1 Carbon turnover in the rhizosphere and why plants release carbon in soil Yakov Kuzyakov [email protected]Soil Science of Temperate Ecosystems Georg-August-University Göttingen 3 http://kfrserver.natur.cuni.cz/globe/others.htm Units: Petagrams (Pg) = 10 15 gC – Pools: Pg – Fluxes: Pg/year Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2 Global C cycle 4 Element contents in soils (mg/kg), the Earth crust and sediments (Sparks 2003) 2,200 260 30-1,600 700 S 670 1,000 35-5,300 800 P 470 25 200-5,000 2,000 N 33 50 2-250 30 Cu 52 80 2-750 50 Ni 2 1.5 0.1-40 1.2 Mo 95 75 1-900 90 Zn 770 950 20-10,000 1,000 Mn 5,700 23,000 150-25,000 5,000 Na 20,000 21,000 80-37,000 14,000 K 14,000 23,000 400-9,000 5,000 Mg 66,000 41,000 700-500,000 15,000 Ca 29,400 480 7,000-500,000 20,000 C (total) 41,000 41,000 2,000-550,000 40,000 Fe 72,000 82,000 10,000-300,000 71,000 Al 245,000 277,000 250,000-410,000 330,000 Si 486,000 474,000 - 490,000 O Sediments (mean) Earth crust (mean) Soils Median Range Element Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2 5 Main differences between soils and soil parent material • High C and N content 2 biophilic elements • High content of chemical energy available for microorganisms • High activity of microorganisms • Nutrients are available for plants • ……… Fertility! the ability of soil to maintain plant growth: – Water – Nutrients – Oxygen Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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Main differences between soils and soil parent material
• High C and N content � 2 biophilic elements• High content of chemical energy available for microorganisms• High activity of microorganisms• Nutrients are available for plants• … … …
Fertility!the ability of soil to maintain plant growth:– Water
– Nutrients
– Oxygen
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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Very high C and N content!
Haplic Phaeozem
Fertility!
Importance of roots
7
Sources of organic C in soils
Plants ( ∑ > 99%)
• Dead / Litter:– Above ground
• Leaves• Shoots
– Below ground • Roots
• Living:– Rhizodeposition
(organic C released by living roots)
Other sources ( ∑ << 1%)• Algae • CaCO3• Organic C of some rocks (shist)
Contribution of root-C to soil-C is ~ 2.5 timeshigher than of shoot-C(Rasse et al. 2005 Plant and Soil)
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Functions of root C in the rhizosphere• Modification of soil chemistry
– pH and redox– Organic acids composition– Chelating substances for nutrient solubilizaiton – Detoxication of Al3+ and Fe3+ (at low pH)
• Symbiosis with soil microbes– N2 fixing bacteria:
• Defense against pathogens (allelopathics)• Improving of soil structure• Lubricator to decrease soil impendance• .......
Substrate and energy for symbiosis
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Trade: C for N
Plants provide C as energy for
• Rhizobia and Frankia to fix atmospheric N2
• Associative bacteria to fix atmospheric N2
• Mycorrhizal fungi to acquire N and P from soil
• Rhizospheric bacteria to accelerate SOM mineralization for Nmin release
• ... ... ... ... ... ...
C costs of N aquisition
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Legumes:N2 fixation and
shoot dry matter
Herridge et al., Plant & Soil 2008
Trade: C for N
15 kg N / Mg DM
25 kg N / Mg DM
50 g DM for 1 g N
Vance & Heichel 1991
1 g N ~ 6 g C1 Mol N ~ 7 Mol C
1 g N ~ 2.5-4 g CWarembourg & Roumet 1989
Symbiotic N 2 fixation
20 g C for 1 g N
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Trade: C for N
Plants provide C as energy for
• Rhizobia and Frankia to fix atmospheric N2
• Associative bacteria to fix atmospheric N2
• Mycorrhizal fungi to acquire N and P from soil
• Rhizospheric bacteria to accelerate SOM mineralization for Nmin release
• ... ... ... ... ... ...
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Mycorrhiza of young pine
Smith & Read 14
Trade: C for N
Plants provide C as energy for
• Rhizobia and Frankia to fix atmospheric N2
• Associative bacteria to fix atmospheric N2
• Mycorrhizal fungi to acquire N and P from soil
• Rhizospheric bacteria to accelerate SOM mineralization for Nmin release
• ... ... ... ... ... ...
