Biodegradation 10: 43–50, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 43 Effect of nitrogen and phosphorus addition on phenanthrene biodegradation in four soils Carol R. Johnson & Kate M. Scow ∗ Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA ( ∗ author for correspondence, e-mail: kmscow@ucdavis.edu) Accepted 21 October 1998 Key words: biodegradation, nitrogen, nutrients, phenanthrene, phosphorus Abstract Phenanthrene mineralization rates were found to vary widely among four soils; differences in soil nutrient lev- els was one hypothesis to explain this variation. To test this hypothesis, phenanthrene mineralization rates were measured in these soils with, and without, added nitrogen and phosphorus. Mineralization rates either remained unchanged or were depressed by the addition of nitrogen and phosphorus. Phenanthrene degradation rates remained unchanged in the soil which had the highest indigenous levels of nitrogen and phosphorus and which showed the largest increase in phosphorus levels after nutrients were added. The soils in which degradation rates were depressed had lower initial phosphorus concentrations and showed much smaller or no measurable increase in phosphorus levels after nutrients were added to the soils. To understand the response of phenanthrene degradation rates to added nitrogen and phosphorus, it may be necessary to consider the bioavailability of added nutrients and nutrient induced changes in microbial metabolism and ecology. Introduction Microbial activity in soil is generally considered to be limited by the amount of available carbon and not by the levels of inorganic nutrients (Alexander 1994). A large influx of carbon compounds, as occurs dur- ing an oil spill, can reverse this situation creating an environment where biodegradation of the added carbon compounds is limited by nutrient availabil- ity. Numerous studies have been published in which nitrogen and/or phosphorus addition stimulated pol- lutant degradation; however, the effects of nutrients on pollutant degradation rates in soil are not consis- tent (Bossert & Bartha 1984; Alexander 1994; Baker & Herson 1994). Pollutant degradation rates in soil may be unaffected by added nutrients or may actu- ally decline relative to controls (Manilal & Alexander 1991; Morgan & Watkinson 1992). Stimulation of pol- lutant degradation rates may not appear until days or weeks after nutrients are added (Jobson et al. 1974; Bossert & Bartha 1984). Different aspects of pollutant degradation rates such as lag time, initial rate, and degradation extent may be affected singly or as a group by nutrient additions (Thorton-Manning et al. 1987; Swindoll et al. 1988). Differences in the bioavailability of nutrients added to soil may explain a portion of the observed differences in the response of degradation rates to ni- trogen and phosphorus supplements. Just as sorbed carbon substrates are generally considered to be un- available to soil microbes, sorbed inorganic nutrients also may be less bioavailable than nutrients dissolved in the soil solution. While nitrate is very soluble in soil, ammonium is retained by soil cation exchange sites on clays (Bohn et al. 1985). When added to soil, phosphate is adsorbed quickly to iron and aluminum oxide surfaces and may form precipitates with iron, aluminum, manganese, and calcium (Brady 1990). The objective of this study was to determine the effect of added nitrogen and phosphorus on the biodegradation rates of low concentrations of phenan- threne in four soils. These soils were selected from a set of seven soils for which phenanthrene degra- dation rates had been measured over a period of six