Influence of soil pH on the abundance and distribution of core and intact polar lipid-derived branched GDGTs in soil

Branched glycerol dialkyl glycerol tetraether (GDGT) lipids occur in soils worldwide and are presumed to be produced by soil bacteria. They form the base of the MBT/CBT proxy (methylation of branched tetraethers/cyclisation of branched tetraethers) for palaeoclimate reconstruction. The degree of cyclisation and methylation, expressed in the CBT and MBT indices, has been shown to relate to soil pH, and to both temperature and soil pH, respectively. To evaluate the direct impact of soil pH on the abundance and distribution of branched GDGTs, either present as core lipids or derived from intact polar lipids (IPLs), long term (>45 years) soil pH manipulation plots, with a pH range of 4.5-7.5, were analyzed. The CBT index values of both core and IPL-derived branched GDGTs was significantly (r2 = 0.93, P = 0.0004 and r2 = 0.93, P = 0.0005, respectively) linearly related to actual soil pH, confirming the direct influence of pH on the distribution of branched GDGT. In contrast, the MBT index was only influenced to a minor extent by the changes in soil pH. Branched GDGT abundances decreased with increasing soil pH, supporting the idea that Acidobacteria may be a possible source of these orphan membrane lipids.