Insights into the molecular transformation in the dissolved organic compounds of agro-waste-hydrochars by microbial-aging using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Hydrochar and Its Dissolved Organic Matter Aged in a 30-Month Rice-Wheat Rotation System: Do Primary Aging Factors Alter at Different Stages?

Hydrochar, recognized as a green and sustainable soil amendment, has garnered significant attention. However, information on the aging process in soil and the temporal variability of hydrochar remains limited. This study delves deeper into the interaction between hydrochar and soil, focusing on primary factors influencing hydrochar aging during a 30-month rice-wheat rotation system. The results showed that the initial aging of hydrochar (0-16 months) is accompanied by the development of specific surface area and leaching of hydrochar-derived dissolved organic matter (HDOM), resulting in a smaller particle size and reduced carbon content. The initial aging also features a mineral shield, while the later aging (16 to 30 months) involves surface oxidation. These processes collectively alter the surface charge, hydrophilicity, and composition of aged hydrochar. Furthermore, this study reveals a dynamic interaction between the HDOM and DOM derived from soil, plants, and microbes at different aging stages. Initially, there is a preference for decomposing labile carbon, whereas later stages involve the formation of components with higher aromaticity and molecular weight. These insights are crucial for understanding the soil aging effects on hydrochar and HDOM as well as evaluating the interfacial behavior of hydrochar as a sustainable soil amendment.

Hydrothermal carbonization of biogas slurry and cattle manure into soil conditioner mitigates ammonia volatilization from paddy soil

Hydrothermal carbonization of biogas slurry and animal manure into hydrochar could enhance waste recycling waste and minimize ammonia (NH3) volatilization from paddy fields. In this study, cattle manure-derived hydrochar prepared in the presence of Milli-Q water (CMWH) and biogas slurry (CMBSH), and biogas slurry-based hydrochar embedded with zeolite (ZHC) were applied to rice-paddy soil. The results demonstrated that CMBSH and ZHC treatments could significantly mitigate the cumulative NH3 volatilization and yield-scale NH3 volatilization by 27.9-45.2% and 28.5-45.4%, respectively, compared to the control group (without hydrochar addition), and significantly correlated with pH and ammonium-nitrogen (NH4+-N) concentration in floodwater. Nitrogen (N) loss via NH3 volatilization in the control group accounted for 24.9% of the applied N fertilizer, whereas CMBSH- and ZHC-amended treatments accounted for 13.6-17.9% of N in applied fertilizer. The reduced N loss improved soil N retention and availability for rice; consequently, grain N content significantly increased by 6.5-14.9% and N-use efficiency increased by 6.4-16.0% (P < 0.05), respectively. Based on linear fitting results, NH3 volatilization mitigation resulted from lower pH and NH4+-N concentration in floodwater that resulted from the acidic property and specific surface area of hydrochar treatments. Moreover, NH3-oxidizing archaea abundance in hydrochar-treated soil decreased by 40.9-46.9% in response to CMBSH and ZHC treatments, potentially suppressing NH4+-N transformation into nitrate and improving soil NH4+-N retention capacity. To date, this study applied biogas slurry-based hydrochar into paddy soil for the first time and demonstrated that ZHC significantly mitigated NH3 and increased N content. Overall, this study proposes an environmental-friendly strategy to recycle the wastes, biogas slurry, to the paddy fields to mitigate NH3 volatilization and increase grain yield of rice.

Release characteristics of hydrochar-derived dissolved organic matter: Effects of hydrothermal temperature and environmental conditions

Much research has been done on the preparation and application of hydrochars, but research on the release characteristics of hydrochar-derived dissolved organic matter (HDOM) is very limited; clarifying the release characteristics of HDOM is important for understanding and adjusting the environmental behaviour of hydrochar. Herein, the potential release of HDOM from rice straw-derived hydrochars prepared at different hydrothermal temperatures was investigated under various potential environmental conditions for the first time. The total release quantity and humification degree of HDOM decreased with increasing hydrothermal temperature. The critical dividing line for various hydrothermal reactions, decomposition and polymerization, was in the range of 240 °C-260 °C. Alkaline condition increased the HDOM release amount (up to 299 mg g^-1), molecular weight (as high as 423 Da) and molecular diversity (8857 compounds) from rice straw-derived hydrochars. The unique substances of HDOM released under alkaline condition were mainly distributed in lipids-like substances, CRAM/lignins-like substances, aromatic structures, and tannins-like substances, while few unique substances were found under acidic condition. Additionally, CRAM/lignins-like substances were the most abundant in all HDOM samples, reaching 82%, which were relatively stable and could achieve carbon sequestration in different environments. The findings provided a new insight on understanding the potential environment behaviors of hydrochar.

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.

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.

