Abstract
Nitrogen (N) uptake by spring ephemeral communities has been proposed as a mechanism that retains N within northern hardwood forests during the season of maximum loss. To understand better the importance of these plants in retaining N, we followed the movement of ^1^5NH"4^+ and ^1^5NO"3^- into plant and microbial biomass. Two days following isotope addition, microbial biomass represented the largest labile pool of N and contained 8.5 times as much N as Allium tricoccum L. biomass. Microbial immobilization of ^1^5N was 10-20 times greater than uptake by A. tricoccum. Nitrification of ^1^5NH"4^+ was five times lower in cores containing A. tricoccum compared to those without the spring ephemeral. Spring N retention within northern hardwood forests cannot be fully explained by plant uptake because microbial immobilization represented a significantly larger sink for N. Results suggest that plant and microbial uptake of NH"4^+ may reduce the quantity of substrate available for nitrification and thereby lessen the potential for NO"3^- loss via denitrification and leaching.