Biochar type and pyrolysis temperature effects on soil quality indicators and structural stability

Quality of soils of the arid zones with low organic matter can be improved through the application of natural amendments especially biochar from various available feedstocks. The objective of this study was to evaluate the impacts of corn residue and poultry manure and their biochars on soil organic carbon (SOC), hot-water soluble carbohydrates (HWSC), basal soil respiration (BSR),and structural stability determined by HEMC (high-energy moisture characteristic). A sandy loam soil in pots were thoroughly mixed with 1, 2 and 4% w/w of corn residues (CR) and poultry manure (PM) feedstock and their biochars prepared at 350 and 650 °C of slow pyrolysis. Maize seeds were planted in pots and grown until physiological maturity when soil characteristics were measured. Treatments considerably altered the means of studied soil quality indicators, and increased SOC (1.5-10 times) and HWSC (1-7 times), and HEMC indices: volume of drainable pores ratio (VDPR, 1.5- 3.5 times), and stability ratio (SR, 1-3 times). Increasing pyrolysis temperature, regardless of the type and rate of feedstock, significantly decreased the SOC, BSR and percent of water-stable aggregates, and consequently structural stability indices. Contribution of both PM feedstock and its biochars was less effective than the CR ones (particularly the biochar produced at higher pyrolysis temperature), due to elevated sodium adsorption ratio (SAR) associated with higher slaking, physico-chemical dispersion and lower aggregate and structural stability.

Grazing gradient detection and assessment in arid rangelands of central Iran using remote sensing and soil-vegetation characteristics

One key tool to manage fragile ecosystems in arid lands is knowledge of land degradation. For this purpose, field-based soil and vegetation characteristics were related to remote sensing information extracted from Landsat OLI moving standard deviation index (MSDI), normalised difference vegetation index (NDVI), soil-adjusted vegetation index (SAVI) and modified soil-adjusted vegetation index (MSAVI) in degraded (watering points and livestock camps) and reference sites of arid rangelands of Iran. Values of spectral indices along grazing gradients at a distance of 50–1500m from the centre of degradation were also extracted and analysed. Statistical analysis (t-test and ordination analysis) shows that MSDI is a sensitive index of landscape heterogeneity and, unlike other indices, exhibits a significant difference between degraded and reference sites. MSDI was sensitive to grazing gradients around watering points and livestock camps, and declined with increasing distance from these areas, although this varied between different sites due to grazing intensity and characteristics of soil and vegetation cover. Soil analysis showed that the amount of organic carbon, mean weight diameter (MWD), geometric mean diameter (GMD) and soil stability indicator (SSI) at reference sites were significantly higher than those at degraded areas. Ordination analysis confirmed the results of t-test regarding the high potential capability of MSDI in separating degraded areas. This simple and practical index can be used to determine ecological thresholds and investigating rangeland condition in arid ecosystems.

Effect of crude oil-induced water repellency on transport of Escherichia coli and bromide through repacked and physically-weathered soil columns

Knowledge of the transport and fate of pathogenic Escherichia coli, especially in the areas contaminated with crude oil, is required to assess contamination of shallow groundwater resources. The present study aims to investigate the effect of crude oil-mediated water repellency on the movement of nalidixic acid-resistant Escherichia coli strain (E. coli NAR) and bromide (Br) as an inert tracer in two soil types. The soils were contaminated at three levels of 0, 0.5 and 1% w/w of total petroleum hydrocarbons (TPHs) using crude oil. Steady-state saturated flow in the soil columns was controlled using a tension infiltrometer. Leaching experiments were conducted through the columns of repacked (un-weathered) and physically-weathered clay loam (CL) and sandy loam (SL) soils. The columns leachate was sampled at specific times for 4 pore volumes. The shape of breakthrough curves for the E. coli NAR and Br depended on soil texture and structure and the TPHs level. Preferential flow in the crude oil-mediated water-repellent soils facilitated the transport of contaminants especially E. coli NAR. Filtration coefficient and relative adsorption index of bacteria were greatest in the repacked CL soils and were lowest in the weathered SL soils. Discontinuity of soil pores and lower flow velocity resulted in greater filtration of E. coli NAR in the repacked CL soil than other treatments. Physical weathering induced the formation of aggregates which reduced soil particle surfaces available for retention of water-repellent oil and contaminants. Movement of both bacteria and Br tracer in the weathered SL soil with 1% TPHs was higher than other treatments. This finding was attributed to low specific surface area, continuity of the pores and water repellency-mediated preferential pathways in the weathered SL soil columns. Our findings implied that shallow groundwater resources could be very sensitive to microbial contamination particularly in the oil-mediated water-repellent soils.

Impact of calcium carbonate and temperature on survival of Escherichia coli in soil

Spreading of waste organic matter on agricultural lands is considered to enhance soil microbial activities and physical properties and improves soil nutrient status. However, organic wastes have also been shown to be a source of microbial contaminants including pathogens. Related risks are governed by pathogens' survival and transport particularities. We evaluated the significance of high levels of CaCO3, common in arid and semi-arid soils, on survival of Escherichia coli NAR at different temperatures. Amendments of 0, 5, 10, 15 or 25 g CaCO3 were mixed into variable soil amounts to obtain 100 g soil-CaCO3 mixtures. Both sterile and non-sterile soil mixtures were tested. Suspensions of a nalidixic acid-resistant E. coli strain (E. coli NAR) were added to the mixtures at a rate of 10(6) cell g(-1) soil. Mixtures were incubated at 4, 15, or 37 °C at the soil's field capacity for water (i.e. 0.13 g g(-1)). Each treatment was tested in triplicate. Persistence of culturable E. coli NAR was verified throughout the incubation period. The recovery rates of culturable E. coli NAR were significantly correlated to CaCO3 concentrations (P < 0.05). Incubation temperature (T) was the most significant factor (P < 0.01). In non-sterile mixtures the largest decline in survival rates of E. coli NAR was measured for treatments with larger CaCO3 content (i.e. 15 and 25%). Interaction of temperature and CaCO3 was significant for E. coli NAR die-off. Sterilization of soil caused non-uniform fluctuations in the effect of treatments. The maximum calculated decay rate for E. coli NAR was 0.83 d(-1) for the 15 g CaCO3 non-sterile mixture incubated at 37 °C while the minimum was 0.09 d(-1) for the control unamended sterile soil incubated at 15 °C. A combination of high temperature, large CaCO3 concentrations and a non-sterile, biologically active soil created the least favorable conditions for E. coli survival.

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.

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.