Retention and Spatial Distribution of Mineral Nitrogen in the Forest Floor of an Aerially Fertilized Watershed

A small, 113 ha forested watershed in New Brunswick was fertilized with 115 kg-N/ha ammonium nitrate (NH 4 NO 3 ) using fixed-wing aircraft. Eight weeks later, the forest floor of the entire basin was sampled systematically and the NH 4- and NO 3- N contents of the forest floor were determined. At this time the forest floor had received 36 mm of precipitation, and contained about half of the applied NH 4- N fertilizer (i.e., 22.7 kg-N/ha, or 407 ppm), but lateral displacements by runoff of the NH 4 + -ions following the topographic drainage patterns had not occurred. The nitrate content of the forest floor was more mobile and, therefore, more variable than NH 4- N, because the NO 3- N contents were 6.4 N-kg/ha (or 96.2 ppm). Lateral NO 3- N runoff displacements also occurred because the spatial NO 3- N/NH 4- N ratio distribution varied about its mean value of 0.36 from 0.0 to 6.0. The fertilizer distribution patterns resulted in positively skewed histograms which could be fitted with a two-parameter gamma distribution model. This model predicts that (i) improvements in fertilizer uniformity should be obtainable by repeated fertilizer applications each involving the entire target area, (ii) the observed spatial fertilizer distribution pattern, within limits, should be independent of the size and shape of the forest floor samples examined for fertilizer content. Forest Sci. 28:613-626.