Freshwater-irrigated Soils Research Articles

Hidden Resistome: Enrichment Reveals the Presence of Clinically Relevant Antibiotic Resistance Determinants in Treated Wastewater-Irrigated Soils.

Treated-wastewater (TW) irrigation transfers antibiotic-resistant bacteria (ARB) to soil, but persistence of these bacteria is generally low due to resilience of the soil microbiome. Nonetheless, wastewater-derived bacteria and associated antibiotic resistance genes (ARGs) may persist below detection levels and potentially proliferate under copiotrophic conditions. To test this hypothesis, we exposed soils from microcosm, lysimeter, and field experiments to short-term enrichment in copiotroph-stimulating media. In microcosms, enrichment stimulated growth of multidrug-resistant Escherichia coli up to 2 weeks after falling below detection limits. Lysimeter and orchard soils irrigated in-tandem with either freshwater or TW were subjected to culture-based, qPCR and shotgun metagenomic analyses prior, and subsequent, to enrichment. Although native TW- and freshwater-irrigated soil microbiomes and resistomes were similar to each other, enrichment resulted in higher abundances of cephalosporin- and carbapenem-resistant Enterobacteriaceae and in substantial differences in the composition of microbial communities and ARGs. Enrichment stimulated ARG-harboring Bacillaceae in the freshwater-irrigated soils, whereas in TWW-irrigated soils, ARG-harboring γ-proteobacterial families Enterobacteriaceae and Moraxellaceae were more profuse. We demonstrate that TW-derived ARB and associated ARGs can persist at below detection levels in irrigated soils and believe that similar short-term enrichment strategies can be applied for environmental antimicrobial risk assessment in the future.

Changes in alluvial soil quality under long-term irrigation with two marginal water sources in an arid environment

The current work aimed at verifying the modifications in chemical, physical, fertility, and environmental quality of alluvial soils southeast of the Nile Delta of Egypt following 25-years irrigation using two marginal water sources; agricultural drainage water (ADW) and sewage effluent water (SEW). Two sites irrigated with ADW and SEW were chosen and compared with a nearby site irrigated with the Nile freshwater. At each site, ten samples of irrigation water and adjacent (around 50 m) topsoil (0 – 30 cm) were collected and analyzed. Marginal water irrigation caused slight changes in soil properties. The soil chemical quality index was adversely affected through increasing the pH, electrical conductivity, and exchangeable sodium percentage. Available micro-nutrients (Fe, Mn, Zn, and Cu) and P in SEW-irrigated soils were increased. The physical quality index was also increased, but mainly due to inherent soil properties (particle size distribution) rather than irrigation water. Parent materials governed soil total contents of Cr, Co and Ni, while contents of Cd, Cu, Pb, and Zn were closely related to irrigation water. The environmental quality index showed increasing trend in the ADW-irrigated soils, while a decrease in the SEW-irrigated soils. The marginal water irrigation resulted in positive changes in soil quality index exhibiting increases of 14 and 21 % in the ADW- and SEW-irrigated soils over the Nile freshwater-irrigated soils, respectively. It is recommended to perform periodic monitoring for irrigation water and soil quality in the studied area. Proper in-situ remediation scenarios and on-farm practices should be considered in future management.

