Elevated CO2 and O3 alter soil nitrogen transformations beneath trembling aspen, paper birch, and sugar maple

Nitrogen cycling in northern temperate forest ecosystems could change under increasing atmospheric CO2 and tropospheric O-3 as a result of quantitative and qualitative changes in plant litter production. At the Aspen Free Air CO2-O-3 Enrichment (FACE) experiment, we previously found that greater substrate inputs to soil under elevated CO2 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 O-3. 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 NH4+ immobilization (+36%). These results indicate greater rates of N turnover under elevated CO2, but do not indicate a negative feedback between elevated CO2 and soil N availability. Elevated O-3 decreased gross N mineralization (-16%) and had no effect on NH4+ immobilization, indicating reduced N availability under elevated O-3. The effects of CO2 and O-3 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 O-3 could lead to a negative feedback on N availability.