Mycorrhizal fungi modify decomposition: a meta-analysis.

It has been proposed that ectomycorrhizal fungi can reduce decomposition while arbuscular mycorrhizal fungi may enhance it. These phenomena are known as the 'Gadgil effect' and 'priming effect', respectively. However, it is unclear which one predominates globally. We evaluated whether mycorrhizal fungi decrease or increase decomposition, and identified conditions that mediate this effect. We obtained decomposition data from 43 studies (97 trials) conducted in field or laboratory settings that controlled the access of mycorrhizal fungi to substrates colonized by saprotrophs. Across studies, mycorrhizal fungi promoted decomposition of different substrates by 6.7% overall by favoring the priming effect over the Gadgil effect. However, we observed significant variation among studies. The substrate C : N ratio and absolute latitude influenced the effect of mycorrhizal fungi on decomposition and contributed to the variation. Specifically, mycorrhizal fungi increased decomposition at low substrate C : N and absolute latitude, but there was no discernable effect at high values. Unexpectedly, the effect of mycorrhizal fungi was not influenced by the mycorrhizal type. Our findings challenge previous assumptions about the universality of the Gadgil effect but highlight the potential of mycorrhizal fungi to negatively influence soil carbon storage by promoting the priming effect.

Los hongos ectomicorrízicos puden reducir la descomposición mientras que los hongos micorrízico­arbusculares pueden potenciarla. Ambos fenómenos son conocidos como "Gadgil effect" y "priming effect", respectivamente. Sin embargo, no es claro cuál predomina mundialmente. En este trabajo evaluamos si los hongos micorrízicos disminuyen o promueven la descomposición, e identificamos las condiciones que regulan este efecto. Para ello, recopilamos datos de descomposición de 43 estudios (97 observaciones) realizados en condiciones de campo o laboratorio que controlaron el acceso de los hongos micorrízicos a sustratos colonizados por saprótrofos. Los hongos micorrízicos promovieron la descomposición de diferentes sustratos en un 6.7%. Sin embargo, observamos una variación significativa entre estudios. La relación C : N del sustrato y la latitud influyeron en el efecto de los hongos micorrícicos sobre la descomposición y contribuyeron a la variabilidad. Específicamente, los hongos micorrízicos aumentaron la descomposición a valores bajos de C : N del sustrato y latitud, pero no hubo un efecto discernible en valores altos. Inesperadamente, el tipo de micorriza no influyó en el efecto de los hongos micorrízicos. Nuestros hallazgos cuestionan la universalidad del Gadgil effect, y resaltan el potencial de los hongos micorrízicos para influir negativamente en el almacenamiento de carbono del suelo al promover el priming effect.

Investigating the eco-evolutionary response of microbiomes to environmental change.

Microorganisms are the primary engines of biogeochemical processes and foundational to the provisioning of ecosystem services to human society. Free-living microbial communities (microbiomes) and their functioning are now known to be highly sensitive to environmental change. Given microorganisms' capacity for rapid evolution, evolutionary processes could play a role in this response. Currently, however, few models of biogeochemical processes explicitly consider how microbial evolution will affect biogeochemical responses to environmental change. Here, we propose a conceptual framework for explicitly integrating evolution into microbiome-functioning relationships. We consider how microbiomes respond simultaneously to environmental change via four interrelated processes that affect overall microbiome functioning (physiological acclimation, demography, dispersal and evolution). Recent evidence in both the laboratory and the field suggests that ecological and evolutionary dynamics occur simultaneously within microbiomes; however, the implications for biogeochemistry under environmental change will depend on the timescales over which these processes contribute to a microbiome's response. Over the long term, evolution may play an increasingly important role for microbially driven biogeochemical responses to environmental change, particularly to conditions without recent historical precedent.

Decomposition responses to climate depend on microbial community composition.

