Nitrate Assimilation by Herbaceous Ground Flora in Late Successional Forests

Abstract

(1) Nitrate reductase (NR) activity was measured in two plant species to study the mechanism of nitrogen (N) retention by spring ephemeral and herbaceous ground flora. (2) Studies were conducted in a maple-beech and a river flood plain forest that had an abundant coverage of spring ephemeral and herbaceous ground flora. NR activity was determined for leaf and root tissues of Allium tricoccum L. and Asarum canadense Ait. by an in vivo tissue infusion procedure. Potential net N mineralization and nitrification were measured in soil by aerobic laboratory incubation. (3) NR activity was low in both species and confined to leaf tissue. Rates of NO3 reduction were comparable with those reported for ericaceous species typically associated with habitats where nitrification is minimal. Potential nitrification, however, was high in the soils of both forests, with 99% of all inorganic N present as NOy -N after a fourteenweek incubation. (4) Nitrate fertilization was used to induce NR activity in six Asarum canadense clones within each forest. In two separate experiments, deionized water was added to one plot (control) while the second was treated with 60 kg N ha-l as NOf . NR activity was significantly greater in fertilized plots. However, induced activities were low and did not significantly contribute to plant N nutrition. (5) Results suggest that Allium tricoccum and Asarum canadense have a limited ability to assimilate NO-. The discrepancy between leaf NR activity and soil nitrification potential may have resulted from the lack of root competition and stimulation of net mineralization in the laboratory incubation. Alternatively, the NR assay may underestimate NOassimilation in these late successional species. Plant-nitrifier competition may be an important process influencing NOy loss in forested ecosystems and may partially explain the mechanism of N retention by ground flora communities.