The effect of modified biochar on methane emission and succession of methanogenic archaeal community in paddy soil

Co-application of digestate and biochar reduced greenhouse gas emissions in paddy soil through enhanced denitrification and anaerobic methane oxidation

Digestate from food waste (FW) has been identified as a promising nutrient resource for agriculture. However, applying digestate directly to soil often produces considerable greenhouse gas (GHG) emissions. As a soil amendment, biochar has demonstrated potential for mitigating GHG emissions. At present, the effect of biochar on GHG emissions and the associated regulatory mechanisms in paddy soils amended with digestate remains unclear. A 45-day soil incubation was conducted with different nitrogen substitution ratios of urea by digestate, coupled with biochar application: CK (100 % urea), D0U100 (100 % urea + biochar), D50U50 (50 % urea, 50 % digestate + biochar), and D100U0 (100 % digestate + biochar). Results indicated that the co-application of biochar and digestate significantly reduced N2O accumulation by 44.99 %-80.39 % compared to CK, primarily due to a decrease in soil NO3--N content and an increase in soil pH, which together significantly improved the distribution of the nosZ gene involved in denitrification. The increase in the abundance of Conexibacter, Symbiobacterium, Anaerolinea, and Candidatus_Solibacter further contributed to N2O reduction. Furthermore, the co-application led to a 21.68 %-38.15 % reduction in CH4 accumulation compared to CK. Biochar increased the abundance of methanotrophic bacteria, such as Methylococcaceae, Methyloligellaceae, and Methylomirabilaceae. Co-application increased the abundance of nitrate-reducing bacteria Symbiobacterium and Anaerolinea, thereafter facilitating nitrite-dependent anaerobic methane oxidation (AOM) dominated by Methylomirabilaceae. Additionally, sulfate-dependent and Iron(III)-dependent AOM likely further contributed to CH4 reduction. Overall, this study proposed a low-carbon management strategy for FW digestate and GHG emissions mitigation of paddy soil.

Accelerating electron transfer reduces CH4 and CO2 emissions in paddy soil

As an accelerated electron transfer device, the influence of microbial electrochemical snorkel (MES) on soil greenhouse gas production remains unclear. Electron transport is the key to methane production and denitrification. We found that the N2O amount of the MES treatment was comparable to the control however the cumulative CO2 and CH4 emissions were reduced by 50% and 41%, respectively. The content of Fe2+ in MES treatment increased by 31%, which promoted the electron competition of iron reduction to methanogenesis. Furthermore, the competition among iron-reducing, nitrifying and denitrifying bacteria reduced the abundance of methanogens by 19-20%. Additionally, the MES treatment decreased the abundance of genes associated with hydrogen methanogenesis pathway by 6-19%, and inhibited the further conversion of acetyl-CoA into CH4 for acetoclastic methanogenesis. This study reveals effects of accelerating electron transfer on greenhouse gas emission, and provides a novel strategy for reducing greenhouse gas emissions in paddy soil.

Utilizing conductive materials for reducing methane emissions in postharvest paddy rice soil microcosms

Paddy fields are a major anthropogenic source of global methane (CH4) emissions, a powerful greenhouse gas (GHG). This study aimed at gaining insights of different organic and inorganic conductive materials (CMs) - biochar, fungal melanin, and magnetite - to mitigate CH4 emissions, and on their influence on key microbial populations, mimicking the postharvest season throughout the degradation of rice straw in microcosms under anaerobic conditions encompassing postharvest paddy rice soils from the Ebro Delta, Spain. Results showed that fungal melanin was the most effective CM, significantly reducing CH4 emissions by 29 %, while biochar amendment also reduced emissions by 10 %. Magnetite slightly increased CH4 production (3 %), but this result was non-significant compared to unamended control microcosms. All treatments (with and without CM) displayed the acetoclastic methanogenesis pathway according to isotopic signature of δ13C-CH4, δ13C-CO2 and δ2H-CH4. In the presence of CMs, the archaeal populations showed a major abundance of Methanobacteria, Methanosarcina, and Bathyarchaeia. Furthermore, linear discriminant analysis effect size (LefSe) revealed specific positive linkages between fungal melanin and electroactive bacteria like Geobacter, biochar with Clostridia, and magnetite with Thiobacillus, and specifically related with archaea, particularly Bathyarchaeia. Biochar may diversify volatile fatty acids (VFA) utilization leading to a final mitigation of cumulative CH4 emissions through complex microbial interactions in the later stages of incubation. In contrast, fungal melanin increased VFA production, while delaying CH4 production, and may have diverted the electron flow towards melanin quinone reduction, suppressing methanogenesis by oxidizing organic compounds. These results suggest that CMs might facilitate specific potential direct interspecies electron transfer (DIET) between syntrophic electroactive bacteria (i.e. Geobacter, Clostridia) and electroactive methanogens such as Methanosarcina and Methanobacteria, but also with alternative microbial populations with the potential for hampering methanogenesis in a certain extent.

