Using biomarker lipids to reconstruct soil fertility through time

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 forms the basis of the paleotemperature proxy MBT’_5ME. However, MBT’_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 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 ongoing soil formation influenced by climate, vegetation and/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 erosion 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; De Jonge et al., 2024) that cover a large gradient in mean annual temperature (0-28 ℃), seasonality, and soil chemical parameters. Supplemented with 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 as proxies for calcium (and summed bases) and iron (and summed metals) [r2=0.61-0.68, p<0.001] concentrations using GDGTs in soils. As GDGTs are preserved after burial, their presence in paleosol sequences allow reconstruction of ancient topsoil fertility (specifically, calcium and iron) through time, even after the mineralogy of the original topsoil has changed upon further weathering.

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).

De Jonge, C. et al. The impact of soil chemistry, moisture and temperature on branched and isoprenoid GDGTs in soils: A study using six globally distributed elevation transects. Organic Geochemistry 187, 104706 (2024).

von Fromm, S.F., et al. Continental-scale controls on soil organic carbon across sub-Saharan Africa. SOIL 7, 305–332 (2021).

Yang, H., Pancost, R. D., Jia, C. & Xie, S. The Response of Archaeal Tetraether Membrane Lipids in Surface Soils to Temperature: A Potential Paleothermometer in Paleosols. Geomicrobiology Journal 33, 98–109 (2016).