Stabilization of carbon through co-addition of water treatment residuals with anaerobic digested sludge in a coarse textured soil.

Coarse textured soils have low potential to store carbon (C) due to lack of mineral oxides and have low clay content to protect C from biodegradation and leaching. This study evaluated the potential of stabilizing C by adding metal oxyhydroxide-rich water treatment residuals (WTRs) to an aeolian pure sand (<5% clay) topsoil amended with anaerobic digestate (AD) sludge. The AD sludge was applied at 5% (w/w) with aluminum based WTR (Al-WTR) and iron based WTR (Fe-WTR) co-applied at 1:1 and 2:1 WTR:AD (w/w) ratios and incubated at room temperature for 132 days. The cumulative mineralized C was normalized to the total organic C of the treatments. Co-addition with Al-WTR showed to be more effective in stabilizing C through decreased cumulative mineralized C by 48% and 57% in 1Al-WTR:1AD and 2Al-WTR:1AD, respectively, compared to AD sludge sole amendment. Co-application with Al-WTR also decreased permanganate oxidizable C by 37% and dissolved organic C by 51%. Co-application with Fe-WTR did not decrease the concentration of these labile C pools to the same extent, possibly due to the selective use of Fe-WTRs to treat organic-rich raw water. This makes it less effective in stabilizing C in a pure sand relative to Al-WTR due to chemical instability of the Fe-organic complexes. The Al-WTR provides a promising co-amendment to increase C sequestration in pure sands when co-applied with biosolids. The co-amendment approach will not only facilitate C sequestration but also contributes to waste management, aligning to the objectives of a circular economy.

Characterization of clay and nanoclay extracted from a semi-arid Vertisol and investigation of their carbon sequestration potential.

Mitigation of global climate change by means such as soil carbon (C) sequestration has become an important area of research. Soil organic matter (SOM) that is stabilized with clay minerals is the most persistent in soils. Currently, little is known regarding the C sequestration ability of nanoclay extracted from Vertisols in semi-arid regions. Therefore, the aim of this study was to extract and characterize nanoclay and bulk clay from a Vertisol from Iran, in terms of physicochemical surface properties and resistance of SOM to chemical oxidation. The clay fractions were studied before and after H2O2 treatment by total C analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS), pyrolysis gas chromatography mass spectrometry (GC-MS), Fourier transform infrared (FTIR) spectroscopy, specific surface area analysis, and zeta potential. TEM and SEM images showed that the diameter of the extracted nanoclays was 16-46 nm and their morphology was more porous than bulk soil clay. The nanoclay had a much greater specific surface area (111.9 m2 g-1) than the bulk clay (67.9 m2 g-1). According to total C, FTIR, and zeta potential results, the nanoclay was enriched with 1.4 times more C than the bulk clay after peroxide treatment, indicating enhanced soil C stabilization in the nanoclay. About 45% of the peroxide-resistant SOM in the nanoclay was associated with N-containing compounds, indicating that these compounds contribute to SOM stability. The results demonstrate important role of nanoclay in soil C sequestration in Vertisols.

Investigation of interaction effects of biochars and silicon on growth and chemical composition of Zea mays L. in a Ni-polluted calcareous soil.

Biochars are cost effective, carbonaceous amendments that can immobilize potentially toxic elements (PTEs) in soils. Application of silicon (Si) has been shown to mitigate the effect of soil PTEs on plants. Studies on the interaction effects of Si and biochars in PTE-contaminated soils are limited. Thus, the aim of this study was to investigate the interaction effects of biochars, from sheep manure (SMB) and rice husk (RHB) produced at 300 and 500 °C, and 2 levels of Si (as sodium (Na) metasilicate), on growth and chemical composition of corn (Zea mays) in a nickel (Ni)-polluted calcareous soil. The combined application of Si and biochars significantly reduced soil available Ni (17-32%) and the corn shoot Ni concentrations (29-58%), associated with soil pH increase (r = 0.56-0.60, P < 0.01). Application of SMB resulted in greater soil pH increases compared to RHB, and increased soil electrical conductivity (EC) to saline levels, attributed to its higher ash content. Increasing Si application levels also increased soil pH and EC values. Application of all the biochars resulted in significant biomass increases, with RHB having the most positive effect. Despite the positive effect on soil Ni immobilization, the combined application of Si and biochars generally resulted in a decrease in corn shoot biomass yields compared to biochars alone. The biomass decrease was attributed to the significantly higher soil sodicity and pH in the combined treatments which resulted in suppression of macro and micronutrient uptake by the corn. Although the combination of biochar and Na metasilicate was effective at immobilizing soil Ni, future studies should rather employ other essential basic cation metasilicates.

