Dissolved organic nitrogen contributes significantly to leaching from furrow-irrigated cotton-wheat-maize rotations

Leaching of nitrogen (N) in intensive irrigated agriculture can be a significant loss pathway. Though many studies have focussed on losses of mineral N, and in particular nitrate, dissolved organic N (DON) has received less coverage. In the present study, over a 5-year period (2008-2013), 740 kg N ha(-1) fertiliser was applied to an irrigated cotton-wheat-maize rotation on a cracking clay (grey Vertosol). Deep drainage from the undisturbed soil profile at the site was measured at 2.1 m below the soil surface using a variable tension lysimeter. In total, 108 mm of drainage occurred during the 5 years and the majority of the drainage and the irrigations occurred during the cotton seasons. The majority of the N loss occurred during the first 3-4 irrigations and neither the N loss nor its composition were affected by the product or timing of the fertiliser application. The N in the drainage was composed of 12.8 kg NOx-N ha(-1), 8.7 DON-N and 0.1 NH4+-N kg ha(-1), which shows that DON is an important component (40%) of the deep drainage N from irrigated Vertosol cotton production systems. Overall the total N flux lost via deep drainage represents 3% of the applied N fertiliser.