Bacteria and fungi drive decomposition, a fundamental process in the carbon cycle, yet the importance of microbial community composition for decomposition remains elusive. Here, we used an 18-month reciprocal transplant experiment along a climate gradient in Southern California to disentangle the effects of the microbial community versus the environment on decomposition. Specifically, we tested whether the decomposition response to climate change depends on the microbial community. We inoculated microbial decomposers from each site onto a common, irradiated leaf litter within "microbial cages" that prevent microbial exchange with the environment. We characterized fungal and bacterial composition and abundance over time and investigated the functional consequences through litter mass loss and chemistry. After 12 months, microbial communities altered both decomposition rate and litter chemistry. Further, the functional measurements depended on an interaction between the community and its climate in a manner not predicted by current theory. Moreover, microbial ecologists have traditionally considered fungi to be the primary agents of decomposition and for bacteria to play a minor role. Our results indicate that not only does climate change and transplantation have differential legacy effects among bacteria and fungi, but also that bacterial communities might be less functionally redundant than fungi with regards to decomposition. Thus, it may be time to reevaluate both the role of microbial community composition in its decomposition response to climate and the relative roles of bacterial and fungal communities in decomposition.

Embracing a new paradigm for temperature sensitivity of soil microbes.

The temperature sensitivity of soil processes is of major interest, especially in light of climate change. Originally formulated to explain the temperature dependence of chemical reactions, the Arrhenius equation, and related Q10 temperature coefficient, has a long history of application to soil biological processes. However, empirical data indicate that Q10 and Arrhenius model are often poor metrics of temperature sensitivity in soils. In this opinion piece, we aim to (a) review alternative approaches for characterizing temperature sensitivity, focusing on macromolecular rate theory (MMRT); (b) provide strategies and tools for implementing a new temperature sensitivity framework; (c) develop thermal adaptation hypotheses for the MMRT framework; and (d) explore new questions and opportunities stemming from this paradigm shift. Microbial ecologists should consider developing and adopting MMRT as the basis for predicting biological rates as a function of temperature. Improved understanding of temperature sensitivity in soils is particularly pertinent as microbial response to temperature has a large impact on global climate feedbacks.

Temperature sensitivities of extracellular enzyme Vmax and Km across thermal environments.

The magnitude and direction of carbon cycle feedbacks under climate warming remain uncertain due to insufficient knowledge about the temperature sensitivities of soil microbial processes. Enzymatic rates could increase at higher temperatures, but this response could change over time if soil microbes adapt to warming. We used the Arrhenius relationship, biochemical transition state theory, and thermal physiology theory to predict the responses of extracellular enzyme Vmax and Km to temperature. Based on these concepts, we hypothesized that Vmax and Km would correlate positively with each other and show positive temperature sensitivities. For enzymes from warmer environments, we expected to find lower Vmax , Km , and Km temperature sensitivity but higher Vmax temperature sensitivity. We tested these hypotheses with isolates of the filamentous fungus Neurospora discreta collected from around the globe and with decomposing leaf litter from a warming experiment in Alaskan boreal forest. For Neurospora extracellular enzymes, Vmax Q10 ranged from 1.48 to 2.25, and Km Q10 ranged from 0.71 to 2.80. In agreement with theory, Vmax and Km were positively correlated for some enzymes, and Vmax declined under experimental warming in Alaskan litter. However, the temperature sensitivities of Vmax and Km did not vary as expected with warming. We also found no relationship between temperature sensitivity of Vmax or Km and mean annual temperature of the isolation site for Neurospora strains. Declining Vmax in the Alaskan warming treatment implies a short-term negative feedback to climate change, but the Neurospora results suggest that climate-driven changes in plant inputs and soil properties are important controls on enzyme kinetics in the long term. Our empirical data on enzyme Vmax , Km , and temperature sensitivities should be useful for parameterizing existing biogeochemical models, but they reveal a need to develop new theory on thermal adaptation mechanisms.

Experimental warming alters potential function of the fungal community in boreal forest.

Fungal community composition often shifts in response to warmer temperatures, which might influence decomposition of recalcitrant carbon (C). We hypothesized that evolutionary trade-offs would enable recalcitrant C-using taxa to respond more positively to warming than would labile C-using taxa. Accordingly, we performed a warming experiment in an Alaskan boreal forest and examined changes in the prevalence of fungal taxa. In a complementary field trial, we characterized the ability of fungal taxa to use labile C (glucose), intermediate C (hemicellulose or cellulose), or recalcitrant C (lignin). We also assigned taxa to functional groups (e.g., free-living filamentous fungi, ectomycorrhizal fungi, and yeasts) based on taxonomic identity. We found that response to warming varied most among taxa at the order level, compared to other taxonomic ranks. Among orders, ability to use lignin was significantly related to increases in prevalence in response to warming. However, the relationship was weak, given that lignin use explained only 9% of the variability in warming responses. Functional groups also differed in warming responses. Specifically, free-living filamentous fungi and ectomycorrhizal fungi responded positively to warming, on average, but yeasts responded negatively. Overall, warming-induced shifts in fungal communities might be accompanied by an increased ability to break down recalcitrant C. This change in potential function may reduce soil C storage under global warming.

