Knoke, K. E. 1997. Assessment of the origin and fate of nitrate from soil lysimeters using stable nitrogen isotopes. Thesis, Michigan State University, East Lansing, Michigan, USA.

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The concentration and d15N of nitrate in groundwater samples collected from two mid-Michigan lysimeters beneath farmland were determined on a weekly basis from March to November of 1993. The soil within one of the lysimeters was tilled and the other was not tilled. Nitrate concentrations in the samples from the tilled lysimeter were approximately twice those of the untilled lysimeter throughout the study. The nitrogen stable isotopic abundances varied similarly on a seasonal basis in the tilled and untilled lysimeters. The d15N values ranged between -3.9 0/00 in the spring to 9.4 0/00 in fall (September). An increasing trend in the d15N of nitrate from both lysimeters occurred from spring to late summer, that was likely the result of fractionation due to denitrification and mineralization of the less readily mineralizable organic fraction. A decrease in d15N was observed from the middle of September to a early November that reflected a decrease in the influence of microbial denitrification on the isotopic composition of nitrate.

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