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Methods for identifying and following microorganisms in the environment such as soils, water and plant association have been highly developed in recent years. In this study, we used rep-PCR for the characterization of Rhizobium bacteria isolated from legume trees native of Uruguay which permitted to follow them in plant inoculation assays. Among the primers used, ERIC and BOX1AR, the latter allowed the differentiation of isolates from different legumes and the finding of a high level of homology among rhizobia that nodulate the same legume species. Besides, it demonstrated that the profiles of the isolates recovered from nodules of Acacia caven, were identical to the ones used as inoculants; this confirmed the efficiency of these methods to follow these diazotrophs in the environment.

Herbicides have a considerable influence on soil microorganisms and soil biochemistry. These influences are likely to be reflected in soil fertility and plant growth. The effects of atrazine, linuron and 2,4-D amine were studied on soil microflora in a field trial with sorghum in Río Cuarto, Argentina. Atrazine and linuron were applied before sowing and before emergency, and 2-4,-D amine as post-emergence herbicide. Dehydrogenase activity with TTC (tri-Cl-phenyl-tetrazolium) as electron acceptor, nitrogen mineralization by the steam-distillation method (ammonia and nitrate) and enumerations of cellulose-decomposing microorganisms and dinitrogen fixing genus Azotobacter on selective mediums, were studied. The dehydrogenase activity did not show conclusive effects of herbicide action. The small differences at 20 and 71 days after sowing, fluctuated around the control value. The nitrogen mineralization was also barely affected by treatments. At 20 days after sowing, all plots with herbicide accumulated less mineral nitrogen than the control, but only those treated atrazine before emergence (2 kg/ha) differed significantly (5%). At 71 days a small stimulation of nitrification by linuron was observed. These differences disappeared at the end of the trial (3 1/2 months). The microbial population of cellulose decomposers was very sensitive to herbicides. This restriction seems to depend on unfavorable food conditions for these microorganisms in a soil without weeds, or it is due to enzyme inhibition by pesticides. This group was inhibited by all treatments in the same manner at flowering time. The nitrogen fixing Azotobacter, which is stimulated in the rhizosphere of grasses, was not affected by these chemicals.