Journal of Geophysical Research: Biogeosciences

The Arctic is warming rapidly, causing permafrost thaw and accelerating the release of greenhouse gases. Rapid thaw features such as retrogressive thaw slumps are increasing in frequency and severity across the Arctic; however, their associated greenhouse gas emissions are poorly constrained. Current estimates of emissions from retrogressive thaw slumps rely largely on laboratory incubations and carbon stock estimates rather than in-situ field measurements. Here we directly quantify methane and carbon dioxide fluxes from the exposed headwall of an active retrogressive thaw slump. We show that thaw immediately releases biogenic methane and carbon dioxide, originating from gases trapped within the frozen soil matrix. Microbial transcription of methyl-coenzyme M reductase suggests archaea carrying out methanogenesis at subzero temperatures are the source of trapped methane. Carbon emissions varied by an order of magnitude among cryostratigraphic units, reflecting differences in geomorphologic history, organic carbon and nitrogen content, and microbial community composition. Carbon emissions were highest from organic-rich paleo cryosols from the Late Holocene that contained abundant methanogenic archaea. We estimate that [~]300 kg C (CO2 equivalents) is emitted annually from the headwall of this small thaw slump (surface area of [~]1200 m2). Considering the thousands of active slumps and extensive coastal permafrost erosion across the northern continuous permafrost zone, such features may represent a growing natural source of GHG emissions. These findings indicate that current permafrost carbon feedback models underestimate GHG release by omitting the direct release of trapped gases stored in permafrost.

Evidence from the Cassini mission confirmed that Saturn's moon Enceladus hosts a subsurface alkaline ocean where rock-water reactions may generate redox disequilibria capable of supporting microbial metabolisms. To investigate potential microbial survival under simulated Enceladus ocean conditions, we used thermodynamic modeling to develop a salt formulation consistent with one possible Enceladus ocean composition and supplemented it with putative microbial energy sources to create a growth medium. The medium was inoculated with samples from diverse ocean world analog environments on Earth to determine which microorganisms could persist under Enceladus-like conditions. The microorganisms persisting in this geochemically bounded medium were heterotrophic, metabolically versatile bacteria with low carbon requirements. Genomic and physiological analyses further showed the presence of multiple stress-response pathways, sodium- based bioenergetic systems, osmoregulation strategies, and other adaptations consistent with survival in alkaline, low-nutrient settings. These results suggest that some stress-tolerant heterotrophic bacteria may serve as useful model organisms for life in Enceladus' subsurface ocean. These findings demonstrate the value of geochemically modeled media as a framework for constraining habitability, identifying relevant biosignatures, and probing potential microbial survival strategies beyond Earth.

Mangrove forests are major blue carbon ecosystems but are often characterized by low surface methane (CH4) emissions. Such low emissions, however, do not necessarily indicate weak methanogenesis, because CH4 production may be offset by internal CH4 consumption before reaching the atmosphere. Although previous community, genomic, and transcriptomic studies have implicated methylotrophic methanogenesis in mangrove sediments, direct taxon-resolved evidence linking methylated carbon assimilation to potentially active methanogens remains limited. Here, we combined methanogenic activity assays, DNA stable isotope probing (DNA-SIP), mcrA and 16S rRNA gene analyses, and phylogenetic comparisons to identify potentially active methanogens across saline-influenced mangrove soils. The results showed that CH4 production potentials were consistently dominated by methylotrophic pathways (1.86-2.78 g CH4 g-1 soil hr-1) across all sites. DNA-SIP, together with consistent community patterns in fresh soils, indicated the potential activity of methylotrophic and mixotrophic methanogens under saline conditions. Methanolobus-affiliated methanogens were associated with salinity, Na+, Cl-, and NH4+, whereas Methanosarcina and unclassified Methanosarcinaceae were linked to soil soluble organic carbon availability and water content, indicating niche differentiation among active methanogenic groups. Phylogenetic analyses incorporating reference sequences from diverse environments further showed that potentially active mangrove methanogens were dominated by saline-associated lineages. Together with our previous methanotrophic evidence from the same sites, these findings suggest that low CH4 emissions from mangrove blue carbon ecosystems can mask substantial internal CH4 cycling sustained by active methanogenesis and CH4 consumption.

