Rapid warming in the Arctic threatens to amplify climate change by releasing the region's vast stocks of soil carbon to the atmosphere. Increased nutrient availability may exacerbate soil carbon losses by stimulating microbial decomposition or offset them by increasing primary productivity. The outcome of these competing feedbacks remains unclear. Here we present results from a long-term nutrient addition experiment in northern Alaska, United States, coupled with a mechanistic isotope-tracing experiment. We found that soil carbon losses observed during the first 20 years of fertilization were caused by microbial priming and were completely reversed in the subsequent 15 years by shrub expansion which promoted an increasingly efficient carbon-nitrogen economy. Incorporating long-term stoichiometric responses in Earth system models will improve predictions of the magnitude, direction and timing of the Arctic carbon-climate feedback. Arctic warming is thought to lead to large losses in soil carbon stocks. Here a 35-year-long fertilization experiment in Alaska shows that increased shrub productivity and changes in plant-microbial feedbacks may eventually reverse trends of carbon loss and restore the soil carbon sink.

ABSTRACT Rapid climate change in the Arctic is altering microbial structure and function, with important consequences for the global ecosystem. Emerging evidence suggests organisms in higher trophic levels may also influence microbial communities, but whether warming alters these effects is unclear. Wolf spiders are dominant Arctic predators whose densities are expected to increase with warming. These predators have temperature-dependent effects on decomposition via their consumption of fungal-feeding detritivores, suggesting they may indirectly affect the microbial structure as well. To address this, we used a fully factorial mesocosm experiment to test the effects of wolf spider density and warming on litter microbial structure in Arctic tundra. We deployed replicate litter bags at the surface and belowground in the organic soil profile and analyzed the litter for bacterial and fungal community structure, mass loss, and nutrient characteristics after 2 and 14 months. We found there were significant interactive effects of wolf spider density and warming on fungal but not bacterial communities. Specifically, higher wolf spider densities caused greater fungal diversity under ambient temperature but lower fungal diversity under warming at the soil surface. We also observed interactive treatment effects on fungal composition belowground. Wolf spider density influenced surface bacterial composition, but the effects did not change with warming. These findings suggest a widespread predator can have indirect, cascading effects on litter microbes and that effects on fungi specifically shift under future expected levels of warming. Overall, our study highlights that trophic interactions may play important, albeit overlooked, roles in driving microbial responses to warming in Arctic terrestrial ecosystems. IMPORTANCE The Arctic contains nearly half of the global pool of soil organic carbon and is one of the fastest warming regions on the planet. Accelerated decomposition of soil organic carbon due to warming could cause positive feedbacks to climate change through increased greenhouse gas emissions; thus, changes in ecological dynamics in this region are of global relevance. Microbial structure is an important driver of decomposition and is affected by both abiotic and biotic conditions. Yet how activities of soil-dwelling organisms in higher trophic levels influence microbial structure and function is unclear. In this study, we demonstrate that predicted changes in abundances of a dominant predator and warming interactively affect the structure of litter-dwelling fungal communities in the Arctic. These findings suggest predators may have widespread, indirect cascading effects on microbial communities, which could influence ecosystem responses to future climate change.

Understanding how soil nitrogen (N) turnover responds to temperature and soil moisture alterations is important for accurately predicting responses of soil N availability and plant productivity to global climate change. However, few studies had explored such questions under natural conditions. We investigated effects of temperature and moisture on soil gross N turnover with an annual scale field experiment in an Inner Mongolian grassland and a laboratory incubation experiment. In the field experiment, on the annual scale both gross ammonification and nitrification rates showed a hump-shaped response to temperature, with maximum rates occurring around 10 degrees C and 5 degrees C, respectively. Gross ammonification rates did not respond to soil moisture, and gross nitrification rates decreased first and then increasing with soil moisture, which the threshold was 35% water holding capacity (WHC). Totally, soil temperature explained more variation than moisture in gross ammonification (0.30 VS 0.04) and nitrification (0.24 VS 0.14) rates. On the seasonal scale, both soil temperature and moisture showed generally significant effects on gross N turnover. Grazing had no effects on soil temperature and moisture sensitivity of gross N turnover on the annual scale, but increased the sensitivity of gross ammonification to soil moisture during the spring freeze-thaw period. In the laboratory incubation experiment, gross ammonification and nitrification rates increased with the increase of temperature, although there was no difference between 20 degrees C and 30 degrees C. Gross ammonification rates reached a maximum at 50% WHC, while gross nitrification rates did not respond to soil moisture until 100% WHC. Soil temperature explained more variation than moisture in gross ammonification (0.71 VS 0.01) and nitrification (0.37 VS 0.25) rates. Our results imply that climate warming may have greater impact on gross nitrogen turnover compared to changes in rainfall, however it remains great uncertainty due to the low explanation of soil temperature and moisture.