Spatial distribution of metals in soils under agriculture in the Jordan Valley

The aim of this study was to evaluate levels, sources, and spatial distribution of 6 metals in soils under agriculture in the Jordan valley. A total of 127 surface soil samples (101 irrigated with treated wastewater and 26 with fresh water) were collected at a density of one sample per 2 km2, and concentrations of As, Cd, Cr, Cu, Pb, and Zn were analyzed. In addition, clay content, pH, salinity, sodicity, organic carbon, total phosphorus and total nitrogen were measured for all samples. Principal component analysis and Pearson correlation matrix were used to identify possible metal sources. Average soil metal levels in treated wastewater were higher than in fresh water except for Cd. The mean values of the heavy metals in treated wastewater irrigated soils were 1.07, 2.87, 27.39, 14.46, 43.93, and 97.21 mg kg−1, while average metal contents in fresh water irrigated soils were 0.61, 3.22, 18.68, 8.03, 33.66, and 65.87 mg kg−1. The mean concentrations of all metals were below the maximum permissible concentration according to WHO/FAO threshold values for soils under agriculture. Multivariate and geostatistical analyses suggested that soil metals were mainly derived from agricultural practices (fertilizer, organic manure, and treated wastewater use) and parent materials. Results showed that Zn, and Cu were mainly derived from fertilizer use, As mainly from soil parent materials, and Cd, and Cr, and Pb mainly from treated wastewater and organic fertilizer use. Documenting metal levels in these soils may help in understanding their sources and assisting to take certain undertaking measures and actions to protect soil quality. Caution should be taken into consideration to decrease the excessive use of inorganic fertilizers and manage more efficiently TWW use.

Water Salinity Should Be Reduced for Irrigation to Minimize Its Risk of Increased Soil N₂O Emissions.

To reveal the effect of irrigation salinity on soil nitrous oxide (N2O) emission, pot experiments were designed with three irrigation salinity levels (NaCl and CaCl2 of 1, 2.5 and 4 g/L equivalence, Ec = 3.6, 8.1 and 12.7 ds/m), either for 0 kg N/ha (N0) or 120 kg N/ha (N120) nitrogen inputs. N2O emissions from soils irrigated at different salinity levels varied in a similar pattern which was triggered by soil moisture dynamics. Yet, the magnitudes of pulse N2O fluxes were significantly varied, with the peak flux at 5 g/L irrigation salinity level being much higher than at 2 and 8 g/L. Compared to fresh water irrigated soils, cumulative N2O fluxes were reduced by 22.7% and 39.6% (N0), 29.1% and 39.2% (N120) for soils irrigated with 2 and 8 g/L saline water, while they were increased by 87.7% (N0) and 58.3% (N120) for soils irrigated with 5 g/L saline water. These results suggested that the effect degree of salinity on consumption and production of N2O might vary among irrigation salinity ranges. As such, desalinating brackish water to a low salinity level (such as 2 g/L) before it is used for irrigation might be helpful for solving water resources crises and mitigating soil N2O emissions.

Mapping potential risks of long-term wastewater irrigation in alluvial soils, Egypt

In Egypt, wastewater has been used for irrigation in areas with fresh water scarcity; however, continuous applications may cause potential risks. Thus, the current study aims to map the spatial distribution of soil contamination and human risks of long-term wastewater irrigation due to the exposure of heavy metals. Soils from nine sites in Al-Qalyubia Governorate, Egypt, were sampled and analyzed. Wastewater irrigation resulted in a buildup of heavy metals in soils compared to Nile fresh water-irrigated soil. The pollution index (PI) showed the decreasing order of Cd > Zn > Ni > Cu > Co > Pb > Cr. The soils were out of the safe domain, as the integrated pollution index of Nemero’s (IPIN) exceeded the safe limit of 0.7. The enrichment factor (EF) exceeded 1.0, proving anthropogenic effects. The geo-accumulation index (Igeo) indicated high threats associated with Cd. The calculated hazard index (HI) indicated that humans exposed to such contaminated soils would have a potential health risk, particularly children. It is recommended to perform a treatment to the wastewater used in irrigation to avoid such threats.

Changes in Soil Properties in Response to Irrigation of Potato by Urban Wastewater

