Summary Ferralsols have a macrostructure that is weak to moderate and a strong microstructure consisting of near spherical microaggregates. We have studied the evolution of the structure under changing land use in two Ferralsols under a native Cerrado and under pasture sowed after recent clearing by measuring bulk density. We studied the microaggregates' characteristics and resulting porosity using scanning electron microscopy and mercury porosimetry, respectively. Microaggregates 50–300 μm in diameter are attributed to termites. They were in a much closer arrangement from the surface to 1 m depth under pasture than under Cerrado. Larger microaggregates, 500–1000 μm in diameter, that are closely packed, resulted from earthworm activity. They were more numerous under pasture than under Cerrado. Thus within a few years, clearing and grazing have caused a dramatic decrease in microstructure that is one of the rare favourable characteristics of Ferralsols for agriculture. This evolution that affects the subsoil to 80–90 cm cannot be attributed to compaction by mechanical deforestation alone but seems to result from a change in the faunal activity in the soil. Finally, the packing of the clay particles within the microaggregates was similar whatever the type of microaggregate and land use. Our results accord with earlier studies showing that the porosity resulting from clay packing varies little in Ferralsols in which the fine material is mainly kaolinite. Evolution de la porosité et de la microagrégation dans des Ferralsols argileux du Cerrado Brésilien après déforestation et installation d'un pâturage Résumé Les Ferralsols possèdent une macrostructure qui est faiblement à modérément développée et une microstructure très nette sous la forme de microagrégats approximativement sphériques. Nous avons étudié l'évolution de la structure lors d'un changement de mode d'usage du sol dans deux Ferralsols argileux sous végétation naturelle de type Cerrado et sous pâturage semé après déforestation. Nous avons mesuré la densité apparente et décrit la microstructure et la porosité qui en résulte en microscopie électronique à balayage et en porosimétrie au mercure. Des microagrégats de 50 à 300 μm de diamètre ont été attribués à l'activité des termites; leur assemblage est plus compact jusqu'à 1 m de profondeur sous pâturage que sous Cerrado. Des microagrégats de plus grande taille, de 500 à 1000 μm de diamètre, et en assemblage compact ont aussi été observés. Ils résultent de l'activité de vers de terre et sont plus nombreux sous pâturage que sous Cerrado. Ainsi en quelques années, la mise en place d'un pâturage après déforestation a pour effet une décroissance importante de la microagrégation qui est l'une des rares caractéristiques favorables des Ferralsols pour l'agriculture. Cette transformation qui affecte le sol en profondeur ne peut pas être attribuée à un compactage lors la déforestation mécanisée mais apparaît être en revanche la conséquence d'une évolution de l'activité de la faune du sol. Enfin, la porosité résultant d...

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The hyperarid core of the Atacama Desert, Chile, is possibly the driest and most life-limited place on Earth, yet endolithic microorganisms thrive inside halite pinnacles that are part of ancient salt flats. The existence of this microbial community in an environment that excludes any other life forms suggests biological adaptation to high salinity and desiccation stress, and indicates an alternative source of water for life other than rainfall, fog or dew. Here, we show that halite endoliths obtain liquid water through spontaneous capillary condensation at relative humidity (RH) much lower than the deliquescence RH of NaCl. We describe how this condensation could occur inside nano-pores smaller than 100 nm, in a newly characterized halite phase that is intimately associated with the endolithic aggregates. This nano-porous phase helps retain liquid water for long periods of time by preventing its evaporation even in conditions of utmost dryness. Our results explain how life has colonized and adapted to one of the most extreme environments on our planet, expanding the water activity envelope for life on Earth, and broadening the spectrum of possible habitats for life beyond our planet

Fractal parameters of soils become increasingly important in understanding and quantifying transport and adsorption phenomena in soils. It is not known yet how soil degradation affects fractal characteristics of soil pores. We estimated pore surface area fractal parameters from Hg porosimetry data on samples of a Udic Argiboroll, a Typic Haploboroll, and a Ustolic Orthid before and after simulated soil degradation. Three or four distinct intervals with different fractal dimensions were found in the range of pore radii from 4 nm to 5 µm. This was attributed to differences in composition of soil particles of different sizes. The simulated degradation caused an increase in fractal dimensions in one or more fractal intervals, thus manifesting the increased roughness and irregularity of the pore surfaces. The interval of the smallest radii had the highest average fractal dimension, close to 3; some estimated values were >3, probably due to the compressibility of bulk material and air entrapment. Values of the fractal dimension in this interval increased after cyclic wetting‐drying but were not affected by organic matter oxidation. Smaller fractal dimensions were found in the next interval of radii. Here average fractal dimension increased markedly after organic matter oxidation and grew slightly after cyclic wetting‐drying, reflecting the loss of bonds between particles. The range of largest radii included two fractal intervals after cyclic wetting‐drying and one fractal interval for all other samples. Neither organic matter oxidation nor cyclic wetting‐drying significantly affected the boundaries between the fractal intervals.

The hyperarid core of the Atacama Desert, Chile, is possibly the driest and most abiotic place on Earth, yet endolithic microorganisms thrive inside halite pinnacles that are part of ancient salt flats. The existence of this microbial community in an environment that excludes any other life forms suggests biological adaptation to high salinity and desiccation stress, and indicates an alternative source of water for life other than rainfall, fog or dew. Here we show that halite endoliths obtain liquid water through spontaneous capillary condensation at relative humidity (RH) much lower than the deliquescence RH of NaCl. We describe how this condensation occurs inside nano-pores smaller than 100 nm, in a newly identified halite phase that is intimately associated with the endolithic aggregates. This nano-porous phase helps retain liquid water for long periods of time by preventing its evaporation even in conditions of utmost dryness. Our results explain how life has colonized and adapted to one of the most extreme environments on our planet, expanding the water activity envelope for life on Earth, and broadening the spectrum of possible habitats for life beyond our planet