Warming temperatures are accelerating permafrost thaw and changing tundra vegetation, where woody shrubs are displacing sedges. Shrubs, such as Betula nana, and sedges, such as Eriophorum vaginatum, exhibit distinct life strategies including unique root-associated, or rhizosphere microbial communities. As permafrost thaws it unlocks previously unavailable carbon and nutrient sources resulting in deeper roots and a translocation of rhizosphere communities. Because permafrost microbial communities contain lower diversity and biomass than rhizosphere communities, the coalescence of rhizosphere and permafrost microbial communities could alter soil organic matter (SOM) degradation rates and increase greenhouse gas emissions. To identify metabolic strategies across distinct rhizosphere and permafrost microbial communities we conducted an isotope tracing incubation experiment. We inoculated thawed permafrost with shrub and sedge rhizosphere communities while adding exudates or water daily and compared this to an uninoculated control. After 46 days, we spiked samples with 18O enriched water or 13C enriched exudates and measured isotope incorporation into microbial DNA with quantitative stable isotope probing (qSIP). Our results indicate that exudate additions had little effect on uninoculated permafrost communities but the addition of exudates and rhizosphere inoculants had a compounding effect on respiration rates. We found that soils inoculated with shrub rhizosphere communities contained a mixture of exudate and SOM degraders while soils inoculated with sedge rhizosphere communities contained mainly SOM degraders. Finally, we found that individual microbial taxa exhibited maximum growth rates under specific combinations of microbial inoculant communities and exudate addition treatments. Our results reveal that microbial niches are strongly influenced by substrate preferences and community context, and suggest that a reduction in sedges and an expansion of shrubs may provide a mechanism by which permafrost carbon losses are mitigated through corresponding shifts in microbial communities and their substrate preferences.

Increasing terrestrial carbon storage can reduce atmospheric carbon dioxide levels as a climate mitigation strategy. Agricultural soil offers potential for persistent soil carbon sequestration. Mineral-associated organic carbon (MAOC) is generally more persistent than particulate organic carbon (POC), but it is unclear how much influence soil health and management can have on MAOC relative to climate and edaphic conditions. Soil samples from 196 agricultural fields on 77 farms throughout Vermont were fractionated by size to understand how these factors affect the amount and proportions of POC and MAOC. Our results reveal a significant effect of crop type, climate, soil texture, and aggregate stability on these carbon fractions. Hay and pasture fields had on average 69

Overstory trees rely on arbuscular mycorrhizal (AM) or ectomycorrhizal (EcM) fungi for establishment and growth. However, we know very little about how timber harvesting affects the biotic (e.g., mycorrhizal fungi) and abiotic (e.g., nutrient availability) legacies left behind by AM and EcM trees. These belowground mycorrhizal legacies, including potential shifts in abundance of mycorrhizal fungi, may differentially affect seedling success. To address this knowledge gap, we investigated how the legacy of AM and EcM forest stands affected seedling survival and growth, soil fungal communities, and soil chemistry including nutrient availability and pH following timber harvesting in a temperate hardwood forest. We established 16 plots, half of which were AM- and half of which were EcM-dominated. Eight of the 16 plots were harvested and then planted with four AM- and four EcM-associated seedling species in a split-plot design. The eight unharvested control plots allowed us to compare changes in fungal biomass, and soil pH and nutrients in the harvested plots to a baseline. AM fungal biomass decreased in both AM and EcM plots only in the first fall after timber harvesting, while EcM fungal biomass continued to decrease in both AM and EcM plots over three growing seasons. The mycorrhizal legacy also affected seedling success; AM seedlings had higher survival when planted in AM legacy plots, but there was no effect on EcM seedlings. AM and EcM seedling growth was unaffected by legacy. Patterns in foliar nitrogen (N) showed that although AM seedlings closer to plot edges may have acquired more N through mycorrhizal fungi relative to EcM seedlings at plot edges, they also had lower percent N, indicating a tradeoff between increased access to mycorrhizal networks and competition with proximity to live roots. Alternatively, this mycorrhizal legacy effect could have been driven by preexisting soil characteristics. Given that some AM tree species are sensitive to soil calcium (Ca), higher soil extractable Ca in AM legacy plots suggests a complex set of mechanisms driving seedling success beyond potential access to mycorrhizal fungi.

