Linking the chemical nature of soil organic carbon and biological binding agent in aggregates to soil aggregate stability following biochar amendment in a rice paddy

Ectomycorrhizal fungi and biochar promote soil recalcitrant carbon increases under arsenic stress

Globally, vast mining areas (∼31,396.3 km2) hold significant potential for soil carbon sequestration. However, the sequestration capacity of mine soils is closely linked to contamination characteristics and restoration strategies. Arsenic, a highly toxic metalloid prevalent in acid mine soils, affects carbon turnover through its interactions with soil compounds. Nevertheless, the underlying mechanisms remain inadequately understood. This study introduces a phytobial remediation approach combining ectomycorrhizal fungus (Suillus luteus) inoculated into Pinus massoniana and biochar as a soil amendment. Results demonstrated that S. luteus extended apoplastic spaces to absorb arsenic into root cell intervals while encapsulating organic matter into aggregates. Biochar further promoted recalcitrant carbon formation, significantly increasing aggregate-carbon, particulate organic carbon (POC), and mineral-associated organic carbon (MAOC) by 3.15-, 1.74-, and 2.33-fold, respectively, compared to controls. Distinct hyphosphere microbiomes were observed in the combined treatment (BS), with enhanced microbial diversity, enzyme activity, carbon-sequestration genes, and necromass production, indicating the pivotal role of soil microorganisms in stable carbon pool formation. These synergistic effects not only facilitated arsenic detoxification but also significantly contributed to carbon stabilization.

Microstructure and Microorganisms Alternation of Paddy Soil: Interplay of Biochar and Water-Saving Irrigation

Biochar application and controlled irrigation (CI) enhance water conservation, lower emissions, and increase crop yields. However, the synergistic effects on the relationship between paddy soil microstructure and microbiome remain poorly understood. This study investigates the impact of different irrigation regimes and biochar applications on soil physicochemical properties, soil microstructure, and the composition and functions of soil microorganisms in paddy soil. The CA treatment (CI with 60 t/hm2 biochar) showed higher abundances of Mycobacteriaceae, Streptomycetaceae, Comamonadaceae, and Nocardioidaceae than the CK treatment (CI without biochar), which was attributed to two main factors. First, CA increased the pore throat equivalent radius (EqR), throat surface area (SAR), total throat number (TTN), volume fraction (VF), and connected porosity (CP) by 1.47-9.61%, 7.50-25.21%, 41.55-45.99%, 61.12-73.04%, and 46.36-93.75%, respectively, thereby expanding microbial habitats and providing refuges for microorganisms. Second, CA increased the cation exchange capacity (CEC), mean weight diameter (MWD), soil organic carbon (SOC), and total nitrogen (TN) by 22.14-25.06%, 42.24-56.61%, 22.98-56.5%, and 9.41-87.83%, respectively, reinforcing soil structural stability and carbon storage, which promoted microbial community diversity. FK (flood irrigation without biochar) showed no significant correlations with these environmental factors. Compared to CK soil metabolites at Level 2 and Level 3, FK exhibited higher levels of the citrate cycle, indicating that changes in water and oxygen environments due to CI reduced soil organic matter decomposition and carbon cycle. CA and CK strongly correlated with the soil microstructure (VF, CP, TTN, SAR, EqR), and CA notably enhanced soil metabolites related to the synthesis and degradation of ketone bodies, suggesting that biochar can mitigate the adverse metabolomic effects of CI. These results indicate that biochar application in CI paddy fields highlights the critical role of soil microstructure in microbial composition and function and better supports soil sustainability.

