Within the limitations of our study, with few exceptions, if there was an effect of weed management on densities of soil arthropods, it seemed to be driven by the absence of weeds rather than the application of the herbicide. However, a cluster analysis indicated that the composition of arthropod communities below ground tended to differ between hand-weeded and glyphosate plots. Overall, the field site itself had a stronger effect on arthropods than weed management treatments, which is likely related to differences in soil properties such as texture.
Soil nutrients were primarily influenced by field site location and weed presence. Initially, we expected that applying an insecticide would have a negative impact on soybean plant growth parameters due to the disruption of beneficial arthropods (associated with nutrient recycling, etc.). However, we found that in some cases there was no impact of the insecticide or that plant parameters were actually higher in plots where insecticide was applied. Currently, we believe this is because the insecticide suppressed populations of pest arthropods that were negatively affecting N-fixing soybean root nodules. This benefit outweighed the contributions of the beneficial soil arthropod fauna.
This study demonstrates that soil arthropods can impact soybean plants, and that soil conditions and plant diversity can alter arthropod density and/or species composition. Because what happens below ground can affect what happens above ground, it would be beneficial to understand more about how specific soil arthropods positively and negatively impact soybean production in North Dakota.