Root-Associated N 2 Fixation (Acetylene Reduction) by Enterobacteriaceae and Azospirillum Strains in Cold-Climate Spodosols

Abstract

N 2 fixation by bacteria in associative symbiosis with washed roots of 13 Poaceae and 8 other noncultivated plant species in Finland was demonstrated by the acetylene reduction method. The roots most active in C 2 H 2 reduction were those of Agrostis stolonifera, Calamagrostis lanceolata, Elytrigia repens , and Phalaris arundinacea , which produced 538 to 1,510 nmol of C 2 H 4 ·g −1 (dry weight)· h −1 when incubated at pO 2 0.04 with sucrose (pH 6.5), and 70 to 269 nmol of C 2 H 4 · g −1 (dry weight)·h −1 without an added energy source and unbuffered. Azospirillum lipferum, Enterobacter agglomerans, Klebsiella pneumoniae , and a Pseudomonas sp. were the acetylene-reducing organisms isolated. The results demonstrate the presence of N 2 -fixing organisms in associative symbiosis with plant roots found in a northern climatic region in acidic soils ranging down to pH 4.0.