| Concentrations of atmospheric carbon dioxide (CO2) and tropospheric ozone (O3t) are rising rapidly, due mainly to the burning of fossil fuels and deforestation. These atmospheric changes have the potential to alter the quantity, chemistry and species composition of leaf litter. Changes in litter production characteristics could in turn affect decomposition processes by changing substrate availability to soil microbial communities, thereby influencing nutrient release rates and soil carbon formation. Aspen-FACE (Free-Air Carbon Dioxide Enrichment) experiment in Rhinelander, WI, which was established in 1997 and the first open-air facility to examine the responses of forest trees to interacting CO2 and O3, offers a unique ecosystem platform to study these impacts. In 2003, naturally senesced leaf litter from aspen (Populus tremuloides Michx) and birch (Betula papyrifera Marsh)-aspen communities previously treated with mixtures of CO2 and O3 at the Aspen FACE site was used in a 935-day in situ litter incubation study. The individual and combined effects of elevated CO2 and O3 on litter chemistry, fluxes, decay rates and mean residence times of carbon and mineral nutrients were assessed. Although only small changes in litter chemistry were observed, when combined with gas treatment effects on litter biomass production (positive under elevated CO2 and negative with O3), input levels of N, soluble sugars, condensed tannins, soluble phenolics, cellulose and lignin to forest soils could be substantially altered. Elevated CO2 significantly increased the fluxes to soil of all of all nutrients (N, P, K, S, Mg, Ca, Cu, Mn, and Zn) while O3 had the opposite effect. Gas treatments had little effect on litter nutrient release rates, except for decreasing Ca and B release under elevated CO2 and decreasing N and Ca release under elevated O3. Elevated CO2 significantly reduced litter mass loss (-10 %) in the first year, but increased litter mass loss (+46 %) in the second year. Elevated O3 reduced litter mass loss (-13 %) in the first year, and had no effect on mass loss in the second year. The mean residence time of birch/aspen litter (3.1 years) was significant lower than that of pure aspen (4.8 years).;To examine how changes in litter biochemistry and production under elevated CO2 influence microbial activity and soil C formation, a 230-day microcosm incubation was conducted .Litter and soil were collected from the aspen community under control and elevated CO2 treatments in the Aspen FACE experiment. The base level of litter addition, 1 g pulverized litter mixing with 40 g soil, simulated natural aspen litterfall under control treatment (230 g C˙m-2˙ yr-1) at Aspen FACE experiment. To evaluate the possible litter production variation, five mass addition levels were performed: 0 g (blank soil), 0.5 g, 1.0 g, 1.4 g and 1.8 g litter from control and elevated CO2 treatment into 40 g soil from their original treatment. The results indicate that small decreases in litter [N] under elevated CO2 had minor impacts on microbial C, microbial N and dissolved organic C. Increasing mass addition resulted in higher total C and new C accumulating in whole soil and mineral soil fractions, associated with higher cumulative C loss by respiration and greater breakdown of old C. Higher mass addition led to more total N retained in whole and mineral soil, but also greater C sequestration per unit N. Combining the results of microbial activity and soil C turnover, litter chemistry and production changes under elevated CO2 can be expected to alter soil C and N cycling and increase soil C storage in north temperate deciduous forests. |
