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Methane emission hotspots from a drained peat soil under dairy grazing

Methane (CH4) is a greenhouse gas that is emitted from natural peatland ecosystems due to their high water tables. However, large areas of natural peatlands have been drained for agricultural purposes, resulting in a reduction in overall methane emissions. However, where soil is saturated, such as within or adjacent to drainage ditches, methane emission can remain high. The aim of this research was to determine the magnitude of soil- and drainage ditch-derived methane emissions from a drained Waikato peatland under dairy grazing and where and when these emissions occur. Gamma Farm is a pastoral dairy farm located on the remnants of the Moanatuatua peatland and drained by shallow surface “spinner” drains that discharge into deeper field-border drains. The paddocks were classified into four different landforms based on the location and hydrology of the drainage features. These landforms are crown, slope, ditch edge and drainage ditch. To adequately determine the spatial and temporal variation in methane fluxes chamber measurements of methane fluxes were undertaken along a transect across the width of the study site, approximately perpendicular to the border drains. A campaign approach was used, with chamber measurements being undertaken from autumn through to winter to capture methane fluxes under different environmental conditions. To measure seasonal and annual-scale methane emissions an eddy covariance flux tower was used. Based on chamber measurements drainage ditches were shown to have average methane emissions of 0.071 ± 0.005 mg CH4 m-2 h-1 . Conversely, the soil of the crown, slope and ditch edges were shown to be a net methane sink, with average net methane oxidation of ‒0.019 ± 0.006, ‒0.01 ± 0.008 and ‒0.023 ± 0.006 mg CH4 m-2 h-1 respectively. Weighting the chamber measurements by landform area it was concluded that the study site was primarily a minor net methane sink. However, eddy covariance measurements indicated that the study site was a net source of methane with annual emissions of 44.72 kg CH4 ha-1 yr-1 . This large discrepancy between the chamber and eddy covariance measurements is likely caused by the large spatial and temporal scale differences between the two measurement techniques. In addition, it was found that there was no relationship between soil methane fluxes and the soil temperature, Olsen-P, and nitrate concentration. However, methane fluxes were shown to decrease as the ammonium concentration and depth to the water table increased. In addition, methane fluxes decreased as volumetric moisture content (VMC) decreased, but at low VMCs (<40%) methane fluxes tended towards zero. For the water-borne methane fluxes there was no relationship found between methane fluxes and any measured variable (pH, dissolved oxygen, electrical conductivity, water temperature, water depth, nitrate concentration or dissolved phosphorus). However, at longer time scales such as monthly averages of eddy covariance measurements, methane fluxes were positively correlated with soil temperature and air temperature. Additionally, both seasonal and diurnal cycles in the eddy covariance methane fluxes were observed.
Type of thesis
Hamill, J. (2019). Methane emission hotspots from a drained peat soil under dairy grazing (Thesis, Master of Environmental Sciences (MEnvSci)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13481
The University of Waikato
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