Manganese (Mn) is a biologically essential and economically critical element; yet how the environment shapes its behavior, and thereby its role in terrestrial biogeochemical cycles, remains largely unknown. Mn exists in three oxidation states in soils-Mn(II), Mn(III), and Mn(IV)-each possessing distinct reactivities that shape their biogeochemical roles. It is unclear what soil processes drive Mn abundance and the distribution of its oxidation states. We investigated Mn dynamics on a Hawaiian rainfall gradient to evaluate how mean annual rainfall, water availability (the difference between mean annual rainfall and mean annual evapotranspiration), soil moisture, soil pH, and soil redox conditions influence Mn oxidation states, lability, and plant uptake. We found that mean annual rainfall is the most consistent variable predicting the distribution of Mn and its oxidation states along the gradient. Grass Mn and soil Mn(II) increased with rainfall, and total soil Mn and soil Mn(IV) generally decreased with increasing rainfall. Mn(III) trends effectively split into two groups: labile Mn(III) and persistent soil Mn(III). Persistent Mn(III) pools in soils were most abundant in the driest and wettest sites; in contrast, labile Mn(III) was most abundant in the middle of the gradient, from similar to 850-2400 mm mean annual rainfall. These findings illustrate that rainfall defines where Mn is most concentrated, bioavailable, potentially reactive, and likely to influence terrestrial biogeochemical cycles.

Wildfire smoke contains metal-laden ultrafine particles (less than 0.25 µm) systematically overlooked in air quality monitoring, representing a hidden hazard. We show that these ultrafine particles dominate smoke composition, accounting for >60% of particle mass and carrying toxic metals including chromium, nickel, and titanium at sizes down to 5 nm, small enough to penetrate lung barriers and enter the bloodstream. Unexpectedly, ultrafine metal abundance is decoupled from conventional PM₂.₅ measurements but correlates strongly with underlying geology, revealing that burned lithology controls smoke toxicity. With wildfires intensifying globally and smoke exposure affecting hundreds of millions annually, current exposure assessments based solely on PM₂.₅ mass miss the most hazardous fraction of smoke. Our findings necessitate the incorporation of size-resolved metal composition into air-quality forecasting and public health protection.

Groundwater nitrate contamination is largely attributed to fertilizer and intensive livestock manure inputs in agricultural systems. California's Salinas Valley is an area where regional policy is aimed at reducing nitrate leaching. Nonlegume winter cover crops can help decrease nitrate leaching by scavenging residual soil nitrogen (N) during winter fallow periods following the cropping season. However, the ability of fall-incorporated cover crops to decrease nitrate leaching and recycle N to subsequent cash crops is unknown. We conducted a 112-day laboratory soil incubation experiment using Merced rye (Secale cereale) cover crop shoot biomass, with four carbon-to-nitrogen (C/N) ratios (10, 14, 19, and 30), at three temperatures (10°C, 15°C, and 20°C). Destructive soil sampling was done at six intervals during the incubation to measure plant-available nitrogen. Rye biomass with the lowest C/N ratio (10) had the highest average nitrogen mineralization (Nmin) rate (56%) at the warmest temperature (20°C). Conversely, biomass with the highest C/N (30) showed net nitrogen immobilization at 10°C and 15°C during the incubation, transitioning to net mineralization only at 20°C. We found a linear correlation between soil temperature and nitrogen mineralization (at Day 112) for higher C/N ratios. Furthermore, doubling the soil mineral nitrogen content had a negligible impact on the percent mineralization of the C/N 30 residue. These results provide useful information to help farmers and policymakers understand mineralization dynamics from fall-, winter-, or spring-terminated cereal cover crops.

Community based participatory research (CBPR) is an approach to research that equitably involves community members, academic researchers, and stakeholders in all aspects of the research process. The goal is to enact social change and improve the well-being of the communities involved. CBPR is widely adopted in the fields of psychology and public health but to a lesser extent in the fields of Earth and environmental sciences (EES). Few clear frameworks or examples of successful implementation of CBPR in environmental research has limited its adoption in the field. We seek to fill that gap by providing a case study of a long-term project in Oakland, California. Academic researchers assessed soil contamination in urban school gardens in partnership with the Oakland Unified School District and a local environmental justice organization, Communities for a Better Environment. The effectiveness of the CBPR approach is illustrated in generating scientifically robust data while empowering community members to actively contribute to the research process. To encourage adoption of the CBPR approach in the EES fields, we provide eight strategies employed to foster meaningful collaboration between scientists and community members. Furthermore, the study introduces a novel framework, C.I.R.C.L.E, which outlines the core principles needed to build and sustain strong community partnerships and successfully implement CBPR in research. This paper contributes to the growing body of literature on the integration of community perspectives in environmental research and provides a blueprint for future endeavors seeking to address environmental issues through collaborative, community-driven approaches.

