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.

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.

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.

Response of antioxidative enzymes and apoplastic bypass transport in Thlaspi caerulescens and Raphanus sativus to cadmium stress.

A hydroponics experiment using hyperaccumulator Thlaspi caerulescens (alpine pennycress) and non-specific accumulator Raphanus sativus (common radish) was conducted to investigate the short-term effect of increasing Cd concentrations (0, 25, 50, 75, 100 microM) on metal uptake, chlorophyll content, antioxidative enzymes, and apoplastic bypass flow. As expected, T. caerulescens generally showed better resistance to metal stress, which was reflected by higher Cd accumulation within plant tissues with no signs of chlorosis, or wilt. Glutathione reductase (GR) and superoxide dismutase (SOD) activities in fresh leaves were monitored as the plant metal-detoxifying response. In general, both plant species exhibited an increase trend of GR activity before declining at 100 microM likely due to excessive levels of phytotoxic Cd. SOD activity exhibited almost a similar variation pattern to GR and decreased also at 100 microM Cd. For both plant species, fluorescent PTS uptake (8-hydroxy-1,3,6-pyrenetrisulphonic acid) increased significantly with metal level in exposure solutions indicating that Cd has a comparable effect to drought or salinity in terms of the gain of relative importance in apoplastic bypass transport under such stress conditions.

Plant response to heavy metal toxicity: comparative study between the hyperaccumulator Thlaspi caerulescens (ecotype Ganges) and nonaccumulator plants: lettuce, radish, and alfalfa.

Thlaspi caerulescens (alpine pennycress) is one of the best-known heavy metal (HM) hyperaccumulating plant species. It exhibits the ability to extract and accumulate various HM at extremely high concentrations. In this hydroponic study, the performance of T. caerulescens (ecotype Ganges) to accumulate Cd, Zn, and Cu was compared with that of three nonaccumulator plants: alfalfa (Medicago sativa), radish (Raphanus sativus), and lettuce (Lactuca sativa). Plants were exposed to the separately dissolved HM salts for 7 days at a wide range of increasing concentrations: 0 (control: 1/5 Hoagland nutrient solution), 0.1, 1, 10, 100, and 1000 microM. The comparative study combined chemical, physiological, and ecotoxicological assessments. Excessive concentrations of HM (100 and 1000 microM) affected plant growth, photosynthesis, and phytoaccumulation efficiency. Root exudation for all plant species was highly and significantly correlated to HM concentration in exposure solutions and proved its importance to counter effect toxicity. T. caerulescens resisted better the phytotoxic effects of Cd and Zn (at 1000 microM each), and translocated them significantly within tissues (366 and 1290 microg g(-1), respectively). At the same HM level, T. caerulescens exhibited lower performances in accumulating Cu when compared with the rest of plant species, mainly alfalfa (298 microg g(-1)). Root elongation inhibition test confirmed the selective aptitude of T. caerulescens to better cope with Cd and Zn toxicities. MetPLATE bioassay showed greater sensitivity to HM toxicity with much lower EC(50) values for beta-galactosidase activity in E. coli. Nevertheless, exaggerated HM concentrations coupled with relatively short exposure time did not allow for an efficient metal phytoextraction thus a significant reduction of ecotoxicity.

Solid-phase bioassays and soil microbial activities to evaluate PAH-spiked soil ecotoxicity after a long-term bioremediation process simulating landfarming.

The residual ecotoxicity of long-term bioremediated soils concomitantly spiked with three PAHs at four levels (15, 75, 150, 300 mg Sigma 3 PAHs kg(-1) soil) was evaluated using physico-chemical analyses, solid-phase bioassays and soil microbial activities. The pot-scale bioremediation process consisted of weekly moderate waterings in the presence or absence of sewage sludge compost (SSC) under greenhouse conditions. After 15 months, anthracene and pyrene were almost completely degraded whereas benzo[a]pyrene was still persisting, most apparently in SSC-amended soil treatments. However, no apparent toxic effects of the residual PAHs could be detected. SSC application at 40 t ha(-1) was performed to valorize the biowaste and stimulate PAH biodegradation but caused soil salinization and pH reduction at the end of the bioremediation process. Consequently, SSC-amended soils were characterized by strong phytotoxicity to lettuce and had adverse effects on the ostracod Heterocypris incongruens. Despite the smaller number of culturable bacterial populations in SSC-amended soils, soil enzymatic activities were not affected by the organic amendment and residual PAHs; and the bioremediation efficiency was likely to be more limited by the bioavailability of PAHs rather than by the total number of PAH-degraders.