Impact of bentonite on greenhouse gas emissions during pig manure composting and its subsequent application

Additives were widely investigated to retain the nutrients and mitigate the greenhouse gas emissions (GHGs) during manure composting. However, the sustained effects of additives on the GHGs emissions following incorporation of composts to soil were scarcely explored. This study evaluated the effects of bentonite added at the beginning of pig manure composting on the GHGs emissions during two successive processes, i.e., composting and soil incubation amended with composting products. Addition of bentonite did not hinder the composting process and alter the total CO2 emission. On the other hand, reduction by about 17% and 29% for CH4 and N2O emission, respectively, was achieved in the presence of bentonite during composting. Incorporation of the final composting products to soil enhanced significantly the soil C and N of various forms, and gas emissions of CO2 and N2O. However, no significant differences were observed between bentonite-manure co-compost and manure-only compost application except for the N2O emission. Compared to the manure-only compost, compost amended with bentonite reduced N2O loss by around 6.8%, but not statistically significant. This study confirmed that addition of bentonite at the composting stage can mitigate the GHGs emission considering both composting and compost application stages, with all reductions occurring at the composting stage.

Liquid-solid ratio during hydrothermal carbonization affects hydrochar application potential in soil: Based on characteristics comparison and economic benefit analysis

Returning straw-like agricultural waste to the field by converting it into hydrochar through hydrothermal carbonization (HTC) is an important way to realize resource utilization of waste, soil improvement, and carbon sequestration. However, the large-scale HTC is highly limited by the large water consumption and waste liquid pollution. Here, we propose strategies to optimize the liquid-solid ratio (LSR) of HTC, and comprehensively evaluate the stability, soil application potential, and economic benefits of corn stover-based hydrochar under different LSRs. The results showed that the total amount of dissolved organic carbon of hydrochars increased by 55.0% as LSR reducing from 10:1 to 2:1, while the element content, thermal stability, carbon fixation potential, specific surface area, pore volume, and functional group type were not obviously affected. The specific surface area and pore volume of hydrochar decreased by 61.8% and 70.9% as LSR reduced to 1:1, due to incomplete carbonization. According to the gray relation, hydrochar derived at LSR of 10:1 and followed by 2:1 showed greatest relation degree of 0.80 and 0.70, respectively, indicating better soil application potential. However, reducing LSR from 10:1 to 2:1 made the income of single process production increased from -388 to 968 ¥, and the wastewater generation decreased by 80%. Considering the large-scale application of HTC in fields for farmland improvement and environmental remediation, the comprehensive advantages of optimized LSR will be further highlighted.

Application of spray-dried erythromycin fermentation residue as a soil amendment: antibiotic resistance genes, nitrogen cycling, and microbial community structure

Erythromycin fermentation residue (EFR) after spray drying could be reused as a soil amendment. However, the effects of spray-dried EFR on antibiotic resistance genes (ARGs), nitrogen cycling, and microbial community structure in soil are rarely reported. In this study, a pot experiment was conducted by adding spray-dried EFR to soil. For the application of 1.0% spray-dried EFR, the residual erythromycin (ERY) could be rapidly removed with the half-life of 22.2 d; the total relative abundance of ARGs increased at first, but decreased to the initial level of the control group in the end; genes related to ammonium assimilation (glnA, gltB, gltD), ammonification (gdhA, gudB, cynT, cynS, ncd2), denitrification (narI, narG, narH), assimilatory nitrate reduction (nirA, nasA), and dissimilatory nitrate reduction (nirD) were enriched; soil microbial community structure presented temporary variation. Network analysis showed significant negative correlations between ARGs and nitrogen cycling genes. The addition of 6.0% spray-dried EFR resulted in the amplification of ARGs and inhibition of nitrogen cycling. This work provides new insights into the effects of spray-dried EFR on ARGs, nitrogen cycling, and microbial community structure within the fertilized soil.

Hydrothermal carbonization of kitchen waste: An analysis of solid and aqueous products and the application of hydrochar to paddy soil

