C. Limmer et Hl. Drake, EFFECTS OF CARBON, NITROGEN, AND ELECTRON-ACCEPTOR AVAILABILITY ON ANAEROBIC N-2-FIXATION IN A BEECH FOREST SOIL, Soil biology & biochemistry, 30(2), 1998, pp. 153-158
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.