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

Abstract

Summary 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).

Reduction of nutrient losses with eroded sediments by post-fire soil stabilisation techniques

“…As for C, stabilisation treatments had little effect on sediment N concentration, as also indicated Thomas et al (2000) for a litter treatment, lowest values being observed with mulching. Accumulated losses of N in seeding and control plots largely exceeded the usual range in burnt soils (14-57kgha~1 year -1 : Debano andConrad 1978: Helvey et al 1985;Soto et al 1997;Baird et al 1999;Thomas et al 1999;Gimeno-Garcia et al 2000;Lane et al 2008), being only below the highest values reported for high severity wildfires (100-129kg ha -1 : SotoefaZ. 1997; Spigel and Robichaud 2007); with mulching the mass of total N eroded was only half of the lowest values.…”

“…Stabilisation treatments had small effects on sediment C concentration, with the lowest values in mulching plots. Except for with mulching, the accumulated amount of C in the sediments was 5-fold those after high and moderate intensity experimental fires in eastern Spain (Gimeno-Garcia et al 2000) and it slightly exceeded the highest value (900kgha _1 ) observed the first year after high severity wildfires (Baird et al 1999;Spigel and Robichaud 2007). This result suggested that, whereas low intensity prescribed fires reduce nutrient losses by volatilisation relative to wildfires, they expose available nutrients to erosion (de Koff et al 2006).…”

“…Despite universal recognition of erosion as a prevalent post-fire process, most studies have focussed on nutrient losses in solution, with less emphasis on the importance of nutrients attached to sediments and, therefore, little is known concerning nutrient losses associated with the eroded material (de Koff et al 2006). Shortly after a fire, compared with soil surface, debris leaving a burnt area is enriched in organic matter, P, mineral N and exchangeable cations (Carreira and Niell 1995;Andreu et al 1996;Thomas et al 1999) and, consequently, nutrient losses with sediments can substantially exceed those lost in solution (Debano and Conrad 1978;Gimeno-Garcia et al 2000;Smith et al 2011). The increase in nutrients in eroded sediments has been related to fire intensity (Andreu et al 1996;Gimeno-Garcia et al 2000), but it was also observed after low intensity prescribed fires (de Koff et al 2006).…”

“…Post-fire erosion rates are highly variable (reported values range from 1 to 240 Mg ha" ; Robichaud et al 2006;Moody and Martin 2009) and are strongly related to the percentage of bare soil (Vega et al 2005); they generally decrease by an order of magnitude with each year of recovery (Robichaud et al 2000(Robichaud et al , 2006, leading to the greatest nutrient losses in the first 4-12 months after fire (Gimeno-Garcia et al 2000;de Koff et al 2006;Robichaud et al 2006). Therefore, post-fire soil stabilisation techniques (e.g.…”

Rainfall influence on plot-scale runoff and soil loss from repeated burning in a Mediterranean-shrub ecosystem, Valencia, Spain

“…The temperatures on the soil surface and their duration were measured in 1995, and also in 2003 fires by means of thermosensitive paints and thermocouples (Gimeno-García et al, 2007). In 1995, statistically significant differences, between T1 and T2, were observed on the average soil surface temperatures (439ºC and 232ºC, respectively) and on the average values of residence time in soil of temperatures greater than 100ºC (36 minutes in T1 and 17 minutes in T2) (Gimeno-García et al, 2000, 2004.…”