A substantial fraction of marine sediments experience elevated temperatures due to burial or hydrothermal activity. These conditions can fundamentally reshape both microbial activity and the chemical nature of sedimentary organic matter (OM). Laboratory incubations have demonstrated that moderate heating of marine sediments can lead to the production of labile organic compounds such as acetate, however, it remains unclear whether heating alters the bioavailability of the remaining OM pool. In this study, we experimentally tested the effect of temperature on the bioavailability of OM through a series of bioreactor experiments using deeply buried sediment collected from Guaymas Basin (Gulf of California). We measured acetate concentrations in sterilized Guaymas Basin sediments before and after artificial heating (70{degrees}C for 7 days) to quantify abiotic acetate generation. We then conducted incubations of a model marine bacterium with sterilized, artificially heated sediment and tracked respired CO2 production and its associated 13C and 14C signatures. Our study revealed that sediment depth and hydrothermal history strongly control abiotic acetate production, with higher acetate yields from shallower, cooler sediments. Respiration rates in control and heated sediment incubations were nearly identical, indicating that heating does not measurably alter the bioavailability of bulk sedimentary OM. Moreover, the {delta}13C values of respired CO2 were indistinguishable between control and heated sediment incubations while the {Delta}14C values were more depleted in the first 24 hours in incubations with heated sediment. This transient offset suggests that low-temperature heating mobilizes a small pool of older material due to desorption of mineral-bound OM without altering overall bioavailability. Our findings shed light on the role of thermal alteration in shaping carbon cycling in marine sediments by influencing how OM is made available to sedimentary microorganisms.

Rock surface weathering is a critical element in the process of early soil formation, in which lichens are thought to play a significant role. Crustose lichens, with a large area of rock-surface contact, are generally considered more influential in rock weathering, while foliose and fruticose growth forms, with more developed three-dimensional structure and less rock-surface contact, are rarely considered in this context. Here, we test the extent to which all three growth forms contribute to granitic rock surface weathering in Maritime Antarctic ecosystems, by quantifying rock hardness beneath foliose (n = 2 species), fruticose (n = 2) and crustose lichens (n= 5). Our data confirm that foliose lichens reduced rock surface hardness by 9%, to a lesser extent than crustose and foliose lichens (40% and 31% reduction, respectively). To disentangle whether these effects result from lichen-induced weathering or lichen preference for pre-weathered rock, we also analyzed a dated deglaciation sequence on granitic rocks from the Morteratsch Glacier forefield in the Swiss Alps. At this location, the impact of crustose lichens on rock substrate hardness generally increased with time since exposure from glacial retreat and with lichen thallus size. We conclude that lichen presence on rock surfaces significantly reduces rock hardness, with crustose lichens having a greater impact than foliose and fruticose forms, highlighting the potential role of lichens of all three growth forms in driving substrate breakdown and shaping early-stage ecosystem processes in polar and alpine regions.

Biochar amendments are increasingly applied to improve soil physical functioning and support carbon dioxide removal, but their effects on intrinsic soil thermal properties remain poorly characterised. We conducted the first global systematic meta-analysis of 19 independent studies, 231 control-biochar comparisons, and 529 property-specific effect sizes to test how biochar changes soil heat transfer and storage. Biochar reduced thermal conductivity by 17.6% (95% CI, -22.7 to -12.2), thermal diffusivity by 11.0% (-14.5 to -7.3), and volumetric heat capacity by 8.3% (-12.3 to -4.1). Gravimetric heat capacity showed no significant overall response (+3.3%; -7.6 to 15.4) but was supported by fewer studies. Negative responses were directionally consistent for thermal conductivity, diffusivity, and volumetric heat capacity. Moderator analyses showed that responses were most consistently associated with post-application bulk density and changes in bulk density, while application rate modulated response magnitude and soil texture constrained context dependence. Co-variation among thermal conductivity, thermal diffusivity, and volumetric heat capacity matched expected physical dependencies, indicating coordinated structural reorganisation rather than independent shifts in isolated parameters. These estimates describe intrinsic conductive and storage properties; field-scale soil temperature responses may also be modified by albedo, evaporation, vegetation, and surface energy balance. Improved integration of soil thermal measurements with moisture dynamics, structural changes, and carbon cycling is essential to accurately represent biochar effects in soil and land-surface models.

