Experimental Analysis: What are possible reasons for this increase in N₂O production?
This may have no correlation to your project but many insecticides, bacteriocides, and herbicides need a "solvent" or "transfer medium" such as water to be effective. Even when using ethanol in the laboratory against a variety of bacteria 100% ethanol becomes almost ineffective. 

I would take a look at running round-up again by itself versus 95% roundup 5% water, and 90% roundup and 10% water and I am sure you may seem some results that will aid your experiment.

Good Luck!