Variation of soil properties with sampling depth in two different light-textured soils after repeated applications of urban sewage sludge

Semi-arid agricultural soils have increasingly been subjected to urban sewage sludge (USS) applications due to accelerated soil depletion and shortages in manure supply. Research studies addressing USS reuse have mostly been conducted in cropping systems and focused on changes in topsoil properties of a given texture. Therefore, sludge-soil interactions could be largely influenced by the presence of plants, soil particle composition and depth. In this field study, two agricultural soils (sandy, S and sandy loam, SL) received simultaneously four annual USS applications of 40, 80, and 120 t ha−1 year−1 in absence of vegetation. Outcomes showed the increase of carbon and macronutrients in both soils proportionally to USS dose especially in the topsoil profile (0–20 cm). Subsoil (20–40 cm) was similarly influenced by sludge rates, showing comparable variations of fertility parameters though at significant lower levels. The depth-dependent improvement of soil fertility in both layers enhanced the microbiological properties accordingly, with significant variations in soil SL characterized by a higher clay content than soil S. Besides, positive correlations between increases in sludge dose, salinity, trace metals, and enzyme activities in both soils indicate that excessive sludge doses did not cause soil degradation or biotoxic effects under the described experimental conditions. In particular and despite high geoaccumulation indices of Ni in both soils and profiles, the global concentrations of Cu, Ni, Pb, and Zn were still below threshold levels for contaminated soils. In addition, the maintenance of pH values within neutral range and the increase of organic matter content with respect to control would have further reduced metal availability in amended soils. Therefore, we could closely investigate the effects of texture and depth on the intrinsic resilience of each soil to cope with repetitive USS applications.This research study was financially supported by the Ministry of Higher Education and Scientific Research of Tunisia

Monitoring the variation of soil quality with sewage sludge application rates in absence of rhizosphere effect

Agricultural soils in semi-arid regions have frequently been degraded due to adverse climatic conditions, organic matter depletion, and poor farming practices. To enhance soil quality, this study examines the reuse of sewage sludge (SS) as an available source of organic matter in a typical Mediterranean sandy-loam soil. Accordingly, we studied the cumulative effect of two annual applications of 40, 80 and 120 tons of sludge per ha on soil quality in absence of vegetation. The dose-dependent improvement of organic matter content was the most significant event that reflected sludge application rates, and consequently influenced other soil properties. Accordingly, soil structural stability increased by 13.3%, 28.8% and 59.4% for treatments SS-40, SS-80 and SS-120 respectively as compared to unamended control. Structural stability improvement was also confirmed by the dose-dependent variation of other edaphic factors including calcium content, the microbial quotient as well as Welt and C:N ratios. These parameters are involved in cementing soil aggregates by cation bridging, the formation of microbial mucilage, and clay-humic complexes. Soil magnetic susceptibility (SMS) was measured in situ as a possible rapid tool to evaluate soil condition. SMS showed significant correlation with sludge dose and stability amelioration testifying to the aggregation role that can play Al2O3 and particularly Fe2O3 minerals added by the hematite-rich sludge. Besides, analytical results and field observations revealed no trends of soil salinization or acidification by excessive sludge amounts. By avoiding the rhizosphere effect, outcomes could reflect the resilience and intrinsic capacity of the soil to cope with excessive sludge loads.This study was financially supported by a research grant from the Tunisian Ministry of Higher Education and Scientific Research. The authors would like to thank the National Sanitation Utility (ONAS) for providing urban sewage sludge. The technical support of Rym Ghrib is hereby acknowledged

Correction: Mahmoudi et al. Effect of Surface and Subsurface Drip Irrigation with Treated Wastewater on Soil and Water Productivity of Okra (<i>Abemoschus esculentus</i>) Crop in Semi-Arid Region of Tunisia. <i>Agriculture</i> 2022, <i>12</i>, 2048

The authors wish to make the following corrections to the original paper [...

Effect of Surface and Subsurface Drip Irrigation with Treated Wastewater on Soil and Water Productivity of Okra (<i>Abemoschus esculentus</i>) Crop in Semi-Arid Region of Tunisia

Under semi-arid conditions, irrigated agriculture faces hard competition for water. It is against this backdrop that appropriate management of irrigation techniques and water resources becomes a major concern. This study investigated the effect of surface (SDI) and subsurface drip irrigation (SSDI) with domestic treated wastewater (TWW) and fresh water (FW) on soil water dynamics, salinity, yield, and mineral nutrition of okra. The experimental design was set-up based on two adjacent plots according to the water quality: Fresh Water (FW) T1 and domestic Treated Wastewater (TWW) T2. Results showed that measured soil water contents (SWCs), under TWW treatment (T2), were greater than their corresponding measurements under FW (T1), and in particular at 35 cm depth. Meanwhile, for both water qualities, soil Electrical Conductivity (EC) registered at 5 cm depth was higher than those measured at 35 cm, with values ranging from 0.14 to 0.36 mS·cm−1 and from 0.20 to 0.47 mS·cm−m for T1 and T2, respectively. Regarding crop yield, a statistically significant increase (p = 0.05) in okra fresh yield was observed when TWW was used. Fresh yield in SDI was 2.55 t·ha−1 and 3.9 t·ha−1 in T1 and T2, respectively. Nevertheless, results indicated that lateral depth did not significantly affect okra fresh yield. Moreover, a significant higher irrigation water productivity (WPirrig) with TWW (1.08 ± 0.26 and 1.23 ± 0.18 kg m−1) was observed, which was nearly double those obtained with FW (0.72 ± 0.33 to 0.78 ± 0.18 kg m−1). Appropriate use of SSDI with TWW stands as an irrigation management technique to improve yield and irrigation water productivity of okra crops

