The effects of added nitrogen deposition on light-saturated photosynthesis and respiration in a semi-arid environment

Background/Question/Methods

Atmospheric nitrogen in Southern California has increased due to anthropogenic activity; most N inputs fall as dry deposition in the fall and summer months.  Because nitrogen is often considered a limiting resource for growth in the coastal sage scrub (CSS), we expect that semi-arid shrublands exposed to high levels of added N will have increased levels of light saturated photosynthesis and respiration.  We have been measuring light-saturated photosynthesis and dark respiration in plots exposed to ambient and elevated (50 kgN ha^-1 yr^-1) N addition using a portable photosynthesis system (LI-6400) for two species, Salvia mellifera and Artemisia californica, that are characteristic of Southern Californian CSS.  Leaf nitrogen levels were also analyzed among the control and treated plots to determine expected links between leaf N concentration and leaf gas exchange. 

Results/Conclusions

Previous results revealed that light saturated photosynthesis increased as leaf nitrogen levels increased due to N addition to the soil; however, treatment differences were at times equivocal, and presumably, longer exposure to added N would lead to larger differences in leaf gas exchange over time.  These data will help determine how anthropogenic N addition will affect potential rates of leaf gas exchange in these N limited semi-arid shrublands.