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Nitrogen-15-Labeled Nitrate Transport in a Soil with Fissured Shale Substratum¹

ABSTRACT

Published data on the fate of ¹⁵N-labeled fertilizer applied to 4 Coshocton, Ohio, monolith lysimeters showed a series of declining NO₃^–-N concentration maxima in the lysimeter percolate during 3 winter drainage periods subsequent to the single 336 kg/ha fertilizer application. A possible explanation of these concentration maxima as arising from the leaching of previously immobilized and subsequently mineralized N was quantitatively inadequate. In this study, we sought an alternative explanation of these observations.

The lowest meter in the 2.44-m deep monolith profile of Keene silt loam consisted of fissured shale bedrock, and analysis of mass conservation of ¹⁵N-labeled inorganic N in the fissured layer provided an explanation of these observed annually periodic concentration maxima. Some of the calculated flux of labeled N into the top of the fissured layer was shown to be stored within the layer, and to reappear in subsequent annual winter drainage periods. Two possible mechanisms, which allowed such temporary storage, were identified: storage within the pore space of the crack system, and storage in micropores of the weathered rock by lateral diffusion from solution contained in the cracks.

Key Words: NO₃^– leaching • monolith lysimeters • preferential solute movement • preferential flow • soil macropores • lateral diffusion • solute storage • chemical transport model

¹ Contribution from the School of Australian Environmental Studies and the USDA, Agricultural Research Service, in cooperation with the Texas Agricultural Experiment Station, Texas A&M University.

² Soil Scientists and Honors Student, respectively. C. W. Rose and I. Phillips located at the School of Australian Environmental Studies, Griffith University, Nathan, Queensland, Australia 4111, and F. W. Chichester, at the USDA-ARS, Grassland, Soil & Water Research Laboratory, P.O. Box 748, Temple, TX 76503.

Received for publication May 29, 1982.