Using biomarker lipids to reconstruct soil fertility through time
Authors: De Jonge Cindy , Guo Jingjing , Hallberg Petter , Griepentrog Marco , Smittenberg Rienk , Peterse Francien , Boeckx Pascal , Dercon Gerd 
Published: February 2023

Glycerol dialkyl glycerol tetraethers (GDGTs) are ubiquitous membrane-spanning lipids with a wide environmental distribution. In soils, branched GDGTs are produced by a possibly large diversity of bacteria. The relative abundance of methyl groups attached to the central alkyl chains is at the basis of the paleotemperature proxy MBT&#8217;5ME. However, MBT&#8217;5ME values in soils can also be directly influenced by pH (De Jonge et al., 2021). A second group of compounds, the isoprenoid GDGTs, are produced by archaea. They have been used only sparsely as environmental proxies in soils, although they are at the base of the marine paleotemperature proxy TEX86. In soils, a recent compilation by Yang et al. (2016) illustrates that the temperature dependency of TEX86 is sometimes present, but potentially influenced by other soil (chemistry) parameters.In addition to temperature, other soil parameters are expected to vary with time, even on a Holocene timescale. For instance, soil mineral fertility (specifically, the concentration of exchangeable cations) will vary following climate or land use changes. As soil mineral fertility will impact the soil nutrient status for vegetation, and impact the soil capacity to store organic carbon (von Fromm et al., 2021), it is a relevant parameter to reconstruct over time. However, as soil fertility of surface soils will decrease during eroision or burial, this parameter can currently not be reconstructed quantitatively.To investigate the potential of GDGTs as soil fertility proxies, branched and isoprenoid GDGTs were measured in soils from 5 elevation transects (Austria, Bolivia, China, Indonesia and Tanzania, n=74) that cover a large gradient in mean annual temperature (0-28 &#8451;), seasonality, and soil chemical parameters. Supplemented with climate (temperature and precipitation) data, we evaluate both changes in absolute concentration and relative distribution of the GDGTs. Of the chemical parameters, exchangeable calcium and exchangeable iron are shown to correlate with the absolute abundance of several branched (6 methyl brGDGTs) and isoprenoid (crenarchaeol isomer) GDGT compounds. Based on these relations we have developed ratios to quantify calcium (and summed bases) and iron (and summed metals) [r2=0.61-0.68, p<0.001] using GDGTs in soils. As GDGTs are stable on geological timescales, their presence in paleosoil sequences will thus allow us to reconstruct changes in surface soil fertility (specifically, calcium and iron) through time, even after the mineralogy of the original topsoil has changed.Based our promising preliminary data we propose that GDGT ratios to reconstruct soil mineral fertility should be developed further using well-characterized modern soils. In addition, we look forward to testing our proxies on paleosoils by starting new collaborations.&#160;De Jonge, C. et al. The influence of soil chemistry on branched tetraether lipids in mid- and high latitude soils: implications for brGDGT- based paleothermometry. Geochimica et Cosmochimica Acta (2021).von Fromm, S.F., et al. Continental-scale controls on soil organic carbon across sub-Saharan Africa. SOIL 7, 305&#8211;332 (2021).Yang, H., et al. The Response of Archaeal Tetraether Membrane Lipids in Surface Soils to Temperature: A Potential Paleothermometer in Paleosols. Geomicrobiology Journal 33, 98&#8211;109 (2016).