Effects of elevated atmospheric carbon dioxide on root growth, turnover and decomposition in a scrub oak ecosystem.

摘要

Atmospheric carbon dioxide levels are increasing and predicted to double this century. The implications of this rise on vegetation structure and function are not well understood. Measurement of root growth response to elevated atmospheric carbon dioxide is critical to understanding soil carbon input. I investigated the effects of elevated carbon dioxide on fine root growth and decomposition using open top chambers with both ambient and elevated (700 PPM) CO2 treatments in an oak-palmetto scrub ecosystem at Kennedy Space Center, Florida. Minirhizotron tubes were installed in each elevated and control chamber to allow observation of roots. Each tube was sampled for root length density (mm cm-2) every three months. Carbon dioxide enrichment of the chambers began May 15, 1996. By December 1998 root length density (RLD) increased to 19.1 mm cm-2 for the control chambers and 37.7 mm cm-2 for the enriched chambers in the top 101-cm of soil. Root distribution was unchanged under elevated carbon dioxide. Fine root production increased with elevated carbon dioxide and mortality was unaffected over 33 months. Root length elongation increased significantly over a one-month period in June 1997. I also measured the effects of elevated carbon dioxide on the decomposition rates of roots grown in ambient and elevated carbon dioxide. Fine root decomposition rates were obtained from root litterbags incubated from December 1996 to December 1998 and showed no significant treatment effect. Initial percent mass loss varied from 10.3% to 13.5% after three months; 55.5% to 38.3% of original mass had been lost after 828 days. A period of nitrogen immobilization occurred in both fine roots and rhizomes in the elevated CO2 treatment, which is potentially a mechanism for nitrogen conservation for this system in an elevated CO 2 world. Significant fine root length-mass relationships were applied to minirhizotron measurements and a 180% increases in root biomass was calculated at the end of the study. The increased rates of fine root growth coupled with no change in decomposition rate suggest a potential increased rate of carbon input into the soil.