New study finds substantial loss of carbon, nitrogen from burned soils—and connections to warming climate

US Forest Service, Pacific Northwest Research Station: …New research conducted by PNW Research Station scientists and their colleagues on the 2002 Biscuit Fire is the first to document the toll of wildfire on forest soils—namely, the loss of significant amounts of carbon and nitrogen and 1 full inch of the upper soil layer. The work also raises an intriguing question: might the missing fine soil have been transported away in the fire’s massive smoke plume, such as the one seen in this satellite image from July 29, 2002?

….. .A new study led by the Pacific Northwest (PNW) Research Station addresses this critical information gap and represents the first direct evidence of the toll wildfire can take on forest soil layers. It draws on data from the 2002 Biscuit Fire, which scorched some 500,000 acres in southwest Oregon, including half of a pre-existing study’s experimental plots, which had been studied extensively before the fire. The result was a serendipitous and unprecedented opportunity to directly examine how wildfire changes soil by sampling soils before and after a wildfire. The study appears in the November issue of the Canadian Journal of Forest Research.

…[The team] … found that the combustion of the organic layer at the soil’s surface, including woody debris, caused intense, 1,300 °F-plus temperatures, which, in turn, displaced considerable amounts of carbon and nitrogen from the underlying mineral soil layer and left mostly ash behind. What was more surprising to the researchers was how these organic materials may have been lost. Some carbon and nitrogen were lost as gases—consisting mostly of carbon dioxide, nitrogen dioxide, and water vapor—and some in an inch of fine mineral-soil particles, which disappeared and left behind a crust of rocks.

…The loss of topsoil and carbon from soil can negatively affect a range of processes, Bormann said, including nutrient retention and water infiltration. In the absence of special nitrogen-fixing plants, which are capable of converting atmospheric nitrogen into nitrogen compounds for growth, losses of nitrogen in the order of what he and his colleagues documented would require at least a century to be reversed.

Equally disconcerting is the role these released organic materials might have on the atmosphere, especially in the face of a warming climate. The burning of soil by wildfire may contribute to global warming, in the short term, by releasing carbon as a greenhouse gas and, in the long term, by reducing soil productivity through losses of organic matter and nutrients….

The 2002 Biscuit Fire, image via MODIS/NASA