A log lying on the ground will rot slowly, and the minerals and nitrogen compounds that were incorporated into from the soil and the carbohydrates made by photosynthesis will be released gradually back into the soil. These nutrients can then be taken up by other plants. Over time, the log may also function as a home for many creatures, and may eventually be a "nurse tree" for a seed that lands on it and sprouts.
If a fire burns the log, the nutrients are released suddenly rather than gradually. The carbohydrates, for example fibers and resins, will undergo a combustion reaction and be released directly into the air as carbon dioxide. The minerals will remain as ash, which is likely to blow away on the wind or be washed away by rain or melting snow, as opposed to returning to the soil. Consequently fires can cause the nutrient cycle to be short circuited.