Effect of Surface and Subsurface Drip Irrigation with Treated Wastewater on Soil and Water Productivity of Okra (Abemoschus esculentus) Crop in Semi-Arid Region of Tunisia

Under semi-arid conditions, irrigated agriculture faces hard competition for water. It is against this backdrop that appropriate management of irrigation techniques and water resources becomes a major concern. This study investigated the effect of surface (SDI) and subsurface drip irrigation (SSDI) with domestic treated wastewater (TWW) and fresh water (FW) on soil water dynamics, salinity, yield, and mineral nutrition of okra. The experimental design was set-up based on two adjacent plots according to the water quality: Fresh Water (FW) T1 and domestic Treated Wastewater (TWW) T2. Results showed that measured soil water contents (SWCs), under TWW treatment (T2), were greater than their corresponding measurements under FW (T1), and in particular at 35 cm depth. Meanwhile, for both water qualities, soil Electrical Conductivity (EC) registered at 5 cm depth was higher than those measured at 35 cm, with values ranging from 0.14 to 0.36 mS&middot;cm&minus;1 and from 0.20 to 0.47 mS&middot;cm&minus;m for T1 and T2, respectively. Regarding crop yield, a statistically significant increase (p = 0.05) in okra fresh yield was observed when TWW was used. Fresh yield in SDI was 2.55 t&middot;ha&minus;1 and 3.9 t&middot;ha&minus;1 in T1 and T2, respectively. Nevertheless, results indicated that lateral depth did not significantly affect okra fresh yield. Moreover, a significant higher irrigation water productivity (WPirrig) with TWW (1.08 &plusmn; 0.26 and 1.23 &plusmn; 0.18 kg m&minus;1) was observed, which was nearly double those obtained with FW (0.72 &plusmn; 0.33 to 0.78 &plusmn; 0.18 kg m&minus;1). Appropriate use of SSDI with TWW stands as an irrigation management technique to improve yield and irrigation water productivity of okra crops

Co-occurrence of antibiotic and metal resistance in long-term sewage sludge-amended soils: influence of application rates and pedo-climatic conditions

Urban sewage sludge (USS) is increasingly being used as an alternative organic amendment in agriculture. Because USS originates mostly from human excreta, partially metabolized pharmaceuticals have also been considered in risk assessment studies after reuse. In this regard, we investigated the cumulative effect of five annual USS applications on the spread of antibiotic-resistant bacteria (ARB) and their subsequent resistance to toxic metals in two unvegetated soils. Eventually, USS contained bacterial strains resistant to all addressed antibiotics with indices of resistance varying between 0.25 for gentamicin to 38% for ampicillin and azithromycin. Sludge-amended soils showed also the emergence of resistome for all tested antibiotics compared to non-treated controls. In this regard, the increase of sludge dose generally correlated with ARB counts, while soil texture had no influence. On the other hand, the multi-antibiotic resistance (MAR) of 52 isolates selected from USS and different soil treatments was investigated for 10 most prescribed antibiotics. Nine isolates showed significant MAR index (≥ 0.3) and co-resistance to Cd, As and Be as well. However, events including an extreme flash flood and the termination of USS applications significantly disrupted ARB communities in all soil treatments. In any case, this study highlighted the risks of ARB spread in sludge-amended soils and a greater concern with the recent exacerbation of antibiotic overuse following COVID-19 outbreak.Open Access funding provided by the Qatar National Library. This research study was financially supported by the Ministry of Higher Education and Scientific Research of Tunisia (Grant #11–18)

Use of Electrical Resistivity Tomography to Study the Impact of Long-Term Application of Sewage Sludge: Case Study of Oued Souhil Experimental Station, Nabeul (Tunisia)