Unraveling the synergistic effect of biochar and potassium solubilizing bacteria on potassium availability and rapeseed growth in acidic soil

Potassium (K) is an essential macronutrient for plant growth. However, its bioavailability is low in acidic soils. Excessive K fertilization deteriorates the soil health, thus highlighting the need for sustainable alternatives. In previous studies, biochar application has been proven to be an effective amendment. Meanwhile, various potassium solubilizing bacteria (KSB) have been identified in soil that contributes to K bioavailability. However, their interaction under combine (co) application in acidic soil and its effects on K availability remain poorly understood. Therefore, a pot experiment was conducted to investigate the synergistic effect of co-application of rice straw biochar (BC) and KSB consortium on K availability to promote rapeseed growth. The treatment plan consisted of CK (control), recommended K fertilizer, 2 % BC (2 % w/w), KSB consortium, KSB consortium + 2 % BC (2 % w/w). Results of soil analysis conducted after crop maturity showed that co-application of 2 % BC and KSB consortium significantly improved the soil pH and organic matter contents by 0.62 and 12.52 units respectively, relative to CK. Meanwhile, soil available nutrients were greatly enhanced, as available K content increased by 52.1 %, which indicated that co-application of 2 % BC and KSB consortium could facilitate the better conversion of different forms of soil K and make it available for plant uptake. Furthermore, it also improved extracellular enzymatic activities (26.7-71.6 %) and soil bacterial community (Actinobacteriota and Firmicutes). These improvements greatly enhanced plant biomass (46 %) and yield (31 %). Overall results proved that co-application of 2 % BC and KSB effectively enhanced K availability for sustainable plant growth. Still, there is a need to identify the most efficient KSB strains that, in conjugation with BC, reduce the K fertilizer usage.

Cadmium distribution and availability in different particle-size aggregates of post-harvest paddy soil amended with bio-based materials

Research on the use of organic materials as soil amendments for the remediation of Cd-contaminated agricultural land exists. However, the mechanisms based on which organic materials affect the distribution and availability of Cd in soil aggregates remain unclear. Here, Cd-contaminated paddy soil and different bio-based materials were used for rice pot experiments. Rhizosphere soils were separated into six particle sizes. Cd fractions were analyzed with BCR sequential extraction and specific functional groups associated with Cd were characterized using XPS. We found that bio-based materials promoted the formation of large aggregates to different extents. Cd tended to be enriched in fine- and coarse-grained soil particles, which is mainly related to the soil organic matter. Bio-based materials reduced the relative content of the weak-acid extractable fraction and increased the relative content of the reducible fraction, resulting in soil Cd immobilization. Soil dissolved organic matter (DOM) was the key factor affecting the distribution and availability of Cd in soil aggregates and different organic matter and Cd-binding functional groups in aggregates altered the Cd availability in soil. The results provide insight and guidance for understanding the cadmium immobilization mechanism and screening appropriate materials in the remediation of agricultural land.

Influence of Pseudomonas aeruginosa-based biopolymer on mitigating soil erosion and heavy metal dispersion

Extreme weather phenomena caused by climate change have exacerbated soil erosion and the subsequent dispersion of pollutants. Pseudomonas aeruginosa is known to contribute to the remediation of polluted water and reduce the geochemical mobility of heavy metals in contaminated soil. However, studies on the influence of biopolymers produced by soil microbes and P. aeruginosa on physical soil properties and soil erosion are limited. We aimed to investigate the influence of soil microbes on the mitigation of soil erosion and geochemical dispersion of heavy metals using a naturally occurring microbial substance, P. aeruginosa-based biopolymer (PBB). The PBB comprised carboxyl, hydroxyl, and amine surface functional groups; consequently, the biopolymer effectively sequestered Cd (maximum sorption capacity qm = 45.7 mg/g), Cu (qm = 26.7 mg/g), Pb (qm = 64.9 mg/g), and Zn (qm = 26.1 mg/g) in the solution. The PBB amendment of the soil improved the physical properties associated with soil erosion, increasing soil aggregation stability and shear strength by 41.6% and 36.8%, respectively. The extraction of heavy metals from soil via synthetic precipitate leaching decreased by 54.2% following the PBB amendment, and a negative correlation was observed between soil aggregate stability and heavy metal extraction, indicating that this microbial substance could immobilize pollutants by adsorbing cationic metal ions and inhibiting water-induced disaggregation. In the soil erosion experiments, soil loss and heavy metal extraction decreased by 70.9% and 43.8%, respectively, following the PBB amendment. These aggregation and sorption effects of the PBB underscore the potential of soil microbes to mitigate soil erosion and immobilize the geochemical dispersion of heavy metals, thereby contributing to the conservation of soil and water quality in areas surrounding contaminated slopes and heavy metal-contaminated areas, such as cut slopes, agricultural fields, mine dumps, and dams.

