RESULTSManagement and development of the crop

Seasonal course of the fluxes

Unit of Biosystem PhysicsJérôme Elisabeth, Beckers Yves, Bodson Bernard, Moureaux Christine, Aubinet Marc

Carbon exchange in a Belgian grassland with cattle

This research is 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 - BelgiumTel : +32 (0)81 62 26 25 - Fax : +32 (0)4 73 62 24 39 e-mail : Elisabeth.Jerome@ulg.ac.be

OBJECTIVESLong term objectives : - To compute carbon and GHG balance of a grazed managed grassland,- To propose mitigation scenarios in order to improve the GHG balance.This poster : - To analyze a first year of carbon dioxide (CO2) fluxes measurements.

EXPERIMENTAL SITESituation : Belgium, DorinneType : Permanent grazed grasslandRuminant livestock system : intensiveBreed of cattle : “Blanc Bleu Belge” (BBB) Total surface area : 4.3 haSlope : moderate (1 to 2 %)

MEASUREMENTSCO2 fluxes measurements :

- Eddy correlations = 3D sonic anemometer and fast infrared gas analyzerMicrometeorology : - Radiation, soil and air temperature, soil and air humidity, etc. Beginning of the measurements : 12th of May 2010Grass canopies development : - Weekly measurements of the height of grass = manually by measuring the centre height of a plate of 0.25 m2 dropped onto the canopy, ± 50 points on the field - Regular samples of biomass = quadrate of 0.25 m², ± 20 samples

Fig. 1: Carbon exchange in a grazed grassland.

Response of the turbulent fluxes to climate

Nighttime fluxes

Daytime fluxes

Nighttime flux response to temperature- Sensitivity of respiration to temperature : E0 = 285 ± 44 [K]- Respiration at reference temperatureR10 = 6.37 ± 0.5 [µmol/m²/s] high respiration rate compared to reference values (maximum values ≈ 6 µmol/m²/s for an intensive managed grassland during growing phase: Ammann et al., 2007)- Influence of summer drought- No clear livestock impact on respiration flux

- Nighttime and daytime fluxes follow primarily temporal variation of T° and radiation.- Maximum daytime fluxes during spring (before cut).- Harvest stopped assimilation during 10 days.- Restoration of assimilation during summer but at a lower rate : impact of drought and cattle ? - Since October : fluxes dominated by respiration.Spring – June: C accumulation in plant biomass Cumulated value reached -100 g. C/m².No clear short term impact of cattle. After 10 months, positive C budget (source of ≈ 200 g. C/m²) BUT : early spring not included.

Amax evolution- Strong variations between periods due to climate and management (cutting event, consecutive re-growth, drought). - Most intensive growth during period 1 corresponds to a Amax value of 33 µmol/m²/s. - Effect of drought : limited values (18 µmol/m²/s) during periods 4-6. Amax increase in period 7 after drought end. - No significant difference between periods with and without cattle.

Rd evolution

- Similar (but much lower) variations between periods compared to Amax.

- Very high (up to 10 µmol/m²/s) values.

- Probable (but limited – 25%) impacts of cut and drought.

- No clear difference between periods with and without cattle.

Amax // Rd10

- Good correlation between Amax and Rd10

- Harvest: lowest values heterotrophic respiration Importance of autotrophic respiration in other period

Rotational grazing : average stocking rate = 2 LU/haBiomass : - Before the cut : 3000 kg DM/ha - After the cut (with or without cattle) : ≈ 500 kg DM/ha (height of grass = 5 cm) forage complement Impact of management Impact of presence of cattle

Fig. 2: Experimental site and set up.

Fig 8: Dependence of daytime flux on radiation : overall fit (12th of May – 2nd June 2010). Data are filtered for stationarity.

Fig. 5: Daily mean of nighttime and daytime fluxes between the 12th of May 2010 and the 28th of February 2011.

CONCLUSIONS- High respiration values < > Low assimilation values.- No clear short term influence of cattle on the fluxes.- Cumulated fluxes : at present, the net flux is positive :

The site behaves as a source BUT : - The year is not complete- The results are much influenced by drought- The budget must include forage complement

PERSPECTIVES- Complete C budget over the year. To better analyze cattle influence on CO2 flux, we have to integrate results of :

C content of biomass and feces samples, Intake by cattle, C export and import (harvest and manure).- Study in details the impact of the drought during summer 2010 comparison with 2011

This work is in progress

-10 0 10 20 300

5

10

15

20

Air temperature [°C]

Nig

httim

e flu

x [µ

mol

/m²/s

]

CattleNo CattleLloyd and Taylor fitBin average

Fig 7: Dependence of nighttime respiration on air temperature : overall fit (12th of May 2010 – 28th of February 2011). Data are filtered for u* and stationarity.

Particular conditions in late June and July- Mean daily T° up to 25°C- No precipitation Limited soil moisture content Impact of drought

Fig. 3: Climatic conditions at Dorinne (12th of May 2010 and 28th of February 2011).

Table 1: Management

0 500 1000 1500 2000-35

-25

-15

-5

5

15

Radiation [µmol/m²/s]

Day

time

flux

[µm

ol/m

²/s]

2.

1.

Fig 4: Temporal course of canopy development and stocking rate over the season of 2010.

Fig 9: Evolution of assimilation at light saturation and daytime respiration for different periods of the season. Values are deduced from daytime flux/radiation response. Error bars represent 95% confidence intervals.

1+2. 2.

Fig 10: Relationship between normalized daytime respiration at 10°C and assimilation at light saturation. Dots represent estimation for different periods. Solid line shows linear regression.

Harvest

PPFD and high precipitations

Fig. 6: Cumulative Net Ecosystem Exchange between the 12th of May 2010 and the 28th of February 2011.