Biochar addition accelerates the humification process by affecting the microbial community during human excreta composting

Biochar addition plays an important role in manure composting, but its driving mechanism on microbial succession and humification process of human excreta composting is still unclear. In the present study, the mechanism of biochar addition was explored by analysing the humification process and microbial succession pattern of human excreta aerobic composting without and with 10% biochar (HF and BHF). Results indicated that BHF improved composting temperature, advanced the thermophilic phase by 1 d, increased the germination index by 49.03%, promoted the growth rate of humic acid content by 17.46%, and raised the compost product with the ratio of humic acid to fulvic acid (HA/FA) by 16.19%. Biochar regulated the diversity of fungi and bacteria, increasing the relative abundance of Planifilum, Meyerozyma and Melanocarpus in the thermophilic phase, and Saccharomonospora, Flavobacterium, Thermomyces and Remersonia in the mature phase, which accelerates the humification. Bacterial communities' succession had an obvious correlation with the total carbon, total nitrogen, and temperature (P < 0.05), while the succession of fungal communities was influenced by the HA/FA and pH (P < 0.05). This study could provide a reference for the improvement of on-site human excreta harmless by extending the thermophilic phase, and facilitating the humification in human excreta compost with biochar addition.

Roles of Natural Phenolic Compounds in Polycyclic Aromatic Hydrocarbons Abiotic Attenuation at Soil-Air Interfaces through Oxidative Coupling Reactions

Little information is available on the roles of natural phenolic compounds in polycyclic aromatic hydrocarbons (PAHs) attenuation at dry soil-air interfaces. The purpose of this study was to determine the roles of model phenolic constituents of soil organic matter (SOM) on the abiotic attenuation of PAHs. The phenolic compounds can significantly change the attenuation rates of PAHs, among which hydroquinone was the most effective in promoting anthracene and benzo[a]anthracene attenuation. Product identification and sequential extraction experiments revealed hydroquinone enhanced the formation of oxidative coupling products and promoted the incorporation of PAHs into humic analogues, thereby reducing potential risks to humans and ecosystems. Electron paramagnetic resonance spectroscopy analyses showed both PAHs and phenolic compounds could donate electrons to Lewis acid sites of soil minerals, resulting in the generation of persistent free radicals (PFRs). PFRs could promote the generation of ·OH to enhance PAH oxidation and could cross-couple with PAHs, resulting in high-molecular-weight oxidative coupling products. This study revealed for the first time the reaction mechanism between PAHs and phenolic components of SOM under relatively dry conditions and provided new insights into promoting PAHs detoxification in soils but also a potential strategy to increase the organic carbon sequestration.

Exolaccase Propels Humification to Decontaminate Bisphenol A and Create Humic-like Biostimulants

Exolaccase-propelled humification (E-PH) helps eliminate phenolic pollutants and produce macromolecular precipitates. Herein, we investigated the influencing mechanism of 12 humic precursors (HPs) on exolaccase-enabled bisphenol A (BPA) decontamination and humification. Catechol, vanillic acid, caffeic acid, and gentian acid not only expedited BPA removal but also created large amounts of copolymeric precipitates. These precipitates had rich functional groups similar to natural humic substances, which presented great aromatic and acidic characteristics. The releasing amounts of BPA monomer from four precipitates were 0.08-12.87% at pH 2.0-11.0, suggesting that BPA-HP copolymers had pH stability. More excitingly, certain copolymeric precipitates could stimulate the growth and development of radish seedlings. The radish growth-promotion mechanisms of copolymers were involved in two aspects: (1) Copolymers interacted with root exudates to accelerate nutrient uptake; (2) Copolymers released auxins to provoke radish growth. These results may provide an innovative strategy for decontaminating phenolic pollutants and yielding humic-like biostimulants in E-PH.

