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Deals with a study which investigated the relative roles of iron and silicic acid availabilities in regulating silicic uptake rates across the Polar Frontal Zone in the Pacific Sector of the Southern Ocean during the United States Joint Global Ocean Flux Study Southern Ocean program. Methods; Results; Discussion.

We investigated the effect of CO{sub}2{end} concentration and soil nutrient availability during growth on the subsequent decomposition and nitrogen (N) release from litter of four annual grasses that differin resource requirements and native habitat. Vulpia microstachys is a native grass found on California serpentine soils, whereas Avena fatua, Bromus hordaceus, and Lolium multiflorum are introduced grasses restricted to more fertile sandstone soils (Hobbs & Mooney 1991). Growth in elevated CO{sub}2{end} altered litter C:N ratio, decomposition, and N release, but the direction and magnitude of the changes differed among plant species and nutrient treatments. Elevated CO{sub}2{end} had relatively modest effects on C:N ratio of litter, increasing this ratio in Lolium roots (and shoots at high nutrients), but decreasing C:N ratio in Avena shoots. Growth of plants under elevated CO{sub}2{end} decreased the decomposition rate of Vulpia litter, but increased decomposition of Avena litter from the high-nutrient treatment. The impact of elevated CO{sub}2{end} on N loss from litter also differed among species, with Vulpia litter from high-CO{sub}2{end} plants releasing N more slowly than ambient-CO{sub}2{end} litter, whereas growth under elevated CO{sub}2{end} caused increased N loss from Avena litter. CO{sub}2{end} effects on N release in Lolium and Bromus depended on the nutrient regime in which plants were grown. There was no overall relationship between litter C:N ratio and decomposition rate orN release across species and treatments. Based on our study and the literature, we conclude that the effects of elevated CO{sub}2{end} ondecomposition and N release from litter are highly species-specific.These results do not support the hypothesis that CO{sub}2{end} effects on litter quality consistently lead to decreased nutrient availability in nutrient-limited ecosystems exposed to elevated CO{sub}2{end}. [ABSTRACT FROM AUTHOR]