Semi-arid regions support the majority of global wheat production, but they are threatened by continued soil erosion and increased risk of drought and extreme temperatures from climate change. Enhancing soil organic matter (SOM) is a key adaptation strategy for sustaining agriculture in these regions because it can improve water retention and nutrient cycling. When seeking to increase SOM, growers face a dilemma, known as the 'soil carbon (C) dilemma' (Janzen, 2006): How can SOM be increased, while also increasing the release of nutrients that accompanies decomposition? Our observational study examined whether growers could address this dilemma through intensification with diverse grain crops. We specifically tested whether incorporating legumes into a continuous rotation influences the form and amount of SOM as well as productivity in farms of the central Great Plains region of the U.S. by contrasting three, no-till rotation systems: 1) conventional wheat-fallow; 2) continuous grain-only rotations, and 3) continuous grain rotations that incorporate a legume crop. We sampled on-farm fields and experimental agricultural research station plots that had received one of these rotations for at least eight years. We compared yield, soil organic C (SOC), and nitrogen (N) stocks in bulk soil to 1 m, as well as SOC and N in particulate organic matter (POM) and mineral-associated organic matter (MAOM) in surface soils (0-10 cm) among the different rotations. We also estimated the long-term effects of the rotations on SOC fractions through a simulation of each rotation with the MEMS 1.0 model, verified with measured data.We found that intensifying the rotation with continuous grains led to 1.5-fold increase in aggregate size but did not change SOC stocks. Incorporating a legume to the continuous grain rotation resulted in 1 Mg C ha(-1) more SOC on average in surface soil compared to wheat-fallow rotations. No significant soil changes among rotations were observed at depth. Longer-term simulations of these cropping systems suggest that including legumes could allow for 10% greater SOC gains over time compared to wheat-fallow. Since legume incorporation also increased crop yields across the full rotation cycle, our results support our hypothesis that growers can solve the 'soil C dilemma' by simultaneously increasing the quantity and N-content of soil input through continuous cropping with both grains and legumes.

Soil health is a promising lens through which to approach land management, having the potential to serve as a descriptor of biophysical processes and as an effective communication tool across stakeholders. However, this potential has been largely unrealized due to difficulty in quantitatively assessing soil health and linking those assessments to outcomes. Here we discuss many multiple persistent obstacles to quantitative soil health assessment and outline a suite of analyses to help address those obstacles. Specifically, we propose a quantitative approach to developing and selecting soil health indicators that help connect management-induced changes in soil health to specific outcomes (e.g., yield or water quality). To demonstrate the utility of this approach, we perform a small case study using published data from North Carolina and New York cropping systems. Additionally, we outline how this approach is scalable and flexible enough to integrate future soil health metric development. The proposed approach stands to provide a quantitative, empirical basis for future measurement, assessment, and interpretation of soil health.

Plant roots add carbon (C) -rich rhizodeposits to the soil, which can alter microbial activity and nitrogen (N) cycling with implications for N availability and uptake by plants. We evaluated root architecture, rhizodeposit C, and microbial community structure and function across a breeding gradient of twelve winter wheat genotypes and examined how these rhizosphere traits were related to the availability and uptake of N from fresh cover crop residues in the soil. We traced wheat-derived C into soil and microbial pools using continuous isotopic labelling (13C–CO2) and applied 15N labelled plant residues to quantify plant and microbial uptake of residue-derived N. Wheat genotypes differed in root C allocation patterns, influencing N cycling. Thicker roots released more C into soil, which enhanced N mineralization through stimulation of the microbial biomass. Microbial biomass increased N-cycling enzyme activity and residue N-uptake by wheat. Microbial communities did not differ between wheat genotypes but were strongly related to patterns in root C allocation, and several genera showed strong relationships with root C deposition and N uptake. The microbial community associated with extractable root-derived C was structurally different from the community associated with residue N uptake, indicating that N-cycling and mineralization was not necessarily carried out by the same microbial community members as those stimulated by rhizosphere C inputs. Our results indicate that differential patterns of rhizodeposition and associated belowground C allocation strategies in winter wheat can alter microbial communities and influence cycling and plant availability of residue N. Ecologically-based nutrient management in agricultural systems should consider the role of crop root traits and associated microbiomes to optimize soil nutrient dynamics.

