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 15 N 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.

Community composition of aquatic fungi across the thawing Arctic.

Thermokarst activity at permafrost sites releases considerable amounts of ancient carbon to the atmosphere. A large part of this carbon is released via thermokarst ponds, and fungi could be an important organismal group enabling its recycling. However, our knowledge about aquatic fungi in thermokarstic systems is extremely limited. In this study, we collected samples from five permafrost sites distributed across circumpolar Arctic and representing different stages of permafrost integrity. Surface water samples were taken from the ponds and, additionally, for most of the ponds also the detritus and sediment samples were taken. All the samples were extracted for total DNA, which was then amplified for the fungal ITS2 region of the ribosomal genes. These amplicons were sequenced using PacBio technology. Water samples were also collected to analyze the chemical conditions in the ponds, including nutrient status and the quality and quantity of dissolved organic carbon. This dataset gives a unique overview of the impact of the thawing permafrost on fungal communities and their potential role on carbon recycling.

Declining fungal diversity in Arctic freshwaters along a permafrost thaw gradient.

Climate change-driven permafrost thaw has a strong influence on pan-Arctic regions, via, for example, the formation of thermokarst ponds. These ponds are hotspots of microbial carbon cycling and greenhouse gas production, and efforts have been put on disentangling the role of bacteria and archaea in recycling the increasing amounts of carbon arriving to the ponds from degrading watersheds. However, despite the well-established role of fungi in carbon cycling in the terrestrial environments, the interactions between permafrost thaw and fungal communities in Arctic freshwaters have remained unknown. We integrated data from 60 ponds in Arctic hydro-ecosystems, representing a gradient of permafrost integrity and spanning over five regions, namely Alaska, Greenland, Canada, Sweden, and Western Siberia. The results revealed that differences in pH and organic matter quality and availability were linked to distinct fungal community compositions and that a large fraction of the community represented unknown fungal phyla. Results display a 16%-19% decrease in fungal diversity, assessed by beta diversity, across ponds in landscapes with more degraded permafrost. At the same time, sites with similar carbon quality shared more species, aligning a shift in species composition with the quality and availability of terrestrial dissolved organic matter. We demonstrate that the degradation of permafrost has a strong negative impact on aquatic fungal diversity, likely via interactions with the carbon pool released from ancient deposits. This is expected to have implications for carbon cycling and climate feedback loops in the rapidly warming Arctic.

A tipping point in carbon storage when forest expands into tundra is related to mycorrhizal recycling of nitrogen.

Tundra ecosystems are global belowground sinks for atmospheric CO2 . Ongoing warming-induced encroachment by shrubs and trees risks turning this sink into a CO2 source, resulting in a positive feedback on climate warming. To advance mechanistic understanding of how shifts in mycorrhizal types affect long-term carbon (C) and nitrogen (N) stocks, we studied small-scale soil depth profiles of fungal communities and C-N dynamics across a subarctic-alpine forest-heath vegetation gradient. Belowground organic stocks decreased abruptly at the transition from heath to forest, linked to the presence of certain tree-associated ectomycorrhizal fungi that contribute to decomposition when mining N from organic matter. In contrast, ericoid mycorrhizal plants and fungi were associated with organic matter accumulation and slow decomposition. If climatic controls on arctic-alpine forest lines are relaxed, increased decomposition will likely outbalance increased plant productivity, decreasing the overall C sink capacity of displaced tundra.

Sample Preparation for Fungal Community Analysis by High-Throughput Sequencing of Barcode Amplicons.

Fungal species participate in vast numbers of processes in the landscape around us. However, their often cryptic growth, inside various substrates and in highly diverse species assemblages, has been a major obstacle to thorough analysis of fungal communities, hampering exhaustive description of the fungal kingdom. Recent technological developments allowing rapid, high-throughput sequencing of mixed communities from many samples at once are currently having a tremendous impact in fungal community ecology. Universal DNA extraction followed by amplification and sequencing of fungal species-level barcodes such as the nuclear internal transcribed spacer (ITS) region now enable identification and relative quantification of fungal community members across well-replicated experimental settings. Here, we present the sample preparation procedure presently used in our laboratory for fungal community analysis by high-throughput sequencing of amplified ITS2 markers. We focus on the procedure optimized for studies of total fungal communities in humus-rich soils, wood, and litter. However, this procedure can be applied to other sample types and markers. We focus on the laboratory-based part of sample preparation, that is, the procedure from the point where samples enter the laboratory until amplicons are submitted for sequencing. Our procedure comprises four main parts: (1) universal DNA extraction, (2) optimization of PCR conditions, (3) production of tagged ITS amplicons, and (4) preparation of the multiplexed amplicon mix to be sequenced. The presented procedure is independent of the specific high-throughput sequencing technology used, which makes it highly versatile.

Site-dependent N uptake from N-form mixtures by arctic plants, soil microbes and ectomycorrhizal fungi.

Soil microbes constitute an important control on nitrogen (N) turnover and retention in arctic ecosystems where N availability is the main constraint on primary production. Ectomycorrhizal (ECM) symbioses may facilitate plant competition for the specific N pools available in various arctic ecosystems. We report here our study on the N uptake patterns of coexisting plants and microbes at two tundra sites with contrasting dominance of the circumpolar ECM shrub Betula nana. We added equimolar mixtures of glycine-N, NH4+ -N and NO3(-) -N, with one N form labelled with 15N at a time, and in the case of glycine, also labelled with 13C, either directly to the soil or to ECM fungal ingrowth bags. After 2 days, the vegetation contained 5.6, 7.7 and 9.1% (heath tundra) and 7.1, 14.3 and 12.5% (shrub tundra) of the glycine-, NH4+ - and NO3 (-) -(15)N, respectively, recovered in the plant-soil system, and the major part of 15N in the soil was immobilized by microbes (chloroform fumigation-extraction). In the subsequent 24 days, microbial N turnover transferred about half of the immobilized 15N to the non-extractable soil organic N pool, demonstrating that soil microbes played a major role in N turnover and retention in both tundra types. The ECM mycelial communities at the two tundras differed in N-form preferences, with a higher contribution of glycine to total N uptake at the heath tundra; however, the ECM mycelial communities at both sites strongly discriminated against NO3 (-) . Betula nana did not directly reflect ECM mycelial N uptake, and we conclude that N uptake by ECM plants is modulated by the N uptake patterns of both fungal and plant components of the symbiosis and by competitive interactions in the soil. Our field study furthermore showed that intact free amino acids are potentially important N sources for arctic ECM fungi and plants as well as for soil microorganisms.