INTRODUCTION 106 MATERIALS AND METHODS Soils Sampling Procedure Nodule Isolates Isolation Physiological testing 113 Environmental stress Plant infection cest 113 RESULTS AND DISCUSSION Nodule Serology 115 Physiological Testing 119 Environmental stress 119 pH 119 Temperature 119 NaCl 119 1 GENERAL THTRODUCTION Soybean productivity can be greatly enhanced by inoculation of seeds in soils that lack bradyrhizobia. In soils with established populations, however, the competition between native and added bradyrhizobia is thought to be a major factor that inhibits inoculation responses and successful establishment of more desirable inoculum strains. Research is needed to better understand bradyrhizobial competition and ecology in soil. These studies investigated factors affecting competition between native and introduced Bradyrhizobium japonicum in Iowa soils. The first section entitled "Literature review" outlined and summarized previous research. This section contains Introduction, Competition, Fertility, Field Surveys, Methodology, and References Cited subsections and includes tables within the text. This manuscript will not be submitted for publication. The second section entitled "Competition of introduced Bradyrhizobium japonicum in Iowa soils" evaluated the competitiveness of inoculum strains in field studies over a 3-yr period to determine if introduced strains could compete against native bradyrhizobia in forming nodules on soybeans and, if so, whether their presence in the nodules contributed to improved plant growth. This section contains Abstract, Introduction, Materials and Methods, Results and Discussion, Summary, and References Cited subsections and has tables and a figure included in the text. This manuscript will be submitted to Agronomy Journal for publication. 2-4 The third section entitled "Serology of bradyrhizobia in Iowa soybean nodules as influenced by N additions and cultivars" considered N fertilizer, cultivars, and commercial inoculum as management factors that may affect the presence of bradyrhizobial serogroups in soybean nodules in the field. This section has the same subsection format as Section 2, except that Section 3 does not have a Summary. Tables are also included in the text. It has not been decided at the time of printing if this section will be submitted for publication. The fourth section is entitled "Field survey of soybean bradyrhizobia in Iowa soils" and characterized serogroups of native bradyrhizobia in field-grown soybean nodules from 32 soils across central Iowa over a 3-yr period. Nodule isolates were evaluated for a) tolerance to environmental stresses of pH, temperature, and salt concentrations, and b) their nodulation and Ng-fixation efficiency under laboratory conditions. This section has the same subsection format as Section 2 and has tables included in the text. It has not been decided at the time of printing if this section will be submitted for publication. An overall summary and discussion of results follows these sections in "Summary and Conclusions." Appendices A...