Land disposal of dredged sediments from an urbanized tropical lagoon: toxicity to soil fauna

Urban tropical lagoons are commonly impacted by silting, domestic sewage and industrial wastes and the dredging of their sediments is often required to minimize environmental impacts. However, the ecological implications of land disposal of dredged sediments are still poorly investigated in the tropics. Aiming to contribute to filling this gap, an ecotoxicological evaluation was conducted with dredged sediments from Tijuca Lagoon (Rio de Janeiro, Brazil) using different lines of evidence, including soil and sediment characterization, metal determination, and acute and avoidance bioassays with Eisenia andrei. Two different dredged sediment samples, a sandy sediment and another muddy one, were obtained in two distinct and spatially representative sectors of the Tijuca Lagoon. The sediments were mixed with an artificial soil, Ferralsol and Spodosol to obtain doses between 0 (pure soil) and 12%. The sediment dose that caused mortality (LC50) or avoidance responses (EC50) to 50% of the organisms was estimated through PriProbit analysis. Metal concentrations and toxicity levels were higher in the muddy sediment (artificial soil LC50 = 3.84%; Ferralsol LC50 = 4.58%; Spodosol LC50 = 2.85%) compared to the sandy one (artificial soil LC50 = 10.94%; Ferralsol LC50 = 14.36%; Spodosol LC50 = 10.38%), since fine grains tend to adsorb more organic matter and contaminants. Mortality and avoidance responses were the highest in Spodosol due to its extremely sandy texture (98% of sand). Metal concentrations in surviving earthworms were generally low, except sodium whose bioaccumulation was high. Finally, the toxicity is probably linked to marine salts, and the earthworms seem to accumulate water in excess to maintain osmotic equilibrium, increasing their biomass.

Introducing N2-fixing trees (Acacia mangium) in eucalypt plantations rapidly modifies the pools of organic P and low molecular weight organic acids in tropical soils

Many studies have shown that introducing N₂-fixing trees (e.g. Acacia mangium) in eucalypt plantations can increase soil N availability as a result of biological N₂ fixation and faster N cycling. Some studies have also shown improved eucalypt P nutrition. However, the effects of N₂-fixing trees on P cycling in tropical soils remain poorly understood and site-dependent. Our study aimed to assess the effects of planting A. mangium trees in areas managed over several decades with eucalypt plantations on soil organic P (Po) forms and low molecular weight organic acids (LMWOAs). Soil samples were collected from two tropical sites, one in Brazil and one in the Congo. Five different treatments were sampled at each site: monospecific acacia, monospecific eucalypt, below acacias in mixed-species, below eucalypts in mixed-species as well as native vegetation. Po forms and LMWOAs were identified in sodium hydroxide soil extracts using ion chromatography and relationships between these data and available P were determined. At both sites, the concentrations of most Po forms and LMWOAs were different between native ecosystems and monospecific eucalypt and acacia plots. Also, patterns of Po and LMWOAs were clearly separated, with glucose-6-P found mainly under acacia and phytate and oxalate mainly under eucalypt. Despite the strongest changes occurred at site with a higher N₂ fixation and root development, acacia introduction was able to change the profile of organic P and LMWOAs in <10 years. The variations between available Pi, Po and LMWOA forms showed that P cycling was dominated by different processes at each site, that are rather physicochemical (via Pi desorption after LMWOAs release) at Itatinga and biological (via organic P mineralization) at Kissoko. Specific patterns of Po and LMWOAs forms found in soil sampled under acacia or eucalypt would therefore explain the effect of acacia introduction in both sites.

How do methane rates vary with soil moisture and compaction, N compound and rate, and dung addition in a tropical soil?

