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Harnessing the capacity of PGPR with or without AMF for improved plant use efficiensy of chemical fertilizers and organic maure

Adesemoye, A. O.. 2009. Auburn University, PhD

Abstract

The basis for the application of fertilizers (manure and chemical) is to make up for soil nutrient deficiencies and maintain soil fertility towards improved crop yield. Fertilizers could exacerbate environmental problems such as pollution of groundwater, leaching, nutrient runoff, soil salinization, greenhouse effect, global warming, etc which are major concerns. Alternatives that will halt this trend and which will have applications in different parts of the world are needed. Plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhiza fungi (AMF) are important biofertilizers that could be used in an environmentally benign manner to improve plant nutrient use efficiency. My first objective was to determine if PGPR or PGPR plus AMF will enhance N, P, and K uptake in plants with: (i) inorganic fertilizer application and (ii) organic fertilizer (chicken litter) application. A three-year field study was conducted with field corn from 2005 to 2007 in Sand Mountain, Alabama. Microbial inoculants, which included a formulated PGPR product, AMF, and their combination, were evaluated across two tillage systems (no-till and conventional till) and two fertilization regimes (poultry litter and ammonium nitrate). Data were collected on plant height, yield (dry weight of ears and silage), and nutrient content of corn grain and silage. In addition, nutrient content of soil was determined, and bioavailability of soil nutrient was measured with plant root simulator PRS™ probes. Results showed that inoculantspromoted plant growth and yield. For example, grain yield (kg ha -1 ) in 2007 for inoculants were 7,717 for AMF, 7,260 for PGPR AMF, 7,313 for PGPR, 5,725 for Control, and for fertilizer were 7,470 for Poultry litter and 6,537 for NH4NO3. Nitrogen content per gram of grain tissues was significantly enhanced in 2006 by inoculant, fertilizer, and their interactions. Significantly higher amounts of N, P, K were removed from the plots with inoculants, based on total nutrient content of grain per plot. The second objective was to determine (i) ifreduced rates of inorganic fertilizer coupled with microbial inoculants (PGPR or PGPR plus AMF) will produce plant growth, yield, and nutrient uptake equivalent to that obtained with full rates of the fertilizer and (ii) to what minimum level the fertilizer could be safely reduced. The microbial inoculants used in this greenhouse study were a mixture of PGPR strains Bacillus amyloliquefaciens IN937a and Bacillus pumilus T4, a formulated PGPR product, and the AMF, em>Glomus intraradices. Results showed that supplementing 75% of the recommended fertilizer rate with inoculants produced plant growth, yield, and nutrient (N and P) uptake that were statistically equivalent to full fertilizer rate without inoculants. When inoculants were used with lower rates of fertilizer, the beneficial effects were not noted; however, inoculation with the mixture of PGPR and AMF at 70% fertility consistently produced the same yield as the full fertility rate without inoculants. My third objective was to use 15 N tracer techniques to demonstrate that a model PGPR system (Bacillus amyloliquefaciens IN937a and Bacillus pumilus T4) can enhance plant uptake of N using different rates of depleted ammonium sulphate (15NH4)2SO4. Results showed that the dry biomass of plants which received 70% to 90% of recommended N fertilizer with PGPR inoculation was comparable to plants that received full rates of fertilizer without PGPR. Also, atom % 15 N per gram of tomato tissues decreased as the amount of fertilizer increased and PGPR inoculation had significant impacts on the values. For example, the atom % 15 N abundance in plants that received 80% fertilizer plus PGPR was 0.1146, which was significantly lower than 0.1441 for plants that received 80% fertilizer without PGPR. In conclusion, the results support the idea that inoculants can aid plant nutrient use efficiency. In the long-term, results will have applications in sustainable use of fertilizers. In the short term, the integrated system and models that were developed would have practical applications for farmers. I recommend further studies using the models developed in this study as a launch pad to understand better, the intricacies of the actual flow of N, P, and K in PGPR-AMF-plant interactions.