Win-win: Application of sawdust-derived hydrochar in low fertility soil improves rice yield and reduces greenhouse gas emissions from agricultural ecosystems

Characteristics and phytotoxicity of hydrochar-derived dissolved organic matter: Effects of feedstock type and hydrothermal temperature

The dissolved organic matter (DOM) with high mobility and reactivity plays a crucial role in soil. In this study, the characteristics and phytotoxicity of DOM released from the hydrochars prepared from different feedstocks (cow manure, corn stalk and Myriophyllum aquaticum) under three hydrothermal carbonization (HTC) temperatures (180, 200 and 220°C) were evaluated. The results showed that the hydrochars had high dissolved organic carbon content (20.15 to 37.65 mg/g) and its content showed a gradual reduction as HTC temperature increased. Three fluorescent components including mixed substance of fulvic acid-like and humic acid-like substances (C1, 30.92%-58.32%), UVA humic acid-like substance (C2, 25.27%-29.94%) and protein-like substance (C3, 11.74%-41.92%) were identified in hydrochar DOM by excitation emission matrix spectra coupled with parallel factor analysis. High HTC temperature increased the relative proportion of aromatic substances (C1+C2) and humification degree of hydrochar DOM from cow manure, while it presented adverse effects on the hydrochar DOM from corn stalk and Myriophyllum. aquaticum. The principal component analysis suggested that feedstock type and HTC temperature posed significant effects on the characteristics of hydrochar DOM. Additionally, seed germination test of all hydrochar DOM demonstrated that the root length was reduced by 8.88%-26.43% in contrast with control, and the germination index values were 73.57%-91.12%. These findings provided new insights into the potential environmental effects for hydrochar application in soil.

Reuse of straw in the form of hydrochar: Balancing the carbon budget and rice production under different irrigation management

Hydrochar is proposed as a climate-friendly organic fertilizer, but its potential impact on greenhouse gas (GHG) emissions in paddy cultivation is not fully understood. This two-year study compared the impact of exogenous organic carbon (EOC) application (rice straw and hydrochar) on GHG emissions, the net ecosystem carbon budget (NECB), net global warming potential (net GWP), and GHG emission intensity (GHGI) in a rice pot experiment using either flooding irrigation (FI) or controlled irrigation (CI). Compared with FI, CI increased ecosystem respiration by 23 - 44 % and N2O emissions by 85 - 137 % but decreased CH4 emissions by 30 - 58 % (p < 0.05). Since CH4 contributed more to net GWP than N2O, CI reduced net GWP by 16 - 220 %. EOC amendment increased crop yield by 5 - 9 % (p < 0.05). Compared with CK, hydrochar application increased initial GHG emission, net GWP and GHGI in the first year, while in the second year, there was no significant difference in net GWP and GHGI between CI-hydrochar and CK. Compared with straw addition, hydrochar amendment reduced net GWP and GHGI by 20 - 66 % and 21 - 66 %; and exhibited a lower net CO2 emission when considering the energy input during the hydrochar production. These findings suggest that integrated CI-hydrochar practices would be a sustainable and eco-friendly way for organic waste management in rice production as it holds potential to enhance the NECB and SOC sequestration of rice production, while also offsetting the extra carbon emissions from organic inputs.

Soil application of activated hydrochar derived from sewage sludge enhances plant growth and reduces nitrogen loss

Sewage sludge treatment and disposal is a considerable environmental and economic burden, and is considered a major global challenge. Here, sewage sludge treatment and disposal were studied with a focus on hydrothermal carbonization and the use of hydrochar (HC) as a soil amendment after Fenton-reaction activation. The underlying hypothesis was that enhanced adsorption of nutrients (e.g., ammonium) by activated HC (AHC) increases their availability, thus enhancing plant growth and reducing environmental impacts such as greenhouse gas emission and N leaching relative to conventional soil-amendment techniques. The impact of AHC on lettuce plant growth, N leaching, ammonia volatilization, soil trace-gas emissions, and respiration was studied in a net-house planting experiment. Four treatments were tested in quadruplicate using sandy loam soil with addition of either AHC, urea fertilizer, or AHC plus urea, and a control with no amendment. Activation-induced changes in AHC surface properties (indicated by SEM and XPS analyses) resulted in an NH4+ adsorption capacity 60 % higher than that of untreated HC. The AHC + urea soil treatment yielded the most enhanced plant growth, followed by urea and AHC treatments with comparable growth rates. Least growth occurred in the control with no amendment. Nitrogen loss through gas emissions, per kg of lettuce, was lowest with AHC + urea treatment, although its mean N emission as nitrous oxide (N2O) was notably higher at 2.3 mg N2O-N kg-1 than for other treatments (∼0.4 mg N2O-N kg-1). Dissolved-N leaching was reduced by up to four times with AHC treatment due to its higher NH4+ adsorption capacity, indicating reduced environmental impact of the AHC amendment. AHC application is therefore considered a sustainable soil amendment, enhancing plant growth and reducing N loss and sewage environmental impact.

