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Prescribed Fire Effects on Soil Fertility

longleaf-pine-regrowth
Longleaf pines grow on the Harrison Experimental Forest study site three years after the prescribed burn. Photo by John Butnor, USFS.

USDA Forest Service researcher John Butnor wondered how dormant-season prescribed fire affects forest soil fertility in the months after a burn. Do nutrients from burned pine straw, grasses, and woody debris remain in the forest?

Others have studied soil a year or more after a prescribed burn. Butnor’s research compares soil chemistry before burning and 1, 3, 6, and 12 months after.

After one month, the researchers saw that the fire had consumed much of the forest floor, increased soil pH, and caused a pulse of carbon, nitrogen, and elemental nutrients to flow to the near surface soils. At three months postburn, coinciding with the start of the growing season, retention of nutrients by soil peaked.

By six months, the amount of these elements plummeted.

After 12 months, oxidation-resistant carbon levels had dropped. And underneath, soils at 2–4 inches (5–10 cm) were missing some of the carbon, copper, iron, nitrogen, potassium, phosphorus, magnesium, and manganese that they had pre-burn—the exact opposite of findings in other southern pine ecosystem studies.

Butnor wrote in 2012 that this plantation had low soil carbon, nitrogen, and phosphorus. He hypothesizes that this poor fertility may have combined with the fire’s flush of carbon and nutrients to increase decomposition.

Bottom line: Your results may vary. Soil carbon and nutrients may take more time to recover on this lower coastal plain site in southeastern Mississippi than in other southern pine forests. Burning late in the dormant season could sync the pulse of nutrients with the start of growth in spring and may increase forest productivity.

Read the article in Forests. For more information, email John Butnor at john.butnor@usda.gov.

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