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

Abstract

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.

The Primacy of Temporal Dynamics in Driving Spatial Self-organization of Soil Redox Patterns

“…7A). As a result of the spatial segregation of OM and Fe(III) in the gray and orange layers created by pattern formation, a potential capacity of ~ 0.86 t C ha -1 mineralization by Fe(III) is lost (estimated from (46)).…”

“…Upland soils are often well-aerated. In subtropical forests, such as Calhoun in the Southeastern U.S. Piedmont, the marked seasonality of rainfall, transpiration, and temperature restricts anoxic conditions to only short periods within wet years (45, 46) (Figs. 7 and S4).…”

Interacting Bioenergetic and Stoichiometric Controls on Microbial Growth

“…therein, Dukovski et al (2021) ]. New models are exploring the potential of bioenergetics to study how microbial metabolic diversity and spatial heterogeneity of resources interact and shape community dynamics and resource niches in the complex soil environment ( Araujo Granda et al, 2016 ; Jayathilake et al, 2017 ; Borer et al, 2019 ; Gogulancea et al, 2019 ; Li et al, 2019 ; Ben Said et al, 2020 ; Calabrese et al, 2020 ; Dal Co et al, 2020 ; Desmond-Le Quéméner et al, 2021 ). Bioenergetics can thus be a useful complement to traditional biogeochemical models describing only the dynamics of C and nutrients.…”

Micronutrients in Food Production: What Can We Learn from Natural Ecosystems?

“…Iron primarily exists in ferrous (Fe II) and ferric (Fe III) forms, which can bind to clay fractions abiotically via cation exchange, or precipitate to form oxides, hydroxides and oxyhydroxides, which can render Fe unavailable under neutral or alkaline conditions [25,29]. Iron is widely used in biotic microbial redox reactions to yield energy for organic carbon degradation [25,34]. The reduction of ferric Fe(III) to ferrous Fe(II), which is more soluble and, therefore, readily available, primarily occurs under anaerobic conditions in water-saturated soils.…”

“…The reduction of ferric Fe(III) to ferrous Fe(II), which is more soluble and, therefore, readily available, primarily occurs under anaerobic conditions in water-saturated soils. However, under aerobic conditions, the oxidation of soluble Fe(II) to the more insoluble Fe(III) prevails, reducing Fe availability [25,30,34]. Consequently, agricultural management practices that influence soil pH, redox potential, saturation, or aeration can affect the availability of Fe in soils.…”