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Climate change affects ecosystems considerably worldwide, but as warming is happening at an accelerated pace at higher latitudes, it is essential to study how warming affects ecosystem structure and function in Arctic and sub-Arctic ecosystems.In this research, we looked at changes in the aboveground biomass (AGB) of two Icelandic sub-Arctic grasslands located at the ForHot site. The ForHot site is an exceptional studying site where the soils are naturally warmed. Thus, making it an important natural laboratory to assess and learn more about the long-term effect of global warming. At the research site, one grassland ecosystem has soils that have been warmed for over 60 years (long-term warming; LTW) and the other since 2008 (medium-term warming; MTW), when an earthquake disrupted geothermal channels in the underlying bedrock. Fifty permanent survey plots were established in the autumn of 2012 along the two grassland soil temperature gradients (ranging from 0 to +18°C).We assessed how vegetation structure (non-vascular; AGBnon-vasc and vascular plants; AGBvasc) and the ecosystem's maximum aboveground total biomass(AGBtot) were affected by different levels of soil warming over multiple studied years (i.e. 2013, 2016, 2018, 2020, 2021 and 2022).Our preliminary results showed unexpectedly relatively small changes in AGBtot with warming. We hypothesise that changes in AGBvasc would typically induce opposite changes in AGBnon-vasc, probably because of light competition. When looking separately at the vegetation structures, for AGBvasc, the duration of soil warming induced contrasted responses between MTW and LTW grasslands. That is, in the MTW grassland, there were no changes for most years (p > .05) and strong negative responses (p < .05) with warming in seasonally maximum AGBvasc for other years. Whereas in the LTW grassland, warming generally increased AGB for most years (p < .05), and also a strong negative response as observed in the MTW for the respective years despite statistically not significant. This strong negative response could be because of untypically high AGB production in control (unwarmed) plots during those years and less positive reactions with different levels of soil warming. We will show some potential drivers (environmental variables) for those unexpected temporal variations in the warming response. AGBnon-vasc, such as lichens and mosses, have an unclear pattern across the soil warming gradient in both grassland ecosystems.

The terrestrial soil organic matter (SOM) pool size depends on the balance between SOM formation and stabilization of decomposing plant litter relative to mineralization as CO2. Decomposition and mineralization processes are to large extents mediated by microbial decomposer communities. In addition, labile fractions released by decomposing litter can be stabilized in mineral associations (MAOM). High latitude ecosystems are particularly affected by global warming. Increasing temperatures can stimulate litter decomposition and increase nutrient mineralization, thereby increasing nutrient (e.g. nitrogen) availability for plants allowing higher productivity and subsequent plant organic matter inputs. On the other hand, if warming accelerates SOM decomposition stronger than formation processes it can cause large carbon and nutrient losses.Tracing 13C/15N labelled above-ground litter we aimed to disentangle if soil warming changes the balance between litter derived SOM formation through microbial communities, over particulate organic matter (POM) and MAOM stabilization across a decadal geothermal soil warming gradient (from ambient up to +5 °C) in a grassland in Iceland. In addition, we added three levels of inorganic N to disentangle potential direct warming from indirect (over plant feedbacks) effects of increased warming on SOM dynamics. We found that over the course of two years warming accelerated litter decomposition rates and litter-derived carbon turnover by the microbial community, and we could recover more litter-derived carbon recovered in particulate organic matter (POM) nor MAOM. Nitrogen additions triggered a faster decomposition of structural above-ground litter compounds, but did not influence carbon turnover in different soil fractions. On the other, hand we found that overall the absolute amount of soil carbon decreased in response to warming. We therefore conclude that direct warming not only increased SOM formation, but also decomposition and mineralization processes, and – at least in the studied grassland – plant inputs may not have counterbalanced warming induced losses.

