Ericoid mycorrhizal fungi mediate the response of ombrotrophic peatlands to fertilization: a modeling study

Carbon dioxide and methane gas exchange following sphagnum moss harvesting in boreal peatland

Understanding the impacts of Sphagnum moss harvesting on peatland carbon (C) balance is crucial due to its potential rise as an anthropogenic land use. We studied eight nutrient-poor peatlands in Finland, harvested between 2015 and 2021, focusing on net ecosystem exchange of CO2 (NEE) and methane (CH4) emissions. The greenhouse gas fluxes were measured to evaluate the sustainability of harvesting practices. Results showed significant variability in Sphagnum regeneration, with wet strip-harvested sites achieving 2-28% re-establishment in 2-8 years, while drier clear-harvested sites saw minimal spontaneous regeneration in 1-6 years. In addition to vegetation succession, GHG emissions were moisture dependent. In wet sites CH4 emissions increased along with time since harvesting and Eriophorum vaginatum (L.) cover, while dry sites exhibited overall lower CH4 fluxes. Younger (1-2 years post-harvest), dry sites were significant CO2 sources due to low photosynthetic activity. Older dry site with sparse ericoid shrub vegetation acted as CO2 sink. Wet sites initially had lower CO2 sink capacity, but this increased as E. vaginatum spread, and reached a plateau when Sphagnum mosses emerged, highlighting the importance of suitable water table levels for efficient CO2 sequestration.

Melanized root-associated fungi: key players in plant-soil systems

Melanized root-associated fungi are a group of fungi that produce melanized structures and form root associations, including different mycorrhizal and endophytic symbioses with plants. They are pervasive across terrestrial ecosystems and play an important role in the prevailing soil carbon (C) and nutrient cycling syndromes through direct and indirect mechanisms, where they may strongly modulate plant-microbe interactions and structure root and soil microbiomes. Furthermore, melanized root-associated fungi can confer on plants an enhanced ability to tolerate abiotic and biotic stressors such as drought, extreme temperatures, heavy metals, and pathogen attacks. We propose that melanized root-associated fungi are a cohesive and ecologically relevant grouping that can be an indicator of plant-soil system functioning, and considering them will advance research on plant-soil interactions.

Responses of vascular plant fine roots and associated microbial communities to whole-ecosystem warming and elevated CO2 in northern peatlands

Warming and elevated CO2 (eCO2) are expected to facilitate vascular plant encroachment in peatlands. The rhizosphere, where microbial activity is fueled by root turnover and exudates, plays a crucial role in biogeochemical cycling, and will likely at least partially dictate the response of the belowground carbon cycle to climate changes. We leveraged the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment, to explore the effects of a whole-ecosystem warming gradient (+0°C to 9°C) and eCO2 on vascular plant fine roots and their associated microbes. We combined trait-based approaches with the profiling of fungal and prokaryote communities in plant roots and rhizospheres, through amplicon sequencing. Warming promoted self-reliance for resource uptake in trees and shrubs, while saprophytic fungi and putative chemoorganoheterotrophic bacteria utilizing plant-derived carbon substrates were favored in the root zone. Conversely, eCO2 promoted associations between trees and ectomycorrhizal fungi. Trees mostly associated with short-distance exploration-type fungi that preferentially use labile soil N. Additionally, eCO2 decreased the relative abundance of saprotrophs in tree roots. Our results indicate that plant fine-root trait variation is a crucial mechanism through which vascular plants in peatlands respond to climate change via their influence on microbial communities that regulate biogeochemical cycles.

