Seasonal and spatial variation in the potential for iron reduction in soils of the Southeastern Piedmont of the US

Hodges, Caitlin; Mallard, J. M.; Markewitz, Daniel; Barcellos, Diego; Thompson, Aaron

doi:10.1016/j.catena.2019.03.026

Cited by 15 publications

(16 citation statements)

References 50 publications

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“…Previous work has shown that wet-dry cycles, which can induce reducing and oxidizing conditions, may cause reduction of Fe in non-crystalline phases and subsequent increase in Fe crystallinity (Thompson et al, 2006). Similar observations have been reported for an upland soil experiencing substantial precipitation events (Hodges et al, 2019). Al phases were not affected by moisture (Figure 3A) because they are pH-dependent but not redox-dependent (Bertsch and Bloom, 2018).…”

Section: Long-term Moisture Shaped Mineral Composition and C Interactions With Mineralssupporting

confidence: 85%

Higher Long-Term Soil Moisture Increases Organic Carbon Accrual Through Microbial Conversion of Organic Inputs

Shabtai¹,

S²,

Inagaki³

et al. 2021

Preprint

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High long-term soil moisture may either stimulate or inhibit soil organic carbon (SOC) losses through changes to mineral and chemical composition, and resultant organo-mineral interactions. Yet, the trade-off between mineralization and accrual of SOC under long-term variation in unsaturated soil moisture remains an uncertainty. In this study, we tested the underexplored relationships between long-term soil moisture and organo-mineral chemical composition, and its implications for SOC persistence. The results provide new insights into SOC accrual mechanisms under different long-term moisture levels commonly observed in well-drained soils. Differences in long-term mean volumetric water content ranging from 0.4 - 0.63 (v/v) on fallow plots in an experimental field in New York, USA, were positively correlated with SOC contents (R2 = 0.228; P = 0.019, n = 20), mineral-associated organic matter (MAOM) (R2 = 0.442; P = 0.001; n = 20) and occluded particulate organic matter (oPOM) contents (R2 = 0.178; P = 0.033; n = 20). Higher long-term soil moisture decreased the relative content of sodium pyrophosphate extractable Fe (R2 = 0.33; P < 0.005; n = 20), increased that of sodium dithionite extractable Fe (R2 = 0.443; P < 0.001; n = 20), and increased the overall importance of non-crystalline Al pools (extracted with sodium pyrophosphate and hydroxylamine extractable) for SOC retention. Higher long-term soil moisture supported up to a four-fold increase in microbial biomass (per unit SOC), and lower C:N ratios in MAOM fractions of high-moisture soils (from C:N 9.5 to 9, R2 = 0.267, P = 0.011, n =20). This was reflected by a 15% and 10% greater proportion of oxidized carboxylic-C to aromatic-C and O-alkyl C, respectively, as measured with 13C-NMR, and a more pronounced FTIR signature of N-containing proteinaceous compounds in high-moisture MAOM fractions, reflective of microbial metabolites. SOC accrual increased with increasing soil moisture (P = 0.019), exchangeable Ca2+ (P = 0.013), and pyrophosphate-extractable Al content (P = 0.0001) and Al/Fe ratio (P = 0.017). Taken together, our results show that high long-term soil moisture resulted in SOC accrual by enhancing microbial conversion of plant inputs to metabolites that interact with reactive minerals.

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Section: Long-term Moisture Shaped Mineral Composition and C Interactions With Mineralssupporting

confidence: 85%

Higher Long-Term Soil Moisture Increases Organic Carbon Accrual Through Microbial Conversion of Organic Inputs

Shabtai¹,

S²,

Inagaki³

et al. 2021

Preprint

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“…In subtropical forests, such as Calhoun in the Southeastern U.S. Piedmont, the marked climatic seasonality greatly constrains anaerobic processes, which likely contribute to biogeochemical processes only in those months with higher rainfall and lower evaporative demand. This finding is supported by recent experiments in Calhoun, where the potential for Fe reduction was measured over the course of a year by means of steel 10.1029/2020JG005894 IRIS (Indicator of Reduction of Iron in Soils) probes (Hodges et al, 2019). The hydrologic regime at Calhoun allows potential Fe reduction to reach only about 10% of the theoretical maximum in January, while over a year a maximum of 60 mmol Fe kg −1 can potentially be reduced.…”

Section: Discussionsupporting

confidence: 71%

“…We have chosen a tropical forest in Luquillo (Puerto Rico) and a subtropical forest in Calhoun (South Carolina, USA), two experimental forests for which comprehensive hydrological and biogeochemical observations are available. Their humid climates and soil moisture regimes are such that surface soils experience periodic anoxic conditions due to high soil moisture levels, triggering anaerobic processes (Hodges et al, 2019; O'Connell et al, 2018). In addition, their soils are highly weathered and low in base saturation, but they contain high concentrations of Fe minerals, which remain available as electron acceptors in absence of oxygen (Markewitz & Richter, 2000; Scatena, 1989).…”

