Bio-geochemical markers surrogated to fire induced hydrophobicity. Model system: Doñana National Park sandy soils

Abstract

[EN]: Forest fires are a major factor of disturbance in many terrestrial ecosystems, especially in European areas under Mediterranean type of climate. This is due to the confluence of specific climatic, ecological and socio-economic conditions. Fire produces important changes in soil organic matter (SOM) both qualitatively and quantitatively, which, in turn, affect relevant physical, chemical and biological properties of the soil. These changes also affect a large number of related biotic and abiotic factors and processes. The main objective of this PhD Thesis is to deepen the knowledge of the impact of forest fires on SOM in relation to changes in soil water repellency, using advanced techniques of molecular analysis. Due to the high number of variables that may influence soil water repellency and the chemical alteration of organic matter after fire, we chose the sandy soils of the Doñana National Park for this study, with well-known and relatively simple composition. In any case, we have followed the classical scheme of comparison of burnt soils with unaffected soils (control), under the same geomorphological and climatic characteristics. Water repellency is one of the main edaphic properties affected by forest fires. This physical property reduces the affinity of soil for water, which carries important hydrological, geomorphological and ecological consequences. Fire-induced changes of soil water repellency can be due to numerous factors, although it is generally accepted that the alteration of SOM and, in particular, of its more labile fraction (lipid fraction) is the main variable involved in this process. Due to their environmental implications, there are currently countless works on the effects of fire on SOM and water repellency, which are reviewed in chapter 1. However, the current state of knowledge shows some gaps and aspects insufficiently studied, due either to the complexity of the soil system or to the lack of adequate analytical techniques. Numerous scientific studies indicate that both the type of vegetation and the chemical composition of SOM strongly influence soil water repellency. These studies have focused mainly on the study of complete soils both at the surface and at different depths. However, available information on the influence of organic matter and vegetation on the degree of hydrophobicity of different soil physical fractions is limited. This aspect is studied in detail in chapter 3. In particular, the relation between soils under different vegetation cover, dominated by cork oaks (Quercus suber), eagle fern (Pteridium aquilinum), pine (Pinus pinea) and rockrose (Halimium halimifolium), the amount and quality of organic matter, and water repellency in four particle sieve fractions (1-2, 0.25-1, 0.05-0.25 and <0.05 mm). We observed that the degree of water repellency was significantly different, both among soils under different vegetation cover and among different sieve fractions, with soils under cork oak showing the highest severity of water repellency. In addition, we found a clear relation between the amount of SOM and the degree of water repellency. On the other hand, the molecular analysis of the organic matter from sieve fractions by analytical pyrolysis techniques let us find a relation between the quality of MOS and soil water repellency, the presence of long-chained fatty acids and the degree of humification (evolution) of SOM in the different fractions. drophobicity in sandy soils.

The impact of fire on water repellency and SOM was studied especially in soils under cork oaks, due to the greater organic contribution of this type of vegetation, the severe soil water repellency and its pyrophilic character. For the most detailed study, the number of studied sieve fractions was expanded to 6 (1-2, 0.5-1, 0.25-0.5, 0.1-0.25, 0.05-0.1 and <0.05 mm), including also the complete sample. It is known that impacts caused by forest fires on soils are related to changes in SOM. Fire favors the modification or formation of new chemical structures, besides contributing to mass outputs and inputs, such as the contribution of fresh biomass or more or less carbonized residues. This idea has predominated in the focus of a great number of research works, which have aimed to the study of complete soils or some of their horizons. However, the knowledge about the effect of fire on soil granulometric fractions is little known and that is why we consider some relevant questions, such as i) does the chemical composition of organic matter from different sieve fractions vary?, ii) does fire cause the same impact on all fractions?, or iii) what chemical reactions does fire induce in the different particle size fractions? In chapters 4, 5 and 6, we try to give answers to these questions, by means of detailed studies of the molecular composition of the organic matter present in the different fractions. With this aim, we used advanced analytical techniques such as mass spectrometry of isotopic relations of carbon and hydrogen (13C and 2H, respectively) (chapter 4), analytical pyrolysis (chapter 5) and ultra-high resolution mass spectrometry (chapter 6). The study of the isotopic composition of 13C confirms the existence of two compartments of organic matter with different degrees of evolution. The larger sieve fractions contain slightly evolved organic matter, impoverished in 13C and with δ13C values not different from leaf biomass, while finer fractions showed a more evolved organic material, enriched in 13C. Fire produced no changes in this trend, although an increase in the 13C content was observed in all affected fractions. This process may be explained by selective removal of light compounds (lower 13C content) or incorporation of charred residues. The study of the isotopic composition of 2H showed the existence of two differentiated water compartments in the upper centimeters of soil and dependent on the size of particles. No homogeneous behavior of the 2H composition after the fire was observed...

