Northern Prairie Wildlife Research Center
Volatilization sends carbon, hydrogen, and oxygen, (C, H, and 0) into the atmosphere, along with varying amounts of sulfur (S), and phosphorus (P) depending on the composition of the organic matter burned and the degree of combustion (Raison 1979).
Nutrients in mineral form are affected by the changing physical properties of soil particles due to heating and subsequent cooling. When micaceous minerals and clays dehydrate or fracture, the solubility of elements such as P and potassium (K) can increase or decrease (White et al 1973).
Chemical changes at mineral surfaces can be caused by alkaline or alkaline earth compounds from the heated minerals or by organic matter combustion. Solubility of P or K can increase or decrease depending on the chemical compounds formed when the material cools. Rapid heating and cooling may break a mineral apart as it expands or contracts. Fresh unweathered surfaces could release P and K more rapidly than weathered surfaces.
Amounts of N lost range from 30 to 33 lb/A (34 to 37 kg/ha) with 2,000 to 3,000 lb/A (2,240 to 3,360 kg/ha) of fuel (Sharrow and Wright 1977b). N decline has also been noted for litter, mor, and A- 1 horizons when temperatures exceeded 200 C (White et al 1973).
Although there is ample evidence that N in organic matter is volatilized, some authors report an increase in total soil N (which would include organic N, nitrate, and ammonia) after a fire (Vlamis and Gowans 1961; Vlamis et al 1955; White and Gartner 1975).
Nitrate levels have also risen after a fire (Kramer 1973; Christensen 1976; Sharrow and Wright 1977a; Worcester 1979). Schripsema (1977) found nitrate and ammonia declined in August following a winter burn; total N was also lower on a spring burn.
Researchers have seen an increase in ammonia after burning (White and Gartner 1975; Christensen 1976; Worcester 1979). Schripsema (1977) thought lower levels of ammonia and nitrate may have reflected increases in plant uptake.
The reported increases in all forms of N could be due to stimulation of legumes (Mayland 1967), the washing of charred surface material into the soil (Metz et al 1961), formation of ash which increases growth of nitrifying bacteria (Burns 1952), and increased growth of nitrogen-fixing microorganisms (Isaac and Hopkins 1937). Nitrifying bacteria are protected from heat and recover quickly to produce nitrates from organic matter (Sharrow and Wright 1977a).
Ammonia increases have also been attributed to increases in biological activity after heating (Walker and Thompson 1949; Jenkinson 1966; Simon-Sylvestre 1967). Ammonifying bacteria can withstand heat up to 212 F (100 C), while nitrifiers die at 127- 142 F (53-58 C) (Raison 1979). Certain forms of N increase or decrease, depending on fire intensity.
Heat also intensifies the physiochemical processes which lead to the decomposition of nitrogen-containing organic matter and to the release of ammonia from soil minerals (Arefyeva and Kolesnikov 1964). Ammonia loss peaks at 482-572 F (250-300 C), which might explain why ammonia could increase while organic N decreases as a result of volatilization at 392 F (200 C) (Raison 1979).
A guide to determine N loss is the appearance of the ash. Up to 392 F (200 C), material is charred. At 392-752 F (200-400 C), grayish ash skeleton becomes apparent. At 752-932 F (400-500 C), the litter and mor become grayish ash while the A-1 horizon becomes reddish or grayish (White et al 1973).
White and Gartner (1975) found an increase in available P only if temperatures did not exceed 392 F (200 C). They also speculated that, as in the case of ammonia, soil moisture and heat determine the extent of the increase in P availability.
Losses may be due to surface erosion (Wells et al 1979), movement below the root zone from leaching (Stark 1979), dilution effects of increased runoff (DeBano and Conrad 1978), and losses in fly ash (DeByle 1976).
These findings confirm that actual effects on soil nutrients at any given site will be variable depending on the condition of the vegetation, character of the soil and topography, and climatic factors (Vogl 1974).
Timing of the burn and pH level of the existing soil may be important. Vlamis et al (1955) found pH to rise on neutral but not acid soils. Owensby and Wyrill (1973) found a larger increase in pH from winter and midspring burning than after late spring burns. This rise in pH is because mineral substances are released as oxides or carbonates that usually have an alkaline reaction (Schripsema 1977). This is supported by others who have found that ash is dominated by carbonates of alkaline and alkaline earth metals (Youngberg 1953; Daubenmire 1968). Mayland (1967) found pH to be 0.5 higher, and Christensen (1976) found no change at all.
There is also the possibility of pH rising 0.5 to 0.4 but only persisting for 1 or 2 years (Wright and Bailey 1982).
Litter removal and a dark surface cause soil temperatures to increase (Sharrow and Wright 1977a). After a fire, higher temperatures shorten the oxidative process and are believed to be the main effect fire has on surface organic matter (Harvey et al 1976).
This coincides with Hulbert (1969), who stated that the major short-term effect of fire is the removal of litter instead of fire-induced nutrient changes. He found that burned and clipped plots responded in the same manner, which suggested that the removal of mulch explained renewed vigor in burned stands.
Increases in available nutrients have often, but not always, been attributed to ash accretion (Tyron 1948; Ahlgren and Ahlgren 1960; Smith 1970).
These factors, by themselves or acting together, determine availability of soil nutrients and plant benefits from fire. The interaction of these factors needs to be understood so that fire can be a better tool in grassland management for wildlife, livestock, and forage production.