Changes in organic matter,nitrogen, phosphorus and cations in soil as a result of fire and water erosion in a Mediterranean landscape

Fire affects large parts of the dry Mediterranean shrubland, resulting in erosion and losses of plant nutrients. We have attempted to measure these effects experimentally on a calcareous hillside representative of such shrubland. Experimental fires were made on plots (4 m × 20 m) in which the fuel was controlled to obtain two different fire intensities giving means of soil surface temperature of 439°C and 232°C with temperatures exceeding 100°C lasting for 36 min and 17 min. The immediate and subsequent changes induced by fire on the soil's organic matter content and other soil chemical properties were evaluated, together with the impact of water erosion. Seven erosive rain events, which occurred after the experimental fires (from August 1995 to December 1996), were selected, and on them runoff and sediment produced from each plot were measured. The sediments collected were weighed and analysed. Taking into account the variations induced by fire on the soil properties and their losses by water erosion, estimates of the net inputs and outputs of the soil system were made. Results show that the greatest losses of both soil and nutrients took place in the 4 months immediately after the fire. Plots affected by the most intense fire showed greater losses of soil (4077 kg ha^?1) than those with moderate fire intensity (3280 kg ha^?1). The unburned plots produced the least sediment (72.8 kg ha^?1). Organic matter and nutrient losses by water erosion were related to the degree of fire intensity. However, the largest losses of N‐NH₄⁺ and N‐NO₃^– by water erosion corresponded to the moderate fire (8.1 and 7.5 mg N m^?2, respectively).