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Tillage practices have a significant influence on the soil hydro-physical properties. The objective of this work was to evaluate the effect of tillage on the α (a scaling factor) and n (a pore size distribution parameter) van Genuchten soil water retention curve parameters during an 18-month long fallow period in a semiarid dryland. Three different tillage systems employed during 23 years of trials were compared: conventional (CT), reduced (RT) and no-tillage (NT). Measurements of soil bulk density (ρb) and the soil water retention curve θ(ψ) were performed at 0–10, 10–20 and 20–30cm soil depths. The θ(ψ) was determined with the Time Domain Reflectometry (TDR)-pressure cells at the following pressure heads: 0.5, 1.5, 3, 10, 50, 100, 500 and 1500kPa. From these data, α, n and the SDexter index were evaluated. The 0–40cm depth soil volumetric water content, θ, was also measured in the field using the TDR technique. Compared with CT and RT, NT had the highest θ values during all the fallow period. No significant influence of soil depth on θ(ψ) was observed in all tillage treatments at each sampling date. Although under consolidated soil conditions no significant differences in ρb and the water content at saturation (θs) were observed among tillage treatments, NT had the highest and lowest values of ρb and θs, respectively. The loosening of soil due to tillage practices in CT and RT significantly decreased ρb and increased θ at the wet-end section of θ(ψ). Post-tillage rainfall resulted in significant decreases in θs, α and the maximum value of the pore size distribution (PSDmax). The different soil structure created by mouldboard ploughing (CT) and chiselling (RT) explained the higher PSDmax under RT than CT. The most important changes in θ(ψ) followed the first copious effective rainfall events (>10mm) after tillage. These facts enabled the soil to recover the pre-tillage water retention curve shapes and the van Genuchten parameters pre-tillage values. Effective rainfall events in the late fallow had a minor effect on the water retention curve. Although tillage tended to increase n, this change was not significant. The SDexter index, which was also affected by tillage, was greater than 0.035 during all the fallow period, indicating good soil physical quality.

Estimation of the soil–water retention curve, θ(h), on undisturbed soil samples is of paramount importance to characterise the hydraulic behaviour of soils. Although a method of determining parameters of the water retention curve (α, a scale parameter inversely proportional to mean pore diameter and n, a measure of pore size distribution) from saturated hydraulic conductivity (Ks), sorptivity (S) and the β parameter, using S and β calculated from the inverse analysis of upward infiltration (UI) has been satisfactorily applied to sieved soil samples, its applicability to undisturbed soils has not been tested. The aim of the present study was to show that the method can be applied to undisturbed soil cores representing a range of textures and structures. Undisturbed soil cores were collected using stainless steel cylinders (5cm internal diameter×5cm high) from structured soils located in two different places: (1) an agricultural loam soil under conventional, reduced and no tillage systems; and (2) a loam soil under grazed and ungrazed natural shrubland. The α and n values estimated for the different soils using the UI method were compared with those calculated using time domain reflectometry (TDR) pressure cells (PC) for pressure heads of –0.5, –1.5, –3, –5, –10 and –50kPa. To compare the two methods, α values measured with UI were calculated to the drying branch of θ(h). For each treatment, three replicates of UI and PC calculations were performed. The results showed that the 5-cm high cylinders used in all experiments provided accurate estimates of S and β. Overall, the α and n values estimated with UI were larger than those measured with PC. These differences could be attributed, in part, to limitations of the PC method. On average, the n values calculated from the optimised S and β data were 5% larger than those obtained with PC. A relationship with a slope close to 1 fitted the n values estimated using both methods (nPC=0.73 nUI+0.49; R2=0.78, P<0.05). The results show that the UI method is a promising technique to estimate the hydraulic properties of undisturbed soil samples.

