Variability and quantity of organic matter in multiples wildfires affect soil

Abstract

Wildfires represent perturbations that affect a big variety of ecosystems. However, due to the fact that during a forests fire fuels are not always distributed homogeneously within the affected area, a considerable variation of the quantity and quality has to be taken into account during sampling. In order to elucidate to which extend this affects a study of the alteration of soil organic matter (SOM) due to forest fire in the hilly area of the Sierra de Aznalcóllar, Andalusia, samples from several spots of various sites with different relief, fire history and vegetation were compared with respect to their C and N contents and the composition of their SOM. For the first study, five randomly chosen spots of five different sites including burnt and unburnt soils, either under pine or oak, were collected. In a second sampling, three places of a fire-unaffected area, and sites having experienced one and two vegetation fires were selected. In this approach composite samples were taken. Whereas the pH values showed low variability among the samples of the same site, the fireaffected soils which were only recently affected by fire, showed a slight but significant increase if compared to the soils having recovered for several years. This is best explained by the liming effect of charcoal input. The composite samples of the second study had average C contents between 5.5 and 5.7 % with standard deviations (SD) between 0.57 and 1.51. Higher SDs were obtained in the first study, confirming that taking composite samples can efficiently reduce variability. Solid-state 13C NMR spectroscopy of the soils of the first study revealed comparable composition of the SOM among the samples of each site but notable differences between those of different areas. The latter can be best explained by charcoal input into the soil at the site which was only recently affected by fire. This addition resulted in an increase of the aromaticity. However, with prolonged recovery time of the soils their aromatic C content decreased by approximately 50% but remained still slightly higher than that of the control soils. In our study difference in the vegetation did not significantly affect the SOM composition. A comparable pattern was observed for the soils of the second study.