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1. Changing sub-Arctic tundra vegetation upon permafrost degradation: impact on foliar mineral element cycling

   https://doi.org/10.5194/bg-19-2333-2022  

   Mauclet, Elisabeth; Agnan, Yannick; Hirst, Catherine; Monhonval, Arthur; Pereira, Benoît; Vandeuren, Aubry; Villani, Maëlle; Ledman, Justin; Taylor, Meghan; Jasinski, Briana L.; et al (January 2022, Biogeosciences)

   Abstract. Arctic warming and permafrost degradation are modifying northernecosystems through changes in microtopography, soil water dynamics, nutrientavailability, and vegetation succession. Upon permafrost degradation, therelease of deep stores of nutrients, such as nitrogen and phosphorus, fromnewly thawed permafrost stimulates Arctic vegetation production. Morespecifically, wetter lowlands show an increase in sedges (as part ofgraminoids), whereas drier uplands favor shrub expansion. These shifts inthe composition of vegetation may influence local mineral element cyclingthrough litter production. In this study, we evaluate the influence ofpermafrost degradation on mineral element foliar stocks and potential annualfluxes upon litterfall. We measured the foliar elemental composition (Al,Ca, Fe, K, Mn, P, S, Si, and Zn) of ∼ 500 samples of typicaltundra plant species from two contrasting Alaskan tundra sites, i.e., anexperimental sedge-dominated site (Carbon in Permafrost Experimental Heating Research, CiPEHR) and natural shrub-dominated site(Gradient). The foliar concentration of these mineral elements was species specific, with sedge leaves having relatively high Si concentration andshrub leaves having relatively high Ca and Mn concentrations. Therefore,changes in the species biomass composition of the Arctic tundra in responseto permafrost thaw are expected to be the main factors that dictate changesin elemental composition of foliar stocks and maximum potential foliarfluxes upon litterfall. We observed an increase in the mineral elementfoliar stocks and potential annual litterfall fluxes, with Si increasingwith sedge expansion in wetter sites (CiPEHR), and Ca and Mn increasing withshrub expansion in drier sites (Gradient). Consequently, we expect thatsedge and shrub expansion upon permafrost thaw will lead to changes inlitter elemental composition and therefore affect nutrient cycling acrossthe sub-Arctic tundra with potential implications for further vegetationsuccession. 

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2. Impacts of Arctic Shrubs on Root Traits and Belowground Nutrient Cycles Across a Northern Alaskan Climate Gradient

   https://doi.org/10.3389/fpls.2020.588098  

   Chen, Weile; Tape, Ken D.; Euskirchen, Eugénie S.; Liang, Shuang; Matos, Adriano; Greenberg, Jonathan; Fraterrigo, Jennifer M. (December 2020, Frontiers in Plant Science)

   null (Ed.)

   Deciduous shrubs are expanding across the graminoid-dominated nutrient-poor arctic tundra. Absorptive root traits of shrubs are key determinants of nutrient acquisition strategy from tundra soils, but the variations of shrub root traits within and among common shrub genera across the arctic climatic gradient are not well resolved. Consequently, the impacts of arctic shrub expansion on belowground nutrient cycling remain largely unclear. Here, we collected roots from 170 plots of three commonly distributed shrub genera ( Alnus , Betula , and Salix ) and a widespread sedge ( Eriophorum vaginatum ) along a climatic gradient in northern Alaska. Absorptive root traits that are relevant to the strategy of plant nutrient acquisition were determined. The influence of aboveground dominant vegetation cover on the standing root biomass, root productivity, vertical rooting profile, as well as the soil nitrogen (N) pool in the active soil layer was examined. We found consistent root trait variation among arctic plant genera along the sampling transect. Alnus and Betula had relatively thicker and less branched, but more frequently ectomycorrhizal colonized absorptive roots than Salix , suggesting complementarity between root efficiency and ectomycorrhizal dependence among the co-existing shrubs. Shrub-dominated plots tended to have more productive absorptive roots than sedge-dominated plots. At the northern sites, deep absorptive roots (>20 cm depth) were more frequent in birch-dominated plots. We also found shrub roots extensively proliferated into the adjacent sedge-dominated plots. The soil N pool in the active layer generally decreased from south to north but did not vary among plots dominated by different shrub or sedge genera. Our results reveal diverse nutrient acquisition strategies and belowground impacts among different arctic shrubs, suggesting that further identifying the specific shrub genera in the tundra landscape will ultimately provide better predictions of belowground dynamics across the changing arctic. 

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3. Long-Term Warming in Alaska Enlarges the Diazotrophic Community in Deep Soils

   https://doi.org/10.1128/mBio.02521-18  

   Feng, Jiajie; Penton, C. Ryan; He, Zhili; Van Nostrand, Joy D.; Yuan, Mengting M.; Wu, Liyou; Wang, Cong; Qin, Yujia; Shi, Zhou J.; Guo, Xue; et al (February 2019, mBio)

