Pintamaan geokemiaa selittävät tekijät muuttuvassa tundraympäristössä

Ongoing climate change is altering sensitive tundra environments. However, the impacts of these changes on topsoil geochemistry are still not fully understood even though their feedbacks might have the potential to change even the global climate system. Therefore, it is of great importance to study the relative importance of climatic and other environmental factors affecting topsoil geochemistry. This thesis examines the factors and processes regulating topsoil geochemistry and its sensitivity to climate change. The aim of the research was also to link local observations to regional models and predictions in order to improve the quality of digital soil maps. Field data (n=429) was collected from a landscape-scale research area (72 km²) in northern Norway (69° 90' N and 26° 20' E). Soil samples were analyzed in the laboratory and the concentrations of total nitrogen, phosphorous, total carbon, calcium, iron and pH were measured. Geochemical variables were modelled against environmental data comprising of soil moisture and derivatives of a climate model, an elevation model, Landsat-satellite images and aerial images. The relationships between the geochemical and environmental data were described using NMDS ordination method and modelled, predicted and simulated with generalized boosted models (GBM) and structural equation modelling (SEM). The distribution of the topsoil geochemistry based on the GBM model predictions was similar between the geochemical variables: there was a decrease of nutrient concentrations with increasing elevation and distance to tundra streams and with increasing mountain birch abundance. Topsoil geochemistry varied markedly over short distances, especially in mountain birch forest where the extreme ends of the geochemical gradient could be found even within a 100 m radius. Climate was the most important factor governing all the geochemical variables based on the GBM models but it affected topsoil geochemistry partly through soil moisture, the role of which was at times bigger than the role of climate-driven NPP's which was studied in the SEM models. The relative importance of topography and geology on topsoil geochemistry was small in the GBM models. The predictive power of the GBM and SEM models predicting nitrogen, phosphorous, carbon and calcium concentrations were relatively good in the treeless tundra whereas the model fit of iron and pH models were poor. The results emphasize the role of climate and soil moisture as controlling forces in the organic cycle and small local impacts of topography affecting the leaching of nutrients. Topsoil nutrients and pH have distinct relationships with the environmental variables with topsoil nitrogen, carbon, calcium and partly phosphorous behaving similarly and iron and pH in different ways based on the NMDS, GBM and SEM results. The low importance of topographic and geologic variables in the GBM models resulted in considerable topsoil geochemical changes in relation to the climate simulations made with GBM models. According to the simulations, topsoil geochemical concentrations increased with decreasing snow cover and increasing temperatures with a tipping point of 1,5 °C where the biggest changes are occurring. Climate sensitivity of the nutrients was highest on ridges and in valleys with carbon showing the biggest temperature sensitivity. Changes in snow cover affected topsoil geochemistry less than climate simulations but created more heterogeneous concentration differences. Topsoil will likely continue to act as an important global sink for geochemical variables in changing tundra environments.Ilmastonmuutos muuttaa