Rapid Incorporation of Plant-derived Lipids into Soil Organic Matter in Response to Elevated CO2 and Warming in a Spruce-dominated Ombrotrophic Bog

Abstract:

Globally, more than one third of soil organic matter (SOM) is stored in peatlands, despite occupying less than 3% of the land surface. Rising atmospheric CO₂ concentration and temperature are predicted to stimulate the decomposition of the stored carbon, potentially turning peatlands from net carbon sinks to net carbon sources and thus causing positive feedbacks to future climate. The magnitude and rate of the decomposition response remains uncertain, as peat SOM decomposition is mediated by biogeochemical processes as well as substrate quality and quantity with distinct responses to environmental change drivers.

In this study we assessed how 0–9°C of deep peat warming (0-200 cm) with ambient or elevated CO₂ (+500 ppm) affect the quantity and quality of SOM in the climate change manipulation experiment SPRUCE (Spruce and Peatland Responses Under Changing Environments) in Minnesota USA. We assessed how warming and elevated CO₂ affect the degradation of plant and microbial residues as well as the incorporation of these compounds into SOM. Specifically, we analyzed free extractable n-alkanes and fatty acids combined with compound-specific stable carbon isotope (δ¹³C) analysis.

We observed a 6‰ offset in δ¹³C between bulk SOM and n-alkanes, which were uniformly depleted in δ¹³C when compared to bulk organic matter. Such an offset between SOM and n-alkanes is common and confirms previous findings. Surprisingly, already after 4 years of deep peat warming and 2 years of elevated CO₂ a strong depth-specific response became visible with changes in SOM quantity and quality. In the upper 0-30 cm depth, the δ¹³C values of bulk organic matter and of individual n-alkanes increase concurrently with increasing temperatures in both temperature and temperature x elevated CO₂ treatments, but not below 40 cm depth. Our results suggest that n-alkanes, which typically turnover slower than bulk SOM, undergo a fast transformation under relatively short period of simulated warming. Further separation of the lipids into fatty acids will reveal in more detail the interlink and incorporation of these compounds into SOM.

It remains to be seen how fast the deep peat will respond to rising temperatures and atmospheric CO₂ concentrations, and how this large carbon reservoir will respond to the changing environmental conditions.