Assessing climate change effects on soils usually involves conducting comparisons of biogeochemical processes under projected future conditions against ambient ones. This is typically achieved through incubation experiments utilizing today’s soils. However, a significant limitation of relying on present-day soils is the oversight of the ongoing evolution of soils in terms of geochemistry and microbiology over several years in response to future climatic conditions. This study challenges the traditional approach by asking: Can climate change experiments accurately replicate future biogeochemical processes and their outcomes using soils with today's geochemistry and microbiome? To address this question, we collected oxic and anoxic soils from experimental climate studies, exposed to both present-day and concurrently predicted future climate conditions. We reintroduced these soils with varying climate histories to both sets of climatic conditions (ambient/future), employing a crossover design. This unique experimental setup enables us to discern which biogeochemical processes are influenced by the soil’s historical context and which are contingent on the specific incubation conditions imposed. For the oxic soil, with an eight-year night temperature increase of up to 2°C coupled with altered precipitation patterns (a 10% increase in spring and autumn, a 20% decrease in summer), our findings indicate a notable influence of soil history on soil respiration, surpassing impacts of the incubation climate. This implies that the historical context of the soil wielded a stronger influence than the specific incubation conditions in shaping organic matter pools and turnover within oxic soils. Conversely, iron(III) reduction, as a pivotal indicator of geochemical evolution, was primarily regulated by incubation conditions related to soil moisture rather than being dictated by the soil’s historical background. In the anoxic soil, with a one-year treatment of temperature increases of 4°C and doubled atmospheric CO2, a more pronounced reductive iron(III) dissolution occurred in the soil with the future climate history compared to soils with today’s history. This observation suggests that, over the course of soil history, a larger pool of reducible iron became available to microorganisms in soil with a future climate history than in those with today's soil history. Interestingly, the release of arsenic from these ageing iron minerals was higher in soils with a future climate history compared to today’s soils. This indicates that studies investigating arsenic mobility and its impact on crop performances using present-day soils may underestimate the potential environmental consequences of arsenic. Additionally, the history of future soil conditions favoured greater microbial growth than the incubation conditions. However, soil respiration deviated from this pattern, with a predominant increase attributed to the future incubation climate and, to a lesser extent, influenced by soil history. Complementary data on compositional variations in soil organic matter (LDI-FT-ICR MS) and microbial community (16S rRNA amplicon sequencing) assessing differences based on soil history and short-term experimental conditions will also be presented for both soils. Our findings indicate that soil history plays a differential role for biogeochemical processes and outcomes of the future with biogeochemical outcomes and temporal trajectories possibly being over- or underinterpreted when studies on climate change utilize present-day soils.

Wildfires significantly contribute to ambient air pollution, yet our understanding of how wildfire smoke influences specific chemicals and their resulting concentration in smoke remains incomplete. We combine 15 years of daily species-specific PM2.5 concentrations from 700 air pollution monitors with satellite-derived ambient wildfire smoke PM2.5, and use a panel regression to estimate wildfire smoke's contribution to the concentrations of 27 different chemical species in PM2.5. Wildfire smoke drives detectable increases in the concentration of 25 out of the 27 species with the largest increases observed for organic carbon, elemental carbon, and potassium. We find that smoke originating from wildfires that burned structures had higher concentrations of copper, lead, zinc, and nickel relative to smoke from fires that did not burn structures. Wildfire smoke is responsible for an increasing share of ambient concentrations of multiple species, some of which are particularly harmful to health. Using a risk assessment approach, we find that wildfire-induced enhancement of carcinogenic species concentrations could cause increases in population cancer risk, but these increases are very small relative to other environmental risks. We demonstrate how combining ground-monitored and satellite-derived data can be used to measure wildfire smoke's influence on chemical concentrations and estimate population exposures at large scales.

