Subarctic Grassland Research Articles

Soil warming increases the number of growing bacterial taxa but not their growth rates.

Soil microorganisms control the fate of soil organic carbon. Warming may accelerate their activities putting large carbon stocks at risk of decomposition. Existing knowledge about microbial responses to warming is based on community-level measurements, leaving the underlying mechanisms unexplored and hindering predictions. In a long-term soil warming experiment in a Subarctic grassland, we investigated how active populations of bacteria and archaea responded to elevated soil temperatures (+6°C) and the influence of plant roots, by measuring taxon-specific growth rates using quantitative stable isotope probing and 18O water vapor equilibration. Contrary to prior assumptions, increased community growth was associated with a greater number of active bacterial taxa rather than generally faster-growing populations. We also found that root presence enhanced bacterial growth at ambient temperatures but not at elevated temperatures, indicating a shift in plant-microbe interactions. Our results, thus, reveal a mechanism of how soil bacteria respond to warming that cannot be inferred from community-level measurements.

Microorganisms in subarctic soils are depleted of ribosomes under short-, medium-, and long-term warming.

Physiological responses of soil microorganisms to global warming are important for soil ecosystem function and the terrestrial carbon cycle. Here, we investigate the effects of weeks, years, and decades of soil warming across seasons and time on the microbial protein biosynthesis machineries (i.e. ribosomes), the most abundant cellular macromolecular complexes, using RNA:DNA and RNA:MBC (microbial biomass carbon) ratios as proxies for cellular ribosome contents. We compared warmed soils and non-warmed controls of 15 replicated subarctic grassland and forest soil temperature gradients subject to natural geothermal warming. RNA:DNA ratios tended to be lower in the warmed soils during summer and autumn, independent of warming duration (6weeks, 8-14years, and > 50years), warming intensity (+3°C, +6°C, and +9°C), and ecosystem type. With increasing temperatures, RNA:MBC ratios were also decreasing. Additionally, seasonal RNA:DNA ratios of the consecutively sampled forest showed the same temperature-driven pattern. This suggests that subarctic soil microorganisms are depleted of ribosomes under warm conditions and the lack of consistent relationships with other physicochemical parameters besides temperature further suggests temperature as key driver. Furthermore, in incubation experiments, we measured significantly higher CO2 emission rates per unit of RNA from short- and long-term warmed soils compared to non-warmed controls. In conclusion, ribosome reduction may represent a widespread microbial physiological response to warming that offers a selective advantage at higher temperatures, as energy and matter can be reallocated from ribosome synthesis to other processes including substrate uptake and turnover. This way, ribosome reduction could have a substantial effect on soil carbon dynamics.

Responses of soil hexapod communities to increasing nitrogen in a subarctic grassland

The warming of boreal ecosystems accelerates decomposition and increases nitrogen (N) availability. The impact of increased N on subarctic soil fauna communities, however, remains poorly understood. We investigated the response of soil hexapods to a N addition experiment in a subarctic grassland. We characterized the soil hexapod communities using environmental DNA metabarcoding and analyzed the levels of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), microbial carbon (Cmic), and microbial nitrogen (Nmic). N addition increased DON and Nmic, while DOC and Cmic pools remained unchanged. Furthermore, N addition caused shifts in soil hexapod community compositional diversity between control and N plots in herbivore and microbivore taxa. The levels of DON and Nmic strongly correlated with these shifts, explaining 54% and 45% of the compositional variability, respectively. This study demonstrates a clear link between N availability and shifts in soil hexapod communities, associated to changes in microbial and dissolved N pools in subarctic grasslands.

Decadal soil warming decreased vascular plant above and belowground production in a subarctic grassland by inducing nitrogen limitation.

