Carbon balance of an intensive grazed grassland

Unit of Biosystem Physics

Jérôme Elisabeth1, Beckers Yves2, Bodson Bernard3, Moureaux Christine3, Aubinet Marc1

1 _{University of Liege, Gembloux Agro-Bio Tech, Unit of Biosystem Physics, 8 Avenue de la Faculté, B-5030 Gembloux, Belgium -} 2 _{University of Liege, Gembloux Agro-Bio Tech, Animal Science Unit, 2 Passage des Déportés, B-5030 Gembloux, Belgium -}

3 _{University of Liege, Gembloux Agro-Bio Tech, Crops Science Unit, 2 Passage des Déportés, B-5030 Gembloux, Belgium.}

Carbon balance of an intensive grazed grassland

This research was funded by The « Direction Generale opérationnelle de l’Agriculture, des Ressources naturelles et de l’Environnement - Région Wallonne » Project n° D31-1235, January 2010 - December 2011

Contact Person: Jérôme Elisabeth - University of Liege – Gembloux Agro-Bio Tech (GxABT) - Unit of Biosystem Physics, 8 Avenue de la Faculté - 5030 Gembloux - Belgium

Tel : +32 (0)81 62 24 90 - Fax : +32 (0)81 62 24 39 e-mail : [email protected]

1. OBJECTIVES

Long term objectives:

•To compute carbon and GHG balances of a grazed managed grassland •To propose mitigation scenarios in order to improve the GHG balance

This poster:

•To analyze the yearly carbon budget of the grassland

2. EXPERIMENTAL SITE

•Situation: Belgium, Dorinne (l50° 18’ 44’’ N; 4° 58’ 07’’ E; 248 m asl.) •Climate: temperate oceanic

•Mean annual temperature: 10°C •Annual precipitation: 800 mm

•Type: permanent grassland •Surface: 4.2 ha

•Slope: moderate (1 to 2 %)

3. GRASSLAND MANAGEMENT

•Rotational grazing: mean annual stocking rate = 1.6 LU ha-1

•Intensive management

4. CARBON BUDGET ESTABLISHMENT

5.2 Impact of climate and management on CO

flux

6. CONCLUSIONS

•High respiration value < > low assimilation value  The site behaved as a net source of CO₂

•The NBP is not significantly different from zero

 To conclude about the sink or source activity of the plot,

long term measurements are necessary

6. CONCLUSIONS

•High respiration value < > low assimilation value  The site behaved as a net source of CO₂

•The NBP is not significantly different from zero

 To conclude about the sink or source activity of the plot,

long term measurements are necessary

7. PERSPECTIVES

•Second year of measurements: comparison between the C budget  Impact of climate on the C budget?

•To analyze the drought impact in summer 2010 and spring 2011  H₂O fluxes

•Measurements of N₂O and CH₄  full GHG budget

NBP=NEE+C_NBP,import+C_{NBP,complement}+C_NBP,export+C_NBP,CH4+C_NBP,lw+C_NBP,leach

Table 1: List of management activities from 12 May 2010 to 12 May 2011. -20 -10 0 10 20 C f lu xe ( g C m -2 d ay -1 ) a) TER GPP -10 -5 0 5 10 N E E ( g C m -2 d ay -1 ) _b)

Jun10 Aug10 Oct10 Dec11 Feb11 Apr11

-150 0 150 300 C u m u la ti ve N E E ( g C m -2 ) c)

5. RESULTS

5.1 Climate

5.3 Seasonal course of the fluxes

5.4 Carbon balance and related fluxes

deduced from daytime flux/radiation response. Error bars represent 95% confidence intervals.

Table 2: Measurement methods of carbon fluxes.

Fig. 1: Instantaneous stocking rate (livestock unit per hectare, LU ha-1_{) between 12 May 2010 and 12 May}

2011).

Jun10 Aug10 Oct10 Dec11 Feb11 Apr11

0 1 2 3 4 5 6 In st an ta n eo u s st o ck in g r at e (L U h a -1 )

Table 3: annual carbon fluxes at Dorinne grassland site (12 May 2010 -12 May 2011).

