Root production and demography in a California annual grassland under elevated atmospheric carbon dioxide
Authors: Higgins, PAT, RB Jackson, JM des Rosiers, and CB Field
This study examined root production and turnover in a California grassland during the third year of a long-term experiment with ambient (LO) and twice-ambient atmospheric CO2 (HI), using harvests, ingrowth cores, and minirhizotrons. Based on harvest data, root biomass was 32% greater in the HI treatment, comparable to the stimulation of aboveground production during the study year. However, the increase in photosynthesis that typically occurs under elevated CO2 is considerably larger than the combined increase in above and belowground biomass. One possible explanation is increased root turnover, which could be a sink for the additional fixed carbon. Cumulative root production in ingrowth cores from both treatments harvested at 4 dates was 2-3 times that in the single harvested cores, suggesting substantial root turnover within the growing season. Minirhizotron data confirmed this result, demonstrating that production and mortality occurred simultaneously through much of the season. As a result, cumulative root production was 54%, 47% and 44% greater than peak standing root length for the no chamber (X), LO, and HI plots respectively. Elevated CO2, however, had little effect on rates of turnover (i.e., rates of turnover were equally high in the LO and HI plots throughout most of the year). Elevated CO2 increased monthly production of new root length (59%) only at the end of the season (May-October) when root growth had largely ceased in the LO plots but continued in the HI plots. This increase in production coincided with end-of season differences in soil moisture between the LO and HI plots. Total standing root length was not affected by CO2 treatment. Root mortality was unaffected by elevated CO2 in all months except April, in which plants grown in the HI plots had higher mortality rates. Together, these results demonstrate that root turnover is considerable in the grassland community and easily missed by destructive soil coring. However, increased fine root turnover under elevated CO2 is apparently not a major sink for extra photosynthate in this system.