Assessing Two Diagnostic Methods for Enumeration of Nitrate Reducing and Denitrifying Bacteria in Soil-plant Root Associations
1977
Volz, M. G.
Most probable number (MPN) determinations were made of NO₃⁻ reducing- and denitrifying bacteria in soil samples taken at time of crop harvest from fallow sites and from root zones of fertilized field plots of tomato (Lycopersicon esculentum L.), soybean (Glycine max L. Merr.), or corn (Zea mays L.) plants cultivated in pure stand or in association with nutsedge (Cyperus esculentus L.), a competitive weed. Diagnostic methods utilized were inoculated Difco NO₃⁻ broth and Giltay's medium (citrate-nitrate-asparagine + mineral salts), incubated with and without the exclusion of atmospheric O₂, respectively. The NO₃⁻ broth method usually yielded both numerically larger and significantly different (5% level) MPNs for NO₃⁻ reducers (5 of 7 cases larger, 3 significant) and denitrifiers (6 of 7 cases larger, 3 significant) from a given soil inoculum when compared with identical enumerations performed with Giltay's medium. When selected soil inocula were incubated anaerobically in Giltay's medium, MPNs of neither NO₃⁻ reducers nor denitrifiers were increased significantly as compared with the aerobic procedure. Therefore, larger MPNs of soil NO₃⁻ reducers and denitrifiers observed when NO₃⁻ broth is utilized are probably more due to improved nutritional environs than to ambient oxygen status. As a result, a more complete estimate of these soil bacterial populations, especially those from the vicinity of plant roots is thought to be obtained using the NO₃⁻ broth procedure.
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