Diversity of Microbial Communities Associated with Rhizosphere Soils of Selected Rice Varieties Grown In Sri Lanka

The diversity and activity of microbial communities in the rhizosphere of irrigated rice soils influence the soil fertility and nutrient use efficiency.  A study was undertaken to assess the diversity of rhizosphere microbial communities as affected by the rice variety grown in irrigated paddy fields in Sri Lanka.  Three replicates of rhizosphere soil samples were collected from 14 traditional varieties and two new improved varieties grown in an irrigated paddy field at Maha-Illuppallama after 90 days of transplanting. Soil pH, organic C, extractable P (Olsen), NH₄⁺ and NO₃^- were measured. Phospholipids fatty acid analysis (PLFA) was used to determine the microbial community composition.  

The pH and organic C of studied rhizosphere soils ranged from 5.62 to 6.33 and 0.7% to 6.2 % respectively. Available P and N varied in a wide range. Rhizosphere soils of all varieties were dominated by fatty acid 16:0, indicating high population of Gram positive bacteria. The ratio between Gram positive and Gram negative bacteria varied from 0.77 to 1.82. The ratio of cyclopropyl fatty acids (cy 17:0 and cy 19:0) to cis monounsaturated fatty acids (16:1w7c and 18:1w7c) indicated that no environmental stress existed at the time of sampling. Fungal biomarker PLFA (18:2w6) was observed in all except for three varieties. Microbial community in variety ‘Rathawalu' clustered separately from all the other analyzed rhizospheres according to principal component analysis (PCA). The PLFA of the rhizosphere of two improved varieties did not differ from many traditional varieties. The major variable governing the disparity among community structures of different habitats appeared to be the fatty acids of Gram positive bacteria as indicated by PCA1. Therefore, this study provides evidence that microbial community composition differs between tested rice varieties despite the similarity in soil conditions and agronomic practices under which they were grown.