I did some web searching on glyphosate, the active ingredient in RoundUp. It gave me the willies to hear about its danger to health, but I also learned a bit about the breakdown of 'chemical' fertilizer, herbicide, and insecticide. They are supposed to be designed to break down by natural processes. Some studies quantified the conditions for optimum breakdown and thankfully those are the conditions we are creating with organic gardens; lots of organic material, pH in the range of 6.2 to 7, and active biological conditions. Within a matter of weeks to months, the concentrations of chemicals would drop by something like 98%. If you're starting out with a mild degree of chemical contamination, it will be mostly gone by the time your plants grow. I think the act of composting will destroy most of the chemicals, and tilling that compost into the soil will take away most of the rest.

I believe that 2 mechanisms are mineralization and denitrification- although they are normally described in soils or water. In the first- N forms (simple anions, or from proteins) are transformed into NH4+, and NH3 is lost because it is volatile. Normal thermal/ light induced degradation and bacterial action can cause this to happen (going on memory here). In denitrification NO3- is directly converted to N2 as part of the nitrogen cycle. You may remember basic fishtank chemistry where NH4+ --> NO2- --> NO3- . I guess this is the opposite of that. Going on memory there. However- green leaves turning brown, fresh cut green grass drying out and turning brown- both are accompanied by loss of N as a volatile. I'm sure that autocatalytic breakdown of the proteins in the greens would cause loss of a variety of N bearing compounds. Manures and urine often contain urea, which has a very simple pathway to NH3 conversion.

Wood chip mulch is far better. What they do is promote fungal growth which is essential to trees' health. Grass clipping tend to promote bacterial growth. Grass clipping is helpful though when you put fresh grass clipping in a bucket of dechlorined water for a day then pour around wood chip mulch to help release nitrogen to feed the trees. Protozoa grow rapidly in the bucket feeding on grass clippings and they chew through fungi in the wood chips releasing nutrients to the roots. Of course, it is pointless if somebody uses all kinds of chemicals on the lawn around the trees.

I'll offer a bit of real life experience...... Lawn company used weed killer with 2-4D as an ingredient. 8 weeks later, after many mowings with removal of all clippings, I saved some grass clippings and allowed them to dry on driveway. After drying, I used clippings as mulch on some potted plants Shortly after, the plants began curling up as if they had Cucumber Mosaic Virus. The lack of cucumber beetles and a little research later suggested possible herbicide damage. I removed the grass clippings and flushed a ton of water through the potted plants. Each recovered and produced a good supply of fruit. Tomato plants are known to detect minute traces of 2-4D. They are able to detect levels that lab equipment can't. Hope this helps, Lee