Transcript Trace Gases - Arizona State University

Rhizosphere interactions under elevated CO2:
Impact on soil organic carbon dynamics
Shuijin Hu
North Carolina State University
Raleigh, NC 27695
Email: [email protected]
An Overview of Recent and Ongoing
Research Projects
Carbon & nitrogen dynamics
in agroecosystems
Microbes & plant
competition
Plant-Microbe
Interactions
Microbial diversity &
ecosystem stability
Microbial responses
to climate change
Air temperature has
increased ca. 0.6 oC
Air temperature is
predicted to increase
another 2-5 oC in the
next 100 years
The increasing atmospheric CO2 is correlated
with the temperature rise
Global warming has
some major
consequences
Global climate change: Atmospheric CO2 has
been increasing since the Industrial Revolution
One central goal of global change research is
to understand:
whether and how terrestrial ecosystems can
sequester more organic C.
Why ecosystem C sequestration for
mitigation of climate change?
Active C pools on
the Earth surface:
1. Air CO2-C: 750 ×
1015 g
2. Biomass-C: 550-650
× 1015 g
3. Soil organic C:
1500-2100 × 1015
g
Elevated CO2 stimulates photosynthesis
and net primary production –
Increases short-term C inputs
Herrick & Thomas. 2001
Prerequisites for long-term ecosystem C
sequestration under elevated CO2
1. Plants can effectively
acquire available nutrients;
2. Mechanisms exist to
sustain N supply for plants;
3. Microbial decomposition
is “contained”.
Plants are primarily nutrient-limited but microbes are C-limited
1. Can plants acquire available nutrients
more effectively under elevated CO2?
The prevailing paradigm in 1990’s was:
Microbes outcompete plants for acquiring nutrients in soil.
Elevated CO2
Plants
C inputs
Microbes
Available N
Organic N
Elevated CO2 alters the plantmicrobial competition in favor
of plant N utilization.
Hu et al. 2001. Nature
Nutrient Limitation of Ecosystem C Sequestration
Luo et al. 2004. Bioscience
2. Are there mechanisms that sustain N
supply for plants under elevated CO2?
Can CO2-stimulation of plant growth be sustained
over time?
To a large degree, it will depend on whether plants can
acquire sufficient nutrients from the organic pool.
Elevated CO2
Plants
C inputs
Microbes
Available N
+
Elevated CO2 increased plant N
acquisition from soil organic N
pool.
Organic N
Hu et al. 2005. Global Ch. Biol.
Zak et al. 2011. Ecology Letters.
Drake et al. 2011. Ecology Letters
The Summary
Plants are more effective in nutrient acquisition
under elevated than ambient CO2.
Next Question
How does elevated CO2 increase plant
nutrient acquisition from soil?
Elevated CO2
(15NH4)2SO4
NO3–
NO3–
bacteria
fungi
Residues
NO3–
PO43K+
NH4+
Ca2+
Mg2+
PO43Hu et al. 2005, Global Change Biol.
Indeed, one major finding over the last two
decades is: Elevated CO2 increases soil fungi,
particularly mycorrhizal fungi.
Then the question is:
Why?
Treseder, 2004. New Phytologist
Ectomycorrhzae
Arbuscular Mycorrhizae
1. Mycorrhizae are symbiotic associations between
plant roots and fungi;
2. Over 80% of terrestrial plants form mycorrhizae
with fungi;
3. Plants allocate up to 20% of photosynthates to
mycorrizal fungi under ambient CO2 and up to 3540% under elevated CO2.
AM fungi protect organic C from microbial attack
Scanning electron micrograph of a VA mycorrhizal fungus with particles of
clay firmly attached (left) and VA mycorrhizal fungi binding microaggregates
into a stable macroaggregate (Tisdall and Oades 1979).
The current paradigm
of elevated CO2 impact on soil C
Elevated CO2
Plant Growth
Mycorrhizae
Extraradical
Fungal Hyphae
Cell Wall Materials
(Chitin)
Glomalin
Polysaccharides
Soil Aggregation
Carbon Sequestration
Rillig et al., 1999, Nature;
Antoninka et al. 2009, GCB.
Treseder & Allen, 2000, New Phytol.
Wilson et al. 2009. Ecol. Letters
Major issues related to the current paradigm
1. The current paradigm is largely based on
correlative information, rather than direct
evidence;
2. Emerging evidence suggests that AM fungi may
increase decomposition of organic residues
(Hodge et al. 2001, Nature; PNAS 2010; Tu et al.
2006, Global Change Biology).
Hodge et al. 2001
Can CO2-stimulation of AM fungi increase
decomposition of organic matter in soil?
