The effect of drought and nitrogen fertiliser addition on nitrate leaching risk from a pasture soil; an assessment from a field experiment and modelling

BACKGROUND A combination of field experiment and modelling tested the hypothesis that dry summers increase the risk of nitrogen (N) leaching from pasture owing to a combination of: soil N accumulation in a dry summer; slow recovery of drought‐affected pasture in the autumn; and the resultant inefficient use of fertiliser N by the pasture. RESULTS In the experiment, pasture response to urea and apparent N recovery in autumn after the drought was half that of irrigated pasture (7 vs 13 kg dry matter kg −1 N; 28 vs 52% apparent recovery; P < 0.05). There was more soil mineral N at the start of drainage ( P < 0.001) as a result of this inefficient fertiliser N use. Modelling of pasture growth in six different drought years demonstrated that subsequent N leaching risk after rewetting was inversely related to pasture N uptake during rewetting in the autumn. CONCLUSION When the period between post‐drought pasture recovery and the onset of drainage is short, N leaching risk increases. Nitrogen leaching is determined by the type of autumn (slow or fast growing conditions before drainage) and the amount of fertiliser N applied. The latter can be managed by a farmer, but the former cannot. © 2018 Society of Chemical Industry