herkkiä tundraympäristöjä, mutta muutosten vaikutuksia pintamaan geokemiaan ei vielä tiedetä. Pintamaan geokemia, eli ravinteisuus ja pH, on tärkeässä roolissa ilmastonmuutoksen kannalta, sillä se voi synnyttää globaaliin ilmastoon ja tundraympäristöön vaikuttavia takaisinkytkentöjä. Vielä on epäselvää, onko ilmasto tärkein pintamaan geokemiaa säätelevä tekijä vai voisivatko muut tekijät, kuten topografia ja maaperän kosteus puskuroida tulevia muutoksia. Tämä tutkimus selvittää pintamaan geokemiaa sääteleviä tekijöitä ja prosesseja sekä pintamaan geokemian herkkyyttä ilmastonmuutokselle. Lisäksi tutkimus kytkee paikalliset havainnot laajempiin alueellisiin ennusteisiin. Aineisto kerättiin ympäristögradienteiltaan kattavalta maisemamittakaavan 72 km² tutkimusalueelta (n=429) Pohjois-Norjasta (noin 69° 90' N ja 26° 20' E). Geokemia-aineisto koostui laboratoriossa määritetyistä pintamaan typen, fosforin, hiilen, kalsiumin ja raudan pitoisuuksista sekä maaperän pH:sta. Geokemiamuuttujia tutkittiin alueellisen mallinnuksen keinoin ympäristöaineistolla, joka koostui maaperän kosteudesta ja johon johdettiin muuttujia ilmasto- ja korkeusmallista, Landsat-satelliittikuvista sekä ilmakuvista. Aineiston hajontaa kuvattiin NMDS-gradienttianalyysimenetelmän avulla ja alueellinen mallintaminen ja ennustaminen sekä ilmastonmuutoksen vaikutusten simulointi toteutettiin rakenneyhtälömalleilla (SEM) ja yleistetyillä luokittelupuumenetelmillä (GBM). Geokemiamuuttujien alueelliset jakaumat GBM-malleilla tehdyissä ennusteissa olivat samankaltaisia: pitoisuudet olivat korkeita laaksoissa ja alarinteillä, melko alhaisia ylänköalueilla ja tunturikoivikoissa ja alhaisia tunturien laella, mutta paikallista vaihtelua oli paljon erityisesti tunturikoivikossa. Ilmaston rooli pintamaan geokemiaa selittävänä tekijänä oli GBM-malleissa suuri, mutta SEM-malleissa havaittiin, että paikallinen tekijä maaperän kosteus, joka oli osin ilmaston säätelemä, vaikutti paikoin suuremmissa määrin pintamaan geokemiaan. Topografisten ja hydrologisten tekijöiden vaikutus pintamaan geokemiaan oli GBM-malleissa vähäinen, jolloin ne eivät puskuroineet vahvasti pintamaan geokemian muutoksia GBM-malleilla tehdyissä ilmastonmuutossimulaatioissa, joissa havaittiin pintamaan ravinteisuuden kasvavan keskilämpötilojen noustessa ja lumisuuden vähentyessä. GBM-mallien selityskyky oli kohtuullisen hyvä etenkin puuttomalla paljakalla typpi-, fosfori-, hiili- ja kalsiumpitoisuuksia analysoitaessa, ja tutkimuksessa onnistuttiin linkittämään näiden ravinteiden paikalliset havainnot alueellisiin ennusteisiin. Lisäksi SEM-mallit osoittautuivat potentiaaliseksi ja monipuoliseksi menetelmäksi maaperämallinnuksessa, ja menetelmän avulla havaittiin, että paikalliset tekijät ja niiden vaihtelu pystyvät puskuroimaan ilmaston suurta vaikutusta. Tulokset indikoivat ilmaston ja maaperän kosteuden kontrolloiman orgaanisen aineksen kierron ja paikallisesti topografian ja hydrologisten olosuhteiden kontrolloiman huuhtoutumisen merkitystä pintamaan geokemian säätelijämekanismina. Pintamaan ravinteet ja pH reagoivat ympäristömuuttujiin kuitenkin eri tavalla: ympäristötekijöiden ja geokemian välinen suhde NMDS-analyysissä ja GBM- ja SEM-malleissa oli samanlainen typellä, hiilellä, kalsiumilla ja osin fosforilla, mutta rauta ja pH käyttäytyivät eri lailla. Pintamaan geokemia, ja etenkin pintamaan hiili on herkkä ilmastonmuutokselle, ja kynnysarvona suurimmille muutoksille ilmastonmuutossimulointien mukaan oli jo 1,5 °C lämpötilannousu. Tundraympäristöjen pintamaan geokemiapitoisuudet kasvavat ilmaston lämmetessä erityisesti tunturien huipuilla ja laaksoissa, jolloin tundraympäristöt pysyvät tämän tutkimuksen mukaan tärkeänä ravinnevarastona

