Acclimation of photosynthesis, respiration and ecosystem carbon flux of a wetland on Chesapeake Bay, Maryland to elevated atmospheric CO2 concentration.

摘要

Acclimation of photosynthesis and respiration in shoots and ecosystem carbon dioxide fluxes to rising atmospheric carbon dioxide concentration (Ca) was studied in a brackish wetland. Open top chambers were used to create test atmospheres of normal ambient and elevated Ca (= normal ambient + 34 Pa CO2) over mono-specific stands of the C3 sedge Scirpus olneyi, the dominant C3 species in the wetland ecosystem, throughout each growing season since April 1987. Acclimatization of photosynthesis and respiration were evaluated by measurements of gas exchange in excised shoots. The impact of elevated Ca on the accumulation of carbon in the ecosystem was determined by ecosystem gas exchange measurements made using the open top chamber as a cuvette. ElevatedCa increased carbohydrate and reduced Rubisco (ribulose bisphosphate carboxylase) and soluble protein concentrations as well as photosynthetic capacity (A) and dark respiration (Rd; dry weight basis) in excised shoots and canopies (leaf area basis) of Scirpus olneyi. Nevertheless, the rate of photosynthesis was stimulated by 53% in shoots and 30% in canopies growing in elevated Ca compared with normal ambient concentration. Elevated Ca inhibited Rd measured in excised shoots (-19 to -40%) and in seasonally integrated ecosystem respiration (Re; -36 to -57%). Growth of shoots in elevated Ca was stimulated by 14-21%, but this effect was not statistically significant at peak standing biomass in midseason. Although the effect of elevated Ca on growth of shoots was relatively small, the combined effect of increased number of shoots and stimulation of photosynthesis produced a 30% stimulation in seasonally integrated gross primary production (GPP). The stimulation of photosynthesis and inhibition of respiration by elevated Ca increased net ecosystem production (NEP = GPP - Re) by 59% in 1993 and 50% in 1994. While this study consistently showed that elevated Ca produced a significant increase in NEP, a correspondingly large pool of carbon below ground wasnot identified.