EFFECTS OF A FIRE ON RUNOFF AND EROSION ON MEDITERRANEAN FOREST SOILS IN SE-SPAIN^

From 1985 to 1990, precipitation, runoff and soil erosion have been studied on experimental plots in a locality of Alicante (SE Spain). A prescribed burning was carried out in September of 1989, (maximum temperature was moderate). In order to know soil evolution, soil was sampled three times: before fire, one day after fire and six months later. One day after fire, a significant increase in organic matter content, total nitrogen, available phosphorus and the cations: K + , Mg 2+ and N + , was found. On the other hand, Ca 2+ and C.E.C. showed an opposite pattern. The modified values after fire tended to go bacic to the initial levels in the case of organic matter, phosphorus, Na + , Ca 2+ and C.E.C. Annual runoff after fire is significantly lower than in the year before fire, whilst average runoff in the year after fire is only significantly different when the most erosive year (October 1987-September 1988) is not considered. The runoff decrease will be related with a lower average precipitation after fire. There are no significant differences in the sediment yield between the year before and after the fire. The nutrient outputs and runoff decrease is greater than the nutrient inputs and precipitation decrease after fire. Nutrient output in runoff after fire ranges between 8 to 35 % of the previous year, whilst volume of runoff is only 3 %, implying a greater concentration.