Soil water dynamics and biomass production of young rooibos (Aspalathus linearis) plants.

Rooibos (Aspalathus linearis) is endemic to certain regions of the Western- and Northern Cape of South Africa, where it is also commercially grown. Being low-rainfall regions, information on the soil water balance of rooibos is essential, but such data is limited. Consequently, the effect of inorganic fertilisation and soil depth on soil water dynamics in a young rooibos plantation at Nardouwsberg, Western Cape were studied. Soil water content of plots planted to unfertilised and fertilised plants as well as that of bare soil were determined over the duration of the 2016/17 season. All treatments were replicated on shallow and deep soils sites and plant growth was determined at the end of the season. At the end of the study, the profile soil water content and evapotranspiration of the bare and planted plots were similar which prove that fallowing (water harvesting) is not an option in the sandy soils of this region. With the exception of the 20-30 cm root zone of the planted plots at the deep site, the water content decreased to levels below the permanent wilting point in the soil profile during summer. It was concluded that rooibos plants could survive through an adapted root system. A further survival method was proposed, involving moisture moved through evaporation from the deeper soil layers into the drying-front in the ~ 10-30 cm soil layer where a condensation-evaporation cycle enables rooibos to harvest small amounts of water. The highest shoot biomass with the longest taproot resulted from the unfertilised treatment on the deep soil thanks to higher soil water content, whereas the shoot and root biomass of the fertilised treatment at both sites were low due to high P soil concentration. This study revealed that unfertilised plants on deeper soils result in higher rooibos production under drought conditions.

Chemical speciation and release kinetics of Ni in a Ni-contaminated calcareous soil as affected by organic waste biochars and soil moisture regime.

Biochars vary widely in properties and have been shown to have variable effects on potentially toxic element(s) stabilization in soil. This is the first study to examine the interaction effects of biochar and soil moisture regime on Ni stabilization in a Ni-contaminated calcareous soil. Three different organic waste (cow manure, municipal compost and licorice root pulp) biochars produced at two temperatures (300 and 600 °C) were applied (3% wt.) to a Ni-contaminated calcareous soil and incubated at field capacity and saturated conditions for 70 d. Sequential chemical fractionation and Ni release kinetics were then performed. All applied biochars, especially the high-temperature biochars, were significantly able to enhance Ni stabilization in the studied soil. In particular, the biochars significantly decreased Ni content in the water-soluble and exchangeable fractions (10-42% decrease), while increasing the immobile residual fraction (13-38% increase). The biochars also significantly decreased the rate and cumulative amount of EDTA-extractable Ni from the calcareous soil. Among the studied biochars, the cow manure and municipal compost biochars produced at 600 °C were the most effective at reducing Ni mobility factor (27-28% decrease) and initial release rate (42-49% decrease), likely due to their high ash content and pH, which promotes Ni sorption in soil. Soil moisture regime was not found to significantly affect the Ni mobility factor or rates of Ni release from the calcareous soil but did, however, affect certain soil Ni chemical fractions. Soil water saturation significantly decreased Ni in the Mn (4%) and non-crystalline Fe oxides (17%) fractions, while increased the crystalline Fe oxide fraction (3%), attributed to reductive dissolution of Mn and Fe oxide crystallinity enhancement. Saturation also significantly enhanced Ni in the residual fraction (4%), attributed to the associated pH increase and potential sulfide formation. The results of this study demonstrate that high temperature, ash-rich, and alkaline biochars are most effective at Ni immobilization, and that soil water saturation can further enhance Ni in the residual fraction.

Characterisation, adsorption and desorption of ammonium and nitrate of biochar derived from different feedstocks.

