Impacts of fire, fire-fighting chemicals and post-fire stabilisation techniques on the soil-plant system

Resumen

Nowadays fires pose a serious threat for many ecosystems as they have significant ecological and economic impacts. Despite the potential ecological impacts of fire-fighting chemicals (FFCs), in Galicia their use has increased in the last decades. Deleterious effects of FFCs on water bodies are well known, whereas the impacts of these chemicals on soils and terrestrial plants have not been so thoroughly studied. Since recently, effective post-fire rehabilitation measures to prevent erosion are implemented in areas that have been severely affected by fires. In Galicia, straw mulching in steep burnt areas effectively reduces soil losses through erosion, although there is almost no information on the effects that straw addition can have on other important ecosystem processes such as biogeochemical cycling. In order to effectively extinguish fires without aggravating their consequences and to protect burnt ecosystems from further degradation, it is paramount to assess when and where these measures should be applied and to optimize them in terms of resources and funds invested. Therefore, the main objectives and results of this thesis can be summarized as follows. To assess how fire and three FFCs affect the soil-plant system in the long term (10 years). Burnt plots with ammonium polyphosphate (BS+Ap) had the highest levels of soil available P and plant total P in all studied plants. Pines in unburnt plots were higher and wider than in burnt treatments, except for BS+Ap, where the tallest and widest trees were found, although half of them were either death (the second highest mortality after burnt plots with Firesorb, BS+Fi) or had a distorted trunk. BS+Ap was the treatment with strongest effects on plant height and cover and was the only treatment where Genista triacanthos was absent. Consequently, it is concluded that both fire and ammonium polyphosphate application had significant effects on the soil-plant system 10 years after burning. To optimize straw mulch application, so it is economically viable, and to assess the effects of its application on soil nutrient dynamics. More specifically, the aim was to evaluate the effects of straw mulching on some soil and sediment properties; and to assess the efficiency of two straw mulching application strategies in reducing nutrient losses in eroded sediments. Soil and sediment samples changed with time after the fire regardless treatment, decreasing progressively the differences between successive sampling dates. During the first post-fire year, less than 500 kg ha-1 of sediments were eroded and mulching had no effect on the total mass of lost sediments and nutrients. We conclude that the erosion rate was rather low that year due to moderate precipitation rates and therefore mulching did not significantly reduce soil erosion. Nevertheless, the concentration of Mo, Mn and Zn in sediments exceeded reference levels for ecosystem protection and can lead to deficiency problems for on-site vegetation and to soil and water pollution off-site. To evaluate the effects of fire and straw mulching on soil gross N transformation rates. By means of two 15N tracing experiments, gross N transformation rates were quantified (Ntrace model) to assess the effects of different burning severities and straw mulching on the N cycle. In the first experiment soils were preincubated under optimal temperature and humidity during 3 and 6 months to accelerate the evolution of the soil-straw mixture. Burning opened up the N cycle and NO3- accumulated, increasing the potential for ecosystem N losses. In the short term, straw mulching slightly mitigated the effects of fire on the N cycle and no effects of straw were recorded in the longer term. The second experiment included soils from both medium and highly burnt spots which were preincubated for 3 and 6 months, but outdoors so weather conditions were as close as possible to those in the field. Fire severity impacts on SOM quantity and quality and on soil microbial communities greatly affected the MIT, and increased MIT combined with high soil moisture probably limited autotrophic nitrification and promoted DNRA and denitrification. For the first time, DNRA was reported in burnt soils and it was of similar magnitude to the values reported for the unburnt control. The partitioning of NO3- reduction between DNRA and denitrification in burnt soils might have implications on N retention within the soil system and for greenhouse gas emissions. Straw mulching, by the extra supply of organic matter in the short term and by facilitating plant growth in the longer term, had contrasting consequences (t=3 vs t=6 months) on the N cycle of moderately burnt soils.