Denitrification is the process by which certain bacteria convert nitrate (NO3-) in the soil to nitrogen gas (N2), which is released into the atmosphere. The condition that encourages denitrification is a low soil oxygen level. This is because the bacteria that carry out denitrification require an oxygen-free environment to function. When there is not enough oxygen in the soil, the bacteria will turn to using nitrate as an alternative electron acceptor, leading to the conversion of nitrate to nitrogen gas. Therefore, option A - low soil oxygen - is the correct answer. The other options are not related to the process of denitrification.