Hot Spots of Inorganic Nitrogen Availability in an Alpine-Subalpine Ecosystem, Colorado Front Range
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Inorganic nitrogen (N) availability hot spots have been documented in many ecosystems, but major uncertainties remain about their prevalence, timing, and causes. Using a novel mathematical definition of hot spots, spatially explicit measurements of KCl-extractable inorganic N, 2-week soil incubations in the field, ion-exchange resins deployed for 1 year, and a set of associated biotic and abiotic variables, we investigated inorganic N availability hot spots within a 0.89 km2 alpine-subalpine ecosystem in the Colorado Front Range. Measurements of KCl-extractable NH4 + and NO3 − taken on multiple dates showed that hot spots of N availability were present in some but not all parts of the study site and that hot spot location varied over the course of the season. Ion-exchange resins showed that over a 1-year period hot spots were important contributors to resin-available N at the landscape level, with 14% of resin locations accounting for 58% of total resin-extractable inorganic N. The KCl-extractable and resin-available inorganic N measurements showed that although spatial variation in the timing of hot spots (that is, hot moments) spreads the influence of short-term hot spots across the landscape to some extent, spatial variation in inorganic N availability is still important when integrated over 1 year. Resin-available N was poorly correlated with the biotic and abiotic variables that we measured, though we did observe that hot spots of resin-available N were twice as common below tree and shrub canopies than in herbaceous areas. Beyond this relationship with canopy structure, neither KCl-extractable nor resin-available inorganic N hot spots were closely related to plant species identity. Instead, the most effective predictor of KCl-extractable NH4 + was the size of the soil organic matter (SOM) N pool, with nearly all hot spots appearing in soils that had greater than 1.4% SOM N.
Key wordsnutrient availability Lorenz curve spatially explicit inequality Niwot Ridge LTER random forest model disproportion
For funding, we thank NSF DGE 0202758, NSF DEB 0423662, NSF DEB 0808275, the John W. Marr Ecology Fund, and the Department of Ecology and Evolutionary Biology. For helpful suggestions on this manuscript, we thank Alan Townsend, Carol Wessman, Tim Seastedt, and Mark Williams, Michael Weintraub, and Zachary Rinkes. For their help in the field, lab, and/or planning stages of this project, we thank Courtney Meier, John Murgel, Carly Baroch, Brendan Whyte, Anna Lieb, Jaclyn Darrouzet-Nardi, Jeanette Darrouzet-Nardi, Chris Darrouzet-Nardi, Kathy Buehmann, Lisa Gerstenberger, David Knochel, Russ Monson, Stuart Grandy, Courtney Meier, Andy Thomspon, Todd Ackerman, and the numerous volunteers on the July 9th and 30th 2008 field days as well as during the snow sampling effort. We thank Chris Seibold and the Kiowa Lab assistants for help with nutrient analyses. Logistical support was provided the University of Colorado’s Mountain Research Station. Finally, we thank the reviewers, whose careful consideration and suggestions greatly improved this manuscript.
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