Lignin Metabolism by Selected Fungi and Microbial Consortia for Plant Stimulation: Implications for Biologically Active Humus Genesis

Plant lignin is regarded as an important source for soil humic substances (HSs). Nonetheless, it remains unclear whether microbial metabolism on lignin is related to the genesis of unique HS biological activities (e.g., direct plant stimulation). Here, selected white-rot fungi (i.e., Ganoderma lucidum and Irpex lacteus) and plant litter- or mountain soil-derived microbial consortia were exploited to structurally modify lignin, followed by assessing the plant-stimulatory activity of the lignin-derived products. Parts solubilized by microbial metabolism on lignin were proven to exhibit organic moieties of phenol, carboxylic acid, and aliphatic groups and the enhancement of chromogenic features (i.e., absorbance at 450 nm), total phenolic contents, and radical-scavenging capacities with the cultivation times. In addition, high-resolution mass spectrometry revealed the shift of lignin-like molecules toward those showing either more molar oxygen-to-carbon or more hydrogen-to-carbon ratios. These results support the findings that the microbes involved, solubilize lignin by fragmentation, oxygenation, and/or benzene ring opening. This notion was also substantiated by the detection of related exoenzymes (i.e., peroxidases, copper radical oxidases, and hydrolases) in the selected fungal cultures, while the consortia treated with antibacterial agents showed that the fungal community is a sufficient condition to induce the lignin biotransformation. Major families of fungi (e.g., Nectriaceae, Hypocreaceae, and Saccharomycodaceae) and bacteria (e.g., Burkholderiaceae) were identified in the lignin-enriched cultures. All the microbially solubilized lignin products were likely to stimulate plant root elongation in the order selected white-rot fungi > microbial consortia > antibacterial agent-treated microbial consortia. Overall, this study supports the idea that microbial transformation of lignin can contribute to the formation of biologically active organic matter. IMPORTANCE Structurally stable humic substances (HSs) in soils are tightly associated with soil fertility, and it is thus important to understand how soil HSs are naturally formed. It is believed that microbial metabolism on plant matter contributes to natural humification, but detailed microbial species and their metabolisms inducing humic functionality (e.g., direct plant stimulation) need to be further investigated. Our findings clearly support that microbial metabolites of lignin could contribute to the formation of biologically active humus. This research direction appears to be meaningful not only for figuring out the natural processes, but also for confirming natural microbial resources useful for artificial humification that can be linked to the development of high-quality soil amendments.

The Emergence of Extracellular Electron Mediating Functionality in Rice Straw-Artificial Soil Mixture during Humification

This study aimed to elucidate the origin of extracellular electron mediating (EEM) functionality and redox-active center(s) in humic substances, where they are ubiquitously distributed. Here, we show the emergence of EEM functionality during the humification of rice straw in artificial soil (kaolin and sand) with a matric potential of -100 cm at 20 °C for one year. We used the dechlorination activity of an EEM material-dependent pentachlorophenol-dechlorinating anaerobic microbial consortium as an index of the EEM functionality. Although rice straw and its mixture with artificial soil did not initially have EEM functionality, it emerged after one month of humification and increased until six months after which the functionality was maintained for one year. Chemical and electrochemical characterizations demonstrated that the emergence and increase in EEM functionality were correlated with the degradation of rice straw, formation of quinone structures, a decrease in aromatic structures, an increase in nitrogenous and aliphatic structures, and specific electric capacitance during humification. The newly formed quinone structure was suggested as a potential redox-active center for the EEM functionality. These findings provide novel insights into the dynamic changes in EEM functionality during the humification of organic materials.

Degradation or humification: rethinking strategies to attenuate organic pollutants

The fate of organic pollutants in environmental matrices can be determined by degradation and humification. The humification process represents a promising strategy to remove organic pollutants, particularly those resistant to degradation. In contrast to the well-studied degradation process, the contribution and application prospects of the humification process for organic pollutant removal has been underestimated. The recent progress in synthesizing artificial humic substances (HS) has made directed humification of recalcitrant organic pollutants possible. This review focuses on degradation and humification of organic matter, especially recalcitrant organic pollutants. Challenges in understanding the contribution, underlying mechanisms, and artificial synthesis of HS for removing organic pollutants are also critically discussed. We advocate further investigating the humification of organic pollutants in future studies.

