Soils are inherently variable in the physical and chemical properties that determine yield potential. Apparent electrical conductivity (ECa) measurements allow mapping of natural patterns of soil conditions across a field. Our first objective was to identify these soil properties that influenced ECa in an irrigated cornfield in Nebraska. The variability observed was related to soil properties that influenced soil water availability in the 0- to 90-cm: total C and clay content, total dissolved solids and depth of topsoil. Our second objective was to determine whether specific soil microbiological groups were associated with the soil properties that caused spatial variability in this field. Soil microbial populations mediate the processes of organic matter turnover and nutrient cycling. Soil microbial communities were related to diverse C pools. Bacterial and actinomycetes biomarkers covaried mainly with fine particulate organic matter (POM), while fungal and mycorrhizal biomarkers responded to coarse POM distribution. We also evaluated the distribution of soil microbial groups at a smaller scale. Row cultivation increased labile C pools and soil microbial biomass in the row position, independently of the ECa classification. Four weeks after row cultivation we observed a shift in the relative concentration of arbuscular mycorrhizal (AM) fungal biomarkers in the row, but we found no clear differences in abundance of a specific group. The spatial variability in soil microbial groups may be accompanied by changes, over short periods of time, in AM fungi dynamics. In corn, more than 50% of the P is absorbed after tasselling. AM fungi form symbiotic relationships with most plants and play an important role in plant P nutrition. Two fields in Nebraska were selected to study the influence of available P in AM fungi during the reproductive stages of corn. The concentration of AM fungal biomarkers increased over time, and confirmed the C allocation from the plant to the symbiont during this period. Furthermore, hyphae were as efficient as roots and hyphae in reducing the P concentration from the soil matrix. It is likely that AM fungi play an important role in P uptake later in the crop season.
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