Effects of elevated atmospheric CO2 and tropospheric O3 on nutrient dynamics: decomposition of leaf litter in trembling aspen and paper birch communities

Atmospheric changes could strongly influence how terrestrial ecosystems function by altering nutrient cycling. We examined how the dynamics of nutrient release from leaf litter responded to two important atmospheric changes: rising atmospheric CO₂ and tropospheric O₃. We evaluated the independent and combined effects of these gases on foliar litter nutrient dynamics in aspen (Populus tremuloides Michx) and birch (Betula papyrifera Marsh)/aspen communities at the Aspen FACE Project in Rhinelander, WI. Naturally senesced leaf litter was incubated in litter bags in the field for 735 days. Decomposing litter was sampled six times during incubation and was analyzed for carbon, and both macro (N, P, K, S, Ca, and Mg) and micro (Mn, B, Zn and Cu) nutrient concentrations. Elevated CO₂ significantly decreased the initial litter concentrations of N (−10.7%) and B (−14.4%), and increased the concentrations of K (+23.7%) and P (+19.7%), with no change in the other elements. Elevated O₃ significantly decreased the initial litter concentrations of P (−11.2%), S (−8.1%), Ca (−12.1%), and Zn (−19.5%), with no change in the other elements. Pairing concentration data with litterfall data, we estimated that elevated CO₂ significantly increased the fluxes to soil of all nutrients: N (+12.5%), P (+61.0%), K (+67.1%), S (+28.0%), and Mg (+40.7%), Ca (+44.0%), Cu (+38.9%), Mn (+62.8%), and Zn (+33.1%). Elevated O₃ had the opposite effect: N (−22.4%), P (−25.4%), K (−27.2%), S (−23.6%), Ca (−27.6%), Mg (−21.7%), B (−16.2%), Cu (−20.8%), and Zn (−31.6%). The relative release rates of the nine elements during the incubation was: K ≥ P ≥ mass ≥ Mg ≥ B ≥ Ca ≥ S ≥ N ≥ Mn ≥ Cu ≥ Zn. 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. We conclude that elevated CO₂ and elevated O₃ will alter nutrient cycling more through effects on litter production, rather than litter nutrient concentrations or release rates.