The site behaved as a small source of carbon, BUT:

•The NBP value is not significantly different from zero

•NEE < 10% (TER and GPP)  a small relative change in one of these fluxes may strongly modify the net budget

•It was obtained under particular climatic conditions, characterised by drought during summer 2010 and spring 2011

•Annual GPP and TER considerably larger than any other fluxes

•Annual C inputs (≠ GPP) =194 g C m-2 y-1 ≈ NEE

•Annual C export ≈ 25% of C inputs

Balance between C imports and C exports created

a large departure of NBP from NEE: Cumulative

NBP = 57 ±58 g C m-2 y-1

•Considering uncertainties: NPP = C_intake + C_NBP,export

•Cattle respiration (R_l) is lower than 10 % of TER

Fig. 6: (a) Daily totals of Total Ecosystem

Respiration (TER), Gross Primary Productivity

(GPP), (b) Net Ecosystem CO₂ Exchange (NEE) and (c) cumulative NEE. Fluxes are presented over one year of measurements (12 May 2010 – 12 May 2011).

•Spring 2010:

High accumulation of C in the system: fluxes dominated by photosynthesis

GPP reached its maximal value by the end of May

•Cutting:

GPP decreased due to the removal of photosynthetic material  abrupt decline of NEE

•Start of July:

Dry conditions precluded C accumulation.

GPP declined more than TER as the soil dried out  source of CO₂

in mid July

•Mid August - end of September:

Better climatic conditions  CO₂ neutral (Figure 3c).

•From October:

Lower temperatures and radiation  assimilation decreased and net fluxes dominated by TER

End of November:

GPP ≈ 0, TER declined to low levels and NEE >0  end of January

•Since the end of January:

TER and GPP gradually increased  start of March: CO₂ sink

 Cumulative NEE = 172 ±53 g C m-2 y-1 

the site behaved as a net CO₂ source

Summer 2010 (June – early August):

•High temperatures

•Very few precipitations  Limited soil moisture content

Nighttime flux

•High respiration values

•No short term response of nighttime respiration to

temperature

•No clear livestock impact on respiration flux

A_max evolution

•Variations between periods due to climate and management (cut, consecutive re-growth, drought). •Most intensive growth during

period 1: A_max = 33 µmol m-2 s-1.

•Effect of drought: limited values

(≈ 18 µmol m-2 s-1.) during

periods 4-6 and 12-13

R_d evolution

•Similar (but much lower) variations between

periods compared to A_max.

•Very high (up to 10 µmol m-2 s-1) values.

 No significant difference between periods with and without cattle

Fig. 3: Daily means of air temperature (TA) and soil temperature at 2 cm depth (TS), (b) Soil moisture at 5 cm depth and rainfall and c) Daily means of Photosynthetically Photon Flux Density (PPFD) meaasured at Dorinne (12 May 2010 – 12 May 2011).

Spring 2011 (March – early May):

•Exceptionally sunny conditions

•Minimal and maximal daily temperature higher or equal to normal

•Very little precipitation: <60 mm (200 mm in normal)

 Limited soil moisture content at the beginning of May -10 0 10 20 30 a) T em p er at u re ( °C ) TA TS 0 0.2 0.4 0.6 b) S o il m o is tu re ( m -3 m -3 ) 0 2 4 6 8 10 12 R ai n fa ll ( m m )

Jun10 Aug10 Oct10 Dec11 Feb11 Apr11 0 200 400 600 800 P P F D ( µ m o l m -2 s -1 ) _c) -10 0 10 20 30 a) T em p er at u re ( °C ) TA TS 0 0.2 0.4 0.6 b) S o il m o is tu re ( m -3 m -3 ) 0 2 4 6 8 10 12 R ai n fa ll ( m m )

Jun10 Aug10 Oct10 Dec11 Feb11 Apr11 0 200 400 600 800 P P F D ( µ m o l m -2 s -1 ) _c) -10 0 10 20 30 a) T em p er at u re ( °C ) TA TS 0 0.2 0.4 0.6 b) S o il m o is tu re ( m -3 m -3 ) 0 2 4 6 8 10 12 R ai n fa ll ( m m )

Jun10 Aug10 Oct10 Dec11 Feb11 Apr11 0 200 400 600 800 P P F D ( µ m o l m -2 s -1 ) _c)

Fig. 5: Dependence of nighttime respiration on soil temperature:

overall fit (12 May 2010 – 12 May 2011). Data are filtered for u_* and stationarity. 0 5 10 15 20 25 0 5 10 15 20

Soil temperature at 2 cm depth (°C)

N ig h tt im e fl u x (µ m o l m -2 s -1 ) No Cattle Cattle

Lloyd and Taylor fit Bin average

Fig. 2: Carbon balance of a grazed grassland.