Chemistry and long-term decomposition of roots of Douglas-fir grown under elevated atmospheric carbon dioxide and warming conditions

Elevated atmospheric CO, concentrations and warming may affect the quality of litters of form plants and their subsequent decomposition in ecosystems, thereby potentially affecting the global carbon cycle. However, few data on root tissues are available to test this feedback to the atmosphere. In this study, we used fine (diameter <= 2 mm) and small (2-10 mm) roots of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings that were grown for 4 yr in a 2 x 2 factorial experiment: ambient or elevated (+ 180 ppm) atmospheric CO2 concentrations, and ambient or elevated (+3.8 degrees C) atmospheric temperature. Exposure to elevated CO2 significantly increased water-soluble extractives concentration (%WSE), but had little effect on the concentration of N, cellulose, and lignin of roots. Elevated temperature had no effect on substrate quality except for increasing %WSE and decreasing the %lignin content of fine roots. No significant interaction was found between CO, and temperature treatments on substrate quality, except for %WSE of the fine roots. Short-term (<= 9 mo) root decomposition in the field indicated that the roots from the ambient CO, and ambient temperature treatment had the slowest rate. However, over a longer period of incubation (9-36 mo) the influence of initial substrate quality on root decomposition diminished. Instead, the location of the field incubation sites exhibited significant control on decomposition. Roots at the warmer, low elevation site decomposed significantly faster than the ones at the cooler, high elevation site. This study indicates that short-term decomposition and long-term responses are not similar. It also suggests that increasing atmospheric CO2 had little effect on the carbon storage of Douglas-fir old-growth forests of the Pacific Northwest.