><mi mathvariant="normal">CO</mi><mtext>2-eq</mtext></msub><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">kg</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="92pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="3404ea53ae88124f3c211523f4fcccc3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00008.svg" width="92pt" height="17pt" src="bg-18-5085-2021-ie00008.png"/></svg:svg></span></span> compared to coastal wetlands with 144 to 884 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><msub><mi mathvariant="normal">CO</mi><mtext>2-eq</mtext></msub><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">kg</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="92pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="716ab3705b9260c90ed57e50f15f05b0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-5085-2021-ie00009.svg" width="92pt" height="17pt" src="bg-18-5085-2021-ie00009.png"/></svg:svg></span></span> (where <span class="inline-formula">CO_2-eq</span> is <span class="inline-formula">CO₂</span> equivalent). Restoring unproductive sugarcane land or pastures (especially ponded ones) to coastal wetlands could provide significant GHG mitigation.</p>