Hydrothermal carbonization (HTC) technology can potentially be used to safely and sustainably utilize kitchen waste (KW). However, the characteristics of HTC solid products (hydrochar) and aqueous products (HAP) based on different types of KW have not yet been clarified. Here, four types of KW, cellulose-based (CL), skeleton-based (SK), protein-based (PT), and starch-based (ST) KW, were used for HTC at 180 °C, 220 °C, and 260 °C. The basic physicochemical properties and structures of hydrochars and HAP were analyzed, and the effects of different hydrochars on rice growth were characterized. HTC decreased the H/C and O/C of KW. All hydrochars were acidic (3.12 to 6.78) and the pH values increased with the HTC temperature, while high HTC temperature reduced the porosity of hydrochars. HTC promoted the enrichment of total carbon (up to 78.1 %), total nitrogen (up to 62.6 %), and total phosphorus (up to 171.6 %) in KW. More carbon (60.7-88.0 %) and nitrogen (up to 87.4 %) were present in the hydrochars than in the HAP. The relative content of C_1s increased and O_1s decreased in CL and ST hydrochars as the HTC temperature increased, while the opposite pattern was observed for SK and PT hydrochars. The dissolved organic matter (DOM) of different hydrochars and HAP were mainly humus-like substances. The biodegradability of the DOM in HAP was often higher than the corresponding hydrochar, and their DOM biodegradability increased with the HTC temperature. The content of heavy metals from different hydrochars did not exceed the relevant thresholds of fertilizer standards. Rice grain yield increased by 3.7-11.1 % in the hydrochar treatments without phosphate fertilizer addition compared with the control treatment. The results of this study provide new theoretical and empirical insights into the potential for HTC technology to be used for the recycling of KW and its products in the agricultural environment.

Investigating antibiotics, antibiotic resistance genes in soil, groundwater and vegetables in relation to agricultural field - Applicated with lincomycin mycelial residues compost

Antibiotic mycelial residue, a kind of organic bio-waste, after composting with the subsequent land application is an effective way to achieve its resource utilization. However, its influences on soil quality and ecological safety in the practical agricultural field and related environmental media, e.g., groundwater and vegetables, remain investigated. In the present study, a field experiment with vegetable plants was conducted to study the influences of lincomycin mycelial residue compost (LMRC) on soil quality, and antibiotics and ARGs' fate. In particular, soil physicochemical properties and microbial community composition were analyzed. Moreover, antibiotics and ARGs' evolution in soil, vegetable, and groundwater were determined. The results showed that the LMRC amendment enhanced soil fertility with the increases of organic matter, total nitrogen, and available P/K. Enzyme activities except catalase and urease were promoted, and they were positively related to the LMRC application ratio. Soil microbial community composition presented temporary shifts as LMRC added, and the low application amount soil showed no significant difference with control at the end of the experiment. Similarly, lincomycin concentration in soil was far lower than the background, and it decreased below the predicted no-effect concentration in groundwater. Besides, the detected lincomycin in pakchoi grew in 0.5% and 1% LMRC amended soil was lower than acceptable daily intake (30 μg/kg). Low application rate (0.5%) of LMRC caused no significant changes of tested ARGs in soil, vegetables, and groundwater. Information obtained from this study provides reasonable application strategies for LMRC that with environmental acceptable antibiotic and ARGs.

Thermal dehydration of urban biosolids with green liquor dregs from pulp and paper mill

This study aims to investigate the sewage sludge (SS) drying process by incorporating a by-product formed in the pulp and paper industry (green liquor dregs - GLD) as drying adjuvant. Through an innovative approach, the drying kinetics was modelled, and the final dried product was tested for soil applications. After the characterization phase, small cylinders of SS without and with 0.15 g GLD g SS^-1_wb (SS_GLD) were dried in isothermal conditions at 70, 100, and 130 °C. The experimental data were fitted by Fick's second law and thin-layer models, and good results were achieved in both cases (R² > 0.98 and RMSE <0.05). Although only a slight improvement was observed, the best conditions for drying biosolids were found with the addition of GLD at 130 °C, where the drying rate was 30.88 gH₂O kg^-1 SS_wb min^-1. In this condition, a decrease of 8% in the energy required for moisture evaporation was observed. Regarding the phytotoxicity tests with Lepidium sativum L. (garden cress), 60% improvement in EC₅₀ was noticed for the sample with GLD when compared to the raw SS. The dried product SS_GLD complied with the regulations for soil applications regarding the potentially toxic metals (Cr, Ni, Cu, Pb, Zn, and Cd), and promoted an increase in soil organic matter and pH. However, further and in-depth studies should be conducted to assess the potential of SS_GLD application in agricultural soil.