Below and aboveground vegetation dynamics are crucial in understanding how climate warming may affect terrestrial ecosystem carbon cycling. In contrast to aboveground biomass, the response of belowground biomass to long-term warming has been poorly studied. Here, we characterized the impacts of decadal geothermal warming at two levels (on average +3.3°C and +7.9°C) on below and aboveground plant biomass stocks and production in a subarctic grassland. Soil warming did not change standing root biomass and even decreased fine root production and reduced aboveground biomass and production. Decadal soil warming also did not significantly alter the root-shoot ratio. The linear stepwise regression model suggested that following 10 yr of soil warming, temperature was no longer the direct driver of these responses, but losses of soil N were. Soil N losses, due to warming-induced decreases in organic matter and water retention capacity, were identified as key driver of the decreased above and belowground production. The reduction in fine root production was accompanied by thinner roots with increased specific root area. These results indicate that after a decade of soil warming, plant productivity in the studied subarctic grassland was affected by soil warming mainly by the reduction in soil N.

Soil warming duration and magnitude affect the dynamics of fine roots and rhizomes and associated C and N pools in subarctic grasslands.

The response of subarctic grassland's below-ground to soil warming is key to understanding this ecosystem's adaptation to future climate. Functionally different below-ground plant organs can respond differently to changes in soil temperature (Ts). We aimed to understand the below-ground adaptation mechanisms by analysing the dynamics and chemistry of fine roots and rhizomes in relation to plant community composition and soil chemistry, along with the duration and magnitude of soil warming. We investigated the effects of the duration [medium-term warming (MTW; 11 years) and long-term warming (LTW; > 60 years)] and magnitude (0-8.4 °C) of soil warming on below-ground plant biomass (BPB), fine root biomass (FRB) and rhizome biomass (RHB) in geothermally warmed subarctic grasslands. We evaluated the changes in BPB, FRB and RHB and the corresponding carbon (C) and nitrogen (N) pools in the context of ambient, Ts < +2 °C and Ts > +2 °C scenarios. BPB decreased exponentially in response to an increase in Ts under MTW, whereas FRB declined under both MTW and LTW. The proportion of rhizomes increased and the C-N ratio in rhizomes decreased under LTW. The C and N pools in BPB in highly warmed plots under MTW were 50 % less than in the ambient plots, whereas under LTW, C and N pools in warmed plots were similar to those in non-warmed plots. Approximately 78 % of the variation in FRB, RHB, and C and N concentration and pools in fine roots and rhizomes was explained by the duration and magnitude of soil warming, soil chemistry, plant community functional composition, and above-ground biomass. Plant's below-ground biomass, chemistry and pools were related to a shift in the grassland's plant community composition - the abundance of ferns increased and BPB decreased towards higher Ts under MTW, while the recovery of below-ground C and N pools under LTW was related to a higher plant diversity. Our results indicate that plant community-level adaptation of below ground to soil warming occurs over long periods. We provide insight into the potential adaptation phases of subarctic grasslands.

Individual and interactive effects of warming and nitrogen supply on CO2 fluxes and carbon allocation in subarctic grassland.

Climate warming has been suggested to impact high latitude grasslands severely, potentially 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 belowground 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. On a 10-year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13 CO2 pulse-labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C-N interactions in the C dynamics of subarctic ecosystems in a warmer world.

Responses of soil hexapod communities to warming are mediated by microbial carbon and nitrogen in a subarctic grassland

Warming in subarctic ecosystems will be two-fold higher compared to lower latitudes under current climate change projections. While the effects of warming in northern ecosystems on plants and microorganisms have been extensively studied, the responses of soil fauna have received much less attention, despite their important role in regulating key soil processes. We analyzed the response of soil hexapod communities in a subarctic grassland exposed to a natural geothermal gradient in Iceland with increases of +3 and + 6 °C above ambient temperature. We characterized hexapod communities using environmental DNA (eDNA) metabarcoding. We analyzed the amounts of microbial carbon (Cmic), microbial N (Nmic), dissolved organic C (DOC) and dissolved organic N (DON) and then assessed whether these variables could help to account for the compositional dissimilarity of ground hexapod communities across temperatures. The increases in soil temperature did lead to changes in the composition of hexapod communities. The compositional differences caused by +6 °C plots were correlated with a decrease in Cmic and Nmic, soil DOC and DON. Our results highlight the response of soil hexapods to warming, and their interaction with microbial biomass ultimately correlated with changes in the availabilities of soil C and N.

