1. Increasing our fundamental knowledge of the fate and transport of carbon, nutrients, metals and contaminants

1.1. Response of natural soil and sediment systems to environmental changes

A focus of our research is to understand the coupled iron, sulfur, nitrogen and carbon cycles in human-altered environments, especially in water-saturated, carbon-rich environments, such as wetland soils and littoral sediments. Intermittent estuaries are common in Mediterranean climate regions. To understand and quantify the coupled sulfur and iron cycling in intermittent estuaries, we focused our research on Pescadero Estuary (California), where recurring fish kills have occurred since 1995. We showed that Pescadero sediments behave as acid-sulfate soils. More specifically, the transition from closed to open state leads to a degradation in water quality, which is extensive enough to affect fish health, especially through the oxidation of sediment sulfides, which drives water acidification and the release of elevated levels of metals (Largier et al., 2016; Richards and Pallud, 2016, Richards et al., 2018a, b).

Wetlands play a vital role in terrestrial carbon sequestration, but the sensitivity of their carbon stocks to disturbance remains uncertain. Carbon cycling in soils is partly controlled by their chemistry, especially by the chemistry of iron, due to the interactions between iron mineral phases and organic matter. We developed a mechanistic and kinetic understanding of the role that iron-oxides play in the fate and transport of dissolved organic carbon (DOC) in subalpine wetland ecosystems (Schilling et al., 2019; Pallud et al., 2020) and our findings suggest that temperature-related increase in iron reduction will not generate additional release of DOC from soils to rivers (Pallud et al., 2020, Daugherty et al., 2022).

1.2. Coupled transport and environmental geochemistry of selenium

Selenium is an essential micronutrient, but also an environmental contaminant, especially in California due to irrigated agriculture. We analyzed the history and current developments in science, policy, and management of irrigation-induced selenium contamination in California (Kausch and pallud, 2013b). Whereas selenium transformation pathways and the microorganisms involved have been well studied under simplified laboratory conditions, an integration of the multiple biogeochemical and physical processes that govern selenium cycling and fate in natural ecosystems is lacking. By combining insight into process controls at the microscale (Kausch et al., 2012; Kusch and Pallud, 2012; Kausch and Pallud, 2013a) with measurements of bulk reaction rates (VillaRomero et al., 2013; Schilling et al., 2018), elemental isotope fractionation (Schilling et al., 2020; Wasserman et al., 2021; Dwivedi et al., 2022) and field-scale reactive transport modeling, we worked on estimating the environmental controls on selenium, and constraining potential future environmental hazards. Using this multi-scale unique approach, we showed that sediment organic carbon content and microbially-mediated selenate reduction, which outweighs current dissolved selenium inputs to the lake, are the main drivers of selenium retention in littoral sediments of the Salton Sea (VillaRomero et al., 2013; Schilling et al., 2018). We also developed a dynamic 2D reactive transport model of selenium cycling in idealized aggregates and found that selenium retention in soil scales with aggregate size (Kausch and Pallud, 2013a).

Additionally, we improved understanding of how bacterial and abiotic selenium oxidation affects selenium mobilization during chemical weathering (Goff et al., 2019; Wasserman et al., 2021; Dwivedi et al., 2022). More specifically, we showed that Se(0) solubilization by the bacterium JG17 we isolated from seleniferous soils occurs via the bacterial release of inorganic sulfur compounds that chemically dissolve elemental selenium (Goff et al., 2019), suggesting that the production of reactive sulfur metabolites by soil microorganisms can promote selenium mobilization during chemical weathering. Selenium isotope fractionation has been used for constraining redox conditions and microbial processes in both modern and ancient environments, but our knowledge of isotopic fractionation during oxidative dissolution of selenium and of the controls on fractionation during selenium microbial reduction is based on a limited number of studies. Our work suggests that oxidative weathering of selenium-bearing minerals, previously thought to induce minimal isotopic fractionation, tends to produce an isotopically heavy selenium weathering flux (Wasserman et al., 2021; Dwivedi et al., 2022). Such results have implications for the development of reliable models that describe modern and ancient selenium biogeochemical cycles.

2. Managing soils for a sustainable future

2.1. Soil health and remediation in urban systems

My lab investigates soil health and soil remediation in urban systems, including in the context of urban agriculture. As heavy metal contamination is gaining recognition as a widespread problem, exposure to such contaminants presents a growing human health risk, especially in urban environments where both industry and people are most concentrated. Soil rehabilitation, while important for human and ecosystem health, is challenged by soil heterogeneity, the presence of multiple contaminants, and a lack of proven methods.

A large number of California low-income urban residents live in food deserts, especially in the Alameda County, where almost a third of the population is food-insecure. Urban agriculture is one solution to improve the health and food environments in cities, however, its application depends partly on the health of the available soils. Through a community-based participatory project carried out in 24 urban farms in the East Bay, we observed that most farms have high soil fertility and use practices that emphasize high spatial and temporal biodiversity, however farmers face issues related to insect and weed pressures, as well as problems linked to soil contamination and water use efficiency (Altieri et al., 2015). We also developed the first comprehensive guide aimed at aiding urban agriculture practitioners in how to sample and interpret soil sample results for risk from exposure to contaminants (Matzen et al., accepted).

Arsenic contamination more specifically is a widespread problem in urban soils, due to historical use of arsenical pesticides. Considering that soil is a finite resource, remediation methods that are cost effective, sustainable, and broadly applicable are urgently needed. Plant-based removal of arsenic from soil, or phytoextraction, with the arsenic-hyperaccumulating fern Pteris vittata is an emerging in situ technology potentially suitable for moderately contaminated soils. However, current remediation times are prohibitively long and heterogeneity in soil conditions affects availability of arsenic. Our work suggested that the presence of the P. vittata ferns used to remove arsenic leads to higher concentrations of leached arsenic, a troubling finding for phytoextraction (Matzen et al., 2020). We calculated the first mass balance that compares arsenic uptake in P. vittata with leaching during growth and showed that variations in soil arsenic availability and nutrient content lead to both metabolic and ecosystem costs during phytoextraction (Matzen et al., 2022b). We showed that high arsenic availability leads to resource reallocation away from biomass production towards arsenic tolerance and hyperaccumulation, leading to lower transpiration, greater infiltration, and greater leaching of available arsenic to potentially impact other ecological receptors. We also showed that phytoextraction with P. vittata is limited to specific soil and climate conditions, narrower than those under which P. vittata grows in the wild and that environmental factors including soil and climate are more important than fertilization to phytoextraction outcomes (Matzen et al., 2022a). Our work suggests that phytoextraction as currently practiced is limited to soils with low arsenic concentrations and that P. vittata cultivation is climate-limited to a zone smaller than its range as a wild species (Matzen et al., 2023).