How do boreal forest soils store carbon?

Abstract

Boreal forests store a globally significant pool of carbon (C), mainly in tree biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms underlying C stabilization are not well understood. Here, recently discovered mechanisms behind SOM stabilization, their level of understanding, interrelations, and future directions in the field are provided. A recently unraveled mechanism behind C stabilization via interaction of root‐derived tannins with fungal necromass emphasizing fungal necromass chemistry is brought forth. The long‐lasting dogma of the stability of SOM on minerals is challenged and the newest insights from the field of soil fauna and their influence on SOM stabilization are provided. In conclusion, mechanisms unraveled during the last decade are crucial steps forward to draw a holistic view of the main drivers of SOM stabilization.

The functional role of ericoid mycorrhizal plants and fungi on carbon and nitrogen dynamics in forests

“…However, given the multiple mechanisms that contribute to SOM persistence, the directionality of the effects of ErM plant and fungal traits remains unclear. For instance, interactions between plant and fungal litter inputs are thought to increase the longerterm persistence of particulate SOM through the formation of tannin-organic N complexes (Clemmensen et al, 2013(Clemmensen et al, , 2015Adamczyk et al, 2019b;Adamczyk, 2021). Further, and in contrast to the growth efficiency-recalcitrance relationship that favors the flow of more labile substrates via the microbial biomass to mineral surfaces, both labile and recalcitrant components of fungal necromass can sorb directly to mineral surfaces efficiently (See et al, 2021).…”

“…Although the humification and selective preservation models of SOM persistence have been refuted (Lehmann & Kleber, 2015), the initial biochemical traits of plant and fungal inputs – along with products arising through their decomposition – might increase the mean residence time of surface SOM pools, thereby favoring SOM accumulation under ErM plants relative to other plant mycorrhizal types (Adamczyk, 2021). For instance, ErM plants generally have higher concentrations of aromatic macromolecules, such as polyphenolic compounds and lignin, whose oxidative decomposition creates substrates with progressively more random molecular structures.…”

Precipitation-derived effects on the characteristics of proteinaceous soil organic matter across the continental United States

“…In addition to minerals, proteinaceous amino acids produced by microorganisms can bind to root-derived compounds, thus prolonging their presence in the environment (21). For instance, root-derived condensed tannins would transform fungal residues into more persistent forms and slow decomposition (22).…”

Decomposition of soil organic matter by ectomycorrhizal fungi: Mechanisms and consequences for organic nitrogen uptake and soil carbon stabilization

“…These metabolites work in concert with extracellular enzymes, and most likely ECM fungi have evolved mechanisms to keep the reactants and their products close to the hyphae. Extracellular polymeric substances (EPS) may be important functional components that assure that extracellular decomposition reactions occur at the right time and in the right place (Op De Beeck et al, 2020, 2021.…”

“…For example, the production of fungal necromass might have a profound effect on soil C stabilization , and the release of low-molecularweight compounds by mycorrhizal fungal exudation may stimulate the decomposition activities of saprotrophic microorganisms (Phillips et al, 2012). Recent research has shown that soil C sources might be stabilized not only by association with mineral particles but also by interactions with root-derived tannins (Adamczyk, 2021). Moreover, competition with free-living saprotrophic fungi may potentially suppress SOM decomposition (the so-called Gadgil effect) (Fernandez and Kennedy, 2016).…”