Fire activity, including wildfires and urban fires, is increasing in frequency and severity, significantly impacting soil-borne metals such as chromium (Cr), which can be transformed from benign Cr(III) to toxic Cr(VI) during heating. However, the reaction pathway of Cr(VI) formation during wildfires remains unclear. We investigated the impacts of Fe-bearing minerals on the fire-induced formation of Cr(VI). Magnetite (Fe3O4) synthesized and doped with Cr(III), and Fe and Cr rich soils were heated up to 800 °C to investigate temperature-dependent transformations. For the synthetic system, Cr(III) oxyhydroxide (CrOOH) was oxidized to metastable Cr(VI) trioxide (CrO3) up to 600 °C, which spontaneously converted to Cr(III) oxide (Cr2O3) with increasing temperature to 800 °C. In the soil samples, Fe-bearing minerals reacted with Cr(III) hydroxide [Cr(OH)3] and chromite [FeCr2O4; Cr(III)] to form Cr(VI) and magnetite up to 600 °C, which react with each other with increasing temperature and reduce Cr(VI) to form chromite and hematite (α-Fe2O3). These findings highlight the role of Fe-bearing minerals in controlling the Cr(VI) formation and reduction pathway during fires. Our results have implications for understanding how wildfires contribute to the formation of toxic metals in soils, providing valuable insights for predicting the risks posed by wildfires.

Soil contamination with metals and metalloids is a growing environmental concern, impacting soil ecosystems. Exogenous metal(loid)s are retained in the soil matrix via adsorption, structural incorporation, and precipitation, imposing stress on soil microbiomes, potentially influenced by climate. It remains unclear whether introduced metal(loid)s bind similarly to native ones and how quickly soil microbiomes adapt under today's and future climate conditions. We incubated soils spiked with 0.7 mg kg-1 cadmium or 15 mg kg-1 arsenic under today's and future climate scenarios (IPCC SSP 3-7.0: +400 ppmv CO2, +4°C). After 38 days, spiked As and Cd did not integrate into soil minerals like native counterparts but preferentially associated with more reactive minerals. Spiked As became more recalcitrant over time, an effect enhanced under future conditions. Spiked Cd remained reactive during incubation, independent of climate conditions. Prokaryotic abundances increased faster in metal(loid)-spiked soil under future conditions with distinct soil prokaryotic community structures emerging in response to metal(loid)s and climate. Despite this, key functions like Fe(III) reduction were maintained. Communities nearly stabilized within 38 days across climate conditions. These findings suggest that exogenous metal(loid)s may require years to achieve native-level binding, while soil microbes adapt functionally within weeks, even under climate change.

Groundwater quality is critical for safe drinking water and irrigation supplies but can be threatened by geogenic toxins that are difficult to predict. In the arid, high desert San Luis Valley (SLV), Colorado, a groundwater basin serves as the primary water supply with observed arsenic concentrations exceeding the maximum contaminant level (MCL) of 10 μg/L set by the U.S. EPA. However, the sources and processes responsible for As occurrence are unclear. Through a community-engaged sampling effort, we collected 244 groundwater samples and measured major/trace element concentrations. Long-term land subsidence and depth-resolved sediment texture were computed at the same locations. We tested three plausible geochemical processes responsible for As release: (1) overpumping-induced dewatering of As-bearing clays (proxied by land subsidence), (2) pH-promoted desorption as well as reductive dissolution of As(V)/Fe(III) (hydr)oxides, and (3) incursion of higher-As geothermal fluids (proxied by lithium, boron, tungsten, and molybdenum) into groundwater. We find that statistics, statistical/machine learning, and aqueous thermodynamics all agree that geothermal fluid mixing within the aquifer is the main source of dissolved As. Our findings suggest that overpumping draws higher-As thermal fluid from the bottom of the aquifer to pumping depth, leading to increased concentrations of As in drinking/irrigation water supplies at wells.

Anoxic microsites, zones of oxygen depletion in otherwise well-aerated soils, serve as prominent controls on several biogeochemical cycles (e.g., carbon, nitrogen, iron). However, relatively little is known about the spatiotemporal distribution of anoxic microsites and thus little is known about their biogeochemical influence. Here, we use time-integrative measures of past anoxia (i.e., electrochemical measurements and quantification of anaerobic functional genes) to determine how the spatial distribution of anoxic microsites varies between aggregate interiors and bulk soils in soils of two distinct textures across multiple depths in a California grassland. We found greater evidence of anoxia in topsoils vs. subsoils and finer vs. coarse-textured soils. Counter to many traditional depictions of soil aggregates, we observed that aggregate interiors showed equal or less evidence of anoxic microsites than bulk soils. Across the entire dataset, our combined proxies for anoxic microsite prevalence were strongly and positively correlated with organic C concentration (R2 = 0.80), highlighting the importance of soil organic C availability and microbial oxygen demand in creating anoxic microsites. Our results contribute to a growing body of evidence that soil oxygen demand (i.e., microbial respiration) can play a more prominent role in anoxic microsite formation than soil oxygen supply, provoking questions about the suitability of using aggregate size and moisture as lone proxies for soil oxygen availability.

