Williams et al., 2012
RESEARCH BRIEF 11
Frequent prescribed fire (5 fires per decade) may reduce soil organic matter and carbon thus increasing bulk density and reducing soil porosity and water holding capacity
Burning frequencies of 2.5 fires per decade or greater reduced the nitrogen content of leaf litter
Fire frequencies less than 2.5 fires per decade had little measureable effect on the soil physical characteristics
Fire occurred frequently on landscapes in south-central North America for millennia. Fire suppresion associate with Euro-American settlement has caused an increased prevalence of fire-intolerant, non-oak species and closed forest canopies. In recent years prescribed fire has been increasingly employed to reverse these changes, increase plant diversity of these forests, and reduce fuel levels. Fire may affect forest litter and soil microbial communities. Long-term effects of repeated fires on soils have rarely been examined, though previous studies have shown that certain physical properties can be altered after burning (see Chapter 2 in DeBano et al. 2005). Due to the heterogeneous distribution of fuels and soil water content, studies often report mixed results regarding physical soil properties. After a fire, it is known that many nutrients, including some forms of nitrogen, become available to plants through ash. Soil microbial community dynamics can fluctuate depending on fire severity, soil water content, and fuel loading. In a study of an oak savanna that assessed short term effects of prescribed fire, gram-positive and negative bacteria were unaffected while fungi decreased (see Ponder et al.2009).
This study examined the effects of long-term periodic prescribed burning on the litter characteristics, physical soil properties, and soil microbial community in xeric oak forests of the Okmulgee Wildlife Management Area in eastern Oklahoma. Soil and litter measurements were made in areas that were subjected to prescribed burning at frequencies of 0 (unburned), 2.5, and 5 fires per decade (FPD) over the prior 20 years using low-intensity dormant season prescribed fires. Litter samples and soil cores were taken from five plots in each treatment type.
A decrease in litter nitrogen (N) by 20-21% was found in treatments of 2.5 and 5 FPD, with no changes in litter carbon (C) or lignin. Ratios of nitrogen to carbon and lignin were increased due to lower nitrogen levels in the litter. Soil pH, N, C:N, nitrate nitrogen (NO3-N), ammonia nitrogen (NH3-N), potassium (K), phosphorous (P), and soil water content did not differ between burned and unburned plots. In the 5 FPD treatment, soil organic matter decreased by 60%, organic carbon decreased by 65%, and bulk density increased by 20%. Mycorrhizal and saprophytic fungi, non-specific microbes, and grampositive bacteria did not differ between burned and unburned plots. Gram-negative bacteria decreased 20% in the 5 FPD treatment.
The authors stated that the observed reduction in litter N was likely due to oxidation loss from the litter and that this may lower decomposition rates and plant tissue quality. Similar to other studies, changes in litter N occurred independently of total soil N, which showed no change in burned areas. Soil organic matter (decaying biotic materials) and organic carbon (carbon stored in the soil) were significantly reduced in the 5 FPD treatment, adding to the varying results in the literature. Soil organic carbon levels are known to be reduced by fire in some studies but unchanged in others. Soil organic matter, in a study by Phillips et al. (2000), did not change with 2 fires per decade, yet it was reduced 31-41% with annual burning. The same study reported higher soil bulk densities when exposed to annual and biennial low intensity prescribed fires. Diminution of soil organic matter and carbon leads to a collapse in soil structure, which results in decreased soil porosity and water holding capacity, and a higher risk of erosion.
Soil microbial communities were affected by frequent burning but with no clear mechanism identified. The authors suggested that the decrease in gram-negative bacteria are a product of indirect effects of prescribed burning. Burning increased plant cover in treatment areas of 5 FPD and this may, in turn, affect nutrient and substrate availability for microbes.
The authors advised resource managers to consider these results and the potential consequences on the soil ecosystem when applying frequent fire. They indicated that xeric forests can sustain prescribed burning frequencies of up to 2.5 FPD without major changes in the soil properties they considered. They also suggest that future research should examine annual net primary production resulting from plant community changes and the subsequent effects on soil organic matter, organic carbon, and microbial communities.
Ryan J. Williams, Stephen W. Hallgren, and Gail W.T. Wilson. Forest Ecology and Management, 265 (2012): 241-247
FOR FURTHER READING
DeBano, L.F., Neary, D.G., Folliott, P.F., 2005. Soil physical properties, Chapter 2. In:Neary, D.G., Ryan, K.C., DeBano, L.F. (Eds.), Wildland Fire in Ecosystems: Effects of Fireon Soil and Water. United States Department of Agriculture, Forest Service, GeneralTechnical Report RMRS-GTR-42-vol. 4. Washington, D.C. pp. 29-52.
Phillips, D.H., J.E. Foss, E.R. Buckner, R.M. Evans, and E.A. FitzPatrick 2000. Responseof surface horizons in an oak forest to prescribed burning. Soil Science Society AmericaJournal., 64(2000):754-760.
Ponder, Jr., F., M. Tadros, and E.F. Loewenstein 2009. Microbial properties and litter and soil nutrients after two prescribed fires in developing savannas in an upland MissouriOzark Forest. Forest Ecology Management, 257(2009):755-763.
The Oak Woodlands and Forests Fire Consortium seeks to provide fire science to resource managers, landowners, and the public about the use, application, and effects of fire in the region. www.oakfirescience.com
This research brief was funded by The Joint Fire Science Program. http://www.firescience.gov/