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 Ge rmany. These areas are known to be extremely low in inorganic carbon. The o bjective of this work was to determine if roots of bulbous rush with iron p laque formation acquire higher concentration of carbon for photosynthesis t han roots without iron plaque. Microscopic examination of the microbial com ponent 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 i nterstitial space with a chemical composition differing widely from lake wa ters and pore-water sediments. The rates of exudation release by roots with out iron plaque was minute but important to an estimation of the turnovers rates of dissolved organic carbon under the plaque. This protective environ ment allows for bacterial rapid recycling of carbon exuded by roots back to the plant. Thus, dissolved inorganic carbon in the root-plaque interstitia l space is higher than in the surrounding water and the pore-water sediment s. The differences in the delta C-13 and biomass production among plants wi th and without iron plaque suggests that iron plaque and presumably the mic roscale phenomena may be important in allowing bulbous rush to maintain a p ositive carbon balance in the low carbon environment of acidic mining lakes . (C) 2001 Elsevier Science B.V. All rights reserved.