Nitrogen interactions and photosynthetic responses to CO2: Work plan for biocon experiment/physiological measurements at cedar creek

摘要

Nitrogen plays a critical role in photosynthetic function, which in turn can affect many ecosystem processes through its effects on plant growth and ecosystem carbon cycles. As a result of its central role in photosynthetic enzymes, leaf N status can affect the magnitude of photosynthetic enhancement by elevated CO(sub 2). It is now well-recognized that species responses to elevated CO(sub 2) may be different when species are grown in isolation vs. in a mixed community. Part of this effect may result from effects on leaf N itself as a result of species competition for N in N-limited ecosystems, and pad of the effect is simply a function of dilution of leaf nutrients in leaves with greater carbohydrates as a result of elevated CO(sub 2), However, photosynthetic efficiency itself may be affected if N-limited plants reallocate N within leaves away from carboxylation functions under elevated CO(sub 2) compared to ambient plants (Drake et al. 1997). Thus while there is cause to believe that there are interactive effects of N and CO(sub 2) on species photosynthetic physiology, there is little experimental data to support or reject this idea, particularly in realistic ecosystems under field conditions. Biotic interactions, most notably the presence of N-fixing plants, can affect ecosystem N stocks and carbon cycling via effects of N on photosynthetic function (Chapin et al. 1997, Hooper and Vitousek 1997). If photosynthetic responses of leaves and ecosystems are largely mediated through canopy N, then biotic as well as inorganic N sources will affect the magnitude of these responses. Under elevated CO(sub 2) there is evidence from the Swiss FACE experiment that growth and photosynthetic function are most responsive to CO(sub 2) in species mixtures including N-fixers (Hebeisin et al. 1997, Liischer et al. 1998, S.P. Long, pers. comm.). However, in that experiment there were confounding management factors and species diversity effects per se could not be tested. Still, photosynthetic studies showed that CO(sub 2)-induced photosynthetic adjustments in the Swiss FACE experiment were greater under low N and in monocultures than in the mixture of grasses and the N-fixing species clover (S.P. Long, pers. comm.). Effects of species diversity and N-fixers in specific on plant CO(sub 2) responses in interaction with N have important implications for predicting ecosystem responses to elevated CO(sub 2) under a variety of site conditions, and may also temper management for mitigation of ecosystem CO(sub 2) responses.