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Forest, Range & Wildland Soils

Resin Adsorption of Carbon and Nitrogen as Influenced by Season and Time Since Fire

Dep. of Ecosystem and Conservation Sciences, The Univ. of Montana, Missoula, MT 59812. M.D. MacKenzie, current address: 442 Earth Science Building, Dep. of Renewable Resources, Univ. of Alberta, Edmonton, AB, Canada T6G 2E3. T.H. DeLuca, currently at The Wilderness Society, 503 West Mendenhall Bozeman, MT 59715

^* Corresponding author (tom_deluca{at}tws.org)

Natural disturbance, such as fire, represents a long-term displacement of ecosystem dynamics, while seasonal changes, such as spring thaw and summer drought, represent a short-term perturbation superimposed on the longer disturbance cycle. In spite of the importance in determining forest ecosystem function of these two forms of perturbation, no previous studies have simultaneously investigated the effects of season and fire exclusion on soil C and N dynamics in the inland Northwest. Fire exclusion has resulted in decreased N mineralization due to changes in stand structure and decreased litter quality. However, C and N dynamics are also affected by seasonal fluctuations in soil temperature and moisture. We used ionic and non-ionic resin capsules to quantify indices of labile C and N including ninhydrin reactive N (NR-N–amino N), ammonium (NH₄⁺), nitrate (NO₃^–), anthrone reactive C (AR-C–hexose sugars), and phenols (low molecular weight aromatics). Repeated measures ANOVA showed that most soil biochemical properties varied significantly with time, season, and their interaction. However, only a few of the variables displayed linear trends with increasing time since fire (TSF), including NR-N, NH₄⁺–N, and NO₃^––N for winter and spring measurements, and AR-C for the spring measurement, all of which exhibited decreasing levels. Scatter plots showed that NR-N and AR-C were sequentially higher with each season, but decreased with TSF and that inorganic N was sequentially lower for each season and also decreased by TSF, except for spring NO₃^– levels on recently burned sites. None of the variables exhibited a significant difference during the summer measurement, likely as a result of seasonal drought, which emphasizes the importance of sampling time.

Abbreviations: AR-C, anthrone reactive carbon • DON, dissolved organic nitrogen • NH₄⁺, ammonium • NO₃^–, nitrate • NR-N, ninhydrin-reactive nitrogen • SOM, soil organic matter • TSF, time since fire