Wildfires in NW Patagonia: long-term effects on a Nothofagus forest soil

In NW Patagonia, Argentina, poor regeneration after fires of Nothofagus pumilio (Poepp. and Endl.) Krasser forests located in drier environments has been attributed to probable edaphic changes. We studied the long-term effects of wildfire on the mineralogical, physical, chemical and biological properties of a soil developed from volcanic-ashes under N. pumilio forests. Soils from six small patches burned in January 1996 and of undisturbed forest were sampled at a depth of 0–10 cm in March 1996, 1997, 1998 and 2000. As biological soil indicators we assessed N in microbial biomass (N-MB) and potential N mineralization (pNmin). Chemical properties were analyzed for all 4 years in dry samples, N-BM and pNmin in rewetted samples in 1996 and 1997, and in field-moist samples in 1998 (pNmin) and 2000 (N-MB). Additionally, we measured soil moisture twice during the growing seasons of 1998 and 2000 at 0–16 cm, and mineralogical and physical properties once in 1998. The main effects of fire were: (i) a significant increase in pH, electrical conductivity, extractable P and cations (Ca, Mg, Na, K) and a significant decrease in organic C and total N; 4 years after the fire, C and N were still, respectively, 52 and 20% lower, pH was one unit higher, and electrical conductivity and extractable P were twice as high as in the unburned control; (ii) a considerable decrease in N-MB (>90%), without significant recovery in subsequent years; (iii) an increase of pNmin at the beginning of the incubation period, decreasing afterwards to only 4–44% the levels in the unburned soil; and (iv) a decrease of 31% in field capacity and 56% in soil moisture. No mineralogical changes in the amorphous soil components were observed. Although volcanic soils show a high capacity to stabilize organic matter, buffer pH, retain P and store water, the magnitude of the changes of all soil properties indicated that the intensity of the fire was very high, and might have a powerful effect on seedling emergence and survival.