Warming shifts the biomass distribution of soil microarthropod communities

Climate warming is often more detrimental to large body sized organisms than small body sized organisms. Yet, how such differential effects of warming at organismal levels affect aggregate community properties, such as community biomass, remains little understood. Here, using geothermally warmed sub-Arctic grassland soils, we investigate how total biomass (product of density and individual body mass) of two major groups of soil microarthropods (Collembola and mites), which are composed of both large and small body sized species, shift in warmed soils when warmed by ∼3–∼6 °C. Our results show that total biomass of Collembola significantly decreased in warmed soils predominantly due to a decline in the density of large body sized species. In contrast, total mite biomass showed a unimodal response to warming. As a result, there was a shift towards mite biomass dominated microarthropod communities in warmed soils. Within Collembola, the deep soil living eu-edaphic functional group declined the most in total biomass, whereas the unimodal response in mites was most pronounced in oribatid mites. Our study highlights that warming induced shifts in total community biomass of soil microarthropods are likely due to greater detrimental effects of warming on several large body sized Collembola.

Soil carbon loss in warmed subarctic grasslands is rapid and restricted to topsoil

Abstract. 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 (&gt;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 &gt;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.

Down-regulation of the bacterial protein biosynthesis machinery in response to weeks, years, and decades of soil warming.

How soil microorganisms respond to global warming is key to infer future soil-climate feedbacks, yet poorly understood. Here, we applied metatranscriptomics to investigate microbial physiological responses to medium-term (8 years) and long-term (>50 years) subarctic grassland soil warming of +6°C. Besides indications for a community-wide up-regulation of centralmetabolic pathways and cell replication, we observed a down-regulation of the bacterial protein biosynthesis machinery in the warmed soils, coinciding with a lower microbial biomass, RNA, and soil substrate content. We conclude that permanently accelerated reaction rates at higher temperatures and reduced substrate concentrations result in cellular reduction of ribosomes, the macromolecular complexes carrying out protein biosynthesis. Later efforts to test this, including a short-term warming experiment (6 weeks, +6°C), further supported our conclusion. Down-regulating the protein biosynthesis machinery liberates energy and matter, allowing soil bacteria to maintain high metabolic activities and cell division rates even after decades of warming.

Long-term warming reduced microbial biomass but increased recent plant-derived C in microbes of a subarctic grassland

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.

Responses of Soil Insect Communities to Warming are Mediated by Shifts in Microbial Carbon and Nitrogen in a Subarctic Grassland

Responses of Soil Insect Communities to Warming are Mediated by Shifts in Microbial Carbon and Nitrogen in a Subarctic Grassland

Negative priming of soil organic matter following long-term in situ warming of sub-arctic soils

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.

Vegetation structure modulates ecosystem and community responses to spatial subsidies

AbstractEcosystem responses to external inputs of nutrients and organisms are highly variable. Theory predicts that ecosystem traits will determine the responses to spatial subsidies, but evidence for how vegetation structure can modulate those effects is lacking. We investigated how vegetation structure (i.e., leaf area index [LAI] and vegetation height) influenced the ecosystem and community responses to insect spatial subsidies in a subarctic grassland. Our experiment consisted of a 2 × 2 manipulation where in one treatment we either blocked flying insects over a 2‐yr period in 1‐m2 plots near the shore of Lake Mývatn, Iceland, where deposition of aquatic adult midges (Diptera: Chironomidae) to land is high, or left control plots accessible to flying midges. In the second treatment, grassland vegetation was cut (tall vs. short) at the start of each season and then allowed to regrow. We then measured litter decomposition and arthropod composition and density within each plot (n = 6 replicates × 4 treatments). Midge‐exclusion cages reduced midge deposition by 81% relative to the open plots. Vegetation cutting initially reduced LAI and vegetation height by 3× and 1.5×, respectively, but these were not different by the end of the second‐growing season. We found that vegetation structure modulated the effects of midge subsides on litter decomposition, with taller canopies intercepting more insect subsidies than shorter ones, leading to 18% faster litter decomposition. In contrast, the short‐vegetation plots intercepted fewer subsidies and had higher temperatures and sunlight, resulting in no effects of midges on decomposition. However, by the end of the experiment when all vegetation structure characteristics had converged across all plots, we found no differences in decomposition between treatments. The effects of midge subsidies on arthropod composition depended on the vegetation structure, suggesting that arthropods might also be responding to the structural effects on spatial subsidies. Our findings indicate that vegetation structure can modify the abiotic environment and the quantity of subsidies entering a recipient ecosystem as aerial insects, resulting in ecosystem‐ and community‐level responses. Thus, changing vegetation structure via habitat disturbances will likely have important implications for ecosystem functions that rely on spatial subsidies.

