Plant Growth-Promoting Rhizobacteria Allow Reduced Application Rates of Chemical Fertilizers
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Microb Ecol (2009) 58:921–929 DOI 10.1007/s00248-009-9531-y PLANT MICROBE INTERACTIONS Plant Growth-Promoting Rhizobacteria Allow Reduced Application Rates of Chemical Fertilizers A. O. Adesemoye & H. A. Torbert & J. W. Kloepper Received: 24 December 2008 / Accepted: 29 April 2009 / Published online: 23 May 2009 # Springer Science + Business Media, LLC 2009 Abstract The search for microorganisms that improve recommended rate, the beneficial effects were usually not soil fertility and enhance plant nutrition has continued to consistent; however, inoculation with the mixture of PGPR attract attention due to the increasing cost of fertilizers and AMF at 70% fertility consistently produced the same and some of their negative environmental impacts. The yield as the full fertility rate without inoculants. Without objectives of this greenhouse study with tomato were to inoculants, use of fertilizer rates lower than the recom determine (1) if reduced rates of inorganic fertilizer mended resulted in significantly less plant growth, yield, and coupled with microbial inoculants will produce plant nutrient uptake or inconsistent impacts. The results suggest growth, yield, and nutrient uptake levels equivalent to that PGPR-based inoculants can be used and should be those with full rates of the fertilizer and (2) the minimum further evaluated as components of integrated nutrient level to which fertilizer could be reduced when inocu management strategies. lants were used. The microbial inoculants used in the study were a mixture of plant growth-promoting rhizo bacteria (PGPR) strains Bacillus amyloliquefaciens Introduction IN937a and Bacillus pumilus T4, a formulated PGPR product, and the arbuscular mycorrhiza fungus (AMF), Fertilizers are essential components of modern agriculture Glomus intraradices. Results showed that supplementing because they provide essential plant nutrients. However, 75% of the recommended fertilizer rate with inoculants overuse of fertilizers can cause unanticipated environmental produced plant growth, yield, and nutrient (nitrogen and impacts. One example of the negative impacts of fertilizer phosphorus) uptake that were statistically equivalent to the is the “dead zone” in the Gulf of Mexico where nutrients full fertilizer rate without inoculants. When inoculants washing from fertilized farms across the Mississippi Basin were used with rates of fertilizer below 75% of the cause oxygen starvation, leading to an almost lifeless area in the gulf [27]. One potential way to decrease negative environmental impacts resulting from continued use of A. O. Adesemoye (*) : J. W. Kloepper Department of Entomology & Plant Pathology, chemical fertilizers is inoculation with plant growth- Auburn University, promoting rhizobacteria (PGPR). These bacteria exert 209 Life Science Building, beneficial effects on plant growth and development [5], Auburn, AL 36849, USA and many different genera have been commercialized for e-mail: [email protected] use in agriculture. One of the important mechanisms for J. W. Kloepper these beneficial effects is PGPR-elicited enhanced nutrient e-mail: [email protected] availability and nutrient use efficiency. In a recent review, H. A. Torbert Glick et al. [16] observed that some PGPR may influence USDA Agricultural Research Services, plant growth by synthesizing plant hormones or facilitat National Soil Dynamics Laboratory, ing uptake of nutrients from the soil through different 411 South Donahue Drive, Auburn, AL 36849, USA direct mechanisms such as atmospheric nitrogen (N) e-mail: [email protected] fixation, solubilization of phosphorus (P), and synthesis 922 A. O. Adesemoye et al. of siderophores for iron sequestration making nutrients Results from some past studies have suggested ineffec more available to plants. tiveness of PGPR using single-strain inoculations [14, 25], Chemical fertilizers often have low use efficiency, but mixtures of strains provided more consistency [8, 18, meaning that only a portion of the applied nutrients are 36]. Some levels of interactions have been reported by co taken up by plants [17]. For example, P is precipitated after inoculating PGPR with AMF [4, 6, 7, 31, 39]. Some addition to soil, thus becoming less available to plants [17]. studies, mostly with single elements, have suggested that In addition, applied N can be lost through nitrate leaching, PGPR are more effective when nutrients become limiting resulting in contamination of groundwater [9]. Microbial [12, 38]. Reported here are the results of a study that inoculants have shown some promise in increasing nutrient included single elements (N and P) as well as conventional availability. For example, previous reports