Identifying multidisciplinary research gaps across Arctic terrestrial gradients

Peer reviewe

Relationships between above-ground plant traits and carbon cycling in tundra plant communities

The trait composition and trait diversity of plant communities are globally applicable predictors of ecosystem functioning. Yet, it is unclear how plant traits influence carbon cycling. This is an important question in the tundra where vegetation shifts are occurring across the entire biome, and where soil organic carbon stocks are large and vulnerable to environmental change. To study how plant traits affect carbon cycling in the tundra, we built a model that explained carbon cycling (above-ground and soil organic carbon stocks, and photosynthetic and respiratory fluxes) with abiotic conditions (air temperature and soil moisture), and the averages and within-community variabilities of three above-ground traits: plant height, leaf dry matter content (LDMC) and SLA. These functional parameters were represented by abundance-weighted means and standard deviations of species traits. The data were collected from an observational study setting from northern Finland. The explanatory power of the models was relatively high, but a large part of variation in soil organic carbon stocks remained unexplained. Average plant height was the strongest predictor of all carbon cycling variables except soil carbon stocks. Communities of larger plants were associated with larger CO2 fluxes and above-ground carbon stocks. Communities with fast leaf economics (i.e. high SLA and low LDMC) had higher photosynthesis, ecosystem respiration and soil organic carbon stocks. Within-community variability in plant height, SLA and LDMC affected ecosystem functions differently. Variability in SLA and LDMC increased CO2 fluxes and soil organic carbon stocks, while variability in height increased the above-ground carbon stock. The contributions of within-community trait variability metrics to ecosystem functioning within the study area were about as important as those of average SLA and LDMC. Synthesis. Plant height, SLA and LDMC have clear effects on tundra carbon cycling. The importance of within-community trait variability highlights a potentially important mechanism controlling the vast tundra carbon pools that should be better recognized. More research on root traits and decomposer communities is needed to understand the below-ground mechanisms regulating carbon cycling in the tundra.Peer reviewe

Dwarf Shrubs Impact Tundra Soils : Drier, Colder, and Less Organic Carbon

In the tundra, woody plants are dispersing towards higher latitudes and altitudes due to increasingly favourable climatic conditions. The coverage and height of woody plants are increasing, which may influence the soils of the tundra ecosystem. Here, we use structural equation modelling to analyse 171 study plots and to examine if the coverage and height of woody plants affect the growing-season topsoil moisture and temperature (< 10 cm) as well as soil organic carbon stocks (< 80 cm). In our study setting, we consider the hierarchy of the ecosystem by controlling for other factors, such as topography, wintertime snow depth and the overall plant coverage that potentially influence woody plants and soil properties in this dwarf shrub-dominated landscape in northern Fennoscandia. We found strong links from topography to both vegetation and soil. Further, we found that woody plants influence multiple soil properties: the dominance of woody plants inversely correlated with soil moisture, soil temperature, and soil organic carbon stocks (standardised regression coefficients = - 0.39; - 0.22; - 0.34, respectively), even when controlling for other landscape features. Our results indicate that the dominance of dwarf shrubs may lead to soils that are drier, colder, and contain less organic carbon. Thus, there are multiple mechanisms through which woody plants may influence tundra soils.Peer reviewe

The current state of CO2 flux chamber studies in the Arctic tundra : A review

The Arctic tundra plays an important role in the carbon cycle as it stores 50% of global soil organic carbon reservoirs. The processes (fluxes) regulating these stocks are predicted to change due to direct and indirect effects of climate change. Understanding the current and future carbon balance calls for a summary of the level of knowledge regarding chamber-derived carbon dioxide (CO2) flux studies. Here, we describe progress from recently (2000-2016) published studies of growing-season CO2 flux chamber measurements, namely GPP (gross primary production), ER (ecosystem respiration), and NEE (net ecosystem exchange), in the tundra region. We review the study areas and designs along with the explanatory environmental drivers used. Most of the studies were conducted in Alaska and Fennoscandia, and we stress the need for measuring fluxes in other tundra regions, particularly in more extreme climatic, productivity, and soil conditions. Soil respiration and other greenhouse gas measurements were seldom included in the studies. Although most of the environmental drivers of CO2 fluxes have been relatively well investigated (such as the effect of vegetation type and soil microclimate on fluxes), soil nutrients, other greenhouse gases and disturbance regimes require more research as they might define the future carbon balance. Particular attention should be paid to the effects of shrubification, geomorphology, and other disturbance effects such as fire events, and disease and herbivore outbreaks. An improved conceptual framework and understanding of underlying processes of biosphere-atmosphere CO2 exchange will provide more information on carbon cycling in the tundra.Peer reviewe

