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Loess-derived soils of the northern Paris basin are prone to surface Beneficial effects of sewage sludge application have structure degradation leading to erosion, flooding, and pollution. Conbeen widely acknowledged, even though Metzger and comitantly, recycling of municipal solid waste (MSW) has been recog-Yaron (1987) pointed out that controversial results nized as an important environmental issue. The aim of this study was to test the impact of compost application on soil surface structure could be found in the literature. Literature is scarce on degradation and on the resulting runoff and erosion processes. Aggrethe effects of compost on soil structure. In their review gates (0-30 mm) from a silty loam Typic Hapludalf were mixed with on the changes in soil physical properties because of a MSW compost at a rate of 15 g kg Ϫ1 (dry matter). Repacked seedbeds organic waste applications, Khaleel et al. (1981) prewere exposed to a 19 mm h Ϫ1 simulated rainfall for 60 min. Morphologsented 17 papers, only two dealing with MSW composts ical evolution of the soil surface was monitored using sequential photo-(Mays et al., 1973; Epstein et al., 1976). They did not graphs. Crust and seedbed microstructures were studied after 4, 15, find a significant difference between the various types and 60 min of rainfall, using thin sections from resin-impregnated of wastes. Regardless of waste type, both long-term and replicates. Runoff was measured every five minutes, and aliquots short-term studies indicated a significant linear relationwere sampled for sediment concentration. In control seedbeds, surface ship between reduction in bulk density and increase in crusts quickly developed and the whole seedbed slumped because of aggregate coalescence through deformation in a viscous state. Com-soil organic C. The decrease in bulk density has been post application delayed crust formation and prevented seedbed ascribed to the dilution effect resulting from the mixing slumping. This, in turn, delayed runoff from 2.5 to 9.2 mm of cumulaof the soil with less dense organic material added (Khative rainfall. Sediment concentration in the incipient runoff was deleel et al., 1981; Tester, 1990). Structural changes recreased from 36.4 to 11 g L Ϫ1. This could be ascribed to the stabilization sulting from interactions between added organic matter of the aggregate framework, which allowed the particles detached and soil material were seldom suggested, but a close from the top of surface aggregates to illuviate a few millimeters deeper. examination of the data published in the literature sup-In a highly unstable soil, MSW compost application was efficient in ports this hypothesis. Changes in soil structure (Guisquicombating soil surface structure degradation and its consequences on ani et al., 1995), as well as, changes in macro and mesorunoff and erosion.

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Hardsetting has been related to two main processes: (i) development of strength when the soil is still moist (-100 kPa) due to matric suction acting within interparticle and interaggregate bridges and (ii) temporary cementation of dry soil by poorly ordered silica and aluminosilicates. In both cases, hardsetting of a seedbed should depend on geometric aspects of macro-and micro-structure. This study deals with aggregate breakdown and/or deformation on wetting and with the structural changes which follow on drying. Repacked seedbeds of a hardsetting red-brown earth were wetted by capillary rise. Seedbeds with a coarse and a fine aggregate size distribution were examined. Before and after wetting, the bulk density of the seedbeds was measured at 5-mm increments using a gamma probe. Replicate samples were air dried, gamma scanned and impregnated. Binary images of pore space ( > 107 pm) of vertical faces were used to generate depth functions of structure attributes, including macroporosity. Bulk density measurements combined with image analysis resulted in successful structure characterization. Thin-section observations were used to interpret the quantified changes in terms of physical processes. In both coarse and fine seedbeds the physical processes which determined structural change occurred upon wetting rather than on drying. Coalescence of aggregates under plastic conditions, partly due to overburden pressure and enhanced by microcracking and partial slaking, occurred at the bottom of the coarse seedbed. In the fine seedbed, the fine material agglomerated due to matric suction. These different processes led to rather similar, microporous microstructures which exhibited similar strength properties after drying.

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Exponential and sigmoidal functions have been suggested to describe the bulk density profiles of crusts. The present work aims to evaluate these conceptual models using high resolution X‐radiography. Repacked seedbeds from two soil materials, air‐dried or prewetted by capillary rise, were subjected to simulated rain, which resulted in three types of structural crusts, namely, slaking, infilling, and coalescing. Bulk density distributions with depth were generated using high‐resolution (70 μm), calibrated X‐ray images of slices from the resin‐impregnated crusted seedbeds. The bulk density decreased progressively with depth, which supports the suggestion that a crust should be considered as a nonuniform layer. For the slaking and the coalescing crusts, the exponential function underestimated the strong change in bulk density across the morphologically defined transition between the crust and the underlying material; the sigmoidal function provided a better description. Neither of these crust models effectively described the shape of the bulk density profiles through the whole seedbed. Below the infilling and slaking crusts, bulk density increased linearly with depth as a result of slumping. In the coalescing crusted seedbed, the whole seedbed uniformly collapsed and most of the bulk density change within the crust could be ascribed to slumping (0.33 g cm−3) rather than to crusting (0.12 g cm−3). Finally, (i) X‐radiography appears as a unique tool to generate high resolution bulk density profiles and (ii) in structural crusts, bulk density profiles could be modeled using the existing exponential and sigmoidal crusting models, provided a slumping model would be coupled.