Changes in soil inorganic nitrogen as related to atmospheric nitrogenous pollutants in southern California

The deposition of nitrogenous pollutants has serious implications for ecosystem function and stability. Research in temperate ecosystems has indicated a wide range of ecological responses, yet very little is known about arid ecosystems. In this study, measurements of atmospheric and soil concentrations of the plant-available NO-3 and NH+4 were evaluated to identify a potential gradient in nitrogen (N) deposition. The evaluations were conducted in coastal sage scrub, a semi-arid vegetation type native to the lower elevations of southern California. The summer atmospheric concentrations of nitrate (NO-3) and ammonium (NH+4) were determined at five locations on the Perris Plain of southern California. The atmospheric influences varied from direct interception of pollution generated in the Los Angeles Basin at the northern end of the gradient to a site 70 km south lacking any direct Los Angeles influence. The summer atmospheric concentrations of NO-3 varied more than three-fold along the gradient. Ammonium concentrations followed a similar pattern, but the gradient was less steep. Winter concentrations were very low for both compounds. The summer soil surface NO-3 concentrations were near the detection limits at low pollution sites but in the range of 50–60 μg N g-1 soil under highly polluted conditions. Wet deposition was found to be a minor contributor of plant-available N, suggesting that dry deposition may be a consequential source of plant-available N. The detection of significant changes in inorganic, plant-available N in the upper layer of soils is enhanced by the unique environmental conditions and vegetation of southern California. This study suggests that the coastal sage scrub ecosystem is experiencing significant changes in N fertility that may contribute to changes in plant species composition. The data also show that this semi-arid ecosystem provides a unique opportunity to assess many physical, chemical and biological responses to dry deposition alone.