The implementation of soil health-promoting practices, such as cover cropping and compost application, has important implications for nutrient cycling and management in agroecosystems. At the same time, plant belowground carbon (C) allocation patterns can influence nutrient cycling and availability in soil through changes to the microbial community, but the effects may depend on the crop genotype and management practices in place. We evaluated belowground C allocation patterns using C-13 labeling and root architecture in two genotypes of winter wheat (Triticum aestivum) with different levels of exudation and belowground allocation strategies in soils with contrasting compost amendment legacy (108.7 Mg ha(-1) every 2 years over 10 years vs. no compost). We also measured microbial community structure and function in the rhizosphere and quantified uptake of residue-derived N from N-15-labelled cover crop residues. We found an interactive effect between soil management and genotype, where in the no-compost soil, the high-exudation genotype (Snowmass) increased exudation by over 4-fold, while the low-exudate genotype (Byrd) increased only 2-fold. While we did not observe genotype differences in rhizosphere enzyme activity or dissolved N pools, residue N uptake was 1.8 times greater for Snowmass in the compost-amended soil. There were more rhizosphere microbial taxa associated with the high-exudate genotype (Snowmass); nine bacterial and seven fungal families were indicative of Snowmass, versus one bacterial and four fungal families for Byrd. Our results suggest that the high-exudation strategy can influence the rhizosphere microbial community, and lead to greater short-term residue N uptake in high SOM soil. By directly linking root architecture, exudation, microbial communities, and N mineralization and uptake dynamics, this work demonstrates that plasticity in root C allocation is genotype-specific and influences microbial communities and nutrient cycling depending on the soil health context.

The adoption of conservation agriculture has gained considerable attention due to growing interest in managing soil biological diversity and overall soil health. However, there is limited understanding of how practices such as conservation tillage and residue retention affect soil biota across different spatial scales and are associated with alterations to other soil properties. In this study, we examined changes in soil physicochemical properties, soil macrofauna, and microbial communities at two soil depths (0-10 cm, and 10-20 cm) in a 6-year field experiment manipulating tillage and maize residue management. The following treatments were included in the experiment: no-till with residue retention; no-till with residue harvest; conventional tillage with residue retention; and conventional tillage with residue harvest. Macrofauna taxa were identified and estimated visually, while bacterial and fungal communities were identified and analyzed using high throughput sequencing and multivariate statistics including non-metric dimensional scaling and indicator species analysis. Soil physicochemical properties measured include soil carbon and nitrogen, soil moisture, permanganate oxidizable carbon, available soil phosphate, pH, electrical conductivity and aggregate stability. Residue retention increased macrofauna abundance and diversity across soil depths and tillage treatments. Fungal diversity was also highest under residue retention in the topsoil (0-10 cm), while bacterial diversity was generally higher under conventional tillage. Residue retention was the main driver of macrofauna and microbial community composition, while an interaction between tillage and residue management indicated that the effect of tillage on microbial communities was most pronounced when residues were retained. Soil carbon and nitrogen, aggregate stability, permanganate oxidizable carbon, soil moisture content, available soil phosphate and soil electrical conductivity were all enhanced under residue retention in the topsoil. Indicator species analysis suggested that macrofauna taxa belonging to Annelida, Aranaea and Chilopoda together with bacterial phyla Fibrobacteres and fungal phyla Rozellomycota were indicators for no-till combined with residue retention, while Coleoptera, Spirochaetae and Basidiomycota were indicators for conventional tillage with residue retention. Multivariate analyses suggested that total macrofauna abundance, soil carbon and pH were strongly associated with bacterial and fungal community composition in the topsoil layer. Co-inertia analysis indicated significant covariation between soil physicochemical, macrofauna, bacterial and fungal datasets, suggesting a strong association between different soil parameters and cascading effects of management on multiple soil properties. Our findings demonstrate that residue retention enhances soil biological, physical and chemical properties and that communities of soil macroand microorganisms tend to respond in similar ways to these management interventions.

