Manganese availability is negatively associated with carbon storage in northern coniferous forest humus layers

Carbon sequestration below ground depends on organic matter input and decomposition, but regulatory bottlenecks remain unclear. The relative importance of plant production, climate and edaphic factors has to be elucidated to better predict carbon storage in forests. In Swedish forest soil inventory data from across the entire boreal latitudinal range (n = 2378), the concentration of exchangeable manganese was singled out as the strongest predictor (R-2 = 0.26) of carbon storage in the extensive organic horizon (mor layer), which accounts for one third of the total below ground carbon. In comparison, established ecosystem models applied on the same data have failed to predict carbon stocks (R-2 <0.05), and in our study manganese availability overshadowed both litter production and climatic factors. We also identified exchangeable potassium as an additional strong predictor, however strongly correlated with manganese. The negative correlation between manganese and carbon highlights the importance of Mn-peroxidases in oxidative decomposition of recalcitrant organic matter. The results support the idea that the fungus-driven decomposition could be a critical factor regulating humus carbon accumulation in boreal forests, as Mn-peroxidases are specifically produced by basidiomycetes.Peer reviewe

Links between boreal forest management, soil fungal communities and below-ground carbon sequestration

1. Forest management has a potential to alter below-ground carbon storage. However, the underlying mechanisms, and the relative importance of carbon input and decomposition in regulation of soil carbon dynamics are poorly understood.2. We examined whether interactive effects of forest fertilization and thinning on carbon stocks in the topsoil of boreal forests were linked to changes in fungal community composition, biomass and enzyme activities, in a long-term fertilization and thinning experiment distributed across 29 Pinus sylvestris forests along a 1,300 km latitudinal transect in Sweden.3. Nitrogen fertilization increased fungal biomass, particularly towards the north and mainly by promoting root-associated Ascomycetes, but the response was moderated by thinning. Fungal biomass correlated positively with carbon stocks in the organic topsoil. However, ectomycorrhizal Cortinarius species were reduced in abundance by fertilization and correlated negatively with carbon stocks.4. Plausibly, increased soil carbon stocks after fertilization are linked to increased input of carbon in the form of root-associated mycelium combined with the loss of ectomycorrhizal decomposers within the genus Cortinarius. These fungal responses to fertilization may mediate a natural climate solution by promoting carbon sequestration in the organic topsoil, but the effect of fertilization may also be undesired from a biodiversity perspective

Contrasting plant–soil–microbial feedbacks stabilize vegetation types and uncouple topsoil C and N stocks across a subarctic–alpine landscape

Global vegetation regimes vary in belowground carbon (C) and nitrogen (N) dynamics. However, disentangling large-scale climatic controls from the effects of intrinsic plant–soil–microbial feedbacks on belowground processes is challenging. In local gradients with similar pedo-climatic conditions, effects of plant–microbial feedbacks may be isolated from large-scale drivers. Across a subarctic–alpine mosaic of historic grazing fields and surrounding heath and birch forest, we evaluated whether vegetation-specific plant–microbial feedbacks involved contrasting N cycling characteristics and C and N stocks in the organic topsoil. We sequenced soil fungi, quantified functional genes within the inorganic N cycle, and measured 15N natural abundance. In grassland soils, large N stocks and low C : N ratios associated with fungal saprotrophs, archaeal ammonia oxidizers, and bacteria capable of respiratory ammonification, indicating maintained inorganic N cycling a century after abandoned reindeer grazing. Toward forest and heath, increasing abundance of mycorrhizal fungi co-occurred with transition to organic N cycling. However, ectomycorrhizal fungal decomposers correlated with small soil N and C stocks in forest, while root-associated ascomycetes associated with small N but large C stocks in heath, uncoupling C and N storage across vegetation types. We propose that contrasting, positive plant–microbial feedbacks stabilize vegetation trajectories, resulting in diverging soil C : N ratios at the landscape scale.publishedVersio

Fungal ecological strategies reflected in gene transcription - a case study of two litter decomposers.

