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

Abstract

Abstract 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 C 2 H 4 ·g −1 DW·h −1 after preincubation at pO 2 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 −1 h −1 . Oat, rye and wheat roots from plots with organic or mineral N fertilizers had activities between 1.3 and 7.3 nmol·g −1 h −1 at flowering, which were not correlated with their Azospirillum populations (10 2 ‐10 7 ·g −1 after preincubation). Winter wheat and barley roots given 0, 40, 80 and 120 kg. ha −1 NH 4 NO 3 ‐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 C 2 H 4 g −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 N 2 ‐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.