PARENT SESSION
Symposium #8: Nitrogen deposition in the Western U.S.... Is there cause for concern?.
Sponsored by ESA Rocky Mountain Chapter
Organized by: H.M. Rueth and J.S. Baron.
Tuesday, August 7, 2001. 8:00 AM to 12:00 PM. Madison Ballroom A

Effects of N deposition on plants and soil microorganisms on an urban to rural gradient in southern California.

Allen, Edith¹, Egerton-Warburton, Louise¹, Sigüenza, Concepcion¹, Sirulnik, Abby¹, Padgett, Pamela¹, ¹

ABSTRACT- Southern California has up to 45 kg/ha/yr N deposition, the highest in the nation. We measured the effects of N deposition on vegetation and soil microorganisms on a gradient from the Riverside area southward 50 km. Most of the atmospheric N that is plant-available is in the form of nitrate, originating from automobile exhaust. Extractable N in soil along the gradient was 87 to 15 g/g. We selected nine sample sites dominated by coastal sage scrub on decomposed granite soils. The shrubland had an understory of exotic Mediterranean annuals all along the gradient. To test the hypothesis that N is increasing the productivity of exotic annuals, we fertilized plots following a 1993 wildfire at Lake Skinner with 60 kg/ha/yr of ammonium nitrate. Grass productivity increased with N during wet years but not during dry years. Shrub cover increased gradually after the fire but was lower in the fertilized plots by the forth year. The species richness of arbuscular mycorrhizal fungi decreased with elevated soil N along the gradient and in fertilized plots, and large-spored species dropped out. Fatty acid methyl ester soil profiles indicated changes in soil bacteria and fungi that were similar for soils with N deposition or fertilization. Soil microorganisms were consistent indicators of elevated soil N, while there was variability in the plant community along the gradient dependent on other local factors.

KEY WORDS: coastal sage scrub, FAME, mycorrhizae