The Iron Redox Engine Drives Carbon and Nitrogen Cycling in Terrestrial Ecosystems

Whendee L. Silver1, Steven J. Hall1, Daniel Liptzin2, Wendy H. Yang1

1. Environmental Science, Policy, and Management University of California, Berkeley

2. Agricultural Sustainability Institute University of California, Davis

NSF Division of Environmental Biology NSF Critical Zone Observatory NSF GeoSciences

• Iron (Fe) is the most abundant redox active metal on Earth

• Many soils are rich in iron minerals

• Redox happens • Fe redox dynamics can have a significant impact on biogeochemistry in terrestrial ecosystems

0 1000 2000 3000 Day of Study

0

7

14

21

Soil

Oxy

gen

(%)

Redox happens…. even in upland soils

Silver et al. 2012

Fe(III)

Fe(II) (aq)

Fe oxidation

DissimilatoryFe reduction

Oxic

Anoxic

How does the Fe redox engine drive C and N biogeochemistry?

Fe(III)

Fe(II) (aq)

(CH2O)n

CO2

Fe oxidation

DissimilatoryFe reduction

Anaerobic respiration

Oxic

Anoxic

Carbon oxidation coupled to Fe reduction

Highly weathered tropical forest soils are typically Fe-rich. Tropical forests have the highest rates of soil respiration globally. How much carbon is oxidized by Fe reduction in tropical forest soils?

Fe reduction fuels considerable soil respiration

Dubinsky et al. 2010

Iron reduction

• likely cycles on 2-week intervals in humid tropical forests associated with fluctuating redox

• rates of 78-133 µg Fe(II) g-1 d-1 could oxidize 100-171 g C m-2 y-1

• equivalent to 23-39 % of litterfall production

Liptzin et al. 2010

Fe(III)

Fe(II) (aq)

Fe oxidation

Oxic

Anoxic

Iron oxidation can produce free radicals

O2

…OH• O2•-

Production of free radicals

The free radicals can attack organic molecules

…OH• O2•-

Production of free radicals

Oxidized C

CH2O

H2O

Iron - oxygen reactions: “Fenton chemistry” Fe(II) and O2 can generate strong oxidants, including OH⋅

Are these reactions important in natural ecosystems?

r2 = 0.84, p < 0.01

0 1 2 3 4 5 6 7 8 Fe(II) (mg g-1)

0.00

0.02

0.04

0.06

0.08

0.10

0.12 O

xida

tive

Act

ivity

Hall and Silver in prep

Fe(II) was strongly positively correlated with oxidative activity…..

Oxi

dativ

e Act

ivity

0

0

.05

0.1

0

0

.15

Ridge1 Ridge2 Cloud1 Cloud2

Control Autoclave

Hall and Silver in prep

…even when the soils were dead…

µg Fe(II) mL-1

Oxi

dativ

e Act

ivity

0.0

0

.2

0.

4

0.6

0

.8

Time (min)

2 4 6 8 10

Hall and Silver in prep

…and when there was no soil at all!

Fe redox reactions have the potential to drive N cycling. Can Fe reduction be coupled to ammonium oxidation?

Feammox: Fe coupled anaerobic NH4+ oxidation

NH4+

Fe(II) (aq) Fe(III)

N2, NO2-

N2 ΔGr = -245 kJ mol-1

NO2- ΔGr = -165 kJ mol-1

Feammox produced 30N2 in a tropical forest soil

Yang et al. submitted

0.0

0.5

1.0

1.5

2.0 30

N2 pr

oduc

tion

(µg

N g

-1 d

-1) P < 0.001

N = 8

Direct 30N2 production accounted for 55 to 79% of Feammox.

P = 0.08 N = 8

30N

2 pro

duct

ion

(µg

N g

-1 d

-1)

0.0

0.5

1.0

1.5

2.0

Feammox

• could oxidize 2 to 4 kg NH4+-N ha-1 y-1 with just 1 % of

Fe reduction in surface soil (0.23 µg Fe(II) g-1 d-)

• equivalent to tropical forest denitrification (1-4 kg N ha-1 y-1)

•Iron reduction can oxidize significant amounts of organic carbon in fluctuating redox soils

•Iron oxidation may also be responsible for carbon oxidation, where minerals “masquerade” as enzymes

•Iron reduction coupled to ammonium oxidation can result in nitrogen losses that can be both locally and globally significant

Iron redox cycling

Steven Hall Tuesday 10:20 am MW 2004

Dan Liptzin Thursday 8:45 am MW 2002

Wendy Yang Thursday Afternoon poster B34E

Thursday 4:30 pm MW 2002