Influence of Plant-Growth Promoting Rhizobacteria on Arthropod Populations, Forage Biomass, and Soil Health in Bermudagrass Pastures
Type of DegreeMaster's Thesis
Entomology and Plant Pathology
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Bermudagrass (Cynodon dactylon (L.) Pers.) is a warm-season, perennial grass commonly used in forage production systems in the southeastern United States. Drought tolerance and pronounced yield response to high rates of nitrogen fertilizer applications make bermudagrass one of the preferred species for hay production and grazing pastures. With growing concerns of negative environmental impacts resulting from high levels of nitrogen fertilizer applications, it is important to explore biological alternatives for fertilizers in the forage industry. Plant growth-promoting rhizobacteria (PGPR) represent a potential sustainable alternative to chemical fertilizers that could significantly reduce the amount of nitrogen in forage production. The overall purpose of this project was to evaluate the potential of PGPR as a biostimulant to reduce chemical inputs in bermudagrass hay fields and grazing pastures. Plant growth promoting rhizobacteria with and without nitrogen fertilizers were compared to full rates of nitrogen fertilizers to evaluate differences in forage biomass, arthropod populations, and soil health among treatments. Chapter I is a detailed review of literature pertaining to bermudagrass, major forage pests, nitrogen fertilizers, and PGPR. This review provides background on bermudagrass, its improved varieties, and management practices utilized in hay production and grazing systems in Alabama. An overview of forage pest management, including sections discussing history and biology of the fall armyworm (Spodoptera frugiperda) and the bermudagrass stem maggot (Atherigona reversura), are also included. A section discussing negative environmental impacts of nitrogen fertilizers precede a section discussing PGPR, how they work, and their potential for reducing chemical inputs in the agricultural industry. In Chapter II, a 2-year, large-scale field evaluation in bermudagrass fields comparing effects of PGPR and nitrogen fertilizers was conducted. Three grazed locations and three hay locations throughout the state of Alabama were established with three replications per location and six treatments per replication. The six treatments include a blend of PGPR (Blend 20), a single strain PGPR (DH44), Blend 20 plus a ½ rate of nitrogen fertilizer, DH44 plus a ½ rate of nitrogen fertilizer, a full rate of nitrogen fertilizer, and a control plot. These treatments allowed comparisons of PGPR to a full rate of nitrogen, and an evaluation of the effect of PGPR when applied with a half rate of nitrogen fertilizer. Full and half rates of N for bermudagrass were based on extension recommendations in Alabama. Two applications of treatments and four harvests were completed each year. At harvests, arthropod populations, forage heights, and forage dry weight data were collected. Treatments significantly affected biomass weights at location managed for hay production and grazing. Forage weight from plots treated with the full rate of nitrogen fertilizer and both PGPR plus ½ rate treatments was significantly greater than control plots. These results indicate that PGPR plus a ½ of nitrogen fertilizer can yield similar results as a full rate of nitrogen fertilizer in fields utilized for hay production and grazing systems. In Chapter III, a 1-year, large-scale field evaluation in bermudagrass pastures comparing effects of PGPR and nitrogen fertilizers was conducted. Two sites at the E.V. Smith Research Center (Macon County, Alabama) were selected and three replicates of six treatments were established on each site. The treatments were Blend 20, DH44, Blend 20 plus a ½ rate of nitrogen fertilizer, DH44 plus a ½ rate of nitrogen fertilizer, a full rate of nitrogen fertilizer, and an untreated control. These treatments allowed comparisons of PGPR to a full rate of nitrogen, and an evaluation of the effect of PGPR when applied with a half rate of nitrogen fertilizer. Two applications of each treatment were applied to plots and were evaluated four times. At harvests, arthropod populations, forage heights, and forage dry weight data were collected. In September 2020, soil cores and samples were also collected from each plot to analyze and compare populations of the soil mesofauna and soil respiration as indicators of soil health. There were no significant treatment differences for forage biomass or arthropod populations at either site. Soil respirations and populations of mesofauna were numerically greater in plots treated with PGPR but were not statistically significant in oribatid populations.