Abstract
Fires are predicted to increase in Arctic regions due to ongoing climate change. Tundra fires can alter carbon and nutrient cycling and release a substantial amount of greenhouse gases with global consequences. Yet, the long-term effects of tundra fires on carbon (C) and nitrogen (N) stocks and cycling are still unclear. Here we used a space-for-time approach to investigate the long-term fire effects on C and N stocks and cycling in soil and aboveground living biomass. We collected data from three large fire scars (> 44, 28 and 12 years old) and corresponding control areas and used linear mixed-effects models in a Bayesian framework to analyse how the stocks and cycling were influenced by fire. We found that tundra fires did not affect total C and N stocks because a major part of the stocks was located belowground in soils, which were largely unaltered by fire. However, fire had a strong effect on stocks in the aboveground vegetation, mainly due to the reduction of the lichen layer. Fire reduced N concentrations in graminoids and herbs on the younger fire scars, which affected respective C / N ratios and indicated an increased post-fire competition between vascular plants. Aboveground plant biomass was depleted in 13C in all three fire scars. This could be related to a lower 13C abundance in CO2 in the ambient air because of increased post-fire decomposition, providing a source of 13C-depleted CO2. In soil, the relative abundance of 13C changed with time after fire because of the combined effects of microbial decomposition and plant-related fractionation processes. Our results indicate that in lichen-rich subarctic tundra ecosystems, the contribution of fires to the release of additional carbon to the atmosphere might be relatively small as soil stocks appear to be resilient.
Highlights
Arctic regions are warming faster than the worldwide average, with strong impacts on the global carbon cycle (Post et al, 2019; Schuur et al, 2015)
We found that tundra fires did not affect total C and N stocks because a major part of the stocks was located belowground in soils, which were largely unaltered by fire
Our results indicate that in lichen-rich subarctic tundra ecosystems, the contribution of fires to the release of additional carbon to the atmosphere might be relatively 15 small as soil stocks appear to be resilient
Summary
Arctic regions are warming faster than the worldwide average, with strong impacts on the global carbon cycle (Post et al, 2019; Schuur et al, 2015). The cover of vascular plants increases after fires at the expense of previously dominating cryptogams, which cannot profit from the enhanced nitrogen availability as they do not reach deep soil layers or cannot take up nitrogen from the soil (Bret-Harte et al, 2013; Jandt et al, 2008; Narita et al, 2015; Turetsky et al, 2012) Such profound changes in vegetation cover, in turn, influences ecosystem functioning and processes, such as carbon and nitrogen cycling and C and N 40 stocks (Longton, 1997; Sancho et al, 2016; Turetsky, 2003). We hypothesized that: 1) Fire leads to decreased ecosystem C and N stocks 2) Fire affects long-term C and N cycling, evident through a) an increased N concentration and a decreased C/N ratio in vascular plants 60 after fire b) a depletion of 13C in aboveground biomass and a relative enrichment of 13C in soil after fire c) increased relative abundance of 15N in leaves of vascular plants on the younger fire scars
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