To determine the importance of microorganisms in regulating the retention of anthropogenic NO3, we followed the belowground fate and flow of 15NO3 in a mature northern hardwood forest, dominated by Acer saccharum Marsh. Total recovery of added 15N (29.5 mg 15N/m2 as NaNO3) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0v10 cm depth) was 93% two hours following application of the 15NO3 but rapidly dropped to  29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for 15NO3 (21% in forest floor; 16% in soil microbial biomass). After one month, total 15N recovery varied little (24v18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four-month experiment, the main fates of the 15N label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%). Temporal changes in the 15N enrichment (atom % excess 15N) of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO3 cycling in this forest. The 15N enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO3. In contrast, the 15N enrichment of soil organic matter did not peak until day 1, presumably because much of the added 15N cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the 15N enrichment of root biomass (,0.5-mm diameter and 0.5v 2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the 15N enrichment of soil organic matter (day 1) and, more importantly, NH4 1 (day 2). From these data we conclude that microorganisms are immediate, short-term sinks for anthropogenic NO3. Although the long-term fate of NO3 additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short-term factor influencing patterns of NO3 retention in this ecosystem.