Extraction of agricultural plastic greenhouses based on a U-Net Convolutional Neural Network Coupled with edge expansion and loss function improvement

IMPLICATION STATEMENT

Compared to traditional interpretation methods, which suffer problems such as heavy workloads, small adaptation ranges and poor repeatability, deep learning network models can better extract target features from remote sensing images. In this study, we used GF-7 image data to improve the traditional U-Net convolutional neural network (CNN) model. The Canny operator and Gaussian kernel (GK) function were used for sample edge expansion, and the binary cross-entropy and GK functions were used to jointly constrain the loss. Finally, APGs were accurately extracted and compared to those obtained with the original model. The results indicated that the APG extraction accuracy of the U-Net network was improved through the expansion of sample edge information and adoption of joint loss function constraints.

Recent advances in environmental and agricultural applications of hydrochars: A review

Hydrochar is a carbonaceous material that is generated through the process of hydrothermal carbonization (HTC) from biomass, which has garnered considerable attention in recent years owing to its potential applications in a diverse range of fields, such as environmental remediation and agriculture. Hydrochar is produced from a diverse range of biomass waste materials and retains exceptional properties, including high carbon content, stability, and surface area, making it an optimal candidate for various enviro-agricultural applications. Moreover, it delves into the production process of hydrochar, with explicit emphasis on the optimization of certain properties during the production of hydrochar from bio-waste. Furthermore, the potential of hydrochar as an adsorbent and catalyst support for heavy metals and dyes was extensively explored, along with a soil remediation potential that can improve the physical, chemical and biological properties of soil. This comprehensive review aims to provide a thorough overview of hydrochar with a particular focus on its production, properties, and prospective applications. The significance of hydrochar is accentuated and the growing need for alternative sources of energy and materials that are environmentally sustainable is highlighted in this paper. Besides, the consequence of hydrochar on soil properties such as water-holding capacity, nutrient retention, and total soil porosity, as well as its influence on soil chemical properties such as cation exchange capacity, electrical conductivity, and surface functionality is scrutinized.

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Cattle manure hydrochar posed a higher efficiency in elevating tomato productivity and decreasing greenhouse gas emissions than plant straw hydrochar in a coastal soil

Rehabilitation of degraded soil health using high-performance and sustainable measures are urgently required for restoring soil primary productivity and mitigating greenhouse gas (GHG) emission of coastal ecosystems. However, the effect of livestock manure derived hydrochar on GHG emission and plant productivity in the coastal salt-affected soils, one of blue carbon (C) ecosystems, was poorly understood. Therefore, a cattle manure hydrochar (CHC) produced at 220 °C was prepared to explore its effects and mechanisms on CH4 and N2O emissions and tomato growth and fruit quality in a coastal soil in comparison with corresponding hydrochars derived from plant straws, i.e., sesbania straw hydrochars (SHC) and reed straw hydrochars (RHC) using a 63-day soil column experiment. The results showed that CHC posed a greater efficiency in reducing the global warming potential (GWP, 54.6 % (36.7 g/m2) vs. 45.5-45.6 % (22.2-30.6 g/m2)) than those of RHC and SHC. For the plant growth, three hydrochars at 3 % (w/w) significantly increased dry biomass of tomato shoot and fruit by 12.4-49.5 % and 48.6-165 %, respectively. Moreover, CHC showed the highest promotion effect on shoot and fruit dry biomass of tomato, followed by SHC ≈ RHC. Application of SHC, CHC and RHC significantly elevated the tomato sweetness compared with CK, with the order of CHC (54.4 %) > RHC (35.6 %) > SHC (22.1 %). Structural equation models revealed that CHC-depressed denitrification and methanogen mainly contributed to decreased GHG emissions. Increased soil phosphorus availability due to labile phosphorus supply from CHC dominantly accounted for elevated tomato growth and fruit production. Comparably, SHC-altered soil properties (e.g., decreased pH and increased total carbon content) determined variations of GHG emission and tomato growth. The findings provide the high-performance strategies to enhance soil primary productivity and mitigate GHG emissions in the blue C ecosystems.

