Project at a Glance

Title: Air pollution and changes in forest nitrogen status: fog and rain deposition and nitrogen losses from forested watersheds in the San Bernardino mountains.

Principal Investigator / Author(s): Fenn, Mark E

Contractor: USDA Forest Service

Contract Number: 95-329

Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Ecosystem Impacts


The primary objective of this project was to address knowledge gaps in our understanding of the effects of N deposition on forest ecosystems in California. This study was carried out at key sites located across an air pollution gradient in the San Bernardino Mountains. The research entailed three major components: measuring nitrogen (N) deposition to the forest in fog and throughfall, determining spatial and temporal patterns of nitrate (NO3-) export in strearnwater, and quantification of trace gas fluxes from soil at a high and a low-to-moderate N deposition site. We estimate that N deposition in fog contributed 21 percent of the total annual N deposition at Barton Flats (1.0 kg ha-1 yr -l) and 43 percent at Camp Paivika (13.4 kg N ha-l yr"1. Although fog water inputs were low compared to rain and snow, N deposition in fog was significant because of the high ionic concentrations in fog and the frequency of occurrence, especially in the western San Bernardino Mountains. Annual throughfall deposition of N was 19 kg ha-l yr-l at Camp Paivika (CP) and 2.9 at Barton Flats (BF). Total forest stand-level N deposition was estimated to be 31 and 5 kg ha-1 yr-l at CP and BF, based on literature values of throughfall underestimation of total atmospheric N deposition. The throughfall data from this study demonstrated that N inputs that are associated with N saturated sites in the San Bernardino Mountains are similar to levels reported to cause N saturation or elevated streamwater nitrate export in more mesic forests.

We report streamwater nitrate (NO3) concentrations from 19 sampling sites across the deposition gradient in the SBM for December 1995 through December 1997. Sampling frequency varied from biweekly during the first winter rain season to predominantly monthly thereafter. Six streams (seven sampling sites) are in Devil Canyon (DC), a high-pollution area on the western end of the range. Twelve of the streams sampled are located in a horseshoe pattern surrounding the San Gorgonio Wilderness (SGW) on the eastern end of the SBM, a region which is characterized by relatively low-to-moderate N deposition. Streamwater NO3 concentrations in DC are the highest values reported in North America for undisturbed watersheds. The streams in DC exhibited a wide range of temporal NO3- concentration profiles. Concentrations in the primary stream draining western DC peaked at 350 ~ L -1 in December 1997 and minimum baseflow NO3 concentrations were nearly always greater than 70 p.eq L-1. Nitrate concentrations at the source of a spring-fed stream averaged 70 (:1:0.9 s.e.) p.eq L-1 in DC and 10.3 (:1:1.3) p.eq L-1 in the SGW with little seasonal variation. Base flow NO3- concentrations in five of the six streams in DC were greater than base flow concentrations in all of the SGW streams. In the SGW, only the five catchments on the southwestern and western end (streams 1-5), where N deposition is greatest, consistently exported levels of NO3- well above detection limits. Peak NO3- concentrations in DC and in the SGW occurred after large winter storms and a short-term large spike in NO3- export (10-370 p.eq L -1) in SGW streams 1-5 was observed after slight thundershower activity in July 1997. Streamwater NO3- concentrations in the 19 sampling sites corresponded with levels of N deposition across the N deposition gradient. Greater than expected spatial variation in streamwater NO3- concentrations within DC, an area of high N deposition, was attributed to differences in N processing within the upland catchments and riparian zone. Further studies of the factors responsible for the variability in streamwater NO3- in DC may provide useful.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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