Rainfall and soil redox cycling: can we predict biogeochemical - - PowerPoint PPT Presentation

Rainfall and soil redox cycling: can we predict biogeochemical thresholds?

Steven Hall, Whendee Silver May 2010 CZO meeting

Redox Cycling:

• Definition: the rapid (hours to days) conversion of

chemical species from oxidized to reduced forms, and back again QUESTIONS

• How do static vs. pulsed rainfall regimes affect redox

cycling in soil?

• Over what time scales does redox cycling occur?
• Can redox cycling help us better understand trace gas

emissions and other critical zone processes?

A Redox Primer

• pE is a measure of electron

abundance, or “redox potential”

• pE values are associated with

distinct terminal electron accepting processes (TEAPs)

• High pE ~ oxidizing
• low pE ~ reducing

pH = 5

Redox and Soils

• Knowing which redox

reactions dominate could help us predict greenhouse gas fluxes in the field

• Could also predict mineral

transformations, rock weathering, mercury methylation...

Redox Potential in the Field

• O2 is highly dynamic over

space and time

• Influence of rainfall?
• A redox ladder or a milieu
• f redox-sensitive

processes?

• Is the TEAP model useful

in tropical forest soils?

Soil O2 over time (Silver et al. 1999, Biogeochemistry)

First Hypotheses:

• 1: Repeated periods of intense rainfall

followed by rainfall exclusion will drive redox cycles between oxidized and reduced chemical species

• 2: Fluctuating rainfall regimes will increase the

total number of redox cycles relative to continuous rainfall or rainfall exclusion

• 3: Increased redox cycling will decrease soil-

atmosphere fluxes of CH4 and CO2 but increase fluxes of N2O

Redox Reactions of Interest:

• Iron reduction Fe(III) -> Fe(II):
• Can account for up to 44% of C mineralization (Dubinsky et al.

2010), and can occur under an aerobic headspace (Liptzin and Silver 2009)

• Can suppress CH4 production (Teh et al. 2008)
• Nitrate reduction (NO3- -> N2O, N2, or NH4+...)
• Various pathways for ecosystem nitrate loss or retention
• Several new mechanisms unraveled in Luquillo soils, but temporal

dynamism still poorly described

Redox Reactions of Interest:

• Manganese reduction (Mn(IV) -> Mn(II):
• Likely suppresses CH4 production, but not yet measured in Luquillo

soil

• Thermodynamically more favorable than iron reduction
• Sulfate reduction (SO42- -> H2S)
• Low rates recently measured in three Luquillo soils (2 nmol g-1 d-1)
• Likely suppresses CH4 production
• Co-occurs with mercury methylation

Field experiments

• Bisley watershed
• 32-day field campaigns
• Throughfall exclusion,

supplemental watering by treatment:

• (1) Daily watering
• (2) No watering
• (3) Four-day watering cycles
• (4) Eight-day watering cycles
• (5) Ambient control

Instrumentation and Sampling

• PVC collars
• Buried soil gas

chamber, O2 sensors

• Tension

lysimeters

• Soil moisture

probes

1.5 x 1.5-m throughfall exclusion plot Area of water application

Measurements

• Soil moisture and O2 concentration data

(continuous)

• Soil atmosphere concentrations of CO2, CH4, N2O

(daily)

• Soil surface fluxes of CO2, CH4, N2O (every eight

days)

• Soil porewater analysis of Mn, Fe, NO3-, NH4+, SO42-

, DOC, DON; soil extractions of the same species (every eight days)

• Soil extracellular enzyme activity

Goals

• Building a “bottom-up” framework for incorporating

redox cycling into models of soil greenhouse gas fluxes

• Predicting threshold effects of global change on

biogeochemical cycling

Complex effects of redox dynamics

• Oxidative extracellular

enzymes (e.g. phenol

• xidase) may represent an

“enzymatic latch” on soil C (Freeman et al. 2001)

• Fluctuating redox could

affect the activity of

• xidative extracellular

enzymes

• Effects on enzyme

synthesis and persistence are unknown

Lignified organic matter

Oxidative enzyme attack

+O2

• O2

Rapid degradation Slow degradation more CO2