Nitrous oxide gas, which is given off by the soil as excess nitrates are eaten by bacteria, is drawing more attention from scientists who are examining greenhouse gases associated with farming systems. Until recently, nitrous oxide received little notice, except at the dentist office or the drag strip. (Nitrous oxide is the “laughing gas” sedative used by dentists — and the horsepower enhancer sometimes used by tractor pullers and drag racers.)
Scientists had thought that nitrous oxide was produced primarily in wetlands. But no one knew for sure, because it is difficult to measure, Robertson says.
That changed after new analytical instruments became available and scientists developed the labor-intensive procedures needed to test for nitrous oxide. They learned that virtually all soils produce this powerful greenhouse gas. How much is produced depends heavily on how much nitrogen is in the soil.
The greenhouse gas testing procedure involves plopping a box about a foot square on each plot, then taking gas samples about two hours later. This is done at multiple locations in each plot throughout the year, since nitrous oxide off-gases at varying rates, depending on the location and the time of year.
All the annual crop management systems in Robertson's study produced nitrous oxide in a narrow range centering on a GWP of about 500 lbs./acre. That included the organic system, whose nitrogen was supplied by manure, not supplemental fertilizer nitrogen.
This shows that the source of the nitrate does not matter. “Organic farming is good for the environment in one sense [because it reduces pesticide use], but there is little evidence that it keeps nitrogen out of the environment,” Robertson says.
Various strategies can be used to reduce nitrous oxide off-gassing without reducing yields, he says. These include side-dressing, use of time-release fertilizers and other approaches to supplying nitrogen when the crop can use it.
“There is relatively moderate nitrous oxide production until you saturate the crop with nitrogen,” he says. “If you spoon-feed nitrogen, you can reduce the amount of nitrous oxide a lot without affecting yield.”