ABSTRACTThis study evaluated the physicochemical changes in the soil of potato field that was irrigated by fresh water, differentially diluted wastewater and undiluted wastewater (hereafter called wastewater). The potato crop was cultivated for consecutive three seasons under fertilized and unfertilized conditions. The wastewater contained higher concentrations of organic carbon (C), nitrogen (N), phosphorus (P), potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), sulfur (S), zinc (Zn) and boron (B) and lower concentrations of heavy metals. In this study, properties of wastewater-irrigated soil were compared with fresh water-irrigated soil. The application of wastewater reduced the bulk density of the surface soil by 2.83% and augmented the porosity by 6.02%. The unsaturated hydraulic conductivity and water retention capacity of the soil were improved under wastewater irrigation. Soil pH increased due to wastewater application but decreased, to a smaller extent, due to fertilizer application. Soil EC increased both with wastewater and fertilizer application; both parameters changed significantly in the top 0–15 cm soil layer. But, at the deeper layers, they were not affected by wastewater application. The organic C, total N, available P and S of the soils increased significantly (p = 0.05) when potato field was irrigated with raw wastewater. The organic C increased by 23.80% under wastewater irrigation in the top soil layer. The N content of the soil showed similarities with the organic C contents. Exchangeable Na, K, Ca, Mg; Zn and B of the soil also increased significantly with wastewater application. So, irrigation with urban wastewater is suggested to improve soil fertility as well as to ease pressure on the fresh water in the area of water scarcity.

Physicochemical Properties of Soil under Wheat Cultivation by Irrigation with Municipal Wastewater in Bangladesh

ABSTRACTIrrigation with wastewater provides the opportunity to solve the problems of its disposal, reuse and water conservation. Freshwater, differentially diluted wastewater and undiluted wastewater (hereafter called wastewater) were used to grow wheat in sandy loam soil under fertilized and unfertilized conditions at the experimental farm of Bangladesh Agricultural University, Bangladesh. Fresh groundwater and wastewater of Mymensingh municipality were used to irrigate a wheat field for three consecutive years to examine the effects of wastewater application on soil properties. In this study, the properties of wastewater-irrigated soil were compared with freshwater-irrigated soil. The application of wastewater reduced the bulk density of the surface soil by 1.92% and augmented the porosity by 5.89%. The unsaturated hydraulic conductivity and water retention capacity of the soil were improved under wastewater irrigation. Soil pH increased due to wastewater application but decreased, to a smaller extent, due to fertilizer application. Soil electrical conductivity (EC) increased both with wastewater and with fertilizer application; both parameters changed significantly in the 0–20 cm soil layer. However, at the deeper layers, they were not affected by wastewater application. The organic carbon (C) and total nitrogen (N) level of the soils were higher under wastewater irrigation than under freshwater-irrigated soil. The organic C increased by 23.93% under wastewater irrigation in the top 20 cm soil layer. The N content of the soil showed similarities with the organic C contents. Available P and S concentrations were greater in the soil irrigated with wastewater compared with the soil irrigated with freshwater. The exchangeable cations – sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg) – also increased significantly with wastewater application. Thus, farmers are advised for irrigation with municipal wastewater to ease pressure on freshwater and to improve soil fertility.

Anticonvulsant Drugs in the Agroenvironment: Fate and a Potential Bioremediation Tool

Introduction. Water shortage is a major concern in arid and semiarid regions, where the growing demand for irrigation water led to the use of reclaimed wastewater for agriculture. However, treated wastewater may contain active pharmaceuticals which are not being removed during wastewater treatment and are introduced to agricultural fields, where they can be taken up by crops. Although pharmaceuticals are found in the environment at very low concentrations, this is of health concern since the consequences of chronic exposure to a mixture of various compounds and their degradation products is largely elusive. We focus on the anticonvulsant drugs carbamazepine and lamotrigine which are widely used and found in soils and water. The aim of this study was to asses the microbial degradation of carbamazepine and lamotrigine in soil and by a pure culture of the fungus Pleurotus ostreatus. Methods. Carbamazepine and lamotrigine were incubated for 90 days in soils originally irrigated with either fresh water or treated wastewater. In addition, both compounds were incubated separately with pure cultures of P. ostreatus grown in liquid medium for 30 days. Removal rate of carbamazepine and lamotrigine was monitored using LC/MS or HPLC. Results. When incubated in soil, concentration of carbamazepine and lamotrigine remained constant during the incubation period, both in fresh water irrigated soil and soils irrigated with treated wastewater. However, when incubated in a pure liquid culture of P. ostreatus, >99% of the added carbamazepine and 70% of the added lamotrigine were removed. Conclusions. Our data show that carbamazepine and lamotrigine are persistent in soils when introduced via treated wastewater. However, pure cultures of the fungus P. ostreatus were able to remove a large amount of the added pharmaceuticals. This work highlights the need in new methods for removal of pharmaceuticals from treated wastewater and the potential of using P. ostreatus for this purpose.

