Fertilizer produces far more greenhouse gas than expected

With an overload of fertilizer, soil microbes on farms may belch unexpectedly high levels of nitrous oxide, a greenhouse gas with 300 times as much heat-trapping power as carbon dioxide. The finding may help explain why agricultural nitrous oxide emissions are much higher than some scientists had predicted and could give clues for how to curb farm pollution.

Soil microbes have long been known to convert nitrogen-rich crop fertilizers, including manure and synthetic fertilizers, into nitrous oxide. After more than 1,000 field experiments, climate scientists calculated in the mid-2000s that the dirt dwellers spew about one kilogram of the greenhouse gas for every 100 kilograms of fertilizer, or roughly 1 percent. Researchers generally thought that emissions would scale up linearly: doubling fertilizer would double the emissions of gas.

But the predictions didn’t match up with real-world numbers. Estimating regional and global fluxes of atmospheric nitrous oxide levels a few years ago, researchers pegged the microbial conversion of fertilizer to gas at somewhere between 1.75 and 5 percent. Either the initial calculations were off or there were unknown sources of nitrous oxide, says biogeochemist Phil Robertson of Michigan State University in East Lansing. The latter was unlikely, he adds.

From cornfield experiments in Michigan in 2005, Robertson and colleagues found that the relationship between the amount of fertilizer and production of greenhouse gas is not always linear. When farmers apply larger amounts of fertilizer than initially tested, the relationship appears exponential — applying 200 kilograms of fertilizer could result in four kilograms of gas, for instance.  Most of the original field experiments, he says, didn’t look at microbial gas conversion with excess fertilizer, far more than crops need. In such situations, the crops take up as much nitrogen as they can, and the rest goes to soil microbes, Robertson says. “They effectively go to town on that nitrogen,” he says.

To see if the finding held up on a global scale, Robertson and colleagues reanalyzed more than 200 experiments, each examining nitrous oxide emissions of multiple fertilizer levels, across 84 locations worldwide. The results confirm that excess fertilizer can exponentially boost the emissions of microbes, the team reports June 9 in the Proceedings of the National Academy of Sciences. From this finding, Robertson infers that some farmers are using far more fertilizer than they need to, yielding more nitrous oxide than scientists had expected.

The good news is that the study provides a clear path to lower nitrous oxide emissions, says soil scientist Timothy Griffis of the University of Minnesota in St. Paul. There are many easy methods for assessing how much nitrogen-based fertilizer is right for a crop field, so addressing overuse could be simple, he says.  

But the exponential relationship between fertilizer and gas still doesn’t entirely account for the emission discrepancy, says soil scientist Rod Venterea of the U.S. Department of Agriculture in St. Paul. There must be other factors upping the nitrous oxide in the atmosphere, he says, including unknown emissions from nitrogen fertilizer after it leaves farms via streams and erosion.