Contribution of winter processes to soil nitrogen flux in taiga forest ecosystems
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We measured annual net nitrogen (N) mineralization, nitrification, and amino acid production in situ across a primary successional sequence in interior Alaska, USA. Net N mineralization per gram dry soil increased across the successional sequence, but with a sharp decline in the oldest stage (black spruce). Net N mineralization expressed per gram soil organic matter exhibited the opposite pattern, suggesting that soil organic matter quality decreases significantly across succession. Net N mineralization rates during the growing season from green-up (early May) through freeze-up (late September–early October) accounted for approximately 60% of the annual inorganic N flux, whereas the remaining N was released during the apparent dormant season. Nitrogen release during winter occurred primarily during October–January with only negligible N mineralization during early spring in stands of willow, alder, balsam poplar and white spruce. By contrast, black spruce stands exhibited substantial mineralization after snow melt during early spring. The high rates of N mineralization in late autumn through early winter coincide with high turnover of fine root biomass in these stands, suggesting that labile substrate production, rather than temperature, is a major controlling factor over N release in these ecosystems. We suggest that the convention of restricting measurements of soil processes to the growing season greatly underestimate annual flux rates of inorganic nitrogen in these high-latitude ecosystems.
KeywordsAlaska Biogeochemistry Boreal forests Nitrogen mineralization Nitrogen cycling Subarctic ecosystems
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We wish to thank L. Oliver for expert assistance in the laboratory. We also thank W.␣Davis and Jamie Hollingsworth Bonanza Creek LTER staff, for providing climate data used to interpret this study. The research was funded by USDA Ecosystems NRICGP 99-35101-7859, National Research Initiative Competitive Grants Program and the Bonanza Creek LTER program. This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-0137896. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
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