Hydroxyapatite reduces potential Cadmium risk by amendment of sludge compost to turf-grass grown soil in a consecutive two-year study

Recycling of sludge compost to soil as conditioner is generally regarded as the best means of disposal. However, concerns regarding heavy metal residues and sludge toxicity have recently received increasing public attention. Cadmium (Cd) is a mobile metal commonly found in sludge; therefore, the risk posed by Cd contaminated sludge should be carefully assessed. In this report, the effects of addition of hydroxyapatite (HAP) with sludge compost amendment on potential Cd risk were investigated. The results of consecutive two years showed that exchangeable Cd content in treatment of sludge compost with 1.5% HAP decreased by 6.0% compared with single sludge compost treatment, and residual Cd increased by 7.6%. Compared with single sludge compost, the incremental rate of exchangeable Cd dropped by 38.3% and the reductive rate of residual Cd increased by 37.7% in response to 1.5% HAP addition, indicating that HAP played a role of decreasing Cd phytoavailability. The HAP reduced the amount of Cd uptaken by turf-grass in both root and leaf. Moreover, HAP remarkably improved the quality of turf grass grown in amended soil, including leaf greenness, green maintainable period and root strength. However, HAP did not attenuate the downward mobility of Cd. Taken these together, these findings indicated that HAP can be used as a potential candidate to control surface Cd risk of sludge compost amended soil rather than that from leachate.

Selenium forms and methods of application differentially modulate plant growth, photosynthesis, stress tolerance, selenium content and speciation in Oryza sativa L

Selenium (Se) is an essential microelement for humans and a beneficial element for plants. Recently, biofortification with Se has emerged as a key strategy to increase crop Se content. Nonetheless, Se species matters a lot as inorganic Se species is mostly toxic to human health. In this study, we investigated the effects of different forms and mode of Se application on Se accumulation and speciation in rice. The results showed that root application of Se remarkably increased Se accumulation, photosynthetic rate, biomass accumulation and tolerance to cadmium stress in rice as compared to foliar application. However, the stimulatory effects of Se varied depending on the Se species used for root feeding. At vegetative stage, root application of Se-(Methyl) selenocysteine caused the highest water extractable Se content in leaves with major contribution from organic Se species such as Se-amino acid and non-amino acid organic Se. Further investigation at reproductive stage revealed that foliar application of sodium selenite (Na₂SeO₃) resulted in the highest total Se content in rice seeds which was largely attributed to inorganic Se. In contrast, the root application of Na₂SeO₃ led to the maximum accumulation of organic Se compounds which are advantageous to human health. Moreover, the root application of Se increased antioxidant capacity and selectively enhanced amino acids and essential element content in rice grain. This study deepens our understanding of the Se species in Se-enriched rice and suggests that root application of Se may ensure the safe intake of Se through rice.

Bioavailability of phosphorus, other nutrients and potentially toxic elements from marginal biomass-derived biochar assessed in barley (Hordeum vulgare) growth experiments

Biochars produced from marginal biomass feedstocks are a potential source of recycled nutrients for agriculture, but may also contain potentially toxic elements (PTEs) which can cause phytotoxicity. We assessed the potential for nutrient recycling from such materials against potential environmental risks in 17 biochars containing high concentrations of various PTEs and nutrients. Methods for investigating the risk of biochar-derived PTEs were developed and assessed. Short-term (21days) growth experiments with barley (Hordeum vulgare) in 5% biochar/sand mixtures were used to present the 'worst-case scenario' of high dose and low pH buffering. We compared plant nutrient and PTE concentrations with amounts extracted from the same biochars using 1M NH₄NO₃ or 0.01M CaCl₂ (buffered and unbuffered, respectively) and Mehlich 3 to analyse whether such extractions could be used to predict bioavailability. The yields of barley grown with biochars "EPOCAD550", and "WLB550" were significantly higher than the control (p<0.05). Total phosphorus (P) concentration in above-ground biomass was higher than the control for the EPOCAD550 treatment (p<0.01). Both buffered and unbuffered 0.01M CaCl₂ biochar extractions were significantly positively correlated with plant leaf concentration for six of the 18 elements investigated, more than any of the other extractions. CaCl₂ extractions provided the most representative assessment of element bioavailability from marginal biochars compared to more resource-intensive growth experiments. Our results provide new insights into the bioavailability of elements in biochar and the standardisation of methods which accurately assess this attribute, which is necessary for promoting use of biochars from marginal biomass for recycling nutrients from wastewater and to agricultural production.

Soil amendment with olive mill wastes: impact on groundwater

Two sets of soil lysimeters were amended with solid and liquid olive mill wastes and the composition of leachate was analysed. Five treatments were carried out using: olive mill wastewater (OMW) at two different rates (80 and 320 m(3)/ha); OMW pre-treated by catalytical digestion with MnO2; compost obtained by exhausted olive pomace; freshwater as the control. Electric conductivity, pH, potassium, total polyphenols and nitrates were monitored in the leachate as indexes of potential groundwater contamination. The study demonstrated that the impact of all the selected amendments on groundwater was the minimum. OMW was safely applied to soil even at four times the rate allowed by the Italian law, and pre-treatment by catalytical digestion was not necessary to further reduce the impact on groundwater. The application of olive pomace compost was equally safe.