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of shrinkage curves on undisturbed core samples allowed improvement of the knowledge of soil shrinking behav-The availability of methods for quasi-continuous measurements of ior, and to develop shrinkage curve models with differsoil shrinkage curves allowed the development of new models. The ent sets of parameters, as reviewed in Braudeau et al. exponential (XP) model allows the calculation of the volume of two pore phases in the soil, namely macro and micropore volumes. The (1999). micropore volume is identified with the pore volume of the soil clay Shrinkage curves generally present a typical sigmoid matrix according to the assumption that the maximum swelling of the shape, with linear and curvilinear parts separated by clay matrix (MS) is the point of minimum water content of the structransition points as drawn in Fig. 1. Similar to clay paste, tural shrinkage (Point D). This is discussed using undisturbed and a SL and an AE are observed on structured soil samples repacked soil samples with various clay contents and clay types. The (e.g., McGarry and Malafant, 1987; Tariq and Durnford, slope of the shrinkage curves as a function of equivalent saturated-1993b). However, the slope of the shrinkage curve for pore radius show a transition in pore type around a 10-m pore radius, water content greater than AE, called normal shrinkage where smaller and more deformable pores start to desaturate. This or basic shrinkage as discussed by Mitchell (1992), is corresponds to the fitted D point and is close to the size of the largest generally not equal to one (e.g., Lauritzen and Stewart, pores in the clay matrix or clay-silt phase reported in the literature. The calculated micropore volume and micropore swelling properties

International audiencePredicting soil hydraulic properties and understanding their temporal variability during the irrigated cropping season are required to mitigate agro-environmental risks. This paper reports field measurements of soil hydraulic properties under two drip irrigation treatments, full (FT) and limited (LT). The objective was to identify the temporal variability of the hydraulic properties of field soil under high-frequency water application during a maize cropping season. Soil hydraulics were characterized using the Beerkan infiltration method. Seven sets of infiltration measurements were taken for each irrigation treatment during the cropping season between June and September 2007. The first set was measured two weeks before the first irrigation event. The results demonstrated that both soil porosity and hydraulic properties changed over time. These temporal changes occurred in two distinct stages. The first stage lasted from the first irrigation event until the root system was well established. During this stage, soil porosity was significantly affected by the first irrigation event, resulting in a decrease in both the saturated hydraulic conductivity Ks and the mean pore effective radius ξm and in an increase in capillary length αh. These hydraulic parameters reached their extreme values at the end of this stage. This behavior was explained by the “hydraulic” compaction of the surface soil following irrigation. During the second stage, there was a gradual increase in both Ks and ξm and a gradual decrease in αh when the effect of irrigation was overtaken by other phenomena. The latter was put down to the effects of wetting and drying cycles, soil biological activity and the effects of the root system, which could be asymmetric as a result of irrigation with only one drip line installed for every two plant rows. The processes that affected soil hydraulic properties in the two irrigation treatments were similar. No significant change in ξm and αh was observed between FT and LT. However, as a result of daily wetting and drying cycles, which were strongest in LT, the soil in this treatment was found to be more conductive than that of FT. This showed that most of the changes in pore-size distribution occurred in the larger fraction of pores. The impact of these temporal changes on the dimensions of the wetting bulb was studied using a simplified modeling approach. Our results showed that there were marked differences in the computed width and depth of wetting bulb when model input parameters measured before and after irrigation were used. A temporal increase in capillary length led to a more horizontally elongated wetting bulb. This could improve both watering and fertilization of the root zone and reduce losses due to deep percolation. As a practical result of this study, in order to mitigate agro-environmental risks we recommend applying fertilizers after the restructuration of tilled soil. Further studies using improved models accounting for temporal changes in soil hydraulic properties are needed

International audienceRecent studies underlined the likeness of soil water retention (WRC) and shrinkage curves (ShC) with respect to their shapes. This paper aims at experimentally discussing the possible use of the same equations to fit them. The WRC (on the tensiometric range) and ShC were simultaneously determined on a series made of 28 undisturbed soil cores collected in surface horizons from a wide variety of soil types, with clay content ranging from 8.5 to 65% and of 30 repacked soil samples of various clay contents and mineralogies. The van Genuchten (VG) closed-form equation of WRC and the VG modified equation of ShC, with five and three parameters, respectively, were found to fit well to both curves, but they did not properly reproduce the observed linear parts and sloping ends of both curves, and the dissymmetric shapes of the ShC as well. The exponential shrinkage model XP fitted significantly better to both the WRC with five parameters and the ShC with eight parameters. It is shown that the transition points of the XP equations independently fitted on the ShC and WRC curves occur at the same gravimetric water content, thus illustrating the likeness of the curves with respect to their shape. The WRC was estimated with a reasonable accuracy from the water content of the ShC transition points plus some measured suction values

Summary The long‐term effects of intermittent flooding on soil properties were studied in field experiments on a Vertisol cropped with rice in Senegal. The dominant clay minerals were smectite and kaolinite. When the soil was reduced after flooding, its cation exchange capacity (CEC) increased to twice that of its oxidized, unflooded state. Mössbauer spectroscopy showed an increase in smectite structural FeII upon reduction, which explained a part of the increase in CEC. The rest of the increase was attributed to the removal of iron oxyhydroxide coatings by reductive dissolution. The reduction and dissolution of oxides under the field conditions were substantiated by analysis of the surfaces of vermiculites buried in the Ap horizons of the cropped and the non‐cropped soils. The redox‐induced CEC changes were found to be reversible after 22 cycles of rice cropping. Nevertheless, the structural Fe and free Fe contents of the rice field Ap horizon were less than those of soil in uncropped neighbouring land, suggesting that inundation induced weathering and eluviation of the minerals. The observed changes in CEC and related redox reactions may substantially modify proton, anion and cation balances in intermittently flooded soils.