Laboratory determination of soil hydraulic characteristics (HC) is performed on cylindrical confined soil cores, while resin‐coated small natural aggregates are prevalent for determining the Soil Shrinkage Characteristic Curve (SSCC). Because of the different geometry and volume of clods and cores, incorporation of shrinkage in HC of clay soils could be affected by the use of SSCC determined on natural aggregates or on confined cores. The objectives of this study were (i) to investigate differences between the SSCC obtained on resin‐coated natural aggregates (volume V = 20–30 cm3) and on cylindrical confined cores having a volume (650 cm3) close to that of cores used for HC laboratory determination, (ii) to test the performance of different models proposed for analytical interpretation of the SSCC, and (iii) to derive regression equations predicting the SSCC from routinely measured soil physical properties. Using twenty‐one Sicilian soils of variable shrink‐swell behavior, we found significantly larger specific volume (n), indicating less shrinkage, in the cylindrical confined cores. The investigation also proved the good fitting of a two‐line model to the measured SSCC and the possibility to predict the basic shrinkage line from the clay content. These results suggest that incorporation of shrinkage in HC of clay soils should be based on the SSCC measured or predicted on cores with geometry and dimensions as those used for routine laboratory measurement of HC.

No abstract

Purpose The aim of the present study was to investigate structure alterations of a sandy-clay soil upon addition of different amounts of biochar (f bc ). Materials and methods All the f bc samples were analyzed by high energy moisture characteristic (HEMC) technique and 1 H nuclear magnetic resonance (NMR) relaxometry. HEMC was applied in order to evaluate aggregate stability of biocharamended soil samples. 1 H NMR relaxometry experiments were conducted for the evaluation of the pore distributions through the investigation of water dynamics of the same samples.Results and discussion The HEMC technique revealed improvement in aggregate stability through measurements of the amount of drainable pores and the stability ratio. The latter increased as the amount of biochar was raised up. The 1 H NMR relaxometry revealed a unimodal T 1 distribution for both the sole sandy-clay soil and the biochar. Conversely, a bimodal T 1 distribution was acquired for all the different f bc samples. Conclusions Improvement in aggregate stability was obtained as biochar was progressively added to the sandy-clay soil. A dual mechanism of water retention has been hypothesized. In particular, intra-aggregate porosity was indicated as the main responsible for molecular water diffusion when f bc comprised between 0 and 0.33. Conversely, inter-aggregate porosity resulted predominant, through swelling processes, when f bc overcame 0.33.

An exchangeable sodium percentage (ESP) >15 is traditionally considered to affect soil structural and hydraulic characteristics. Recent investigations show that both this critical value and the concept of critical threshold need reconsideration, because soil degradation often takes place even at lower ESP in dilute solutions, and soil behavior at increasing ESP appears to be a continuum. This study was carried out to analyze the response of two Sicilian Typic Haploxererts to ESP values up to 15, at a low cationic concentration. The investigation was carried out on aggregate stability, rating of soil shrink‐swell potential, and both saturated and unsaturated hydraulic conductivity. The specific purpose was to verify if a critical ESP threshold exists, or if the hypothesis of a continuum behavior is more appropriate. The high‐energy moisture characteristic technique was used for determining aggregate stability, and Brasher's method for the shrinkage characteristic. The constant‐head method was used for determining saturated hydraulic conductivity, and the parameter estimation method based on one‐step outflow experiments for unsaturated hydraulic conductivity. Almost linear relationships were found between the investigated soil properties and ESP, indicating no critical ESP threshold; furthermore, the results obtained indicate that an effective hazard of soil quality degradation can be forecast even in a 2 to 5 ESP range at a low cationic concentration. This is a basic indication for irrigation management aimed to combat and prevent degradation of soil quality.

tainable irrigation strategies finalized to protect soil from salinization (Crescimanno et al., 2004). In Sicily, the increasing scarcity of quality water is leading to irriga-In Sicily, the increasing scarcity of good quality water tion with saline water in soils having a considerable susceptibility to coupled with intensive use of soil under semiarid to arid cracking. Irrigation systems involving high application rates are used climatic conditions, is leading to irrigation with saline in these irrigated areas, and bypass flow during irrigation is thus prevalent. Adoption of management practices accounting for cracking water on soils having a high shrink-swell potential and is therefore necessary to prevent salinization and land degradation. susceptibility to cracking (Crescimanno and Provenzano, In this paper, water flow and solute transport in a Sicilian cracking 1999). These soils are irrigated in the summer season, soil irrigated with saline water was simulated by using the soil-waterwhen the cracks open up, by sprinkler systems, which inatmosphere-plant environment (SWAP) model, and the simulated volve high application rates. Because of these high apresults compared with measured values of soil moisture and salinity. plication rates, bypass flow, that is, the rapid transport The soil hydraulic parameters were obtained by inverse method based of water and solutes via macropores or cracks to subsoil on multi-step outflow experiments, adopting two different sets of hyor to groundwater (Bouma, 1991; Crescimanno, 2001), draulic parameters/functions, that is, (i) the van Genuchten-Mualem, is prevalent during irrigation. (VGM model) and (ii) the Brutsaert retention equation coupled with Laboratory investigations performed on undisturbed the hydraulic conductivity model proposed by Gardner (B-G model). soil columns sampled from these areas showed that sali-The results obtained using field measurements from four soil profiles nization or leaching occurred during bypass flow dependof a cracking clay soil showed that SWAP provided accurate predicing on the concentration of the applied solution compared tions of water content, , when the soil hydraulic properties were expressed according to the B-G model. Using the B-G hydraulic pa-with the concentration of the pore solution (Crescimanno rameters/functions, the model was calibrated with reference to the and De Santis, 2004); the efficiency of salt-leaching was dispersivity (L dis). A calibration value of about 20 cm was found for the found to depend on crack volume (Crescimanno et al., four different profiles. In the conditions occurring in the Sicilian area 2002). The low values of the sodium adsorption ratio where we are focusing our attention, the predictive errors associated (SAR) of irrigation water, and the low values of the exwith the simulated EC sat , can be considered acceptable if the purpose changeable sodium percentage (ESP) measured in these of application is to predict the influence of salinity on crop yield. soils, indicated no risk of sodication under ...