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: We used a combination of experiment and modelling to test the hypothesis that dry summers could increase the risk of nitrogen (N) leaching from pasture in the following winter due to a combination of: soil N accumulation after 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 N fertiliser response and apparent N recovery in autumn after the drought was half that of previously irrigated pasture (7 vs. 13 kg DM kg N-1; 28 vs 52% recovery; P<0.05) with more soil mineral N at the start of drainage (P<0.001) as a result of this inefficient use of fertiliser N. When comparing six drought years, modelling demonstrated a strong inverse relationship between N leaching risk and pasture N uptake in autumn and winter. 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 rewetting after a summer drought) and the amount of fertiliser N applied. The latter can be managed by a farmer, but the former cannot.