When the soil becomes compacted, the amount of air inside the soil is reduced. Initially, microbes use oxygen, but when they run out of oxygen, they start to take nitrogen from the soil and use it to breathe. At this point, they emit either nitrogen gas, N2, or nitrous oxide, N2O.
Work by Nimlesh Balaine in New Zealand on soil compaction has shown that as soils become more compacted, they could lose 10% to 60% of the nitrogen applied in the form of N2 or N2O. This is a huge amount.
As soils become more compacted, the likelihood of this happening becomes greater,especially in the context of climate change, where rainfall that used to occur once every two years 20 years ago is now occurring up to four times a year. This means that the soil remains wetter for longer and is more oxygen deficient, which means that more and more nitrogen is likely to be emitted at the beginning of the season.
We aren't sure yet, but according to the indicators on the situation in Quebec in the recent study on soil health, 60% of the soil was below this value.