The effects of fire on soil physical and chemical properties are highly dependent upon the severity of the fire. Low-severity broadcast-burn prescribed fires often have minimal negative impacts if the fuel loads are light and duration is short whereas high severity wild fires can result in substantial losses of soil organic matter and nitrogen and losses of fertility. Regardless of severity, volatile elements such as carbon, nitrogen and sulfur (C, N, and S, respectfully) are readily released to gaseous phases even at low burn temperatures and fire always results in some loss of at least C and N from the ecosystem (Hosking, 1938; Knight, 1966; Raison et al., 1985; Tiedemann, 1987; Caldwell et al., 2002). Other elements such as phosphorus, potassium, calcium and magnesium (P, K, Ca, and Mg, respectfully), require much greater burn temperatures to volatilize and either remain on site in the form of ash or are transported by post-fire erosional processes (Malmer, 1996; Raison et al., 1985; Weast, 1980).
Even though there is usually a net loss of total N by volatilization during fire, soil available (inorganic) often increase following moderate fires because of heat-induced decomposition of organic N in the soil. With severe fire, however, both mineral N and organic N (as well as organic C) are reduced in the mineral soil by volatilization.
In our studies of fire effects in the Sierra Nevada mountains, we have found that prescribed fires in volcanic (andic) soils have had little or no effect on soil chemistry or water quality. We have concluded from these studies that the main effect of prescribed fire is on the volatile loss of N from the burned forest floor.
On the other hand, we found substantial increases in nitrate, ammonium, and especially sulfate leaching and significant losses of total N from the soil following a wildfire that coincidentally burned through some of our prescribed fire research plots on a site with granitic soils.
This raises the question as to what degree the contrasting responses in the prescribed fire and wildfire studies are due to differences in fire severity as opposed to differences in soil parent material, the latter of which are quite considerable. The andic soils are much finer textured, higher in organic matter and N content, and have < 5% of the extractable P content that is typical of granitic soils. In order to sort out the effects of fire severity vs. parent material effects on these responses, we would ideally conduct studies of prescribed fire on a granitic soil and wildfire studies on andic soils.
We now have an opportunity to partially fill these gaps: a program of prescribed fire, including slash pile burning, has been initiated in Little Valley, a 1200 ha parcel of university-owned property just east of Lake Tahoe in Nevada dominated by granitic soils. Slash burning commenced in Jeffrey pine (Pinus jeffreyi) and lodgepole pine (Pinus contorta) forests of the valley in the autumn of 2006, and is projected to continue annually for at least the next decade.
Of special interest to us is the fact that this program of prescribed fire includes slash pile burning, which will cause large local variations in fire severity. Severe fire under slash piles can cause significant changes in soil chemistry and water quality, losses of soil organic matter and N and perhaps even changes in soil mineralogy (Ulery et al., 1996).
We have observed severe effects of slash pile burning in some areas of the Sierra Nevada, where former slash piles are devoid of all vegetation and soils take on a reddish hue suggestive of low organic matter and mineral alteration. While slash piles occupy only a small proportion of land area, cumulative watershed effects could be significant over the landscape. This is obviously a situation that we want to avoid, and the proposed research here aims to determine optimal slash pile sizes so as to avoid these undesirable consequences.
In conjunction with the studies on prescribed fire, we also are following up on one of the most interesting aspects of our recent research: the identification of nutrient-rich waters flowing on top of the mineral soil through the litter layer. These waters often have extremely high concentrations of mineral N and P, with or without the effects of fire, and may be vitally important to changes in the quality of sensitive surface waters such as Lake Tahoe.
As with the other responses, we found much greater increases in mineral N and P in runoff from the wildfire on granitic soil than from the prescribed fires in andic soils. Thus we also lack data on the effects of prescribed fire on overland flow from granitic soils, and we plan to measure runoff in the treatments proposed below.
We are also determining the degree to which this overland/litter interflow actually discharges directly into down-gradient tributaries as opposed to preferentially infiltrating. We have long been aware of sporadic but widely distributed areas of preferential infiltration in upland Sierran watersheds (Burcar et al., 1994) and the potential for such infiltration creating “hot spots” and “hot moments” of nutrient availability over the landscape, as have been noticed in other systems.
By focusing on nutrient delivery via specific hydrologic flow paths (e.g., preferential infiltration of nutrient laden overland flow), we can develop a better spatio-temporal characterization for the application of nutrient transport models following biomass management activities such as prescribed fire.