Mycorrhizal mediation of soil carbon in permafrost regions depends on soil nutrient stoichiometry and physical protection

Mycorrhizal associations are considered as one of the key drivers for soil carbon (C) accumulation and stability. However, how mycorrhizal associations influence soil organic C (SOC) and its fractions (i.e., particulate organic C [POC] and mineral-associated organic C [MAOC]) remain unclear. In this study, we examined effects of plant mycorrhizal associations with arbuscular mycorrhiza (AM), ectomycorrhiza (ECM), and their mixture (Mixed) on SOC and its fractions as well as soil stoichiometric ratios across 800-km transect in permafrost regions. Our results showed that soil with only ECM-associated trees had significantly higher SOC and POC compared to only AM-associated tree species, while soil in Mixed plots with both AM- and ECM- associated trees tend to be somewhat in the middle. Using structural equation models, we found that mycorrhizal association significantly influenced SOC and its fraction (i.e., POC, MAOC) indirectly through soil stoichiometric ratios (C:N, C:P, and N:P). These results suggest that selecting ECM tree species, characterized by a "slow cycling" nutrient uptake strategy, can effectively enhance accumulation of SOC and its fractions in permafrost forest ecosystems. Our findings provide novel insights for quantitatively assessing the influence of mycorrhiza-associated tree species on the management of soil C pool and biogeochemical cycling.

Ericoid shrub encroachment shifts aboveground-belowground linkages in three peatlands across Europe and Western Siberia

In northern peatlands, reduction of Sphagnum dominance in favour of vascular vegetation is likely to influence biogeochemical processes. Such vegetation changes occur as the water table lowers and temperatures rise. To test which of these factors has a significant influence on peatland vegetation, we conducted a 3-year manipulative field experiment in Linje mire (northern Poland). We manipulated the peatland water table level (wet, intermediate and dry; on average the depth of the water table was 17.4, 21.2 and 25.3 cm respectively), and we used open-top chambers (OTCs) to create warmer conditions (on average increase of 1.2°C in OTC plots compared to control plots). Peat drying through water table lowering at this local scale had a larger effect than OTC warming treatment per see on Sphagnum mosses and vascular plants. In particular, ericoid shrubs increased with a lower water table level, while Sphagnum decreased. Microclimatic measurements at the plot scale indicated that both water-level and temperature, represented by heating degree days (HDDs), can have significant effects on the vegetation. In a large-scale complementary vegetation gradient survey replicated in three peatlands positioned along a transitional oceanic-continental and temperate-boreal (subarctic) gradient (France-Poland-Western Siberia), an increase in ericoid shrubs was marked by an increase in phenols in peat pore water, resulting from higher phenol concentrations in vascular plant biomass. Our results suggest a shift in functioning from a mineral-N-driven to a fungi-mediated organic-N nutrient acquisition with shrub encroachment. Both ericoid shrub encroachment and higher mean annual temperature in the three sites triggered greater vascular plant biomass and consequently the dominance of decomposers (especially fungi), which led to a feeding community dominated by nematodes. This contributed to lower enzymatic multifunctionality. Our findings illustrate mechanisms by which plants influence ecosystem responses to climate change, through their effect on microbial trophic interactions.

Peat loss collocates with a threshold in plant-mycorrhizal associations in drained peatlands encroached by trees

Drainage-induced encroachment by trees may have major effects on the carbon balance of northern peatlands, and responses of microbial communities are likely to play a central mechanistic role. We profiled the soil fungal community and estimated its genetic potential for the decay of lignin and phenolics (class II peroxidase potential) along peatland drainage gradients stretching from interior locations (undrained, open) to ditched locations (drained, forested). Mycorrhizal fungi dominated the community across the gradients. When moving towards ditches, the dominant type of mycorrhizal association abruptly shifted from ericoid mycorrhiza to ectomycorrhiza at c. 120 m from the ditches. This distance corresponded with increased peat loss, from which more than half may be attributed to oxidation. The ectomycorrhizal genus Cortinarius dominated at the drained end of the gradients and its relatively higher genetic potential to produce class II peroxidases (together with Mycena) was positively associated with peat humification and negatively with carbon-to-nitrogen ratio. Our study is consistent with a plant-soil feedback mechanism, driven by a shift in the mycorrhizal type of vegetation, that potentially mediates changes in aerobic decomposition during postdrainage succession. Such feedback may have long-term legacy effects upon postdrainage restoration efforts and implication for tree encroachment onto carbon-rich soils globally.