Boreal forest soil chemistry drives soil organic carbon bioreactivity along a 314-year fire chronosequence

Abstract

Abstract. Following wildfire, organic carbon (C) accumulates in boreal forest soils. The long-term patterns of accumulation as well as the mechanisms responsible for continuous soil C stabilization or sequestration are poorly known. We evaluated post-fire C stock changes in functional reservoirs (bioreactive and recalcitrant) using the proportion of C mineralized in CO2 by microbes in a long-term lab incubation, as well as the proportion of C resistant to acid hydrolysis. We found that all soil C pools increased linearly with time since fire. The bioreactive and acid-insoluble soil C pools increased at a rate of 0.02 MgC ha−1 yr−1 and 0.12 MgC ha−1 yr−1, respectively, and their proportions relative to total soil C stock remained constant with time since fire (8 % and 46 %, respectively). We quantified direct and indirect causal relationships among variables and carbon bioreactivity to disentangle the relative contribution of climate, moss dominance, soil particle size distribution and soil chemical properties (pH, exchangeable Mn and Al, and metal oxides) to the variation structure of in vitro soil carbon bioreactivity. Our analyses showed that the chemical properties of Podzolic soils that characterise the study area were the best predictors of soil carbon bioreactivity. For the FH horizon (O-layer), pH and exchangeable Mn were the most important (model-averaged estimator for both: 0.34) factors directly related to soil organic C bioreactivity, followed by time since fire (0.24), moss dominance (0.08) and climate and texture (0 for both). For the mineral soil, exchangeable aluminum was the most important factor (model-averaged estimator: −0.32), followed by metal oxide (−0.27), pH (−0.25), time since fire (0.05), climate and texture (~ 0 for both). Of the four climate factors examined in this study (i.e., mean annual temperature, growing degree-days above 5 °C, mean annual precipitation and water balance) only those related to water availability, and not to temperature, had indirect effect (FH horizon) or a marginal indirect effect (mineral soil) on soil carbon bioreactivity. Given that predictions of the impact of climate change on soil carbon balance are strongly linked to the size and the bioreactivity of soil C pools, our study stresses the need to include the direct effects of soil chemistry and the indirect effects of climate and soil texture on soil C decomposition in Earth System Models to forecast the response of boreal soils to global warming.

Manganese limitations and the enhanced soil carbon sequestration of temperate rainforests

“…The initial evidence for the role of Mn in SOC cycles came from litter decomposition studies where mass loss rates were positively correlated to initial Mn concentrations of needles and leaves (Berg et al 1996;Berg et al 2010). Across northern latitudes there is further correlative evidence that SOC accumulation, particularly organic horizons (forest oors), increases with declining exchangeable Mn (Stendahl et al 2017;Kranabetter 2019;Andrieux et al 2020). In landscapes with substantial nitrogen (N) pollution and poorly-buffered soils, such as parts of the eastern United States, the enhanced accumulation of SOC observed with N deposition has similarly been linked to a reduction in Mn availability (van Diepen et al 2015;Whalen et al 2018), with polluted soils lacking some of the Agaricomycetes likely responsible for MnP production (Edwards et al 2011;Morrison et al 2016;Entwistle et al 2018b).…”