Correct estimation of the soil-water retention curve (WRC) is of paramount importance to characterise the hydraulic behaviour of soils. This paper studies the influence of two different soil-wetting processes (waterlogging soil, WP; capillary rise to saturation, CRP) on the estimate of the WRC. The two procedures were applied on undisturbed loam soil samples with three degrees of soil structure: (i) consolidated soils under conventional tillage (CT), reduced tillage (RT) and no tillage (NT); (ii) freshly tilled soil under CT and RT; and (iii) CT and RT after secondary tillage plus some intense rainfalls events. WRCs were estimated with time-domain reflectometry (TDR) pressure cells and volumetric water content was measured at saturation conditions (for the WP method) and at pressure heads of 0.5, 1.5, 3, 5, 10, 50, 100, 500 and 1500kPa. The same cores were used to determine the soil bulk density (ρb), which was subsequently used to estimate the saturated water content under CRP. The ρb value of the consolidated soil under NT was significantly higher (P<0.001) than under CT and RT. No effect of the wetting process on the WRC of consolidated soils was observed. Only the freshly tilled soil samples under RT were significantly affected by the wetting process. In these cases, the water draining after WP collapsed the more unstable soil macropores and increased the volume of the smaller ones. However, this effect was minimised by the CRP method, which prevented the collapse of the more unstable soil pores. This work demonstrates that the soil-wetting process may have an important effect on the characterisation of the water-holding capacity on freshly tilled soils.

Aggregate density (ρ) is defined as the relationship between the mass and the volume occupied by an aggregate. Previous studies have characterised ρ on large to medium-sized soil aggregates (>4mm diameter); however, little information is available for smaller aggregates (<4mm). The objective of this study was to test the viability of the photogrammetry (PHM) technique to determine the volume and subsequent ρ of small soil aggregates (1–8mm diameter). The method uses a standard digital camera that photographs a rotating aggregate and reconstructs its three-dimensional surface and the corresponding volume. To validate the method, the volume estimated with PHM on rough stones of different sizes (1–16mm diameter) was compared with the corresponding volume measured by the Archimedes’ principle. The method was tested on soil aggregates 1–8mm in diameter, collected from two sites under conventional and conservation tillage treatments. The strong correlation (R2>0.99, P<0.0001) between the volumes estimated on rough stones with the PHM and Archimedes methods demonstrates that this technique can be satisfactorily used to estimate the volume and, consequently, the ρ of small soil aggregates. The results showed an increase in ρ with decreasing aggregate size. A general trend of increasing ρ with the degree of soil disturbance by tillage was also observed.

In semiarid agroecosystems of the Ebro valley (NE Spain) soils are characterized by low soil organic matter (SOM) and a weak structure. In this study we investigated the individual and combined effect of tillage system (no‐tillage, NT; reduced tillage, RT; conventional tillage, CT) and cropping system (barley–fallow rotation at the Peñaflor site, PN‐BF and continuous barley at the Peñaflor site, PN‐BB) on soil organic carbon (SOC) storage as well as the physical protection of SOM fractions by soil aggregates in three long‐term experimental sites. In both cropping systems, total SOC content was more than 30% higher in NT compared with CT in the 0‐ to 5‐cm depth. The suppression of fallowing in the PN‐BB cropping system led to a greater SOC stabilization only in NT. In all the three sites, greater proportion of water‐stable macroaggregates (>250 μm) was found under NT than under CT in the 0‐ to 5‐cm depth. Macroaggregate organic C concentration (250–2000 μm) was greater in NT compared with CT in the BB cropping system, but did not differ with tillage treatment in the PN‐BF rotation. Greater proportion of microaggregates within macroaggregates in NT compared with CT was only found in the Agramunt site (AG). However, greater C stabilized inside these microaggregates was observed in AG, Selvanera site (SV), and PN‐BB in the 0‐ to 5‐cm depth. The results of this study demonstrate that in the semiarid Mediterranean agroecosystems of the Ebro valley, the adoption of NT together with the suppression of long‐fallowing period can significantly increase the amount of SOC stabilized in the soil surface and improve soil structure and aggregation.

