Fungal succession and the future of carbon cycling in Northern bogs

Abstract

Anthropogenic atmospheric pollution and emissions remain major causes of climate change, with serious projected harmful consequences to biological systems and our way of life. With carbon as the main greenhouse gas culprit and peatlands as the largest terrestrial store of carbon, understanding future effects on peatland carbon cycling is of utmost importance. Northern Sphagnum-dominated ombrotrophic bogs will remain in focus as temperate and boreal climatic regions are disproportionately at risk of temperature increases, shifts in precipitation and nutrient deposition. To assess the net shifts in carbon flux and conserve peatlands as carbon sinks, fungi central to soil mineralization and nutrient cycling processes as saprotrophic decomposers and plant symbionts, might play a pivotal role and should hence be further studied. This review will first summarise the direct and indirect plant-mediated effects of warming, drier conditions and nutrient deposition on fungal composition and activity. The review will then tie the literature together and discuss the impact of enviro-climatic shifts on key fungal decomposition and mycorrhizal functions contributing to peatland carbon cycling. Northern bogs are experiencing direct and indirect increases in saprotrophic activity, from oxidation and metabolic limitation alleviations, showing a strong potential in contributing to net carbon loss. Climatedriven vegetation transitions towards vascular plant dominance, such as ericoid shrubs, is improving carbon input quality and indirectly facilitating saprotrophic activity by their more decomposable litter and labile carbon exudates. However, a concomitant rise in root-associating ectomycorrhizal species may mediate a nutrient limitation function and the competition between plants and decomposers. Hence, effective plant-fungal nutrient recycling dynamics may to a certain extent limit decomposition, and in tandem with recalcitrant necromass accumulation, drive carbon storage despite the loss of Sphagnum spp.. Nonetheless, it remains unclear if fungal species assemblages under unprecedented climate change will ensure its ongoing and important role, particularly under increasing nutrient deposition decoupling plants from the symbiosis. Further research is required to understand the plant-fungal biogeochemical processes and future functional redundancies in the accumulation of organic matter and decomposition-limiting factors.