Project at a Glance

Title: Ecosystem level alterations in soil nutrient cycling: an integrated measure of cumulative effects of acidic deposition on a mixed conifer forest in Southern California.

Principal Investigator / Author(s): Miller, Paul R

Contractor: USDA Forest Service

Contract Number: 92-335

Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Acid Deposition, Ecosystem Impacts


This project was proposed and approved as a supplement to ARB Contract No.A032-180, "Assessment Acidic Deposition and Ozone Effects on Conifer Forests in the San Bernardino Mountains." The final for A032-180 was submitted in March 1996. The four tasks to be completed by this project (ARB Contract No.92-335) were: (1) Continue dry deposition monitoring at Barton Flats for several months during one summer of 1994; (2) Calibrate the Nutrient Cycling Model (NuCM) for the Barton Flats area and run simulations to examine some possible effects of five levels of nitrogen deposition over a 40-year period Determine the relationship between stomatal conductance and ozone uptake for Jeffrey pines of three classes during diurnal and seasonal cycles; and provide a one-time assessment of the variability of state conductance at several Sierra Nevada sites; (4) Conduct an intensive sampling study during July 18 to 31, 1993 in order to examine the fine structure of events or processes including, meteorology, atmospheric chemistry, pollutant flux to foliage, tree physiology, and the efflux of CO2 and NO from soils. The investigators elected to include the data from the additional dry deposition monitoring in the sun 1994 as part of the report for Contract No. A032-180. The only portion of that work included in this was the data from the intensive sampling period (summarized below). The Nutrient Cycling Model (NuCM) was formerly appropriate for humid forests of the eastern Unit but this project succeeded in calibrating it for Barton Flats, a "dry" ponderosa pine ecosystem. The was used to simulate the effects of varying levels of atmospheric N deposition ranging from 0.14 to 6.83 kmol ha-1 yr-1 (1.9 to 95.6 kg ha-1 yr-1). Simulation results indicated a significant increase in NO3,-leaching, reduced N retention, and a cessation of growth response to N input at deposition levels greater than 2.73 kmol ha-1 yr-1 (38.2 kg ha-1 yr-1). Increasing simulated N deposition levels from 0.14 to 0.68 and 1.37 kmol ha-1 yr-1 (1.9 to 9.5 and 19.5 kg ha-1 yr-1) reduced base cation leaching because of an anion shift from NO3 to HCO3- and SO42- caused by lower soil solution pH. Increasing deposition levels resulted in reduce exchangeable base cation levels in the soil, especially at the highest deposition level (6.83 kmol ha-1 especially with respect to K+).

These simulation results are not offered as quantitative predictions of what specific response will occurred specific N deposition loadings but rather as indicators of what is expected with changing N loadings upon our best current knowledge of nutrient cycling within this ecosystem. The results of these simulations clearly identify key indicators of long-term deposition effects and suggest several hypotheses that COL tested in either a field or laboratory setting.

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

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