Print

Jamison, Austin E.

Plant and Soil Science

2004

MS

Excess nitrogen (N) deposition has been targeted as a major environmental problem, especially in the northeastern United States. Concerns over the long term deposition of N on forested ecosystems are increasing as some forested watersheds have begun exporting higher levels of nitrate. Stream water nitrate has been shown to originate from the soil nitrification process, so factors which influence or control soil nitrification rates (NR) are of interest in understanding watershed N export. This study examined 10 small, forested watersheds in the northeastern U.S. to investigate the relationship of topographic, vegetative, and soil controls with soil NR. Soil sampling was done in the spring and fall of 2001-2003 to determine NR and soil properties. Topographic and vegetative characteristics at each sampling point were also measured. Soil properties such as higher soil moisture, higher pH, and lower C:N ratios were most often correlated with higher NR. Topographic analyses indicate lower slope positions and higher elevations to be occasionally correlated with higher NR, possibly because of greater C and N accumulation and higher soil moisture contents. The C:N ratio and vegetation composition can explain differences between watersheds in terms of NR. Conifers were often associated with lower NR and higher C:N ratios while sugar maples dominated in areas of higher NR and lower C:N ratios. Soil properties were more often correlated with NR than were topographic properties. When conifers make up more than 50% basal area in study plots, NR was lower (p < 0.0001) and C:N ratios higher (p < 0.0001) when compared to plots dominated by sugar maple, yellow birch, or American beech. Our results demonstrate the difficulty of finding similar controls on NR between watersheds, but do point to a regionwide influence of the C:N ratio and vegetation on NR.