A.A. Mahboubi

Influence of organic waste type and soil structure on the bacterial filtration rates in unsaturated intact soil columns

Organic wastes are considered to be a source for the potentially pathogenic microorganisms found in surface and sub-surface water resources. Following their release from the organic waste matrix, bacteria often infiltrate into soil and may be transported to significant depths contaminating aquifers. We investigated the influence of soil texture and structure and most importantly the organic waste properties on the transport and filtration coefficients of Escherichia coli and total bacteria in undisturbed soil columns. Intact soil columns (diameter 16 cm and height 25 cm) were collected from two soils: sandy clay loam (SCL) and loamy sand (LS) in Hamadan, western Iran. The cores were amended with cow manure, poultry manure and sewage sludge at a rate of 10 Mg ha(-1) (dry basis). The amended soil cores were leached at a steady-state flux of 4.8 cm h(-1) (i.e. 0.12 of saturated hydraulic conductivity of the SCL) to a total volume of up to 4 times the pore volume of the columns. The influent (C(0)) and effluent (C) were sampled at similar time intervals during the experiments and bacterial concentrations were measured by the plate count method. Cumulative numbers of the leached bacteria, filtration coefficient (lambda(f)), and relative adsorption index (S(R)) were calculated. The preferential pathways and stable structure of the SCL facilitated the rapid transport and early appearance of the bacteria in the effluent. The LS filtered more bacteria when compared with the SCL. The effluent contamination of poultry manure-treated columns was greater than the cow manure- and sewage sludge-treated ones. The difference between cow manure and sewage sludge was negligible. The lambda(f) and S(R) values for E. coli and total bacteria were greater in the LS than in the SCL. This indicates a predominant role for the physical pore-obstruction filtration mechanisms as present in the poorly structured LS vs. the retention at adsorptive sites (chemical filtration) more likely in the better structured SCL. While the results confirmed the significant role of soil structure and preferential (macroporous) pathways, manure type was proven to have a major role in determining the maximum penetration risk of bacteria by governing filtration of bacteria. Thus while the numbers of bacteria in waste may be of significance for shallow aquifers, the type of waste may determine the risk for microbial contamination of deep aquifers.

Soil water availability for plants as quantified by conventional available water, least limiting water range and integral water capacity

There are different approaches to define the soil available water (SAW) for plants. The objectives of this study are to evaluate the SAW values of 12 arable soils from Hamadan province (western Iran) calculated by plant available water (PAW), least limiting water range (LLWR) and integral water capacity (IWC) approaches and to explore their relations with Dexter’s index of soil physical quality (i.e., S-value). Soil water retention and mechanical resistance were determined on the intact samples which were taken from the 5–10 cm layer. For calculation of LLWR and IWC, the van Genuchten-Mualem model was fitted to the observed soil water retention data. Two matric suctions (h) of 100 and 330 cm were used for the field capacity (FC). There were significant differences (P < 0.01) between the SAW values calculated by PAW100, PAW330, LLWR100, LLWR330 and IWC. The highest (i.e., 0.210 cm3 cm−3) and the lowest (i.e., 0.129 cm3 cm−3) means of SAW were calculated for the IWC and LLWR330, respectively. The upper limit of LLWR330 for all of the soils was h of 330 cm, and that of LLWR100 (except for one soil that was air-filled porosity of 0.1 cm3 cm−3) was h of 100 cm. The lower limit of LLWR330 and LLWR100 for five soils was h of 15,000 cm and for seven soils was mechanical resistance of 2 MPa. The IWC values were smaller than those of LLWR100 for two soils, equal to those of LLWR100 for three soils and greater than those of LLWR100 for the rest. There is, therefore, a tendency to predict more SAW using the IWC approach than with the LLWR approach. This is due to the chosen critical soil limits and gradual changes of soil limitations vs. water content in the IWC calculation procedure. Significant relationships of SAW with bulk density or relative bulk density were found but not with the clay and organic matter contents. Linear relations between IWC and LLWR100 or LLWR330 were found as: IWC = −0.0514 + 1.4438LLWR100, R2 = 0.83; and IWC = −0.0405 + 2.0465LLWR330, R2 = 0.84, respectively (both significant at P < 0.01). Significant relationships were obtained between the SAW values and S indicating the suitability of the index S to explain the availability of soil water for plants even when complicated approaches like IWC are considered. Overall, the results demonstrate the importance of the choice of the approach to be used and its critical limits in the estimation of the soil available water to plants.