Long-term soil warming and nitrogen (N) availability have been shown to affect microbial biomass and community composition. Altered assimilation patterns of recent plant-derived C and changes in soil C stocks following warming as well as increased N availability are critical in mediating the direction and magnitude of these community shifts. A 13C pulse labelling experiment was done on a warming gradient in an Icelandic grassland (Sigurdsson et al. 2016), to investigate the role of recent plant-derived C and warming on the microbial community structure and size. We observed an overall increase of microbial 13C (e.g., root-exudate) uptake, while warming led to significant microbial biomass loss in all microbial groups. The increase of microbial 13C uptake with warming differed between microbial groups: an increase was only observed in the general and Gram-positive bacterial phospholipid fatty acid (PLFA) markers and in the PLFA and neutral lipid fatty acid (NLFA) markers of arbuscular mycorrhizal fungi (AMF). Nitrogen addition of 50 kg ha-1 y-1 for two years had no effect on the microbial uptake, microbial biomass or community composition, indicating that microbes were not N limited, and no plant-mediated N addition effects occurred. Additionally, we show that both warming and soil C depletion were responsible for the microbial biomass loss. Soil warming caused stronger loss in microbial groups with higher 13C uptake. In our experiment, warming caused a general reduction of microbial biomass, despite a relative increase in microbial 13C uptake, and altered microbial community composition. The warming effects on microbial biomass and community composition were partly mediated through soil C depletion with warming and changes in recent plant-derived C uptake patterns of the microbial community.

Global warming may lead to carbon transfers from soils to the atmosphere, yet this positive feedback to the climate system remains highly uncertain, especially in subsoils (Ilyina and Friedlingstein, 2016; Shi et al., 2018). Using natural geothermal soil warming gradients of up to +6.4 ∘C in subarctic grasslands (Sigurdsson et al., 2016), we show that soil organic carbon (SOC) stocks decline strongly and linearly with warming (−2.8 t ha−1 ∘C−1). Comparison of SOC stock changes following medium-term (5 and 10 years) and long-term (>50 years) warming revealed that all SOC stock reduction occurred within the first 5 years of warming, after which continued warming no longer reduced SOC stocks. This rapid equilibration of SOC observed in Andosol suggests a critical role for ecosystem adaptations to warming and could imply short-lived soil carbon–climate feedbacks. Our data further revealed that the soil C loss occurred in all aggregate size fractions and that SOC stock reduction was only visible in topsoil (0–10 cm). SOC stocks in subsoil (10–30 cm), where plant roots were absent, showed apparent conservation after >50 years of warming. The observed depth-dependent warming responses indicate that explicit vertical resolution is a prerequisite for global models to accurately project future SOC stocks for this soil type and should be investigated for soils with other mineralogies.

Priming is the change of microbial soil organic matter (SOM) decomposition induced by a labile carbon (C) source. It is recognised as an important mechanism influencing soil C dynamics and C storage in terrestrial ecosystems. Microbial nitrogen (N) mining in SOM and preferential substrate utilisation, i.e., a shift in microbial carbon use from SOM to more labile energy sources, are possible, counteracting, mechanisms driving the priming effect. Climate warming and increased N availability might affect these mechanisms, and thus determine the direction and magnitude of the priming effect. Hence, these abiotic factors can indirectly affect soil C stocks, which makes their understanding crucial for predicting the soil C feedback in a warming world. We conducted a short-term incubation experiment (6 days) with soils from a subarctic grassland that had been subjected to long-term geothermal warming (55 years) by 2-4°C above unwarmed soil. Soil samples were amended with 13C-labelled glucose and 15N-labelled NH4NO3. We found a significantly negative relationship between in situ warming and cumulative primed C, with negative priming in the warmed soils. The negative priming suggests that preferential substrate utilisation was a key mechanism in our experiment. Our results indicate that changes in SOM characteristics associated with the in situ warming gradient can play a major role in determining the rate and direction of the priming effect. Additionally, we found that neither microbial N limitation nor N addition affected the priming effect, providing evidence that in our experiment, N mining did not lead to positive priming.