Soil moisture and compaction, and source of N and bovine urine can reduce methane (CH₄) rates from agricultural soils. However, the magnitude of the effect is unknown in tropical soil under different conditions, as well as the potential of different urine-N concentration, volume, and sources of N in such an effect. This study aimed to investigate the effects of different soil conditions (moist, dry, compacted, moist-dung, moist-dung-compacted), N concentration in urine (2.5, 5.0, 10.0, and 15.0 g N L^-1), volume of urine (25, 50, 100, and 200 ml kg^-1 dry soil), and source of N (ammonium, nitrate, and urea) on CH₄ emissions. A tropical Ferralsol soil from marandu-grass pasture was incubated during 106 days and the CH₄ concentration determined by gas chromatography. The CH₄ rates varied significantly according to the soil conditions when manipulated the urine-N (p < 0.01) and averaged 0.75, - 0.50, 1.14, 6.23, and 8.17 μg C-CH₄ m^-2 h^-1for the moist, dry, compacted, moist-dung, and moist-dung-compacted soil, respectively, and, not responded to the level of N (p = 0.73) averaging 2.57 μg C-CH₄ m^-2 h^-1. When evaluated, the volumes of urine cumulative CH₄ averages were - 0.52, - 1.24, - 0.88, 14.48, and 18.56 μg C-CH₄ m^-2 h^-1 for the moist, dry, compacted, moist-dung, and moist-dung-compacted, respectively. Soils were affected by soil treatments (p < 0.001) but not by urine volumes (p = 0.30). The source of N did not influence the CH₄ rates (p = 0.1) averaging 0.88, - 1.26, and - 1.19 μg C-CH₄ m^-2 h^-1 respectively, for urea, nitrate, and ammonium. The CH₄ fluxes in tropical Ferralsols are controlled by the soil characteristics and dung addition.

Bisphenol S Adsorption Behavior on Ferralsol and Biochar Modified Soil with Dissolved Organic Matter

Bisphenol S (BPs) has been found in a variety of common consumer products surrounding human living, despite the fact that it could damage the human digestive system and genital system. In China, straw-returning to the field is a common soil improvement technology used to increase the concentration of dissolved organic matter (DOM), which plays an important role in the natural environment as a microreactor of contaminants. Additionally, the biochar obtained by the straw is an effective soil conditioner. DOM is a key influencing factor when biochar is employed as the conditioner of BPs contaminated soil. However, the BPs adsorption behavior on the Ferralsol affected by DOM and biochar is also unclear. Hence, DOM was prepared and the effect of DOM on the BPs adsorption behavior on soil and biochar modified soil was investigated. DOM was characterized by Elemental analysis, Fourier transforming infrared spectra (FT-IR), and three-dimensional excitation-emission matrix spectra (3D-EEM). The results of the adsorption experiments indicated that both biochar and DOM could improve the BPs adsorption capacity in Ferralsol, while DOM suppressed the BPs adsorption capacity of biochar modified soil, indicating that DOM and BPs could not be applied at the same time for BPs adsorption.

Spatial variability and seasonal toxicity of dredged sediments from Guanabara Bay (Rio de Janeiro, Brazil): acute effects on earthworms

The toxicity of dredged sediments from Guanabara Bay (Rio de Janeiro, Brazil) was evaluated using acute bioassays with Eisenia andrei and metal determination. The sediments were collected in August 2014 (winter) and February 2015 (summer) and in five areas distributed along the Bay: Port of Rio de Janeiro, Port of Niterói, Meriti River mouth, Iguaçu River mouth, and the Environmental Protection Area (APA) of Guapimirim. The sediments were mixed with a ferralsol (a representative Brazilian tropical soil) in proportions varying between 0 (pure soil) and 30%. The acute bioassays with E. andrei followed a standard protocol (ISO 11268-2:2012). Total metal determination in the sediments was performed by ICP-OES. The medium lethal earthworm concentration (LC₅₀) was estimated through PriProbit analysis. The sediments from the APA of Guapimirim, which is a control area at the Guanabara Bay, were the only ones whose total metal concentrations were in agreement with the limits established by Brazilian law for land disposal of dredged sediments. However, the sediments collected in the APA of Guapimirim were the most toxic ones among the study areas due to very high contents of salts in these materials. Winter sediments were generally more toxic compared to the summer ones due to the increase of metal concentrations and salt precipitation to bottom sediments during the winter. The exceptions were (i) the sediments from APA of Guapimirim, where the toxicity in the summer (LC₅₀ = 3.99%) and winter (LC₅₀ = 4.60%) were relatively similar to each other, since the toxicity is linked to salt in excess; and (ii) the Iguaçu River mouth, where the presence of mangrove areas might be associated with the filtering of pollution sources (winter LC₅₀ = 12.67%; summer LC₅₀ = 11.58%). In the Port of Rio de Janeiro, LC₅₀ obtained in the winter (7.30%) was almost three times lower than that found in the summer (19.64%). The sediments from Meriti River mouth showed the highest total metal concentrations, were the most toxic sediments among the study areas (excluding the APA of Guapimirim), and its winter LC₅₀ (6.64%) was almost twice lower than that obtained in the summer (12.55%). By following the same tendency, summer LC₅₀ (17.52%) found for the sediment collected in the Port of Niterói was also higher than the value found in the winter (12.34%). Finally, the dredged sediments from Guanabara Bay were toxic to earthworms in mixtures with pure ferralsol and winter samples were generally more toxic than the summer ones, in agreement with the increase of metal and salt concentrations during the winter.

