RAMIRAN International ConferenceIn Portugal, as in other Mediterranean countries, fire events are common primarily due to climate and progressive
rural abandon, and contribute largely to desertification. Top layers of soil are constituted mostly by organic residues
and decomposed organic matter, and are the most sensitive layers to fire damages. Soil organic matter is an
important factor on physical, chemical and biological properties of soil. Quantity and duration of heat transfer and
soil thermal conductivity, which varies according to soil conditions (humidity, organic matter, texture, etc), are the
main responsible factors for the more or less physical, chemical and biological changes in soil after fire (Neary et
al,1999). A soil temperature of only forty degrees is enough to initiate biological changes that may begin with plant
and roots death caused by dehydration, and protein degradation. Microbial populations have different thermal
resistances and most are killed between 50 and 120 ºC. Organic matter (OM) losses may occur at low temperatures,
through the volatilization of volatile compounds which may occur between 100º and 180 ºC or OM distillation at
200 to 315 ºC (DeBanno et al, 1998). Under 200 ºC, decomposition of resistant compounds as hemicelullose and
lignin starts (Chandler et al, 1983, cit by González-Pérez et al, 2004). At 300 ºC, decarboxylation and loss of
Oxigen-containing functional groups (as phenols) occur in the Humic (HA) and Fulvic acids (FA) (Knoepp, 2005).
Humin content increases, due to formation of aromatic compounds, because the more soluble fraction, the FA
fraction, is transformed into insoluble acids like HA, and this last ones suffer dehydratation and decarboxylation,
becoming insoluble – Black Carbon – resembling the Humin fraction (González-Pérez et al, 2004, Tinoco et al,
2006, Hatten and Zabowski, 2009). Above 450 ºC, all OM is loss.
The main purpose of this work was to evaluate the effect of soil heating in forest soils, using a natural
burned (B) and an unburned (U) soils and thermally treated soils at different temperatures (65, 105 and 250ºC).
Further, we examined the benefic effects of application of organic residues to soils that had been exposed to fire, we
added digested pig slurry, municipal solid waste compost and a mixture of both, and evaluated organic matter
composition after 2 months of incubation