Study on the auxin-like activity of organic compounds extracted from corn waste hydrochar prepared by hydrothermal carbonization

This work studied the auxin-like activity of liquid and solid hydrochar from aboveground corn biomass prepared using hydrothermal carbonization (HTC). Understanding the action of organic compounds in regulating plant metabolism is important to develop strategies to improve plant growth and production. Bioassays were performed by testing liquid hydrochar concentrations in the range of 0.0557-5570.0 mg carbon L-1; and solid hydrochar (via extracted dissolved organic matter, DOM) in the range of 0.026-2600.0 mg carbon L-1, using seeds of Lactuca sativa. SEM, ATR-FTIR, and Py-GC/MS were applied to assess the effect of HTC on hydrochar production/composition. Liquid hydrochar presented an intense bioactivity, completely inhibiting the germination of testing seeds at higher concentrations. Liquid hydrochar also was considerably more bioactive. Py-GC/MS allowed the identification of the molecules involved in IAA-like effects: carboxylic acids (linear and aromatic) and amino acids. The concentration of more bioactive molecules, rather than their simple presence in the hydrochar fraction, determined the bio-stimulating effect, besides an excellent linear regression between the auxin-like effect and the concentration of active molecules.

Hydrochar more effectively mitigated nitrous oxide emissions than pyrochar from a coastal soil of the Yellow River Delta, China

Application of char amendments (e.g., pyrochar or biochar, hydrochar) in degraded soils is proposed as a promising solution for mitigating climate change via carbon sequestration and greenhouse gases (GHGs) emission reduction. However, the hydrochar-mediated microbial modulation mechanisms underlying N₂O emissions from coastal salt-affected soils, one of essential blue C ecosystems, were poorly understood. Therefore, a wheat straw derived hydrochar (SHC) produced at 220 °C was prepared to investigate its effects on N₂O emissions from a coastal salt-affected soil in the Yellow River Delta and to distinguish the microbial regulation mechanisms in comparison with corresponding pyrochar pyrolyzed at 500 °C (SPC) using a 28-day soil microcosm experiment. Compared with SPC, the acidic SHC (pH 4.15) enriched in oxygenated functional groups, labile C and N constituents. SHC application more efficiently depressed cumulative soil N₂O emissions (48.4-61.1 % vs 5.57-45.2 %) than those of SPC. SHC-induced inhibition of ammonia-oxidizing gene (amoA)-mediated nitrification and promotion of full reduction of N₂O to N₂ by nitrous oxide reductase gene (nosZ) were the underlying microbial mechanisms. Structural equation models further revealed that SHC-modulated bacterial N-transformation responses, i.e., inhibited nitrification and promoted heterotrophic denitrification, mainly contributed to reduced N₂O emissions, whereas modification of soil properties (e.g., decreased pH, increased total C content) by SPC dominantly accounted for decreased N₂O emissions. These results address new insights into microbial regulation of N₂O emission reduction from the coastal salt-affected soils amended with hydrochar, and provide the promising strategies to enhance C sequestration and mitigate GHG emissions in the blue C ecosystems.