Five steps to assess the impact of CO2–
enhancement of AM fungi on organic C
decomposition
Step 1
A microcosm experiment to assess AMF-mediated organic
C decomposition under different CO2 and N levels
CSTR chambers
Microcosm unit
Isolation of root contribution from fungal effects
on organic C decomposition
13C/15N
Nylon net
(1.6 mm)
labeled materials
Nylon mesh
(20 µm )
Fig. 7 Root- and hyphal-ingrowth cores
Step 2
A microcosm experiment to examine the impact of AMF
identity on AMF-mediated organic C decomposition
under different CO2 levels
AM fungal species or assemblages
A. Acaulospora morrowiae
B. Gigaspora margarita
C. Glomus clarum
D. Assemblage A: The combination of A, B and C
F. Assemblage B: Eight species from field, including A, B & C
Step 3
A field experiment to determine AMF-mediated
organic C decomposition under elevated CO2
Open-top chambers used to simulate atmospheric
CO2 concentrations under future climate scenarios
Result 1:
Elevated CO2 increased mycorrhizal infection of
roots and AMF biomass in soil
Fig. S3. Elevated CO2 stimulated the growth of AMF in roots of
Avena fatua and wheat, and in soil
Result 2:
Higher AMF under elevated CO2 increases decomposition
A: Microcosm Exp. 1;
B. Microcosm Exp. 2;
C. Field Exp.
Cheng et al. 2012. Science
Why does elevated CO2 concentration
increase organic C decomposition?
Our initial hypothesis was:
Elevated CO2 stimulates organic C decomposition because
1. N becomes more limiting,
2. plants under elevated CO2 need to obtain more N, and
3. plants allocate more carbohydrates to prime
decomposition through stimulating saprotrophs.
Does elevated CO2 really reduce N availability?
Result 3:
Elevated CO2 reduces soil NH4+ in N-limiting soils but
increases soil NO3- in the N-rich field soil
A: Microcosm Exp. 1;
B. Microcosm Exp. 2;
C. Field Exp.
Cheng et al. 2012. Science
These results led us to ask:
1. Why do plants not use the increased NO3- under
elevated CO2?
2. Does elevated CO2 lead to plant preference of
soil NH4+ over soil NO3- ?
Fig. 1. Three methods for assessing
nitrate absorption (Absorb) and
assimilation (Assim.) in wheat
and Arabidopsis plants in hydroponic
solutions where the shoots were
exposed to atmospheres containing 380ppm CO2 and 21% O2, 720-ppm CO2
and 21% O2, or 380-ppm CO2 and 2% O2.
Bloom et al. 2010. Science
Step 4
A meta-analysis of elevated CO2 impact on
soil N and plant N acquisition in the
literature
Does elevated CO2 lead to plant preference of soil
NH4+ over soil NO3- ?
Step 4
A meta-analysis of elevated CO2 impact on
soil N and plant N acquisition in the
literature.
1. 38 studies that quantified the concentrations of soil
NH4+ and NO3– and/or the capacity of plant use of NH4+
and NO3– under eCO2;
2. These studies encompassed more than 58 species of
crop, grass, and tree species.
Result 4:
Elevated CO2 reduced plant NO3- uptake and
increased soil NO3- (Net effect %).
40
20
-20
-40
Cheng et al. 2012. Science
Step 5
A field experiment to assess the impact of
nitrification inhibition on AMF-mediated organic
C decomposition under elevated CO2
X
Result 5:
Inhibition of nitrification offsets CO2-enhancement of
AMF-mediated organic C decomposition
Cheng et al. 2012. Science
The Summary
Fig. 4. A conceptual framework
of AMF-mediated decomposition
driven by CO2-enhancement of
plant N acquisition. CO2enhancement of AMF primes
residue decomposition and
ammonium (NH4+) release
and optimizes NH4+ acquisition,
while reducing nitrification.
Cheng et al. 2012. Science
Potential Implications
1.The contribution of arbuscular mycorhizal fungi to
soil C sequestration under future CO2 scenarios may
have been over-estimated;
2.Increasing plant N use efficiency and reducing
decomposition through effective management of soil
N transformations are keys to facilitate soil C
sequestration.
Acknowledgements
Lab members (in the last 6 years): Sean Blosvies*, Xin Chen, Jared Chauncey, Lei
Cheng, Mary Claire Garrison, Natalie Gross*, Anna Johnson, Marissa Lee, Lingli Liu,
Karen Parker, Tomin Sa*, Qinghua Shi*, Cong Tu*, Jinping Wang*, Liang Wang, Yi
Wang, Dolly Watson, Scotty Wells*, Li Zhang*, Yi Zhang*, Lishi Zhou*
Major Collaborators
NCSU: David Shew, Chris Reberg-Horton, Julie Grossman, Frank Louws, Mike Benson,
David Bird, Mike Burton, Nancy Creamer, Marc Cubeta, Ralph Dean, Greg Hoyt,
Paul Mueller, Jean Ristaino, David Ritchie, Tom Rufty, Michelle Schroeder, Wei Shi,
Lane Tredway, Dolly Watson
USDA-ARS: Fitz Booker, Kent Burkey
Funding Agencies:
USDA-NRI: Soil Processes, Pest Management Alternative, Managed Ecosystems
USDA-SARE
USDA_NIFA_ORG
NC Center for Turfgrass Environmental Research & Education
USDA-ARS Plant Research Unit