Climate change, coupled with widespread soil arsenic (As) contamination, is expected to decrease rice yields and increase grain As, threatening food security. One promising mitigation strategy is alternate wetting and drying (AWD) irrigation. However, AWD has not previously been tested under potential future climate conditions. Using rhizoboxes to visualize the rhizosphere, we evaluated the efficacy of AWD for limiting porewater and grain As under both current (daily high of 33 °C and 420 ppmv CO2) and severe warming conditions (daily high of 38 °C and 850 ppmv CO2). Compared to continuous flooding, AWD decreased cumulative As exposure 10 cm below the surface by 8.2× under a 33 °C climate and by 15.9× under a 38 °C climate. Grain total As concentrations decreased by 1.5× with AWD under a 33 °C climate and by 1.3× under a 38 °C climate. Porewater cadmium (Cd) concentrations often increased following drainage but never exceeded 1 μg L-1, and grain Cd concentrations were 14.7× to 119.7× lower than grain As concentrations. Both AWD and the 38 °C and 850 ppmv CO2 climate conditions enhanced root growth. Our findings indicate that AWD may still be an effective As mitigation strategy under severe future climate conditions.

California's Bay-Delta watershed encompasses 40% of the state's runoff and serves water supply and irrigation needs throughout the state. A recently amended policy attempts to rebalance water supply and ecological outcomes by requiring 40% of the flow to remain in-stream in the Tuolumne River and other tributaries between February 1 and June 30 each year. This policy impacts water supply diversions serving millions of customers in the San Francisco Bay Area. This work analyzes possible regional coping responses in the context of climate and policy uncertainty, exploring the effectiveness of various strategies to mitigate climate and policy impacts and improve future water supply outcomes. We find that climate uncertainty has a dominant impact on three measures of water supply performance: the frequency of full storage reserves, minimum storage level, and uncaptured flow remaining. Coping interventions in the ranges considered (0-500,000 acre-feet/0-620 million m3 new storage; 0-60,000 acre-feet per year/0-70 million m3 per year new supplies) can serve to mitigate climate impacts and to fully or partially offset the impact of the in-stream flow policy. Regardless of the policy, coping interventions are needed to avoid critically low storage levels. Our analysis indicates that recharge may be an effective strategy to increase capture of Tuolumne River streamflow, with an expected yield of 0.15-0.51 units of flow captured per unit of recharge capacity added, compared to reservoir expansion, with an expected yield of 0.05-0.07 units of flow captured annually per unit of volume capacity added.

Floodplain soils are vast reservoirs of organic carbon often attributed to anaerobic conditions that impose metabolic constraints on organic matter degradation. What remains elusive is how such metabolic constraints respond to dynamic flooding and drainage cycles characteristic of floodplain soils. Here we show that microbial depolymerization and respiration of organic compounds, two rate-limiting steps in decomposition, vary spatially and temporally with seasonal flooding of mountainous floodplain soils (Gothic, Colorado, USA). Combining metabolomics and -proteomics, we found a lower abundance of oxidative enzymes during flooding coincided with the accumulation of aromatic, high-molecular weight compounds, particularly in surface soils. In subsurface soils, we found that a lower oxidation state of carbon coincided with a greater abundance of chemically reduced, energetically less favorable low-molecular weight metabolites, irrespective of flooding condition. Our results suggest that seasonal flooding temporarily constrains oxidative depolymerization of larger, potentially plant-derived compounds in surface soils; in contrast, energetic constraints on microbial respiration persist in more reducing subsurface soils regardless of flooding. Our work underscores that the potential vulnerability of these distinct anaerobic carbon storage mechanisms to changing flooding dynamics should be considered, particularly as climate change shifts both the frequency and extent of flooding in floodplains globally.