Heavy metals potential health risk assessment through consumption of wastewater irrigated wild plants: A case study

ABSTRACTThis study investigates the potential health risk assessment of heavy metals consumption through wild plants (WPs) irrigated with wastewater and fresh water in the Bannu district, Pakistan. Therefore, soil and WP samples were collected and analyzed for heavy metals by atomic absorption spectrophotometer (Varian FAAS-240). Wastewater irrigated soil showed an elevated level for Cd (5.05–14.50 mg/Kg) and Ni (99.35–104.85 mg/Kg), while Cd (3.35–4.84 mg/Kg) concentration in the fresh water irrigated soil also surpassed the maximum allowable limits (MAL) set by the World Health Organization. Similarly, selected wastewater irrigated plants and fresh water irrigated plants also exceeded the MAL set by the Food and Agriculture Organization for Cd, Pb, and Cr (except for Cr in Vicia sativa). Heavy metal concentrations through WPs consumption were evaluated for the health risk index (HRI). The HRI value for each selected WPs was <1. However, total HRI values through WPs consumption was >1. Therefore, WPs consumption of the study area may pose a potential threat to the local community.

Heavy metal accumulation in soils and grains, and health risks associated with use of treated municipal wastewater in subsurface drip irrigation.

Constant use of treated wastewater (TWW) for irrigation over prolonged periods may cause buildup of heavy metals up to toxic levels for plants and animals, and entails environmental hazards in different aspects. However, application of TWW on agricultural land might be an effective and sustainable strategy in arid and semi-arid countries where fresh water resources are under great pressure, as long as potential harmful effects on the environment including soil, plants, and fresh water resources, and health risks to humans are minimized. The aim of this study was to assess the effect of deep emitters on limiting potential heavy metal accumulation in soils and grains, and health risk under drip irrigation with treated municipal wastewater. A field experiment was conducted according to a split block design with two treatments (fresh and wastewater) and three sub-treatments (0, 15, and 30cm depth of emitters) in four replicates on a sandy loam Calcic Argigypsids, in Esfahan, Iran. The annual rainfall is about 123mm, mean annual ETo is 1457mm, and the elevation is 1590m above sea level. A two-crop rotation of wheat (Triticum spp.) and corn (Zea mays) was established on each plot with wheat growing from February to June and corn from July to September. Soil samples were collected before planting and after harvesting for each crop in each year. Edible grain samples of corn and wheat were collected at harvest. Elemental concentrations (Cu, Zn, Cd, Pb, Cr, Ni) in soil and grains were determined using an atomic absorption spectrophotometer. Results showed that the concentrations of heavy metals in the wastewater-irrigated soils were not significantly different (P > 0.05) compared with the freshwater-irrigated soils. No significant difference (P > 0.05) in heavy metal content in soil between different depths of emitters was found. A pollution load index (PLI) showed that there was no substantial buildup of heavy metals in the wastewater-irrigated soils compared to the freshwater-irrigated soils. Cu, Pb, and Zn concentrations in wheat and corn grains were within the permissible US Environmental Protection Agency (EPA) limits, but concentrations of Cd (in wheat and corn) and Cr (in corn) were above the safe limits of the EPA. In addition, concentrations of Ni in wheat and corn seeds were several folds higher than the EPA standards. A health risk index (HRI) which is usually adopted to assess the health risk to hazard materials in foods showed values higher than 1 for Cd, particularly for wheat grain (HRI >2.5). Results also showed that intake of Cu through consumption of edible wheat grains posed a relatively high potential health risk to children (HRI >1.4), whereas children might also be exposed to health risk from Cd and Cr from corn grains (HRI >1.4). Based on aforementioned results, it can be concluded that the emitter depth in drip irrigation does not play a significant role in the accumulation of heavy metals from TWW in our sandy loam soil. Although their accumulation in the soil was limited and similar to using freshwater, uptake of Cd and Cr by wheat and corn was relatively large and hence resulted in health risk. The results suggest that more attention should be directed towards cultivation of other crops with drip irrigation system for a safe and more productive use of wastewater for irrigation. Alternatively, methods that filter the wastewater before it enters the soil environment might be an option that needs further investigation.

