C. Limmer et Hl. Drake, NON-SYMBIOTIC N-2-FIXATION IN ACIDIC AND PH-NEUTRAL FOREST SOILS - AEROBIC AND ANAEROBIC DIFFERENTIALS, Soil biology & biochemistry, 28(2), 1996, pp. 177-183
The effects of aerobic and anaerobic conditions on nitrogenase activit
ies in forest soils (in particular that of a beech forest) obtained fr
om northeast Bavaria (Germany) were assessed by both C2H2-reduction an
d N-15(2)-fixation methods. Nitrogenase activity occurred without dete
ctable delay under both aerobic and anaerobic conditions; in both case
s, initial rates approximated 0.2 nmol C2H2 reduced g(-1) d.w. soil d(
-1). Prolonged anaerobic conditions yielded rates approximating 6 nmol
C2H2 reduced g(-1) d.w. soil d(-1); in contrast, rates remained relat
ively constant under aerobic conditions independent of the incubation
period. When soils were amended with glucose, significantly higher C2H
2-reduction rates (1-4 mu mol C2H2 reduced g(-1) d.w. soil d(-1)) occu
rred only under anaerobic conditions. Assays with N-15(2) confirmed th
is N-2-fixation activity; based on initial reaction rates with moisten
ed, but otherwise unamended soils, estimated N-2-fixation activity app
roximated 0.2 kg N-fixed ha(-1) y(-1). Compared to activities obtained
under static conditions, shifts from aerobic to anaerobic conditions
greatly enhanced N-2-fixation activities. High anaerobic-to-aerobic ac
tivity ratios were obtained with both pH neutral and weakly acidic (pH
4.5-5.9) hardwood and coniferous forest soils. These consistently hig
h anaerobic N-2-fixation activities indicate that this process is like
ly limited to O-2-deficient microsites in forest soils. No nitrogenase
activity was detected with extremely acidic coniferous forest soils (
pH 2.7-3.3) under either anaerobic or aerobic conditions.