Spatial patterns of denitrification and temporal variation in the factors controlling this process were studied in three forested ecosystems in northern Lower Michigan. Two forest stands were randomly located within each of two well-drained upland forests (sugar maple–red oak/Maianthemum and sugar maple–basswood/Osmorhiza ecosystems) and one swamp ecosystem (silver maple–red maple/Osmunda ecosystem). Potential N mineralization, nitrification, and microbial respiration were measured in each forest stand using a 33-week laboratory incubation. Factors controlling denitrification were investigated in each ecosystem by treating soil samples with factorial combinations of NO3−, C, and Ar (anaerobic conditions). We also investigated the separate production of N2 and N2O during denitrification, and the factors controlling these fluxes, in a different experiment. Seasonal patterns of denitrification were quantified using an intact soil core method. Potential nitrification and microbial respiration were consistently highest in the swamp forest and lowest in the sugar maple–red oak/Maianthemum ecosystem (582 vs. 3 μg NO3−-N•g−1 and 5275 vs. 1254 μg CO2-C•g−1, respectively). Nitrate availability was the most important factor controlling denitrification in the swamp ecosystem, whereas increased soil water content resulted in the greatest response in the upland forests. Although NO3− significantly increased denitrification in the upland ecosystems, water additions elicited an even greater response. In addition, N2O production in the upland forests accounted for 70 to 90% of the total gaseous N loss; N2O accounted for only 25% of this loss in the swamp forest. Mean denitrification (intact soil cores) in the sugar maple–red oak/Maianthemum ecosystem (12 μg N2O-N•m−2•d−1) was significantly lower than rates measured in the sugar maple–bass-wood/Osmorhiza and silver maple–red maple/Osmunda ecosystems (24 and 39 μg N2O-N•m−2•d−1, respectively). Denitrification reached a maximum during June and July in the sugar maple–basswood/Osmorhiza ecosystem, whereas peaks occurred in May and September in the silver maple–red maple/Osmunda ecosystem. Denitrification in the sugar maple–red oak/Maianthemum forest was variable throughout the year and consistently low. Although variability was high, results suggest that denitrification and the factors controlling this process can be predicted using the spatial distribution of ecosystems.

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The sugar maple (Acer saccharum Marshall)–red oak (Quercus rubra L.) and sugar maple–basswood (Tilia americana L.) ecosystems are Lake States forests that differ in net nitrification (5 and 15 g N m−2 yr−1, respectively), but experience equivalent rates of NO−3 leaching following clear‐cut harvest (≈5 g N m−2 yr−1). Our objectives were to determine whether high rates of N leaching are sustained following harvest and whether ecosystem‐specific patterns of biomass accumulation influence NO−3 loss. We studied two stands in each ecosystem and established four research plots in each stand; two plots were clear‐cut in 1991 and two were controls. In 1996, we measured soil solution NO−3 concentration (1‐m depth) and calculated areal losses by a water balance method. We used allometric equations to estimate woody biomass in clearcut plots; herbaceous biomass was clipped. In the sugar maple–red oak ecosystem, NO−3 leaching from 5‐yr‐old clear‐cut plots (0.56 g N m−2 yr−1) was significantly greater than leaching from control plots (0.05 g N m−2 yr−1). In contrast, NO−3 leaching did not differ between control (0.41 g N m−2 yr−1) and 5‐yr‐old clear‐cut (0.02 g N m−2 yr−1) in the sugar maple–basswood ecosystem; however, loss from these clear‐cut plots was significantly lower than that from clear‐cut sugar maple–red oak plots. Five years after harvest, 7.1 Mg ha−1 of aboveground biomass accumulated in clear‐cut sugar maple–basswood plots, almost twice that of clear‐cut sugar maple–red oak plots (3.9 Mg ha−1). Five years after harvest, the highest rates of NO−3 loss occurred in the sugar maple–red oak ecosystem, in which aboveground biomass accumulation was least.