The dissolved organic matter pool is fundamental to regulating soil properties, but there is currently a limited understanding of its molecular composition due to its complexity. Mass spectrometry-based environmental metabolomics is a promising tool for discerning which components of the dissolved organic matter pool may be relevant for and/or responsive to ecosystem functioning. Effectively an ecosystem metabolome integrating biotic signals from macro and microorganisms along with abiotic and physical controls on molecular composition of the organic matter pool. A liquid chromatography time-of-flight mass spectrometry platform with an untargeted (data-driven) processing workflow was used to investigate the ecosystem metabolome in arctic soils from shrub, tussock, and wet sedge vegetation types. It was found that season was a more important driver of DOM molecular composition than vegetation type and that although the largest shift in DOM quantity takes place during a winter thaw, the largest shift in molecular composition took place during the freeze-up, across the late summer to early winter transition. Additionally, facets of soil extraction and sample preparation procedure are reviewed and recommendations on the processes for external validation and quality control in future environmental metabolomics studies are made.

Soil organic matter (SOM) plays a central role in mediating soil productivity through its impacts on nutrient cycling and retention, aggregate stability and water retention. Thus, management techniques or technologies including novel soil amendments could benefit farmers through the accumulation of carbon (C) and other nutrients in SOM. However, these same inputs can also lead to accelerated mineralization of native SOM through the process known as priming. This unresolved paradox may be due to the limited understanding of how different SOM fractions respond to priming and in which direction. In this study, we examine the response of functionally distinct SOM fractions to priming when soils are amended with lactobionate, a low molecular weight sugar acid byproduct of cheese manufacturing. Liquid-based 13C lactobionate was added to an agricultural silty loam soil to study its persistence, priming effects, and response of different SOM fractions to lactobionate over 84 days. Cumulative soil carbon dioxide (CO2) was greater in lactobionate-amended soils versus control and by the end of the experiment, 53% of added lactobionate was mineralized. In total, positive priming of 40% of extant SOM was observed from 14 to 84 days. Lactobionate-induced changes to SOM fractions were determined at days 14, 28, 56 and 84 of the incubation to examine if and how priming altered the distribution of C between fast and slowcycling SOC fractions. In response to lactobionate, the total C content of the water extractable organic matter (WEOM) fraction initially increased by 100% from the dissolved lactobionate we added, but then declined and at a faster rate than other SOM fractions. In addition, the total C of the light-fraction particulate organic matter (LFPOM) fraction also declined. At the same time, we observed total C increases in the slower-cycling sand-sized POM (H-POM) and mineral-associated organic (MAOM) C fractions, in response to lactobionate additions. We also saw a marginal increase in total soil C in the lactobionate-amended soils. Our findings therefore suggest that the application of lactobionate to soils may induce positive priming of the faster cycling LF-POM and WEOM fractions, but also concurrent gains in the H-POM and MAOM C fractions associated with long-term persistence and relative resiliency to disturbance with no net loss of total soil carbon. Thus, the application of low-molecular weight C-based materials such as lactobionate presents an avenue to building more persistent SOM through its impacts on the internal cycling and transformation of SOM fractions.

