ing over 53 percent of the total N in the surface 12 inches 

 (30 cm) of soil prior to burning. Even after the fire, 49 

 percent of the N was still present in the organic layers. We 

 did not find a significant decrease in the total N concentra- 

 tion of the surface litter layer (0-| ) after the fire. We attrib- 

 uted this somewhat surprising result to the variable and 

 spotty nature of the burn. However, the largest N losses 

 occurred in the 0-| layer, which was caused by a 60 percent 

 reduction in average horizon thickness. The depth of the 

 humus horizon (O2) decreased by nearly 30 percent, butthe 

 N concentration of the remaining material increased 

 significantly. Consequently, only neligible amounts of 

 N were lost from this layer. No significant differences were 

 found in the decayed wood (O3) or in the mineral soil. In 

 total, slightly over 100 kg/ha or 6 percent of the N present 

 in the surface 1 2 inches (30 cm) of soil was lost as a result of 

 the prescribed fire. However, this value does not include 

 any N losses from the burning of larger woody slash. 



Table 1.— Nitrogen conient of soil organic layers as affected by prescribed fire 



Soil layer 





Before burn 





Two days after burn 



% N 



Lb N/acre 



kg N/ha 



% N 



Lb N/acre 



kg N/ha 



Litter (O-] ) 



1,19 



114 



128 



0.87NS 



36 



40 



Humus (O2) 



0.71 



302 



339 



.91* 



297 



333 



Decayed wood (O3) 



.46 



421 



472 



.69NS 



410 



460 



Mineral 















- 5 cm 



.12 



205 



230 



.12NS 



205 



230 



5 - 22 cm 



.08 



543 



609 



.08NS 



543 



609 



All layers 





1.586 



1 778 





1,492 



1 672 



NS = no significant difference p>0.05. 

 ' = significant difference 0.01 < p < 0.05. 



were determined by macro-Kjeldahl techniques with 

 salicylic acid added to include NO3 ( (Bremner 1965). 

 Organic matter content was estimated by weight loss-on- 

 ignition (Ball 1964). 



Statistical Analysis 



Data on nitrogen concentrations and populations of 

 autotrophic nitrifying bacteria were analyzed to detect 

 significant differences among horizons before and after 

 the prescribed broadcast fire. One-way and two-way 

 analysis of variance was used to analyze these data. 



RESULTS 



Total Nitrogen 



Since N is oxidized and/or volatilized as a result of fire, 

 considerable changes would be expected in soil N content 

 after the prescribed burn. Losses of N occurred only in 

 the soil organic layers (table 1). The organic fractions are 

 extremely important to the N economy of this site, contain- 



Available Nitrogen 



In contrast to total N contents, NH4 and NO3 concentra- 

 tions increased as a result of the fire. The levels of available 

 N present in the humus layer for 13 months after burning 

 are shown in figure 1. Similar changes in NH4 and NO3 

 concentrations were recorded for the soil mineral layers, 

 but the actual values were much lower than those found 

 in the organic fractions. A large increase in NH4 levels 

 was evident immediately after burning. No postfire change 

 was detected in NO3 values until nearly 3 weeks later when 

 rapid nitrification began. This rise in NO3 concentration 

 was followed by an equally rapid decline over the next 

 week. Leaching of NO3 due to 2.5 inches (6.3 cm) of rain 

 during that period probably caused this loss. In fact, the 

 6 weeks following the fire w6re especially wet, with the 

 area receiving 5.4 inches (13.5 cm) of rain. After this 

 decline, NO3 concentrations again increased substantially. 

 We found that the higher soil NO3 level after burning was 

 associated with a significant population increase of auto- 

 trophic nitrifying bacteria, both in the organic layers and 

 mineral soil (table 2). 



By the middle of October, the NH4 concentrations began 

 to decline and showed a steady decrease during the 

 winter and spring months until preburn levels were 

 reached by the end of the next summer (fig. 1). Nitrate 

 concentrations generally followed a similar pattern, but 

 a spring nitrification peak was evident on the burned site. 

 Such an increase in the nitrification rate may also have 

 occurred in the control soil but could have been obscured 

 by plant N uptake. 



When the NH4 and NO3 results were converted to total 

 amounts of available N present in the surface 12 inches 

 (30 cm) of soil (table 3), the pronounced effect of fire 

 on surface organic horizons was again evident. Available 

 N levels in these layers increased by fourfold within 2 

 days after the fire, followed by a gradual decrease 

 during the next 13 months. The mineral soil showed a 

 similar trend but attained the highest available N values 

 later in the fall, presumably due to leaching of NH4 and 

 NO3 from the organic horizons. 



2 



