Ploughing Reduced N2O Emissions From Forage Fields Compared to Chemical Fallow.

Animal production results in a large landbase of forage fields that are heavily fertilized with animal manures, rich in both carbon and nitrogen, and present a possibility of gaseous nitrogen loss in the form of N₂O. We converted a long-term forage field to an annual cropping system using full inversion tillage with a mouldboard plough and using chemical fallow to represent a no-till system to establish if the act of ploughing these fields would increase or decrease N₂O emissions. In fall 2007, two plots, one with history of fertilization with pig slurry and the other non-amended, were divided into sub-plots and treated with herbicide. They were then either left undisturbed, or turned by mouldboard plough in fall or spring. Static chambers were installed to measure soil CO₂ and N₂O emissions and tension lysimeters (15, 30 and 45 cm) to monitor nitrogen species in the soil solution. Soil samples were taken to monitor changes in solid phase nitrogen fractions by KCl extraction. Over the fallow year the undisturbed-fertilized subplots emitted the most N₂O-N (29.6 kg ha^-1) followed by the fall-ploughed-fertilized (17.4 kg ha^-1), the undisturbed-unfertilized (16.7 kg ha^-1) and the fall-ploughed-unfertilized subplots (8.7 kg ha^-1). Over the summer of 2008 the spring-ploughed subplots emitted 10.1 and 9.2 kg N₂O-N ha^-1 from the fertilized and unfertilized plots respectively.  On poorly drained forage soils, full inversion tillage reduces emissions of N₂O relative to a chemical fallow for an equivalent soil NO₃ content. Tillage may temporarily induce N₂O production through nitrification, but this effect is short-lived. We suspect that placing C and N at depth with inversion tillage of forage fields, favours complete denitrification (N₂ emission), whereas the undisturbed profile retained rich denitrification microsites near the soil surface, thereby favouring emission of N₂O.