Forest Litter Production, Chemistry, and Decomposition following Two Years of Free-air Co2 Enrichment

Increases in tree biomass may be an important sink for CO2 as the atmospheric concentration continues to increase. Tree growth in temperate forests is often limited by the availability of soil nutrients. To assess whether soil nutrient limitation will constrain forest productivity under high atmospheric CO2, we studied the changes in forest litter production and nutrient cycling in a maturing southern U.S. loblolly pine–hardwood forest during two years of free-air CO2 enrichment. The objective of this paper is to present data on the chemistry of green leaves and leaf litter, nutrient-retranslocation efficiency, aboveground litter production, whole-system nutrient-use efficiency, decomposition, and N availability in response to forest growth under elevated CO2. The chemical composition of green leaves and leaf litter was largely unaffected by elevated CO2. Green-leaf nitrogen (N) and phosphorus (P) concentrations were not significantly lower under elevated CO2. N and P retranslocation from green leaves did not increase under elevated CO2; therefore, leaf litter N and P concentrations were not significantly lower under elevated CO2. The concentrations of carbon, lignin, and total nonstructural carbohydrates in litter were not significantly different under elevated CO2. Total aboveground litterfall increased significantly with CO2 fumigation. The increase in litterfall was due to significant increases in loblolly pine leaf litter and bark production. The mass of leaves from deciduous species did not increase with CO2 fumigation. Wholesystem nutrient-use efficiency (aboveground litterfall/nutrient content of litterfall) did not increase as a consequence of forest growth under elevated CO2, but N and P fluxes from vegetation to the forest floor increased significantly. During the second year of CO2 fumigation, the flux of N and P to the forest floor in litterfall increased by 20% and 34%, respectively. The rate of mass loss during one year of decomposition was unaffected by ‘‘litter type’’ (whether the litter was produced under ambient or elevated CO2), nor by the ‘‘site’’ of decomposition (whether the litter was decomposed in the ambient or elevated CO2 plots). N was immobilized in litter during decomposition, whereas P was mineralized. There was no consistent effect of litter type or site on nutrient dynamics in decomposing litter. There was no significant effect of elevated CO2 on the pool size of inorganic N (NH4 and NO3) in the top 7.5 cm of mineral soil. The rate of net N mineralization and nitrification in mineral soil was not significantly different between treatment and control plots. Identifying the source of the nutrients lost in litterfall is critical to the long-term potential growth stimulation of forests under elevated CO2. If the nutrients lost from biomass come from storage (e.g., the movement of nutrients from wood to leaves), then the increase in litter production should decrease over time as slowly replenished nutrient reserves are exhausted. If the nutrients lost in plant litter are replaced by uptake from soils, then it is possible (1) that trees acquire soil nutrients at a rate commensurate with growth stimulated by elevated CO2; and (2) that forest productivity will be stimulated by elevated CO2 in the long term.