Control of amino acid catabolism in soil and direct assimilation by plants

Control mechanisms expressed at the organism-level, such as catabolite repression, could have important consequences to the coordination and control of soil nitrogen mineralization and result in increased nitrogen utilization efficiency in agricultural systems. This thesis examines the applicability of these forms of control to regulation of soil nitrogen-mineralizing enzymes at aggregate and horizon-levels of organization.;Soil histidase activity increased 3-4 fold by the addition of 100 ;Examination of the interrelationship between histidase activity, protease activity, biomass and net mineral-N production under field conditions revealed that integrative measures such as biomass provided better descriptions of mineral-N production. An inverse relationship between NH;The potential for direct plant uptake of amino acids from the rhizosphere was examined. Although maize was shown to be able to incorporate amino acids directly under axenic conditions, catabolism was the dominant fate of amino acid added to rhizosphere soil, with only 2% being assimilated by plants. Plant growth increased the rate of amino acid turnover but decreased the amount catabolized. Addition of carbohydrate was unable to mimic the increased retention of amino acid. The catabolism of amino acid in the rhizosphere suggests that conservative control mechanisms do not predominate even in this relatively energy rich zone.;Biological control of enzyme content is dependent upon the instability of existing enzymes. As a model system for examining the control of soil nitrogen mineralization, soil histidase was found not to be stable in soil. The decline in soil histidase activity following the addition of a biostatic agent (toluene or azide) followed first order kinetics. Two kinetic components were distinguished having half-lives of 3 and 77 hours. The decline in histidase activity was not attributable to enzyme inactivation by the biostatic agent, substrate limitation, or product (NH...