Fire severity effects on soil Carbon and Nitrogen stocks in southern Iberian Peninsula: comparing prescribed and wildfires

It is accepted that prescribed fires generally led to low losses of the organic and mineral superficial layers,especially as compared to the moderate or high intensity burning of wildfires. Consequently, prescribed fires have been largely used to reduce wildfire risk or change the land use. Still, the effects of the fire in the soil C and N pools are not totally understood. The main objective of this research was to assess the effects of prescribed versus wildfires on the soil C and N stocks of Mediterranean forest and shrublands. Additionally, the relation of proxy visual indicators, such as the Soil Burn Severity (SBS) with the maximum temperature reached at different depths was also assessed in the prescribed fires. Pre-/unburned and post-fire/burned organic (L, FH) and mineral soil samples (0-2, 2-5, 5-10 cm) were gathered from three prescribed fires and four wildfires. The three prescribed fires burned at low SBS (Cartaya: 2.2, Doñana20:1.9 and Doñana21: 0.7), two wildfires burned at moderate SBS (Casares: 3.2 and Bermeja-L: 2.7) and twoat high SBS (Monchique: 4.4, Bermeja-H: 3.9). Pre-fire C stocks ranged from 70-15 Mg ha-1 in theprescribed fire sites; and from 60-30 Mg ha-1 in the wildfire sites. All fires dropped the C and N contentson the organic layers, and the effect consistently decreased with increasing the mineral soil depth. The consumption of the C stocks was, on average, 50, 34 and 14%, respectively for Cartaya, Doñana20 and DOñana21 prescribed fires; while in the moderate wildfires it was 26% and 12% (Casares and Bermeja-L) and 38-39% for the high severity wildfires. The consumption of the N stocks on the prescribed sites was high (37 to 13%) if compared to the moderate wildfires (8-2%) but similar to the 35-27% consumption in the high severity wildfires. The pre-fire soil C and N stocks, as well as the conditions at which prescribed fires were carried out should be re-evaluated to prevent high consumptions of the soil organic matter