A review on hydrothermal carbonization of potential biomass wastes, characterization and environmental applications of hydrochar, and biorefinery perspectives of the process

It is imperative to search for appropriate processes to convert wastes into energy, chemicals, and materials to establish a circular bio-economy toward sustainable development. Concerning waste biomass valorization, hydrothermal carbonization (HTC) is a promising route given its advantages over other thermochemical processes. From that perspective, this article reviewed the HTC of potential biomass wastes, the characterization and environmental utilization of hydrochar, and the biorefinery potential of this process. Crop and forestry residues and sewage sludge are two categories of biomass wastes (lignocellulosic and non-lignocellulosic, respectively) readily available for HTC or even co-hydrothermal carbonization (Co-HTC). The temperature, reaction time, and solid-to-liquid ratio utilized in HTC/Co-HTC of those biomass wastes were reported to range from 140 to 370 °C, 0.05 to 48 h, and 1/47 to 1/1, respectively, providing hydrochar yields of up to 94 % according to the process conditions. Hydrochar characterization by different techniques to determine its physicochemical properties is crucial to defining the best applications for this material. In the environmental field, hydrochar might be suitable for removing pollutants from aqueous systems, ameliorating soils, adsorbing atmospheric pollutants, working as an energy carrier, and performing carbon sequestration. But this material could also be employed in other areas (e.g., catalysis). Regarding the effluent from HTC/Co-HTC, this byproduct has the potential for serving as feedstock in other processes, such as anaerobic digestion and microalgae cultivation. These opportunities have aroused the industry interest in HTC since 2010, and the number of industrial-scale HTC plants and patent document applications has increased. The hydrochar patents are concentrated in China (77.6 %), the United States (10.6 %), the Republic of Korea (3.5 %), and Germany (3.5 %). Therefore, considering the possibilities of converting their product (hydrochar) and byproduct (effluent) into energy, chemicals, and materials, HTC or Co-HTC could work as the first step of a biorefinery. And this approach would completely agree with circular bioeconomy principles.

The weathering process of polyethylene microplastics in the paddy soil system: Does the coexistence of pyrochar or hydrochar matter?

This study is based on a particular test site to simulate the weathering process of microplastics (MPs) in paddy soil. A substantial amount of plastic waste, especially MPs, inevitably accumulates in agricultural soil due to the high consumption and short average use of plastics. Recently, MP pollution has become a global environmental concern. However, insight into the soil weathering process of MPs in paddy soil, particularly in the presence of biochar, is lacking. In this study, the physicochemical properties of polyethylene (PE) MPs were determined through a 24-week weathering system conducted in paddy soil, paddy soil with pyrochar, or hydrochar. Moreover, the sorption of original and weathered PE MPs toward three typical pollutants (cadmium/Cd, bisphenol A/BPA, and dimethyl phthalate/DMP) was investigated. The surface of PE MPs was fractured, 1.1-fold rougher, yellow-colored (11.7 units), and 1.8-fold more oxidized after paddy soil weathering. In addition, the crystallinity, negative charge, and stronger hydrophilicity of weathered PE MPs increased compared to original PE MPs. Weathering in a pyrochar or hydrochar system caused fissures, extensive destruction of amorphous areas, and accelerated chemical or bio-oxidation processes for PE MPs, resulting in a more noticeable change in roughness (1.4-2.2-fold), yellow color (12.7-13.7), crystallinity (1.2-1.5-fold), and oxygen content (2.5-3.6-fold). Weathered PE MPs facilitated the sorption with Cd and BPA, attributed to larger specific surface area, abundant polar functional groups, and increased negatively charged sites. However, sorption of DMP to PE MPs was highly influenced by their hydrophobicity, resulting in decreased hydrophobic partition sorption on weathered PE MPs. Overall, paddy soil weathering affected the properties of PE MPs and enhanced sorption of Cd and BPA but reduced sorption of DMP. The coexistence of biochar exacerbated the paddy soil weathering effect. The insight gained from this study assists in better understanding the weathering process of PE MPs in agricultural soils.

Effects of process water obtained from hydrothermal carbonization of poultry litter on soil microbial community, nitrogen transformation, and plant nitrogen uptake

Process water (PW) obtained from hydrothermal carbonization of nitrogen-rich (N-rich) biowaste is proposed to be a renewable resource utilized as a liquid N fertilizer. However, its effects on soil microbial community, N transformation, and plant N uptake are unclear or controversial. In this study, fertilizers were prepared with different percentages of PW (poultry litter, 220 °C 1 or 8 h, PW-S or -L) and urea to supply 160 mg kg^-1 total N in a barren alkali soil. Results showed that the addition of PW relative to pure urea decreased organic N mineralization by low bio-accessibility, increased N loss by high soil pH, and decreased NO₃^--N by low nitrification substrate. It supported the lettuce in health but decreased plant N uptake by low NO₃^--N. It significantly increased the gram-positive bacteria that responded to resistant organic matter, changed the bacterial community to enhance decomposition, detoxification, ureolysis, and denitrification, and to decrease nitrification. Its inhibition effect on nitrification activity was stronger than that on nitrifiers growth. Different from PW-S, the addition of PW-L seriously and significantly decreased seed germination index and fungal biomass that responded to N retaining capacity, respectively. The best fertilizer was 50% urea +50% PW-S that supported the seed germination and seedling growth, and mildly affected microbial community.