Risk Probability of Soil and Groundwater Pollution Due to Leaching Effect of Distillery Effluent Used for Agricultural Purpose

Distillery effluent is posing threat to the different components of the environment due to indiscriminate use of distillery effluent for irrigation of agricultural field, mainly affecting soil and water quality as the chemicals leaches to the groundwater table. The chemical substances transferred in soil profiles is a major environmental concern because even slow transport through the soil may eventually lead to deterioration of groundwater quality. The study reveals the physico-chemical characterisation of soil horizon of agricultural field which was regularly irrigated with distillery effluent for the observation of leaching effect of distillery effluent in soil horizons and ground water quality. The result showed that all the physico-chemical properties of soil horizons have high content of effluent irrigated soil compared to fresh water irrigated soils. It may be concluded that the higher value and indiscriminate use of distillery effluent on land affects the composition of the soil and groundwater quality due to leaching. The soil horizon showed higher value in surface soil while the values were less in lower soil horizons as compared to unaffected soil.

Fate of Graywater Constituents After Long-Term Application for Landscape Irrigation

While interest in and adoption of graywater reuse for irrigation has rapidly grown in recent years, little is known about the long-term effects of graywater irrigation. Concerns exist in relation to the presence of pathogenic organisms, fate of personal care products, and accumulation of salts. The purpose of this research was to evaluate the long-term effects of graywater irrigation to soil quality under real conditions where homeowners may not always apply graywater in a highly controlled manner. Four households from different climatic and geological conditions were selected for sampling (AZ, CA, CO, and TX) where graywater was applied for irrigation for a minimum of 5 years. Soil samples were collected in areas irrigated with graywater and areas irrigated with freshwater within the same yard. Soil cores were taken at depths of 0–15, 15–30, and 30–100 cm and analyzed separately for surfactants, antimicrobials, sodium adsorption ratio (SAR), electrical conductivity (EC), extractable boron, fecal indicator organisms (E. coli, enterococci, and Clostridium perfringens), and soil dehydrogenase activity. In surface soil samples (0–15 cm), the average total surfactant concentration (over all sites) was higher in graywater-irrigated soil (0.078 ± 0.033) compared to freshwater-irrigated soil (0.030 ± 0.025 mg kg−1). This difference was not found to be significant (P > 0.05). Triclosan and triclocarban were detected in surface soil samples at some locations (3.8–6.3 and 3.5–9.1 μg kg−1, respectively), but not in samples deeper than 15 cm. Among the sampling locations, the TX household appeared to be most impacted by graywater, as evidenced by elevated SAR, potentially toxic levels of B, and relatively high numbers of E. coli and enterococci due to 30 years of graywater application for irrigation.

Impact of Treated Wastewater Irrigation on Antibiotic Resistance in Agricultural Soils