Neurospora discreta as a model to assess adaptation of soil fungi to warming.

BACKGROUND: Short-term experiments have indicated that warmer temperatures can alter fungal biomass production and CO2 respiration, with potential consequences for soil C storage. However, we know little about the capacity of fungi to adapt to warming in ways that may alter C dynamics. Thus, we exposed Neurospora discreta to moderately warm (16 °C) and warm (28 °C) selective temperatures for 1500 mitotic generations, and then examined changes in mycelial growth rate, biomass, spore production, and CO2 respiration. We tested the hypothesis that strains will adapt to its selective temperature. Specifically, we expected that adapted strains would grow faster, and produce more spores per unit biomass (i.e., relative spore production). In contrast, they should generate less CO2 per unit biomass due to higher efficiency in carbon use metabolism (i.e., lower mass specific respiration, MSR). RESULTS: Indeed, N. discreta adapted to warm temperatures, based on patterns of relative spore production. Adapted strains produced more spores per unit biomass than parental strains in the selective temperature. Contrary to our expectations, this increase in relative spore production was accompanied by an increase in MSR and a reduction in mycelial growth rate and biomass, compared to parental strains. CONCLUSIONS: Adaptation of N. discreta to warm temperatures may have elicited a tradeoff between biomass production and relative spore production, possibly because relative spore production required higher MSR rates. Therefore, our results do not support the idea that adaptation to warm temperatures will lead to a more efficient carbon use metabolism. Our data might help improve climate change model simulations and provide more concise predictions of decomposition processes and carbon feedbacks to the atmosphere.

Initial phylogenetic relatedness of saprotrophic fungal communities affects subsequent litter decomposition rates.

Ecosystem-level consequences of biodiversity loss of macroorganisms are well understood, while the repercussions of species extirpation in microbial systems are not. We manipulated species richness and phylogenetic relatedness of saprotrophic fungi in situ in a boreal forest to address this issue. Litter decomposition rates (as total mass loss) after 2 months were significantly higher in the least phylogenetically related fungal assemblages. Likewise, cellulose loss was also highest in the most distantly related treatments after 1 year. There were marginal effects of species richness on mass loss that only affected decomposition after 2 months. At the end of 1 year of decomposition, most fungal communities had collapsed from their original diversity to two species, mainly in the Penicillium or Hypocrea clades. Two concurrent processes may explain these results: competition between closely related fungal taxa and phylogenetic conservation in cellulose decomposition. Our results suggest that phylogenetic relatedness of fungal communities may be a more appropriate metric than species richness or community composition to predict functional responses of fungal communities to global change.

Evolutionary histories of soil fungi are reflected in their large-scale biogeography.

Although fungal communities are known to vary along latitudinal gradients, mechanisms underlying this pattern are not well-understood. We used high-throughput sequencing to examine the large-scale distributions of soil fungi and their relation to evolutionary history. We tested the Tropical Conservatism Hypothesis, which predicts that ancestral fungal groups should be more restricted to tropical latitudes and conditions than would more recently derived groups. We found support for this hypothesis in that older phyla preferred significantly lower latitudes and warmer, wetter conditions than did younger phyla. Moreover, preferences for higher latitudes and lower precipitation levels were significantly phylogenetically conserved among the six younger phyla, possibly because the older phyla possess a zoospore stage that is vulnerable to drought, whereas the younger phyla retain protective cell walls throughout their life cycle. Our study provides novel evidence that the Tropical Conservatism Hypothesis applies to microbes as well as plants and animals.

Fungal host specificity is not a bottleneck for the germination of Pyroleae species (Ericaceae) in a Bavarian forest.