Development of chitosan-magnetic sawdust hydrochar for Pb and Zn immobilization process on various soil conditions

A series of 60-day soil immobilized incubations were performed to explore the impacts of various factors (incubation time, chitosan modified magnetic sawdust hydrochar (CMSH) dosages, initial pH values, moisture contents, and humic acid (HA)) on CMSH immobilization of Pb and Zn. DTPA and BCR extraction techniques were undertaken to study the distribution of form transformations of Pb and Zn. CMSH showed significant immobilization ability for both DTPA-Pb and DTPA-Zn, and the highest removal rates were shown to be 57.40% and 90.00% for Pb and Zn respectively. After 60 days of incubation, the residual Pb was enhanced by 34-61% and residual Zn increased by 25-41%, which indicated that CMSH was effective in immobilizing Pb and Zn. Meanwhile, the immobilization efficiency improved with increasing incubation time, CMSH dosage, HA dosage, and initial solution pH. In particular, 5% HA application increased the soil TOC and accelerated the metal stabilization processes, with the residual forms of Pb and Zn eventually reaching a maximum of 73% and 71%, respectively. In addition, the alkaline initial solution promoted the ion exchange, surface complexation reaction, and cationic-π interaction, resulting in a better immobilization of Pb and Zn by CMSH. Finally, according to the orthogonal analysis of BCR results, HA dosage was the major factor affecting Pb and Zn immobilization by CMSH compared to soil pH and moisture content in this study.

Effects of hydrochar derived from hydrothermal treatment of sludge and lignocellulose mixtures on soil properties, nitrogen transformation, and greenhouse gases emissions

In this study, hydrochar samples derived from hydrothermal treatment (HTT) of sludge and sludge-biomass mixtures were applied to a sandy soil and their effects on soil properties, soil nutrients, greenhouse gas (GHG) emissions, and soluble heavy metals were investigated. The application of untreated sludge and hydrochar derived from HTT of sludge at 180 °C led to the highest soluble nitrate, CO₂ and N₂O emissions, followed by the application of hydrochar samples derived from HTT of sludge-biomass mixtures at 180 °C. Although the application of hydrochar samples derived from HTT of sludge alone and sludge-biomass mixtures at 240 °C in sandy soil led to the lowest emissions of CO₂ and N₂O, it resulted in lower levels of soil electrical conductivity (EC), cation exchange capacity (CEC) and soluble phosphorus. The application of hydrochar samples derived from HTT at 240 °C led to the production of CH₄ and lower nitrate-N contents than hydrochar samples derived from HTT at 180 °C. These results indicated that the soils containing hydrochar samples from HTT at 240 °C were anaerobic, which might inhibit the growth of plants. The application of hydrochar samples derived from HTT of sludge-biomass at 180 °C led to significantly improved contents of soil soluble phosphorus (2.56 and 2.84 g kg^-1 soil) and soil nitrate-N (160.2 and 263.2 mg kg^-1 soil) at the end of 60 days of incubation. However, these contents were lower than the contents of soluble phosphorus (3.71 and 4.45 g kg^-1 soil) and nitrate-N (528.3 and 583.2 mg kg^-1 soil) with the application of untreated sludge and sludge derived from HTT of sludge alone at 180 °C. Although more studies are needed to understand the mechanisms and effects on different soils, this study provides useful insights into the application of hydrochar derived from sludge-biomass mixture in soil.