   ABSTRACT Tundra ecosystems are typically carbon (C) rich but nitrogen (N) limited. Since biological N 2 fixation is the major source of biologically available N, the soil N 2 -fixing (i.e., diazotrophic) community serves as an essential N supplier to the tundra ecosystem. Recent climate warming has induced deeper permafrost thaw and adversely affected C sequestration, which is modulated by N availability. Therefore, it is crucial to examine the responses of diazotrophic communities to warming across the depths of tundra soils. Herein, we carried out one of the deepest sequencing efforts of nitrogenase gene ( nifH ) to investigate how 5 years of experimental winter warming affects Alaskan soil diazotrophic community composition and abundance spanning both the organic and mineral layers. Although soil depth had a stronger influence on diazotrophic community composition than warming, warming significantly ( P <  0.05) enhanced diazotrophic abundance by 86.3% and aboveground plant biomass by 25.2%. Diazotrophic composition in the middle and lower organic layers, detected by nifH sequencing and a microarray-based tool (GeoChip), was markedly altered, with an increase of α-diversity. Changes in diazotrophic abundance and composition significantly correlated with soil moisture, soil thaw duration, and plant biomass, as shown by structural equation modeling analyses. Therefore, more abundant diazotrophic communities induced by warming may potentially serve as an important mechanism for supplementing biologically available N in this tundra ecosystem. IMPORTANCE With the likelihood that changes in global climate will adversely affect the soil C reservoir in the northern circumpolar permafrost zone, an understanding of the potential role of diazotrophic communities in enhancing biological N 2 fixation, which constrains both plant production and microbial decomposition in tundra soils, is important in elucidating the responses of soil microbial communities to global climate change. A recent study showed that the composition of the diazotrophic community in a tundra soil exhibited no change under a short-term (1.5-year) winter warming experiment. However, it remains crucial to examine whether the lack of diazotrophic community responses to warming is persistent over a longer time period as a possibly important mechanism in stabilizing tundra soil C. Through a detailed characterization of the effects of winter warming on diazotrophic communities, we showed that a long-term (5-year) winter warming substantially enhanced diazotrophic abundance and altered community composition, though soil depth had a stronger influence on diazotrophic community composition than warming. These changes were best explained by changes in soil moisture, soil thaw duration, and plant biomass. These results provide crucial insights into the potential factors that may impact future C and N availability in tundra regions. 

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4. Rates and controls of nitrogen fixation in postfire lodgepole pine forests

   https://doi.org/10.1002/ecy.70016  

   Heumann, Robert E; Turner, Monica G; Cleveland, Cory C (February 2025, Ecology)

   Abstract Severe, stand‐replacing wildfire substantially depletes nitrogen (N) stocks in subalpine conifer forests, potentially exacerbating N limitation of net primary productivity in many forested regions where fire frequency is increasing. In lodgepole pine (Pinus contortavar.latifolia) forests in the Greater Yellowstone Ecosystem (GYE), long‐term data show surface soil and biomass N stocks are replenished during the first few decades following wildfire, but the source(s) of that N are unclear. We measured acetylene reduction rates in multiple cryptic niches (i.e., lichen, moss, pine litter, dead wood, and mineral soil) in 34‐year‐old lodgepole pine stands in the GYE to explore the rates, temporal patterns, and climate controls on cryptic N fixation. Acetylene reduction rates were highest in late May (0.376 nmol C₂H₄g⁻¹ h⁻¹) when moisture availability was high compared with early August and mid‐October when moisture was relatively low (0.112 and 0.002 nmol C₂H₄g⁻¹ h⁻¹, respectively). We observed modest rates of nitrogenase activity in a few niches following a mid‐summer rain event, suggesting that moisture is an important factor regulating field‐based N fixation rates. In a laboratory experiment, moss responded more strongly to temperature and moisture variation than all other niches. Acetylene reduction rates in dead wood increased with temperature but not moisture content. No other niches showed clear responses to either moisture or temperature manipulation. Together, the field and laboratory results suggest that frequent asynchrony between favorable temperature and moisture conditions may limit N fixation rates in the field. Overall, total annual cryptic N fixation inputs (mean: 0.26; range: 0.07–2.9 kg N ha⁻¹year⁻¹) represented <10% of the postfire biomass and surface soil N accumulation in the same stands (39.4 kg N ha⁻¹year⁻¹), pointing to a still unknown source of ecosystem N following fire. 

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5. Shrub expansion in the Arctic may induce large‐scale carbon losses due to changes in plant‐soil interactions

   https://doi.org/10.1007/s11104-021-04919-8  

   Parker, Thomas C.; Thurston, Alana M.; Raundrup, Katrine; Subke, Jens-Arne; Wookey, Philip A.; Hartley, Iain P. (June 2021, Plant and Soil)

   Abstract Background Tall deciduous shrubs are increasing in range, size and cover across much of the Arctic, a process commonly assumed to increase carbon (C) storage. Major advances in remote sensing have increased our ability to monitor changes aboveground, improving quantification and understanding of arctic greening. However, the vast majority of C in the Arctic is stored in soils, where changes are more uncertain. Scope We present pilot data to argue that shrub expansion will cause changes in rhizosphere processes, including the development of new mycorrhizal associations that have the potential to promote soil C losses that substantially exceed C gains in plant biomass. However, current observations are limited in their spatial extent, and mechanistic understanding is still developing. Extending measurements across different regions and tundra types would greatly increase our ability to predict the biogeochemical consequences of arctic vegetation change, and we present a simple method that would allow such data to be collected. Conclusions Shrub expansion in the Arctic could promote substantial soil C losses that are unlikely to be offset by increases in plant biomass. However, confidence in this prediction is limited by a lack of information on how soil C stocks vary between contrasting Arctic vegetation communities; this needs to be addressed urgently. 

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