This is just one example of how prescribed fire is often praised without a firm grounding in fact. Fire’s benefits are often exaggerated, perhaps not maliciously but rather out of ignorance. In writing, when we take someone else’s work and claim it as our own, it’s called plagiarism – and that ain’t cool! Shouldn’t we show the same respect to nature’s work? It’s tempting to accept simplified narratives, but doing so misrepresents reality. Digging into science takes time, but it’s deeply rewarding—and it leads to more informed, effective ecosystem management.
Dogma Statement:
Fire breaks down organic matter, returning nutrients to the soil and promoting healthy plant growth. This nutrient recycling is essential for maintaining productive ecosystems.
This assumption isn’t proven. What we do know with certainty is that fire removes thatch, (aka the organic matter). The oft-repeated claim that fire contributes a nutrient “pulse” is murky at best. Nutrient cycling is the domain of soil microorganisms, not flames (Allen et al. 2002). Fire volatilizes nutrients held in vegetation, and smoke and wind often carry them off-site (Hobbs 2002, Marrs 2002, Davy 2002, Raison 1979).
When we dig deeper into the assumption that ash contains nutrients we find it is debated. Some say there is little evidence of this (Vogl 1974, Risser et al. 1981, Dhillion et al. 1987, Hulbert 1986). One researcher proposes ash works indirectly by increasing the microbial activity (Alcaniz et al. 2017). Some suggest that ash is rich in N and P (Montgomery and Bikle 2016, Pyne 1982) or supply specific minerals like magnesium (Certini 2005).
Researchers quantify ash based on whether it is grey (mostly combusted) or black (coarse and incompletely burned) yet concede that most nutrients are transported off the site in smoke or wind (Raison et al. 1985).
Gray vs. Black Ash: What It Tells Us
Gray ash:
- Indicates that combustion is more complete
- Results from high burn temperatures
- Sign of higher heat penetration into the soil with possible negative affects on seeds and microbes
- Nutrients are volatilized
Black ash:
- Combustion of organic matter is incomplete
- Occurs when moisture levels are high and fuels are dense
- Leaves nutrients in the form of organic matter in place
The Role of Microbes—And the Risks of Overdoing It
The few weeks of bare ground warmed by the sun stimulates the microbes to do their work. The flip side of this is if fire is too frequent, there is insufficient organic matter for microbes to work with, essentially starving them (Soong et al. 2015). Additionally, frequent fires change the vegetation, favoring grasses and reducing diversity. That’s a tradeoff that needs to be weighed carefully.
Practical Limits—and A Call for Caution
The only way to know with certainty, especially since the research has found varying results, is to test whatever remaining ash there is. Unless you’re involved in a research project, it’s not practical to analyze post-burn ash for nutrient content. Most landowners and managers won’t send in samples to a lab after every prescribed fire. Though costly and labor-intensive, this adds little practical value beyond satisfying curiosity and confirming whether the burn produced nutrient-rich ash. And none of this even touches on the different dynamics of burning logs or pile burns, which produce larger quantities of ash over longer combustion times.
The point of this discussion is to expand our understanding of fire’s role and learn the facts. We should not claim with certainty that burns recycle nutrients when the evidence is mixed and often contradictory. Being realistic about the purpose and value of burns is important. What prescribed fire does and doesn’t do helps us become better land stewards.
References
Alcañiz, M., L. Outeiro, M. Francos, and X. Úbeda. 2018. Effects of prescribed fires on soil properties: A review. Science of the Total Environment 613: 944-957.
Allen, Michael F., David A. Jasper, and John C. Zak. 2002. Micro-organisms. Pages 257-278 in Martin R. Perrow and Anthony J. Davy (eds), Handbook of Ecological Restoration. New York: Cambridge University Press.
Certini, Giacomo. 2006. Effects of fire on properties of forest soils: a review. Oecologia 143: 1-10.
Davy, Anthony J. 2002. Establishment and manipulation of plant populations and communities in terrestrial systems. Pages 223-241 in Martin R. Perrow and Anthony J. Davy (eds), Handbook of Ecological Restoration. New York: Cambridge University Press.
Dhillion, Shivcharn S., Roger C. Anderson, and Anthony E. Liberta. 1988. Effect of fire on the mycorrhizal ecology of little bluestem (Schizachyrium scoparium). Canadian Journal of Botany 66(4): 706-713.
Hobbs, Richard J. 2002. The ecological context: a landscape perspective. Pages 25-46 in Martin R. Perrow and Anthony J. Davy (eds), Handbook of Ecological Restoration. New York: Cambridge University Press.
Hulbert, L. C. 1986. Fire effects on tallgrass prairie. Pages 38-42 in G.K. Clambey. & Pemble, R. H. (eds), Proceedings Ninth North American Prairie Conference. Fargo, ND: Tri-College University Center for Environmental Studies, North Dakota State University.
Marrs, Robert H. 2002. Manipulating the chemical environment of the soil. Pages 155-183 in Martin R. Perrow and Anthony J. Davy (eds), Handbook of Ecological Restoration. New York: Cambridge University Press.
Montgomery, David R. and Ann Bikle. 2016. The hidden half of nature: The Microbial Roots of Life and Health. New York City: W.W. Norton and Co.
Pyne, Stephen J. 1982. Fire in America. Seattle and London: University of Washington Press.
Raison, Robert John. 1979. Modification of the soil environment by vegetation fires, with particular reference to nitrogen transformations: a review. Plant and soil 51: 73-108.
Raison, Robert John, P. K. Khanna, and P. V. Woods. 1985. Mechanisms of element transfer to the atmosphere during vegetation fires. Canadian Journal of Forest Research 15(1): 132-140.
Risser, P.G., E.C. Birney, H.D. Blocker, S.W. May, W.J. Parton, and J.A. Wiens. 1981. The True Prairie Ecosystem. Stroudsburg, PA: Hutchinson Ross Publishing Co.
Soong, Jennifer L. and Francesca Cotrufo. 2015. Annual burning of a tallgrass prairie inhibits C and N cycling in soil, increasing recalcitrant pyrogenic organic matter storage while reducing N availability. Global Change Biology 21: 2321-2333.
Vogl, Richard J. 1974. Effects of Fire on Grasslands. Pages 139-194 in T.T. Kozlowski and C.E. Ahlgren (eds), Fire and Ecosystems. New York: Academic Press.
After watching a movie called The Need to Grow, I learned of something called biochar. Years later, it came full circle to my need to grow native woodland spring ephemerals and other native species that need a rich layer of organic matter. Prairie burns with low-intensity fires naturally produce biochar. No wonder the richest soil on earth was the Midwest prairies of precolonization times. It’s now one of my solutions for the buckthorn in and around my property. It produces excellent biochar that can be charged and mixed into any soil to supercharge it.
Biochar has lots of benefits but isn’t widely used in restoration work…yet. I have experimented a bit with it.
Ash remaining on the top after a burn isn’t guaranteed. Studies that measure it show most is blown away.
I was at a burn yesterday, and it was very low intensity in most places. I saw a lot of pyrolized plant material left, which should end up in the soil as char.
Observation doesn’t guarantee process. We can’t assume the ash has become incorporated, especially under windy, dry conditions.