Microbial nitrogen transformations in the root environment of barley

Abstract

To determine the influence of barley roots on microorganisms and N-transfonning processes in soil, numbers of nitrifiers and potential nitrification and denitrification rates were measured every week for 5 wks. The barley plants were grown in growth chambers in which the root-containing soil layer (A) was separated from three outer soil layers (B, C, D). The numbers and biomass of bacteria, numbers of flagellates and amoebae, total and FDA-active hyphal lengths, microbial biomass carbon and respiration were also determined. The numbers of ammonium oxidizers were positively correlated with root biomass but did not differ significantly between soil layers. Potential ammonium oxidation was stimulated in the root-layer, while potential nitrite oxidation was stimulated in the B- and C-layers. The denitrification activity (measured anaerobically in the presence of excess No − 3) was positively correlated with root biomass in the A-layer. Denitrification activity in the B-layer was positively correlated with the water content of the soil. When roots grew near the nets separating the root layer from the other layers, denitrification activity was stimulated in the next layer (B). We propose that nitrite oxidation in the root zone partly depends on the reduction of nitrate. This would explain why nitrite-oxidizer numbers were usually several orders of magnitude higher than ammonium-oxidizer numbers. Bacterial numbers decreased between wks 1 and 5. Increases in bacteria, naked amoebae and flagellates in all layers between wks 2 and 3 indicated that bacteria were produced until wk 3. There were no signs of bacterial production after wk 3. The total length of hyphae and the length of FDA-active hyphae were not significantly different between layers. However, both of these parameters, as well as total microbial biomass carbon and respiration, were consistently highest in the A-layer.