have suggested water-soluble NPK fertilizer and the interaction of a two- positive impacts of microbes on N uptake involving strain mixture of PGPR with AMF. nonlegume biological fixation [4, 13, 22, 37, 41]. Also, inoculation with some microbes, including arbuscular mycorrhiza fungi (AMF), resulted in P solubilization or enhanced plant uptake of fixed soil P and applied phosphate Materials and Methods resulting in higher crop yield [2–4, 7, 10]. The main mechanism resulting in increased availability of inorganic P Sources of Inoculants appears to be through the action of organic acids synthe sized by inoculants [34]. Plant growth-promoting rhizobacteria used included two Because essential plant nutrients are taken up from single strains that have been used in previous studies [26, the soil by roots [29], good root growth is considered a 36, 42]. The two PGPR strains, B. amyloliquefaciens prerequisite for enhanced plant development. Many IN937a and B. pumilus T4, were obtained from the culture PGPR systems cause stimulation of root growth [9, 25], collection of the Department of Entomology and Plant sometimes via production of phytohormones by the plant Pathology, Auburn University, Auburn, AL, USA and used or the bacteria [25, 38]. If promotion of root growth by as spore preparations. Another inoculant used was a PGPR could be achieved with high frequency in the field, commercial PGPR formulation, which consisted of many PGPR may be potential tools for increasing nutrient PGPR Bacillus strains with the trade name Plant Growth uptake. Activator (PGA; Organica, Norristown, PA, USA). In Two key questions arise from some of the past studies: Is addition, the arbuscular mycorrhiza fungi used was Glomus it possible to reverse the current trend of applying large intraradices obtained from Becker Underwood (Ames, IA, amounts of fertilizers by supplementing reduced fertilizer USA). with inoculants? Can the potential benefits of PGPR and/or AMF in plant nutrient uptake be utilized by combining Test for Nitrogen Fixation by PGPR Strains them with reduced levels of fertilizers? The overall hypothesis is that PGPR or combinations of PGPR and The two PGPR strains were tested for their capacity to fix AMF with fertilizers will improve the use efficiency of N using JNFb medium [30]. The strains were streaked onto fertilizers and lead to a reduction in the amount of fertilizer JNFb agar plates and stab-inoculated into JNFb semisolid usage. medium in test tubes. After incubation for 48 h at 28°C, the The objectives in this study were to determine (1) if color of agar and liquid medium was examined in reduced rates of inorganic fertilizer coupled with microbial comparison to a positive control consisting of the N- inoculants (PGPR or PGPR plus AMF) will produce plant fixing strain Azosprillum brasilense obtained from the growth, yield, and nutrient uptake levels equivalent to those bacterial culture collection at Auburn University. with full rates of the fertilizer and (2) the minimum level to which fertilizer could be reduced when inoculants were Experimental Design and Preliminary Studies used. To achieve these objectives, experiments were designed using single strains as well as formulated PGPR All assays reported in this paper were conducted in the products with or without AMF coupled with different greenhouse at the Plant Science Research Center, Auburn fertilizer regimes. For the PGPR strains, a two-strain University using tomato (Solanum lycopersicum, formerly mixture was used which included Bacillus amyloliquefa Lycopersicon esculentum) cultivar Juliet (Park Seed, ciens IN937a and Bacillus pumilus T4. The strains were Anderson, SC, USA). The growth medium was a mixture previously reported to elicit significant effects on root of one part field soil and three parts sand. After seeding, development, plant growth, biocontrol, and/or induced water was applied twice daily, and the greenhouse systemic resistance [23, 32, 36, 42]. temperature was maintained at 21–25°C. PGPR Allow Reduced Fertilizers 923 The overall experimental design was a randomized Tests with a Two-Strain Mixture, AMF, and Hoagland complete block with variations in the number of blocks Solution depending on each test. The main blocks were inoculant types, while fertilizer rate was the subfactor. Inoculant type The spore suspension of the two PGPR strains (B. included PGPR alone, PGPR+AMF, AMF alone, and no amyloliquefaciens IN937a and B. pumilus T4) were diluted inoculants. The fertilizer used was water-soluble Peters appropriately and mixed together. The concentration was Professional® 20:10:20 Peat-Lite Special (Buddies Plant adjusted to log 5 cfu/ml and used for inoculation. At Food, Ballinger, TX, USA). The 100% fertilizer rate was planting, 1 ml of the bacterial suspension was applied onto −1 1.25 g L which was the