Optimizing soil organic carbon (SOC) sequestration in pasture systems requires understanding the carbon storage capacity across the entire soil profile. Though SOC concentrations typically decline with depth, the large volume of subsurface soil means it can constitute more than half of the total SOC pool. Therefore, it is important to consider deeper soil layers beyond those typically sampled (i.e., 0-30 cm). Here, we quantified the SOC stocks to 1 m on 12 Vermont (USA) pastures. This SOC stock averaged 12.9 kg C m-2 (range 8.8-19.2), which is consistent with both SOC storage in US grasslands and average stocks across Vermont, including forests. Environmental variables, including temperature, precipitation, and pH, were generally poor predictors of SOC stocks, though there was a positive relationship between clay content and surface SOC. We found that surface SOC stocks in these pastures were not a good predictor of deeper SOC stocks. Variability in SOC stocks, which increased with depth, may instead be influenced by untested factors. Depth to bedrock and accurate bulk density measurements substantially influence total SOC estimates and are important considerations for quantifying SOC. Our results suggest that the labor-intensive collection of deep soil cores is necessary to accurately quantify soil carbon. Pastures are tractable landscapes for managing soil carbon compared to forests, and intensive sampling provides a baseline for evaluating soil carbon sequestration in these systems.

Rising temperatures are accelerating permafrost thaw, exposing large soil organic carbon (SOC) stocks to microbial decomposition with implications for global climate. Understanding how permafrost carbon is stored and protected through associations with minerals is critical for predicting its vulnerability to decomposition upon thaw. However, how landscape age, substrate chemistry, and soil depth influence mineral associations remain relatively unexplored. We investigated organo-mineral associations in active layer and permafrost soils across a landscape age and geochemical gradient on Alaska's North Slope, spanning three deglaciated (similar to 11,500-125,000 years) and one unglaciated site. Using selective dissolution extractions, X-ray diffraction, and M & ouml;ssbauer spectroscopy, we characterized minerals and their relationship with SOC. The three recently deglaciated sites had low soil pH that decreased with age and greater abundances of pyrophosphate- and oxalate-extractable Al and Fe, whereas the oldest unglaciated site exhibited near-neutral pH, greater pyrophosphate-extractable Ca, and differences in mineralogy. Across sites, SOC was positively associated with Al and Fe mineral phases, with stronger relationships in acidic soils. Pyrophosphate-extractable Ca also showed strong relationships with SOC at the acidic sites (up to similar to 10 & times; greater), suggesting that Ca-mediated protection may operate beyond traditionally recognized high-pH soils. Permafrost soils showed depth-related changes in pH, SOC, and Fe mineralogy, suggesting chemically active, heterogeneous layers may shape mineral dynamics and associated carbon. Our results highlight how landscape age, parent material, and depth create distinct geochemical environments that govern mineral-organic associations. As thaw exposes soil to new conditions, these mineral-mediated protection mechanisms may be altered, potentially affecting the permafrost carbon-climate feedback.

Abstract. We report direct evidence of an inorganic carbon pool that is sensitive to permafrost thaw–siderite. Notably, siderite was absent in the seasonally thawed active layer above permafrost, implying this inorganic carbon reservoir may be lost upon thaw. Assuming siderite weathers quickly once permafrost thaws, we estimate siderite weathering could release carbon equivalent to about 10% of permafrost organic carbon losses over the next half-century. However, studies are needed to understand how widespread siderite is and to quantify its actual weathering rate. This study is submitted as a LESSONS Report because it documents a surprise result that opens up opportunities for new science.

Mycorrhizal fungi are important drivers of soil organic matter dynamics, but it can be difficult to isolate the effects of the fungi themselves from covarying traits of their host trees. For example, many trees with an evergreen leaf habit associate with ectomycorrhizal (ECM) fungi, while many deciduous tree species associate with arbuscular mycorrhizal (AM) fungi. Because leaf habit influences the quantity and quality of organic matter inputs to soil, it is often an important factor in soil carbon and nitrogen dynamics, and thus can mask the effects of mycorrhizal fungi on soil organic matter processes. We evaluated how tree mycorrhizal associations and leaf habit separately influence the amount and composition of mineral-associated organic matter (MAOM) and particulate organic matter (POM) in forest soils in New Hampshire and Vermont, USA. We measured carbon (C) and nitrogen (N) concentrations and C/N ratios of three soil density fractions beneath six tree species that vary in mycorrhizal association and leaf habit. We found lower concentrations of MAOM C and N beneath evergreen vs. deciduous trees, but only for tree species associating with AM fungi. Further, MAOM C/N was higher beneath evergreen trees and beneath trees with ECM fungi rather than AM fungi. These results add to the growing body of support for mycorrhizal fungi as mediators of soil organic matter dynamics, suggesting that the MAOM fraction is more sensitive to leaf habit beneath AM-associated versus ECM-associated trees. Because MAOM decomposition is thought to be less responsive than POM decomposition to changes in soil temperature and moisture, differences in the tendency of AM- and ECM-dominated forests to support MAOM formation and persistence may lead to systematic differences in the response of these forest types to ongoing climate change.

