The Vernal Dam: Plant‐Microbe Competition for Nitrogen in Northern Hardwood Forests

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 5 NH 4 + and 1 5 NO 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 5 N was 10—20 times greater than uptake by A. tricoccum. Nitrification of 1 5 NH 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.