Microbial communities interplay with their environment through their functional traits that can be a response or an effect on the environment. Here, we explore how a functional trait-the decomposition of organic matter, can be addressed based on genetic markers and how the expression of these markers reflect ecological strategies of two fungal litter decomposer Gymnopus androsaceus and Chalara longipes. We sequenced the genomes of these two fungi, as well as their transcriptomes at different steps of Pinus sylvestris needles decomposition in microcosms. Our results highlighted that if the gene content of the two species could indicate similar potential decomposition abilities, the expression levels of specific gene families belonging to the glycoside hydrolase category reflected contrasting ecological strategies. Actually, C. longipes, the weaker decomposer in this experiment, turned out to have a high content of genes involved in cell wall polysaccharides decomposition but low expression levels, reflecting a versatile ecology compare to the more competitive G. androsaceus with high expression levels of keystone functional genes. Thus, we established that sequential expression of genes coding for different components of the decomposer machinery indicated adaptation to chemical changes in the substrate as decomposition progressed

Modeling Microbial Adaptations to Nutrient Limitation During Litter Decomposition

Microbial decomposers face large stoichiometric imbalances when feeding on nutrient-poor plant residues. To meet the challenges of nutrient limitation, microorganisms might: (i) allocate less carbon (C) to growth vs. respiration or excretion (i.e., flexible C-use efficiency, CUE), (ii) produce extracellular enzymes to target compounds that supply the most limiting element, (iii) modify their cellular composition according to the external nutrient availability, and (iv) preferentially retain nutrients at senescence. These four resource use modes can have different consequences on the litter C and nitrogen (N) dynamics–modes that selectively remove C from the system can reduce C storage in soil, whereas modes that delay C mineralization and increase internal N recycling could promote storage of C and N. Since we do not know which modes are dominant in litter decomposers, we cannot predict the fate of C and N released from plant residues, in particular under conditions of microbial nutrient limitation. To address this question, we developed a process-based model of litter decomposition in which these four resource use modes were implemented. We then parameterized the model using ∼80 litter decomposition datasets spanning a broad range of litter qualities. The calibrated model variants were able to capture most of the variability in litter C, N, and lignin fractions during decomposition regardless of which modes were included. This suggests that different modes can lead to similar litter decomposition trajectories (thanks to the multiple alternative resource acquisition pathways), and that identification of dominant modes is not possible using “standard” litter decomposition data (an equifinality problem). Our results thus point to the need of exploring microbial adaptations to nutrient limitation with empirical estimates of microbial traits and to develop models flexible enough to consider a range of hypothesized microbial responses

Lack of thinning effects over inter-annual changes in soil fungal community and diversity in a Mediterranean pine forest

Predicted changes in global climate might negatively affect the soil microbiome and associated ecosystem processes in Mediterranean forests. Forest treatments, such as forest thinning, have been suggested to mitigate climate change impacts on vegetation by reducing competition between trees, thus increasing water availability. Studies addressing the combined effects of climate and forest thinning on belowground fungal communities are still scarce, being fundamental to elaborate adaptive strategies to global warming. The aim of this study was to evaluate the tree density reduction effects on soil fungal communities and their response to inter-annual changes in weather conditions. The temporal dynamics of soil fungal communities in relation to these two drivers (i.e., forest management and weather conditions) were studied from 2009 until 2014 in a set of 12 pairs of thinned and un-thinned plots dominated by Pinus pinaster Ait. Thinning (from 30% up to 70% reduction in stand basal area) was conducted in 2009 and soil fungal community composition was studied during 4 years. Here, we used autumn precipitation and temperature to describe the impact of inter-annual weather changes. We used Pacific Biosciences sequencing of fungal ITS2 amplicons to study fungal communities in soil samples. Forest thinning did not significantly affect fungal community composition nor fungal species richness and diversity, indicating that the soil fungal community is resistant to forest thinning regardless of its intensity. However, fungal species composition changed progressively across years, both at the species level and with regards to functional guilds. These changes in community composition were partly driven by inter-annual variation in precipitation and temperature, with free-living fungi increasing in abundance under wetter conditions, and symbiotic fungi being more prominent under drier and colder conditions. The results indicate that mycorrhizal communities in Mediterranean forest ecosystems can resist forest thinning, if enough trees and functional roots from thinned trees are retained.info:eu-repo/semantics/acceptedVersio

