Glyphosate is one of the most widely used herbicides in agriculture with predictions that 1.35 million metric tons will be used annually by 2017. With the advent of glyphosate tolerant (GT) cropping more than 10 years ago, there is now concern for non-target effects on soil microbial communities that has potential to negatively affect soil functions, plant health, and crop productivity. Although extensive research has been done on short-term response to glyphosate, relatively little information is available on long-term effects. Therefore, the overall objective was to investigate shifts in the rhizosphere bacterial community following long-term glyphosate application on GT corn and soybean in the greenhouse. In this study, rhizosphere soil was sampled from rhizoboxes following 4 growth periods, and bacterial community composition was compared between glyphosate treated and untreated rhizospheres using next-generation barcoded sequencing. In the presence or absence of glyphosate, corn and soybean rhizospheres were dominated by members of the phyla Proteobacteria, Acidobacteria, and Actinobacteria. Proteobacteria (particularly gammaproteobacteria) increased in relative abundance for both crops following glyphosate exposure, and the relative abundance of Acidobacteria decreased in response to glyphosate exposure. Given that some members of the Acidobacteria are involved in biogeochemical processes, a decrease in their abundance could lead to significant changes in nutrient status of the rhizosphere. Our results also highlight the need for applying culture-independent approaches in studying the effects of pesticides on the soil and rhizosphere microbial community.

In commercial agriculture, populations and interactions of rhizosphere microflora are potentially affected by the use of specific agrichemicals, possibly by affecting gene expression in these organisms. To investigate this, we examined changes in bacterial gene expression within the rhizosphere of glyphosate-tolerant corn (Zea mays) and soybean (Glycine max) in response to long-term glyphosate (PowerMAX™, Monsanto Company, MO, USA) treatment. A long-term glyphosate application study was carried out using rhizoboxes under greenhouse conditions with soil previously having no history of glyphosate exposure. Rhizosphere soil was collected from the rhizoboxes after four growing periods. Soil microbial community composition was analyzed using microbial phospholipid fatty acid (PLFA) analysis. Total RNA was extracted from rhizosphere soil, and samples were analyzed using RNA-Seq analysis. A total of 20-28 million bacterial sequences were obtained for each sample. Transcript abundance was compared between control and glyphosate-treated samples using edgeR. Overall rhizosphere bacterial metatranscriptomes were dominated by transcripts related to RNA and carbohydrate metabolism. We identified 67 differentially expressed bacterial transcripts from the rhizosphere. Transcripts downregulated following glyphosate treatment involved carbohydrate and amino acid metabolism, and upregulated transcripts involved protein metabolism and respiration. Additionally, bacterial transcripts involving nutrients, including iron, nitrogen, phosphorus, and potassium, were also affected by long-term glyphosate application. Overall, most bacterial and all fungal PLFA biomarkers decreased after glyphosate treatment compared to the control. These results demonstrate that long-term glyphosate use can affect rhizosphere bacterial activities and potentially shift bacterial community composition favoring more glyphosate-tolerant bacteria.

Biochar is known to ameliorate soil fertility and increase crop yield; however, information regarding its effects on soil chemical and biological properties remains limited. The experiment was conducted to study the short-term impacts of different types of biochar on soil C fractions, enzyme activities, and microbial community structure at depositional and eroded landscape positions at different sampling times [before planting, after planting, and after harvesting of soybean (Glycine max L.)]. Three biochar materials, produced from C-optimized gasification of corn (maize, Zea mays L.) stover (CS), ponderosa pine (Pinus ponderosa Lawson and C. Lawson) wood residue (PW), and switchgrass (Panicum virgatum L.) (SG), and dairy manure (DM) and mixture of dairy manure and pinewood biochar (DMP), were applied at a rate of 10 Mg ha −1 to depositional and eroded landscape positions. Data showed that the application of DMP and sole application of DM treatments significantly increased the labile C fractions at the depositional and eroded landscapes. The addition of DM and DMP increased the activities of β-glucosidase and urease enzymes, and those are involved in C and nitrogen cycling at depositional and eroded landscape positions. There were no significant differences between different biochar materials. However, there was an increase in soil microbial community structure in the DM and DMP treatments at both the landscape positions. In conclusion, our study revealed that DMP and sole application of DM influenced the soil labile C pool, enzyme activities, and microbial community structure at both the landscape positions for different sampling times. Article Highlights• Application of DMP and DM significantly increased the labile C fractions at both the landscape positions. • DM and DMP application increased the soil enzymatic activities. • DM and DMP treatments significantly increased the soil microbial community structure.

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Glyphosate is a broad-spectrum herbicide used extensively worldwide to control broadleaf weeds in agriculture. Research suggests that repeated application causes a change in soil microbial properties which could be affecting soil quality and productivity. Although glyphosate is generally regarded as having relatively low environmental impact, after 10 or more years of widespread use, field observations by farmers and emerging research suggest that long-term glyphosate tolerant (GT) cropping is having cumulative and non-target effects on soils and crop productivity. There is very little information on the effects of GT residue when added to soils. Therefore, the objective was to determine shifts in the soil microbial community during GT residue decomposition in soils with and without a history of glyphosate exposure. Soybean residues from a simulated long-term GT cropping system were used in laboratory incubation. The experiment was a 2x3x4 factorial design with 2 soils (with or without glyphosate), three residue types (leaf, stem,or root), and 4 soybean residue treatments (GT residue exposed to glyphosate with potassium salt carrier, GT residue exposed to glyphosate with isopropylamine salt carrier, untreated GT genotype, and untreated non-GT genotype). These soils were profiled using phospholipid fatty acid analysis to determine shifts in soil microbial community structure due to the addition of GT residue to soil. The results showed that microbial shifts during decomposition of GT soybean residue varied between soils with or without long-term exposure to glyphosate. There was also a trend that GT material that had been exposed to glyphosate cause a differential shift in the communities over GT residue that had not been exposed to glyphosate. Commercially available glyphosate formulations have two major types of salt carriers; potassium salt and isopropylamine salt which could be a factor besides glyphosate in affecting the chemistry of GT residues and subsequently microbial response during decomposition. However, the results showed that carrier did not significantly affect PLFA profiling in soils regardless of the soil's history of glyphosate exposure. Ratios of saturated to monounsaturated PLFAs are used as indicators of microbial stress. Our results showed that soil history of glyphosate exposure significantly affected microbial stress. There were also significant differences in stress between glyphosate residue treatments in soil with a history of glyphosate exposure.