Identification of fire severity in peat deposits using carbon and nitrogen isotopes

Peatlands are believed to be important as significant long-term global carbon sinks of atmospheric CO2. The ignition of flammable dry peat substrate, either naturally or anthropogenically, may lead to periods of extensive peat fires, the dynamics of which we aim to clarify. However, existing records are largely historically limited or do not accurately identify fire severity and nutrient status. In this study, we test the isotope compositions of carbon and nitrogen to determine fire severity archived in peat deposits. Eight fire periods were determined using microcharcoal analysis. High fire severity results in the alteration of organic matter to pyrogenic organic matter resulting in the depletion of δ13Corg (~1‰), however the δ13Corg remains enigmatic due to variation of organic sources and its combustion efficiencies. Following heavy loss of nitrogen by high fire severity, regrowing vegetation gains nitrogen via atmospheric fixation, subsequently resulting in a lower δ15Ntot (-1–1‰). During the undisturbed accumulation of peat, residual ash from past fires and the depth of the water table can be factors for enrichment of nitrogen. This condition led to N2-fixation by microbial activity and nitrification processes, resulting in higher δ15Ntot values as well more luxuriant plant growth, along with variable values of δ13Corg. This improves the current understanding of the carbon and nitrogen isotopes fractionation pathways during fires and are potentially useful proxies to identify fire severity.