Soils play a crucial role in the fight against climate change. Through proper management, they can contribute significantly to atmospheric carbon (C) drawdown. On the other hand, greenhouse gas emissions from soils may trigger strong positive feedbacks to global warming. One key challenge is that soil C accrual requires nitrogen (N), but the addition of N fertilizers has cascading impacts on nitrous oxide (N2O) emissions and atmospheric and water pollution. Robust understanding and models are needed to predict outcomes of climate and land-use changes on soil C-N biogeochemistry to guide solutions, policies, and investments. The world is looking to ecosystem ecologists to advise large-scale efforts to co-manage soil C and N stocks. Are we ready for the challenge? Globally, plant C inputs to soils need to increase and soil C outputs via microbial C mineralization need to decrease, while N mineralization and internal N recycling need to be maintained to support plant productivity and avoid detrimental environmental impacts. Do we know where and how to achieve these outcomes? As the field of ecosystem ecology has matured, we have gained a deep understanding of the interconnected C, N, and water cycles in terrestrial ecosystems. We can articulate and model many of the underlying mechanisms that drive the coupling and decoupling of these cycles. Yet, the complexity of the ecosystems and the diverse conditions in which they operate impede us from predicting how these mechanisms interact to drive emergent processes and patterns. Unraveling how soil C storage and N recycling will respond to climate change in specific locations and under specific land uses is a major challenge. We need an overarching theoretical framework to address this challenge and, to be truly transformative, this new framework must be quantitatively translated into mathematical models of C, N, and water dynamics that are verifiable and able to accurately reproduce biogeochemical dynamics from the poles to the tropics. A confluence of recent developments is leading us toward a breakthrough in our ability to model and predict soil C and N processes at the ecosystem scale. First, our mechanistic understanding of soil organic matter (SOM) dynamics has been greatly advanced (Basile-Doelsch et al., 2020) enabling a new generation of SOM models based on measurable pools and fluxes (Zhang et al., 2021 and references therein). However, these advances have been largely C-centric with less attention focused on understanding the shifts in soil N dynamics across ecosystems in response to climate and interactions with C availability. Second, there is an emerging emphasis on SOM dynamics in subsoils (i.e., soils below the top c. 20–30 cm; or below the A horizon), which hold more than half of the total SOM but have not yet been explicitely incoporated into most ecosystem models. Finally, the establishment of large-scale research and observatory networks provide data and samples to parameterize and test empirical and conceptual models across ecosystems and soil depths, creating exciting new opportunities. Based on current understanding, we propose advancing our MEMS framework, which links plant input to microbial processing and mineral-associated SOM formation (Cotrufo et al., 2013), into a new broader conceptual and quantitative framework. Our In-N-Out framework identifies a hierarchical and interactive structure of controls on soil C and N cycling (Figure 1a). We expect resource limitation to be the major constraint on biogeochemical processes. Plant versus microbial activity limitation is a key determinant of soil C storage and N recycling, driving the dynamic coupling/decoupling of C and N cycles, within ranges imposed by the relatively constrained C:N stoichiometry of microbial and faunal biomass, and organic matter pools in soil (Cleveland & Liptzin, 2007). Our framework represents multiple hypotheses that can be empirically tested using quantifiable proxies (Figure 1b) and represented in ecosystem models (e.g., Zhang et al., 2021). Our understanding of bulk soil C storage has significantly advanced (Basile-Doelsch et al., 2020; Wiesmeier et al., 2019). However, while we can now identify the drivers of soil organic C at different spatial scales, we lack a hierarchical structure of controls operating across different ecosystems. We posit that these specific controls are the result of interactions between climate, vegetation, and soil traits. In particular, we expect climate to be the first overarching control under extreme climates, but vegetation, microbial and soil traits to become direct drivers under mesic climates (Figure 1a). In soils, organic C and N are highly intertwined. Organic C is stored in a myriad of different chemical compounds, many of which contain N and/or are formed through microbial activity that demands N. Furthermore, SOM contains more N per unit C than plant biomass. Thus, soil organic C storage is linked to N availability and can result in N immobilization in SOM with feedbacks to plant productivity. Conversely, when N is available in excess of plant and microbial demand, the C and N cycles decouple, and N is lost from the system. Soil organic matter stocks, their formation, persistence, and response to N availability and disturbances can be better described if SOM is broadly divided into a particulate organic matter (POM) and a mineral-associated organic matter (MAOM) pool (Lavallee et al., 2020). Light POM is predominantly of plant origin and thus contains many structural C-compounds with low N content. It persists (<50 years) in soil because of microbial inhibition to decomposition through various mechanisms including inherent POM biochemical recalcitrance, physical protection in aggregates, and other climatic and environmental constraints (e.g., limiting temperatures, water, O2, or nutrients). MAOM has a higher share of microbial products richer in N. It persists (10–1000 years) in soil because of chemical bonding to minerals and physical protection in fine aggregates. We hypothesize that at ecosystem scales the relative accumulation of POM reflects microbial limitation and leads to limitations of N availability (e.g., peat soils), while the relative dominance of MAOM reflects plant productivity limitation, and opening of the N cycle (e.g., dryland soils; Figure 1a). Ecosystems where plant inputs are balanced by microbial outputs have an equal share of POM and MAOM, maintaining soil C stocks and recycling N to sustain productivity. Additionally, bulk soil, POM, and MAOM C:N stoichiometry can be used as a proxy for the N demand of C storage (Figure 1b). The degree to which N is recycled versus lost within soils can be quantified with stable isotopes. The natural abundance of the 15N:14N isotopes in soil relative to atmospheric N2 has been used as an indicator of terrestrial N cycling dynamics because N loss pathways (nitrification, denitrification, and ammonia volatilization) fractionate against the heavier 15N isotope while biological N fixation does not (Hogberg, 1997). Thus, the 15N enrichment of SOM can be used as a proxy for the degree of openness of the N cycle (Figure 1b), or the degree of C and N cycle decoupling. Hotter, drier climates tend to have more open N cycling than cooler, wetter environments (Amundson et al., 2003). Within climate regions, vegetation can influence soil δ15N because different plant N acquisition strategies. Vegetation is more likely to influence total δ15N in soils with larger proportions of N in POM, which reflects more recent and less processed litter inputs, than systems with larger proportions of N in MAOM, which has undergone microbial transformations and is more consistently enriched in 15N. We expect microbial-limited, POM-dominated soil to have a lower 15N enrichment than plant input-limited, MAOM-dominated soil (Figure 1a). There have been limited analyses of the relative structure of these multiple drivers on N cycling in terrestrial systems at different soil depths and their interaction with soil C dynamics, leading to an important knowledge gap. We also hypothesize (Figure 1a) that subsurface SOM dynamics are inherently C input-limited and more strongly affected by soil traits (e.g., texture and mineralogy) than climate (Mathieu et al., 2015). Physicochemical and biological properties differ markedly between subsoils and topsoils and, while the exact delineation between topsoil and subsoil can vary across soil types, there is an increasing evidence that models of soil C and N storage and cycling should consider topsoils and subsoils separately (Zhang et al., 2021). Subsoils typically have a lower organic matter content and are characterized by 14C ages thousands of years greater than the surface, lower C/N ratio, and a higher natural abundance of the heavy C and N isotopes than topsoils (Rumpel & Kogel-Knabner, 2011). Microbial biomass and activity are lower in subsoil than in topsoil and subsoil microbes rely more on SOM as a C source than on fresh plant inputs. Deep SOM is thought to be made mostly of microbial products. Plant inputs are limited and lignin, and POM generally does not accumulate in subsoil, despite lower litter mass loss rates having been reported at depth. These findings point to a higher degree of SOM recycling at depth. Additionally, when subsoils are treated with de-mineralizing agents (i.e., hydrofluoric acid), they lose a much higher proportion of their C than topsoils, suggesting a higher degree of mineral association at depth (Rumpel & Kogel-Knabner, 2011). We encourage the use of large datasets from ecosystems around the world to test this hierarchical framework of C and N cycling controls, using the proposed proxies (Figure 1b) and others. We hope the In-N-Out framework can help advance understanding of the feedbacks between climate, land cover, and soil C and N cycling, enabling us to more accurately predict outcomes of climate and land-use changes on soil biogeochemistry to guide solutions, policies, and investments. This work was supported by NSF DEB Award #2016003. No data are presented.

