Influence of nitrogen on the number of N 2-fixing and total bacteria in the rhizosphere

Abstract

Various grass and herbaceous plant species (e.g. Arrhenatherum elatius L., Phleum pratense L., Planlago media L., Taraxacum officinale WEB.) were cultivated in a freshly collected soil from an unfertilized grassland area with its natural population of N 2-fixing bacteria. Diazotrophs and total bacteria at the root surface (at outer plus “inner” surface as recovered in root homogenate) and bacteria in the rhizosphere soil (0–3 mm from root surface) were counted by the most probable number (MPN) method. Addition of mineral N (NH 4NO 3) to the soil decreased its nitrogenase activity (assayed by acetylene reduction) and also the proportion of diazotrophs among the heterotrophs. The result was similar when N was supplied via the leaves, probably owing to an increased release of N-containing compounds with lower C:N ratios from the root. The decrease in the proportion of diazotrophs suggests a competitive suppression by non-diazotrophs in the presence of combined N. Correspondingly, the increase in the MPN of N 2-fixing bacteria found during plant development is interpreted as the result of a decrease in N in the soil, particularly in the rhizosphere, by uptake and in addition, a decrease in the N-concentration of the plant with a concomitant change in the C:N ratio of compounds released from the root. The proportion of diazotrophs among total bacteria varied greatly but was lower for bacteria living on or within the roots than for bacteria in the rhizosphere soil. Most of the N 2-fixing bacteria were members of the genera Klebsiella and Enterobacter. Species of Azospirillum represented only a small proportion of the total counts.