Time domain reflectometry (TDR) is a technique that allows simultaneous estimates of apparent permittivity (εa) and hence volumetric water content (θ) and bulk electrical conductivity (σa). Difficulties arise for θ and σadetermination, however, when uncoated TDR probes (UP) are used in highly conductive media. This work shows that σacan be estimated in highly conductive media using a TDR probe coated with a high‐dielectric insulator (CP). To this end, the Dalton method for σaestimations was applied to a 10‐cm‐long three‐rod TDR probe insulated with a 0.2‐mm‐thick epoxy‐ceramic composite coating with a relative permittivity, εr, of 32.3. This method was calibrated on different NaCl–water solutions (0–15 dS m−1) and compared with the standard long‐time TDR method for σaestimations using an UP. The method was subsequently used for determining σain four different soils with different values of θ and σa(0–6 dS m−1) and again compared with the standard TDR procedure. The low error (RMSE = 1.5) for the comparison between the εameasured with the CP and that calculated with the analytical solution for coaxial probes indicates that the CP is accurate enough for εaestimations. For σavalues <4 dS m−1, the UP allows determinations of εaand the most accurate estimations of σausing the standard TDR method (R2= 0.99). For higher σavalues, however, estimations of εawere only possible using the CP, where σawas satisfactorily determined (R2= 0.99) using the Dalton method. To this end, a previous calibration between the real and the apparent bulk electrical conductivity estimated with the CP was required.

Under semiarid conditions, soil quality and productivity can be improved by enhancing soil organic matter content by means of alternative management practices. In this study, we evaluated the feasibility of no‐till (NT) and cropping intensification as alternative soil practices to increase soil organic C (SOC). At the same time, we studied the influence of these management practices on two SOC fractions (particulate organic matter C, POM‐C, and the mineral‐associated C, Min‐C), in semiarid agroecosystems of the Ebro River valley. Soil samples were collected from five soil layers (0–5‐, 5–10‐, 10–20‐, 20–30‐, 30–40‐cm depth) during July 2005 at three long‐term tillage experiments located at different sites in the Ebro River valley (northeast Spain). Soil bulk density, SOC concentration and content, SOC stratification ratio, POM‐C, and Min‐C were measured. Higher soil bulk density was observed under NT than under reduced tillage (RT), subsoil tillage (ST), or conventional tillage (CT). At the soil surface (0–5‐cm depth), the highest total SOC concentration, POM‐C, and Min‐C were measured under NT, followed by RT, ST, and CT, respectively. In the whole soil profile (0–40 cm), similarly, slightly greater SOC content was measured under NT than under CT with the exception of the Selvanera site, where deep subsoil tillage combined with moldboard plowing accumulated more SOC than NT. In semiarid Mediterranean agroecosystems where CT consists in moldboard plowing, NT is a viable management practice to increase SOC.

Losses of soil organic carbon (SOC) have contributed to CO2emissions from soils to the atmosphere and to global climate change. We hypothesized that in semiarid agroecosystems of the Mediterranean region, a shift from the traditional management system (including conventional tillage [CT] and a cereal–fallow rotation) to a more conservative system, including no‐till (NT) and continuous cropping, could reduce CO2emissions during the cropping season. Thus, in this study, we studied the effects of tillage and cropping systems on C inputs and soil CO2fluxes during three cropping seasons at three different sites in the Ebro River valley (northeast Spain). Carbon inputs ranged from 650 to 6000 kg ha−1and seasonal average CO2flux ranged from 0.10 to 1.76 g CO2m−2h−1Differences in rainfall led to marked differences in C inputs and soil fluxes among growing seasons. Although differences among tillage treatments were weak, CO2fluxes under NT were always lower. Intensification of cropping systems led to an increase in C input. A move from CT to NT together with cropping intensification is suitable to increase C inputs and to reduce soil CO2fluxes in semiarid Mediterranean agroecosystems.