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Landscape-Scale Variations in Leached Nitrate: Relationship to Denitrification and Natural Nitrogen-15 Abundance

Dep. of Chemistry, Univ. of Massachusetts-Lowell, One University Ave., Lowell, MA 01854

Peter J. Sandercock, Daniel J. Pennock and Chris Van Kessel^*

Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, SK, Canada S7N 0W0

*Corresponding author.

ABSTRACT

Evidence suggests that the natural ¹⁵N abundance (¹⁵N) of leached NO₃ can be related to denitrification activity. In addition, spatial patterns for denitrification and leached NO₃ are controlled by land-scape-scale interactions between topography and hillslope hydrology. Thus, this study was conducted to assess whether a landscape-scale relationship exists between denitrification activity and the ¹⁵N of deep-leached (>120 cm) NO₃ in an undulating landscape developed on silty glacio-lacustrine sediments overlying a clay loam till of late Wisconsinan age, under semiarid conditions. The highest concentrations of NO₃ occurred in shoulder and upper-level landscape positions and the lowest concentrations in footslope and lower-level positions. In areas of the landscape where water accumulation was at a maximum, denitrification rates were highest and there was no accumulation of NO₃ in the upper 3 m of the solum. Conversely, in areas where water accumulation and denitrification activity were low, there was a buildup of NO₃ below the root zone and within 3 m of the soil surface. The relationship between seasonal denitrification activity and the concentration of leached NO₃ was characterized by a negative correlation (r_s = –0.464**). Moreover, values of ¹⁵N-NO₃ were highest in areas of the landscape where rates of denitrification were highest and there was a negative correlation (r_s = –0.638***) between the concentration of deep-leached NO₃ and its ¹⁵N. These data support the hypothesis that 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 ¹⁵N value of leached NO₃-N.

Contribution no. R774 of the Saskatchewan Center for Soil Research.

Received for publication July 24, 1995.

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