Mycorrhizal fungi form symbiotic relationships with most plant species, facilitating nutrient acquisition while consuming a significant fraction of the plant's photosynthetic carbon (C), which we define as the mycorrhizal C cost. Drivers of the mycorrhizal C cost, which is crucial for predicting environmental impacts on plant productivity, remain under-explored and difficult to quantify. Ecosystem models that incorporate mycorrhizae can offer insights into mycorrhizal C cost dynamics, but their predictions have rarely been validated against empirical data. In this study, we used the Myco-CORPSE model, which explicitly simulates mycorrhizal processes alongside soil carbon and nitrogen cycling, to investigate the drivers of mycorrhizal C cost in temperate forests. Applying this model to over 1,800 forest inventory plots across the eastern United States, we found that the simulations matched published data, showing higher C allocation to ectomycorrhizal (ECM) fungi (16.0% of net primary production (NPP)) compared to arbuscular mycorrhizal (AM) fungi (5.8% of NPP). Further analysis showed that mixed forests, co-dominated by both AM and ECM trees, allocated less C to mycorrhizal fungi compared to forests dominated by either AM or ECM fungi alone, due to complementary nutrient acquisition strategies. Elevated Nitrogen (N) deposition and higher temperatures reduce mycorrhizal C costs, favoring AM strategies. Conversely, elevated CO2 (eCO(2)) increased plant N demand and mycorrhizal C costs, favoring ECM strategies that access organic N sources. These findings underscore the critical role of mycorrhizal functional diversity in plant nutrient acquisition and C dynamics, providing new insights into how mycorrhizal symbioses respond to global change.

The rhizosphere contains diverse groups of bacteria and fungi living near plant roots and mycorrhizal hyphae whose composition and function are key drivers of ecosystem and biogeochemical processes. Despite rich literature on rhizosphere communities, no studies have examined the drivers of rhizosphere communities across plants or soil types in the tundra. We collected 513 root samples from 141 individual plants representing six plant species and three mycorrhizal association types across four glacial histories in Northern Alaska. Glacial drifts ranged from 11000 to 4.5 million years since deglaciation representing a gradient in glacial history and mineralogical weathering. We found that glacial history, a strong proxy for soil mineralogy, explained the most variation in rhizosphere bacterial communities (13.3%) while interactions between glacial history and host plants explained the most variation in fungal rhizosphere communities (11.6%). We found strong correlations between ectomycorrhizal and rhizosphere communities across spatial scales and sites for the shrub Betula nana (30.7%-54.7% correlated), and that ectomycorrhizal composition was most similar among root fragments of the same plant, followed by plants at the same site, and plants at different sites. This work serves to advance the ecological understanding of rhizosphere and ectomycorrhizal communities in response to shrubification.

An important control on long-term soil organic carbon (SOC) storage is the adsorption of SOC by short-range-ordered (SRO) minerals. SRO are commonly quantified by measuring oxalate-extractable metals (Mox = Alox + ½ Feox), which many studies have shown to be positively correlated with SOC. It remains uncertain if this organo-mineral relationship is robust at the global scale, or if capturing regional differences is needed to maximize model accuracy. We used a global synthesis of Alox and Feox data to test their role in controlling SOC abundance across regions. We compiled 37,344 individual soil horizon measurements, with soil depth ranging between 0 and 200 cm, from 11,122 profiles. We used the Holdridge Life Zones, which are characterized by biotemperature, precipitation, and potential evapotranspiration, to group the soil profiles by their climatic conditions that also correlate with other important soil-forming factors. Based on linear mixed-effects models, we found a positive relationship between Mox and SOC across regions and depths, accounting for 49