Hydrochar amendments stimulate soil nitrous oxide emission by increasing production of hydroxyl radicals and shifting nitrogen functional genes in the short term: A culture experiment

The application of waste biomass-derived hydrochar to soil may cause extremely intensive nitrous oxide (N₂O) fluxes that can challenge our current mechanistic understanding of the global nitrogen cycle in the biosphere. In this study, two waste biomasses were used to prepare cyanobacterial biomas-derived hydrochar (CHC) and wheat straw-derived hydrochar (SHC) for short-term incubation experiments to identify their effects and mechanisms of waste biomass-derived hydrochar on soil N₂O efflux, with time-series samples collected for N₂O efflux and soil analysis. The results showed that CHC and SHC caused short-term bursts of N₂O effluxes without nitrogen inputs. Moreover, the enrichment of exogenous organics and nutrients at the hydrochar-soil interface was identified as the key factor for enhancing N₂O fluxes, which stimulated microbial nitrification (i.e., increased gene copy number of ammonia oxidizing bacteria) and denitrification (i.e., increased gene copy number of nitrate and N₂O reducing bacteria) processes. The concentrations of Fe (II) and hydroxyl radicals (HO^•) were 6.49 and 5.63 times higher, respectively, in the hydrochar layer of CHC than SHC amendment. Furthermore, structural equation models demonstrated that HO^•, as well as soil microbiomes, played an important role in driving N₂O fluxes. Together, our findings provide a deeper insight into the assessment and prognosis of the short-term environmental risk arising from agricultural waste management in integrated agriculture. Further studies under practical field application conditions are warranted to verify the findings.

Applications of soft computing techniques for prediction of pollutant removal by environmentally friendly adsorbents (case study: the nitrate adsorption on modified hydrochar)

Artificial intelligence has emerged as a powerful tool for solving real-world problems in various fields. This study investigates the simulation and prediction of nitrate adsorption from an aqueous solution using modified hydrochar prepared from sugarcane bagasse using an artificial neural network (ANN), support vector machine (SVR), and gene expression programming (GEP). Different parameters, such as the solution pH, adsorbent dosage, contact time, and initial nitrate concentration, were introduced to the models as input variables, and adsorption capacity was the predicted variable. The comparison of artificial intelligence models demonstrated that an ANN with a lower root mean square error (0.001) and higher R² (0.99) value can predict nitrate adsorption onto modified hydrochar of sugarcane bagasse better than other models. In addition, the contact time and initial nitrate concentration revealed a higher correlation between input variables with the adsorption capacity.

Clay-hydrochar composites return to cadmium contaminated paddy soil: Reduced Cd accumulation in rice seed and affected soil microbiome

Excess cadmium (Cd) in rice precipitated by Cd contamination in paddy soils is a global human health threat and rational response is urgently needed. In this study, attapulgite-modified hydrochar (CA) and the montmorillonite-modified hydrochar (CM) were utilized in Cd-contaminated paddy soils at 0.5% (w/w) and 1% (w/w) application rates to investigate the effects of these clay-hydrochar composites on rice growth and soil Cd availability. The results show that the utility of CA and CM resulted in a significant increase in rice yield, especially at 1% application rate, which extended rice yield by 46.7-50.0% compared to 0.5% application rate. This is related to the Cd fixation and nutrient sequestration of the acidic functional groups on the surface of CA and CM. Additionally, CA-1% and CM-1% reduced the Cd concentration in rice seeds by 26.9-28.1% relative to the control. Notably, CA-1% showed the capacity to passivate Cd at the early stage of rice transplanting, lowering the proportion of Cd in the ion exchange state by 41.6% compared to the control, and this passivation effect persisted until the late stage of transplanted rice. The soil microbial community consequences showed that CA and CM did not significantly change the horizontal composition of the soil bacterial phylum and species diversity, indicating that CA and CM had excessive soil microbial adaptability. Moreover, results of correlation and Canonical Correspondence Analysis confirm that microbial genera responded significantly to the soil Cd morphologies, revealing the importance of CA and CM in the remediation of Cd-contaminated soils by influencing microorganisms. Our findings provide clay-hydrochar composites as a low-cost approach to effectively mitigate soil Cd contamination and improve the security and quality of rice.