Biochar modification accelerates soil atrazine biodegradation by altering bacterial communities, degradation-related genes and metabolic pathways

Atrazine is one of the most used herbicides, posing non-neglectable threats to ecosystem and human health. This work studied the performance and mechanisms of surface-modified biochar in accelerating atrazine biodegradation by exploring the changes in atrazine metabolites, bacterial communities and atrazine degradation-related genes. Among different types of biochar, nano-hydroxyapatite modified biochar achieved the highest degradation efficiency (85.13 %), mainly attributing to the increasing pH, soil organic matter, soil humus, and some enriched indigenous bacterial families of Bradyrhizobiaceae, Rhodospirillaceae, Methylophilaceae, Micrococcaceae, and Xanthobacteraceae. The abundance of 4 key atrazine degradation-related genes (atzA, atzB, atzC and triA) increased after biochar amendment, boosting both dechlorination and dealkylation pathways in atrazine metabolism. Our findings evidenced that biochar amendment could accelerate atrazine biodegradation by altering soil physicochemical properties, microbial composition and atrazine degradation pathways, providing clues for improving atrazine biodegradation performance at contaminated sites.

The interplay of soil physicochemical properties, methanogenic diversity, and abundance governs methane production potential in paddy soil subjected to multi-decadal straw incorporation

Straw incorporation holds significant promise for enhancing soil fertility and mitigating air pollution stemming from straw burning. However, this practice concurrently elevates the production and emission of methane (CH4) from paddy ecosystems. Despite its environmental impact, the precise mechanisms behind the heightened CH4 production resulting from long-term straw incorporation remain elusive. In a 32-year field experiment featuring three fertilization treatments (CFS-chemical fertilizer with wheat straw, CF-chemical fertilizer, and CK-unamended), we investigated the impact of abiotic (soil physicochemical properties) and biotic (methanogenic abundance, diversity, and community composition) factors on CH4 production in paddy fields. Results revealed a significantly higher CH4 production potential under CFS treatment compared to CF and CK treatments. The partial least squares path model revealed that soil physicochemical properties (path coefficient = 0.61), methanogenic diversity (path coefficient = -0.43), and methanogenic abundance (path coefficient = 0.29) collectively determined CH4 production potential, explaining 77% of the variance. Enhanced soil organic carbon content and water content, resulting from straw incorporation, emerged as pivotal factors positively correlated with CH4 production potential. Under CFS treatment, lower Shannon index of methanogens, compared to CF and CK treatments, was attributed to increased Methanosarcina. Notably, the Shannon index and relative abundance of Methanosarcina exhibited negative and positive correlations with CH4 production potential, respectively. Methanogenic abundance, bolstered by straw incorporation, significantly amplified overall potential. This comprehensive analysis underscores the joint influence of abiotic and biotic factors in regulating CH4 production potential during multi-decadal straw incorporation.

Advances, trends and challenges in the use of biochar as an improvement strategy in the anaerobic digestion of organic waste: a systematic analysis

A recently strategy applied to anaerobic digestion (AD) is the use of biochar (BC) obtained from the pyrolysis of different organic waste. The PRISMA protocol-based review of the most recent literature data from 2011-2022 was used in this study. The review focuses on research papers from Scopus® and Web of Knowledge®. The review protocol used permits to identify 169 articles. The review indicated a need for further research in the following challenges on the application of BC in AD: i) to increase the use of BC in developing countries, which produce large and diverse amounts of waste that are the source of production of this additive; ii) to determine the effect of BC on the AD of organic waste under psychrophilic conditions; iii) to apply tools of machine learning or robust models that allow the process optimization; iv) to perform studies that include life cycle and technical-economic analysis that allow identifying the potential of applying BC in AD in large-scale systems; v) to study the effects of BC on the agronomic characteristics of the digestate once it is applied to the soil and vi) finally, it is necessary to deepen in the effect of BC on the dynamics of nitrogen and microbial consortia that affect AD, considering the type of BC used. In the future, it is necessary to search for new solutions in terms of the transport phenomena that occurs in AD with the use of BC using robust and precise mathematical models at full-scale conditions.

Biochar amendment reassembles microbial community in a long-term phosphorus fertilization paddy soil

