Nitrogen Pools and Fluxes in Grassland Soils Sequestering Carbon

1
Nitrogen Pools and Fluxes in Grassland Soils
Sequestering Carbon

• R.T. Conant
• K. Paustian
• S.J. Del Grosso
• W.J. Parton
• Natural Resource Ecology Laboratory
• Colorado State University

2
Lit. review of impacts of improved pasture
management on soil C

• Requirements of papers included
• Management is main influence on soil C
• Paired plot with management control or measured
  changes over time
• Soil C (wet oxidation, dry oxidation, LOI)
• Conversion from cultivation to grassland
• Literature survey
• 112 references
• 308 data points
• 18 countries (see map)
• Evaluated influence of
• Management (meta-analysis)
• Climate (MAT, MAP, PET)
• Depth
• Time
• Native vegetation

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Soil C sequestration with changes in grassland
management
4
Potential regional to global C sequestration in
grasslands
460
69.8
71.5
45.5
8.0
2.5
5
C sequestration vs. N sequestration in grasslands
6
C sequestration vs. N sequestration in
grasslands by management change (kg C/N ha-1
yr-1)
Native to pasture conversion Location
expmnts. C seq. Nseq. Costa Rica La
Selva (3) -0.176 0.02 New Zealand Oropi (1) 0.1
0 0.06 Taupo (1) 0.74 0.08 Tiniroto (1) 1.06 0.1
0 Tirau (1) 0.34 0.02 New Plymouth (1) 0.89 0.08
Eggmont (1) 0.39 0.17 Hamilton (1) 0.52 0.05 Mat
apiru (1) 0.74 0.07 Tokumaru (1) 0.39 0.03 Waito
(1) 0.77 0.06 Average 0.49 0.06 all NZ
conversions included fertilization/sowing
improved species
Fertilization Location studies C seq. Nseq. New
Zealand Lake Taupo (1) 0.09 0.06 Australia Kybybol
ite (7) 0.29 0.05 Pnjarra-Waroona
(1) 0.63 0.04 Average 0.31 0.05
Grazing Location C seq. Nseq. Canada Manyberries
(3) 0.09 0.01 UK Holme Knott (3) 0.22 0.01 USA Che
yenne (6) 0.50 0.04 Mandan (5) -0.18 -0.01 Avera
ge 0.17 0.01
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Changes in C cycling with improved pasture
management
CO2
Improved management (fertilization, grazing, etc.)
Initial
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N sequestration with improved pasture
management Where does the N come from?
N2O
?
?
Improved management (fertilization, grazing, etc.)
Initial
NO-3
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Daycent trace gas sub-model
H2Osoil, pH, Tsoil, Texture
D/Do, PPT
Ngasnit
NH4
N2O
nitrification
NO3-
NOx
N2
denitrification
Ngasden
H2Osoil, C, Texture
D/Do, NO3C
DAYCENT Model Validations
N2O Emissions
Soil Organic C
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C sequestration vs. N2O fluxes at CPER Daycent
simulations
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Grazing at Mandan, ND and Cheyenne, WY Model
results
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Calculation of Net Global Warming Potential (for
C sequestration and N2O flux)
Costa Rica GWP calculation C seq. GWP -? N2O
GWP Net GWP of conversion
Grazing/general GWP calculation C seq. GWP (CO2
sequestered) -? N2O GWP (model output
280) Net GWP of conversion
Conversion/fertilization GWP calc C seq.
GWP - ? N2O GWP Net GWP of conversion N2O
flux N fixation rate (Goh et al. 1999) N
conversion factor (average of 26 values from 7
publications) for fertilization
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Changes in grasslands management Net Global
warming potential
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Conclusions

• Changes in soil N are closely related to changes
  in soil C
• across multiple treatments
• whether or not N is added (grazing studies)
• N2O responses to changes in management
• Carbon sequestration with improved management
• is a viable GHG reduction mechanism
• but, N2O must be considered
• Changes in grassland management lead to decreased
  GWP in most cases