Low-Cost Detection of Methane Gas in Rice Cultivation by Gas Chromatography-Flame Ionization Detector Based on Manual Injection and Split Pattern

Rice cultivation is one of the most significant human-created sources of methane gas. How to accurately measure the methane concentration produced by rice cultivation has become a major problem. The price of the automatic gas sampler used as a national standard for methane detection (HJ 38-2017) is higher than that of gas chromatography, which greatly increases the difficulty of methane detection in the laboratory. This study established a novel methane detection method based on manual injection and split pattern by changing the parameters of the national standard method without adding any additional automatic gas samplers. The standard curve and correlation coefficient obtained from the parallel determination of methane standard gas were y = 2.4192x + 0.1294 and 0.9998, respectively. Relative standard deviation (RSD, <2.82%), recycle rate (99.67−102.02%), limit of detection (LOD, 0.0567 ppm) and limit of quantification (LOQ, 0.189 ppm) of this manual injection method are satisfying, demonstrating that a gas chromatography-flame ionization detector (GC-FID), based on manual injection at a split ratio (SR) of 5:1, could be an effective and accurate method for methane detection. Methane gases produced by three kinds of low-methane rice treated with oxantel pamoate acid, fumaric acid and alcohol, were also collected and detected using the proposed manual injection approach Good peak shapes were obtained, indicating that this approach could also be used for quantification of methane concentration.

Hydrothermal treatment as a complementary tool to control the invasive Pampas grass (Cortaderia selloana)

The rapid spread of invasive Pampas grass (PG) is having not only ecosystems impact, but also significant economic and social effects. The tonnes of bulky waste from the plant disposal require proper treatment to avoid seed dispersal, greenhouse gas emissions and landscape damage. In the pursuit of zero-waste management, hydrothermal treatment (HT) appears as a challenging alternative. The possibility of mobile HT systems offers an alternative to accomplish on-site both the PG waste management and the application of the resulting by-products within a circular framework. As a first step, this research shows that, without a prior drying step, the hydrothermal treatment at 100-230 °C under autogenous water vapor pressure for only 30 min allows safe seeds inertization, while a stable carbon-enriched solid and an aqueous stream are generated. Prolonging the process for 2 h has no profitable effects. As the reaction temperature increases, the PG residue is converted into a material with 49-58 wt% of carbon, 41-32 wt% of oxygen and 3-4 wt% of ash. The pH (~6.3), low electrical conductivity (1.21-0.86 dS/m), high carbon content, open porosity (5-8 m²/g) and improved performance in seed germination and in the early growth test suggest the potential of HT-solids derived at 100-120 °C as amendment to sequester carbon in the soil and improve its physico-biological properties. The phytotoxicity detected in the peat/lignite-like solids obtained at 200-230 °C limits its application in soil, but calorific values of 22-24 MJ/kg indicate their suitability as CO₂-neutral fuel. The agrochemical analysis of the liquid by-products indicates poor value on their own, but their use supplemented with compost may be an option.

How does biochar aging affect NH3 volatilization and GHGs emissions from agricultural soils?

Biochar has been considered as a potential tool to mitigate soil ammonia (NH₃) volatilization and greenhouse gases (GHGs) emissions in recent years. However, the aging effect of biochar on soils remains elusive, which introduces uncertainty on the effectiveness of biochar to mitigate global warming in a long term. Here, a meta-analysis of 22 published works of literature with 217 observations was conducted to systematically explore the aging effect of biochar on soil NH₃ and GHGs emissions. The results show that, in comparison with the fresh biochar, the aging makes biochar more effective to decrease soil NH₃ volatilization by 7% and less risk to contribute CH₄ emissions by 11%. However, the mitigation effect of biochar on soil N₂O emissions is decreased by 15% due to aging. Additionally, aging leads to a promotion effect on soil CO₂ emissions by 25% than fresh biochar. Our findings suggest that along with aging, particularly the effect of artificial aging, biochar could further benefit the alleviation of soil NH₃ volatilization, whereas its potential role to mitigate global warming may decrease. This study provides a systematic assessment of the aging effect of biochar to mitigate soil NH₃ and GHGs, which can provide a scientific basis for the sustainable green development of biochar application.