Influence of dissolved organic matter on sorption and desorption of MCPA in ferralsol

MCPA (4-chloro-2-methylphenoxyacetic acid) is an acidic herbicide, widely used in paddy fields. The presence of dissolved organic matter (DOM) modifies the sorption-desorption of herbicides in soils. In this study, effects of DOM on sorption- desorption of MCPA were tested using three typical ferralsol soil types from China: rhodic ferralsol, haplic ferralsol and paddy soil. DOM preparations were extracted from the paddy soil (DOM_P), from a compost mixture of cassava stems with chicken manure (DOM_C), and from rice straw (DOM_R). Sorption-desorption of MCPA in the tested soil types was shown to follow pseudo first-order kinetics, and the calculated isotherm data fitted well with a Freundlich equilibrium model in the range of the studied concentrations. MCPA was weakly sorbed by the soils, producing low Freundlich coefficient values (K_f) (0.854 to 4.237). The presence of DOM reduced the K_f whereby DOM_C had the strongest and DOM_R the weakest effect. Presence of DOM also promoted MCPA desorption from the soils, again with DOM_C having the strongest effect and DOM_R the weakest. DOM coating changed the soil particle surface, as demonstrated by electron microscopy, and DOM also directly interacted with MCPA, as shown by Fourier-transform infrared spectroscopy. The experimental data were interpreted to suggest a competing sorption of DOM to ferralsol and an increased solubility of MCPA in the presence of DOM. The results indicate that the environmental risk of MCPA leaching to groundwater and surface flow is increased by presence of DOM, for instance as a result of organic fertilizer use.

Benefits of biochar, compost and biochar-compost for soil quality, maize yield and greenhouse gas emissions in a tropical agricultural soil

Soil quality decline represents a significant constraint on the productivity and sustainability of agriculture in the tropics. In this study, the influence of biochar, compost and mixtures of the two on soil fertility, maize yield and greenhouse gas (GHG) emissions was investigated in a tropical Ferralsol. The treatments were: 1) control with business as usual fertilizer (F); 2) 10 t ha(-1) biochar (B)+F; 3) 25 t ha(-1) compost (Com)+F; 4) 2.5 t ha(-1) B+25 t ha(-1) Com mixed on site+F; and 5) 25 t ha(-1) co-composted biochar-compost (COMBI)+F. Total aboveground biomass and maize yield were significantly improved relative to the control for all organic amendments, with increases in grain yield between 10 and 29%. Some plant parameters such as leaf chlorophyll were significantly increased by the organic treatments. Significant differences were observed among treatments for the δ(15)N and δ(13)C contents of kernels. Soil physicochemical properties including soil water content (SWC), total soil organic carbon (SOC), total nitrogen (N), available phosphorus (P), nitrate-nitrogen (NO3(-)N), ammonium-nitrogen (NH4(+)-N), exchangeable cations and cation exchange capacity (CEC) were significantly increased by the organic amendments. Maize grain yield was correlated positively with total biomass, leaf chlorophyll, foliar N and P content, SOC and SWC. Emissions of CO2 and N2O were higher from the organic-amended soils than from the fertilizer-only control. However, N2O emissions generally decreased over time for all treatments and emission from the biochar was lower compared to other treatments. Our study concludes that the biochar and biochar-compost-based soil management approaches can improve SOC, soil nutrient status and SWC, and maize yield and may help mitigate greenhouse gas emissions in certain systems.