Mitigation of Potential Adverse Effects of Transgenic Crop Production for Long-Term Improvement of Soil Health

Highlights and Major Findings

• Soil glyphosate residues are variably distributed across a given field.
• Glyphosate residues persist in soil more than four years after last application.
• Glyphosate seems to be concentrated in soils under soybean production.
• Glyphosate movement offsite from adjacent GM-corn field occurred based on detection in waterway in north field.
• Glyphosate appears to be dissipated (degraded) quickly in grass waterway due to perennial vegetation supporting diverse and abundant soil microbiome and high microbial activity.
• Glyphosate persistence in soil is complex involving many soil properties including soil texture, soil organic carbon (organic matter), pH, and several cation nutrients; however, the properties vary among soil samples making interpretation difficult.
• Glyphosate appears to affect nutrient content and sufficiency in both corn and soybean likely due to effects of soil residues on nutrient immobilization and decreased plant uptake.
• Glyphosate residues appear to detrimentally affect photosynthesis in corn and soybean based on SPAD measurements.
• Glyphosate drift may be an additive factor in reducing nutrient metabolism and photosynthesis in corn.
• Soil glucosidase activity seems to negatively correlate moderately well with soil glyphosate content, suggesting it could be a basis for a potential assay for residual glyphosate impacts on soil health and crop productivity in the transition period

Management implications based on our findings:

• Maintaining sufficient levels of soil micronutrients is critical for both plant nutrition and soil microbial activity during transition period.
• Periodic soil testing for micronutrients is suggested due to inherent insufficiency levels of specific ones (Cu, Mo) and potential chelation (immobilization) activity of glyphosate and AMPA residues. This can be accomplishments with appropriate soil amendments and/or foliar applications.

• The restoration of microbial activity and abundance in soils may require several years and/or incorporation of perennial crops or cover crops during transition from transgenic to non-transgenic cropping systems. Possible use of biostimulants of known effectiveness for enhancing plant growth promoting rhizosphere microorganisms should also be considered.
• Herbicide drift, especially Roundup, needs to be avoided from contacting non-GM corn and soybean.
• Implementation of more diverse crop rotations and establishment of cover crops in the offseason will help improve soil microbiome diversity and soil properties such as organic matter, active C and aggregate stability that are critical in improving soil health.
• Addition of organic materials such as livestock manure will also benefit soil biological activity and soil properties.
• Installation of grassed waterways and other conservation practices can improve soil microbiome diversity, especially those groups able to degrade herbicides such as glyphosate to quickly dissipate these chemicals and reduce further degradation of productive soils and the environment.
• Avoid overuse of alternative herbicides for managing glyphosate-resistant weeds as these can be detrimental to soil biology, especially if multiple applications are made at various times during the crop season.