Atmospheric CO2 concentration is increasing, largely due to anthropogenic activities. Previous studies of individual free-air CO2 enrichment (FACE) experimental sites have shown significant impacts of elevated CO2 (eCO2) on soil microbial communities; however, no common microbial response patterns have yet emerged, challenging our ability to predict ecosystem functioning and sustainability in the future eCO2 environment. Here we analyzed 66 soil microbial communities from five FACE sites, and showed common microbial response patterns to eCO2, especially for key functional genes involved in carbon and nitrogen fixation (e.g., pcc/acc for carbon fixation, nifH for nitrogen fixation), carbon decomposition (e.g., amyA and pulA for labile carbon decomposition, mnp and lcc for recalcitrant carbon decomposition), and greenhouse gas emissions (e.g., mcrA for methane production, norB for nitrous oxide production) across five FACE sites. Also, the relative abundance of those key genes was generally increased and directionally associated with increased biomass, soil carbon decomposition, and soil moisture. In addition, a further literature survey of more disparate FACE experimental sites indicated increased biomass, soil carbon decay, nitrogen fixation, methane and nitrous oxide emissions, plant and soil carbon and nitrogen under eCO2. A conceptual framework was developed to link commonly responsive functional genes with ecosystem processes, such as pcc/acc vs. soil carbon storage, amyA/pulA/mnp/lcc vs. soil carbon decomposition, and nifH vs. nitrogen availability, suggesting that such common responses of microbial functional genes may have the potential to predict ecosystem functioning and sustainability in the future eCO2 environment.

