other Soil Properties 



The effects of the prescribed fire on other soil properties 

 were also monitored. Soil acidity decreased over one pH 

 unit in the O2 layer following the burn (fig. 2). We also 

 found decreased soil acidity in the mineral horizons but 

 to a lesser degree. At the end of the next growing season 

 the pH of the O2 horizon in the burned soil was still higher 

 than in the adjacent uncut stand. A sampling of the area 

 4 years after the burn showed that the pH of the O2 had not 

 yet returned to preburn status. In contrast, soil acidity in 

 the mineral horizons on the burned site decreased to 

 original values by the following fall. 



There was no apparent effect of the fire on organic 

 matter content of the surface mineral layer, which 

 averaged 4.2 percent before and after the fire. Fire did 

 not cause any appreciable development of water repellent 

 layers in the mineral soil. Prior to the fire the surface of 

 only one of the 30 mineral cores showed evidence of water- 

 repellent properties. After the burn nine of the cores gave 

 a positive water repellency test but only on the soil surface. 

 This development was temporary, since at the end of 6 

 weeks no water repellency was found. 



X 



Prescribed fire 

 September 8 



Cleorcut & burn 



Control 



SEPT. 



OCT. 



1 



DEC. 



FEB. 



APR. JUNE AUG. OCT. 

 1975 1976 



Figure 2. --Effects of a prescribed fire on pH in the humus layer (O2) of a 

 clearcut Douglas-fir/western larch stand. 



DISCUSSION 



Investigations concerned with the effects of fire on soil 

 N status have often been contradictory. Some have 

 reported losses of soil N, while others have indicated 

 either no change, or in a few cases, N gains after a fire 

 (Wells and others 1979). Many of these contradictions 

 undoubtedly come from differences inherent in study 

 design, sampling technique, and analytical methods. 

 Others come from actual site differences as reflected in 

 the type, amount, and condition of forest fuel present. 



The intensity and duration of a fire are important 

 variables that affect N losses (Knight 1966). The moist 

 fuel conditions on the Coram site prior to the fire resulted 

 in a generally "cool burn" (Artley and others 1978). Such 

 a fire would account for the rather small N losses from the 

 forest floor and the lack of organic matter change in the 

 mineral soil. These results contrast with those of DeByle 

 (1976), who found a significant reduction of organic 

 matter in the surface mineral layer and N losses exceeding 

 180 lb/acre (200 kg/ha) following a prescribed fire on a 

 similar clearcut Douglas-fir/western larch site. In this 

 instance, the fire was more intense and fuel consumption 

 more complete than occurred at the Coram site. The 

 relatively high proportion of N present in the surface 

 organic matter as compared to the mineral soil makes this 

 timber type highly susceptible to N losses during a fire. 

 Similar soil conditions exist in ponderosa pine stands in 

 the Southwest (Campbell and others 1977; Welch and 

 Klemmedson 1975). 



As noted earlier, N would be added to burned sites by 

 inputs from precipitation and biological N-fixation by soil 

 microorganisms. The N gains from both these sources 

 over a stand rotation of 100 to 150 years on the Coram 

 site would more than replenish the N losses due to this 

 fire, as well as the N removed in the timber harvest 

 (Jurgensen and others 1979; Stark 1979). Consequently, 

 no long-term site depletion of soil N is expected to result 

 from the effects of the prescribed burn. 



Increases in the concentration and total amounts of 

 available soil N were found after burning. Similar fire- 

 related gains in NH4 and/or NO3 content have also been 

 reported for other conifer sites in the Northern Rocky 

 Mountain Region (Hooker and Tisdale 1974; Orme and 

 Leege 1976; Skujins^). Higher NH4 levels result from an 

 immediate release of N when the organic matter is burned 

 followed by a partial mineralization of remaining organic 

 N by the soil microflora (Mroz and others 1980). Microbial 

 activity is stimulated by the release of available carbon 

 sources and mineral salts from the burned organic matter, 

 and by the decrease in soil acidity (Ahlgren 1974). 



In contrast to the NH4 release pattern, nitrification 

 showed a definite 3-week lag following the fire. The 

 initial low nitrification rates may be related to the inability 

 of the nitrifying microflora to compete with the other soil 

 microorganisms for available NH4 (Jones and Richards 



Skujins, J. 1977. Effect of modified slash disposal practices on the bio- 

 chemistry of soils. Final rep.. Study FS-INT-1203, 145 p. Intermt. For. 

 and Range Exp. Stn., Ogden, Utah, 



4 



