Effects of plant-bacterial-amoebal interactions on plant uptake of nitrogen under field conditions

Summary

Bacterial biomass and numbers of bacterivorous naked amoebae were estimated daily in soil associated with barley roots and, to avoid the influence of roots, in soil from a field in its fifth summer under bare fallow. The estimates were associated with two rainfall events and were started just before the first. Increases in bacteria were observed after each rainfall, and bacterial production was about the same size for both treatments. A peak in naked amoebae followed each burst of bacterial production in the root-associated soil, whereas in the fallowed soil protozoan production was low after the first rainfall and undetectable after the second. The bacterial populations in the fallowed soil had yet to decline by the end of the 12-day study, probably because grazing pressure by protozoa was low. Calculations based (1) on short-term decreases in bacterial biomass in soil close to roots or (2) on the amount of C added to the soil by plants over the growing season indicated that N released via bacterial-protozoan interactions contributed 10%–17% of the N taken up by the fertilized barley.