Biochar is known to be a highly adsorptive material, especially when the biochar is altered by activation to further increase its sorption ability. Little information, however, is available on the potential reversibility of both ammonium (NH4+) and nitrate (NO3-) sorption on the inherent biochar pH. The objective of our study was to characterise biochars made using different pyrolysis conditions from five various plant materials and rubber tyre, and to use them to investigate the biochar properties responsible for NH4+ and NO3- adsorption and desorption. The rubber tyre, maize stover and sugarcane pith were the weakest adsorbing biochars (5.7-7.8 mg g-1) and best described by the Freundlich adsorption isotherm. The grape pip, grape skin and pine wood biochars had adsorption capacities in the range 8.3-9.4 mg NH4+ g-1 and best described by a linear adsorption isotherm at 100 mg L-1. The NH4+ adsorption results were associated with physisorption which implies that they can act as slow release NH4+ fertilisers if NH4+ is bioavailable. The six biochars had NO3- adsorption capacities in the range 15.2-15.9 mg g-1 and were well fitted to the linear adsorption isotherm at 100 mg L-1. All six biochars had a stronger NO3- removal affinity (82-89%) compared to NH4+ (33-39%). Adsorbed nitrate was not desorbable (0.01-0.23%) compared to adsorbed NH4+ which was 53-60% desorbable. The desorption result was possibly due to NO3- competing redox reactions or NO3- being too strongly adsorbed for extraction. Desorption of NH4+ was associated with biochar net negative pH values and volatilisation of ammonia.

The microbiology of rebuilding soils with water treatment residual co-amendments: Risks and benefits.

Water treatment residual (WTR) is composed of sludges from the potable water treatment process, currently largely destined for landfill. This waste can be diverted to rebuild degraded soils, aligning with the UN's Sustainable Development Goals 12 (Consumption and Production) and 15 (Terrestrial Ecosystems). Biosolids are tested against stringent pathogen guidelines, yet few studies have explored the microbial risk of WTR land application, despite anthropogenic impacts on water treatment. We explored the microbial risks and benefits of amending nutrient-poor sandy soil with WTRs. Our results showed that the culturable pathogen load of wet and dry WTRs did not warrant pre-processing before land application, according to South African national quality guidelines, with fecal coliforms not exceeding 104 colony forming units per gram dry weight in wet sludges sampled from four South African and Zimbabwean water treatment plants and decreasing upon drying and processing. There was no culturable pathogenic (fecal coliforms, enterococci, Salmonella, and Shigella) regrowth in soil incubations amended with dry WTR. However, the competition (microbial load and diversity) introduced by a WTR co-amendment did not limit pathogen survival in soils amended with biosolids. Application of WTR to nutrient-poor sandy soils for wheat (Triticum aestivum L.) growth improved the prokaryotic and eukaryotic culturable cell concentrations, similar to compost. However, the compost microbiome more significantly affected the bacterial beta diversity of the receiving soil than WTR when analyzed with automated ribosomal intergenic spacer analysis. Thus, although there was a low pathogen risk for WTR amendment in receiving soils and total soil microbial loads were increased, microbial diversity was more significantly enhanced by compost than WTR.

The effect of soil moisture regime and biochar application on lead (Pb) stabilization in a contaminated soil.

Soil application of biochars has been shown to effectively immobilize potentially toxic elements (PTEs). Soil water regime can also affect PTE availability. No previous studies have examined the interactive effect of biochars and soil water regime on Pb availability. Therefore, this study investigated the effect of high and low temperature (300 and 600°C) biochars derived from cow manure (CB), municipal compost (MB) and licorice root pulp (LB) applied at 3 wt%, under two soil moisture regimes (field capacity (FC) and saturation (ST)) on Pb release kinetics and chemical fractions in a Pb-contaminated calcareous soil. Results showed that CB and MB treatments significantly enhanced Pb stabilization compared to LB, attributed to their favorable chemical properties (high P, ash, carbonate, oxidizable C content and high pH) which could promote Pb conversion into stable chemical fractions. Immobilization of Pb was enhanced under saturated conditions compared to FC by the treatments, which is attributed to increased soil pH, reduction of metal oxides and possible formation of sulfides. The most significantly effective treatments were the CB300, CB600 and MB600 treatments under ST, as indicated by significant decrease in soil Pb mobility factor from 29.1% (CL+FC) to 21.2-22.9%, and 11.7-16.3% increase in non-EDTA-extractable Pb. Results of this study demonstrate that combined application of high ash biochars and soil water saturation significantly enhances Pb immobilization in calcareous soil.