Interactions of bacteria, protozoa and plants leading to mineralization of soil nitrogen

The capacity of bacteria and protozoa to mineralize soil nitrogen was studied in microcosms with sterilized soil with or without wheat plants. The effect of small additions of glucose or ammonium nitrate or both, twice a week was also tested. Plant dry weight and N-content, number of microorganisms and biomass plus inorganic N were determined after 6 weeks. The introduction of plants profoundly influenced the N transformations. In the presence of root-derived carbon, much more N was mineralized from the organic matter and immobilized mainly in plant biomass. “Total observable change in biomass N plus inorganic N” was negative in the unvegetated soils without additions, while a mineralization of 1.7 mg N microcosm−1 was observed in microcosms with wheat plants grown with bacteria only. When protozoa were included, the N taken up by plants increased by 75%. Sugar additions resulted in an 18% increase of total N in the shoots when protozoa were present, but had no significant effect in the absence of grazers. Plants with the same root weight were more efficient in their uptake of inorganic N when protozoa were present. Plants grown with protozoa also had a lower R/S ratio, indicating a less stressed N availability situation. The lowest ratio was found with N additions in the presence of protozoa. The results indicate that, with energy supplied by plant roots or with external glucose additions, soil bacteria can mineralize N from the soil organic matter to support their own growth. Grazing of the bacteria is necessary to make bacterial biomass N available for plant uptake.