LANDSCAPE-SCALE VARIATIONS IN LEACHED NITRATE - RELATIONSHIP TO DENITRIFICATION AND NATURAL N-15 ABUNDANCE

Evidence suggests that the natural N-15 abundance (delta(15)N of leach ed NO3 can be related to denitrification activity. In addition, spatia l patterns for denitrification and leached NO3 are controlled by lands cape-scale interactions between topography and hillslope hydrology. Th us, this study was conducted to assess whether a landscape-scale relat ionship exists between denitrification activity and the delta(15)N Of deep-leached (>120 cm) NO3 in an undulating landscape developed on sil ty glacio-lacustrine sediments overlying a clay loam till of late Wisc onsinan age, under semiarid conditions. The highest concentrations of NO3 occurred in shoulder and upper-level landscape positions and the l owest concentrations in footslope and lower-level positions. In areas of the landscape where water accumulation was at a maximum, denitrific ation rates were highest and there was no accumulation of NO3 in the u pper 3 m of the solum. Conversely, in areas where water accumulation a nd denitrification activity were low, there was a buildup of NO3 below the root zone and within 3 m of the soil surface. The relationship be tween seasonal denitrification activity and the concentration of leach ed NO3 was characterized by a negative correlation (r(s) = -0.464(*)) . Moreover, values of delta(15)N-NO3 were highest in areas of the land scape where rates of denitrification were highest and there was a nega tive correlation (r(s) = -0.638(**)) between the concentration of dee p-leached NO3 and its delta(15)N. These data support the hypothesis th at topographic and hydrologic controls on denitrification and leaching are reflected in the localized removal of NO3 from the soil profile, which is reflected in the delta(15)N value of leached NO3-N.