SULFATE REDUCTION IN LAKE-SEDIMENTS INHABITED BY THE ISOETID MACROPHYTES LITTORELLA-UNIFLORA AND ISOETES-LACUSTRIS

Sulfur cycling was examined in sediments inhabited with the isoetids L ittorella uniflora a and Isoetes lacustris in the oligotrophic soft-wa ter Lake Kalgaard, Denmark. Based on short-term tracer incubations sul fate reduction was measured along a transect from the short (0.6 m) to profundal sediments (4.6 m). The sulfate reduction rates were low (0. 008-0.8 mmol m(-2) d(-1)) in the sandy shallow sediments with low orga nic content (<1.3 mmol C g(-1) sed DW) and high redox potentials (> 10 0 mV), whereas sulfate reduction was higher at the deeper sites (2.7-4 .6 mmol m(-2) d(-1)) with high organic content (max. 11.5 mmol C g(-1) sed DW) and lower redox potentials (< 100 mV). High concentrations of dissolved organic carbon (DOC) were found in the low particulate orga nic sediments (up to 18.4 mM); and most of the DOC pool consisted of a cetate (40-77%). Reoxidation of sulfides due to root oxygen release wa s probably important at all sites and a positive afflux of sulfate acr oss the sediment-water interface was measured, attaining rates (up to 4.8 mmol m(-2) d(-1)) similar to the sulfate reduction rates. Reoxidat ion of sulfides was also manifested by high fraction (> 80%) of reduce d sulfides bring accumulated as elemental sulfur or pyrite (chromium r educible sulfur, CRS). The largest pools of CRS were found in high org anic sediment with vertical distributions resembling those of the sulf ate reduction rates. The overall effect of isoetid growth on sulfur cy cling in the rhizosphere is a suppression of sulfate reduction in low organic sediments and the governing of sulfide reoxidation in sediment s with higher organic content. (C) 1998 Elsevier Science B.V.