Taxonomic and Functional Diversity of Heterotrophic Protists (Cercozoa and Endomyxa) from Biological Soil Crusts.

Biological soil crusts (biocrusts) accommodate diverse communities of phototrophic and heterotrophic microorganisms. Heterotrophic protists have critical roles in the microbial food webs of soils, with Cercozoa and Endomyxa often being dominant groups. Still, the diversity, community composition, and functions of Cercozoa and Endomyxa in biocrusts have been little explored. In this study, using a high-throughput sequencing method with taxon-specific barcoded primers, we studied cercozoan and endomyxan communities in biocrusts from two unique habitats (subarctic grassland and temperate dunes). The communities differed strongly, with the grassland and dunes being dominated by Sarcomonadea (69%) and Thecofilosea (43%), respectively. Endomyxa and Phytomyxea were the minor components in dunes. Sandonidae, Allapsidae, and Rhogostomidae were the most abundant taxa in both habitats. In terms of functionality, up to 69% of the grassland community was constituted by bacterivorous Cercozoa. In contrast, cercozoan and endomyxan communities in dunes consisted of 31% bacterivores, 25% omnivores, and 20% eukaryvores. Facultative and obligate eukaryvores mostly belonged to the families Rhogostomidae, Fiscullidae, Euglyphidae, Leptophryidae, and Cercomonadidae, most of which are known to feed mainly on algae. Biocrust edaphic parameters such as pH, total organic carbon, nitrogen, and phosphorus did not have any significant influence on shaping cercozoan communities within each habitat, which confirms previous results from dunes.

Long-Term Warming Reduced Microbial Biomass But Increased Recent Plant-Derived C in Microbes of a Subarctic Grassland

Long-Term Warming Reduced Microbial Biomass But Increased Recent Plant-Derived C in Microbes of a Subarctic Grassland

The influence of soil warming on organic carbon sequestration of arbuscular mycorrhizal fungi in a sub-arctic grassland

A substantial portion of grassland photosynthates is allocated belowground to arbuscular mycorrhizal fungi (AMF), but controversy remains about whether this carbon (C) contributes to soil organic carbon (SOC) under warming. The goal of this study was to investigate how AMF biomass and C sequestered by AMF (CNew) are influenced by soil warming. We estimated the AMF biomass and CNew, assumed to be mostly AMF necromass, in mycelial ingrowth bags buried for 1, 2, or 3 years in soil under warming (~+0.5–16.4 °C). The AMF biomass had a positive, curvilinear response to warming gradients after one year of burial. About 107 g C m−2 of CNew accumulated over the three years and ~12% of this C was from glomalin-related soil protein. Modelling suggested the production rate of AMF biomass was 153 g C m−2 yr−1 with a rapid (36–75 days) turnover while AMF necromass turnover was much slower (1.4 ± 0.2 yr−1). Warming duration (7–9 years vs. > 50 years) did not have significant influence on AMF biomass or CNew (P > 0.05). Our results suggest that AMF are more tolerant to increases in temperature than other microbes or fine roots. The dramatic loss of soil C and stable soil aggregates under warming found earlier at this site were not attributed to a decrease in AMF biomass or CNew. Despite a low AMF standing biomass, its contribution to SOC may be substantial.

A systemic overreaction to years versus decades of warming in a subarctic grassland ecosystem.

SummaryTemperature governs most biotic processes, yet we know little about how warming affects whole ecosystems. Here we examined the responses of 128 components of a subarctic grassland to 5-8 or >50 years of soil warming. Warming of >50 years drove the ecosystem to a new steady state possessing a distinct biotic composition and reduced species richness, biomass and soil organic matter. However, the warmed state was preceded by an overreaction to warming, which was related to organisms’ physiologies and was evident after 5-8 years. Ignoring this overreaction yielded errors of more than 100% for 83 variables when predicting their responses to a realistic warming scenario of 1 ºC over 50 years, although some, including soil carbon content, remained stable after 5-8 years. This study challenges long-term ecosystem predictions made from short-term observations, and provides a framework for characterising ecosystem responses to sustained climate change.