Soil microbes form complex interactive networks throughout the soil and plant rhizosphere. These interactions can result in emergent properties for consortia that are not predictable from the phenotypes of constituents in isolation. We used a four-species consortium to assess the capacity of individual microbial species versus different consortia permutations of the four species to contribute to increased P-solubilization using soil incubations and plant growth experiments. We found that as different combinations of bacterial species were assembled into differing consortia, they demonstrated differing abilities to stimulate soil P cycling and plant growth. The combination of all four microbes in the consortia were much more effective at solubilizing P and stimulating plant growth than any of the individual bacterial species alone. This suggests that in vivo functionally synergistic soil microbial consortia can be adept at performing specific ecosystem functions in situ . Improving our understanding of the mechanisms that facilitate synergistic functioning examined in this study is important for maximizing future food production and agroecosystem sustainability.

To meet the nutritional demands of a rapidly growing population in the face of increasing climate variability, innovative tools are needed to rapidly regenerate soil health in agricultural systems. Using food wastes to improve soil health presents a viable opportunity to improve soils and efficiently manage waste. In a previous laboratory study, we found that potassium lactobionate, a byproduct of cheese production, greatly enhanced soil water holding capacity and nutrient availability. To further explore its potential as a soil amendment, we conducted agronomic trials in winter wheat and corn at the USDA-ARS Central Great Plains Research Station in Akron, Colorado. We evaluated lactobionate for potential improvements in key soil health indices, focusing on soil moisture, carbon, and nitrate. Lactobionate was applied at 5 rates and either as broadcast or surface banding depending on the crop, and soil samples were collected from 0 to 5 cm and 5 to 15 cm depths. Four weeks after broadcast application in the wheat trial, we observed a significant increase in soil moisture and microbial biomass in the 5-15 cm-depth and a decrease in soil nitrate at both soil depths and across rates, relative to unamended plots (p < 0.1). We also saw a non-significant 14% increase in corn yield with subsurface banding of lactobionate but no observed changes in other soil properties measured in the corn trial. We found no significant changes in soil pH, total soil carbon and nitrogen, and soil ammonium concentration with lactobionate for both trials. Our observations suggest the potential for lactobionate to modify soil water content, microbial biomass, nitrate, and yield but outcomes varied by crop trial and amendment rates. This implies that while recycling food waste for use as a soil amendment may have benefits for key soil health parameters, the timing, mode and application amount need to be optimized for maximal effects of lactobionate.

Microbiomes play critical roles in ecosystems and human health, yet in most cases scientists lack standardized and reproducible model microbial communities. The development of fabricated microbial ecosystems, which we term EcoFABs, will provide such model systems for microbiome studies.