This study investigates the effect of biochar amendment on microbial community structure and soil nutrient status in paddy soil that has been fertilized for an extended period of time, shedding light on sustainable agricultural practices. A 90-day incubation period revealed that biochar amendment, as opposed to long-term fertilization, significantly influenced the physicochemical properties and microbial composition of the soil. The microcosm experiment conducted using six treatments analyzed soil samples from a long-term rice ecosystem. We employed microbial biomarkers (phospholipid fatty acids, PLFAs; isoprenoid and branched glycerol dialkyl glycerol tetraethers, iGDGTs and brGDGTs; DNA) to assess microbial biomass and community structure. Biochar addition led to a decrease in PLFA biomass (15-32%) and archaeal iGDGT abundance (14-43%), while enhancing bacterial brGDGT abundance by 15-77%. Intact biochar increased archaeal and bacterial diversity, though fungal diversity remained unchanged. However, acid-washed biochar did not result in a uniform microbial diversity response. The abundance of various microbial taxa was changed by biochar amendment, including Crenarchaeota, Proteobacteria, Nitrospira, Basidiomycota, Halobacterota, Chloroflexi, Planctomycetota, and Ascomycota. Soil NH4+-N was found as the primary environmental factor impacting the composition of archaea, bacteria, and fungus in this study. These findings imply that the addition of biochar has a quick influence on the structure and activity of microbial communities, with fungi possibly having a critical role in acid paddy soil. This study contributes valuable knowledge for developing sustainable agricultural practices that promote healthy soil ecosystems. KEY POINTS: • Biochar type and phosphorus fertilization demonstrated an interactive effect on the diversity of archaea, but no such effect was observed for bacteria and fungi. • Soil fungi contribute to approximately 20% of the total phospholipid fatty acid (PLFA) content. • Biochar, especially acid-washed rice straw biochar, increases glucose metabolism in bacteria and archaea and decreases saprophytic fungi.

Effects of salinity on methane emissions and methanogenic archaeal communities in different habitat of saline-alkali wetlands

The increase in temperature caused by global climate change has promoted the salinization of wetlands. Inland saline-alkaline wetlands have an environment of over-humidity and shallow water and are hot spots for CH4 emissions. However, there are few reports on the effect of salinity on CH4 emissions in inland saline-alkaline wetlands. This study conducted simulation experiments of increased salinity to investigate the impact of salinity, habitat, and their interactions on CH4 emissions, as well as to examine the response of methanogenic archaea to salinity. Overall, salinity inhibited CH4 emissions. But there were different responses in the three habitat soils. Salinity decreased the relative abundance of methanogenic archaea and changed the community structure. In addition, salinity changed soil pH and dissolved organic carbon (DOC) and ammonium (NH4+) concentrations, which were significantly correlated with methanogenic archaea. Our study showed that salinity changed the soil physicochemical properties and characteristics of the methanogenic archaeal community, affecting CH4 emissions.

Soil organic carbon regulates CH4 production through methanogenic evenness and available phosphorus under different straw managements

Methane (CH₄) is the main greenhouse gas emitted from rice paddy fields driven by methanogens, for which methanogenic abundance on CH₄ production has been intensively investigated. However, information is limited about the relationship between methanogenic diversity (e.g., richness and evenness) and CH₄ production. Three independent field experiments with different straw managements including returning method, burial depth, and burial amount were used to identify the effects of methanogenic diversity on CH₄ production, and its regulating factors from soil properties in a rice-wheat cropping system. The results showed that methanogenic evenness (dominance) can explain 23% of variations in CH₄ production potential. CH₄ production potential was positively related to methanogenic evenness (R² = 0.310, p < 0.001), which is driven by soil organic carbon (SOC), available phosphorus (AP), and nitrate (NO₃^-) through structure equation model (SEM). These findings indicate that methanogenic evenness has a critical role in evaluating the responses of CH₄ production to agricultural practices following changes in soil properties. The SEM also revealed that SOC concentration influenced CH₄ production potential indirectly via complementarity of methanogenic evenness (dominance) and available phosphorus (AP). Increasing SOC accumulation improved AP release and stimulated CH₄ production when SOC was at a low level, whereas decreased evenness and suppressed CH₄ production when SOC was at a high level. A nonlinear relationship was detected between SOC and CH₄ production potential, and CH₄ production potential decreased when SOC was ≥14.16 g kg^-1. Our results indicated that the higher SOC sequestration can not only mitigate CO₂ emissions directly but CH₄ emissions indirectly, highlighting the importance to enhance SOC sequestration using optimum agricultural practices in a rice-wheat cropping system.

Effects of modified biochars on the shifts of short-chain fatty acid profile, iron reduction, and bacterial community in paddy soil

Biochar is well known as an effective means for soil amendment, and modification on biochar with different methods could improve the benefits for environmental remediation. In this study, two modified biochars were generated with nitric acid (NBC) and hydrogen peroxide (OBC) pretreatment, and a control biochar was produced after washing with deionized water (WBC). The dynamics of short-chain fatty acids (SCFAs), iron concentration and bacterial community in rice paddy soil amended with different biochars or without adding biochar (CK) were studied during 70 days of anaerobic incubation. Compared to CK treatment, the accumulation of SCFAs was largely inhibited by the amendment of biochars. Besides, OBC and WBC increased the accumulation of Fe(II) at the initial stage of incubation. Via 16S rRNA gene sequencing, modified biochars caused significant response of bacterial community in comparison to WBC at Day 0-1, and three biochars favored bacterial α-diversity in the paddy soil at the end of the incubation. Interestingly, positive and negative correlations between NBC and several bacteria taxa (e.g. Geobacter, Fonticella and Clostridium) were observed. The study revealed that modified biochars had significant effects on the shifts of SCFAs, Fe(III) reduction and bacterial diversity, which provides fundamental information for future application of modified biochars in rice cropping ecosystem.