NON-SYMBIOTIC N-2-FIXATION IN ACIDIC AND PH-NEUTRAL FOREST SOILS - AEROBIC AND ANAEROBIC DIFFERENTIALS

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.