Primary production in the Southern Ocean (SO) plays a critical role in regulating the global carbon cycle, yet the physiological mechanisms governing phytoplankton responses to iron (Fe) limitation and variable light remain poorly constrained. Using a custom made incubation system that simulated natural diel solar variability, we examined the interactive effects of Fe availability, light intensity, and photoperiod (continuous vs. variable) on three ecologically important SO phytoplankton: Fragilariopsis cylindrus, Phaeocystis antarctica, and Thalassiosira antarctica. Physiological, photophysiological, and proteomic measurements revealed that Fe availability was the dominant factor regulating growth, carbon production, photosynthetic performance and protein expression across all species. Distinct acclimation strategies emerged: F. cylindrus exhibited marked trade-offs between productivity and photoprotection under Fe stress, consistent with adaptation to stable, low-light, Fe-poor environments; P. antarctica maintained growth by flexibly modulating photoprotective and photosynthetic capacity, reflecting high plasticity suited to dynamic, open-ocean conditions; and T. antarctica expressed a balanced strategy, sustaining productivity and photoprotection simultaneously, characteristic of coastal bloom formers with higher Fe demand. Dynamic light regimes produced smaller, species-specific effects, influencing chlorophyll content and carbon storage primarily in T. antarctica. Correlation and z-score analyses demonstrated that Fe-rich photosynthetic proteins co-varied with biomass production, whereas photoprotective traits clustered independently, underscoring divergent energy-allocation strategies. Together, these results reveal how SO phytoplankton partition resources between productivity and photoprotection under shifting Fe-light regimes, providing mechanistic insight into their ecological niches.

Forests are essential to the global carbon cycle with light use efficiency (LUE) as a key parameter for assessing carbon sequestration capacity. However, the variations and drivers of LUE remain inadequately understood. Using remote sensing data, we analyzed global LUE patterns across five forest types and identified the main drivers. The global average annual LUE of forests is 0.93 {+/-} 0.36 g C MJ-1 during the period 2001-2022, with an increasing trend of 0.0034 g C MJ-1 yr-1. Among forest types, evergreen broadleaf forests exhibited the highest LUE, followed by evergreen needleleaf forests. Deciduous broadleaf forests and mixed forests showed similar levels, while deciduous needleleaf forests exhibiting the lowest LUE. Variations in LUE were jointly driven by plant traits and climatic conditions, with generalized linear models explaining 86% and 98% of spatial and temporal LUE variations, respectively. These findings highlight the critical role of plant traits and climate in shaping forest LUE, providing insights for enhancing carbon cycle models and informing forest management strategies in the context of global change.

Conventional anaerobic digestion emits methane from organic waste. Here, we investigate a sulfate-based alternative that suppresses methane production and generates alkaline solutions that may sequester carbon by carbonate precipitation. Although methanogenesis is known to occur when reduced organic carbon is replete and sulfate is limiting, it remains unclear whether methane emissions during microbial conversion of waste gypsum are primarily driven by community composition or organic availability. By comparing fluxes of electrons from organic matter toward sulfate or methane in microbial communities grown on different organic loads, we show that community composition, microbial growth, and organic availability collectively determine sulfide and methane fluxes. Lower organic loads increase the importance of syntrophic interactions with fermenters and competition between sulfate reducing bacteria and methanogens due to scarcity of substrates. Microbes present in the original sewage sludge reduce less sulfate, produce more methane, and generate less alkalinity compared to the communities enriched by multiple cycles of growth in the presence of sulfate and sewage sludge. The inoculation of communities enriched at low organic loadings in the presence of sulfate decreases the production of methane by enabling the growth of sulfate reducing bacteria from the order Desulfobacterales that can oxidize acetate to CO2 and compete with methanogens for acetate. The use of such enrichments in sludge treatment systems can stimulate the removal of organic substrates and waste gypsum, while suppressing methane production, over timescales comparable to those in the current sludge treatment systems that do not contain sulfate.

