Crusting and erosion of cultivated soils result from aggregate breakdown and the detachment of soil fragments by rain, and the susceptibility of soil to these processes is often inferred from measurements of aggregate stability. Here, theories of aggregate breakdown are reviewed and four main mechanisms (i.e. slaking, breakdown by differential swelling, mechanical breakdown by raindrop impact and physico-chemical dispersion) are defined. Their relative importance depends on the nature of the rain, as well as on the soil's physical and chemical properties. The relations between aggregate breakdown, crusting and water erosion are analysed, and existing methods for the assessment of aggregate stability are reviewed. A unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion. It combines three treatments having various wetting conditions and energies (fast wetting, slow wetting, and stirring after pre-wetting) and measures the resulting fragment size distribution after each treatment. It is designed to compare different soils, or different climatic conditions for a given soil, not to compare time-dependent changes in that soil. StabilitC structurale etCvaluation de la sensibilitk des sols g la battance et B I'Crosion: I: ThCorie et mCthologie RCsumC La battance et I'Crosion des sols cultivCs rCsultent essentiellement de la dCsagrCgation des mottes de terre et du dktachement de fragments sous l'action des pluies. La mesure de la stabilitC structurale est donc souvent utilisCe pour Cvaluer la sensibilite des sols A la battance et B I'Crosionhydrique. On peut distinguer quatre principaux mCcanismes de dCsagrCgation (I'Cclatement, la dCsagrCgation par gonflement diffkrentiel, la dbsagrkgation mCcanique par I'impact des gouttes de pluie, la dispersion physico-chimique) dont I'importance relative dCpend des caractkristiques des pluies et des sols. Aprbs un rappel sur ces mCcanismes, cet article analyse leurs relations avec la battance et 1'Crosion puis dCcrit les caractkristiques des principales mCthodes existant. On propose, B partir de cette analyse, un cadre mCthodologique cohCrent permettant d'Cvaluer la stabilitC structurale des sols en relation avec la battance et 1'Crosion. La mCthode combine trois traitements correspondant B differentes conditions d'humectation et diffkrents niveaux d' Cnergie (humectation rapide par immersion, humectation lente par capillaritk et agitation mCcanique apr&s prkhumectation). La mCthode est adaptCe pour comparer le comportement intrindque de diffkrents sols ou d'un sol donnC dans differentes conditions climatiques, ainsi que pour tester I'influence de traitements ou amendements sur la stabilitk structurale, mais pas pour tester les variations de stabiiitk dans le temps pour un sol donne.

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Soil organic matter is thought to increase aggregate stability by lowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) have been intensively cropped for 40 yr, decreasing the soil organic pool and lowering the soil agregate stability. This study assessed (i) the contribution of organic matter to aggregate stability by decreasing aggregate wettability and (ii) the specific role of clay‐associated organic matter. Soil samples with a C content of 4 to 53 g kg−1 were sampled and soil aggregate stability was measured. Aggregate wettability was assessed by measuring water drop penetration times on individual 3‐ to 5‐mm aggregates. The <2‐μm fractions were extracted without organic matter destruction and their wettability was determined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content Water drop penetration time increased with C contents from 1 to 32 s and was very heterogeneous among individual aggregates from a given soil. The contact angle of water on the clay fraction increased linearly with the C content This change in clay wettability could partly explain the higher water stability of soils rich in C.