Biodegradation and Structural Modification of Humic Acids in a Compost Induced by Fertilization with Steelmaking Slag under Coastal Seawater, as Detected by TMAH-py-GC/MS, EEM and HPSEC Analyses

The compost's humic acid (HA) content decreased when it was fertilized in coastal seawater with steelmaking slag, as confirmed. This study clarified the cause for this change by a detailed analysis of the structural changes in HAs based on the TMAH-py-GC/MS, HPSEC, and 3D-EEM spectra. An increase in the levels of pyrolysates of tannic acid with a low polymerization degree was attributed to the biodegradation of a high polymerized aromatic structure. Moreover, analyses of 3D-EEM, supported by HPSEC, indicated that approximately 20 kDa of the fluorescent matter was generated at the protein-like peaks (Ex/Em = 220/340 and 275/350 nm) in HAs derived from a mixture of compost with steelmaking slag. It would be caused due to the formation of HAs from the bacterial by-product by a catalytic reaction of the steelmaking slag. From these findings reported herein, we conclude that bio-degradation was a major reason for the decreased HA content, and the formation of HA from a part of the degradation products. This would be a reason for the structural modification of HA under the seawater condition.

Potential Hydrothermal-Humification of Vegetable Wastes by Steam Explosion and Structural Characteristics of Humified Fractions

In this work, steam explosion (SE) was exploited as a potential hydrothermal-humification process of vegetable wastes to deconstruct their structure and accelerate their decomposition to prepare humified substances. Results indicated that the SE process led to the removal of hemicellulose, re-condensation of lignin, degradation of the cellulosic amorphous region, and the enhancement of thermal stability of broccoli wastes, which provided transformable substrates and a thermal-acidic reaction environment for humification. After SE treatment, total humic substances (HS), humic acids (HA_s), and fulvic acids (FA_s) contents of broccoli samples accounted for up to 198.3 g/kg, 42.3 g/kg, and 166.6 g/kg, and their purification were also facilitated. With the increment of SE severity, structural characteristics of HA_s presented the loss of aliphatic compounds, carbohydrates, and carboxylic acids and the enrichment of aromatic structures and N-containing groups. Lignin substructures were proved to be the predominant aromatic structures and gluconoxylans were the main carbohydrates associated with lignin in HA_s, both of their signals were enhanced by SE. Above results suggested that SE could promote the decomposition of easily biodegradable matters and further polycondensation, aromatization, and nitrogen-fixation reactions during humification, which were conducive to the formation of HA_s.

Selenium distribution in French forests: Influence of environmental conditions

Selenium is a trace element and an essential nutrient. Its long-lived radioisotope, selenium 79 is of potential radio-ecological concern in surface environment of deep geological repository for high-level radioactive waste. In this study, the influence of environmental, climatic and geochemical conditions on stable Se (as a surrogate of ⁷⁹Se) accumulation was statistically assessed (PCA analysis, Kruskall-Wallis and Spearman tests) based on the analysis of its concentration in litterfall, humus, and soil samples collected at 51 forest sites located in France. Selenium concentrations were in the ranges: 22-369, 57-1608 and 25-1222 μg kg^-1 respectively in litterfall, humus, and soil. The proximity of the ocean and oceanic climate promoted Se enrichment of litterfall, likely due to a significant reaction of wet deposits with forest canopy. Se content was enhanced by humification (up to 6 times) suggesting that Se concentrations in humus were affected by atmospheric inputs. Selenium stock in humus decreased in the order of decreasing humus biomass and increasing turnover of organic matter: mor > moder > mull. Positive correlations between Se content and geochemical parameters such as organic carbon content, total Al and total Fe confirmed the important role of organic matter (OM) and mineral Fe/Al oxides in Se retention in soils.

