1
IMPACTS OF ATMOSPHERIC DEPOSITION OF N ON FRAGILE ENVIRONMENTS: A CASE
STUDY FROM SOUTHERN CALIFORNIA SHRUBLANDS
Edith B. Allen
Department of Botany and Plant Sciences
University of California
Riverside, California, USA
Anthropogenic nitrogen (N) deposition is impacting natural ecosystems globally. The highest
levels of 90 kg ha-1 yr-1 (mainly ammonia/ammonium from agricultural sources) have been measured in the
Netherlands, where native vegetation is experiencing losses of biodiversity. In southern California up to 50
kg ha-1 yr-1 N deposition has been measured in native vegetation types. Most of the N is nitrate originating
from automobile exhaust, and up to 90% falls as summertime dry deposition in this Mediterranean-type
climate. The coastal sage scrub (CSS) of southern California has the been most impacted of local
vegetation types because it lies adjacent to urban areas where it experiences air pollution. The native
shrublands have been invaded by Mediterranean annual grasses, with a loss of native biodiversity and an
increase in the fire cycle. I report here on changes in vegetation, soils, and soil microorganisms in sites that
have received N from automobile emissions for some 50 years.
An urban-to-rural N deposition gradient occurs in western Riverside County, with both higher air
and soil concentrations of nitrate near urban areas. To determine whether the elevated soil N is responsible
for declines in native shrubs and weed invasion, a N fertilization experiment was initiated in spring 1994
following a fall 1993 wildfire. By 2001 there was no significant difference in shrub cover between
fertilized and unfertilized plots. However, grass cover and biomass were higher with N fertilization in most
years. Since grasses are the most flammable component of the vegetation, this indicated that N deposition
may increase the fire frequency and promote a more rapid conversion of native shrubland to exotic annual
grassland.
Mycorrhizal fungi have also been affected by N deposition, and in fact are good indicators of soil
N eutrophication. Arbuscular mycorrhizal (AM) fungi are important to plant growth because they are
beneficial fungi that increase plant nutrient and water uptake. However, the density and species richness of
AM fungi decreased with elevated soil N. The significant of this loss in diversity became apparent from
experiments using mycorrhizal inoculum from N-eutrophied soils to grow California sagebrush and the
exotic annual red brome grass. The N-affected mycorrhizae caused poor growth of California sagebrush
compared to the low-N soils. However, red brome growth was equivalent with both kinds of inoculum.
Thus the mycorrhizae of the native shrub cause a less mutualistic response and would be expected to
decrease the ability of native shrubs to establish over the long term.
We can draw the following conclusions from these studies: 1) N deposition causes elevated soil N
along a N deposition gradient from urban to rural areas,. 2) The exotic grasses have increased biomass with
elevated N, which may result in an increase in the fire cycle. 3) The diversity and density, and the root
infection of mycorrhizal fungi are decreased by elevated soil N. 4) Changes in AM fungi have feedbacks
on the growth of the plants that were detrimental to a native shrub but not to an exotic grass.