Antibiotic resistance (AR) is a global phenomenon with severe epidemiological ramifications. Anthropogenically impacted natural aquatic and terrestrial environments can serve as reservoirs of antibiotic resistance genes (ARG), which can be horizontally transferred to human-associated bacteria through water and food webs, and thus contribute to AR proliferation. Treated-wastewater (TWW) irrigation is becoming increasingly prevalent in arid regions of the world, due to growing demand and decline in freshwater supplies. The release of residual antibiotic compounds, AR bacteria, and ARGs from wastewater effluent may result in proliferation of AR in irrigated soil microcosms. The aim of this study was to assess the impact of TWW-irrigation on soil AR bacterial and ARG reservoirs. Tetracycline, erythromycin, sulfonamide, and ciprofloxacin resistance in soil was assessed using standard culture-based isolation methods and culture-independent molecular analysis using quantitative real-time PCR (qPCR). High levels of bacterial antibiotic resistance were detected in both freshwater- and TWW-irrigated soils. Nonetheless, in most of the soils analyzed, AR bacteria and ARG levels in TWW-irrigated soils were on the whole identical (or sometimes even lower) than in the freshwater-irrigated soils, indicating that the high number of resistant bacteria that enter the soils from the TWW are not able to compete or survive in the soil environment and that they do not significantly contribute ARG to soil bacteria. This strongly suggests that the impact of the TWW-associated bacteria on the soil microbiome is on the whole negligible, and that the high levels of AR bacteria and ARGs in both the freshwater- and the TWW-irrigated soils are indicative of native AR associated with the natural soil microbiome.

Sorption of organic compounds to humin from soils irrigated with reclaimed wastewater

Humin is an important fraction of soil organic matter that is known to tightly bind many organic compounds. The objective of this study was to evaluate the role of humin as a natural sorbent in soils irrigated with freshwater and treated wastewater. To meet this goal, the sorption–desorption behavior of phenanthrene, atrazine and chlorotoluron was studied with humin isolated from Basra and Akko soils sampled from plots irrigated with treated wastewater or freshwater. For the Basra samples, the sorption affinities ( K OC values) of all tested compounds were higher for the humin sample isolated from the freshwater-irrigated soil than for that from the wastewater-irrigated soil. The opposite trend was recorded for the Akko humin samples. Desorption hysteresis was observed with atrazine but not with chlorotoluron or phenanthrene. Lipid extraction significantly decreased phenanthrene sorption affinities ( K OC values) for both Basra humin samples. For the humin isolated from the Akko freshwater-irrigated sample, lipid removal significantly decreased the sorption linearity, but sorption affinity was less affected. For the humin isolated from the Akko wastewater-irrigated sample, lipid extraction did not affect sorption affinity or linearity. Our data suggest that humin lipids constitute a powerful sorption domain and probably block pores or other high-energy adsorption sites within the humin matrix; therefore, their removal promotes desorption hysteresis. Our data show significantly small differences in sorption affinity between the humin and the source soil samples. We therefore suggest that the role of humin as a natural sorbent in soils should be studied on humin samples isolated under mild conditions to avoid the removal of matrix components and alteration of the humin.

Effects of Cd and Pb pollution on soil enzymatic activities and soil microbiota

Based on a representative sampling method and pot experiment with different concentrations of Cd and Pd, the enzymatic activities (urease, phosphatase, catalase, invertase), population of bacteria, fungus and actinomycete in the soil, the Cd and Pd pollution status of soil samples (from the wastewater-irrigated area of Baoding suburb) were appraised. Unitary linear and nonlinear curve-fitting optimization models were applied in the research, and the relationship between Pb and Cd causing pollution and enzymatic activities of the tested soils were discussed. The research may provide a theoretical basis for protecting the environment in the region of Baiyangdian Lake, Hebei province, prevent soil pollution, and ascertain biochemical indexes, which reflect soil heavy metal pollution levels. The research results indicated that: (1) there was obvious accumulation of Pb and Cd in the wastewater-irrigated area, also the accumulation in wastewater-irrigated soil is more than that in fresh water-irrigated soil, and accumulation on surface layer was more than that in the lower layer. Pb and Cd contents in the tested soils exceeded the standards of soil background values for some major cities at home and abroad and the world soil Cd and Pb contents range. This means that the tested soil had reached a lightly polluted level; (2) there existed an obvious negative correlation between soil enzymatic activities and Pb and Cd contents in wastewater-irrigated soil, where the soil urease and catalase activities decreased obviously with the increase of Pb and Cd contents in soil. Therefore, the urease and catalase can be considered as biochemical indexes that reflect the degree of soil Pb and Cd pollution; (3) the pot experiments indicated that the influence of Cd on soil enzymatic activities was greater than that of Pb. Generally, the effect of Cd on soil phosphatase, urease, catalase is more obvious than that on invertase, while Pb has a more obvious effect on invertase than Cd; (4) pot experiments of triple cropping showed that, Cd and Pb had an obvious inhibiting effect on microbes. The influence sensitivity of Cd and Pb on different microbes was ordered as bacteria > actinomycete > fungus. Cd had a greater inhibiting effect on microbes than Pb, the influence of Cd and Pb on actinomycete and fungus was relatively smaller, therefore, bacteria can be chosen as an important index that reflects the effect of Cd and Pb on microbe sensitivity.