Redox reactions underlie several biogeochemical processes and are typically spatiotemporally heterogeneous in soils and sediments. However, redox heterogeneity has yet to be incorporated into mainstream conceptualizations and modeling of soil biogeochemistry. Anoxic microsites, a defining feature of soil redox heterogeneity, are non-majority oxygen depleted zones in otherwise oxic environments. Neglecting to account for anoxic microsites can generate major uncertainties in quantitative assessments of greenhouse gas emissions, C sequestration, as well as nutrient and contaminant cycling at the ecosystem to global scales. However, only a few studies have observed/characterized anoxic microsites in undisturbed soils, primarily, because soil is opaque and microsites require µm-cm scale resolution over cm-m scales. Consequently, our current understanding of microsite characteristics does not support model parameterization. To resolve this knowledge gap, we demonstrate through this proof-of-concept study that X-ray fluorescence (XRF) 2D mapping can reliably detect, quantify, and provide basic redox characterization of anoxic microsites using solid phase “forensic” evidence. First, we tested and developed a systematic data processing approach to eliminate false positive redox microsites, i.e., artefacts, detected from synchrotron-based multiple-energy XRF 2D mapping of Fe (as a proxy of redox-sensitive elements) in Fe-“rich” sediment cores with artificially injected microsites. Then, spatial distribution of FeII and FeIII species from full, natural soil core slices (over cm-m lengths/widths) were mapped at 1–100 µm resolution. These investigations revealed direct evidence of anoxic microsites in predominantly oxic soils such as from an oak savanna and toeslope soil of a mountainous watershed, where anaerobicity would typically not be expected. We also revealed preferential spatial distribution of redox microsites inside aggregates from oak savanna soils. We anticipate that this approach will advance our understanding of soil biogeochemistry and help resolve “anomalous” occurrences of reduced products in nominally oxic soils.

Context or Problem: Flooded rice systems produce an important staple crop but are a source of methane (CH4) and arsenic (As) exposure. Introducing non-continuous flooding events can decrease seasonal CH4 emissions and grain As concentration. A single mid-season drain (MD: a 7- to 10-day soil drying period during the mid-season) is easier for farmers to manage than multiple drain events (e.g. alternate wetting and drying). Previous research has shown that a MD can accomplish these goals without yield reduction. However, depending on soils and precipitation, the soil drying severity during a MD can vary substantially. Objective or Research Question: The objectives of this study were to compare a MD to the farmer practice under onfarm conditions with respect to yields, greenhouse gas (GHG) emissions and As and cadmium (Cd - which can increase with non-continuous flooding). Second, to quantify the relationship between soil drying severity during the MD and the reduction in CH4 emissions and grain As. Methods: A three-year study with seven on-farm trials was implemented. Each trial had two treatments: a MD and the farmer practice. Grain yields, GHG emissions, and grain As and Cd concentrations were quantified for each treatment. Results: Grain yields were similar between treatments. The MD decreased seasonal CH4 emissions by 20-77% (average of 52%), with the magnitude of reduction being related to soil-drying severity during the MD. Combining previous on-station data with this on-farm data, indicates that for every 1% reduction in soil gravimetric water content (GWC), seasonal CH4 emissions were reduced by 2.5% (r(2) = 0.47). With MD, N2O emissions increased (average = 0.25 kg N2O-N ha(-1)), but accounted for only 3% of the global warming potential. The MD decreased grain As concentration (by 20% on average, but not related to soil drying severity), and there was no effect on grain Cd concentrations. Conclusions: Given these results, an MD has similar GHG mitigation potential to other non-continuous flooding practices that require more drain periods and may be more difficult to implement. These results indicate that a MD is a viable on-farm management practice for GHG mitigation and reducing grain As concentration with limited risk of yield reduction. Implications or Significance: This is the first research that has documented the relationship between soil drying severity and reduction in CH4 emissions across soils. This relationship is a potentially useful tool for on-farm monitoring, but will need to be developed further across more locations.

AbstractClimate change and metals independently stress soil microbiomes, but their combined effects remain unresolved. Here we show that future climate affects soil cadmium through altered soil microbiome and nutrient cycles, with soil pH as critical factor. In soils with pH<7 and during summer temperatures, future climate increased porewater cadmium, shifting total and potentially active taxonomic microbiome structures. Microbial ammonium oxidation released protons liberating cadmium through cation exchange from mineral surfaces. When porewater cadmium levels became toxic to non-cadmium-tolerant bacteria, microbial activity, and nutrient cycling decreased, reducing carbon and nitrogen emissions. In contrast, pH>7 soil show no climate impacts on cadmium mobilization, though imprints on microbiome structure were apparent. Subsequent nutrient cycling increased under future climate, stimulating soil respiration and nitrous oxide release. These findings underscore complex interactions between climate change and soil contaminants affecting the soil microbiome and its activity and highlights potential impacts on crop production, groundwater quality, and climate feedback.