Organic fertilization reduces nitrous oxide emission by altering nitrogen cycling microbial guilds favouring complete denitrification at soil aggregate scale

Agricultural management practices can induce changes in soil aggregation structure that alter the microbial nitrous oxide (N2O) production and reduction processes occurring at the microscale, leading to large-scale consequences for N2O emissions. However, the mechanistic understanding of how organic fertilization affects these context-dependent small-scale N2O emissions and associated key nitrogen (N) cycling microbial communities is lacking. Here, denitrification gas (N2O, N2) and potential denitrification capacity N2O/(N2O + N2) were assessed by automated gas chromatography in different soil aggregates (>2 mm, 2-0.25 and <0.25 mm), while associated microbial communities were assessed by sequencing and qPCR of N2O-producing (nirK and nirS) and reducing (nosZ clade I and II) genes. The results indicated that organic fertilization reduced N2O emissions by enhancing the conversion of N2O to N2 in all aggregate sizes. Moreover, potential N2O production and reduction hotspots occurred in smaller soil aggregates, with the degree depending on organic fertilizer type and application rate. Further, significantly higher abundance and diversity of nosZ clades relative to nirK and nirS revealed complete denitrification promoted through selection of denitrifying communities at microscales favouring N2O reduction. Communities associated with high and low emission treatments form modules with specific sequence types which may be diagnostic of emission levels. Taken together, these findings suggest that organic fertilizers reduced N2O emissions through influencing soil factors and patterns of niche partitioning between N2O-producing and reducing communities within soil aggregates, and selection for communities that overall are more likely to consume than emit N2O. These findings are helpful in strengthening the ability to predict N2O emissions from agricultural soils under organic fertilization as well as contributing to the development of net-zero carbon strategies for sustainable agriculture.

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

Biochar addition promotes soil organic carbon sequestration dominantly contributed by macro-aggregates in agricultural ecosystems of China

Soil aggregates play pivotal roles in soil organic carbon (SOC) preservation and climate change. Biochar has been widely applied in agricultural ecosystems to improve soil physicochemical properties. However, the underlying mechanisms of SOC sequestration by soil aggregation with biochar addition are not well understood at a large scale. Here, we conducted a meta-analysis of 2335 pairwise data from 45 studies to explore how soil aggregation sequestrated SOC after biochar addition in agricultural ecosystems of China. Biochar addition markedly enhanced the proportions of macro-aggregates and aggregate stability, and the production of organic binding agents positively facilitated the formation of macro-aggregates and aggregate stability. Soil aggregate-associated organic carbon (OC) indicated a significantly increasement by biochar addition, which was attributed to direct and indirect inputs of OC from biochar and organic residues, respectively. Biochar stimulated SOC sequestration dominantly contributed by macro-aggregates, and it could be interpreted by a greater improvement in proportions and OC protection of macro-aggregates. Furthermore, the SOC sequestration of soil aggregation with biochar addition was regulated by climate conditions (mean annual temperature and precipitation), biochar attributes (biochar C/N ratio and pH), experimental practices (biochar addition level and duration), and agronomic managements (land type, cropping intensity, fertilization condition, and crop type). Collectively, our synthetic analysis emphasized that biochar promoted the SOC sequestration by improving soil aggregation in agricultural ecosystems of China.

