Effects of Litter Production, Biochemistry and Plant Community Composition on Carbon and Nutrient Cycling under Elevated Carbon Dioxide and Tropospheric Ozone.

Abstract

Elevated CO2 and O3 have the potential to alter the productivity, biochemistry and species composition of leaf litter, which will affect litter decomposition, thereby controlling nutrient release rates and soil carbon formation. To assess those effects, leaf litter was collected from aspen (Populus tremuloides Michx) and birch (Betula papyrifera Marsh) communities in 2003 at Aspen Free-Air Carbon Dioxide Enrichment experiment in Rhinelander, WI. A 935 day in situ litter decomposition study was conducted. The results suggested that small changes in litter chemistry under elevated CO2 and O3 will occur, and combined with changes in litter biomass production could significantly alter the inputs of soluble sugars, condensed tannins, soluble phenolics, cellulose and lignin to forest soils. Elevated CO2 significantly increased the fluxes to soil of all nutrients (N, P, K, S, Mg, Ca, Cu, Mn, and Zn) and elevated O3 had the opposite effect. Atmospheric changes had little effect on 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 with five mass addition levels. 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.