ACTIVITIES OF RHIZOSPHERE MICROORGANISMS AS AFFECTED BY APPLICATION OF ORGANIC AMENDMENTS IN A CALCAREOUS LOAMY SOIL .2. NITROGEN TRANSFORMATIONS

The activities of some nitrogen transforming microorganisms in the rhi zosphere 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 s oil. The asymbiotic nitrogen fixers, either aerobic or anaerobic, as w ell as the nitrifying bacteria, were determined in the rhizosphere and nonrhizosphere soils periodically for 7 weeks. Total nitrogen, organi c carbon, ammonium, and nitrate nitrogen were determined in the same s amples. Results showed that treatment of the calcareous loamy soil wit h organic amendments significantly enhanced the microbial population t ransforming nitrogen (asymbiotic N-2 fixers and nitrifying bacteria) i n the rhizosphere and nonrhizosphere soils of both plants with high va lues for the former during most of the experimental periods. However, the rhizosphere effect was not obvious in the case of Azotobacter dens ities. The highest stimulative effect for the asymbiotic N-2 fixers an d nitrifying bacteria was observed in the soil amended with sludge, es pecially in the early periods of plant growth. Also, the microbial num bers 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 rhizos phere region. Ammonium nitrogen values in the soils amended with organ ic matter were higher in the root regions of both plants than in uname nded soil, indicating a high ammonification rate for the former by het erotrophic microorganisms. Amendment of soil with 2% sludge (narrow C/ N ratio) enhanced the mineralization of organic nitrogen and nitrifica tion processes. Amendment of soil with straw (wide C/N ratio) increase d immobilization of mineral nitrogen. Using the sewage sludge (2% w/w) as an organic amendment to the loamy soil did not retard the prolifer ation and activities of the asymbiotic N-2 fixers and nitrifying bacte ria in the soil but enhanced the processes.