Quantifying nitrate retention processes in a riparian buffer zone using the natural abundance of N-15 in NO3-

Quantifying the relative importance of denitrification and plant uptake to groundwater nitrate retention in riparian zones may lead to methods optimising the construction of riparian zones for water pollution control. The natural abundance of N-15 in NO3 has been shown to be an interesting tool for providing insights into the NO3 retention processes occurring in riparian zones. In this study, N-15 isotope fractionation (variation in delta(15)N of the residual NO3-) due to denitrification and due to plant uptake was measured in anaerobic soil slurries at different temperatures (5, 10 and 15degreesC) and in hydroponic systems with different plant species (Lolium perenne L., Urtica dioica L. and Epilobium hirsutum L.). It was found that temperature had no significant effect on isotope fractionation during denitrification, which resulted in a N-15 enrichment factor epsilon(D) of -22.5 +/- 0.6parts per thousand. On the other hand, nitrate uptake by plants resulted in N-15 isotope fractionation, but was independent of plant species, leading to a N-15 enrichment factor epsilon(P) of -4.4 +/- 0.3parts per thousand. By relating these two laboratory-defined enrichment factors to a field enrichment factor for groundwater nitrate retention during the growing season (epsilon(R) = -15.5 +/- 1.0parts per thousand), the contribution of denitrification and plant uptake to groundwater nitrate retention could be calculated. The relative importance of denitrification and plant uptake to groundwater nitrate retention in the riparian buffer zone was 49 and 51% during spring, 53 and 47% during summer, and 75 and 25% during autumn. During wintertime, high micropore dissolved organic carbon (DOC) concentrations and low redox potentials due to decomposition of the highly productive riparian vegetation probably resulted in a higher denitrification rate and favoured other nitrate retention processes such as nitrate immobilisation or dissimilatory nitrate reduction to ammonium (DNRA). This could have biased the N-15 isotope fractionation and led to a low N-15 enrichment factor for groundwater nitrate retention during wintertime (-6.2 +/- 0.9parts per thousand). In contradiction to what many other studies suggest, it is possible that due to plant decomposition during the winter period other nitrate transformation processes compete with denitrification. Copyright (C) 2003 John Wiley Sons, Ltd.