Many anthropogenic activities are significant sources of greenhouse gasses, however not all emissions are as easy to quantify as the direct burning of fossil fuels (e.g. for transportation or electricity). Other significant sources of greenhouse gasses including agriculture can be harder to quantify due to the increased number of variables involved. In Canada, this sector emits about 8% of total greenhouse gas emissions (Environment Canada, 2015). These emissions come from methane, which is produced primarily from animals, and nitrous oxide, which is primarily from crop focused farming. Varying levels of each may be produced on farms with a dual focus, and are slightly offset by the uptake of carbon dioxide from plants on the farmlands. Quantifying all the sources and sinks to reach a net flux for a farm can be difficult, requiring multiple measuring techniques. In Taylor et al., 2017, they measure the net flux of a beef cattle operation using eddy covariance, as well as gas flux chambers to reach a net flux of the entire farm. In doing so, they hoped to create a better understanding of the complexities involved in measuring complex sources and sinks of greenhouse gasses.
The farm chosen to be studied was located near Brandon, Manitoba, on lands that have been used for agriculture and livestock for over a decade. 264 cattle were on the farm, as well as 329 large grass-legume hay bales which were being stored outdoors until needed for feed. The study covers in detail the livestock and crop management practices, accounting for variability from farm activates such as movement of cattle, herbicide use or harvesting of crops. During 2012 and 2013 measurements were taken using two eddy covariance towers (figure 2), one centrally located and one on the east side of the farm. These were used to measure greenhouse gas flux on the scale of meters to kilometers of the farm, and to measure emissions from the cattle in particular. Flux chambers were also used to measure microsite variation, by positioning the chambers into set locations to measure the flux from specific environments on the farm, such as the respiration and decay of the hay bales. Collars were inserted into the ground or into bales and static vented chambers attached on top to measure the gas flux. The depth of the collars ranged from 10 to 30 cm depending on the location, for example collars on top of feed needed to be deeper to reach the soil surface. A total of 64 chambers were installed, in fields of oats and barley, on top of excess feed, feces and grazing fields. Sampling was done three times a week beginning in the spring, and two times a week in the summer using a syringe and needle through a rubber septum in the lid.
The net greenhouse gas flux of the farm were emissions of 46t CO2 equivalents ha-1 y-1. Of this, methane from cattle was 11t CO2 equivalents ha-1 y-1; methane from soils was 0.06t CO2 equivalents ha-1 y-1; and nitrous oxide from soil, excreta, and fertilizer was 4t CO2 equivalents ha-1 y-1. The net emissions were higher than an average year may have been, as the plant soil system was a net source from the increased use of imported feed during the period measured. The moving cattle herd posed challenges measuring greenhouse gas flux, as a moving herd increased the spatial and temporal variability of the emissions. The cattle impact on the total emissions was greatest for all gasses measured (CO2, N2O, CH4) and especially dominated in the winter, when photosynthesis did not provide a sink for some CO2 emissions. The land contribution to CO2 flux was much smaller than that of the cattle’s, becoming swamped in presence of cattle. The largest source of CH4 was the cattle, and the largest source of N2O was the cattle excreta, and not the predicted flux from fields that contained nitrogen fertilizers. For future studies, the authors suggested using more flux towers as the spatial and temporal variability can be high especially for pastured cattle operations. Chambers were the best method of measuring N2O fluxes; due to their small quantities, more continuous measurements would improve the quality of the data. As of the time of publication by Taylor et al., this had been the first attempt of a whole farm measurement of greenhouse gas fluxes for over a year, and a great step to further understanding how to quantify complex sources of greenhouse gasses.