Priority effects during fungal community establishment in beech wood

Assembly history of fungal communities has a crucial role in the decomposition of woody resources, and hence nutrient cycling and ecosystem function. However, it has not been clearly determined whether the fungal species that arrive first may, potentially, dictate the subsequent pathway of community development, that is, whether there is a priority effect at the species level. We used traditional culture-based techniques coupled with sequencing of amplified genetic markers to profile the fungal communities in beech (Fagus sylvatica) disks that had been pre-colonised separately with nine species from various stages of fungal succession. Clear differences in community composition were evident following pre-colonisation by different species with three distinct successor communities identified, indicating that individual species may have pivotal effects in driving assembly history. Priority effects may be linked to biochemical alteration of the resource and combative ability of the predecessor

Rhizosphere allocation by canopy-forming species dominates soil CO2 efflux in a subarctic landscape

In arctic ecosystems, climate change has increased plant productivity. As arctic carbon (C) stocks are predominantly located below ground, the effects of greater plant productivity on soil C storage will significantly determine the net sink/source potential of these ecosystems, but vegetation controls on soil CO2 efflux remain poorly resolved. To identify the role of canopy‐forming species in below‐ground C dynamics, we conducted a girdling experiment with plots distributed across 1 km2 of treeline birch (Betula pubescens) forest and willow (Salix lapponum) patches in northern Sweden and quantified the contribution of canopy vegetation to soil CO2 fluxes and below‐ground productivity. Girdling birches reduced total soil CO2 efflux in the peak growing season by 53% ‐double the expected amount, given that trees contribute only half of the total leaf area in the forest. Root and mycorrhizal mycelial production also decreased substantially. At peak season, willow shrubs contributed 38% to soil CO2 efflux in their patches. Our findings indicate that C, recently fixed by trees and tall shrubs, makes a substantial contribution to soil respiration. It is critically important that these processes are taken into consideration in the context of a greening arctic since productivity and ecosystem C sequestration are not synonymous

Resistance of subarctic soil fungal and invertebrate communities to disruption of below‐ground carbon supply

The supply of recent photosynthate from plants to soils is thought to be a critical mechanism regulating the activity and diversity of soil biota. In the Arctic, large-scale vegetation transitions are underway in response to warming, and there is an urgent need to understand how these changes affect soil biodiversity and function. We investigated how abundance and diversity of soil fungi and invertebrates responded to a reduction in fresh below-ground photosynthate supply in treeline birch and willow, achieved using stem girdling. We hypothesised that birch forest would support greater abundance of ectomycorrhizal (ECM) fungal species and fauna than willow shrubs, and that girdling would result in a rapid switch from ECM fungi to saprotrophs as canopy supply of C was cut, with a concomitant decline in soil fauna. Birch forest had greater fungal and faunal abundance with a large contribution of root-associated ascomycetes (ericoid mycorrhizal fungi and root endophytes) compared to willow shrub plots, which had a higher proportion of saprotrophs and, contrary to our expectations, ECM fungi. Broad-scale soil fungal and faunal functional group composition was not significantly changed by girdling, even in the third year of treatment. Within the ECM community, there were some changes, with genera that are believed to be particularly C-demanding declining in girdled plots. However, it was notable how most ECM fungi remained present after 3 years of isolation of the below-ground compartment from contemporary photosynthate supply. Synthesis. In a treeline/tundra ecosystem, distinct soil communities existed in contrasting vegetation patches within the landscape, but the structure of these communities was resistant to canopy disturbance and concomitant reduction of autotrophic C inputs