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

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Abstract
Evidence suggests that the natural 15N abundance (δ15N) 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 δ15N 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 (rs = −0.464**). Moreover, values of δ15N‐NO3 were highest in areas of the landscape where rates of denitrification were highest and there was a negative correlation (rs = −0.638***) between the concentration of deep‐leached NO3 and its δ15N. 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 δ15N value of leached NO3‐N.

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