Nitrous oxide emissions from acidic Black Vertosol: effects of residues, nitrogen additions and pH

Abstract

Agricultural soils are a major source of atmospheric nitrous oxide (N₂O), a potent greenhouse gas contributing approximately 6% of the total radiative forcing from anthropogenic greenhouse gas emissions. Vertosols are the major soil type throughout much of the grain-growing areas of northern NSW and southern Queensland in Australia. In these areas, cultivation of grain legumes, retention of crop residues after grain harvest and increased use of fertilizer-nitrogen (N) are common practices, all of which are potentially important sources of N₂O emissions. Vertosols are fine-textured and have poor internal drainage, which can lead to greater periods of saturated, anaerobic conditions conducive to N₂O emissions through denitrification. This underlines the need to better understand the relative contribution of different N sources (i.e. fertilizer-N, crop residue-N, biologically-fixed N₂) to N₂O emissions from cropped Vertosols in order to define the least emitting system. The long-term aim is reduction of N₂O emissions from Australian agricultural systems. In the five experiments reported in this thesis, I examined the relative effects of fertilizer-N rates and biologically-fixed N₂ on N₂O emissions from an acidic Black Vertosol and further examined the effects of residue sources, particularly root versus shoot, and soil pH on N₂O emissions from the same soil.