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TECHNICAL REPORTS

Landscape and Watershed Processes

Nitrogen Deposition Effects on Carbon Storage and Fungal:Bacterial Ratios in Coastal Sage Scrub Soils of Southern California

Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521

^* Corresponding author (crowley{at}ucr.edu).

Received for publication August 25, 2008. The effects of nitrogen (N) across a deposition gradient on bacterial and fungal degradation pathways were studied in southern California coastal sage scrub soils to determine whether elevated N levels alter microbial community structure and organic matter accumulation. Three sites across an N deposition gradient having low, intermediate, and high levels of atmospheric N deposition were studied for 20 mo. Fungi:bacteria (F:B) biomass ratios were determined by phospholipid fatty acid analysis. Plots at each location included control plots receiving ambient N deposition and treatment plots that were fertilized with an additional 50 kg N ha^–1 yr^–1 of slow-release urea. Results showed that organic carbon (C) levels varied seasonally but that F:B ratios were relatively stable and similar across the three locations and over time. Total organic C decreased in response to N additions only at the low N deposition site. The results suggest that organic matter degradation pathways leading to C storage in soils that have been exposed to high levels of atmospheric N deposition are not responsive to additional increases in N and that N effects on organic C in semiarid soils may be significant only in areas with prior low exposure to N pollution.

Abbreviations: F:B ratio, fungi:bacteria ratio • LM, Lake Mathews • MR, Motte Rimrock Reserve • OM, organic matter • PLFA, phospholipid fatty acid • SMR, Santa Margarita Ecological Reserve