Mitigation of methane emission in a rice paddy field amended with biochar-based slow-release fertilizer

Despite improving soil quality and reducing nitrogen (N) loss in paddy soil, replacing chemical fertilizer with organic fertilizer would significantly accelerate greenhouse gas emission in terms of methane (CH₄). The application of slow-release fertilizer has been proposed an effective approach to control CH₄ emissions, in addition to reducing N loss. Yet, the understanding of CH₄ emissions from paddy fields with the additions of different fertilizers is still less known. Therefore, the effects of different fertilizer treatments, including chemical fertilizer treatment (CF), mixed chemical and organic fertilizer treatment (OF), biochar-based slow-release fertilizer treatment (SF), and no fertilizer control treatment (CK) on CH₄ emissions and methanogenic community structure in paddy soils were investigated through a field experiment. Results showed that slow-release fertilizer addition significantly decreased CH₄ emissions by 33.4%, during the whole rice growing season compared to those in OF. The cumulative CH₄ emissions were in a significantly positive relation to soil NH₄⁺-N. Slow-release fertilizer amendment decreased the relative abundances of Methanosarcina and Methanoregula and increased the relative abundances of hydrogenotrophic Methanocella and Rice Cluster I. Reduced CH₄ emissions with slow-release fertilizer amendment might be mainly attributed to the different forms of N in the fertilizer and available potassium (K) in the paddy soil. Our findings produce novel insights into the application of slow-release fertilizer in controlling CH₄ emissions from rice fields.

Clay-hydrochar composites mitigated CH4 and N2O emissions from paddy soil: A whole rice growth period investigation

With the favorable microporous structure and excellent adsorption capacity, clay-hydrochar composites (CHCs) serve as promising materials to mitigate greenhouse gas emissions (GHG) from the paddy fields. Three clays were co-pyrolyzed with hydrochar derived from poplar sawdust to obtain CHCs, which were applied to the paddy fields to investigate the effects on methane (CH₄) and nitrous oxide (N₂O) emissions. Three CHCs were labeled as bentonite-hydrochar composite (BTHC), montmorillonite-hydrochar composite (MTHC), and kaolinite-hydrochar composite (KTHC), respectively. The effects of these three CHCs on GHG emissions were determined by monitoring the dynamic CH₄ and N₂O emissions in the paddy soil column ecosystem during the rice-growing season. The results showed that compared with the control group, three CHCs significantly mitigated CH₄ and N₂O emissions by 21.4%-47.5% and 5.2%-36.8%, respectively. Furthermore, the fluorescent components result displayed CHCs increased humic-like content by 29.62%-59.72%. A structural equation model was used to assess the hypothesis mitigation mechanism, which exemplified that GHG emissions negatively correlated with pmoA and nosZ genes, possibly resulting in the CH₄ and N₂O mitigation. Among the three CHCs, the KTHC amendment mitigated the CH₄ and N₂O emissions by 47.5% and 36.8%, respectively, which was superior to BTHC and MTHC. Hence, it was recommended for application to the field. Overall, this study demonstrates the mitigating effects of CHCs on GHG emissions for the first time, and the reduced CH₄ and N₂O emissions could contribute to increased soil C and N retention for better agricultural nutrients management.

Application of Hydrochar, Digestate, and Synthetic Fertilizer to a Miscanthus x giganteus Crop: Implications for Biomass and Greenhouse Gas Emissions

Miscanthus x giganteus (miscanthus), a perennial biomass crop, allocates more carbon belowground and typically has lower soil greenhouse gas (GHG) emissions than conventional feedstock crops, but best practices for nutrient management that maximize yield while minimizing soil GHG emissions are still debated. This study evaluated the effects of four different fertilization treatments (digestate from a biodigester, synthetic fertilizer (urea), hydrochar from the hydrothermal carbonization of digestate, and a control) on soil GHG emissions and biomass yield of an established miscanthus stand grown on abandoned agricultural land. Soil GHG fluxes (including CH4, CO2, and N2O) were sampled in all treatments using the static chamber methodology. Average biomass yield varied from 20.2 Mg ha−1 to 23.5 Mg ha−1, but there were no significant differences among the four treatments (p > 0.05). The hydrochar treatment reduced mean CO2 emissions by 34% compared to the control treatment, but this difference was only statistically significant in one of the two sites tested. Applying digestate to miscanthus resulted in a CH4 efflux from the soil in one of two sites, while soils treated with urea and hydrochar acted as CH4 sinks in both sites. Overall, fertilization did not significantly improve biomass yield, but hydrochar as a soil amendment has potential for reducing soil GHG fluxes.