UPTAKE AND DISTRIBUTION OF NITROGEN FROM ACIDIC FOG WITHIN A PONDEROSA PINE (PINUS-PONDEROSA LAWS) LITTER SOIL SYSTEM/

The magnitude and importance of wet deposition of N in forests of the South Coast (Los Angeles) Air Basin have not been well. characterized. We exposed 3-yr-old ponderosa pine (Pinus ponderosa Laws.) seedlings growing in native forest soil to acidic fog treatments (pH 3.1) simula ting fog chemistry from a pine forest near Los Angeles, California. Fo g solutions contained either (NH4+)-N-15, (NO3-)-N-15, or unlabeled N. The fog treatments were applied in open-top chambers in six 5-hr expo sures. Soil treatments within each of the fog exposures were bare soil , soil overlain with L- and F-litter, and soil covered with plastic du ring the fog events to prevent fogwater from contacting soil. Seedling s were harvested and samples were collected 15 wk after the fog treatm ents. Uptake of N-15 by roots was by far the dominant pathway for plan t assimilation of fog-deposited N-15. Deposition of N in fog supplied 9.4% and 8.7% of the total N in current-year crown biomass in the litt er-overlay and bare-soil treatments, respectively. Total N concentrati ons in every plant fraction except current-year stems were significant ly higher in the bare-soil treatment than in the plastic-covered soil treatment. Less than 5% of the N-15 deposited directly to the seedling crowns was retained by the plants in the covered-soil treatment, wher eas 57% of the N-15 deposited to the seedling/litter/soil systems was incorporated into plant biomass. The litter layers retained (NH4+)-N-1 5 more effectively than (NO3)-N-15. Data from this study suggest that N deposited from fog may be an important source of N for plant growth in forests of the SCAB where fog occurrence and pollution exposure coi ncide.