Nitrification potentials and landscape, soil and vegetation characteristics in two Central Appalachian watersheds differing in NO3- export

Two watersheds within 1 km of each other in the Central Appalachian mountains of West Virginia have similar management histories and receive 13 kg of N in atmospheric deposition, but NO3- export from one watershed (W4) has increased over the last 30 years, and is now approximately five times greater than NO3- export from the other (W10). We measured net nitrification potentials (NNP) and other landscape, soil, and plant community variables (1) to determine whether differences in leaching could be attributed to differences in NNP, (2) to identify other significant differences between the watersheds, (3) to identify variables that could account for both between- and within-watershed variability in NNP, and (4) to identify readily measured variables that distinguish plots with relatively high or low NNP. NNPs in W4 were 0.84 kg N ha(-1) per day, approximately three times higher than those on W10. Watershed 4 lay at a slightly higher elevation, had gentler slopes, a thinner forest floor, lower C:N in the 0-10 cm soil layer, lower tree density, greater basal area in Acer saccharum, less basal area in Quercus prinus and Amelanchier arborea, more frequent occurrences of A. saccharum seedlings, Laportea canadensis, Polystichum acrostichoides, Trillium sp., Uvularia sessilifolia and Viola spp., and fewer occurrences of Gaultheria procumbens, and Viburnum acerifolum. NNPs were correlated with many soil characteristics related to base cation supply, C:N and water holding capacity (WHC). Several two- and three-variable regression models, which were mostly based on soil characteristics, accounted for a large proportion of the variability in NNP (adjusted R-2 > 0.60), as well as for the difference between watershed means (t-test of residuals indicate no significant difference). A regression model based on basal area of A. saccharum and A. rubrum and the presence or absence of Trillium accounted for 50% of the variability in NNP. At this high-deposition site, plots with soils that had higher pHs, greater base cation supply and WHC, and lower C:N were more susceptible to NO3- leaching and N saturation. (C) 2002 Elsevier Science B.V. All rights reserved.