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

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Distribution of EU grasslands

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• 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