The role of organic carbon excretion by bulbous rush roots and its turnover and utilization by bacteria under iron plaques in extremely acid sediments

Bulbous rush (Juncus bulbosus) is a pioneer species in highly acidic mining lakes (pH 2.5-3) of the Lusatian mining district in the eastern part of Germany. These areas are known to be extremely low in inorganic carbon. The objective of this work was to determine if roots of bulbous rush with iron plaque formation acquire higher concentration of carbon for photosynthesis than roots without iron plaque. Microscopic examination of the microbial component in roots with iron plaque, levels of organic exudates, stable carbon isotopes and biomass production were measured to test this hypothesis. The roots contain iron plaques surrounding a rhizobacterial community and an interstitial space with a chemical composition differing widely from lake waters and pore-water sediments. The rates of exudation release by roots without iron plaque was minute but important to an estimation of the turnovers rates of dissolved organic carbon under the plaque. This protective environment allows for bacterial rapid recycling of carbon exuded by roots back to the plant. Thus, dissolved inorganic carbon in the root-plaque interstitial space is higher than in the surrounding water and the pore-water sediments. The differences in the delta C-13 and biomass production among plants with and without iron plaque suggests that iron plaque and presumably the microscale phenomena may be important in allowing bulbous rush to maintain a positive carbon balance in the low carbon environment of acidic mining lakes. (C) 2001 Elsevier Science B.V. All rights reserved.