Molecular cytogenetics of valuable Arctic and sub-Arctic pasture grass species from the Aveneae/Poeae tribe complex (Poaceae)

BackgroundGrasslands in the Arctic tundra undergo irreversible degradation due to climatic changes and also over-exploitation and depletion of scarce resources. Comprehensive investigations of cytogenomic structures of valuable Arctic and sub-Arctic grassland species is essential for clarifying their genetic peculiarities and phylogenetic relationships, and also successful developing new forage grass cultivars with high levels of adaptation, stable productivity and longevity. We performed molecular cytogenetic characterization of insufficiently studied pasture grass species (Poaceae) from related genera representing two neighboring clades: 1) Deschampsia and Holcus; 2) Alopecurus, Arctagrostis and Beckmannia, which are the primary fodder resources in the Arctic tundra.ResultsWe constructed the integrated schematic maps of distribution of these species in the northern, central and eastern parts of Eurasia based on the currently available data as only scattered data on their occurrence is currently available. The species karyotypes were examined with the use of DAPI-banding, multicolour FISH with 35S rDNA, 5S rDNA and the (GTT)9 microsatellite motif and also sequential rapid multocolour GISH with genomic DNAs of Deschampsia sukatschewii, Deschampsia flexuosa and Holcus lanatus belonging to one of the studied clades. Cytogenomic structures of the species were specified; peculiarities and common features of their genomes were revealed. Different chromosomal rearrangements were detected in Beckmannia syzigachne, Deschampsia cespitosa and D. flexuosa; B chromosomes with distinct DAPI-bands were observed in karyotypes of D. cespitosa and H. lanatus.ConclusionsThe peculiarities of distribution patterns of the examined chromosomal markers and also presence of common homologous DNA repeats in karyotypes of the studies species allowed us to verify their relationships. The obtained unique data on distribution areas and cytogenomic structures of the valuable Arctic and sub-Arctic pasture species are important for further genetic and biotechnological studies and also plant breeding progress.

Reducing allochthonous resources in a subarctic grassland alters arthropod food webs via predator diet and density

AbstractEmergent aquatic insects can be major conduits of resources moving from freshwater to adjacent terrestrial ecosystems. These allochthonous resources can influence the structure and function of adjacent ecosystems, yet their effect on arthropod consumer populations remains largely unknown. In this study, we investigated how flying adult midges influence terrestrial arthropod food webs in subarctic grasslands by blocking midge inputs over four years in 2 × 2‐m plots near the shore of Lake Mývatn, Iceland, where midge abundances are naturally high. We examined responses of terrestrial arthropods by measuring their densities, community composition, and stable isotopes (δ13C) in midge‐exclusion and open (control) plots. Cage treatments significantly reduced midge deposition into exclusion plots (99% reduction) relative to control plots but were designed to allow ground‐active arthropods to move freely among plots. Predator densities (e.g., spiders and harvestmen) were on average 30% lower in exclusion plots relative to control plots, while no other trophic guild (detritivores, omnivores, or herbivores) showed a response to midge exclusion. As a result, blocking midges shifted arthropod communities toward a composition dominated by springtails, mites, and aphids relative to large predators such as wolf spiders and harvestmen. All trophic guilds (detritivores, omnivores, and predators), except for herbivores, were more depleted in δ13C in midge‐exclusion plots, indicating an increased reliance on terrestrial (i.e., plant‐based) resources when midge inputs were blocked. Arthropod predators were the only guild that had both a depleted δ13C and a negative density response to midge exclusion. Apart from predators, these results indicate a weak association between resource type (aquatic vs. terrestrial) and density responses across most arthropod guilds in this grassland system. Overall, our findings suggest that aquatic insect subsidies have significant effects on terrestrial arthropod communities, but predators appear most responsive to insect allochthony in subarctic grasslands.