The effect of physical treatments such as drying, freezing, and heating on acetylene (C2H2) reduction (nitrogen fixation) in 3 types of soils was measured. Most treatments significantly affected ethylene (C2H4) production in the sandy loam. None of the treatments suppressed C2H4formation in the sandy clay loam before 2 days. Muck soil with treatments of air‐drying, freeze‐drying, and freezing did not show different patterns in C2H4production. However, auto claving and oven‐drying resulted in a pronounced increase in formation of C2H4. With exception of the heat treatments, no significant decreases in numbers of non‐symbiotic nitrogen‐fixers in soils were observed.