Experimental climate warming in a French peatland: impact on the abundance and distribution of branched GDGTs
Résumé
Branched glycerol dialkyl glycerol tetraethers (GDGTs) are
complex lipids of high molecular weight, recently discovered in
soils and produced by still unknown bacteria. They are increasingly
used as paleoclimate proxies. Their degree of methylation,
expressed in the MBT, was shown to depend on mean annual air
temperature (MAAT) and to a lesser extent on soil pH, whereas the
relative abundance of cyclopentyl rings of branched GDGTs,
expressed in the (CBT), was related to soil pH. Northern peatlands
contain approximately one third of the world’s organic carbon and
may play an important role in the responses of the global carbon
cycle to climate change. The aim of this work was to study the
effects of experimental climate warming on the abundance and
distribution of branched GDGTs in a Sphagnum-dominated peatland
(French Jura Mountains). Air temperature was experimentally
increased using a warming system consisting of in situ open minigreenhouses
(Open-Top Chambers – OTCs). The effect of the OTCs
was especially apparent in spring and summer, with an increase of
mean and maximal air temperatures of ca. 1 and 3°C respectively.
Branched GDGTs either present as core lipids (CLs; presumed of
fossil origin) or derived from intact polar lipids (IPLs, markers for
living cells) were analysed. Results showed that despite the short
duration of the climate experiment (26 months), branched GDGT
distribution was significantly affected by the temperature rise,
supporting the empirical relationship between MBT and MAAT
established from a large range of soils. The difference in branched
GDGT-derived temperatures between the control and the OTC plots
was in the same range as the increase in maximal temperature
induced by the OTCs in spring and summer, suggesting that branched
GDGT-producing bacteria might be more active during the warmest
months of the year. The OTC treatment had no significant effect on
the abundance of branched GDGTs, mainly present as “fossil” CLs
(70 to 85% of the total extractable branched GDGTs). Furthemore,
no significant differences in branched GDGT distribution were
observed between CLs and IPLs, which both provided higher MBT
and MAAT values for the OTCs. This suggests that the fossil pool of
branched GDGTs has a very fast turnover (less than the 2 year
duration of the experiment) at peat surface and that branched GDGT
distribution may rapidly reflect changes in environmental
conditions.
This work was funded as part of the PEATWARM initiative
through an ANR (French National Agency for Research) grant
(ANR-07-VUL-010).