Wildfire effects on carbon and nitrogen in inland coniferous forests

A ponderosa pine/Douglas-fir forest (Pinus ponderosa Dougl., Pseudotsuga me nziesii (Mirb.) Franco; PP/DF) and a lodgepole pine/Engelmann spruce forest (Pinus contorta Loud., Picea engelmannii Parry ex Engelm.; LP/ES) located on the eastern slopes of the Cascade Mountains in Washington state, USA, we re examined following severe wildfire to compare total soil carbon and nitr ogen capitals with unburned (control) forests. One year after fire, the ave rage C content (60 cm depth) of PP/DF and LP/ES soil was 30% (25 Mg ha(-1)) and 10% (7 Mg ha(-1)) lower than control soil. Average N content on the bu rned PP/DF and LP/ES plots was 46% (3.0 Mg ha(-1)) and 13% (0.4 Mg ha(-1)) lower than control soil. The reduction in C and N in the PP/DF soil was lar gely the result of lower nutrient capitals in the burned Bw horizons (12-60 cm depth) relative to control plots. It is unlikely that the 1994 fire sub stantially affected nutrient capitals in the Bw horizons; however, natural variability or past fire history could be responsible for the varied nutrie nt capitals observed in the subsurface soils. Surface erosion (sheet plus r ill) removed between 15 and 18 Mg ha(-1) of soil from the burned plots. Nut rient losses through surface erosion were 280 kg C ha(-1) and 14 kg N ha(-1 ) in the PP/DF, whereas LP/ES losses were 640 and 22 kg ha(-1) for C and N, respectively. In both forests, surface erosion of C and N was similar to 1 % to 2% of the A-horizon capital of these elements in unburned soil. A bioa ssay (with lettuce as an indicator plant) was used to compare soils from lo w-, moderate- and high-severity burn areas relative to control soil. In bot h forests, low-severity fire increased lettuce yield by 70-100% of controls . With more severe fire, yield decreased in the LP/ES relative to the low-i ntensity burn soil; however, only in the high-severity treatment was yield reduced (14%) from the control. Moderate- and high-severity burn areas in t he PP/DF were fertilized with similar to 56 kg ha(-1) of N four months prio r to soil sampling. In these soils, yield was 70-80% greater than the contr ol. These results suggest that short-term site productivity can be stimulat ed by low-severity fire, but unaffected or reduced by more severe fire in t he types of forests studied. Post-fire fertilization with N could increase soil productivity where other environmental factors do not limit growth.