The Effect of Control Tile Drainage on Soil Greenhouse Gas Emissions from Agricultural Fields in the South Nation Watershed of Ontario

Controlled tile drainage (CTD) is an agricultural management practice with well-documented water quality and agronomic benefits, however, by virtue of its effect upon soil hydrology, CTD could potentially impact soil greenhouse gas (GHG: CO2, CH4, N2O) emissions. This study aimed to determine whether: (1) CTD affects soil GHG emissions throughout a dry (2012) and a wet (2013) growing season for corn, soybean, and forage fields in eastern Ontario, and (2) the location in a field with respect to a tile drain (over tile (OT) versus between tile (BT)) is important in GHG emissions. Non-steady state chambers were used for sampling soil GHG emissions in order to analyze GHG fluxes, the δ13C of soil-respired CO2 (RT), and for separating total soil respiration into its rhizosphere and soil components. There was no significant difference in average GHG emissions from CTD and UTD fields (except for 1/5 field pairs studied for N2O) and from OT and BT locations. The means of δ13C of RT were not statistically different (p>0.05) between 4/5 CTD and UTD field pairs, and between OT and BT locations in 4/5 CTD fields. The mean contributions from rhizosphere respiration and soil respiration did not differ (p>0.05) in 3/4 CTD and UTD field pairs. This lack of difference in GHG emissions is believed to have resulted from their being no difference in surface soil water contents among CTD and UTD fields and among OT and BT locations. It is believed that surface soil moisture did not vary because: (1) the water table was too low in 2012 for effective water table control, and (2) significant precipitation created equally wet surface soil in 2013. In 2013, the surface soil moisture was approximately 10% greater and this may be why there was an approximate 5 kg C/ha/day greater CO2 flux from soybean fields in 2013 than in 2012. δ13C was useful for distinguishing the source of CO2 emissions (rhizosphere versus soil respiration) in CTD fields when the crop and plant δ13C signatures varied. The results are useful for helping to capture the carbon footprint of tile drainage management practices imposed at field-scale