Phosphorus transformations in a ferralsol through ingestion by Pontoscolex corethrurus, a geophagous earthworm

Ce travail analyse les transformations qui conduisent à augmenter le contenu en phosphore inorganique (Pi) assimilable du sol suite à l'ingestion d'un ferralsol par le ver géophase #Pontoscolex corethrurus$. Le contenu en phosphore total, inorganique et organique d'échantillons de sol témoin, de sol non-ingéré mais incubé et de déjections de surface a été mesuré. Le phosphore extractible par NaHCO3 et NaOH a été dosé dans des échantillons ayant préalablement subi un fractionnement granulométrique (plus de 20, 2-20 et moins de 2 micromètres). La teneur en phosphore total est la même dans le sol témoin, le sol non-ingéré et les déjections ; la réhumectation du sol et l'ingestion du sol par le ver diminuent la teneur en phosphore organique et augmentent la teneur en phosphore inorganique. La teneur en argile des déjections (19.7%) est significativement supérieure à celle de l'échantillon témoin (9,1%). Les turricules ont une teneur en phosphore inorganique extractible par NaHCO3 significativement supérieure à celle du sol non-ingéré (5,04 micromètres Pi/g sol contre 3,17). La même tendance est observée dans l'extrait NaOH (45,8 micromètres Pi/g sol dans les déjections contre 27,4 dans le sol non-ingéré). Les phosphates dans les déjections sont essentiellement concentrés dans les fractions granulométriques fines (0-2 micromètres) (8.3% des phosphates extraits contre 61% pour le sol non-ingéré). Ces modifications résultent de l'ingestion sélective par le ver de particules fines et de la minéralisation partielle du phosphore organique. (Résumé d'auteur

Shrinkage of initially very wet soil blocks, cores and clods from a range of European Andosol horizons

In advanced stages of volcanic ash soil formation, when more clay is formed, soil porosity values and soil water retention capacities are large and the soils show pronounced shrinkage on drying. Soil shrinkage is a key issue in volcanic soil environments because it often occurs irreversibly when topsoils dry out after changes from permanent grassland or forest to agriculture. European Andosols have developed in a wide range of climatic conditions, leading to a wide range in intensity of both weathering and organo-mineral interactions. The question arises as to whether these differences affect their shrinkage properties. We aimed to identify common physically based shrinkage laws which could be derived from soil structure, the analysis of soil constituents, the selected sampling size and the drying procedure. We found that the final volumetric shrinkage of the initially field-wet (56-86% of total porosity) or capillary-wet (87-100% of total porosity) undisturbed soil samples was negatively related to initial bulk density and positively related to initial capillary porosity (volumetric soil water content of soil cores after capillary rise). These relationships were linear for the soil clods of 3-8 cm3, with final shrinkage ranging from 21.2 to 52.2%. For soil blocks of 240 cm3 and soil cores of 28.6 cm3 we found polynomial and exponential relationships, respectively, with thresholds separating shrinkage and nearly non-shrinkage domains, and larger shrinkage values for the soil cores than for the soil blocks. For a given sample size, shrinkage was more pronounced in the most weathered and most porous Andosol horizons, rich in Al-humus, than in the less weathered and less porous Andosol horizons, poor in Al-humus. The Bw horizons, being more weathered and more porous, shrank more than the Ah horizons. We showed that the structural approach combining drying kinetics under vacuum, soil water analysis and mercury porosimetry is useful for relating water loss and shrinkage to soil structure and its dynamics. We also found that the more shrinkage that occurred in the Andosol horizon, the more pronounced was its irreversible mechanical chang