In-depth characterization of denitrifier communities across different soil ecosystems in the tundra

Background In contrast to earlier assumptions, there is now mounting evidence for the role of tundra soils as important sources of the greenhouse gas nitrous oxide (N2O). However, the microorganisms involved in the cycling of N2O in this system remain largely uncharacterized. Since tundra soils are variable sources and sinks of N2O, we aimed at investigating differences in community structure across different soil ecosystems in the tundra. Results We analysed 1.4 Tb of metagenomic data from soils in northern Finland covering a range of ecosystems from dry upland soils to water-logged fens and obtained 796 manually binned and curated metagenome-assembled genomes (MAGs). We then searched for MAGs harbouring genes involved in denitrification, an important process driving N2O emissions. Communities of potential denitrifiers were dominated by microorganisms with truncated denitrification pathways (i.e., lacking one or more denitrification genes) and differed across soil ecosystems. Upland soils showed a strong N2O sink potential and were dominated by members of the Alphaproteobacteria such as Bradyrhizobium and Reyranella. Fens, which had in general net-zero N2O fluxes, had a high abundance of poorly characterized taxa affiliated with the Chloroflexota lineage Ellin6529 and the Acidobacteriota subdivision Gp23. Conclusions By coupling an in-depth characterization of microbial communities with in situ measurements of N2O fluxes, our results suggest that the observed spatial patterns of N2O fluxes in the tundra are related to differences in the composition of denitrifier communities.Peer reviewe

Snow is an important control of plant community functional composition in oroarctic tundra

The functional composition of plant communities is a critical modulator of climate change impacts on ecosystems, but it is not a simple function of regional climate. In the Arctic tundra, where climate change is proceeding the most rapidly, communities have not shifted their trait composition as predicted by spatial temperature-trait relationships. Important causal pathways are thus missing from models of trait composition change. Here, we study causes of plant community functional variation in an oroarctic tundra landscape in Kilpisjarvi, Finland. We consider the community-weighted means of plant vegetative height, as well as two traits related to the leaf economic spectrum. Specifically, we model their responses to locally measured summer air temperature, snow conditions, and soil resource levels. For each of the traits, we also quantify the importance of intraspecific trait variation (ITV) for between-community functional differences and trait-environment matching. Our study shows that in a tundra landscape (1) snow is the most influential abiotic variable affecting functional composition, (2) vegetation height is under weak local environmental control, whereas leaf economics is under strong local environmental control, (3) the relative magnitude of ITV differs between traits, and (4) ITV is not very consequential for community-level trait-environment relationships. Our analyses highlight the importance of winter conditions for community functional composition in seasonal areas. We show that winter climate change can either amplify or counter the effects summer warming, depending on the trait.Peer reviewe

Effects of Climate and Atmospheric Nitrogen Deposition on Early to Mid-Term Stage Litter Decomposition Across Biomes

Litter decomposition is a key process for carbon and nutrient cycling in terrestrial ecosystems and is mainly controlled by environmental conditions, substrate quantity and quality as well as microbial community abundance and composition. In particular, the effects of climate and atmospheric nitrogen (N) deposition on litter decomposition and its temporal dynamics are of significant importance, since their effects might change over the course of the decomposition process. Within the TeaComposition initiative, we incubated Green and Rooibos teas at 524 sites across nine biomes. We assessed how macroclimate and atmospheric inorganic N deposition under current and predicted scenarios (RCP 2.6, RCP 8.5) might affect litter mass loss measured after 3 and 12 months. Our study shows that the early to mid-term mass loss at the global scale was affected predominantly by litter quality (explaining 73% and 62% of the total variance after 3 and 12 months, respectively) followed by climate and N deposition. The effects of climate were not litter-specific and became increasingly significant as decomposition progressed, with MAP explaining 2% and MAT 4% of the variation after 12 months of incubation. The effect of N deposition was litter-specific, and significant only for 12-month decomposition of Rooibos tea at the global scale. However, in the temperate biome where atmospheric N deposition rates are relatively high, the 12-month mass loss of Green and Rooibos teas decreased significantly with increasing N deposition, explaining 9.5% and 1.1% of the variance, respectively. The expected changes in macroclimate and N deposition at the global scale by the end of this century are estimated to increase the 12-month mass loss of easily decomposable litter by 1.1-3.5% and of the more stable substrates by 3.8-10.6%, relative to current mass loss. In contrast, expected changes in atmospheric N deposition will decrease the mid-term mass loss of high-quality litter by 1.4-2.2% and that of low-quality litter by 0.9-1.5% in the temperate biome. Our results suggest that projected increases in N deposition may have the capacity to dampen the climate-driven increases in litter decomposition depending on the biome and decomposition stage of substrate.Peer reviewe

