Non-symbiotic N2-fixation in acidic and pH-neutral forest soils: Aerobic and anaerobic differentials

The effects of aerobic and anaerobic conditions on nitrogenase activities in forest soils (in particular that of a beech forest) obtained from northeast Bavaria (Germany) were assessed by both C2H2-reduction and 15N2-fixation methods. Nitrogenase activity occurred without detectable delay under both aerobic and anaerobic conditions; in both cases, 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 relatively constant under aerobic conditions independent of the incubation period. When soils were amended with glucose, significantly higher C2H2-reduction rates (1–4 μmol C2H2 reduced g−1 d.w. soil d−1) occurred only under anaerobic conditions. Assays with 15N2 confirmed this N2-fixation activity; based on initial reaction rates with moistened, but otherwise unamended soils, estimated N2-fixation activity approximated 0.2 kg N-fixed ha−1 y−1. Compared to activities obtained under static conditions, shifts from aerobic to anaerobic conditions greatly enhanced N2-fixation activities. High anaerobic-to-aerobic activity ratios were obtained with both pH neutral and weakly acidic (pH 4.5–5.9) hardwood and coniferous forest soils. These consistently high anaerobic N2-fixation activities indicate that this process is likely limited to O2-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.