Abstract
The activities of some nitrogen transforming microorganisms in the rhizosphere of both alfalfa and wheat growing in a calcareous loamy soil of Saudi Arabia as affected by organic amendments have been studied in a greenhouse experiment. The organic amendments were sludge (narrow C/N ratio) and wheat straw (wide C/N ratio), applied at 2% w/w to the soil. The asymbiotic nitrogen fixers, either aerobic or anaerobic, as well as the nitrifying bacteria, were determined in the rhizosphere and nonrhizosphere soils periodically for 7 weeks. Total nitrogen, organic carbon, ammonium, and nitrate nitrogen were determined in the same samples. Results showed that treatment of the calcareous loamy soil with organic amendments significantly enhanced the microbial population transforming nitrogen (asymbiotic N
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fixers and nitrifying bacteria) in the rhizosphere and nonrhizosphere soils of both plants with high values for the former during most of the experimental periods. However, the rhizosphere effect was not obvious in the case of Azotobacter densities. The highest stimulative effect for the asymbiotic N
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fixers and nitrifying bacteria was observed in the soil amended with sludge, especially in the early periods of plant growth. Also, the microbial numbers were more influenced by their proximity to the alfalfa roots than to wheat roots. Total nitrogen in the rhizosphere and nonrhizosphere soils of alfalfa and wheat significantly increased with application of organic amendments. The increase was especially evident in the rhizosphere region. Ammonium nitrogen values in the soils amended with organic matter were higher in the root regions of both plants than in unamended soil, indicating a high ammonification rate for the former by heterotrophic microorganisms. Amendment of soil with 2% sludge (narrow C/N ratio) enhanced the mineralization of organic nitrogen and nitrification processes. Amendment of soil with straw (wide C/N ratio) increased immobilization of mineral nitrogen. Using the sewage sludge (2% w/w) as an organic amendment to the loamy soil did not retard the proliferation and activities of the asymbiotic N
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fixers and nitrifying bacteria in the soil but enhanced the processes.