Activities of rhizosphere microorganisms as affected by application of organic amendments in a calcareous loamy soil I. Carbon assimilation

The activities of soil microorganisms in the rhizosphere (R) and nonrhizosphere (S) regions of alfalfa and wheat plants, growing in calcareous loamy soil of Saudi Arabia, as influenced by application of organic amendments such as sludge and wheat straw, were studied in pot experiments for 7 weeks. The study showed that counts of aerobic heterotrophic bacteria in the rhizosphere soil of both alfalfa and wheat plants significantly increased due to application of organic amendments accompanied by a high decomposition rate for the carbonaceous materials. Application of sludge to the soil enhanced the rhizosphere microorganisms during all experimental periods as compared with those treated with straw. Straw manure clearly enhanced the aerobic cellulose decomposing bacteria, especially in the rhizosphere region. The total mi‐crobial counts in the rhizosphere of alfalfa were generally higher than those of the wheat plants in the first and second weeks and at the end of the period, especially when sludge was applied as an amendment. The densities of the total fungi, actinomycetes, and sporeformers were slightly affected by the rhizosphere soil, and the R/S ratios rarely exceeded 2.0. Soil treatment with either sludge or straw significantly increased the organic carbon and, consequently, the organic matter of the soil, especially in the rhizosphere region, with relatively higher values for those amended with straw. A high carbon‐to‐nitrogen ratio of the amendment to the soil was associated with slow decay of the organic matter. Amendment of loamy soil with sludge at the common rate (2%) enhanced the microbial flora with no harmful effects to plant growth or soil fertility and can be used safely on agricultural land.