The Role of Lipids on Sorption Characteristics of Freshwater‐ and Wastewater‐Irrigated Soils

The soil lipid fraction can play an important role in the sorption of organic compounds. In this study, the impact of the lipid fraction of freshwater- and wastewater-irrigated soils on the sorption of non- and relatively polar compounds was assessed. Lipid analyses revealed a clear difference between the two lipid fractions. The lipid extract from the wastewater-irrigated soil was consistent with mainly straight paraffinic chain materials; the lipid extract from freshwater-irrigated soil, on the other hand, exhibited stronger signals of aromatics, double bonds, ester, ether, and methyl, in addition to a smaller contribution from methylene protons. Our data suggest that lipid removal induced a stronger increase in the soil's sorption affinity for solutes capable of polar interactions such as atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) and chlorotoluron (N'-(3-chloro-4-methylphenyl)-N,N-dimethylurea) as compared to phenanthrene. Moreover, the level of increase in sorption affinities due to lipid removal was much higher for the freshwater-irrigated soil than for its wastewater-irrigated counterpart, even though the level of lipids in the freshwater-irrigated soil was half that in the wastewater-irrigated one (6 vs. 11% of the total organic C). The higher level of polar functionalities, such as ether and ester moieties, in the lipid fraction from the freshwater-irrigated soil suggests that these extractable compounds compete successfully with the polar solutes (atrazine and chlorotoluron) for specific binding sites in the soil organic matter (SOM). It appears that the composition of the lipid fraction may be a key consideration in unraveling the sorption of organic molecules in soils.

Model Demonstrating the Potential for Coupled Nitrification Denitrification in Soil Aggregates

A model of reactive, multi-species diffusion has been developed to describe N transformations in spherical soil aggregates, emphasizing the effects of irrigation with reclaimed wastewater. Oxygen demand for respiratory activity has been shown to promote the establishment of anaerobic conditions. Aggregate size and soil respiration rate were identified as the most significant parameters governing the existence and extent of the anaerobic volume in aggregates. The inclusion of kinetic models describing mineralization, nitrification, and denitrification facilitated the investigation of coupled nitrification/denitrification (CND), subject to O2 availability. N-transformations are shown to be affected by effluent-borne NH4+-N content, in addition to elevated BOD and pH levels. Their incremental contribution to O2 availability has been found to be secondary to respiratory activity. At the aggregate level, significant differences between apparent and gross rates of N-transformations were predicted (e.g., NH4+ oxidation and N2 formation), resulting from diffusive constraints due to aggregate size. With increasing anaerobic volume, the effective nitrification rate determined at the aggregates level decreases until its contribution to nitrification is negligible. It was found that the nitrification process was predominantly limited to aggregates <0.25 cm. Assuming that nitrification is the main source for NO3- formation, denitrification efficiency is predicted to peak in medium-sized aggregates, where aerobic and anaerobic conditions coexist, supporting CND. In effluent-irrigated soils, the predicted NO2- formation rate in small aggregates is enhanced when compared to freshwater-irrigated soils. The difference vanishes with increasing aggregate size as anaerobic NO2- consumption exceeds aerobic NO2- formation due to the coupling of nitrification and denitrification.