Soil Microbial Community Composition and Microbial Potassium in No-till Soils Under K Deficient Glyphosate-Tolerant Corn and Soybean.

In recent years there has been a growing incidence of K deficiency in corn and soybean in the upper Midwest. This is not reflected in standard soil tests. There is growing evidence that long-term use of glyphosate in glyphosate tolerant-cropping systems (GTS) may affect microbial communities. Preliminary research, limited evidence from the literature, and our understanding of K cycling suggested that glyphosate causes an interaction of soil biology and clay mineralogy that may be inducing crop K deficiency. We hypothesized that GTS causes a microbial shift towards fungal dominance or specific fungal genera, which rapidly take up K and transfer it to non-exchangeable/plant unavailable forms. To test this hypothesis, ten sites in the upper Midwest of the USA have been selected on glyphosate treated corn-soybean rotations at upland non-K deficient sites and on paired, depressional soils that have shown a history of crop K deficiency. Soil samples have been collected for two growing seasons (2007 and 2008) and once in winter 2008. Soils were characterized by relative abundance of clay (%) and clay mineralogy was analyzed using X-ray diffraction (XRD) patterns using oriented clay slides while applying various heat treatments to characterize clay expansion and collapse. We measured soil K pools (non-exchangeable, exchangeable/soluble, and microbial (K_MB)). The soil microbial community composition was analyzed by using a fatty acid (EL-FAME) profiling method and by DNA-PCR-DGGE analyses of 18S rDNA (fungal diversity) and Fusarium-specific DNA targeting a partial region of the translation elongation factor-1 alpha (EF-1 alpha) gene. Some K-deficient sites at lowland positions became dominated by fungi or selective bacteria and at the same time had high K_MB values during the early growing season. This suggests microbial K competition could be inducing K deficiency in corn. Several samples tested for general fungi (18S rDNA) and Fusarium have shown different DGGE fingerprints comparing soils under K deficient and K sufficient crops. Fusarium identification will provide further evidence for a changing biology. Some sites had no differences in soil microbial profiles or K_MB properties between K-deficient and non-deficient corn soils. Although some of sites had higher clay contents in K deficient soils, our overall conclusion, based on XRD analysis, is that mineralogy is not related to K deficient soils. Crop K uptake data will be presented to determine whether there are links between actual K deficiency and microbial community structure, K biomass, or other soil properties.