Antarctic benthic organisms are highly diverse and play a critical role in the Southern Ocean ecosystem. Despite decades of sampling, vast areas of the Antarctic continental shelf remain biologically unsurveyed due to logistical and financial constraints, limiting baseline knowledge essential for effective conservation planning. Species distribution models (SDMs) allow biodiversity to be inferred in the absence of biological data by linking benthic community patterns to environmental predictors. However, the resolution of the environmental predictors, particularly bathymetry, varies significantly between regions, casting doubt about how reliably SDMs can be used to predict into regions where only coarse-resolution data are available. Here, we show that SDMs trained on high-resolution data underestimate Antarctic benthic morphospecies richness by up to 18% when applied to aggregated coarse-resolution environmental data (and up to 50% when using satellite-derived ETOPO bathymetry). Using six systematically degraded versions of high-resolution multibeam bathymetry and annotated seafloor imagery across three Antarctic regions, we evaluate SDM performance both with and without additional environmental variables. High-resolution bathymetry captures terrain complexity most effectively, but we find that the spatial distribution of richness hotspots and the median richness per cell remain consistent, provided models are applied at the same resolution at which they were trained. Our results suggest that while high-resolution bathymetry may enhance local predictions, coarse-resolution data may be more robust for regional-scale predictions, such as those used for Antarctic shelf-wide spatial planning.

Metagenomic characterization of low-biomass Yellowstone National Park (YNP) hot spring waters remains challenging because microbial recovery is influenced by filtration methodology, sample preservation, DNA extraction, and sequencing strategy. We characterized thermophilic microbial communities in alkaline YNP hot spring waters (62-90.5{degrees}C) using three high-temperature-compatible filtration systems (Sterivex, Supor, and polycarbonate membranes), automated onsite DNA extraction ({micro}Titan), and shotgun metagenomic sequencing with Illumina short-read and Oxford Nanopore Technologies (ONT) long-read platforms. Across all filtration systems and sequencing workflows, microbial communities were consistently dominated by Bacteria ([~]90% of reads), whereas Archaea represented <10% of recovered sequences. Dominant microbial populations were reproducibly recovered across all approaches; however, recovery of lower-abundance taxa varied among methods. This variability was most evident in polycarbonate-filtered samples, which exhibited greater replicate-to-replicate variation and less consistent detection of microbial species. Thermocrinis ruber and related Aquificae-associated thermophiles dominated the hottest waters (78.5-90.5{degrees}C), whereas warmer effluent-channel waters (63.5-66.5{degrees}C) contained T. ruber together with photosynthetic taxa, including Synechococcus spp. and Candidatus Thermochlorobacter aerophilum. Archaeal communities were primarily represented by Pyrobaculum- and Thermoproteus-related taxa. Non-metric multidimensional scaling analyses indicated that overall community structure was largely unaffected by filtration or sequencing methodology, whereas alpha-diversity metrics showed that filter selection influenced richness and diversity estimates. These findings identify field-deployable workflows for metagenomic characterization of low-biomass thermophilic aquatic systems and demonstrate the importance of integrating filtration and sequencing strategies for studying extremophile microbiomes under remote sampling conditions. IMPORTANCEAccurate characterization of low-biomass geothermal water microbiomes remains challenging because microbial recovery is strongly influenced by sample handling, filtration efficiency, DNA extraction chemistry, and sequencing methodology. This study demonstrated that Yellowstone National Park alkaline hot spring water microbiomes were consistently dominated by Bacteria (>90% of recovered reads), whereas Archaea represented <10% of community abundance across all filtration systems. Although dominant microbial populations were reproducibly recovered, filtration-device selection influenced the recovery of microbial diversity and low-abundance taxa. By integrating field-deployable onsite DNA extraction with ONT shotgun metagenomic sequencing, this work evaluates practical workflows for studying thermophilic planktonic microbial communities under remote field conditions. These findings are relevant, not only to geothermal microbiology, but also to low-biomass environments in medical, pharmaceutical, and aerospace industries, where rapid onsite processing and contamination-aware workflows are essential for preserving authentic microbial signatures in extreme environments.