Mycorrhizal associations are key drivers of soil biogeochemistry, but previous studies have focused almost exclusively on ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) associations. Ericoid mycorrhizal (ErM) shrubs frequently occur in forest understories and are expanding in response to disturbance, but are rarely considered in biogeochemical frameworks. We investigated the relationships of understory ErM shrubs and overstory trees on carbon (C) and nitrogen (N) in soil organic matter fractions in a southern Appalachian temperate forest. We sampled the 0–10 cm mineral soil layer from 43 plots at the Coweeta Hydrologic Laboratory, across gradients in overstory EcM dominance and understory ErM shrub biomass. Soil C:N ratios increased with both increasing EcM dominance and increasing ErM shrub biomass. However, total particulate organic matter (POM) C, and the proportion of C and N held in POM increased with increasing ErM shrub biomass, but not with increasing EcM dominance. In contrast, mineral-associated organic matter (MAOM) C and N were negatively associated with EcM dominance, but were not related to ErM shrubs. Our findings suggest that ErM shrubs facilitate POM formation while AM trees promote MAOM formation. Because ErM shrub biomass represents a small fraction of total forest biomass, our work provides evidence that ErM shrubs have an outsized effect on soil organic matter, which advocates for their inclusion in mechanistic studies and biogeochemical frameworks.

Mangrove ecosystems are highly productive, sequestering large amounts of carbon from the atmosphere while emitting relatively low levels of greenhouse gases. Large amounts of organic matter and nitrogen from river discharge may stimulate production and emission of greenhouse gases including N _2 O, thus reducing the blue carbon storage capacity of mangrove wetlands. However, it is unclear how the input of organic matter and nitrogen affect the emission fluxes of these greenhouse gases in mangrove wetlands. In this study, we compared the effluxes of N _2 O between the estuarine and non-estuarine mangrove wetlands near the seven major rivers on Hainan Island of China. The averaged N _2 O emission flux in the estuarine mangrove wetlands was 6.9 times than the non-estuarine mangrove wetlands. In the non-estuarine mangrove wetlands N _2 O emission flux increased significantly with total nitrogen content in the sediments. In contrast, organic carbon (OC)/organic nitrogen (ON) ratios were associated with N _2 O emissions in the estuarine mangrove wetlands. As rivers bring in more organic matter with high ON content, more N _2 O emissions occur in estuarine areas. OC in sediments are from different sources between estuary (fresh water dissolved OC (DOC)) and non-estuary (marine DOC) area. A higher OC/ON ratio leads to higher nitrification and lower N _2 O consumption enzyme activities in estuary areas compared with non-estuaries, which contributes extra N _2 O fluxes. Our study provides a direct process assessment of the response of natural N _2 O emissions to sediment chemistry changes caused by river inputs.

The timescales over which soil carbon responds to global change are a major uncertainty in the terrestrial carbon cycle. Radiocarbon measurements on archived soil samples are an important tool for addressing this uncertainty. We present time series (1969-2023) of radiocarbon measurements for litter (Oi/Oe and Oa/A) and mineral (0-10 cm) soils from the Hubbard Brook Experimental Forest, a predominantly hardwood forest in the northeastern USA. To estimate soil carbon cycling rates, we built different autonomous linear compartmental models. We found that soil litter carbon cycles on decadal timescales (Oi/Oe: ~7 years), whereas carbon at the organic-mineral interface (Oa/A), and mineral soil (0-10 cm) carbon cycles on centennial timescales (~104 and 302 years, respectively). At the watershed-level, the soil system appears to be at steady-state, with no observed changes in carbon stocks or cycling rates over the study period, despite increases in precipitation, temperature, and soil pH. However, at the site-level, the Oi/Oe is losing carbon (-15 g C m-2 year-1 since 1998). The observed decline in carbon stocks can be detected when the Oi and Oe layers are modeled separately. This pattern suggests that the rapidly cycling litter layer at the smaller scale is responding to recent environmental changes. Our results highlight the importance of litter carbon as an "early-warning system" for soil responses to environmental change, as well as the challenges of detecting gradual environmental change across spatial scales in natural forest ecosystems.

Tree-mycorrhizal associations are associated with patterns in nitrogen (N) availability and soil organic matter storage; however, we still lack a mechanistic understanding of what tree and fungal traits drive these patterns and how they will respond to global changes in soil N availability. To address this knowledge gap, we investigated how arbuscular mycorrhizal (AM)- and ectomycorrhizal (EcM)-associated seedlings alter rhizodeposition in response to increased seedling inorganic N acquisition. We grew four species each of EcM and AM seedlings from forests of the eastern United States in a continuously 13 C-labeled atmosphere within an environmentally controlled chamber and subjected to three levels of 15 N-labeled fertilizer. We traced seedling 15 N uptake from, and 13 C-labeled inputs (net rhizodeposition) into, root-excluded or -included soil over a 5-month growing season. N uptake by seedlings was positively related to rhizodeposition for EcM- but not AM-associated seedlings in root-included soils. Despite this contrast in rhizodeposition, there was no difference in soil C storage between mycorrhizal types over the course of the experiment. Instead root-inclusive soils lost C, while root-exclusive soils gained C. Our findings suggest that mycorrhizal associations mediate tree belowground C investment in response to inorganic N availability, but these differences do not affect C storage. Continued soil warming and N deposition under global change will increase soil inorganic N availability and our seedling results indicate this could lead to greater belowground C investment by EcM-associated trees. This potential for less efficient N uptake by EcM-trees could contribute to AM-tree success and a shift toward more AM-dominated temperate forests.