Section: Methodsmentioning

confidence: 99%

Theoretical Constraints on Fe Reduction Rates in Upland Soils as a Function of Hydroclimatic Conditions

et al. 2020

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Periods of high soil wetness promote anaerobic processes such as iron (Fe) reduction within soil microsites, with implications for organic matter decomposition, the fate of pollutants, and nutrient cycling. Here we discuss potential Fe reduction rates emerging from an interplay between the timescales of the internal reactions (Fe oxidation and reduction) and external forcings (length of oxic vs. anoxic conditions), and under no organic substrate and microbial population limitations. We compute the upper bound on Fe reduction and the theoretical maximum reduction rate, which would be reached under “resonant conditions,” whereby the timescales of external forcings match the internal timescales of the redox reactions. The variability of soil oxygen is then linked to rainfall frequency and intensity through soil moisture dynamics, allowing us to determine the hydroclimatic conditions that generate oxic/anoxic cycles that most favor Fe reduction. These predictions are applied to an aseasonal tropical (Luquillo, Puerto Rico, USA) and a seasonal subtropical (Calhoun, SC, USA) humid forests. We show that the tropical site maintains a high potential for Fe reduction throughout the year, due to rapid and frequent transitions between predicted oxic and anoxic microsite conditions, with a potential to reduce up to 1,800 mmol kg−1 soil of Fe per year, while a less humid and seasonal climate in the subtropical site limits maximum reduction rates to 60 mmol kg−1 year−1. This analysis paves the way for a global identification of hot spots of potential Fe reduction using readily available hydroclimatic observations.

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“…Other researchers in temperate forested systems have also observed this signal of anaerobic respiration in the subsurface of upland soils. For example, Hodges et al (2019) found significant potential for iron respiration in subsurface soils during summer in a forested catchment in the piedmont of South Carolina; the authors attributed this Fe reduction to soil moisture at depth limiting O 2 diffusion in the clayey B horizon during periods of high root respiration. At Shale Hills, Weitzman and Kaye (2018) documented subsurface N 2 O production, another pathway of anaerobic respiration that could generate the observed high CO 2 (relative to O 2 ) at LRMS.…”

Section: Discussionmentioning

confidence: 99%

Soil CO₂ and O₂ Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

Hodges

Kim

Brantley

et al. 2019

Soil Science Soc of Amer J

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Soil CO2 and O2 cycles are coupled in some processes (e.g., respiration) but uncoupled in others (e.g., silicate weathering). One benchmark for interpreting soil biogeochemical processes affected by soil pCO2 and pO2 is to calculate the apparent respiratory quotient (ARQ). When aerobic respiration and diffusion are the dominant controls on gas concentrations, ARQ equals 1; ARQ deviates from 1 when other processes dominate soil CO2 and O2 chemistry. Here, we used ARQ to understand lithologic, hillslope, and seasonal controls on soil gases at the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania. We measured soil pCO2 and pO2 at three depths from the soil surface to bedrock across catenas in one shale and one sandstone watershed over three growing seasons. We found that both parent lithology and hillslope position significantly affect soil gas concentrations and ARQ. Soil pCO2 was highest (>5%) and pO2 was lowest (<16%) in the valley floors. Controlling for depth, pCO2 was higher and pO2 was lower across all sites in the sandstone watershed. We attribute this pattern to higher macroporosity in sandstone lithologies, which results in greater root respiration at depth. We recorded seasonal variation in ARQ at all sites, with ARQ rising above 1 during July through September, and dipping below 1 in the early spring. We hypothesize that this seasonal fluctuation arises from anaerobic respiration in reducing microsites July through September when the soils are wet and demand for O2 is high, followed by oxidation of reduced species when the soils drain and re‐oxygenate. We estimate that this anaerobic respiration in microsites contributes 36 g C m−2 yr−1 to the soil C flux. Our results provide evidence for a conceptual model of metal cycling in temperate watersheds and point to the importance of anaerobic respiration to the carbon flux from forest soils. Core Ideas Hillslope position and lithology affect soil CO2 and O2 in humid temperate forests. The ratio of CO2 and O2 (ARQ) fluctuates over the growing season. The ARQ fluctuations indicate seasonal metal redox cycling at all hillslope positions. Anaerobic respiration is important to soil CO2 flux during the late growing season.

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Seasonal and spatial variation in the potential for iron reduction in soils of the Southeastern Piedmont of the US

Cited by 15 publications

References 50 publications

Higher Long-Term Soil Moisture Increases Organic Carbon Accrual Through Microbial Conversion of Organic Inputs

Higher Long-Term Soil Moisture Increases Organic Carbon Accrual Through Microbial Conversion of Organic Inputs

Theoretical Constraints on Fe Reduction Rates in Upland Soils as a Function of Hydroclimatic Conditions

Soil CO₂ and O₂ Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

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Seasonal and spatial variation in the potential for iron reduction in soils of the Southeastern Piedmont of the US

Cited by 15 publications

References 50 publications

Higher Long-Term Soil Moisture Increases Organic Carbon Accrual Through Microbial Conversion of Organic Inputs

Higher Long-Term Soil Moisture Increases Organic Carbon Accrual Through Microbial Conversion of Organic Inputs

Theoretical Constraints on Fe Reduction Rates in Upland Soils as a Function of Hydroclimatic Conditions

Soil CO2 and O2 Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations

Contact Info

Product

Resources

About

Soil CO₂ and O₂ Concentrations Illuminate the Relative Importance of Weathering and Respiration to Seasonal Soil Gas Fluctuations