Long-Term CO2 Stimulation of Carbon Influx Into Global Terrestrial Ecosystems: Issues and Approaches

Estimating the additional amount of global photosynthetic arbon influx into terrestrial ecosystems (PG) becomes possible with a leaf-level factor (Y) developed by Luo & Mooney only when an increase in atmospheric CO2 concentration (Ca) is small. Applying the Y factor to study long-term stimulation of PG with a large increase in Ca needs understanding of adjustments in leaf properties, canopy structure and ecosystem nitrogen availability, which could, potentially, feedback to photosynthetic carbon influx. Leaf photosynthetic properties vary greatly with elevated CO2 among species. Aggregation over a group of species, however, shows a small change, suggesting that globally averaged changes in leaf properties may be trivial. Canopy adjustment in elevated CO2 is largely unknown whereas indirect measurements suggest faster development of foliar canopy in elevated than ambient CO2. Biogeochemical feedback of nitrogen on global carbon influx is involved with two general issues: CO2 effects on ecosystem nitrogen availability and interactive ffects of nitrogen and CO2 on photosynthesis. Although nitrogen itself strongly influences photosynthesis, regulation of CO2 effects on photosynthesis by nitrogen is still inconclusive. Ecosystem nitrogen availability is determined by a balance of several nitrogen fluxes, including plant uptake, mineralization, deposition, fixation, denitrification, volatilization and leaching. Elevated CO2 stimulates more plant biomass growth, demanding more nitrogen uptake. Mineralization increased in two studies, decreased in one and was unchanged in one. CO2 stimulation of nitrogen fixation increases nitrogen availability in ecosystems, potentially to match increased photosynthetic potential in the long term. Effects of volatilization, denitrification and leaching are yet to be assessed. Overall, intact ecosystem studies of canopy structure and nitrogen dynamics in elevated CO2 are particularly needed for our quantifying long-term stimulation of global photosynthetic carbon influx.