Spatial and temporal nitrogen deposition and processing in a mediterranean mountain forest, California
Forest health is affected by nitrogen (N) deposition and ecosystem processing. The Kings River Experimental Watersheds in the Sierra Nevada, run by the U.S. Forest Service, provides a long-term mountain research site in California for evaluation of ecosystem N processing. Spatial and temporal patterns of N in deposition, stream and soil water quality, and soils over a 10-year period are discussed. A lichen survey and five years of air-concentration sampling with passive filters provide an extensive sampling array. More intensive sampling occurs within eight experimental watersheds that cover 990 ha; ion-exchange resin samplers provide annual N fluxes to the ground and shallow mineral soils at 460 points. The water chemistry at eight streams is evaluated twice a month along with wet season shallow soil water chemistry at four watersheds. Critical loads of N have been suggested for ecoregions of the United States and are evaluated at this Mediterranean mountain research site during normal, drought, and high precipitation years. Air pollution in the southern Sierra Nevada is significant and often at stress levels for vegetation.
At 3-5 kg N ha-1 yr-1 sensitive lichen species are adversely affected in the Sierra Nevada. At 17 kg N ha-1 yr-1, leaching is expected to surface water although we are not seeing this in the sampled streams on a regular basis. Above ground resin samplers indicate annual loadings of 5 to 11 kg N ha-1 yr-1. Soil leachate N fluxes range from a low of 2-3 kg N ha-1 yr-1 in 2004 to a high of 6-20 kg N ha-1 yr-1 in 2008. Both spatial and between-year variations have been large and will be discussed. The Bull Site watersheds (higher elevation) had significantly greater carbon and nitrogen in the mineral soil, likely because of slower decomposition rates. Although soil ammonium nitrogen was high, the carbon to nitrogen ratio was normal. The higher elevation watersheds received more N deposition than the lower elevation watersheds; this was not expected. Repeat high N fluxes do not appear to be random; some locations repeat two and three years out of five reported here. More of the repeat high values appear to be at watershed ridges.