R ESULTS Management and development of the crop Seasonal course of the fluxes R ESULTS Management and development of the crop Seasonal course of the fluxes Unit of Biosystem Physics Jérôme 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° D , January December 2011 Contact Person: Jérôme Elisabeth - University of Liege – Gembloux Agro-Bio Tech (GxABT) - Unit of Biosystem Physics, 8 Avenue de la Faculté Gembloux - Belgium Tel : +32 (0) Fax : +32 (0) O BJECTIVES Long 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 (CO 2 ) fluxes measurements. E XPERIMENTAL SITE Situation : Belgium, Dorinne Type : Permanent grazed grassland Ruminant livestock system : intensive Breed of cattle : “Blanc Bleu Belge” (BBB) Total surface area : 4.3 ha Slope : moderate (1 to 2 %) M EASUREMENTS CO 2 fluxes measurements : - Eddy correlations = 3D sonic anemometer and fast infrared gas analyzer Micrometeorology : - Radiation, soil and air temperature, soil and air humidity, etc.  Beginning of the measurements : 12 th of May 2010 Grass canopies development : - Weekly measurements of the height of grass = manually by measuring the centre height of a plate of 0.25 m 2 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 : E 0 = 285 ± 44 [K] - Respiration at reference temperature R 10 = 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. A max 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 A max value of 33 µmol/m²/s. - Effect of drought : limited values (18 µmol/m²/s) during periods 4-6. A max increase in period 7 after drought end. - No significant difference between periods with and without cattle. R d evolution - Similar (but much lower) variations between periods compared to A max. - 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. A max // R d10 - Good correlation between A max and R d10 - Harvest: lowest values  heterotrophic respiration  Importance of autotrophic respiration in other period Rotational grazing : average stocking rate = 2 LU/ha Biomass : - 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 (12 th of May – 2 nd June 2010). Data are filtered for stationarity. Fig. 5: Daily mean of nighttime and daytime fluxes between the 12 th of May 2010 and the 28 th of February C ONCLUSIONS - 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 C ONCLUSIONS - 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 P ERSPECTIVES - Complete C budget over the year. To better analyze cattle influence on CO 2 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 P ERSPECTIVES - Complete C budget over the year. To better analyze cattle influence on CO 2 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 Fig 7: Dependence of nighttime respiration on air temperature : overall fit (12 th of May 2010 – 28 th 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 (12 th of May 2010 and 28 th of February 2011). Table 1: Management Fig 4: Temporal course of canopy development and stocking rate over the season of 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 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 12 th of May 2010 and the 28 th of February 2011.