Impact of allelopathic rice seedlings on rhizospheric microbial populations and their functional diversity

Field performance of rice allelopathic potential is indirectly regulated by the microflora in the rhizosphere. The present study aimed to investigate the dynamics of microbial populations and their functional diversities in the seedling rhizospheres of rice cultivars with varied allelopathic activities by employing agar plate bioassay, fumigation and BIOLOG analysis. Rice cultivars significantly affected the microbial carbon content in their associated rhizospheric soil. The microbial carbon contents were ranked in a decreasing order as Iguape Cateto (441.0 mg·kg–1) > IAC47 (389.7 mg·kg–1) > PI312777 (333.2 mg·kg–1) > Lemont (283.8 mg·kg–1) with the nil-rice control soil of 129.3 mg·kg–1. Similarly, the respiration rate of the soils was 1.404, 1.019, 0.671 and 0.488 μgC·g–1· h–1 for PI312777, Iguape Cateto, IAC47 and Lemont, respectively. The respiration rate was only 0.304 μ gC·g–1·h–1 for the control soil. The microbial flora in the rhizospheric soil of different rice cultivars was dominated by bacteria (58.4%–65.6%), followed by actinomycete (32.2%–39.4%) and fungi (2.2%–2.8%). BIOLOG analysis showed that the value of Average Well Color Development (AWCD) differed significantly among rice cultivars. It was always the highest in the rhizospheric soil of the strongly allelopathic rice cv. PI312777, and the lowest in the rhizospheric soil of the poorly allelopathic rice cv. Lemont. The AWCD value reached the maximum in all the sampled soils after 144 hours of incubation. The AWCD values from the rhizospheric soils of PI312777, IAC47, Iguape Cateto and Lemont were 1.89, 1.79, 1.60 and 1.43 times higher than that of the control soil. Principal Component Analysis (PCA) identified 3 principal component factors (PCF) in relation to carbon sources, accounting for 70.1%, 11.3% and 7.0% of the variation, respectively. 19 categories of carbon sources were significantly positively correlated to the 3 principal components. Phenolic acids, carbohydrates, amino acids and amides were significantly correlated to the principal component 1, phenolic acids, carbohydrates and fatty acids to the principal component 2, and carbohydrates and hydroxylic acids to the principal component 3. Amino acids and amides were the two main carbon sources separating the 3 principal component factors. In addition, the total microbial population in the rhizospheric soil was significantly positively correlated with AWCD, microbial biomass carbon, microbial respiration and Shannon index. There was a significantly positive correlation between the total microbial population and the inhibition rate (IR) on the root length of lettuce owing to the different allelopathic activities of the rice cultivars. These results suggest that changes in microbial population, activity and functional diversity in the rhizospheres are highly cultivar-dependent. These changes might play an important role in governing the rice allelopathic activity in the field.