Fire Effects on Soil Nutrients

This discussion of fire effects on soil nutrients will focus on the elements considered essential for plant growth and nutrition needed in relatively large quantities known as macronutrients (Table:Plant Essential Macronutrients). These are the nutrients most likely to impact site productivity and vegetation dynamics and therefore of most interest to forest managers and ecologists.

Nutrient Pool Sizes in Forest Floor and Mineral Soils

Forest fires whether planned or not usually decrease the total site nutrient pool (the total amount of nutrients present) through some combination of oxidation, volatilization, ash transport, leaching, and erosion. For example, a low intensity slash fire resulted in the following reductions in understory and forest floor fuel nutrient pools: 54-75% of N, 37–50% of P, 43–66% of K, 31–34% of Ca, 25–49% of Mg, 25–43% Mn (micronutrient), and 35–54% of B (micronutrient) (Raison et al., 1985). Volatilization and oxidation were the mechanisms responsible for the observed nutrient loss. While the reduction in site nutrient capital intuitively seems detrimental to forest productivity, and it can be, it is important to remember that the amount of nutrients found on site and nutrient availability are not always tightly linked. This is especially true when soil nutrients are considered. In fact, though fire can diminish nutrient pool sizes, nutrient availability often increases, and the pools that are affected most by fire (e.g. fuels) are often insignificant when compared to other nutrient pools such as mineral soils (See Figure). For example, soil fertility can increase after low intensity fires since fire chemically converts nutrients bound in dead plant tissues and the soil surface to more available forms or the fire indirectly increases mineralization rates through its impacts on soil microorganisms (Schoch and Binkley 1986). McKee (1982) found that in unburned Coastal Plain soils, calcium bound to the well developed O horizons in unburned stands might eventually become limiting.

Also important to consider is that some nutrients dynamics are more sensitive to fires than others. The concentration of potassium, calcium, and magnesium ions in the soil can increase or be unaffected by fires whereas nitrogen and sulphur often decrease (Hough 1981). Although the relationship between fire and soil nutrients is complex due to the interactions among many factors, fire intensity is usually the most critical factor driving post-fire nutrient dynamics, with greater nutrient losses occurring with higher fire intensity. Fire intensity has both direct and indirect impacts on many of the mechanisms that affect nutrient pools and cycling. Fire temperature directly determines both the amount and kinds of nutrients that will be volatilized (See diagram: Temperature effects on soil). Consider two elements, N and Ca: N begins volatilizing out of organic matter at only 200º C, whereas Ca must be heated to 1240º C for vaporization to occur (Neary et al. 1999). Nutrients are abundant in surficial organic soil layers, and the amount of these layers consumed is proportional to fire intensity. As an indirect effect, the physical transport of nutrients off site is related to fire intensity. Convective transport of ash varies from 1% in low intensity fires to 11% in high intensity fires (Neary et al. 1999). High intensity fires can also change the physical characteristics of the soil making it more susceptible to nutrient loss through erosion (McColl an Grigal 1977), though in much of the Coastal Plain, the modest topography makes erosion less likely than in Piedmont or mountainous terrain.

The impact of fire on site productivity is also related to intensity. While high intensity fires (which are also more likely to be high severity) tend to decrease site productivity, low intensity fires can increase site productivity (Carter and Foster 2003). In a study of low intensity prescribed fire in loblolly pine stands, Binkley et al. (1992) found that nearly all the fire effects were limited to the forest floor (O horizons) and that the effects were weak; when compared to an unburned stand, nutrient pools in frequently burned stands were unaffected (P, Mg, K, S), increased slightly (Ca), or decreased (N, S). Though the authors found that the N pool decreased in the O horizon, they observed that site productivity was unaffected, possibly due increased mineralization rates in other soil horizons. In a meta-analysis of fire effects on N (a statistical technique that allows an objective synthesis of previous studies) Wan et al. (2001) found that the N pool in fuels is decreased, soil total N pools were unaffected and ammonium and nitrate levels in the soil increased which increased N availability. Reports on the effects of fire on soil N pools have been controversial, both due to the importance of N as it affects site productivity and because of its complicated response.

Further complicating the picture are the interactions among time since fire, vegetation type, sampling techniques and fire intensity. Wan et al. (2001) found that the dynamics of N and fire vary considerably among ecosystems; higher N losses occurred in broad-leaved forests than in coniferous forests. They also found that the magnitude of the effects of fire depended both on how long ago the fire occurred and how the soil was sampled. Even so, the authors argue that the mineral soil N pool is so large (and relatively unaffected by fire) when compared to the organic soil and fuel N pools, that the N lost by consumption is insignificant.

Although the fire-soil nutrient relationship is complicated, some generalities do emerge. Fires typically result in the reduction of fuel and organic soil nutrient pool sizes, increase soil nutrient turnover rates, and redistribute nutrients through the soil profile (Fisher and Binkley 2000). Fire intensity will most likely determine post-fire soil nutrient dynamics. High intensity fires usually decrease nutrient pools more than low intensity fires and can have many other post-fire impacts that lower site productivity. Nutrients pools in the organic soil horizons are more likely to be impacted by fires than those in the mineral horizons. N and S in these pools are particularly sensitive to fires, and tend to diminish when organic soil horizons are consumed regardless of fire intensity, but mineral N concentrations tend to increase and become more available in the soil surface after burning (Wan et al. 2001). Pools of P, K, Mn, Mg, and Ca are generally not as likely to be impacted by low intensity fires, but can be lost after high intensity.