Is Prescribed Fire a Friend or Foe to Soil Health?
On Southern Great Plains rangelands, there is arguably no management practice shrouded with more controversy than prescribed fire. For the most part, it's a love-hate relationship with most agricultural producers and land managers. From the soil health perspective, fire has received a negative reputation largely because of perceptions surrounding organic matter losses. So let's explore the five soil health building management principles and how prescribed fire may effect each of them.
Principle 1: Keep the Ground Covered
Keeping the ground covered is the cornerstone of soil-health-focused management plans on all land uses. The primary reason is to build residue/litter, or a layer of organic material, on the soil surface in an effort to regulate soil temperature. Another important function is to provide organic material that is eventually broken down into litter and cycled through soil organisms, ultimately increasing soil organic matter.
When applying prescribed fire, the primary concern from a soil health manager's perspective is the perceived loss of carbon/organic material via the combustion process. Research by Jim Ansley, Ph.D., provides some insight to how those carbon fluxes respond to prescribed fire on honey mesquite savannahs in the Rolling Plains of Texas (Ansley et al., 2002). Burning across multiple years and measuring carbon dioxide (CO2) fluctuations pre- and post-fire on burned and unburned sites, Ansley found that, on the burned sites, the carbon lost to the fire was recovered in approximately 28 days in above-average rainfall years. This carbon recovery was due to the influx of fresh herbaceous regrowth following the fire. Recovery in dry years took longer, approximately 82 days. Regardless, this suggests the carbon lost in the fire is completely recovered by the end of the first growing season following the burn in both above- and below-average rainfall years (Ansley et al., 2002).
Principle 2: Minimize Disturbance
Minimizing disturbance is another key principle within soil health management systems. This principle largely refers to soil disturbance such as tillage. However, above-ground management practices are also disturbances that can have effects on soils. Plants that grow on a site effect soil properties. These effects can be positive and negative based on the management goal.
Prescribed fire is most certainly a disturbance on the rangeland landscape. Fire is often used to disturb plant communities as a primary objective, especially when meeting many wildlife management goals. For instance, a common wildlife management objective in the Southern Great Plains is to apply prescribed fire in the fall to promote forb growth in the spring. These disturbances are mitigated by managing fire frequency (how often a pasture is burned) and the fire return interval (how long since the last burn). Properly utilizing prescribed fire might not act to immediately minimize disturbance, but it can optimize the effects of disturbance and provide positive management results.
Healthy native range includes a diverse mix of plant species, as demonstrated on rangeland at the Noble Research Institute Coffey Ranch.
Principle 3: Increase Diversity
Increasing diversity in soil health management systems can mean many things. However, it is largely agreed that plant species diversity is the primary driver here. Monocultures are never found in nature. Diverse plant communities provide heterogeneous landscapes that benefit wildlife habitat, nutrient cycling, etc. Plant species diversity additionally provides many below-ground benefits that attract diverse populations of soil micro- and macro-organisms because of their diversity of rooting structure and exudates produced. Plant species diversity is often managed on rangelands with prescribed fire. If properly applied, timing, intensity and frequency of prescribed fire can all have significant positive effects on plant species diversity.
Principle 4: Keep a Live Root in the Ground Year-round
This is an important soil health function because soil biology (primarily bacteria) utilize exudates from growing roots as a primary food source. It is imperative in a soil management system to keep the biology active as long as possible. This principle correlates very closely with plant diversity because keeping a live root in the ground is often accomplished by promoting practices that encourage warm- and cool-season grasses and forbs. Diversity and root availability are further enhanced by promoting annuals and perennials with varied rooting architecture (i.e., fibrous roots vs. tap roots). Prescribed fire can be an effective tool to promote living plants year-round on rangeland by altering application timing, frequency and intensity.
Principle 5: Integrate Livestock
The final soil health building principle is livestock integration. On Southern Great Plains rangelands, no other two practices go together in a more cohesive union than prescribed fire and grazing management. The proper application of these two processes are responsible for molding the region's plant communities, building ecological health and sustaining ranch livelihoods for generations. However, fire suppression and overgrazing can have compounding negative effects on rangeland productivity, function and soil health. With that said, these processes must be applied properly together or negative impacts can occur.
Prescribed fire in the Southern Great Plains is more than a management practice, it is an ecological process that is needed at some frequency for proper rangeland function. Without fire on rangelands, many of the soil health building principles are much harder to achieve.
Ansley, R.J., Dugas, W.A., Heuer, M.L., Kramp, B.A., Bowen Ratio/Energy Balance and Scaled Leaf Measurements of CO2 Flux over Burned Prosopis Savannah. Ecological Applications 12(4). 2002. Pp. 948-961.