Summary Crusting and erosion of cultivated soils result from aggregate breakdown and the detachment of soil fragments by rain, and the susceptibility of soil to these processes is often inferred from measurements of aggregate stability. Here, theories of aggregate breakdown are reviewed and four main mechanisms (i.e. slaking, breakdown by differential swelling, mechanical breakdown by raindrop impact and physico‐chemical dispersion) are defined. Their relative importance depends on the nature of the rain, as well as on the soil's physical and chemical properties. The relations between aggregate breakdown, crusting and water erosion are analysed, and existing methods for the assessment of aggregate stability are reviewed. A unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion. It combines three treatments having various wetting conditions and energies (fast wetting, slow wetting, and stirring after pre‐wetting) and measures the resulting fragment size distribution after each treatment. It is designed to compare different soils, or different climatic conditions for a given soil, not to compare time‐dependent changes in that soil. Stabilité structurale et évaluation de la sensibilité des sols à la battance et à l'érosion: I: Théorie et méthologie Résumé La battance et l'érosion des sols cultivés résultent essentiellement de la désagrégation des mottes de terre et du détachement de fragments sous l'action des pluies. La mesure de la stabilité structurale est donc souvent utilisée pour évaluer la sensibilité des sols àla battance et àl'érosion hydrique. On peut distinguer quatre principaux mécanismes de désagrégation (l'éclatement, la désagrégation par gonflement différentiel, la désagrégation mécanique par l'impact des gouttes de pluie, la dispersion physico‐chimique) dont l'importance relative dépend des caractéristiques des pluies et des sols. Après un rappel sur ces mécanismes, cet article analyse leurs relations avec la battance et l'érosion puis décrit les caractéristiques des principales méthodes existant. On propose, à partir de cette analyse, un cadre méthodologique cohérent permettant d'évaluer la stabilité structurale des sols en relation avec la battance et l'érosion. La méthode combine trois traitements correspondant à différentes conditions d'humectation et différents niveaux d'énergie (humectation rapide par immersion, humectation lente par capillarité et agitation mécanique après préhumectation). La méthode est adaptée pour comparer le comportement intrinsèque de différents sols ou d'un sol donné dans différentes conditions climatiques, ainsi que pour tester l'influence de traitements ou amendements sur la stabilité structurale, mais pas pour tester les variations de stabilité dans le temps pour un sol donné.

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Surface crusting results from aggregate breakdown under raindrop impact. It reduces the infiltration rate and may induce erosion by increasing runoff. Soil crustability and erodibility generally increase as organic carbon content decreases. Samples of topsoil were collected from fields cropped continuously for maize after land clearance at various dates. Organic carbon content ranged 4-30 g kg-' . Aggregate stability was assessed by measuring fragment size distribution after different treatments. Samples were also subjected to simulated rainfall. The size of fragments forming the seal when the rain ceased and the infiltration rate during the rainfall were measured to characterize seal structure and hydraulic properties. Development of the seal and infiltration capacity were related to aggregate stability, which was itself a function of organic carbon content. The relations between aggregate breakdown, crusting and infiltration enabled us to predict soil physical behaviour from measurements of aggregate stability which are easier to make than direct measurements under rainfall. The proposed tests offer the opportunity to evaluate actual or potential soil physical degradation and erosion risks without extensive field measurements.StabilitC structurale et evaluation de la sensibilitC des sols a la battance et a I'brosion hydrique: 11. Application a une sCrie de sols limoneux a teneur en carbone organique variCe RCsumC La battance des sols cultivks rksulte essentiellement de la dksagrkgation des mottes de terre sous l'action des pluies. Elle rkduit l'infiltrabilitk des sols et peut Btre B-l'origine de phknomknes d'krosion hydrique. La sensibilitk des sols A la battance et it l'krosion hydrique augmente gtntralement lorsque leur teneur en carbone organique diminue. On a prBlevB des Bchantillons de sols provenant de parcelles cultivkes en mays depuis leur defrichement datant de huit B cent ans. La teneur en carbone organique est comprise entre 4 et 30 g kg-'. On a mesurk leur stabilitB structurale en utilisant une mkthode qui consiste en une mesurer de la distribution de la taille des fragments rksultants aprks diffkrents traitements. On a Bgalement soumis les Bchantillons it des pluies simulkes, et mesurB la taille des fragments formant la croQte ainsi que I'Bvolution du taux d'infiltration au cours de la pluie. Ces dernikres mesures sont significativement corrBlkes aux mesures de stabilitk structurale, qui sont elle mBmes relikes B la teneur en carbone organique des kchantillons. Les relations entre stabilitk structurale, battance et infiltration permettent de prBvoir le comportement physique des sols a partir des tests de stabilitk, plus rapides et simples it rkaliser que des mesures directes du comportement des sols sous pluies. Les tests proposks permettent une Bvaluation spatiale des risques de dkgradation physique et d'krosion des sols sans mesures extensives de terrain.