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

Abstract

Abstract. Following a 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 the time since fire. The bioreactive and acid-insoluble soil C pools increased at a rate of 0.02 and 0.12 MgC ha−1 yr−1, respectively, and their proportions relative to total soil C stock remained constant with the time since fire (8 % and 46 %, respectively). We quantified direct and indirect causal relationships among variables and C bioreactivity to disentangle the relative contribution of climate, moss dominance, soil particle size distribution and soil chemical properties (pH, exchangeable manganese and aluminum, and metal oxides) to the variation structure of in vitro soil C bioreactivity. Our analyses showed that the chemical properties of podzolic soils that characterize the study area were the best predictors of soil C bioreactivity. For the O layer, pH and exchangeable manganese were the most important (model-averaged estimator for both of 0.34) factors directly related to soil organic C bioreactivity, followed by the 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 of −0.32), followed by metal oxide (−0.27), pH (−0.25), the 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 an indirect effect (O layer) or a marginal indirect effect (mineral soil) on soil C bioreactivity. Given that predictions of the impact of climate change on soil C 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 organic matter decomposition in Earth system models to forecast the response of boreal soils to global warming.

Drivers of legacy soil organic matter decomposition after fire in boreal forests

“…We measured the first three months of decomposition, which is primarily determined by chemical quality and likely did not capture long‐term mechanisms of destabilization. Other short‐term incubations concluded that the chemical quality of organic matter was the primary control regulating the initial decomposition rate of soil C (Andrieux et al, 2020; Knorr et al, 2005; Lee et al, 2012). While mineral‐associated SOM often impacts C decomposition, in our study, soil ash content (proxy for mineral‐associated SOM) was unrelated to soil age and C decomposition rate.…”

Effects of a warmer climate and forest composition on soil carbon cycling, soil organic matter stability and stocks in a humid boreal region

“…The length of these periods depended on rates and CO 2 evolution in order to keep concentrations within the calibration range of the IRGA, a LI-6400 portable photosynthesis system (LI-COR, Lincoln, NE, USA). The methodology used is described in Andrieux et al (2020). At each measurement period, samples were flushed with distilled water, and NO 3 -N and NH 4 -N were analysed by FIA (Quickchem 8500, Lachat Instruments, Loveland, Colorado).…”

“…Stable SOM makes up the bulk of SOM. For example, Andrieux et al (2020) found that only 10 % of total SOM could be considered fast or bioreactive C. To the contrary, several studies that considered the decomposability of SOM have shown a higher content of bioreactive or poorly stabilised soil C in colder soils (Fissore et al, 2009;Ishikuza et al, 2006;Laganière et al, 2015;Norris et al, 2011), suggesting that a cold temperature maintains reservoirs of easily decomposable SOM and consequently that cold soils may be more susceptible to C losses upon warming.…”

“…Balsam fir sites, presumably because they produce greater amounts of litter of higher quality, and sites at the warmest end of the gradient should accumulate more stable SOM in line with the Microbial Efficiency-Matrix Stabilization (MEMS) framework developed in Cotrufo et al (2013), which suggests that a larger input of labile plant material leads to larger and more stable SOM stocks. A larger stock of stable SOM should be apparent in total C stocks because it represents the largest share of total SOM (Andrieux et al, 2020).…”

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).…”