Grasslands are vulnerable to increasing drought with global warming, but process-based models lack the mechanistic knowledge required to predict the magnitude of drought impacts. While a plant hydraulics framework has been successful in advancing process understanding of drought responses in trees, and how drought responses vary across rainfall gradients, similar approaches have rarely been applied to grasses. Here, we quantified the progression of key drought response processes in sixteen dominant perennial grasses (seven C3 and nine C4) with differing climatic origins across eastern Australia. We found that stomatal closure, hydraulic impairment and leaf browning occurred concurrently, in contrast to the progressive sequence typically observed in trees. We also found that drought response traits were not correlated with species climate of origin. The early impairment of leaf hydraulic conductance and leaf browning along with the lack of correlation with climate of origin suggest that grasses may employ fundamentally different strategies to adapt to low water availability than trees. These results highlight the need for grass-specific parameterization of drought responses in process-based models.

Nearly all carbon on Earth today is fixed by the enzyme ribulose-1,5-bisphopshate carboxylase/oxygenase ( rubisco), which converts carbon dioxide (CO2) to sugar phosphates. All rubiscos measured thus far display a kinetic isotope effect (KIE) where 12CO2 is fixed at a faster rate than 13CO2. The relationship between rubiscos KIE and the carbon isotope composition of plants, algae, and organic matter is central to many fields in the Earth sciences, plant biology, and biochemistry. Currently, all applications assume that the KIE does not vary with temperature. Here, we examine this assumption experimentally with in vitro KIE measurements of two rubiscos from phylogenetically distinct host organisms and rubisco protein clades - a Form I rubisco from the plant, Spinacia oleracea (spinach) and a Form II rubisco from the bacterium Rhodosprillium rubrum. We that find that both KIEs decrease linearly by [~]4.5{per thousand} from 10-35{degrees}C with statistically indistinguishable slopes. We place these results into biological and geologic contexts by comparing them to observed variations in the carbon isotope composition of modern terrestrial plants and marine organic carbon, the geologic carbon isotope record, and rubiscos biochemistry. We show that the measured temperature dependencies are sufficiently large to impact our interpretations of the enzymatic processes that drive variations in rubisco KIEs, as well as applications of stable carbon isotopes in the Earth and biological sciences. Significance StatementThe carbon isotope composition of plants, algae, and organic matter are interpreted with models that assume the kinetic isotope effect of the carbon-fixing enzyme rubisco is temperature-independent, even though temperature varies by tens of degrees across the Earth today and in the past. Here, we demonstrate that the kinetic isotope effect of rubisco is temperature-dependent, suggesting that some of this isotopic variation may be due to intrinsic enzyme properties alone. In addition, though the rubiscos we measured are from diverse organisms (plant vs. bacteria), their KIEs show statistically indistinguishable temperature dependencies. This data forms the basis for future thermodynamic models on rubisco biochemistry.

Root-associated microorganisms play a pivotal role in helping plants adapt to drought stress. However, the underlying mechanisms of the rhizospheric microbiome under limiting soil moisture remain largely unresolved. Integrating total and active microbiome analyses enables a more accurate interpretation of microbial responses to climate change-associated water stress. We assessed the effect of reduced rainfall on two wheat genotypes, drought-tolerant (DT) and drought-sensitive (DS), using rainout shelters that allowed 100%, 75%, 50%, and 25% of natural precipitation to reach the crop. At the peak of the growing season, rhizosphere samples were collected for metagenomic (MG) and metatranscriptome (MT) sequencing. In parallel, rhizosphere volatile organic compounds (VOCs) were collected and analysed. Differential expression analysis of metatranscriptomic data using metagenomic abundance as a cofactor was performed by comparing all treatments to the 100% precipitation control. Our results demonstrate that particularly oxidative stress-related transcripts intensify in DS as rainfall decreases. Transcriptomic shifts primarily involved upregulation of transcripts associated with antioxidant (catalase, superoxide dismutase), heat shock proteins (Hsp10, Hsp60, DnaK/DnaJ, GroEL, GroES), as well as microbial functions related to osmoregulation, proline and glycine betaine (PutA, PutP, OpuBB), and plant growth-promoting traits such as auxin production, phosphate solubilization. Moreover, volatile organic compound (VOC) emissions differed significantly between the control and drought treatments, with higher emissions, particularly acetates, in the DS genotype than in the DT genotype. Overall, pronounced drought-induced shifts in active microbial functions and VOC emissions indicate high sensitivity and functional plasticity of the active microbiome, whereas the total microbiome remains robust under medium drought.