Long-term tillage effects on changes in structural properties of two soils in central Ohio

Abstract Soil analyses were done to evaluate changes in percent water stable aggregation (i.e. computed as percent of the total soil dry weight retained by sieves during the standard wet sieving analysis) and mean weight diameter (MWD) of aggregates for different methods of seedbed preparation. These studies were conducted during 1990–1991 in tillage experiments established in 1962 on a Typic Fragiudalfs and an Aeric Ochraqualfs in Ohio. Continuous corn was grown using no-till (NT), chisel plowing (CP) and moldboard plowing (MP) treatments. Soil samples 15 cm deep were obtained for four seasons throughout the 1990–1991 growing seasons. Although results are based on one-year analyses, the data showed significant seasonal trends. For example, the aggregation seasonal means were 38.5%, 34.8%, 32.6% and 23.1% for summer, autumn, spring and winter, respectively. Similarly, the average MWD was 0.93, 0.89, 0.81 and 0.61 mm in summer, autumn, winter, and spring, respectively. Aggregation over both sites and all seasons was 41.4%, 29.6% and 25.8%, and the average MWD was 1.19, 0.71, and 0.53 mm for NT, CP, and MP, respectively. There also existed a tillage×season interaction. Total aggregation for NT was 50.1%, 28.4%, 38.9%, and 48.0%, for CP was 30.8%, 20.1%, 31.5%, and 36.1%, and for MP was 23.4%, 20.8%, 27.5%, and 31.5% for autumn, winter, spring, and summer, respectively. Additional studies are needed to quantify significant seasonal changes in soil structure for planning soil and crop management techniques for minimizing risks of soil degradation by erosion and compaction.

Point pedotransfer functions for prediction of water retention of selected soil series in a semi-arid region of western Iran

This study was conducted to derive point pedotransfer functions (PPTFs) for soil water retention (SWR) in western Iran. Topsoil and subsoil of 63 soil series, which were representative of different regions of Hamadan province, were sampled. Soil water retention was determined by the sand box and pressure plate at matric suctions (h m) of 0, 1, 2, 5, 10, 25, 50, 100, 200, 500, 1000 and 1500 kPa. PPTFs were derived through multiple linear regressions for the topsoils and subsoils. These used particle size distribution, bulk density, organic matter, calcium carbonate and gravel contents as easily-available inputs. To increase the accuracy of the PPTFs, saturated water content was also included as an input variable in a group of PPTFs but they are not better as assessed using the Akaike Information Criterion. All of the PPTFs were statistically significant (p < 0.001) and could be used to predict the SWR. The absolute effect of bulk density on the SWR diminished as h m increased. Bulk density decreased the SWR for low h m and increased it for high h m. In the wet range, organic matter increased the SWR. Clay and silt increased SWR whereas gravel decreased it. The effect of calcium carbonate on SWR was negligible.

Significance of physical weathering of two-texturally different soils for the saturated transport of Escherichia coli and bromide

This study was carried out to investigate the transport of Escherichia coli NAR and bromide (Br) through repacked (R) and weathered (W) soil columns. A suspension containing E. coli NAR and Br were leached and the effluent from the weathered soil columns had greater contaminant concentrations than that from the repacked soil columns. The time to the concentration peak of (C(max)) E. coli NAR and Br increased in the order CL-W < SL-W < SL-R < CL-R. The breakthrough sequence suggests the formation of a heterogeneous soil pore network induced by weathering and the importance of accelerated flow in the weathered columns. The dual-permeability model in HYDRUS-1D software was used to simulate the E. coli NAR and Br transport parameters by inverse modeling. Parameters of the attachment-detachment model were calculated using the dual-permeability model parameters fitted to the BTCs of E. coli NAR. A greater attachment coefficient associated with soil repacking and the finer textured clayey soil demonstrated the importance of adsorbent site and smaller pore spacing in these treatments. Smaller attachment and adsorption isotherm coefficients in weathered soil columns suggest the need for further research to validate this as a predictive model for the risks for vadose zone contaminant transport.

Water Repellency in Calcareous Soils Under Different Land Uses in Western Iran

Abstract Soil wettability and water repellency, two important soil physical properties, play an important role in water retention and water conductivity in arid and semi-arid regions. To date, there is a lack of information on soil water repellency in calcareous soils of western Iran. In this study, soil water repellency and its affecting factors were studied using 20 soil series collected from Hamadan Province, western Iran. The effects of soil properties including organic carbon content (SOC), total nitrogen (TN), C:N ratio, texture, CaCO 3 content, and both fungal and bacterial activities on water repellency were investigated using air-dried, oven-dried and heated soil samples. Water repellency index (WRI) was determined using the short-time sorptivity (water/ethanol) method. To distinguish the actual effects of SOC, a set of soil samples were heated at 300 °C to remove SOC and then WRI was measured on the heated samples. Relative water repellency index (RWRI) was defined as the change of WRI due to heating relative to the oven-dry WRI value. Results of the WRI values showed that the soils were sub-critically water-repellent. Pasture soils had higher WRI values compared to tilled soils, resulting from high SOC and TN, and high activities of bacteria and fungi. It was observed that SOC, TN, fungal activity, and SOC:clay ratio had significant positive impacts on WRI. Strong positive correlations of RWRI with SOC, TN and fungal activity were also observed. Pedotransfer functions derived for predicting WRI showed that the WRI values had an increasing trend with the increases in fungal activity, salinity, alkalinity and fine clay content, but showed a decreasing trend with increasing bacterial activity.