[ES]: Los incendios forestales son un factor de perturbación importante en muchos ecosistemas terrestres, especialmente en las zonas europeas de clima Mediterráneo. Esto se debe a la confluencia de condiciones climáticas, ecológicas y socio-económicas específicas. El fuego produce cambios importantes en la materia orgánica del suelo (MOS) tanto a nivel cualitativo como cuantitativo, que afectan, a su vez, a propiedades físicas, químicas y biológicas relevantes del suelo. Estos cambios afectan también a un gran número de factores y procesos bióticos y abióticos relacionados entre sí. El principal objetivo de esta Tesis Doctoral es el de profundizar en el conocimiento del impacto de los incendios forestales sobre la MOS en relación con los cambios en la repelencia al agua del suelo, empleando para ello técnicas avanzadas de análisis molecular. Debido al elevado número de variables que pueden influir en la repelencia al agua del suelo y la alteración química de la materia orgánica tras el fuego, para este estudio se eligieron los suelos arenosos del Parque Nacional de Doñana, suelos bien conocidos y poco complejos. En todo caso, para conocer el efecto del fuego, se ha seguido el esquema clásico de comparación de suelos quemados con suelos no afectados (control), de idénticas características geomorfológicas y climáticas. La repelencia al agua es una de las principales propiedades edáficas afectadas por los incendios forestales. Esta propiedad física reduce la afinidad del suelo por el agua, lo que conlleva importantes consecuencias hidrológicas, geomorfológicas y ecológicas en los ecosistemas terrestres. Los cambios inducidos por el fuego sobre esta propiedad pueden deberse a numerosos factores, aunque se admite que la principal variable implicada es la alteración que produce en la MOS, y en particular en su fracción más lábil (fracción lipídica) que tiene un marcado carácter hidrofóbico. Debido a sus implicaciones medioambientales, actualmente existen innumerables trabajos sobre los efectos del fuego en la MOS y la repelencia al agua, que se revisan en el capítulo 1. Sin embargo, el estado actual del conocimiento muestra algunas lagunas y aspectos insuficientemente estudiados, bien debido a la complejidad inherente al sistema suelo o bien por la falta de técnicas analíticas adecuadas.

Numerosos trabajos científicos indican que tanto el tipo de vegetación, como la composición química de la MOS influyen marcadamente sobre la repelencia al agua en el suelo. Estos trabajos se han centrado principalmente en el estudio de suelos completos tanto en superficie como a diferentes profundidades. Sin embargo, la información existente sobre la influencia de la materia orgánica y la vegetación en el grado de hidrofobicidad de diferentes fracciones físicas del suelo es limitado. Este aspecto se estudia en detalle en el capítulo 3. En concreto se estudia la relación entre suelos bajo diferentes cubiertas vegetales, dominadas por alcornoques (Quercus suber), helecho águila (Pteridium aquilinum), pino piñonero (Pinus pinea) y jaguarzo (Halimium halimifolium), la cantidad y calidad de la materia orgánica, y la repelencia al agua en cuatro fracciones de tamaño de partículas (1-2, 0.25-1, 0.05-0.25 y <0.05 mm). Observamos que el grado de hidrofobicidad era significativamente diferente, tanto entre los suelos bajo diferente cubierta vegetal como entre las distintas fracciones granulométricas, siendo las pertenecientes a los suelos bajo alcornoque las que presentaron mayor severidad de repelencia al agua. Además se demuestra una relación entre la cantidad de MOS y el grado de repelencia al agua. Por otro lado, el análisis molecular de la materia orgánica de las fracciones mediante técnicas de pirólisis analítica mostró que existe una relación entre la calidad de MOS y la hidrofobicidad, la presencia de ácidos grasos de cadena larga y el grado de humificación (evolución) de la MOS en las diferentes fracciones.......