Plants that produce dust seeds can recruit fungi to meet their earliest requirements for carbon and other nutrients. This germination strategy, termed initial mycoheterotrophy, has been well investigated among the orchid family, but there are numerous other plant lineages that have independently evolved mycoheterotrophic germination strategies. One of these lineages is the tribe Pyroleae (Ericaceae). While the fungi associated with mature plants in Pyroleae have been fairly well documented, their mycobionts at the germination and seedling stages are largely unknown. Here, we use an in situ seed baiting experiment along with molecular fingerprinting techniques and phylogenetic tests to identify the fungi associated with seedlings of two Pyroleae species, Pyrola chlorantha and Orthilia secunda. Our results indicate that similar to adult plants, Pyroleae seedlings can associate with a suite of ectomycorrhizal fungi. Some seedlings harboured single mycobionts, while others may have been inhabited by multiple fungi. The dominant seedling mycobiont of both Pyroleae species was a fungus of unknown trophic status in the order Sebacinales. This taxon was also the only one shared among seedlings of both investigated Pyroleae species. We discuss these results juxtaposed to orchids and one additional Pyrola species in the context of ontogenetic shifts in fungal host specificity for mycoheterotrophic nutrition.

Microbial abundance and composition influence litter decomposition response to environmental change.

Rates of ecosystem processes such as decomposition are likely to change as a result of human impacts on the environment. In southern California, climate change and nitrogen (N) deposition in particular may alter biological communities and ecosystem processes. These drivers may affect decomposition directly, through changes in abiotic conditions, and indirectly through changes in plant and decomposer communities. To assess indirect effects on litter decomposition, we reciprocally transplanted microbial communities and plant litter among control and treatment plots (either drought or N addition) in a grassland ecosystem. We hypothesized that drought would reduce decomposition rates through moisture limitation of decomposers and reductions in plant litter quality before and during decomposition. In contrast, we predicted that N deposition would stimulate decomposition by relieving N limitation of decomposers and improving plant litter quality. We also hypothesized that adaptive mechanisms would allow microbes to decompose litter more effectively in their native plot and litter environments. Consistent with our first hypothesis, we found that drought treatment reduced litter mass loss from 20.9% to 15.3% after six months. There was a similar decline in mass loss of litter inoculated with microbes transplanted from the drought treatment, suggesting a legacy effect of drought driven by declines in microbial abundance and possible changes in microbial community composition. Bacterial cell densities were up to 86% lower in drought plots and at least 50% lower on litter derived from the drought treatment, whereas fungal hyphal lengths increased by 13-14% in the drought treatment. Nitrogen effects on decomposition rates and microbial abundances were weaker than drought effects, although N addition significantly altered initial plant litter chemistry and litter chemistry during decomposition. However, we did find support for microbial adaptation to N addition with N-derived microbes facilitating greater mass loss in N plots than in control plots. Our results show that environmental changes can affect rates of ecosystem processes directly through abiotic changes and indirectly through microbial abundances and communities. Therefore models of ecosystem response to global change may need to represent microbial biomass and community composition to make accurate predictions.

Tracking fungal species-level responses in soil environments exposed to long-term warming and associated drying.

Climate change is affecting fungal communities and their function in terrestrial ecosystems. Despite making progress in the understanding of how the fungal community responds to global change drivers in natural ecosystems, little is known on how fungi respond at the species level. Understanding how fungal species respond to global change drivers, such as warming, is critical, as it could reveal adaptation pathways to help us to better understand ecosystem functioning in response to global change. Here, we present a model study to track species-level responses of fungi to warming-and associated drying-in a decade-long global change field experiment; we focused on two free-living saprotrophic fungi which were found in high abundance in our site, Mortierella and Penicillium. Using microbiological isolation techniques, combined with whole genome sequencing of fungal isolates, and community level metatranscriptomics, we investigated transcription-level differences of functional categories and specific genes involved in catabolic processes, cell homeostasis, cell morphogenesis, DNA regulation and organization, and protein biosynthesis. We found that transcription-level responses were mostly species-specific but that under warming, both fungi consistently invested in the transcription of critical genes involved in catabolic processes, cell morphogenesis, and protein biosynthesis, likely allowing them to withstand a decade of chronic stress. Overall, our work supports the idea that fungi that invest in maintaining their catabolic rates and processes while growing and protecting their cells may survive under global climate change.

Consequences of arthropod community structure for an at-risk insectivorous bird.

