Detailed temporal modelling of carbon and water fluxes from pastures in New Zealand : case study of an experimental dairy farm in the Waikato region : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand
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Date
2017
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Massey University
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Abstract
The terrestrial biosphere is an important pool of carbon, with its size governed by the
opposing processes of CO2 uptake through photosynthesis and release through
respiration. It is therefore critically important to understand and reliably and accurately
model these processes and predict changes in carbon exchange in response to key
drivers. Pasture-based livestock production is particularly important for the New
Zealand’s economy but it is also a main contributor to NZ’s greenhouse gas budget. My
Ph.D. work used half-hourly eddy-covariance (EC) data, previously collected over 2
consecutive years from a grazed pasture in the Waikato region. The main aims of this
study were to assess whether there was any bias in gap-filled eddy covariance
measurements, to assess whether incomplete capture of cow respiration during
grazing events could have led to biased observations, and to quantify the resulting
difference on the net carbon budget of the farm. I approached the work by developing a
new process-based model, CenW_HH, running at a half-hourly time step, to predict the
energy and CO2 exchange of grazed pastures. I implemented and evaluated different
photosynthesis models and upscaling schemes and modelled the energy budget
separately for the canopy, litter layer, and the soil. CenW_HH was then parameterised
and validated with the available EC measurements. The paddocks surrounding the EC
tower were rotationally grazed, which caused heterogeneities in respiratory pulses
when grazing events were in the flux footprint and subsequent vegetation cover on the
different paddocks. To deal with that heterogeneity, the model was run independently
for each individual paddock and a footprint model was used to estimate resultant net
fluxes at the EC tower. Modelled fluxes agreed well with half-hourly observed fluxes as
seen by model efficiencies of 0.81 for net ecosystem productivity, 0.75 for gross
primary production, 0.70 for ecosystem respiration, 0.87 for latent heat flux, 0.76 for
sensible heat flux, 0.94 for net radiation, and 0.92 for soil temperature. CenW_HH was
then used to test for any biases in gap-filled data for times without the presence of
grazing animals, but identified no consistent systematic deviations. Eddy covariance
measurements often failed to capture carbon losses due to cattle respiration, especially
when measurements had to rely on gap-filled data. By replacing gap-filled NEP fluxes
affected by grazing cattle by estimates generated by CenW_HH, the farm carbon
budget was reduced by 31% and 113% (and turning from a positive into a slight
negative balance) in 2008 and 2009, respectively.
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Keywords
Carbon cycle (Biogeochemistry), Pastures, Environmental aspects, New Zealand, Research Subject Categories::NATURAL SCIENCES::Earth sciences