Assessing branched tetraether lipids as tracers of soil organic carbon transport through the Carminowe Creek catchment (southwest England)
oleh: J. Guo, M. Glendell, J. Meersmans, F. Kirkels, J. J. Middelburg, F. Peterse
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2020-06-01 |
Deskripsi
<p>Soils represent the largest reservoir of organic carbon (OC) on land. Upon mobilization, this OC is either returned to the atmosphere as carbon dioxide (<span class="inline-formula">CO<sub>2</sub></span>) or transported and ultimately locked into (marine) sediments, where it will act as a long-term sink of atmospheric <span class="inline-formula">CO<sub>2</sub></span>. These fluxes of soil OC are, however, difficult to evaluate, mostly due to the lack of a soil-specific tracer. In this study, a suite of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are membrane lipids of soil bacteria, is tested as specific tracers for soil OC from source (soils under arable land, ley, grassland, and woodland) to sink (Loe Pool sediments) in a small catchment located in southwest England (i.e. Carminowe Creek draining into Loe Pool). The analysis of brGDGTs in catchment soils reveals that their distribution is not significantly different across different land use types (<span class="inline-formula"><i>p</i>>0.05</span>) and thus does not allow land-use-specific soil contributions to Loe Pool sediments to be traced. Furthermore, the significantly higher contribution of 6-methyl brGDGT isomers in creek sediments (isomerization ratio (IR) <span class="inline-formula">=</span> <span class="inline-formula">0.48±0.10</span>, mean <span class="inline-formula">±</span> standard deviation (SD); <span class="inline-formula"><i>p</i><0.05</span>) compared to that in catchment soils (IR <span class="inline-formula">=</span> <span class="inline-formula">0.28±0.11</span>) indicates that the initial soil signal is substantially altered by brGDGT produced in situ. Similarly, the riverine brGDGT signal appears to be overwritten by lacustrine brGDGTs in the lake sedimentary record, indicated by remarkably lower methylation of branched tetraethers (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi mathvariant="normal">MBT</mi><mrow><mn mathvariant="normal">5</mn><mi mathvariant="normal">ME</mi></mrow><mo>′</mo></msubsup><mo>=</mo><mn mathvariant="normal">0.46</mn><mo>±</mo><mn mathvariant="normal">0.02</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="108pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8fd144d1cb4213ce1d246316942f3cfd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-3183-2020-ie00001.svg" width="108pt" height="15pt" src="bg-17-3183-2020-ie00001.png"/></svg:svg></span></span> in creek bed sediments and <span class="inline-formula">0.38±0.01</span> in lake core sediments; <span class="inline-formula"><i>p</i><0.05</span>) and a higher degree of cyclization (DC <span class="inline-formula">=</span> <span class="inline-formula">0.23±0.02</span> in creek bed sediments and <span class="inline-formula">0.32±0.08</span> in lake core sediments). Thus, in this small catchment, brGDGTs do not allow us to trace soil OC transport. Nevertheless, the downcore changes in the degree of cyclization and the abundance of isoprenoid GDGTs produced by methanogens in the Loe Pool sediment do reflect local environmental conditions over the past 100 years and have recorded the eutrophication history of the lake.</p>