Effect of nitrogen rate and root-zone mix on rhizosphere bacterial populations and root mass in creeping bentgrass putting greens

Bacterial populations in constructed and fumigated golf course putting greens is a topic that has not been widely explored. The objective of this project was to evaluate the root mass and rhizosphere bacterial populations of creeping bentgrass (Agrostris palustris Huds. Crenshaw) greens over time as affected by root-zone mix and nitrogen rate. Individual miniature putting greens (1x0.5 m) were built at ground level. Two root-zone mixes (80%/20% sand-peat and 100% sand) and two N rates (1.0 and 0.5 g N m^–2 week^–1 all year) were evaluated, with four replications of each root-zone x N rate treatment. Quarterly (February, May, August, November) root samples (10 cm deep) were removed from each green for 3 years. Standard dilution plating techniques were used to enumerate fluorescent pseudomonads, gram-positive bacteria, gram-negative bacteria, Stenotrophomonas maltophilia-like bacteria, actinomycetes, heat-tolerant bacteria, and total aerobic bacteria. There was rarely a significant N rate x root-zone mix interaction, although main effects of N rate and root-zone mix did affect microbial populations at some samplings. If differences did exist, bacterial populations were higher in the sand-peat root-zone mix or in the treatments receiving the higher N rate. While a significant decrease in all bacterial populations enumerated was observed between November 1998 and February 1999, no similar decrease was observed for the same time period in 1997–1998 or 1999–2000. In general, there were no obvious population trends in any microbial population over the 3-year sampling period. Populations of total bacteria ranged from a low of 5.4 to a high of 8.3 log₁₀ CFU per gram of root and rhizosphere soil throughout the 3-year sampling period.