Global declines in bird and arthropod abundance highlights the importance of understanding the role of food limitation and arthropod community composition for the performance of insectivorous birds. In this study, we link data on nestling diet, arthropod availability and nesting performance for the Coastal Cactus Wren (Campylorhynchus brunneicapillus sandiegensis), an at-risk insectivorous bird native to coastal southern California and Baja Mexico. We used DNA metabarcoding to characterize nestling diets and monitored 8 bird territories over two years to assess the relationship between arthropod and vegetation community composition and bird reproductive success. We document a discordance between consumed prey and arthropod biomass within nesting territories, in which Diptera and Lepidoptera were the most frequently consumed prey taxa but were relatively rare in the environment. In contrast other Orders (e.g., Hemiptera, Hymenoptera)were abundant in the environment but were absent from nestling diets. Accordingly, variation in bird reproductive success among territories was positively related to the relative abundance of Lepidoptera (but not Diptera), which were most abundant on 2 shrub species (Eriogonum fasciculatum, Sambucus nigra) of the 9 habitat elements characterized (8 dominant plant species and bare ground). Bird reproductive success was in turn negatively related to two invasive arthropods whose abundance was not associated with preferred bird prey, but instead possibly acted through harassment (Linepithema humile; Argentine ants) and parasite transmission or low nutritional quality (Armadillidium vulgare; "pill-bug"). These results demonstrate how multiple aspects of arthropod community structure can influence bird performance through complementary mechanisms, and the importance of managing for arthropods in bird conservation efforts.

Assessing the Influence of Climate on the Spatial Pattern of West Nile Virus Incidence in the United States.

BACKGROUND: West Nile virus (WNV) is the leading cause of mosquito-borne disease in humans in the United States. Since the introduction of the disease in 1999, incidence levels have stabilized in many regions, allowing for analysis of climate conditions that shape the spatial structure of disease incidence. OBJECTIVES: Our goal was to identify the seasonal climate variables that influence the spatial extent and magnitude of WNV incidence in humans. METHODS: We developed a predictive model of contemporary mean annual WNV incidence using U.S. county-level case reports from 2005 to 2019 and seasonally averaged climate variables. We used a random forest model that had an out-of-sample model performance of R2=0.61. RESULTS: Our model accurately captured the V-shaped area of higher WNV incidence that extends from states on the Canadian border south through the middle of the Great Plains. It also captured a region of moderate WNV incidence in the southern Mississippi Valley. The highest levels of WNV incidence were in regions with dry and cold winters and wet and mild summers. The random forest model classified counties with average winter precipitation levels <23.3mm/month as having incidence levels over 11 times greater than those of counties that are wetter. Among the climate predictors, winter precipitation, fall precipitation, and winter temperature were the three most important predictive variables. DISCUSSION: We consider which aspects of the WNV transmission cycle climate conditions may benefit the most and argued that dry and cold winters are climate conditions optimal for the mosquito species key to amplifying WNV transmission. Our statistical model may be useful in projecting shifts in WNV risk in response to climate change. https://doi.org/10.1289/EHP10986.

Life history strategies of soil bacterial communities across global terrestrial biomes.

The life history strategies of soil microbes determine their metabolic potential and their response to environmental changes. Yet these strategies remain poorly understood. Here we use shotgun metagenomes from terrestrial biomes to characterize overarching covariations of the genomic traits that capture dominant life history strategies in bacterial communities. The emerging patterns show a triangle of life history strategies shaped by two trait dimensions, supporting previous theoretical and isolate-based studies. The first dimension ranges from streamlined genomes with simple metabolisms to larger genomes and expanded metabolic capacities. As metabolic capacities expand, bacterial communities increasingly differentiate along a second dimension that reflects a trade-off between increasing capacities for environmental responsiveness or for nutrient recycling. Random forest analyses show that soil pH, C:N ratio and precipitation patterns together drive the dominant life history strategy of soil bacterial communities and their biogeographic distribution. Our findings provide a trait-based framework to compare life history strategies of soil bacteria.

Interactions among lignin, cellulose, and nitrogen drive litter chemistry-decay relationships.

