Quantification of small scale N 2 O emissions and comparison with field-scale emissions of a rotational grazing system

Transcription

1 Agroscope Quantification of small scale N 2 O emissions and comparison with field-scale emissions of a rotational grazing system C. Ammann, K. Voglmeier, M. Jocher, H. Menzi Federal Research Station Agroscope, Zürich & Posieux, Switzerland EmiLi Symposium, Saint Malo, May 2017

2 Introduction Grazing has is considered advantageous concerning: - Cost efficiency - Animal welfare - NH 3 emissions - C-sequestration (?) On the other hand, grazing can lead to increased N 2 O (and CH 4?) emissions, and has a comparatively low N efficiency [from Flechard et al., 2007, AEE] Potential trade-off between mitigation of NH 3 and N 2 O emissions Still large uncertainties for grazing emissions because comprehensive measurements are rare (incl. detailed knowledge about N-input) C. Ammann EmiLi Saint Malo, May

3 Motivation Expected large temporal and spatial (small-scaled) variability of emissions with hot-spots resulting from the uneven distribution of animal excreta (urine and dung patches). represents an inherent problem for quantification, interpretation and modelling. Different sources in IPCC emission calculation for grazed pastures: Micrometeorological methods (eddy covariance, EC) are well suited to quantify the integrated ecosystem emissions of N 2 O over longer periods. Chamber methods are more useful to investigate the contribution of individual sources and their dependences on driving parameters. This study: combination of EC and chamber measurements on a grazed fertilised pasture (field experiment in 2016 results are preliminary) C. Ammann EmiLi Saint Malo, May

4 Experimental site Swiss central plateau, near Posieux 2 ha pasture field with rotational grazing in 12 paddocks 12 dairy cows, full day grazing (milking in barn 2x day -1 ) Experiment over one year (2016) with grazing season Apr-Oct Additional fertilizer applications: (2 x synth. fertiliser, 1 x slurry) wind distribution C. Ammann EmiLi Saint Malo, May

5 Flux measurement systems Field scale measurements with Eddy Covariance (EC) Gill HS sonic and fast QCL analyser (Aerodyne) spatial averaging over ha - varying footprint (wind direction) - includes contributions of several paddocks For annual integration, the spatial coverage is considered representative for the grazing system Small scale measurements with Fast-Box (following A. Hensen, ECN Petten, NL) size: 0.8 x 0.8 x 0.5 m (320 L) non-steady state fast QCL analyser (Aerodyne), 11 L/min sample flow measurement time: ca. 1 2 min Manual operation 1300 individual flux measurements on 46 days (July October) Requires information about urine patch locations C. Ammann EmiLi Saint Malo, May

6 Distribution of urine and dung patches 15 x 15 m (or 10 x 10 m) sub-plot Dung patches: visual identification x old x fresh Urine patches: identification via survey (0.25 m resolution) of soil dielectric conductivity (0-5 cm) Urine patches were marked for chamber measurements (1 20 days after grazing) C. Ammann EmiLi Saint Malo, May

7 Fast-box measurements: temporal development of identified (new) excreta patches and on pasture background areas average results as a function of time since grazing / excreta dropping (vertical bars indicate standard errors) C. Ammann EmiLi Saint Malo, May

8 Upscaling of fast-box measurements estimation of urine/dung excretion by grazing cows from animal N budget calculation (productivity feed intake / N intake subtraction of milk N export N excretion) avg. urine N excretion on pasture: 180 gn cow -1 d -1 Estimation of no. of urine patches from standard literature numbers: 1 urine patch: 2 L volume, 10 gn/l Calculation of N 2 O flux for each paddock (depending on grazing schedule) for - urine patches (exponential decreasing emission x no. of patches - dung pads (constant avg. value x no. of pads) - pasture background (constant avg. value) Integral contributions for a 30 day period: urine patches: 78 gn ha -1 dung pads: 18 gn ha -1 background: 40 gn ha -1 C. Ammann EmiLi Saint Malo, May

9 EC flux processing Analysis and correction of high-frequency damping due to 25 m line sampling and sensor separation: 10-20% (depending mainly on wind speed) Stationarity filtering u * filtering with threshold 0.07 m/s (slightly less strict than for CO 2 ) C. Ammann EmiLi Saint Malo, May

10 EC fluxes and management Large temporal variations; highest fluxes after fertilizer applications harvest mineral fertiliser mineral fertiliser slurry grazing C. Ammann EmiLi Saint Malo, May

11 Comparison of EC and upscaled chamber fluxes Simulation of EC fluxes from fast-box results by applying footprint weights for individual grazing paddocks (effect of soil water content not considered!) C. Ammann EmiLi Saint Malo, May

12 Comparison of EC and upscaled chamber fluxes Fair agreement in magnitude between EC and upscaled fast-box fluxes during grazing phases Differences mainly related to variations in soil water content C. Ammann EmiLi Saint Malo, May

13 Fast-box measurements: driving factors N 2 O emission in urine patches shows significant dependence on soil water content, but no significant effect of soil temperature (not shown) C. Ammann EmiLi Saint Malo, May

14 Cumulative annual emission calculated from EC data with simple gap filling (running mean or mean diurnal cycle) Total annual N input to the pasture: c. 200 kg N ha -1 C. Ammann EmiLi Saint Malo, May

15 Conclusions (preliminary) Fast-box measurements have been performed on a pasture after grazing in parallel to continuous EC flux measurements Urine patches are strong hotspots of N 2 O emissions and contribute a large share to the overall pasture flux during grazing periods simple upscaling leads to fair agreement of average emissions observed by fast-box and EC similar integrated contribution of urine patches and other areas of pasture Strong effect of soil moisture conditions (near soil surface) with optimum at about 80-90% WFPS needs to be included in emission parameterisation (ongoing work) Emission fluxes during grazing phases are lower than after fertilizer application but contribute c. 1/3 to the total annual N 2 O emission but: source attribution of field scale emissions is still challenging; possible interaction of existing excreta hotspots with fertiliser application effect C. Ammann EmiLi Saint Malo, May

16 Thank you for your attention Acknowledgements: The financial support through project grants of the Swiss National Science Foundation (project NICEGRAS) and the Swiss Federal Office for the Environment (project WeidEmi) are gratefully acknowledged. We also thank the many colleagues that supported the field measurements. C. Ammann EmiLi Saint Malo, May

17 Identification of urine patches using soil conductivity C. Ammann EmiLi Saint Malo, May

18 Experimental site Swiss central plateau, near Posieux 2 ha pasture field with rotational grazing in 12 paddocks 12 dairy cows, full day grazing (with milking in nearby barn 2x day -1 ) Experiment over one year (2016) with grazing season Apr-Oct wind distribution C. Ammann EmiLi Saint Malo, May