EFFECTS OF CARBON, NITROGEN, AND ELECTRON-ACCEPTOR AVAILABILITY ON ANAEROBIC N-2-FIXATION IN A BEECH FOREST SOIL

The effects of different energy sources and inhibitors on the nitrogen ase activity of a beech forest soil of north-east Bavaria (Germany) we re determined using the acetylene-reduction method. Simple sugars, cel lobiose and starch greatly stimulated nitrogenase activity under anaer obic,but not under aerobic, conditions; of the substrates tested, cell obiose yielded the highest anaerobic activity. ln contrast, organic ac ids had no appreciable stimulating effect. Glucose-stimulated anaerobi c nitrogenase activity was linked to the formation of butyrate (implic ating the involvement of clostridia) and to the apparent growth of ana erobic N-2-fixing microorganisms. The N-2-fixing microorganisms cultur able under anaerobic conditions in unamended soil approximated 10(5) c ells g(-1) dry wt soil and increased three orders of magnitude after 2 d of anaerobic incubation with glucose. In contrast, no N-2-fixing mi croorganisms were culturable under aerobic conditions. Inhibition of a naerobic nitrogenase activity by supplemental ammonium and nitrate occ urred in both unamended and glucose-amended soils. Nitrate was sequent ially transformed to N2O and N-2, with minor amounts also being reduce d to ammonium. Thus, nitrate appeared to have an indirect inhibitory e ffect on N-2-fixation, the reduction of nitrate being more competitive for available reductant than N-2-fixation. This competitive effect wa s not observed with sulfate. These results suggest that in situ N-2-fi xation in the forest soil examined may be regulated in part by the ava ilability of specific reductant sources and the presence of the compet ing reductant sink nitrate. (C) 1997 Published by Elsevier Science Ltd . All rights reserved.