Plant and soil N response of southern californian semi-arid shrublands after 1 year of experimental N deposition

Large inputs of atmospheric N from dry deposition accumulate on vegetation and soil surfaces of southern Californian chaparral and coastal sage scrub (CSS) ecosystems during the late-summer and early-fall and become available as a pulse following winter rainfall; however, the fate of this dry season atmospheric N addition is unknown. To assess the potential for dry season atmospheric N inputs to be incorporated into soil and/or vegetation N pools, an in situ N addition experiment was initiated in a post-fire chaparral and a mature CSS stand where 10 x 10 m plots were exposed to either ambient N deposition (control) or ambient +50 kg N ha(-1) (added N) added as NH4NO3 during a single application in October 2003. After 1 year of N addition, plots exposed to added N had significantly higher accumulation of extractable inorganic N (NH4-N + NO3-N) on ion exchange resins deployed in the 0-10 cm mineral soil layer and higher soil extractable N in the subsurface (30-40 cm) mineral soil than plots exposed to ambient N. Chaparral and CSS shrubs exposed to added N also exhibited a significant increase in tissue N concentration and a decline in the tissue C:N ratio, and added N significantly altered the shrub tissue delta N-15 natural abundance. Leaching of inorganic N to 1 m below the soil surface was on average 2-3 times higher in the added N plots, but large within treatment variability cause these differences to be statistically insignificant. Although a large fraction of the added N could not be accounted for in the shrub and soil N pools investigated, these observations suggest that dry season N inputs can significantly and rapidly alter N availability and shrub tissue chemistry in Mediterranean-type chaparral and CSS shrublands of southern California.