Minimal set of parameters for evaluation soil microbiota and biological nitrogen fixation in soybean

The objective of this work was to evaluate and validate a minimum set of parameters capable of monitoring, under field conditions, soil microbes and/or the biological N-2 fixation process with the soybean crop. The field trials were performed in commercial fields (2002/2003) and experimental areas (2004/2004), with complete randomized blocks design. Microbial biomass C (MBC) and N (MBN) were adequate for the quantitative evaluation of soil microbes; the parameters were significantly correlated therefore the determination of only one of them is sufficient. Positive and significant correlations were also obtained between the parameters of shoot dry weight (SDW) and MBC and MBN. The analysis of total soil DNA by the denaturating gradient gel electrophoresis method (DGGE) detected qualitative changes in soil microbes, related to the homogeneity of the area, as well as to differences among treatments, areas and harvests. Nodule dry weight (NDW) was the most suitable parameter for the evaluation of nodulation. Significant correlations were obtained between the SDW and the total N accumulated in shoots (TNS). Similar results were obtained between SDW and total N-ureides (TNU). Monitoring of the symbiosis in soils traditionally cropped with soybean and poor on N may be achieved only by the determination of NDW and SDW.