We present a mechanistic model of coexistence among a mycorrhizal fungus and one or two plant species that compete for a single nutrient. Plant-fungal coexistence is more likely if the fungus is better at extracting the environmental nutrient than the plant and the fungus acquires carbon from the plant above a minimum rate. When they coexist, their interaction can shift from mutualistic to parasitic at high nutrient availability. The fungus is a second nutrient source for plants and can promote the coexistence of two plant competitors if one is better at environmental nutrient extraction and the other is better at acquiring the nutrient from the fungus. Because it extracts carbon from both plants, the fungus also serves as a conduit of apparent competition between the plants. Consequently, the plant with the lower environmental nutrient extraction rate can drive the plant with the higher environmental nutrient extraction rate extinct at high carbon supply rates. This model illustrates mechanisms to explain several observed patterns, including shifts in plant-mycorrhizal growth responses and coexistence along nutrient gradients, equivocal results among experiments testing the effect of mycorrhizal fungi on plant diversity, and differences in plant diversity among ecosystems dominated by different mycorrhizal groups.

When digestates from anaerobic digestion of crop residues are added to soil, a considerable body of information indicates that soil organic carbon (SOC) levels are comparable to those when crop residues are left in the field. This occurs although the amount of digestate added to soil is diminished by digestion and implies that digestion increases the proportion of carbon inputs stabilized as SOC. Here we examine the likelihood and implications of these features being manifested for soil application of high lignin-fermentation byproduct (HLFB) from liquid biofuel production. We show that steady-state SOC levels are much less sensitive to crop residue removal with HLFB return than without it, and provide an example supporting the feasibility of foregoing process energy and coproduct revenue when HLFB is returned to the soil. Informed by this review and analysis, we expect with moderate confidence that long-term SOC levels for soils amended with HLFB from some liquid cellulosic biofuel processes will not be substantially lower than those occurring when crop residues are left in the field. We have high confidence that the economically optimum rate of fertilizer nitrogen (N) application and N _2 O emissions will be lower at most sites for HLFB return to the soil than if crop residues were left in the field. We estimate that the per hectare N demand for processing crop residues to liquid biofuels is about a third of the per hectare demand for crop production, giving rise to an opportunity to use N twice and thereby realize cost savings and environmental benefits. These observations support but do not prove the hypothesis that a ‘win-win’ is possible wherein large amounts of liquid biofuel feedstock can be obtained from crop residues while improving the economics and sustainability of food and feed production. A research agenda aimed at exploring and testing this hypothesis is offered.

We measure the age of soil organic matter (SOM) in soil depth profiles using fallout radionuclide (FRN) chronometry. The FRN age model quantifies the well-known lag in ∆14C which is the time between biological carbon fixation and its incorpration into SOM 1–3. The FRN model also reveals sharp excursions in ∆14C at depth to extremely old ages, which we attribute to legacy petrogenic, pedogenic, or passive carbon pools because corresponding soil carbon fluxes based on legacy ∆14C do not reconcile with independent ecosystem measures. FRN ages may thus resolve a foundational uncertainty regarding ages of SOM, agreeing with mass balance of carbon pools and fluxes at both soil profile and global scales 4,5, 𝛿13C experiments at profile and global scales 6,7, compilation of global experimental soil carbon fluxes 8, and independent 35Cl bomb-pulse dating 9. We confirm that the pool of SOM relevant to climate transition is up to 10 times younger than deduced from global ∆14C turnover times. We thereby estimate that temperate and tropical forest soils store ca. 10% of Net Primary Productivity (NPP) over decadal timescales. These observations demonstrate that SOM cycling is more dynamic than deduced from 14C and may respond rapidly to both climate change as well as mitigation efforts aimed at sequestering atmospheric CO2 over annual to decadal timescales.