Nitrobacter plots

Significantly more Nitrobacter inhabited the soil in plots of walnut mixed with European alder than in plots of walnut mixed with autumn-olive or walnut alone. Apparently juglone concentrations were not sufficient to inhibit populations of these nitrifying microorganisms. 

To gain more understanding of the European alder decline and because of our concern about the future growth of black walnut planted with nitrogen-fixing species, a study was initiated to measure soil juglone concentration and to estimate the number of Nitrobacter and Nltrosomonas bacteria in a black walnut plantation containing plots of black walnut alone and in mixture with European alder and autumn-olive. 

Soil samples were collected in mid-November at a distance of 0.9 m from each walnut tree at depths of 0-8, 8-16, and 16-30 cm for juglone estimates. Special effort was taken to avoid locating sample plots near dead or declining European alder trees. Another set of samples was collected at 0.9 m from walnut trees in the same plots at depths of 0-4, 4-8, 8-16, and 16-24 cm for Nitrobacter and Nitrosomonas counts. 

Although the number of Nitrosomonas bacteria was greater than the number of Nitrobacter in each treatment plots, only Nitrobacter differed significantly among treatments (Table II). Both Nitrosomonas and Nitrobacter counts decreased significantly with sampled depths. 

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