Source: Danish Research Centre for Organic Farming
... there are bacteria in the environment that are able to take up naked DNA. The bacteria mainly take up DNA as a nutrient source, but under certain conditions foreign DNA can be incorporated into the bacterial genome. This mechanism is called horizontal gene transfer.
The experiments show that composting of GM plant residues greatly increases the rate of degradation of transgenic DNA compared to the rate for plant residues left in the soil. If this is considered as the only risk factor, composting is a 'DNA-safe' method to treat GM plant residues.
However, even though transgenic plant DNA was not detected in bacterial isolates in our experiments, we cannot conclude that horizontal gene transfer can not take place. The 300 isolates investigated proved to be too low a number to be conclusive.
Thus, it is still an open question if composting constitutes a safe way of disposing of GM plant residues. Furthermore, these experiments give rise to other interesting questions, e.g., whether GM plant materials decomposing in waste piles or deposited in manure yards may be able to transfer genes to indigenous bacteria at the comparably lower temperatures present at these environments.
These questions need to be assessed if the risk associated with the use of GM plants is to be thoroughly investigated.