Microbial population of ryegrass root surfaces: Influence of nitrogen and phosphorus supply

In three experiments perennial ryegrass (Lolium perenne) was grown in pots of soil, with the addition of soluble mineral nutrients to provide contrasting nitrogen and phosphorus supplies. After 7–10 weeks the plants were harvested and the bacteria on the root surface or in the outer cortex were investigated. In two experiments the bacteria of root-free soil were investigated, for comparison.Bacteria which were pleomorphic when cultured were more abundant than rods in the rhizoplane, especially when the plants had received a complete nutrient solution. Only about 1% of the bacteria which were cultured were pseudomonads. Only 6–10% of the rhizoplane bacteria could degrade pectin and no cellulose-degraders were isolated, suggesting that few rhizoplane bacteria can degrade mucigel.In general addition of N or P to the soil had little effect on the total numbers of bacteria in the rhizoplane or the composition of the population. The proportion of bacteria able to degrade chitin was reduced by nutrient addition, but starch and pectin-degraders were not affected, and the proportion able to grow with NH₄NO₃ as their N source changed only a little. These results appear to conflict with previous observations that growth of single bacterial species on L. perenne in sand culture is increased when P is deficient. In the present experiments rhizoplane fungi increased markedly when N or (especially) P was deficient. It is suggested that P deficiency results in increased supply of carbon substrate from the roots, but when mineral nutrients are in short supply fungi are more effective competitors than bacteria for this substrate.