The improvement of phosphorus (P) use efficiency (PUE) is a critical problem in crop production because of phosphorus’ scarcity. Especially in strongly weathered soil with a high P fixation capacity, a low PUE generally limits plant growth. Here, in a 70-day pigeon pea cultivation pot experiment using Ultisols, we evaluated the effects of Rhizobium inoculation (−I/+I) on the plant growth, rhizosphere, bulk soil P dynamics, and plant root P acquisition characteristics, with or without P fertilization (0P: no P application; 50SSP:50 kg P ha−1 with single superphosphate). The combination of Rhizobium inoculation with P fertilization (50SSP + I) increased the plant growth, P uptake, and organic acid content per pot by 63%, 41%, and 130%, respectively, but not without P fertilization (0P + I). The labile and moderately labile inorganic P (NaHCO3-Pi and NaOH-Pi) contents were higher (55% and 44%, respectively) in the rhizosphere soil than those in the bulk soil in the 50SSP + I treatment, indicating the efficient solubilization of the applied P under the 50SSP + I treatment. The fertilized PUE was higher in the 50SSP + I treatment (26%) than that in the 50SSP−I treatment (15%). Thus, these results suggest that Rhizobium inoculation with 50SSP should stimulate plant root P acquisition characteristics, leading to the solubilization of applied P in the rhizosphere and efficient plant P uptake. In conclusion, the 50SSP + I treatment effectively improved the PUE of pigeon peas in strongly weathered soil.

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This study aimed to determine the effect of sulfur (S) application on a root-associated microbial community resulting in a rhizosphere microbiome with better nutrient mobilizing capacity. Soybean plants were cultivated with or without S application, the organic acids secreted from the roots were compared. High-throughput sequencing of 16S rRNA was used to analyze the effect of S on microbial community structure of the soybean rhizosphere. Several plant growth-promoting bacteria (PGPB) isolated from the rhizosphere were identified that can be harnessed for crop productivity. The amount of malic acid secreted from the soybean roots was significantly induced by S application. According to the microbiota analysis, the relative abundance of Polaromonas, identified to have positive association with malic acid, and arylsulfatase-producing Pseudomonas, were increased in S-applied soil. Burkholderia sp. JSA5, obtained from S-applied soil, showed multiple nutrient-mobilizing traits among the isolates. In this study, S application affected the soybean rhizosphere bacterial community structure, suggesting the contribution of changing plant conditions such as in the increase in organic acid secretion. Not only the shift of the microbiota but also isolated strains from S-fertilized soil showed PGPB activity, as well as isolated bacteria that have the potential to be harnessed for crop productivity.

IntroductionLow phosphorus (P) use efficiency (PUE) of fertiliser is a critical problem in sustainable crop production, especially in strongly weathered tropical soils with a high P‐fixation capacity. Both intercropping and rhizobium inoculation have shown to improve the P availability of rhizosphere soil, but the effect of a combined approach of using both intercropping and rhizobium inoculation is still unclear. In this study, we aimed to evaluate the effect of rhizobium inoculation on the soil–plant P dynamics and fertilised PUE under the intercropping system in strongly weathered tropical soil.Materials and MethodsWe conducted an 85‐day cultivation pot experiment with pigeon pea (PP) and maize using highly weathered tropical soil under eight treatments: monocropping (CS) or intercropping, with or without rhizobium (Bradyrhizobium elkanii USDA61) inoculation (−I, +I) and with or without P fertilisation (0P, 50P) (2 × 2 × 2 = 8 treatments). We evaluated the effects of intercropping and rhizobium inoculation on plant growth parameters, P dynamics of the rhizosphere and bulk soil using the Hedley P fractionation method, the amount of organic acid from plant roots as a plant P‐mobilising capacity, and fertilised PUE.ResultsTotal plant P uptake per pot was significantly increased by intercropping but not by combining intercropping and rhizobium inoculation, resulting in better fertilised PUE only in intercropping. The available inorganic P (Pi) and less labile Pi of the soil were higher in the rhizosphere than those in the bulk by intercropping under 50P and were similar in PP + I under 50 P. The amount of organic acid per pot under 50P increased with each treatment, that is, intercropping and rhizobium inoculation, but not with their combination.ConclusionThe intercropping system has a strong potential to improve PUE by stimulating the P‐mobilising capacity of intercropping plant roots, whereas rhizobium inoculation of the intercropping system did not improve PUE in this study.