Crop yield reductions are common in drought-stressed agroecosystems and are likely to become more frequent with climate change. To combat this, soil amendments are often used to enhance soil moisture retention but typically only lead to marginal improvements. Moreover, even as concern over agricultural water use mounts, a large fraction of food is wasted. Diverting more food waste and byproducts back to agricultural fields could reduce waste issues while ameliorating critical water limitations. We evaluated lactobionate, a lactose derivative and major dairy industry byproduct, as a potential soil amendment for enhancing both soil moisture and soil organic carbon (SOC). Lactobionate (LB) is a hydrophilic compound consisting primarily of cations and simple sugar acids. These combined properties could synergistically modify numerous controls on soil-water balances. In a laboratory setting, we compared LB stabilized with various cations (K+, NH4+, and Ca+) across a range of soil types to determine LB effects on soil moisture and SOC retention. All LB amendments increased soil water content relative to unamended soil across a range of soil matric potentials and raised available water content by 37%. Additionally, LB amended soils had on average 70 times more microbial biomass and decreased soil inorganic nitrogen content compared to unamended soils. We found that K+-LB, the most effective amendment, increased soil water content by 100-600% compared to unamended soils and as much as 87% of the increased SOC following LB additions was retained after 2 months. Our results suggest that tapping into novel sources of organic inputs such as LB may be an effective approach for simultaneously enhancing soil moisture and carbon stocks while increasing the economic and energetic value of food production byproducts.

Permafrost thaw is projected to restructure the connectivity of surface and subsurface flow paths, influencing export dynamics of dissolved organic matter (DOM) through Arctic watersheds. Resulting shifts in flow path exchange between both soil horizons (organic-mineral) and landscape positions (hillslope-riparian) could alter DOM mobility and molecular-level patterns in chemical composition. Using conservative tracers, we found relatively rapid lateral flows occurred across a headwater Arctic tundra hillslope, as well as along the mineral-permafrost interface. While pore waters collected from the organic horizon were associated with plant-derived molecules, those collected from permafrost-influenced mineral horizons had a microbial origin, as determined by fluorescence spectroscopy. Using high-resolution nuclear magnetic resonance spectroscopy, we found that riparian DOM had greater structural diversity than hillslope DOM, suggesting riparian soils could supply a diverse array of compounds to surface waters if terrestrial-aquatic connectivity increases with warming. In combination, these results suggest that integrating DOM mobilization with its chemical and spatial heterogeneity can help predict how permafrost loss will structure ecosystem metabolism and carbon-climate feedbacks in Arctic catchments with similar topographic features.

The timing and duration of the plant growing season and its period of peak activity have shifted globally in response to climate change. These changes alter the period of maximum and potential total carbon uptake, especially in highly seasonal environments such as the Arctic. Earlier plant growth has been observed, and if plant senescence remains the same or is delayed, growing season extension will likely lead to greater carbon uptake and growth. We used phenology data from a multifactor climate change experiment to examine how altered seasonality influences the timing and rate‐of‐senescence and to compare direct observations of individual plant senescence with mathematical models of onset‐of‐senescence based on near‐surface remote sensing. Our three‐year experiment in an Arctic tundra ecosystem altered plant microclimates through factorial warming and earlier snowmelt treatments. We found that (1) early snowmelt and warmer temperatures led to earlier remotely sensed onset‐of‐senescence, but did not alter the rate‐of‐senescence, (2) the timing of color change for individual vascular plants did not change in response to the treatments, leading to a mismatch with remotely sensed phenology, and (3) cumulative, phenologically dependent microclimate metrics (e.g., soil cold degree‐days) best predicted the onset‐of‐senescence. Our study highlights the complexity of observing and understanding controls over phenological shifts that affect plant growth and consequently ecosystem functions. Experimental studies that include multiple approaches to observe and model phenological changes and microclimate are critical to develop phenological forecasting models.

Biogeochemical processing of dissolved organic matter (DOM) in headwater rivers regulates aquatic food web dynamics, water quality, and carbon storage. Although headwater rivers are critical sources of energy to downstream ecosystems, underlying mechanisms structuring DOM composition and reactivity are not well quantified. By pairing mass spectrometry and fluorescence spectroscopy, here we show that hydrology and river geomorphology interactively shape molecular patterns in DOM composition. River segments with a single channel flowing across the valley bottom export DOM with a similar chemical profile through time. In contrast, segments with multiple channels of flow store large volumes of water during peak flows, which they release downstream throughout the summer. As flows subside, losses of lateral floodplain connectivity significantly increase the heterogeneity of DOM exported downstream. By linking geomorphologic landscape-scale processes with microbial metabolism, we show DOM heterogeneity increases as a function of fluvial complexity, with implications for ecosystem function and watershed management.