Litter decay rates often correlate with the initial ratios of lignin:nitrogen (N) or lignin:cellulose in litter. However, the chemical and microbial mechanisms that give rise to these patterns are still unclear. To identify these mechanisms, we studied the decomposition of a model plant system, Arabidopsis thaliana, in which plants were manipulated to have low levels of lignin, cellulose, or litter N. Nitrogen fertilizer often increases the loss of cellulose, but it suppresses the breakdown of lignin in plant litter. To understand the mechanisms driving these patterns, we decomposed plants in litterbags for one year in control and N-fertilized plots in an Alaskan boreal forest. We found that litter N had a positive effect on total mass loss because it increased the loss of lignin, N, and soluble C. Lignin had a negative effect on rates of total litter mass loss due to decreases in the loss of cellulose and hemicellulose. Cellulose had a positive effect on lignin loss, supporting the concept of a "priming effect" for lignin breakdown. However, the low-cellulose plants also lost more of their original cellulose compared to the other plant types, indicating that decomposers mined the litter for cellulose despite the presence of lignin. Low-lignin litter had higher fungal biomass and N-acetyl glucosaminidase (NAG, a chitinase) activity, suggesting that lignin restricted fungal growth and may have influenced competitive interactions between decomposers. Nitrogen fertilization increased NAG activity in the early stages of decay. In the later stages, N fertilization led to increased cellulase activity on the litters and tended to reduce lignin losses. The transition over time from competition among decomposers to high cellulase activity and suppressed lignin loss under N fertilization suggests that, in N-limited systems, N fertilization may alter decomposer community structure by favoring a shift toward cellulose- and mineral-N users.

Fungal community composition in neotropical rain forests: the influence of tree diversity and precipitation.

Plant diversity is considered one factor structuring soil fungal communities because the diversity of compounds in leaf litter might determine the extent of resource heterogeneity for decomposer communities. Lowland tropical rain forests have the highest plant diversity per area of any biome. Since fungi are responsible for much of the decomposition occurring in forest soils, understanding the factors that structure fungi in tropical forests may provide valuable insight for predicting changes in global carbon and nitrogen fluxes. To test the role of plant diversity in shaping fungal community structure and function, soil (0-20 cm) and leaf litter (O horizons) were collected from six established 1-ha forest census plots across a natural plant diversity gradient on the Isthmus of Panama. We used 454 pyrosequencing and phospholipid fatty acid analysis to evaluate correlations between microbial community composition, precipitation, soil nutrients, and plant richness. In soil, the number of fungal taxa increased significantly with increasing mean annual precipitation, but not with plant richness. There were no correlations between fungal communities in leaf litter and plant diversity or precipitation, and fungal communities were found to be compositionally distinct between soil and leaf litter. To directly test for effects of plant species richness on fungal diversity and function, we experimentally re-created litter diversity gradients in litter bags with 1, 25, and 50 species of litter. After 6 months, we found a significant effect of litter diversity on decomposition rate between one and 25 species of leaf litter. However, fungal richness did not track plant species richness. Although studies in a broader range of sites is required, these results suggest that precipitation may be a more important factor than plant diversity or soil nutrient status in structuring tropical forest soil fungal communities.

Organic nitrogen uptake by arbuscular mycorrhizal fungi in a boreal forest.

The breakdown of organic nitrogen in soil is a potential rate-limiting step in nitrogen cycling. Arbuscular mycorrhizal (AM) fungi are root symbionts that might improve the ability of plants to compete for organic nitrogen products against other decomposer microbes. However, AM uptake of organic nitrogen, especially in natural systems, has traditionally been difficult to test. We developed a novel quantitative nanotechnological technique to determine in situ that organic nitrogen uptake by AM fungi can occur to a greater extent than has previously been assumed. Specifically, we found that AM fungi acquired recalcitrant and labile forms of organic nitrogen. Moreover, N enrichment of soil reduced plot-scale uptake of these compounds. Since most plants host AM fungi, AM use of organic nitrogen could widely influence plant productivity, especially where N availability is relatively low.

Evolutionary trade-offs among decomposers determine responses to nitrogen enrichment.

Evolutionary trade-offs among ecological traits are one mechanism that could determine the responses of functional groups of decomposers to global changes such as nitrogen (N) enrichment. We hypothesised that bacteria targeting recalcitrant carbon compounds require relatively high levels of N availability to support the construction costs of requisite extracellular and transport enzymes. Indeed, we found that taxa that used more recalcitrant (i.e. larger and cyclic) carbon compounds were more prevalent in ocean waters with higher nitrate concentrations. Compared to recalcitrant carbon users, labile carbon users targeted more organic N compounds, were found in relatively nitrate-poor waters, and were more common in higher latitude soils, which is consistent with the paradigm that N-limitation is stronger at higher latitudes. Altogether, evolutionary trade-offs may limit recalcitrant carbon users to habitats with higher N availability.