Linkages between redox processes and surface soil carbon cycling

Project Description: 

Humid tropical soils are characterized by fluctuating oxygen (O2) concentrations and periodic anaerobosis caused by high soil moisture and biological O2 demand. Iron (Fe) oxides provide an abundant electron acceptor for microbial metabolism under anaerobic conditions. Here, we explored interactions between Fe redox cycling and carbon (C) pools and fluxes.
Key Questions:
1. Does spatial and temporal variation in moisture, O2 availability, and Fe reduction control soil C pools?
2. Does O2 limitation constrain organic matter decomposition via microbial enzymes?
3. Does sequential Fe reduction and oxidation provide a novel mechanism for C decomposition?
We characterized spatial and temporal patterns of Fe, C, microbial enzyme activities, and oxidative capacity using field and laboratory experiments. In the field, patterns of reduced iron (Fe(II)) concentrations varied dramatically over space and time. Mean Fe(II) concentrations were highest on well-drained ridges and generally lower in riparian valleys (Fig 1a), corresponding with patterns of fine root biomass (Fig 1b) and soil C concentrations (Fig 1d), suggesting that biological activity may control Fe reduction, as opposed to soil moisture alone. Notably, C concentrations and hydrolytic enzyme activity (Fig 1c) correlated strongly with Fe(II) concentrations in ridge and slope topographic positions, but not in valleys, suggesting the importance of flooding in controlling valley soil C dynamics.
The “enzymatic latch” hypothesis proposes that anoxic conditions constrain oxidative and hydrolytic enzyme activity. In contrary, we found that activity of five hydrolytic enzymes increased with soil Fe(II) concentrations, an index of anaerobiosis (Fig 1c), providing further evidence of plant/microbial control of soil O2 availability. Furthermore, we found strong relationships between soil oxidative activity (colorimetric assay with L-DOPA) and soil Fe(II) concentrations across a range of forest types in the LCZO (Fig 2a). Solutions of Fe(II) alone generated oxidative activity in the absence of soil (Fig 2b), suggesting an abiotic mechanism, possibly radical species generated via “Fenton reactions.” Fe(II) solutions did not affect hydrolytic activity.
Oxidative activity persisted or increased in autoclaved soils (Fig 2c). Adding Fe(II) solutions to replicate soils at a range of field concentrations stimulated short-term production of CO2 under aerobic conditions (Fig 2d) but had little effect under anaerobic conditions, suggesting the potential importance of sequential Fe

reduction and oxidation as a novel mechanism for stimulating soil C decomposition.
We conducted a field water supplementation experiment to examine the influence of moisture on redox and C dynamics. Daily additions of 60 mm of water over 24 days maintained soils at field moisture capacity (Fig 3a, back side) but did not affect soil O2 concentrations or CO2 fluxes. Soil Fe(II) concentrations were largely static, but some plots showed considerable net Fe reduction (Fig 3b), likely associated with litterfall C inputs during a major storm.

Linkages with LCZO collaborators
We found that nanoscale-order Fe oxides were highest in surface soils on ridges, and decreased with depth and landscape position (Fig. 4), following trends in fine root biomass (Fig. 1b). These landscape-scale measurements of soil Fe provide an ecosystem context for Fe mineralogy and dynamics measurements made by Wilmoth and Thompson (U. of Georgia). They found that specific Mossbauer spectral regions corresponded closely with Fe extractions (citrate/ascorbate). Their numerical model suggested that the frequency of redox oscillations was the best predictor of Fe dynamics, in agreement with our hypothesis that redox fluctuations structure biogeochemical function in this ecosystem.

Variation of soil oxygen concentrations with depth
(with Heather Buss, U. of Bristol)
Soil O2 concentrations fundamentally constrain biogeochemical and weathering processes throughout the soil profile. We examined trends in O2 with depth in granodioritic soils situated on a well-drained ridge in the Icacos Watershed. Concentrations of O2 were measured at 30-minute timesteps for 18 months.
Key questions:
• How do soil oxygen (O2) concentrations vary with depth over time?

• Do O2 concentrations decrease monotonically from the soil surface, or could alternative delivery mechanisms supply atmospheric O2 to deep soils?
Concentrations of O2 typically declined from near-atmospheric levels (21 %) in surface soils to approximately 15% at 5 m depth (Fig 5). Surface soil (0 – 1 m depth) O2 concentrations exhibited consistent temporal fluctuations on the scale of days – weeks (Fig 6). The largest fluctuations occurred at 0.5 m, where concentrations fluctuated as much as 3% over 30 minutes, and varied between 10 – 19 % over time .
Our finding that deep soil O2 concentrations occasionally exceeded surface soil concentrations suggests the potential importance of O2 supply to deep soils via fissures or other unexplored pathways. We will replicate this depth profile at multiple locations in volcanoclastic soils in summer 2012.

Research Location: 
Core Area(s) and/or Keywords: 

redox, carbon dioxide, methane, nitrous oxide, extracellular enzyme, iron, sulfur, manganese, nitrogen, soil moisture,

Source of Funding: 
NSF CZO Program
Dissemination: 
restricted
Contact Information
Person(s) Completing This Form: 
Steven Hall
Investigators: 
a. Whendee Silver b,c. Steven Hall, 608-886-6752
Investigator E-mail Addresses: 
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