Soils store more carbon than the atmosphere and total land plant biomass combined. Soil organic matter (SOM) can be classified into different physical pools characterized by their degree of protection and turnover rates. Usually, these pools are isolated by dividing soils in different water-stable aggregate size classes and, inside these classes, SOM fractions with differing densities and properties: Stable mineral-associated organic matter (MOM) and labile particulate organic matter (POM). Increasing temperatures are known to initially enhance microbial decomposition rates, releasing C from soils which could further accelerate climate change. The magnitude of this feedback depends on which C pool is affected the most by increased decomposition. Since MOM, thought to be the best protected carbon pool, holds most of the soil C, losses from this pool would potentially have the biggest impact on global climate. Experimental results are inconclusive so far, as most studies are based on short-term field warming (years rather than decades), leaving the ecosystem response to decades to century of warming uncertain.We made use of a geothermal warming platform in Iceland (ForHot; https://forhot.is/) to compare the effect of short-term (STW, 5-8 years) and long-term (LTW, more than 50 years) warming on soil organic carbon and nitrogen (SOC, SON) and its carbon and nitrogen isotope composition (δ13C and δ15N) in soil aggregates of different sizes in a subarctic grassland. OM fractions were isolated via density fractionation and ultrasonication both in macro- and microaggregates: Inter-aggregate free POM (fPOM), POM occluded within aggregates (iPOM) and MOM.MOM, containing most of the SOC and SON, showed a similar response to warming for both macro- and microaggregates. Compared to LTW plots, STW plots overall had higher C and N stocks. But warming reduced the carbon content more strongly in STW plot than in LTW plots. δ13C of MOM soil increased with temperature on the STW sites, indicating higher overall SOM turnover rates at higher temperatures, in line with the higher SOC losses. For LTW, δ13C decreased with warming except for the most extreme treatment (+16°C). Warming duration had no impact on iPOM-C. fPOM-C decreased in STW sites with increasing temperature, while it increased on the LTW sites.Overall our results demonstrate warming-induced C losses from the MOM-C-pool, thought to be most stable soil carbon pool. Thus, warming stimulated microbes to decompose both labile fPOM and more stable MOM. After decades of warming, C losses are less pronounced compared to the short-term warmed plots, pointing to a replenishment of the carbon pools at higher temperatures in the long-term. This might be explained by adaptations of the primary productivity and/or substrate-limitation of microbial growth.

Climate warming has been suggested to impact high latitude grasslands severely, causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts soil C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. We hypothesized that warming would increase belowground C allocation, while enhanced N availability would decrease it, and that their interactive effects would be additive.We studied a subarctic grassland located at a natural geothermal soil warming gradient close to Hveragerði, Iceland, which was established by an earthquake in 2008. We chose 14 plots along the gradient with soil warming temperatures ranging from 0 to 10°C above ambient, and fertilized a subset of plots with 50kg ha-1 y-1 of NH4NO3 twice a year prior to the study. We performed 13CO2 canopy pulse labeling for an hour and tracked the 13C pulse through the plant-microbe-soil system and into soil respiration for ten days after labeling.Our preliminary results show that at higher temperatures microbial activity increased, causing higher C turnover and a higher respiration of recently assimilated C from the soil. Warming significantly decreased microbial biomass, however, the recent C allocated from roots to microbes increased. This indicates a higher microbial C-limitation and a tighter root-microbe coupling under warming. Nitrogen addition increased the allocation of recent C to roots, microbial biomass, and soil respiration. The effects of N addition and warming were additive with no interaction. Our results indicate that the microbes in warmed soil may not be N limited, but could be C limited and depend more on the supply of recent C from plants. We conclude that in a future climate with warmer soils, more C may be allocated belowground, however, its residence time may decrease.

The widespread use of perfluoroalkylated acids (PFAAs) has led to a global presence in the environment, in which they accumulate and may cause detrimental effects. Although soils are known sinks for many persistent organic pollutants, still little is known on the behaviour of PFAAs in soils. Furthermore, studies that examine the relationships between PFAA concentrations and soil microbial parameters are scarce. The 3 M fluorochemical plant near Antwerp has been characterized as a PFAAs hotspot. In the present study, we examined the vertical distribution of 15 PFAAs and their associations with multiple physicochemical soil properties along a distance gradient from this hotspot. Additionally, we tested the relationships between PFAA concentrations in the top soil with soil respiration, microbial activity and microbial biomass. Our results show that both perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) concentrations were elevated in the subsurface layer (up to 50 cm), after which concentrations decreased again, suggesting a downward migration of both analytes in the soil. This downward movement might pose a potential threat for the contamination of the groundwater and, consequently, organisms that rely on this water for consumption. The soil concentrations were influenced by multiple physicochemical properties of the soil, which suggests differences in bioavailability and sorption/desorption capacities between different soil types. We did not observe any influence of PFAA contamination in the top soil on microbial activity and biomass nor soil respiration.