The variations of surface wetness recorded by multi-proxies in Yuanchi peatland of the Changbai Mountains from 1962 to 2008

Surface wetness of Yuanchi peatland in the Changbai Mountains were reconstructed by comprehensive analysis on plant residue, testate amoeba and humification, using the age-depth model established with AMS ¹⁴C and ¹³⁷Cs dating methods. The response of surface wetness to climate change was addressed. Results showed that plant residues in the top 50 cm of peat sequence were dominated by mosses. Detrended correspondence analysis (DCA) of plant residue revealed that axis 1 of DCA biplot might represent mire surface wetness (MSW). MSW from 1962 to 2008 as indicated by axis 1 scores of DCA on plant residue were compared with depths to water table derived by testate amoebae-DWT transfer function and humification measurements, and the results showed that the trends of MSW reconstructed by three proxies were largely consistent. They were wetter MSW from 40-50 cm (1962 to 1975), 27-40 cm (fluctuating from wet to dry from 1975 to 1987), and 0-27 cm (drier from 1987 to 2008). Compared with instrumental data from nearby Erdao weather station, the reconstructed MSW was basically consistent with temperature change in this period. The decreases of MSW occurred when mean summer temperature and mean annual temperature were higher than long-term mean value. Our results suggest that the variability in MSW in the recent 46 years predominantly responded weakening of effective precipitation induced by increasing temperature.

Revealing the Inner Dynamics of Fulvic Acid from Different Compost-Amended Soils through Microbial and Chemical Analyses

The formation of fulvic acid (FA), an aromatic compound, is affected by the compost amendment. This study aimed to assess the extent of the humification of FA in soil amended with seven different composts. Results showed that composts improved the FA concentration in soil. Parallel factor (PARAFAC) analysis, combined with hetero-two-dimensional correlation spectroscopy (hetero-2DCOS), indicated that the inner changes in FA components determined the evolution of mineralization. The diversity in the composts used and the dominant microbes present might be responsible for the evolution of different mechanisms of FA transformation. Structural equation models (SEMs) demonstrated that the FA components were transformed directly by microbes, or indirectly via changes in the total organic carbon (TOC) and total nitrogen (TN) contents, C:N ratio, humic substance (HS) levels, and humic acid (HA): FA ratio, which regulate the microbial community structure. Our results will be useful for improving the bioavailability of compost products and realizing sustainable utilization of the soil.

Bioavailability Evaluation of Dissolved Organic Matter Derived from Compost-Amended Soils

In this study, hetero-two-dimensional correlation spectroscopy (hetero-2DCOS) combined with parallel factor analysis (PARAFAC) was employed to reveal the inner changes in the dissolved organic matter (DOM) components derived from soil amended with seven different composts. The dynamics of the four DOM components showed that the fluorescence peaks in each component varied in different directions during mineralization. Structural equation models (SEMs) demonstrated that the compost amendments changed the correlations of the total organic carbon (TOC), total nitrogen (TN), and bacterial community composition with DOM components and strengthened the cooperative function related to transformation of DOM components. The compost sources were further ranked as cabbage waste (CW) > chicken manure (CM), dairy cattle manure (DCM), tomato stem waste (TSW), peat (P) > municipal solid waste (MSW), sewage sludge (SS) by projection pursuit regression (PPR) analysis. It is helpful to improve the bioavailability of compost products to obtain composts with a particular function.

Niche Differentiation between Two Sympatric Cubitermes Species (Isoptera, Termitidae, Cubitermitinae) Revealed by Stable C and N Isotopes

Many African termite species are true soil-feeders: how can they coexist, sometimes with high densities? How do they separate their trophic niches? Preliminary results suggest that two coexisting Cubitermes species forage in different soil layers, and stable C and N isotopes show that they feed on different organic material. Cubitermes aff. ugandensis forages near the soil surface whereas C. aff. sankurensis forages in deeper layers; however, unexpectedly, the former shows a higher δ¹⁵N than the latter, highlighting, for the first time, a trophic niche differentiation between two sympatric true soil feeders bearing different enteric valve patterns.

