Wildfire Effects on Soil Gross Nitrogen Transformation Rates in Coniferous Forests of Central Idaho, USA Akihiro Koyama, 1,3 Kathleen L. Kavanagh, 2 * and Kirsten Stephan 1,4 1 Department of Forest Resources, University of Idaho, PO Box 441133, Moscow, Idaho 83844-1133, USA; 2 Forest Ecology and Biogeosciences, University of Idaho, PO Box 441133, Moscow, Idaho 83844-1133, USA; 3 School of Biological Sciences, Washington State University, Pullman, Washington 99164-4236, USA; 4 Department of Life and Physical Sciences, Lincoln University, 820 Chestnut St., Jefferson City, Missouri 65101, USA ABSTRACT Forest fires often result in a series of biogeochem- ical processes that increase soil nitrate (NO 3 - ) concentrations for several years; however, the dy- namic nature of inorganic nitrogen (N) cycling in the plant–microbe–soil complex makes it chal- lenging to determine the direct causes of increased soil NO 3 - . We measured gross inorganic N trans- formation rates in mineral soils 2 years after wild- fires in three central Idaho coniferous forests to determine the causes of the elevated soil NO 3 - . We also measured key factors that could affect the soil N processes, including temperature during soil incubation in situ, soil water content, pH and car- bon (C) availability. We found no significant dif- ferences (P = 0.461) in gross nitrification rates between burned and control soils. However, microbial NO 3 - uptake rates were significantly lower (P = 0.078) in burned than control soils. The reduced consumption of NO 3 - caused slightly elevated NO 3 - concentrations in the burned soils. C availability was positively correlated with microbial NO 3 - uptake rates. Despite reduced microbial NO 3 - uptake capacity in the burned soils, soil microbes were a strong enough N sink to maintain low soil NO 3 - concentrations 2 years post fire. Soil NH 4 + concentrations between the treat- ments were not significantly different (P = 0.673). However, gross NH 4 + production and microbial uptake rates in burned soils were significantly lower (P = 0.028 and 0.035, respectively) than in the controls, and these rates were positively cor- related with C availability. Our results imply that C availability is an important factor regulating soil N cycling of coniferous forests in the region. Key words: forest fires; gross N transformation; 15 N pool dilution; N limitation; N retention; nitrate; ammonium; coniferous forests; C availability. INTRODUCTION Forest fires are a natural disturbance ubiquitous in many parts of the world including the Inland Pacific Northwest of the USA (Agee 1993). Forest fires influence the structure and composition of forests (Gallant and others 2003), landscape diversity (Romme 1982), and biogeochemical cycles (Raison Received 19 April 2010; accepted 12 August 2010 Author Contributions: A. Koyama conducted field work, lab experi- ments and data analyses, and wrote the manuscript. K.L. Kavanagh and K. Stephan contributed to the overall design of the experiments, field work and provided editorial comments on previous drafts of the manu- script. *Corresponding author; e-mail: katyk@uidaho.edu Ecosystems DOI: 10.1007/s10021-010-9377-7 Ó 2010 Springer Science+Business Media, LLC