C and N cyclesin the rhizosphere
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Rhizosphere:Opposite directions of C and N fluxes
C release
Nuptake
C and N fluxes in opposite
directions
Yaalon 2002
Limitations for microorganisms: • in root-free soil: available C• in rhizosphere: available N
TradeC for N
Continuous input = continuous decomposition
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Labeling: Approach to study processes under steady state
Pulse labeling• Pulse addition of a tracer (13C, 14C, 15N, …)• Chasing of the tracer in various pools
13CO214CO2
13C14C
15N
15N
Parameters• Time lag between assimilation and any flux• C and N flow rates through any pools• Residence time in pools • Dynamics of C, N, … in individual pools:
– turnover times– flux rates � modelling
• known C input � C budget, high resolution & sensitivity
C/N
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Bangor, Wales, UK 2005C input and turnover
of organics in the rhizosphere
21
Fichtelgebirge 2009
Foto: S Heinrich
Drought effects on C fluxes in the rhizosphere
microorganisms of spruce
23
Budget of assimilated Cfor some grasses and crops
in % of total assimilated C
Below ground:
• Roots 10 – 30%
• Root respiration 3 – 6%
• Soil organic matter 1 – 3%
• Microorganisms 1 – 5%
• Rhizodeposition 5 – 15%
Above ground:
• Shoots 25 – 55%
• Shoot respiration 20 – 35%
Kuzyakov & Domanski 2000 JPNSS
Available substrate for microorganisms!
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Janssens et al. 2002 Forest Ecol & Management
Budget of assimilated Cfor trees
in % of total assimilated C
Time scale of the links
between above and below?
Intro Rhizo-C Time Lag Hotspots Priming Elevated C O2
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Photosynthesis is the main source of available substrates in soil
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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200
250
300
350
400
450
500
550
bulk > 2mm 0.25-2mm <0.25mm
Amb Elev
Act
ivity
(nm
ol g
-1 h
-1)
• No changes of pools :– SOM fractions
– microbial biomass
• Changes of microbial activity:– Growth rates– Enzyme activities: β-Glucosidase,
Chitinase , Sulphatase, Phosphotase
C and N turnoverunder elevated CO 2
Dorodnikov et al. 2007 Soil Biology Biochemistry 2009 Global Change Biology
• Elevated CO 2 leads to:1. Higher C input by plants in the soil2. Activation of microbial biomass3. Faster C and N turnover4. Higher nutrient mobilization
� Acceleration of element cycles
Chitinase
Microbail growth rates
Aggregate classes
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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C and N turnover under elevated CO 23 FACE experiments
Hohenheim+150 ppm CO2
canola, wheat
Braunschweig +150 ppm CO2
sugar beet, wheat
Biosphere 2+400/+800 ppm CO2
poplar
FACECO2
plant
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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R2 = 0.76
0.15
0.25
0.35
0.45
300 600 900 1200
Atmospheric CO 2 (ppm)
Spe
cific
gro
wth
rat
e (h
-1)
Braunschweig, Beta vulgarisBraunschweig, Triticum aestivumHohenheim, Brassica napusBiosphere-2, Populus deltoides
1.0
1.2
1.4
1.6
Brassica napus
540 ppm
Triticum aestivum550 ppm
Beta vulgaris 550 ppm
Populus deltoides800 ppm
Populus deltoides
1200 ppm
Rat
io: µ
ele
vate
d C
O 2
/ µ
am
bien
t CO
2
Rhizosphere soil
Root free soil
Microbial growth ratesdepending on
CO2 concentration
Elevated CO 2
Blagodatskaya et al. 2010 Global Change Biology
Rhizosphere hotspots and priming will be more
important in future because of acceleration
of biogeochemical cycles
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
41De Graaff et al. 2006 Global Change Biology Kuzyakov 2011 Nature Climate Change
Priming
Relative increase of
fluxes & pools
22%
in soil:• no changes of pools• strong increase of fluxes
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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C sequestration in soil after 5 (♦), 10 (■), and 15 (▲) months. Means for 6 plants expressed as % of ambient CO2 with no added nutrients∆ 15-month harvest, added nutrients
C sequestration in soil
� Soil microbial respiration after 15 months. Means for 6 species expressed as % of the ambient CO2 control with no added nutrients. ▲ no added nutrients; ∆ added nutrients
Microbial respiration
���� C sequestration in soil decreases
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2
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Photosynthesis:• is the main source of available substrates in soil• links above ground and C turnover in soil• affects belowground processes in short time
• trees: days grasses: hours• initiation of hotspots
N
P
Priming:• mobilization of extra nutrients for plants
N
N
Conclusions
Rhizodeposition is the most important process linking:• above and belowground processes• plants and soils• roots and microorganisms• nutrients and C
Hotspots:• especially in the rhizosphere life time: few days• increase of microbial activity ����
• acceleration of SOM turnover ���� priming effectC
C/N
Release of C in soil by living roots:• ecological importance for symbiosis with microbes
Thanks!
Intro Rhizo-C Time Lag Hotspots Priming Elevated CO 2