Climate warming and land-use changes drive broad-scale floristic changes in Southern Sweden

Land-use changes, pollution and climate warming during the 20th century have caused changes in biodiversity across the world. However, in many cases, the environmental drivers are poorly understood. To identify and rank the drivers currently causing broad-scale floristic changes in N Europe, we analysed data from two vascular plant surveys of 200 randomly selected 2.5 × 2.5 km grid-squares in Scania, southernmost Sweden, conducted 1989-2006 and 2008-2015, respectively, and related the change in frequency (performance) of the species to a wide range of species-specific plant traits. We chose traits representing all plausible drivers of recent floristic changes: climatic change (northern distribution limit, flowering time), land-use change (light requirement, response to grazing/mowing, response to soil disturbance), drainage (water requirement), acidification (pH optimum), nitrogen deposition and eutrophication (N requirement, N fixation ability, carnivory, parasitism, mycorrhizal associations), pollinator decline (mode of reproduction) and changes in CO₂ levels (photosynthetic pathway). Our results suggest that climate warming and changes in land-use were the main drivers of changes in the flora during the last decades. Climate warming appeared as the most influential driver, with northern distribution limit explaining 30%-60% of the variance in the GLMM models. However, the relative importance of the drivers differed among habitat types, with grassland species being affected the most by cessation of grazing/mowing and species of ruderal habitats by on-going concentration of both agriculture and human population to the most productive soils. For wetland species, only pH optimum was significantly related to species performance, possibly an effect of the increasing humification of acidic water bodies. An observed relative decline of mycorrhizal species may possibly be explained by decreasing nitrogen deposition resulting in less competition for phosphorus. We found no effect of shortage or decline of pollinating lepidopterans and bees.

Effects of extreme experimental drought and rewetting on CO2 and CH4 exchange in mesocosms of 14 European peatlands with different nitrogen and sulfur deposition

The quantitative impact of intense drought and rewetting on gas exchange in ombrotrophic bogs is still uncertain. In particular, we lack studies investigating multitudes of sites with different soil properties and nitrogen (N) and sulfur (S) deposition under consistent environmental conditions. We explored the timing and magnitude of change in CO2 (Respiration, Gross Primary Production - GPP, and Net Exchange - NE) and CH4 fluxes during an initial wet, a prolonged dry (~100 days), and a subsequent wet period (~230 days) at 12 °C in 14 Sphagnum peat mesocosms collected in hollows from bogs in the UK, Ireland, Poland, and Slovakia. The relationship of N and S deposition with GPP, respiration, and CH4 exchange was investigated. Nitrogen deposition increased CO2 fluxes and GPP more than respiration, at least up to about 15 kg N ha(-1)  yr(-1) . All mesocosms became CO2 sources during drying and most of them when the entire annual period was considered. Response of GPP to drying was faster than that of respiration and contributed more to the change in NE; the effect was persistent and few sites recovered "predry" GPP by the end of the wet phase. Respiration was higher during the dry phase, but did not keep increasing as WT kept falling and peaked within the initial 33 days of drying; the change was larger when differences in humification with depth were small. CH4 fluxes strongly peaked during early drought and water table decline. After rewetting, methanogenesis recovered faster in dense peats, but CH4 fluxes remained low for several months, especially in peats with higher inorganic reduced sulfur content, where sulfate was generated and methanogenesis remained suppressed. Based on a range of European sites, the results support the idea that N and S deposition and intense drought can substantially affect greenhouse gas exchange on the annual scale.

