Nitrogen composition in furrow irrigated run-off water

Furrow irrigation in cotton growing vertosols is the most preferred method in Australia. After fertilisation, irrigation water interacts with the soil which can dissolve nitrogen (N) compounds into the run-off water. The run-off or tail water that leaves the field is enriched with N and can reduce crop N use efficiency. During 2014−2015 and 2015−2016 N solute concentration in the irrigation water and run-off was measured in a tillage cropping rotation experiment. In the continuous cotton treatments (2014−2015) when urea was broadcast on the surface of furrow irrigated cotton system, 11 % of the applied fertiliser (260 kg N ha−1) was lost from the field in the tail water. Most of the losses from the soil occurred during the first irrigation as nitrate and urea. The irrigation water supplied 10 kg dissolved organic nitrogen (DON) kg N ha−1 to the field. During 2015−2016, when subsurface banding of the urea was used, losses equated to 5 % of applied N, in irrigated continuous cotton treatments. In a second crop treatment, an irrigated maize rotation, the broadcasted urea was leached into the soil by rainfall before a 100 mm irrigation event. The run-off losses were less than the sub surface urea banding and in this treatment were 0.5 % of the applied fertiliser. The study shows that DON-N, NO3-N, NH4-N, Urea-N are dissolved from the soil in cotton production systems and lost to furrow irrigation run-off. This dissolved N maybe denitrified in the cotton irrigation network if the tail water is not reused quickly. The results show that N contributions from irrigation water need to be accounted for overall N budget of the cotton farm to improve the N use efficiency.