Impact of temperature and residence time on sewage sludge pyrolysis for combined carbon sequestration and energy production

Environmental challenges related to sewage sludge call for urgent sustainable management of this resource. Sludge pyrolysis might be considered as a sustainable technology and is anticipated to support measures for mitigating climate change through carbon sequestration. The end products of the process have various applications, including the agricultural utilization of biochar, as well as the energy exploitation of bio-oil and syngas. In this research, sewage sludge was pyrolyzed at 500 °C, 600 °C, 750 °C, and 850 °C. At each temperature, pyrolysis was explored at 1hr, 2hrs, and 3hrs residence times. The ratio (H/Corg)at was tapped to imply organic carbon stability and carbon sequestration potential. Optimum operating conditions were achieved at 750 °C and 2hrs residence time. Produced biochar had (H/Corg)at ratio of 0.54, while nutrients' contents based on dry weight were 3.99%, 3.2%, and 0.6% for total nitrogen (TN), total phosphorus (TP), and total potassium (TK), respectively. Electrical conductivity of biochar was lesser than the feed sludge. Heavy metals in biochar aligned with the recommended values of the International Biochar Initiative. Heat content of condensable and non-condensable volatiles was sufficient to maintain the temperature of the furnace provided that PYREG process is considered. However, additional energy source is demanded for sludge drying.

Granular bacterial inoculant alters the rhizosphere microbiome and soil aggregate fractionation to affect phosphorus fractions and maize growth

The constraint of phosphorus (P) fixation on crop production in alkaline calcareous soils can be alleviated by applying bioinoculants. However, the impact of bacterial inoculants on this process remains inadequately understood. Here, a field study was conducted to investigate the effect of a high-concentration, cost-effective, and slow-release granular bacterial inoculant (GBI) on maize (Zea mays L.) plant growth. Additionally, we explored the effects of GBI on rhizosphere soil aggregate physicochemical properties, rhizosphere soil P fraction, and microbial communities within aggregates. The outcomes showed a considerable improvement in plant growth and P uptake upon application of the GBI. The application of GBI significantly enhanced the AP, phoD gene abundance, alkaline phosphatase activity, inorganic P fractions, and organic P fractions in large macroaggregates. Furthermore, GBI impacted soil aggregate fractionation, leading to substantial alterations in the composition of fungal and bacterial communities. Notably, key microbial taxa involved in P-cycling, such as Saccharimonadales and Mortierella, exhibited enrichment in the rhizosphere soil of plants treated with GBI. Overall, our study provides valuable insight into the impact of GBI application on microbial distributions and P fractions within aggregates of alkaline calcareous soils, crucial for fostering healthy root development and optimal crop growth potential. Subsequent research endeavors should delve into exploring the effects of diverse GBIs and specific aggregate types on P fraction and community composition across various soil profiles.

Chitin and crawfish shell biochar composite decreased heavy metal bioavailability and shifted rhizosphere bacterial community in an arsenic/lead co-contaminated soil

Sustainable management of ever-increasing organic biowaste and arable soil contamination by potentially toxic elements are of concern from both environmental and agricultural perspectives. To tackle the waste issue of crawfish shells and simultaneously minimize the threat of arsenic (As) and lead (Pb) to human health, a pot trial was conducted using chitin (CT), crawfish shell biochar (CSB), crawfish shell powder (CSP), and CT-CSB composite to compare their remediation efficiencies in As/Pb co-contaminated soil. Results demonstrated that addition of all amendments decreased Pb bioavailability, with the greatest effect observed for the CT-CSB treatment. Application of CSP and CSB increased the soil available As concentration, while significant decreases were observed in the CT and CT-CSB treatments. Meanwhile, CT addition was the most effective in enhancing the soil enzyme activities including acid phosphatase, α-glucosidase, N-acetyl-β-glucosaminidase, and cellobiohydrolase, whereas CSB-containing treatments suppressed the activities of most enzymes. The amendments altered the bacterial abundance and composition in soil. For instance, compared to the control, all treatments increased Chitinophagaceae abundance by 2.6-4.7%. The relative abundance of Comamonadaceae decreased by 1.6% in the CSB treatment, while 2.1% increase of Comamonadaceae was noted in the CT-CSB treatment. Redundancy and correlation analyses (at the family level) indicated that the changes in bacterial community structure were linked to bulk density, water content, and As/Pb availability of soils. Partial least squares path modeling further indicated that soil chemical property (i.e., pH, dissolved organic carbon, and cation exchange capacity) was the strongest predictor of As/Pb availability in soils following amendment application. Overall, CT-CSB could be a potentially effective amendment for simultaneously immobilizing As and Pb and restoring soil ecological functions in contaminated arable soils.