Bacterial composition and N 2 ‐fixation of some Egyptian soils cultivated with wheat ( Triticum aestivum L.)

The composition of the microflora, N2-fixing bacteria particularly, in different soils cultivated with wheat in Egypt was investigated in some samples collected from the fields after applying the agricultural practices recommended for wheat cultivation and just before sowing. The influence of carbon sources, mineral nitrogen and water regimes on potential dinitrogen fixation (acetylene reduction assay) in soils was investigated. The bacterial population densities including-N2-fixing organisms were related to a number of environmental factors such as organic matter content. Among diazotrophs, Azotobacter spp. and Azospirillum spp. were encountered in higher densities in comparison with clostridia. Unamended soils showed a lower acetylene-reducing activity (0.5–61.5 nmoles C2H4 g−1 h−1). Addition of glucose (1% w/w) greatly enhanced such activity being the highest (86.9–2846.5 nmoles C2H4 g−1 h−1) in the clay soil with the highest organic carbon content (1.42%). Glucose amendment had no significant influence on acetylene reduction in the saline soil. N2-fixation in barley straw-amended (1%) soils was not much higher than in unamended soils. Concentrations of up to 70 ppm ammonium-nitrogen depressed N2-fixation in soils that received barley straw. Acetylene reduction in submerged soil increased after addition of cellulose. Non-flooded conditions favoured N2-fixation in the fertile clay soil amended with sucrose. Bakterielle Zusammensetzung und N2-Bindung in Weizenboden Agyptens Die Mikroflora agyptischer Weizenboden wurde untersucht. Besonders interessierte die potentielle N2-Bindung (Azetylenreduktion) in Abhangigkeit von verschiedenen C-Quellen, unterschiedlichen Gaben an mineralischem N und dem Wasserregime. Die Dichte der Bakterienpopulationen korreliert mit dem Humusgehalt der Boden (r-0,589x). Azotobacter und Azospirilla traten in hoheren Dichten auf als Clostridia. Nach Strohapplikation (1%) stieg die N2-Bindung kaum; erst Glucose bedingte eine sturmische C2H4-Bildung (bis zu 2846 nmol C2H4 g−1 h−1). Boden ohne zusatzliche C-Quelle, erzeugten nur 0,5–61 nmol C2H4 g−1 h−1. Der salzreiche Boden reagierte jedoch auch auf Glucosezugabe nicht. Die Zugabe von 70 ppm NH4-N reduzierte i.d.R. die N2-Bindung im Vergleich zu den Proben mit 50 ppm NH4-N. Unter anaeroben Bedingungen stieg die N2-Bindung, sofern Cellulose zugegeben wurde. Mit Saccharose versetzte Tonboden wiesen ebenfalls einen Anstieg der N2-Bindung auf.