Figure 8. Molecular ratios from Moose Creek. All bars n=3; Error bars represent SE. ** p=0.05 ** DOC to DON Ratios in Ponderosa Pine Forest Soils of Fire-Excluded and Frequently Burned Sites Rachel J. Brimmer and Thomas H. DeLuca, The University of Montana, Department of Ecosystem and Conservation Sciences Methods Two wilderness sites each having stands of different fire history were used for this study: 0 burn: no fires in 120 years Multiple burn: 3 or more fires in 120 years 4 soil cores (5cm dia.) including litter layer were taken per plot (n=3). Cores were trimmed to 2.5cm mineral soil and pore volume was determined by bulk density estimation. Cores were leached with 5 pore volumes of water Inorganic N (NH 4 + & NO 3 - ), total phenols, DON (persulfate oxidation), DOC (Shimadzu TOC 5000), and ninhydrin-reactive N analyses were performed on the leachates. Potentially mineralizable N (14-day anaerobic incubation) was determined on soils sampled from the same sites. Statistical analyses performed using SPSS All significance values from Mann Whitney U-tests due to data violations of normality. Acknowledgements The authors would like to thank M. Derek MacKenzie, Valerie Kurth, Michael Gundale, Clarice Pina and Joss McKinnon for their assistance and technical advice. Funding for site selection was provided by USDA-NRI ; support for this research was provided by the NSF-DEB Introduction Ponderosa pine forests are estimated to have had a historic fire return interval of years. Forests have experienced fire suppression for ~100 years. Nitrogen availability decreases with increasing time since fire The specific mechanism for this decline is not clear. Previous research has shown fire excluded sites to have less charcoal in the forest floor litter compared to sites experiencing a more typical fire return interval. Charcoal was also shown to adsorb phenolic compounds, a C source or potentially inhibitory class of organic compounds. Charcoal may also influence solution C:N ratios by adsorbing HMW, low N organic compounds. We also investigated the labile amino-N portion of the DON pool. Discussion and Conclusions These two sites display very different solution chemistry likely due to different storage times before leaching; however PMN data indicates a fundamental difference between fire effects on N pools at the two sites. Charcoal content in forest floor litter was less than 100 kg/ha at both 0 burn stands; charcoal content in multiple burn stands was not significantly greater at 23 Mile (~130 kg/ha) but was significantly greater at Moose Creek (~900 kg/ha). DOC is significantly lowered at Moose Creek multiple burn. Total phenols do not differ between stands. Nitrate dominates the inorganic N fraction in the multiple burn stand at 23 Mile and in both stands at Moose Creek. TDN pool is much larger in the multiple burn stand at 23 Mile than in the 0 burn stand. DOC:DON of leachate is significantly lowered at 23 Mile multiple burn. 0 burn stands have high variation in C:N ratios of the soil solution. Though not significant between sites, DOC:NHN (labile amino-N) ratios at multiple burn stands are lower than 0 burn stands; this potentially indicates less inhibition of microbial population by high C:N ratios. Figure 3.Organic and Inorganic nitrogen from 23 Mile. All bars n=3; Error bars represent SE. * p<0.10, ** p <0.05. * * * ** * Figure 4.Organic and Inorganic nitrogen from Moose Creek. All bars n=3; Error bars represent SE. ** p=0.05 ** Figure 7. Molecular ratios from 23 Mile. All bars n=3; Error bars represent SE. NS Figure 6. Carbon to nitrogen ratios from Moose Creek. All bars n=3; Error bars represent SE. NS Figure 5. Carbon to nitrogen ratios from 23 Mile. All bars n=3; Error bars represent SE. ** p < 0.05 ** Figure 1. Potentially mineralizable nitrogen. All bars n=3. Error bars represent SE, * p<0.10, ** p <0.05. Figure 2. DOC and total phenols. All bars n=3. Error bars represent SE. * p<0.10, ** p <0.05. ** * *