Hyporheic community respiration was measured in a riffle-pool-riffle sequence located up- and downstream from a wastewater treatment plant of the River Lahn, Germany. Sediment filled tubes were incubated for several months in the hyporheic zone and then transferred to the laboratory where the oxygen consumption by the hyporheic community colonizing the tubes was measured in a recirculating system. Oxygen consumption in 64 tubes from four sites averaged 0.69 mg O-2 dm(-3) h(-1) with surface water from upstream of the wastewater treatment plant. After the successive amendment of nitrite and ammonium, community respiration increased to 1.14 and 1.36 mg O-2 dm(-3) h(-1), respectively. Nitrate production during the experiments increased from 0.024 mg NO3-N dm(-3) h(-1) to 0.219 and 0.220 mg NO3-N dm(-3) h(-1) with the amendment of nitrite and ammonium, respectively, indicating nitrogen limitation in the hyporheic zone. Respiration and nitrate production correlated strongly with the amount of organic matter accumulated in the tubes during the several months of sediment incubation. Significantly higher respiration and nitrate production after amendment of ammonium were observed for the two sites located downstream of the wastewater treatment plant effluent in comparison to those upstream of the WWTP. This increase indicates a possible impact of wastewater supply to rivers, especially on the hyporheic zone where an important part of organic matter break-down and nutrient recycling occurs in mesotrophic rivers.

N fixation by different faba bean (Vicia faba) cultivars was studied using the natural abundance method. The delta 15N (δ15N) values of the faba beans and the reference plants differed by 4.6–7.0‰. The non-nodulating V. faba cv. F48 seems to be the best reference plant for nodulated and N2-fixing V. faba. Significant differences occurred in the quantity of N2 fixation of six V. faba cultivars. The average fraction of N derived from air (FNdfa) estimated from leaf material ranged between 69 and 80%. Shoot-based estimates of N fixation varied between 200 and 360 kg N ha–1. N fixation was affected more by differences in FNdfa than by differences in total N accumulation. Fixation data calculated with the non-nodulated reference plant V. faba cv. F48 were lower than those calculated with cabbage (Brassica oleracea) and ryegrass (Lolium perenne) as reference plants. Of all reference plants, non-N2-fixing V. faba cv. F48 has a root system and temporal pattern of N assimilation that is the one most similar to that of N2-fixing V. faba plants. Cv. F48 showed senescence as did the other V. faba cultivars after pod-fill was complete, whereas cabbage, ryegrass and camomile had a later senescence period. N fixation during pod-filling appears more important for a good yield than N2 fixation abilities in the earlier growth period. The best V. faba cultivars left about 100 kg N ha–1 in residual material on the field as fertilization for the following crops.