Colonization of Some Polish Soils by Azotobacter spp. at the Beginning and at the End of the 20^th Century

Abstract

The genus Azotobacter comprises aerobic, free-living diazotrophic (with the ability to use N₂ as the sole nitrogen source) soil bacteria, occurring also in other habitats, e.g., water, rhizosphere. The genus Azotobacter includes six species, with A. chroococcum most commonly inhabiting various soils all over the world. The occurrence of other Azotobacter species seems to be much more restricted in nature, e.g., A. paspali can be found only in the rhizosphere of a grass, Paspalum notatum. In temperate climate, soil populations of Azotobacter spp. rarely exceed several thousands cells per gram of neutral or alkaline soils, and in acid (pH <6.0) soils these bacteria are generally absent or occur in very low numbers. With respect to Polish soils, in 1923 Ziemiecka published results of her studies on the occurrence of Azotobacter spp. in soil samples collected in 1917 and 1918 from 28 locations in the former Polish Kingdom. These studies showed that 50% of the examined soils contained Azotobacter spp. In the year 2000 the occurrence of Azotobacter spp. in 31 Polish soils was assessed and results of this study were compared with those published by Ziemiecka. Almost 52% of the soils tested in 2000 were colonized by Azotobacter spp., indicating that intensification of agricultural practices that took place during the past century did not significantly change (50% in 1917–1918) colonization of Polish soils by the studied bacteria. When present, numbers of these bacteria varied widely from several to almost 10,000 cfu g⁻¹ of soil. The results are discussed in relation to the use of mineral fertilizers, particularly nitrogen, and changes in the soil pH.