Environmental DNA (eDNA) has the potential to be a powerful tool in blue carbon science for characterizing and quantifying the contribution of marine macrophytes; but its complex, dynamic relationship with bulk biomass is poorly understood. Here, we used eDNA to examine the degradation dynamics of sugar kelp (Saccharina latissima) in muddy, anaerobic marine sediment. This involved three 16-week incubations; with additions of lyophilized sugar kelp alone, a mix of lyophilized marine macrophytes including sugar kelp, and sugar kelp holdfasts buried in sediment. We used species-specific digital polymerase chain reaction assays for mitochondrial, chloroplast and nuclear markers, and metabarcoding for the 16S and 18S ribosomal RNA genes. In the former two incubations, all sugar kelp eDNA markers showed rapid log exponential declines (up to 98-99%) to asymptotes greater than the unamended controls, even as part of a more complex mix of macrophytes. In contrast, for the buried kelp holdfasts, sugar kelp eDNA increased to an asymptote (by up to [~]15X), which may be reflective of the different nature of added biomass. Overall, we demonstrate substantial preservation of environmental DNA and total organic carbon under anaerobic conditions, and the potential to use environmental DNA to quantify biomass in a blue carbon context.

Internal decay in living trees is an important component of carbon and nutrient cycling as well as species and structural diversity maintenance in forest ecosystems. We used sonic and electrical resistance tomography to evaluate and compare the prevalence and severity of stem decay in 57 living trees among four common species (Acer rubrum L., Nyssa sylvatica Marsh., Quercus rubra L., and Tsuga canadensis (L.) Carriere)) with overlapping and non-overlapping distributions across wetland and upland habitat types at the Harvard Forest in Petersham, MA, USA. Independent of tree size, site identity best explained variation in the prevalence of decay across trees sampled, whereas species identity best explained the severity of decay. We categorized trees as having no decay, incipient decay, active decay, or cavities based on combined sonic and electrical resistance metrics, the latter generated by a custom image analysis application. About 31% of wetland trees exhibited incipient decay (compared to 11% in the upland), whereas about 32% of upland trees exhibited active decay (compared to 10% in the wetland). Our study highlights a new quantitative framework for decay categorization through normalized principal component analysis (PCA) and decay analysis software that complements dual tomographic methodology for future investigations of ecological drivers of decay presence and susceptibility.

Globally, nocturnal lightscapes are now determined by both moonlight and light pollution, or artificial light at night (ALAN). Organisms respond both to changes in moonlight across lunar cycles and to alterations in light conditions due to artificial light. The interaction of natural and artificial light is a critical aspect to incorporate into our understanding of how crepuscular and nocturnal ecology is altered in anthropogenically-modified landscapes. In this manuscript we review the rapidly expanding body of research on ecological impacts of ALAN to (1) assess patterns of lunar data inclusion and (2) summarize documented interactions of moonlight and ALAN. Three-fourths (72%) of 379 papers reviewed did not incorporate moonlight into their statistical analyses and experimental design. Only 12% directly investigated interaction effects of moonlight and ALAN. However, 70% of these studies reported an interactive effect. Considering this stark contrast, as a precursor to our literature review, we present an overview of moonlight and the lunar cycle for biologists. The overarching trend emerging from the literature is that biological impacts of ALAN decrease with increasing moonlight, although the opposite is true in some cases. After summarizing the literature, we present general hypotheses regarding the interaction of the lunar cycle and ALAN. These hypotheses consider the different forms of ALAN encountered by organisms (i.e. skyglow and light sources) and account for the influence of cloud cover. Finally, we suggest best practices for incorporating moonlight into future research on biological impacts of ALAN.

