Wildfires play the role of ecosystem shapers in the majority of terrestrial biomes, deeply modifying soil and other compartments. An enhancement in surface magnetism was observed in fire-affected soils and, considering that weathering leads to the progressive oxidation of iron (Fe) phases, this phenomenon has left an open debate. It has been attributed to the conversion of weakly magnetic oxy-hydroxides into more magnetic phases, or to a relative enrichment in magnetic minerals as a consequence of the mass loss effect caused by combustion. Nevertheless, even small shifts in Fe oxy-hydroxides composition can have a dramatic cascading effect on carbon (C) cycling, Our aims were: i) to combine multiple techniques to monitor the thermal transformations of Fe of oxy-hydroxides in complex soil systems and ii) to clarify the fate of the organic matter (OM) pool with high affinity for Fe phases. Soils (A horizons) collected in the Western Italian Alps were lab-heated up to 300° C and characterized for Fe oxy-hydroxides composition with wet chemistry, XRD, FE-SEM, magnetic measurements and Fe K-edge X-ray absorption spectroscopy. An overall decrease in crystalline degree was detected upon growing temperatures, which would be consistent with the dehydroxylation of goethite (greatest abundance detected at 200° C, as rod-shaped crystals) into disordered proto-hematite. Also, the formation of magnetite in C-rich samples, likely due to the reducing environment provided by OM, could be appreciated above 250° C. Fine-grained pyrogenic ferromagnetic phases were detected alongside with sporadic large spherical assemblages, likely occurring to due to particle aggregation and minimization of surface energy. Finally, a reduction in the OM-bound Fe pool could be appreciated at 300° C. Yet, the pyrophosphate-extracted OM pool was found to dominate, in proportion to total soil OM, at the highest temperatures. This suggests that, despite a great deal of C is loss by combustion, the remaining aromatic OM compounds possibly experience a preferential association with Fe phases. The current investigation was carried out to unravel key-processes naturally occurring in soils after the passage of fire. Considering that climate-change-driven modifications in wildfires regimes are already being experienced worldwide, addressing the transformations occurring after such hazards is crucial to tackle the issue of ecosystems recovery.
A multi-technique approach to detect thermal transformations of Fe oxides in burnt soils
Beatrice Giannetta;
2021-01-01
Abstract
Wildfires play the role of ecosystem shapers in the majority of terrestrial biomes, deeply modifying soil and other compartments. An enhancement in surface magnetism was observed in fire-affected soils and, considering that weathering leads to the progressive oxidation of iron (Fe) phases, this phenomenon has left an open debate. It has been attributed to the conversion of weakly magnetic oxy-hydroxides into more magnetic phases, or to a relative enrichment in magnetic minerals as a consequence of the mass loss effect caused by combustion. Nevertheless, even small shifts in Fe oxy-hydroxides composition can have a dramatic cascading effect on carbon (C) cycling, Our aims were: i) to combine multiple techniques to monitor the thermal transformations of Fe of oxy-hydroxides in complex soil systems and ii) to clarify the fate of the organic matter (OM) pool with high affinity for Fe phases. Soils (A horizons) collected in the Western Italian Alps were lab-heated up to 300° C and characterized for Fe oxy-hydroxides composition with wet chemistry, XRD, FE-SEM, magnetic measurements and Fe K-edge X-ray absorption spectroscopy. An overall decrease in crystalline degree was detected upon growing temperatures, which would be consistent with the dehydroxylation of goethite (greatest abundance detected at 200° C, as rod-shaped crystals) into disordered proto-hematite. Also, the formation of magnetite in C-rich samples, likely due to the reducing environment provided by OM, could be appreciated above 250° C. Fine-grained pyrogenic ferromagnetic phases were detected alongside with sporadic large spherical assemblages, likely occurring to due to particle aggregation and minimization of surface energy. Finally, a reduction in the OM-bound Fe pool could be appreciated at 300° C. Yet, the pyrophosphate-extracted OM pool was found to dominate, in proportion to total soil OM, at the highest temperatures. This suggests that, despite a great deal of C is loss by combustion, the remaining aromatic OM compounds possibly experience a preferential association with Fe phases. The current investigation was carried out to unravel key-processes naturally occurring in soils after the passage of fire. Considering that climate-change-driven modifications in wildfires regimes are already being experienced worldwide, addressing the transformations occurring after such hazards is crucial to tackle the issue of ecosystems recovery.
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