Soil nitrogen availability in some arctic ecosystems in northwest Alaska: Responses to temperature and moisture

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Abstract
We explored patterns in soil nitrogen (N) availability in five ecosystems in the Noatak National Preserve in northwestern Alaska, and the potential response of N availability to changes in soil moisture and temperature. Nitrogen availability appeared to be highest in an ecosystem dominated by Alnus crispa (Ait.) Pursh, and lowest in ecosystems dominated by willows or by cotton grass tussocks. All soils responded strongly to changes in moisture; repeated watering during 30-day incubations raised net N mineralization rates by 1.5 to 3.0 fold. Net N mineralization at 12°C was 5 to 10 fold greater than at 5°C. The responses of net mineralization rates derived from strong effects of treatments on both microbial mineralization (release) of N, and microbial immobilization (uptake) of N. For example, incubation of soils from the alder site at 12°C slightly increased gross mineralization of N, more than doubled microbial immobilization of N, reduced gross nitrate production by more than 80%, and more than tripled nitrate immobilization. The impacts of changing climate on the N supply in these ecosystems will depend upon the effects on the opposing processes of microbial mineralization and immobilization of soil N, and on-site soil warming experiments will be needed to provide realistic assessments of the long-term trends of the supply of N required for plant growth in these ecosystems.