Movement of nitrogen-15 labeled nitrate in large undisturbed columns of poorly drained soil

A laboratory study was conducted with large (20-cm i.d., 110-cm long PVC pipe) intact soil columns to determine the movement of fertilizer NO3- in poorly drained, conventionally tilled soil under simulated low (7.6 cm) and heavy (15.2 cm) rainfall. Soil in the columns was brought to near-maximum water-holding capacity (9 kPa) to simulate the typical field soil moisture regime during the spring. A constant-level water table was imposed at the base of the column to further simulate field conditions of the Drummer silty clay loam (mixed, mesic, Typic Haplaquoll) soil used. Fertilizer was applied in solution at a rate equivalent to 168 kg N ha-1 as 15N-labeled KNO3. Water was then applied in three applications, spaced one wk apart. To minimize the movement of water along the soil-pipe interface, a 3 mm-wide band of air-dried disturbed soil was packed around the core to ensure a seal along the interface. Recovery of fertilizer NO3--N below the water table at the end of the 28-d study was < 0.06% (0.1 kg N ha-1) and 0.5% (0.9 kg N ha-1) of that applied for the low and high treatments, respectively. Denitrification losses were negligible for both water treatments (≤ 1 kg N ha-1). Fertilizer N distribution in the columns indicated significant movement of N beyond estimated water-displacement depths, apparently caused by preferential flow. However, the majority of the N was restricted to the upper portions of the columns. The results indicate that preferential flow of water in poorly drained, conventionally tilled soils during high rainfall periods can lead to the movement of fertilizer N to shallow ground water, but that the amounts are apparently very small.