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C sequestration of a grazed permanent grasslands: uses of complementary methods
for data analyses and interpretation
Katja Klumpp, Juliette M.G. Bloor, Rie Nemoto, Damien Herfurth, Olivier Darsonville
GLOBAL SYMPOSIUM ON SOIL ORGANIC CARBON, Rome, Italy, 21-23 March 2017
Global technical mitigation potential by 2030
Drawn from data in Smith et al., 2007a.
89% of greenhouse gas savingsthrough C sequestration
3
Distribution of EU grasslands
4
• Considerable variation linked to climat, management, vegetation type but also measure technique
Mean 0.8 (±0.16) Mg C /ha.yr
Literature : C storage in grasslands (Mg C/ha.an).
Literature : Equilibrium assumption of C storage in grasslands (Mg C/ha.an).
Smith et al 2014 GBC
• 0 – 40yrs after management (land use) change grasslands may store large amounts of C
• 100-120yrs of constant management before Grasslands attain an equilibrium
• To improve C sequestration potential we would need better understanding of the impacts of climat, management and vegetation.
• Repeated soil sampling Difficulty : - large spatial heterogenity (i.e. large sampling number)- temporal sampling intervals (5-10yrs)- difficult to disentangle climate and management effect
• Eddy covariance gas exchange measurements Difficulty : - no replicate plots - C storage over the whole ecosystem- link measurements to changes in soil C
Methods to measure soil organic carbon changes
Aims
To assess the ability to capture net carbon sequestration of grassland ecosystems.
Method comparison - repeated soil sampling - eddy covariance technique
Further insight where sequestrated C goes, we analysed changes in - spatial distribution of soil C stocks - soil organic matter pools ( i.e. labile, passive, inert, etc)
Intensive (2.8ha) 1.1 LSU ha.yr-1210 g N ha.yr-1 (3 splits)13.7 % clover
7 dominante species (36)Pot Prod: 7.1t DM green.ha.yr-1Standing: 2.6t DM.ha.yr-1
Extensive (3.8ha)0.5 LSU ha.yr-14.4 % clover
7 dominante species (31)Pot Prod: 5.1 t DM green.ha.yr-1Standing: 2.2 t DM.ha.yr-1
Set up in May 2002Grazed May-October
Paired permanent grassland site, F-Laqueuille, Central France Alt. 1050m, mean T 8°C, 1000mm
Flux TowerCO2 + CH4
• Repeated soil sampling 2004, 2004, 20124 layers (0-10, 10-20, 20-40, 40-60cm)
EC technique - Net Carbon Storage
Simplified for temperate managed grasslandsNCS = FNEE - FCH4-C + Fmanure + Fharvest + Fanimal-products + Fleach
(i.e. Allard et al. 2007, Soussana et al 2010):
(NCS or NECB)
[CO2] = C’
Vertical wind = w’CO2 flux = w’ c’
EC-flux towers (spatial ~ 1 to 3ha ) Net Ecosystem Exchange (NEE)
X
X
X
OPOM DOC
Os+c-rSOC
S+A
Physically protected against fast decomposition
rSOC IOM
Chemically resistant
Splitting DPM/RPM ratio calculated by equilibrium
scenario
Splitting BIO/HUM ratio calculated by equilibrium
scenario
Decomposable Plant Material
(DPM)
Resistant Plant Material (RPM)
Microbial Biomass (BIO)
Humified Organic Matter (HUM)
Inert Organic Matter (IOM)
RothC model• Soil organic C pools -> Zimmermann et al. (2007) fractionation method
Analysed Fractions Soil C pools
Net C storage by EC technique covariance
Extensive Intensive
• Mean annual Net C storage (Mg C/ha.yr)
1.80 (±0.5) 2.2 (±0.5)
• Cumulated (10yrs)Net C storage (Mg C/ha) 18.9 19.9
sink
source
• Considerable variation linked to climat and management
dry dry drywet wet wet
– Stock changes over time (0-60cm depth, n=50 )
EXT INTEXT INTExtensif Intensif
2004
2012
C accumulates over time but no difference between grazing treatments 12
2008
2004
2012
2008
Repeated soil sampling
Distribution in the soil profile
Extensive Intensive
• For equal soil mass
SOC stock changes are mainly in deeper soil layers (>40cm)
• For observed soil mass
gainloss gainloss
gainloss gainloss
2004-2008 (….)2008-2012 (----)2004-2012 (__)
Comparison between repeated soil sampling and EC technique
Mg C/ha Extensive IntensiveSoil EC-flux Soil EC-flux
2004-2008 15.8 (±1.0) 8.8 14.6 (±0.8) 7.32008-2012 -0.8 (±1.1) 9.3 5.8 (±0.7) 11.82004-2012 14.9 (±1.0) 18.0 20.4 (±0.8) 19.2Annual mean 1.9 (±0.1) 2.6 (±0.1) 1.9 (±0.1) 2.2 (±0.5)
Both methods are in good agreement for long term comparison.
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EXT
INT
No spatiale dependence due to a grazing gradient
R²=0,96
Distance
Distance
Sem
i-var
ianc
eSe
mi-v
aria
nce
No spatiale dependence
Where did the C go ?Spatial distribution in the 0-10cm soil layer
Where did the C go ?Mean SOC pools (Mg C/ha) over 0-60cm
Extensive Intensive
2008
2012
2008
2012
Little changes in the DPM, RPM and BIO but the HUM and IOM pool
Extensive
2008
2012
Where did the C go ?SOC pools per layer (Mg C/ha)
HUM
IOM
Soil layers
Where did the C go ?Differences in SOC pools between 2008-2012 (Mg C/ha) in the 0-60 cm
Little changes in the DPM, RPM and BIO but the HUM and IOM pool
Summary • Grasslands were a sink of C
• Measured C stock changes matched well between measure methods(2.2 Mg C/ha.yr EC technique vs 1.9 Mg C/ha.yr soil sampling)
• For bulk soil (0-60cm): only little effect was observed between grazing treatments due to high spatial variability
• However, marked C stock changes were observed in deeper soil layer.
• This was confirmed by soil organic matter pools which showed a transfer of C from humified C (HUM) to inert C (IOM) - between years - between soil layers
• EC technique allowed to disentangle climate from management effectseg. intensive treatment had higher net C storage in dry climatic
years, vice versa for the extensive treatment.
• Methods are complementary EC-technique offer to test effective mitigation optionrepeated soil sampling; long term tendencies, baselines, land use…
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Thank you