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							Title                 Activities of Rhizosphere Microorganisms as Affected by
                              Application of Organic Amendments in a Calcareous
                              Loamy Soil. 2. Nitrogen Transformations
Author/s              F. N. Barakah, S. H. Salem, A. M. Heggo and M. A. Bin-Shiha
                      Journal of
Contact Info.
Department
Major
Year of Publication   1995
Publisher             Arid Soil Research and Rehabilitation,
Type of Publication
Full Text (Yes, No)   No
Key Words
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% wt./wt. to
                      the soil. The asymbiotic nitrogen fixers, either aerobic or
                      anaerobic, as well as the nitrfying 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 N2-fixers and nitrifying bacteria) in the
                      rhizosphere and nonrhizosphere soils of both plants with high
                      values for the former during most of' the experiment periods.
                      However, the rhizosphere effect was not obvious in the case of
                      Azotobacter densities. The highest stimulative effect for the
                      asymbiotic N2-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 immobilizaition 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, N2-fixers and nitrifying bacteria in the soil but
enhanced the processes.