Drivers and patterns of iron redox cycling from surface to bedrock in a deep tropical forest soil: a new conceptual model

Abstract

Iron (Fe) reduction and oxidation are important biogeochemical processes coupled to decomposition, nutrient cycling, and mineral weathering, but factors controlling their rates and spatial distribution with depth are poorly understood in terrestrial soils. In aquatic ecosystems, Fe reduction often occurs below a zone of oxic sediments. We tested an alternative conceptual model for Fe redox cycling in terrestrial soils using a deep humid tropical forest soil profile. We hypothesized that Fe reduction in anaerobic microsites scales with depth variation in labile C and Fe availability, as opposed to bulk oxygen (O2). We measured bulk O2 at multiple depths from 0.1 to 5 m quasi-continuously over 18 months and sampled soils from surface to bedrock (~7 m). Median O2 mixing ratios declined from 19.8 ± 1.2 % at 0.25 m to 16.1 ± 1.0 % at 1 m, but did not consistently decrease below 1 m, challenging a recent model of regolith development. Reduced Fe (Fe(II)) extractable in 0.5 M hydrochloric acid was greatest in 0-0.1 m soil and declined precipitously with depth, and did not correspond with visible gleying in B horizons. We observed similar depth trends in potential Fe reduction under anaerobic conditions. Depth trends in Fe(II) also closely mirrored short-term soil respiration and bulk soil C. Labile C stimulated Fe reduction at 0-0.1 m depth, whereas addition of short-range-ordered Fe oxides had no effect. Cultivable Fe-reducing bacterial abundance was four orders of magnitude greater in surface soil (0-0.1 m) than below 1 m. Although cultivable Fe oxidizing bacteria were typically also more abundant in surface soil, addition of labile C and nitrate stimulated Fe oxidizers in deep soil by two orders of magnitude under anaerobic conditions. This implies that infiltration of nitrate (and possibly C) from shallow soil water could potentially promote biotic Fe oxidation, a critical step in bedrock weathering, 7 m below. Together, these data suggest that C, Fe, and nutrient availability increase microbial Fe reduction and oxidation in surface (vs deeper) soil microsites despite high bulk O2, in contrast to the depth segregation of electron accepting processes often observed in aquatic ecosystems. Furthermore, the greatest capacity for Fe redox cycling can occur in A horizons that do not display gleying or mottling. decomposition, nutrient cycling, and mineral weathering, but factors controlling their rates and 21 spatial distribution with depth are poorly understood in terrestrial soils. In aquatic ecosystems, Fe 22 reduction often occurs below a zone of oxic sediments. We tested an alternative conceptual 23 model for Fe redox cycling in terrestrial soils using a deep humid tropical forest soil profile. We 24 hypothesized that Fe reduction in anaerobic microsites scales with depth variation in labile C and 25Fe availability, as opposed to bulk oxygen (O2). We measured bulk O2 at multiple depths from 26 0.1-5 m quasi-continuously over 18 months and sampled soils from surface to bedrock (~7 m). 27Median O2 mixing rati...

Disentangling the long‐term effects of disturbance on soil biogeochemistry in a wet tropical forest ecosystem

“…Our measurements throughout the soil profile and across treatments allowed us to explore relationships between soil C, Fe, and P concentrations, supporting previous work suggesting strong biogeochemical coupling of these elements (Hall, Liptzin, Buss, Deangelis, & Silver, 2016;Townsend, Cleveland, Houlton, Alden, & White, 2011). The highly weathered soils at our site (i.e., Oxisols) are dominated by Fe and Al oxides, which play a key role in both C and P cycling, especially under fluctuating redox conditions (Chacon et al, 2006).…”

Iron reduction: a mechanism for dynamic cycling of occluded cations in tropical forest soils?

“…rice paddies) vs. montane forest soils. The predominance of shallow macropore flow through well-aggregated soils in the Bisley Watershed (McDowell et al 1992) may lead to lower cation leaching losses following Fe reduction, which dominantly occurs within anaerobic microsites in soils with typical O2 mixing ratios of 10 -20% (Hall et al 2013(Hall et al , 2016.…”

“…Although surface soils are rarely completely saturated with moisture due to their high macroporosity (Hall et al 2013), they are suitable environments for Fe reduction due to the combination of high clay and organic matter content, well-developed aggregate structure, warm temperatures that stimulate heterotrophic activity and decrease O2 solubility, and high plant productivity and associated labile C inputs. Key findings include: labile C often controls Fe reduction, leading to higher rates in surface horizons than at depth (Chacon et al 2006;Hall et al 2016); Fe(III) reducing bacteria are highly abundant in surface soil (0.8 x 10 8 cells g -1 ; Dubinsky et al 2010); net Fe(II) production can occur within aggregates under ambient aerobic conditions (Liptzin and Silver 2009); soil Fe(II) concentrations (0.5M HCl extractions) can vary by a factor of two or more within hours to days following rainfall events (Hall et al 2013); soil O2 dynamics vary with precipitation over timescales of days to months (Liptzin et al 2011). Soil data including root biomass, texture, C, and Fe content of samples from this site were reported previously (Hall and Silver 2015).…”

“…Surface leaf litter was removed where present, and soils were quantitatively sampled at depths of 0 -10 and 10 -20 cm at each sampling location using a 6 cm diameter stainless steel cylinder (total n = 118; samples at 10 -20 cm depth could not be collected in two valley plots because of rocks). These depths were selected because they represent the dominant rooting zone in this forest (Silver and Vogt 1993), have much greater cation stocks than deeper soils (Silver et al 1994), and are the dominant zones of Fe reduction due to C limitation at depth (Hall et al 2016). Replicate cores were sampled at each location for soil chemical analyses and fine root biomass measurements.…”

Ecological and genomic attributes of novel bacterial taxa that thrive in subsurface soil horizons

“…Although microbial abundances in deeper soils are relatively 60 low on a per gram soil basis, the cumulative biomass of microbes inhabiting deeper soil horizons can be on par with that living in surface soils, owing to the large mass and volume of subsurface horizons (3,5). Moreover, those microbes living in deeper horizons can play important roles in mediating a myriad of biogeochemical processes, including processes associated with soil C and 65 nitrogen (N) dynamics (9,10), soil formation (11), iron redox reactions (12,13), and pollutant degradation (14).…”