The effect of autumn applied 15N-labelled fertilizer on nitrate leaching in a cultivated soil during winter

Nitrogen (N) fertilizer applied in autumn to arable farm land raises concerns over affects on ground water quality. The contribution of autumn 15N-labelled fertilizer (50 kg N ha-1) to nitrate leaching losses from a cultivated soil (silt loam on sandy loam; Udic Ustochrept) was measured using undisturbed monolith lysimeters (500 mm diameter, 700 mm long) during consecutive winters in Canterbury, New Zealand. The addition of 15N-labelled fertilizer at 50 kg N ha-1 did not significantly increase nitrate leaching losses. Soil-derived-N contributed 78 and 88% (1996 and 1997, respectively) of the nitrate leached beneath fertilized lysimeters. Warmer weather and wetter soil conditions at cultivation and fertilizer application during 1997, compared with 1996, resulted in an increased release of soil-derived-N in 1997. Nitrate leaching and average nitrate concentrations were therefore 41% and 56% higher, respectively, during the winter of 1997 than the winter of 1996. However, fertilizer leaching losses were relatively consistent between years (7.8 and 8.6%). Although not statistically significant, total N leaching losses and average nitrate concentration were 24 to 30% higher below fertilized lysimeters as compared with unfertilized lysimeters, indicating a priming effect of fertilizer on soil N release. During both late winter periods, leachate nitrate concentrations from fertilized and unfertilized lysimeters exceeded World Health Organisation (WHO) limits for drinking water. Higher release of soil-derived-N in 1997 also meant WHO limits were exceeded for 6 weeks longer than in 1996. In conclusion, the application of 15N fertilizer in autumn directly contributed only a small proportion of the total amount of N leached in this cultivated soil. However, the apparent priming effect of autumn applied-N fertilizer has importance on the overall environmental impact of this production system, as the amount of N leached, and extent to which health limits were exceeded, was largely determined by the factors which controlled the release of soil-derived-N.