Conservation management practices: Success story of the Hog Creek and Sturgeon River watersheds, Ontario, Canada

The soil erosion from agricultural watersheds can be reduced by implementation of conservation management practices. In this study, the effectiveness of most popular agricultural best management practices (BMPs) for reducing sediment loads within Hog Creek and Sturgeon River watersheds in Ontario was investigated using measurement of the shift in the sediment rating curves from pre-BMP (1989 to 1993) to post-BMP (2004 to 2008) implementation periods. The data from the water quality monitoring program for the Hog Creek and the Sturgeon River watersheds over this decade of extensive conservation management program implementation showed significant reductions in the sediment loads of 49% for Hog Creek and 41% for the Sturgeon River. The results showed that the most widely adopted BMPs that greatly influenced the overall removal in sediment loads were stream bank fencing, no-till farming, and vegetative buffer strips. Overall, the outcome of the study recommends these promising practices to protect and improve receiving water quality. The practical novel technique presented in this study for quantification of the overall long-term water quality benefits of conservation management practices can be an integral part of an adaptive strategy for a watershed-scale BMP implementation program.

Comparison of three models describing bromide transport affected by different soil structure types

This study was conducted to investigate the effects of soil structure on bromide (Br) transport through three soils with granular, prismatic, and single-grain structures. The breakthrough curve (BTC) of the single-grain structure was sigmoidal, symmetrical and similar to a piston flow, showing the dominance of mass flow. In contrast, the BTCs of the granular and prismatic structures were initially steep, becoming more gradual at high pore volumes (PVs). The stable structure and preferential pathways caused the early breakthrough of Br in the leachate of these columns. The convection–dispersion equation (CDE), mobile–immobile water (MIM), and dual-permeability (DP) models were fitted to observed data using the program HYDRUS-1D. The equilibrium transport model (CDE) was not as successful as non-equilibrium (MIM and DP) models in describing the Br transport in prismatic and granular soil columns, although it was able to describe the Br transport in single-grain column well. Overall, the results demonstrated the importance of soil structure in pollutant transport through soils.

Evapotranspiration models assessment under hyper-arid environment

In this study, we develop more accurate hyper-arid evapotranspiration (ET) models to help improve irrigation water conservation. We examine five ET models (one combination model, three radiation-based models, and one temperature-based model) under hyper-arid condition at three center-pivot fields in the Kingdom of Saudi Arabia. These models were evaluated and calibrated for the alfalfa crop of 2010 and validated for the wheat and potato crops of 2011. The FAO-56 Penman–Monteith (PM) was the most accurate ET model for estimating crop water irrigation needs. The Turc and the Makkink solar radiation-based ET models provided the least accurate estimates even after calibration, while the calibrated Hargreaves–Samani temperature-based model provided the second most accurate estimates for irrigation scheduling in hyper-arid environments. Unlike the FAO-56 PM model, Hargreaves–Samani does not require wind speed or relative humidity data. The most sensitive parameter for this model is air temperature, which is readily available at most sites. The Priestley–Taylor model is highly sensitive to solar radiation data that may not be locally available. The main drawback of the FAO-56 PM model is that it requires extensive list of meteorological data. Weather forecasts are often limited to air temperature data that limit the use of the FAO-56 PM model for irrigation scheduling compared to the calibrated Hargreaves–Samani model.

Long-term Cultivation and Landscape Position Effects on Aggregate Size and Organic Carbon Fractionation on Surface Soil Properties in Semi-arid Region of Iran

ABSTRACT This study was carried out to investigate the effects of long-term cultivation and landscape position on organic carbon content and soil aggregation. Sampling sites were determined based upon land use at the end of 50 years soil use and management, cultivated/annual wheat cropping and grazed pasture, and landscape position in Chaharmahal-va-Bakhtiary province, southwest Iran. Soil samples were collected from the 0–5 cm and 5–15 cm depths in two adjacent fields that have the same slope and aspect. The soil was silty clay at the summit and footslope positions, and was a silty clay loam at the backslope. Wet-sieving analysis and aggregate-size fractionation methods were used to separate the samples into three aggregate fractions (i.e., 2–4.75, 0.25–2, and 0.053–0.25 mm). The treatments were arranged in a factorial design. Land use significantly affected the water-stable aggregate fractions, so that the wet soil stability of the macroaggregates (i.e., 2–4.75 mm) was higher in the pasture, whereas it was greater for the meso-aggregates (i.e., 0.25–2 mm) in the cultivated soils. Cultivation decreased both the wet-aggregate stability and percent of macroaggregates whereas long-term pasture enhanced aggregation. Soil organic carbon (SOC) content within aggregates and primary particles was also significantly influenced by landscape position, land use, and the depth of sampling. The SOC content was higher in clay than those in silt and sand contents. The SOC content decreased as depth increased in all fractions. In general, the highest and lowest wet-stable aggregates were observed on the footslope and backslope positions, respectively.