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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 cm 3 , with final shrinkage ranging from 21.2 to 52.2%. For soil blocks of 240 cm 3 and soil cores of 28.6 cm 3 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 change.

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After growth of doubly labeled (14C and 15N) maize (Zea mays L.), two loamy soils were labeled by root exudation and rhizodeposition, and by direct microbial immobilization of N. Fresh roots were then carefully separated and washed, eliminating organic and organomineral cementing agents by acid and alkaline solubilizing reagents, and the remaining insoluble humin was water dispersed in order to separate coarse, medium, and fine fractions. At harvest time, fresh roots represented 85% of the total C input, and rhizodeposition 15%. Sixty to 70% of the N input was still in living roots at this time, and other organic forms of N were more a result of microbial activity than of rhizodeposition. The largest and most homogeneous organic fraction was the finest insoluble fraction, in which about half of the label for both C and N was found.