Soil salinization following repetitive applications of sewage sludge (S.S.) is a major environmental impediment with severe adverse impacts on agricultural productivity and sustainability, particularly in arid and semiarid regions. Electrical resistivity tomography (ERT) can be easily and rapidly assessed in the field for soil salinity investigations. Accordingly, ERT was conducted on sandy soil (soil S) in a semiarid Region in Tunisia (Oued Souhil, Nabeul) after repetitive S.S. application at the equivalent rates: 0, 40, 80, and 120 t/ha. The study aimed to monitor topsoil and subsoil salinization following the sludge trial. As such, the ERT survey was measured across each field plot of 4 m2 using the Lund System of ABEM with SAS4000 Resistivimeter. Wenner array was applied using 16 electrodes and 0.2 m electrode spacing, which corresponds to approximately 0.6 m depth of investigation. Laboratory analyses were conducted on soil samples taken in the middle of the treated plots at four depths (0–20 cm), (20–40 cm), and (40–60 cm) for a comparison with the ERT survey result. ERT panels showed a general decrease in soil resistivity below (40 cm) with extremely low resistivity in each plot’s corner, indicating a vertical leaching and a lateral migration of conductive materials in all the treatments. However, the laboratory analysis did not confirm these results, showing the decrease in soil electrical conductivity (E.C.) with soil depth in the amended soils. Soil parameters (E.C., O.M.) decreased drastically from the last amendment (in 2017), reaching lower levels than the experimental soil (before sludge trial). This fact could be attributed to the heavy rainfall recorded in Nabeul in September 2018 that caused soil leaching throughout the soil profile, as shown by the ERT panels

Carbon mineralization, biological indicators, and phytotoxicity to assess the impact of urban sewage sludge on two light-textured soils in a microcosm

The agricultural reuse of urban sewage sludge (USS) modifies soil properties depending on sludge quality, management, and pedo-environmental conditions. The aim of this microcosm study was to assess C mineralization and subsequent changes in soil properties after USS addition to two typical Mediterranean soils: sandy (Soil S) and sandy loam (Soil A) at equivalent field rates of 40 t ha−1 (USS-40) and 120 t ha−1 (USS-120). Outcomes proved the biodegradability of USS through immediate CO2 release inside incubation bottles in a dose-dependent manner. Accordingly, the highest rates of daily C emission were recorded with USS-120 (3.7 and 3.9 mg kg−1 d−1 for Soils S and A, respectively) after 84 d of incubation at 25 °C. The addition of USS also improved soil fertility by enhancing soil macronutrients, microbial proliferation, and protease activity. Protease showed significant correlation with N, total organic C, and heterotrophic bacteria, reflecting the biostimulation and bioaugmentation effects of sludge. Soil indices like C/N/P stoichiometry and metabolic quotient (qCO2) varied mostly with mineralization rates of C and P in both soils. Despite a significant increase of soil salinity and total heavy metal content (lead, nickel, zinc, and copper) with USS dose, wheat germination was not affected by these changes. Both experimental soils showed intrinsic (Soil A) and incubation-induced (Soil S) phytotoxicities that were alleviated by USS addition. This was likely due to the enhancement of biodegradation and/or retention of phytotoxicants originating from previous land uses. Urban sewage sludge amendments could have applications in soil remediation by reducing the negative effects of allelopathic and/or anthropogenic phytoinhibitors.This study was supported by a grant from the Ministry of Higher Education and Scientific Research of Tunisia. The authors thank the National Sanitation Office for providing urban sewage sludge

Impact of urban sewage sludge on soil physico-chemical properties and phytotoxicity as influenced by soil texture and reuse conditions

Urban sewage sludge (USS) is increasingly applied to agricultural soils, but mixed results have been reported because of variations in reuse conditions. Most field trials have been conducted in cropping systems, which conceal intrinsic soil responses to sludge amendments due to the rhizosphere effect and farming practices. Therefore, the current field study highlights long-term changes in bare soil properties in strict relationship with soil texture and USS dose. Two agricultural soils (loamy sand [LS] and sandy [S]) were amended annually with increasing sludge rates up to 120 t ha−1 yr−1 for 5 yr under unvegetated conditions. Outcomes showed a USS dose-dependent variation of all studied parameters in topsoil samples. Soil salinization was the most significant risk related to excessive USS doses. Total dissolved salts (TDS) in saturated paste extracts reached the highest concentrations of 37.2 and 43.1 g L−1 in S soil and LS soil, respectively, treated with 120 t USS ha−1 yr−1. This was also reflected by electrical conductivity of the saturated paste extract (ECe) exceeding 4,000 µS cm−1 in both treatments. As observed for TDS, fertility indicators and bioavailable metals varied with soil texture due to the greater retention capacity of LS soil owing to higher fine fraction content. Soil phytotoxicity was estimated by the seed germination index (GI) calculated for lettuce, alfalfa, oat, and durum wheat. The GI was species dependent, indicating different degrees of sensitivity or tolerance to increasing USS rates. Lettuce germination was significantly affected by changes in soil conditions showing negative correlations with ECe and soluble metals. In contrast, treatment with USS enhanced the GI of wheat, reflecting higher salinity tolerance and a positive effect of sludge on abiotic conditions that control germination in soil. Therefore, the choice of adapted plant species is the key factor for successful cropping trials in sludge-amended soils.This project was supported by a national grant from the Ministry of Higher Education and Scientific Research of Tunisia. The authors thank the Center of Material Science at Borj Cedria Technopark, Tunisia, for inductively coupled plasma analysis