Nitrogenase (C2H2) Activity in Roots of Non-Cultivated and Cereal Plants: Influence of Nitrogen Fertilizer on Populations and Activity of Nitrogen-Fixing Bacteria

Root samples of 11 non-cultivated monocotyledonous and 7 dicotyledonous species taken during a wet summer had low mean nitrogenase activities of 10.2 and 7.1 nmol C₂H₄·g^?1 DW·h^?1 after preincubation at pO₂ 0.02, respectively. Maxima of 139–169 nmol·g^?1·h^?1 were observed with Agrostis vulgaris and Agropyron repens on a sandy soil poor in C_org. Three of 6 early, but none of 4 late fodder maize cultivars had a very low activity up to 0.5 nmol·g^?1h^?1. Oat, rye and wheat roots from plots with organic or mineral N fertilizers had activities between 1.3 and 7.3 nmol·g^?1h^?1 at flowering, which were not correlated with their Azospirillum populations (10²-10⁷·g^?1 after preincubation). Winter wheat and barley roots given 0, 40, 80 and 120 kg. ha^?1 NH₄NO₃-N in 0–3 applications had mean activities of 0.08, 4.06, 0.09 and 0.08 nmol or 1.77, 2.67, 0.36 and 0.23 nmol C₂H₄g^?1·h^?1 after flowering, respectively. An appreciable part of this activity could be removed by root washing. In preincubated rhizosphere soil of wheat and barley populations of N₂-fixing, facultative anaerobic Klebsiella and Enterobacter spp. were 10–100 times higher than those of Azospirillum sp., both being higher in O N than in 80 kg N·ha^?1 trials.