The process of microbial degradation of nitrates into atmospheric nitrogen is known as denitrification. Denitrification is a natural process carried out by certain bacteria in the soil and water. These bacteria convert the nitrates, which are nitrogen compounds found in fertilizers and manure, into gaseous nitrogen. This gas is then released into the atmosphere, completing the nitrogen cycle.

Ammonification, option 1, refers to the process of converting organic nitrogen compounds (such as proteins) into ammonia. Nitrification, option 2, is the process by which ammonia is converted into nitrites and then nitrates by different types of bacteria. Putrefaction, option 4, is the decomposition of organic matter by bacteria and fungi. While putrefaction can release nitrogen-containing compounds, it is not specifically focused on the conversion of nitrates into atmospheric nitrogen.

Therefore, the correct answer is denitrification.