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Denitrification is the process that leads to a decrease in available nitrogen in the soil. It occurs when bacteria convert nitrates (NO3) and nitrites (NO2) back into nitrogen gas (N2) or, to a lesser extent, nitrous oxide (N2O), which is then released into the atmosphere. This process is a crucial part of the nitrogen cycle, as it helps to balance nitrogen levels in ecosystems. When denitrification occurs, the nitrogen that was previously available in the forms of nitrates and nitrites for plant and microbial uptake is reduced, causing a decrease in soil nitrogen availability.

The other processes listed contribute to the nitrogen cycle in different ways. Nitrification converts ammonium (NH4) into nitrites and then into nitrates, which increases the nitrogen available for plants. Ammonification involves the decomposition of organic matter, releasing ammonium, thus making nitrogen available in the soil. Nitrogen fixation, performed by certain bacteria, converts atmospheric nitrogen (N2) into ammonia (NH3) or related compounds, increasing soil nitrogen availability. All these processes contribute to enriching the nitrogen content of the soil, in contrast to denitrification, which leads to its reduction.