Inland waters in the Northern Hemisphere are experiencing increased annual runoff due to higher overall precipitation as well as intensified short-term events such as heavy rainfall, floods and storms. These events affect the total loading and variability of inputs of allochthonous, coloured dissolved organic matter (cDOM) and inorganic nutrients into lakes. Previous studies have shown that increased total cDOM and inorganic nutrient loads affect phytoplankton biomass and metabolic rates, but it is unknown how the effects of different cDOM and nutrient pulse scenarios are modified by spatial and seasonal differences in lake characteristics. Here, we conducted a coordinated, standardized mesocosm experiment across three lakes with different ambient cDOM and nutrient concentrations. In two of these lakes, the experiment was implemented in two seasons. The same total amounts of cDOM, nitrate and phosphate were added to all mesocosms, but in pulses that differed in intensity and frequency. We found that pulse intensity and frequency affected chlorophyll a and phycocyanin concentrations and metabolic rates, i.e. gross primary production and respiration, differently. Specifically, more pronounced effects were found in response to the extreme pulse scenario compared to those with more frequent, smaller pulse additions. Furthermore, the effects were mainly temporary and varied more among lakes than between seasons. The clearest differences between the extreme and more gradual runoff scenarios were found in the lake with the lowest background cDOM and nitrate concentrations, likely because lower light limitation and possibly stronger initial N-limitation caused a faster response to the nutrient addition. Our results highlight that both antecedent lake conditions and characteristics of runoff events can affect phytoplankton biomass and metabolic rates and that comparative experimental approaches are needed to reveal the complexity of the responses.

Tropical rainforests generate and maintain their own microclimate regimes and the resultant cooler, more humid and stable environments foster the hyperdiversity typical of these ecosystems. Although the temporal and spatial (horizontal) distributions of microclimates have been relatively well studied, the vertical dimension has received less attention, and little is known about how forest disturbance affects the vertical stratification of microclimates in tropical forests. In this study, we examine how the vertical distribution of temperatures varies between undisturbed and burned Amazonian forests. We installed five vertical transects with temperature dataloggers distributed at 7 different heights to collect data over multiple days during the end of the dry season. We investigated how anthropogenic disturbance (fire) mediates the vertical stratification of microclimate and whether microclimate buffering (i.e, the difference between understorey and canopy temperatures) varies according to the forest structure. We showed that anthropogenic disturbance can cause an inversion in the vertical stratification of microclimates, with burned forests having hotter temperatures (up to 2 {degrees}C) in the understorey than in the canopy during the day - the opposite of what is found in undisturbed forests (typically 3 {degrees}C cooler). During the night, while understorey and canopy temperatures are similar in undisturbed forests, we found that, in burned forests, understorey temperatures were up to 2 {degrees}C cooler than in the canopy. Microclimate buffering by day was best explained by aboveground carbon stocks, with higher temperature buffering in more carbon rich forests. Our study shows that anthropogenic disturbance alters the vertical stratification of temperatures in Amazonian forests, leading to significant temporal changes along the diel cycle. Future research should focus on understanding these changes across a wider range of disturbance regimes, and explore the consequences for biodiversity and ecosystem functions from the canopy to the forest floor.

O_LISpawning salmon transport marine-derived nutrients (MDN) into riparian forests, influencing soil, plant, and animal communities, yet their effects on fungal communities remain poorly understood. C_LIO_LIWe used DNA metabarcoding to examine fungal responses to three spatial patterns of salmon-derived nitrogen (N) in southwest Alaska: (i) patchy inputs from wildlife-deposited carcasses, (ii) a 21-year carcass relocation experiment, and (iii) natural N gradients with distance from streams. C_LIO_LIDecomposing carcasses increased saprotrophic fungal diversity, identifying taxa responsible for salmon carcass decomposition. Long-term carcass relocation reduced diversity of medium-distance fringe ectomycorrhizal fungi (EMF), whereas recent, patchy carcass inputs increased diversity of both medium-distance fringe and long-distance EMF--guilds often associated with low-nutrient environments. Along natural stream N gradients, EMF responses varied markedly within functional guilds and genera, revealing unexpected variation in N sensitivity among closely related taxa. C_LIO_LIPulsed, spatially heterogeneous nutrient inputs enhanced diversity of typically nitrophobic EMF, likely reflecting their capacity to maintain extensive mycelial networks, exploit nutrient hotspots, and mobilize organic N and phosphorus. The diversity of responses along natural N gradients suggests that mechanisms linking EMF traits to nutrient acquisition and tolerance remain unresolved. Our findings emphasize the importance of linking fungal community composition with functional attributes and nutrient dynamics. C_LI