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Plant and Soil 254 35 46 2003 2003 Kluwer Academic Publishers Printed in the Netherlands 35 The effects of fire on soil nitrogen associated with patches of the actinorhizal shrub Ceanothus cordulatus Brian B Oakley1 3 Malcolm P North2 Jerry F Franklin1 1 University of Washington College of Forest Resources Box 352100 Seattle WA 98195 2100 USA 2 Sierra Nevada Research Center Department of Environmental Horticulture University of California Davis CA 95616 USA 3 Corresponding author Received 19 July 2002 Accepted in revised form 20 August 2002 Key words Ceanothus fire Frankia nitrogen Sierra Nevada Abstract Nitrogen is a limiting resource in many temperate forests and nitrogen fixing plants are usually limited to the early stages of post disturbance succession In fire dependent Sierra Nevada forests however Ceanothus cordulatus is relatively abundant even in old growth forest conditions which are at least partly maintained by fire We conducted a field experiment to determine if soil beneath Ceanothus patches represent resource islands of available N which persist after fire Nine plots containing discrete patches of Ceanothus Arctostaphylos patula manzanita chosen as a non N fixing reference species and bare forest floor were subjected to either a low intensity n 3 or highintensity n 3 bum treatment or remained unburned as controls n 3 Soil temperatures during the bum were monitored by a network of thermocouples placed at the surface of the mineral soil and at ca 10 cm depths Soil samples were collected from the organic horizon 0 10 cm and 15 25 cm depths within each patch type immediately before burning and 2 days and 6 and 11 months after Soil moisture total C and N and ammonium and nitrate concentrations were determined in the laboratory Before the burn Ceanothus patches were significantly enriched in total and inorganic N in the organic horizon relative to the other patch types A sharp increase in inorganic N was observed in all patch types and depths immediately



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