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.

Because soil surface structure has a considerable influence on infiltration rate, the sealing process is postulated to have a significant effect on herbicide loss through runoff. We evaluated the effect of degraded soil surface structures on herbicide loss in runoff, and used the experimental data to test the uniform mixing zone concept and two-site sorption kinetics for modeling herbicide transfer to runoff. The experiments were done with simulated rainfall on 10-m2 plots in the field and 0.25-m2 plots in the laboratory after a surface application of 1.5 kg ha(-1) of isoproturon [3-(4-isopropylphenyl)-1,1-dimethylurea] and 0.187 kg ha(-1) of diflufenican [2',4'-difluoro-2-(alpha,alpha,alpha-trifluoro-m-tolyloxy) nicotinanilide]. Isoproturon (IPU) and diflufenican (DFF) concentrations were very high in the first runoff (up to 60 mg L(-1) for IPU and 2 mg L(-1) for DFF) when simulated rainfall was applied 24 h after the treatment. The concentrations decreased very rapidly with total rainfall depth. Degradation of the structural state of the soil surface increased the ratio of pesticide loss to application rate from 0.3 to 10% for IPU and from 0.7 to 7.8% for DFF for a runoff depth of less than 1 mm. The structural state of the soil surface influences the rapidity at which runoff begins after the onset of rain, and the runoff coefficient at steady state. Furthermore, the development of a surface seal seems to limit the depth of soil-runoff interaction and thus influences the dynamics of herbicide mobilization. Concentrations of IPU in the runoff were satisfactorily described with a model incorporating a uniform mixing zone and two-site sorption-desorption.