Arctic soil methane sink increases with drier conditions and higher ecosystem respiration

Arctic wetlands are known methane (CH4) emitters but recent studies suggest that the Arctic CH4 sink strength may be underestimated. Here we explore the capacity of well-drained Arctic soils to consume atmospheric CH4 using >40,000 hourly flux observations and spatially distributed flux measurements from 4 sites and 14 surface types. While consumption of atmospheric CH4 occurred at all sites at rates of 0.092 ± 0.011 mgCH4 m−2 h−1 (mean ± s.e.), CH4 uptake displayed distinct diel and seasonal patterns reflecting ecosystem respiration. Combining in situ flux data with laboratory investigations and a machine learning approach, we find biotic drivers to be highly important. Soil moisture outweighed temperature as an abiotic control and higher CH4 uptake was linked to increased availability of labile carbon. Our findings imply that soil drying and enhanced nutrient supply will promote CH4 uptake by Arctic soils, providing a negative feedback to global climate change

Towards constraining the circumpolar nitrous oxide budget

Arctic soils and sediments are well known for their huge carbon stocks and the significant positive feedback carbon dioxide (CO2) and methane (CH4) emissions can have on climate change. However, the vast amounts of nitrogen (N) and possible emissions of the strong greenhouse gas nitrous oxide (N2O) from Arctic soils are much less considered in this context. Arctic soils have been neglected in global N2O accounting, since their N2O emissions were traditionally thought to be low due to the general N-limitation of biological processes. Recent results suggest, however, that this assumption is unwarranted and needs to be revised. Still, although we know about the risk for increasing N2O emissions from the Arctic with warming, data are available only from a handful of sites and we are lacking any estimate on the circumarctic N2O budget even under the present climate. This presentation will introduce our plan to produce the first circumarctic N2O budget, an important baseline scenario against which changes in circumarctic N2O emissions can be observed with ongoing warming and global change. In order to estimate the first circumarctic N2O budget, we synthesize existing data and organize large-scale surveys of N2O fluxes across the Circumarctic. In our synthesis effort, we collect published and unpublished data on N2O emissions and N2O soil gas concentrations and analyze the data for driving variables and mechanisms underlying the N2O fluxes from various sites with different soil and vegetation characteristics. In addition, we organize measurement campaigns (via the INTERACT remote access program) to quantify N2O fluxes across a wide variety of Arctic sites using a network of collaborator stations with simple, standardized methods, and combine this N2O screening with GIS approaches to scale up the N2O fluxes step-wise from plot to regional and circumarctic levels. Ultimately, these data will be combined with existing data-sets and archived in a database that will be made available for process modelers in order to develop and improve the performance N2O models for permafrost soils. N2O flux data were published in 21 articles from 16 Arctic sites. In the frame of this project, N2O flux measurements were conducted in 2018 at 18 study sites located in Russia, Scandinavia, Svalbard, Canada and Alaska. First analyses show that N2O is released from a range of environmentally distinct sites and at variable magnitudes with soil N content, soil C/N ratios, vegetation cover, water availability, and nutrient content likely playing significant roles. Ultimately, this project will not only provide a valuable input towards the first estimate of the circumarctic N2O budget but also towards understanding the controls of Arctic N2O fluxes which is necessary for future projections. There is urgent need for collaboration among partners in this effort and we would thus like to invite interested researchers to contribute with further published or unpublished data on N2O fluxes/concentrations from Arctic sites to support our synthesis effort. Scientists are also highly requested to sample additional N2O data from “their” Arctic sites with the simple methods introduced here, in order to help us filling large data gaps