Increased plant uptake of nitrogen from 15N-depleted fertilizer using plant growth-promoting rhizobacteria

Harmful environmental effects resulting from fertilizer use have spurred research into integrated nutrient management strategies which can include the use of specific micro-organisms to enhance nutrient use efficiency by plants. Some strains of plant growth-promoting rhizobacteria (PGPR) have been reported to enhance nutrient uptake by plants, but no studies with PGPR have used 15N isotope techniques to prove that the increased N in plant tissues came from the N applied as fertilizer. The current study was conducted to demonstrate that a model PGPR system can enhance plant uptake of fertilizer N applied to the soil using different rates of 15N-depleted ammonium sulfate. The experiments were conducted in the greenhouse with tomato using a mixture of PGPR strains Bacillus amyloliquefaciens IN937a and Bacillus pumilus T4. Results showed that PGPR together with reduced amounts of fertilizer promoted tomato growth compared to fertilizer without PGPR. In addition, atom% 15N per gram of plant tissue decreased as the amount of fertilizer increased, and PGPR inoculation resulted in a further decrease of the atom% 15N values. The atom% 15N 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 and statistically equivalent to 0.1184 for plants that received 100% fertilizer without PGPR. The results demonstrate that increased plant uptake of N applied in fertilizer could be achieved with PGPR as indicated by the differences in 15N uptake. Strains of PGPR that lead to increased nutrient uptake by plants should be evaluated further as components in integrated nutrient management systems.