Elevated CO < inf> 2 and O < inf> 3 alter soil nitrogen transformations beneath trembling aspen, paper birch, and sugar maple

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Nitrogen cycling in northern temperate forest ecosystems could change under increasing atmospheric CO2 and tropospheric O3 as a result of quantitative and qualitative changes in plant litter production. At the Aspen Free Air CO2-O3 Enrichment (FACE) experiment, we previously found that greater substrate inputs to soil under elevated CO 2 did not alter gross N transformation rates in the first 3 years of the experiment. We hypothesized that greater litter production under elevated CO2 would eventually cause greater gross N transformation rates and that CO2 effects would be nullified by elevated O3. Following our original study, we continued measurement of gross N transformation rates for an additional four years. From 1999 to 2003, gross N mineralization doubled, N immobilization increased 4-fold, but changes in microbial biomass N and soil total N were not detected. We observed year-to-year variation in N transformation rates, which peaked during a period of foliar insect damage. Elevated CO2 caused equivalent increases in gross rates of N mineralization (+34%) and NH 4+ immobilization (+36%). These results indicate greater rates of N turnover under elevated CO 2, but do not indicate a negative feedback between elevated CO 2 and soil N availability. Elevated O3 decreased gross N mineralization (-16%) and had no effect on NH 4+ immobilization, indicating reduced N availability under elevated O3. The effects of CO2 and O3 on N mineralization rates were mainly related to changes in litter production, whereas effects on N immobilization were likely influenced by changes in litter chemistry and production. Our findings also indicate that concomitant increases in atmospheric CO2 and O3 could lead to a negative feedback on N availability. © 2007 Springer Science+Business Media, Inc.

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