Humic fractions of forest, pasture and maize crop soils resulting from microbial activity

Humic substances result from the degradation of biopolymers of organic residues in the soil due to microbial activity. The objective of this study was to evaluate the influence of three different ecosystems: forest, pasture and maize crop on the formation of soil humic substances relating to their biological and chemical attributes. Microbial biomass carbon (MBC), microbial respiratory activity, nitrification potential, total organic carbon, soluble carbon, humic and fulvic acid fractions and the rate and degree of humification were determined. Organic carbon and soluble carbon contents decreased in the order: forest > pasture > maize; humic and fulvic acids decreased in the order forest > pasture=maize. The MBC and respiratory activity were not influenced by the ecosystems; however, the nitrification potential was higher in the forest than in other soils. The rate and degree of humification were higher in maize soil indicating greater humification of organic matter in this system. All attributes studied decreased significantly with increasing soil depth, with the exception of the rate and degree of humification. Significant and positive correlations were found between humic and fulvic acids contents with MBC, microbial respiration and nitrification potential, suggesting the microbial influence on the differential formation of humic substances of the different ecosystems.

Impact of laccase enzyme inducers on solid waste compost maturity and stability

The influence of inducers of laccase produced by Pleurotus ostreatus-P1 on the physicochemical properties of solid waste compost was investigated. In order to examine the degree of humification and time period for maturity and stability of compost, three different treatments (T: laccase; T1: laccase+CuSO4; and T2: laccase+CuSO4+CaCl2) were inoculated on solid waste in the initial stage of the composting process. During the composting process, the carbon nitrogen ratio (C/N), nitrate ([Formula: see text]), ammonium nitrogen ([Formula: see text]), cation exchange capacity (CEC), humic acid (HA) and fulvic acid (FA) humification rate (HR), Humification Index (HI), and degree of polymerization (DP) were assessed for the evaluation of compost maturity and stability, and it was found that T2 treatment C/N, CEC, and HI was greater, 46.8%, 37.2%, and 38.46%, respectively, than T at the end of the maturation phase. Optimum degree of maturity was also appraised by the Germination Index, 98.25% was attained by T2, whereas 88.65% and 71.23% by T1 and T, respectively. Significant Pearson correlation was also found between compost characteristics such as C/N, CEC, HA, FA, DP, HI, and HR. In addition, also obstinate the mineralization's of nitrogen under laboratory incubation, using soil amended by 50 and 100 kg-N/ha and examined an increasing trend in nitrogen mineralization with the application rate. Moreover, it was concluded that the laccase inducers increase the disintegration and humification process during solid waste composting.

Plant litter chemistry and microbial priming regulate the accrual, composition and stability of soil carbon in invaded ecosystems

Soil carbon (C) sequestration, as an ecosystem property, may be strongly influenced by invasive plants capable of depositing disproportionately high quantities of chemically distinct litter that disrupt ecosystem processes. However, a mechanistic understanding of the processes that regulate soil C storage in invaded ecosystems remains surprisingly elusive. Here, we studied the impact of the invasion of two noxious nonnative species, Polygonum cuspidatum, which produces recalcitrant litter, and Pueraria lobata, which produces labile litter, on the quantity, molecular composition, and stability of C in the soils they invade. Compared with an adjacent noninvaded old-field, P. cuspidatum-invaded soils exhibited a 26% increase in C, partially through selective preservation of plant polymers. Despite receiving a 22% higher litter input, P. lobata-invaded Pinus stands exhibited a 28% decrease in soil C and a twofold decrease in plant biomarkers, indicating microbial priming of native soil C. The stability of C exhibited an opposite trend: the proportion of C that was resistant to oxidation was 21% lower in P. cuspidatum-invaded soils and 50% higher in P. lobata-invaded soils. Our results highlight the capacity of invasive plants to feed back to climate change by destabilizing native soil C stocks and indicate that environments that promote the biochemical decomposition of plant litter would enhance the long-term storage of soil C. Further, our study highlights the concurrent influence of dominant plant species on both selective preservation and humification of soil organic matter.