1. Unit of Biosystem Physics Jérôme Elisabeth 1, Beckers Yves 2, Beekkerk van Ruth Joran 1, Bodson Bernard 3, Dumortier Pierre 1, Moureaux Christine 3, Aubinet Marc 1 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. Impact of grazing on carbon balance of an intensively grazed grassland in Belgium This research was funded by The « Direction Generale opérationnelle de l’Agriculture, des Ressources naturelles et de l’Environnement - Région Wallonne » Project n° D31-1278, January 2012 - December 2013 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 : Elisabeth.Jerome@ulg.ac.be 1. O BJECTIVES To analyze impact of grazing on CO 2 fluxes and evaluate livestock impact on carbon balance 2. E XPERIMENTAL 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 %) Ruminant livestock system: intensive Breed of cattle: Belgian Blue 3. M ETHODOLOGY 3.1 Carbon budget establishment NBP=F CO2 +F CH4-C +F manure +F import +F harvest +F product +F leach Where: F CO2 is the net carbon dioxide exchange F CH4-C is the C lost through methane emissions by cattle at grazing F manure, F import and F harvest are the lateral organic C fluxes imported in the system through manure application, through complementary feedings and exported from the system through harvest, respectively F product is the lateral organic C fluxes exported from the system as meat: F product =C intake +F import +F CH4-C +C respiration +C excretions Where: C intake is the C intake by cattle at grazing C respiration is the C losses via livestock respiration C excretions is the C losses via excretions 3.2 Confinement experience Each experiment extended over two days: The first day, cattle (27 LU ha -1 ) were confined in the main wind direction area of the eddy covariance set-up (1.76 ha, see below) The second day, it was removed from the plot  We compared filtered half-hourly data made at 24h interval, in the presence of cattle or not, considering that environmental conditions were equivalent: air temperature within 3°C – wind speed within 3 m s -1 - radiation within 75 µmol m -2 s -1 – wind direction within confinement area 5. C ONCLUSIONS The site behaved as a net source of CO 2  impact of climate C balance closed to equilibrium  impact of management 5. C ONCLUSIONS The site behaved as a net source of CO 2  impact of climate C balance closed to equilibrium  impact of management 4. R ESULTS 4.1 CO 2 fluxes evolution 4.2 Carbon balances 4.3 Indirect impact of grazing on CO 2 fluxes 4.4 Direct impact of grazing on CO 2 fluxes Table 2: GPP & TER ≈ 2500 g C m -2  >>> other C fluxes  Upper end of the range of values reported for other grasslands  High potential for biomass production  high carbon uptake and high respired carbon loss Resulting NEE ≈ other C fluxes C inputs: -Year 1: 214 g C m -2 -Year 2: 11 g C m -2 F harvest : -Year 1: 38 g C m -2 -Year 2: /  Balance between C imports and C exports created a large departure of NBP from NEE. It decreased the magnitude of the CO 2 source in Year 1 contrary to Year 2  CO 2 source Year 1 >>> CO 2 source Year 2 but C source Year 1 <<< C source Year 2  C fluxes other than NEE ↓ C source: Schultze et al. (2009)  Key role of management C balance ≈ equilibrium -≠ NBP = 70 g C m -2 <<< ≠ NEE = 100 g C m -2 -≠ C fluxes (other NEE) = 172 g C m -2 >>> ≠ NEE = 100 g C m -2  C accumulation rate Wallonia: -50 to 50 g C m -2 y -1 Table 3: C respiration & C excretions : largest part of the C lost from the herbivores C respiration ≈ 70% C intake : consistent with literature C excretions ≈ 17% C intake F CH4-C ≈ 3.5% C intake  F product was around 30 g C m -2 y -1 with a large associated uncertainty Figure 3: Schematic representation of Dorinne Terrestrial Observatory. Localisation of the micro- meteorological station and eddy-covariance set-up. Black area represents the confinement zone used to analyze the impact of grazing on carbon exchange of the ecosystem. Figure 2: Wind rose for Dorinne Terrestrial Observatory computed with data measured between 12 May 2010 and 12 May 2012 with the micro-meteorological station. Table 1: Measurement methods of carbon fluxes. Figure 1: Carbon balance of a grazed grassland. Table 2: Components of the Net Biome Productivity (NBP) for two years of measurements made at Dorinne Terrestrial