Fig. 2. Soil NO₃–N increase due to manure application frequencies as estimated by ¹⁵N measurements in south-central Wisconsin, 2000. Numbers above each bar represent the percentage of recovered ¹⁵NO₃–N present in the top 30 cm only. For NO₃–N increase in the 0- to 90-cm depth, bars with the same letter are not significantly different at the 0.05 level.