Mobilization of nutrients by fire in a semiarid gorse‐scrubland ecosystem of Southern Spain

Abstract

The effect of fire on ash deposition, litter ignition, and short-term changes in soil nutrient availability and erosion losses was studied after prescribed burning of a semiarid, gorse-scrubland ecosystem of SE Spain. Ash deposition added important amounts of P and cations but negligible amounts of N to the soil. The water-soluble fraction was only 3% for P added in ash but represented 20-60% for cations (Ca, K, Na). On a weight per area basis, 63% of the litter mass and 43% of the litter total N pool were lost by ignition during the fire. However, no significant changes were found for litter P and cations. Litter C/N, C/P, and N/P ratios significantly decreased. A sharp increase in the concentrations of labile P fractions and exchangeable cations was found in surface soils immediately after the fire. However, this fertilization effect lasted only a few weeks at most. Soil total and mineral N showed no significant differences immediately after the fire. Following the first important rain, nitrate concentrations increased sharply from 5 to above 20 μg NO 3 --N g -1 throughout a 3-month sampling period; whereas NH 4 + concentrations decreased. Total erosion losses during that period were significantly higher in the burned and the clipped plots (734 and 845 kg ha -1 , respectively) than in the control plot (46 kg ha -1 ). Erosion debris leaving the burned plot showed the highest nutrient concentrations shortly after the fire, particularly for total and water-soluble P. Such high initial concentrations tended to decrease progressively until they ranked with those found in the clipped plot for equivalent sampling periods. Total nutrient erosion losses from the clipped and burned plots were up to 1 order of magnitude higher than those from the control plot. Although total mass losses were higher after clipping, erosion removed 20% more N and 60% more P from the burned than from the clipped plot, which relates to the higher nutrient concentrations in debris originated at the burned plot. Data strongly suggest the nonconservative character of fire as a mineralizing agent in these steep, semiarid, oligotrophic ecosystems, and point out the risk of soil fertility depletion and desertification that high fire frequency imposes onto these areas.

‘Formiguers’, a historical system of soil fertilization (and biochar production?)

“…But ashes cannot explain the increase in labile P in the outer layer of the soil, because this layer did not apparently mix with them, and can only partially explain the increase in the inner layer of the soil cover, because their amount was so small and because the water-soluble fraction may be only 3% of the total P added in the ashes (Carreira and Niell, 1995). The other process that may be involved in such increase in labile P would be the mineralization of organic P as a result of heating the soil in conditions similar to those of autoclaving (Romanyà et al, 1994;Badía and Martí, 2003a;Serrasolses et al, 2008).…”

“…This mixture would be the reason behind the increase in the total amount of P in the inner layer after burning the 'formiguer'. It would also account for the increase in exchangeable K because a significant fraction (20-60%) of the amount of this cation added in the ashes is in water-soluble forms (Carreira and Niell, 1995).…”

“…The latter three nutrients are not usually scarce in calcareous soils, but K is important in terms of controlling the loss of water by crops during dry periods. While the content of P in ashes may be high, a large proportion is in forms that are not readily available to plants (Carreira and Niell, 1995;Badía and Martí, 2003a). On the other hand, the application of plant ashes is beneficial for acid soils in order to increase the availability of basic cations and other nutrients such as P (Glaser et al, 2002).…”

Evolución de la composición florística Post-fuego en un carrascal de Navarra (n de España)

“…(OJEDA et al, 1996) mientras que otras la retardan como en Arhutus unedo y Myrtus communis (MAZ-ZOLENI, 1989;CANADELL et al, 1991). Por último, la intensidad también afecta al suelo, así a baja intensidad el suelo presenta más cantidad de materia orgánica que protege contra la erosión (TÁRREGA et al, 1996) (SINGH, 1993;TRABAUD, 1994;CARREIRA & NIELL, 1995) que favorecen la restauración de la vegetación (FERNÁNDEZ et ai, 1997).…”

“…Muchas especies perennes sobreviven por su capacidad de rebrote (MALLIK & GIMINGHAM, 1983); las plantas anuales y herbáceas aprovechan los cambios ambientales creados tras el fuego, como son el aumento de luminosidad (PONS, 1992), la creación de huecos vegetacionales (WILLSON, 1992), la ausencia de competencia (MALLIK & GIMINGHAM, 1983) y el aumento de la disponibilidad de nutrientes (SINGH, 1993;TRA-BAUD, 1994;CARREIRA & NIELL, 1995) para germinar y crecer más intensamente, incluso plantas que antes del incendio eran poco abundantes o estaban ausentes (TRABAUD et al, 1997).…”