Observatory (Year 1: 12 May 2010 - 12 May 2011; Year 2: 12 May 2011 - 12 May 2012). Table 3: Components of the carbon balance of herbivores for two years of measurements made at Dorinne Terrestrial Observatory (Year 1: 12 May 2010 - 12 May 2011; Year 2: 12 May 2011 - 12 May 2012). Figure 6: Response of day fitting regression parameters to average stocking rate. Data set covered vegetative period for two years of measurements made at Dorinne Terrestrial Observatory (Start of the experiment to 23 November 2010; 9 March 2011 to 2 December 2011; 9 March 2012 to 12 May 2012). Figure 7: Cumulated Net Ecosystem Exchange response to average stoking rate. One point represents monthly cumulated F CO2 taking duration into account. Data set covered vegetative period for two years of measurements made at Dorinne Terrestrial Observatory (Start of the experiment to 23 November 2010; 9 March 2011 to 2 December 2011; 9 March 2012 to 12 May 2012). Figure 8: a) Nighttime CO 2 flux evolution and b) daytime CO 2 flux response to radiation of two successive days with or without cattle. Errors bars are the random error of measurement reported at a 95% confidence interval. a) b) Cumulative NEE -Year 1 = 123 ± 53 g C m -2 -Year 2 = 23 ± 31 g C m -2  CO 2 source  intensive managed grassland generally CO 2 sinks Temporal evolution -CO 2 sink: May-Jun; Mar-Apr -CO 2 source: Oct-Feb -Less clear evolution: Jun-Oct -May-Jun: accumulation rate two times larger in Year 1 than in Year 2 -Mar-Apr: accumulation rate 2x larger in Year 2 than in Year 1 -Oct-Feb: CO 2 release 2x larger in Year 1 than in Year 2 Significant diminution of ∆GPP max with stocking rate and grazing duration Up to 20 µmol m -2 s -1  Explained 67% of the diminution of ∆GPP max  Reduced grassland production? Between 80% and 90% of above-ground ingested through grazing in Year 1 and Year 2, respectively Significant increase of cumulated F CO2 with stocking rate and duration Up to 85 g C m -2 for the maximal average stocking density and duration observed  Discrimination of indirect impacts of grazing on CO 2 fluxes from flux response to climate possible only after gathering and treating two year of measurements taken under various climatic conditions With cattle: ↓ -GPP max : 34%  Grazing reduced above-ground biomasses  Decreased plant assimilation -R d,10 : 15%  ↓ Autotrophic respiration due to removal biomass >>> ↑ Soil respiration and losses through cattle respiration Without cattle: ↑ -GPP max : 15% -R d,10 : 48%  Grazing effects variable depending on the stocking density, length of the grazing period and timing of grazing and consecutive re-growth periods F CO2 with cattle > F CO2 without cattle TER increased by 44% with cattle  Cattle do not influence flux processes in one day  Cattle respiration only  Could be distinguished and quantified only thanks to the confinement experiment F CO2 average significantly higher with cattle than without R l ≈ 0.14 µmol m -2 s -1 per livestock unit  Consistent with literature R l estimations based on nighttime or daytime data set not significantly ≠ R l < C intake not significantly ≠ than R l < EC measurements  All cattle contribution to CO 2 fluxes included with eddy covariance measurements Considering R l = 0.14 ±0.02 µmol m -2 s -1 LU -1 on average: -Year 1: C respiration = 174 ±28 g C m -2 -Year 2: C respiration = 199 ±32 g C m -2 ≈ C respiration cf. 4.2 Figure 5: Seasonal course of a) gross primary productivity at light saturation (GPP max ) and b) dark respiration (R d.10 ). Values were deduced from daytime data response to radiation. Regressions were performed on a five days window within a period with or without cattle (solid line and dashed line respectively). Data set covered vegetative period for two years of measurement (Start of the experiment to 23 November 2010; 9 March 2011 to 2 December 2011; 9 March 2012 to 12 May 2012). a) b) Direct and an indirect impact of grazing on the CO 2 fluxes:  Significant impact of grazing and length of grazing on photosynthetic capacity  Significant contribution of livestock respiration on respiration fluxes Figure 4: Cumulative Net Ecosystem Exchange for two years of measurements made at Dorinne Terrestrial Observatory (Year 1: 12 May 2010 - 12 May 2011; Year 2: 12 May 2011 - 12 May 2012).