Biostimulant influences on turfgrass microbial communities and creeping bentgrass putting green quality

Immature sand matrix golf putting greens are considered to be inhospitable environments for microorganisms as compared to native soils. Subsequently, turfgrass quality may suffer in the absence of beneficial microbe-plant interactions. The turfgrass industry has responded by marketing a wide array of biostimulant products that claim to improve putting green quality through influences on soil microbial activity. A field study was conducted to determine what influences five commercial biostimulants have on the root-zone microbial community and creeping bentgrass (Agrostis stolonifera L.) quality. A three year old U.S. Golf Association (USGA) specification sand-based putting green (e.g., 80% sand: 20% peat humus by volume) was the test site. Commercially available biostimulants and fertilizer were applied biweekly from May until August 2000. The soil microbial community was characterized using soil enzymes and substrate utilization profiles. Turfgrass quality was determined visually by evaluating color, percentage of localized dry spot (LDS), and overall uniformity. Nutrient uptake levels were monitored to ascertain if increases in quality related to plant health. Visual quality of the putting green was significantly improved (p < 0.05) by the commercial biostimulants. The positive response to biostimulants was not of a nutritional origin. The biostimulants did not effectively alter the putting green microbial community in terms of enzyme activity or substrate utilization. However, a seasonal decline was detected in cellulase activity, which prevailed over any treatment effect, suggesting the root-zone microbial community responded to summer decline of bentgrass roots and concomitant decreases in quantities of root exudates. Visual improvements in putting green quality during the period of summer stress were primarily associated with the incidence of LDS. Visual LDS ratings were significantly reduced (less LDS) by applications of the biostimulants on each observation date (p < 0.05) and over the entire course of the experiment (P < 0.10). Surfactant properties of the biostimulants therefore appeared to play a major role in the improvements in putting green quality. This does not negate the fact that the seaweed extracts and humic acids in the biostimulants may have improved the heat and moisture stress tolerance of the bentgrass once the LDS formed.