Using Potential Molecular Transformation To Understand the Molecular Trade-Offs in Soil Dissolved Organic Matter

Understanding the chemical composition and molecular transformation in soil dissolved organic matter (DOM) is important to the global carbon cycle. To address this issue, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was applied to investigate DOM molecules in 36 paddy soils collected from subtropical China. All the detected 7576 unique molecules were divided into seven compound groups, and nine trade-off relationships between different compound groups were revealed based on principal component analysis and Pearson's correlation. An optimized method was developed to evaluate all potential molecular transformations in DOM samples. The concept of thermodynamics was introduced to evaluate the identified molecular transformations and classify them as thermodynamically favorable (TFP) and thermodynamically limited (TLP) processes. Here, we first tried to understand the molecular trade-offs by using the potential molecular transformations. All the nine trade-offs could be explained by molecular transformations. Six trade-offs had bases of biochemical reactions, and the trade-off-related direct transformations could explain the content variations of carbohydrate-like, condensed aromatic-like, tannin-like, and lignin-like compounds in TLP. More reasonable explanations existed in the TLP rather than TFP, which demonstrated the critical role of external energy in the molecular transformation of soil DOM.

Hydrochar and hydrochar co-compost from OFMSW digestate for soil application: 2. agro-environmental properties

The work concerns the study of the hydrochar from digestate and hydrochar co-compost characterization as amendments. The processes for hydrochar and co-compost production were described in Part 1 of this work (Scrinzi et al., 2022). The amendment properties of hydrochar (produced at 180-200-220 °C for 3 h) and co-composts (25%, 50%, and 75% hydrochar percentage of digestate substitution) were assessed by phytotoxicity, plant growth bioassay, and soil effect. Different seeds species (Lepidium sativum, Cucumis sativus, and Sorghum bicolor sp.) were dosed at increased concentrations using both wet raw amendments and their water extracts. The chemical characterization showed phytotoxic compounds content depending on both the initial feedstock (digestate) and the HTC process; at the same time, the analysis highlighted the reduction of these compounds by composting (organic acid, polyphenols, salt concentration). The dose-response was analyzed by the Cedergreen-Streibig-Ritz model and the half-maximal effective concentration (EC50) was calculated based on this equation. The soil properties and GHG emissions measurements (CH₄, CO₂, N₂O, and NH₃) highlighted the effect on N dynamics and on soil respiration induced by substrates. The HC200 soil application determined a significant impact on CO₂ and N₂O emission and NH₃ volatilization (10.82 mol CO₂/m²; 51.45 mmol N₂O/m²; 112 mol NH₃/m²) and a significant reduction of total N and TOC (46% of TKN and 49% of TOC). The co-compost (75%) showed specific effects after soil application compared to other samples an increase of available P (48%), a greater content of nitrogen (1626 mg/kg dry basis), and a reduction of organic carbon (17%). Our results demonstrate the good quality of co-compost and at the same time the validity of this post-treatment for addressing many issues related to hydrochar use in the soil as an amendment, confirming the suitability of HTC process integration for digestate treatment in anaerobic digestion plants.

Chemical aging of hydrochar improves the Cd2+ adsorption capacity from aqueous solution

Hydrochar (HC) serves as a promising adsorbent to remove the cadmium from aqueous solution due to porous structure. The chemical aging method is an efficient and easy-operated approach to improve the adsorption capacity of HC. In this study, four chemical aging hydrochars (CAHCs) were obtained by using nitric acid (HNO₃) with mass fractions of 5% (N5-HC), 10% (N10-HC), and 15% (N15-HC) to age the pristine HC (N0-HC) and remove the Cd²⁺ from the aqueous solution. The results displayed that the N15-HC adsorption capacity was 19.99 mg g^-1 (initial Cd²⁺ concentration was 50 mg L^-1), which increased by 7.4 folds compared to N0-HC. After chemical aging, the specific surface area and oxygen-containing functional groups of CAHCs were increased, which contributed to combination with Cd²⁺ by physical adsorption and surface complexation. Moreover, ion exchange also occurred during the adsorption process of Cd²⁺. These findings have important implications for wastewater treatment to transform the forestry waste into a valuable adsorbent for Cd²⁺ removal from water.

Potential release of dissolved organic matter from agricultural residue-derived hydrochar: Insight from excitation emission matrix and parallel factor analysis

Potential release quantity and quality of dissolved organic matter (DOM) from hydrochar (HDOM) in various environmental conditions were investigated. Corn cobs were utilized as model agricultural residue to prepare the hydrochar. Four extracts, ultra-pure water, acid solution, alkali solution and salt solution, and two temperatures, 20 °C and 60 °C, were adopted to imitate various environmental conditions. Excitation-emission spectrophotometry with parallel factor analysis was used to evaluate the chemical properties of HDOM. The results showed that the dissolved organic carbon in the HDOM was high, ranging from 46 to 268 mg g^-1. Four components were confirmed in the HDOM: mixed substances of humic-like and protein-like components, marine humic-like substances, terrestrial humic-like substances and tyrosine-like substances. Alkalinity and high temperature conditions could enhance the leaching amount of HDOM, particularly humic-like substances, and change the relative proportion of components and the chemical quality. In addition, values of the fluorescence indexes indicated that the HDOM was high microbial availability. Released HDOM may result in significant impacts in ecosystem functionality. These findings reveal the potential release characteristics of HDOM in the environment, opening new doors to understanding the environmental impacts of hydrochar and guiding its rational application.