Spatial patterns of soil nitrification and nitrate export from forested headwaters in the northeastern United States

Nitrogen export from small forested watersheds is known to be affected by N deposition but with high regional variability. We studied 10 headwater catchments in the northeastern United States across a gradient of N deposition (5.4 - 9.4 kg ha(-1) yr(-1)) to determine if soil nitrification rates could explain differences in stream water NO3- export. Average annual export of two years (October 2002 through September 2004) varied from 0.1 kg NO3--N ha(-1) yr(-1) at Cone Pond watershed in New Hampshire to 5.1 kg ha(-1) yr(-1) at Buck Creek South in the western Adirondack Mountains of New York. Potential net nitrification rates and relative nitrification (fraction of inorganic N as NO3-) were measured in Oa or A soil horizons at 21-130 sampling points throughout each watershed. Stream NO3- export was positively related to nitrification rates (r(2) = 0.34, p = 0.04) and the relative nitrification (r(2) = 0.37, p = 0.04). These relationships were much improved by restricting consideration to the 6 watersheds with a higher number of rate measurements (59-130) taken in transects parallel to the streams (r(2) of 0.84 and 0.70 for the nitrification rate and relative nitrification, respectively). Potential nitrification rates were also a better predictor of NO3- export when data were limited to either the 6 sampling points closest to the watershed outlet (r(2) = 0.75) or sampling points <250 m from the watershed outlet (r(2) = 0.68). The basal area of conifer species at the sampling plots was negatively related to NO3- export. These spatial relationships found here suggest a strong influence of near-stream and near-watershed-outlet soils on measured stream NO3- export.