Dynamics, biodegradability, and microbial community shift of water-extractable organic matter in rice-wheat cropping soil under different fertilization treatments

Recalcitrant components accumulation in dissolved organic matter decreases microbial metabolic quotient of red soil under long-term manuring

Microbial metabolism is closely related to soil carbon dioxide emissions, which in turn is related to environmental issues such as global warming. Dissolved organic matter (DOM) affects many fundamental biogeochemical processes such as microbial metabolism involved in soil carbon cycle, not only directly by its availability, but also indirectly by its chemodiversity. However, the association between the DOM chemodiversity and bioavailability remains unclear. To address this knowledge gap, soils from two agro-ecological experimental sites subjected to various long-term fertilizations in subtropical area was collected. The chemodiversity of DOM was detected by multi-spectroscopic techniques including ultraviolet-visible spectrophotometry, Fourier transform infrared spectroscopy and excitation emission matrices fluorescence spectroscopy. Results showed that long-term manure amendments significantly decreased microbial metabolic quotient (qCO2) by up to 57%. We also observed that long-term manure amendments significantly increased recalcitrant components of DOM (indicated by the aromaticity, humification index, the ratio of aromatic carbon to aliphatic carbon, and the relative abundances of humic-like components) and decreased labile components of DOM. Negatively correlation between the qCO2 and the proportion of recalcitrant components of DOM supported that accumulation in recalcitrant components of DOM increased microbial carbon utilization efficiency. Random forest models also showed the highest contribution of the relative abundances of humic-like components and the aromaticity of DOM in affecting qCO2. Both of the redundancy analysis and structural equation modeling further indicated the decisive role of soil pH in influencing the DOM chemodiversity. Soil pH explained 56.7% of the variation in the chemodiversity of DOM. The accumulation of recalcitrant components in DOM with increasing soil pH might be attributed to the accelerated microbial consumption of bioavailability components and/or to the negative impact on the solubility of bioavailability components. Overall, this research highlights the significance of long-term manure amendments in regulating qCO2 by altering the chemodiversity of soil DOM.

Distribution and survival of pathogens from different waste components and bioaerosol traceability analysis in household garbage room

Household garbage rooms release abundant bioaerosols and are an important source of pathogens; however, information on the distribution and survival patterns of pathogens in different waste components is limited. In this study, a culture method and 16S rRNA high-throughput sequencing were used to determine bacterial communities, culturable pathogens, and human bacterial pathogens (HBPs). The results showed that abundant culturable bacteria were detected in all waste types, and a large number of S. aureus was detected on the surface of recyclable wastes, whereas S. aureus, total coliforms, Salmonella, Enterococcus, and hemolytic bacteria were detected in food waste and other waste. The activities of these detected pathogenic bacteria decreased after 24 h of storage but re-activated within one week. Factors affecting the emergence of pathogens varied with different waste components. Sequencing results showed that Pseudomonas, Acinetobacter, and Burkholderia were abundant in the waste samples, whereas Achromobacter, Exiguobacteriums, Bordetella, and Corynebacterium were the primary pathogens in the bioaerosol and wall attachment. The results of traceability analysis showed that bioaerosol microbes were mainly derived from raw kitchen waste (5.98%) and plastic and paper contaminated with food waste (19.93%) in garbage rooms. In addition, bioaerosols were the main source of microflora in the wall attachment, which possessed high HBP diversity and required more attention. These findings will help in understanding the microbial hazards in different waste components and provide guidance for the control and risk reduction of bioaerosols during waste management and recycling.

Composition of DOM along the depth gradients in the paddy field treated with crop straw for 10 years

Crop straw return is a widely used agricultural management practice. The addition of crop straw significantly alters the pool of dissolved organic matter (DOM) in agricultural soils and plays a pivotal role in the global carbon (C) cycle, which is sensitive to climate change. The DOM concentration and composition at different soil depths could regulate the turnover and further storage of organic C in terrestrial systems. However, it is still unclear how crop straw return influences the change in DOM composition in rice paddy soils. Therefore, a field experiment was conducted in which paddy soil was amended with crop straw for 10 years. Two crop straw-addition treatments [NPK with 50% crop straw (NPK+1/2S) and NPK with 100% crop straw (NPK + S)], a conventional mineral fertilization control (NPK) and a non-fertilized control were included. Topsoil (0-20 cm) and subsoil (20-40 cm) samples were collected to investigate the soil DOM concentration and compositional structure of the profile. Soil nutrients, iron (Fe) fraction, microbial biomass carbon (MBC), and concentration and optical properties (UV-Vis and fluorescence spectra) of soil DOM were determined. Here, we found that the DOM in the topsoil was more humified than that in the subsoil. The addition of crop straw further decreased the humidification degree of DOM in the subsoil. In crop straw-amended topsoil, microbial decomposition controlled the composition of DOM and induced the formation of aromatic DOM. In the straw-treated subsoil, selective adsorption by poorly crystalline Fe(oxyhydr)oxides and microbial decomposition controlled the composition of DOM. In particular, the formation of protein-like compounds could have played a significant role in the microbial degradation of DOM in the subsoil. Overall, this work conducted a case study within long-term agricultural management to understand the changes in DOM composition along the soil profile, which would be further helpful for evaluating C cycling in agricultural ecosystems.

Effects of elevation and geomorphology on cadmium, lead and chromium enrichment in paddy soil and rice: A case study in the Xiangtan basin of China

The distributions of heavy metals in paddy fields and rice along river valleys were studied to explore the key factors affecting the accumulation of heavy metals in the upstream terraces and downstream plains. Results from 975 sampling sites showed that elevation, growing season and soil organic matter (OM) had significant effects on the content of Cd and Pb in topsoil and rice. The content of Cd (0.47-0.66 mg kg-1) and Pb (49.9-68.6 mg kg-1) in paddy fields with low elevation (30-60 m) in the downstream plains was significantly higher than the content of Cd (0.29-0.38 mg kg-1) and Pb (43.9-56.3 mg kg-1) in the upstream terraces with high altitude (60-90 m). In the double-rice production area, late rice generally produced grains with higher Cd and Pb content than early rice. Soil Cd was positively increased with the content of OM, especially in the downstream plains. When elevation was used for principal component analysis, plains with low elevation were grouped together with high content of total and soluble Cd, OM and Pb in soil, as well as high content of Cd and Pb in late rice. Altitude is one of the key factors affecting Cd content in rice. Although content of Cr (93.7-138.0 mg kg-1) was significantly higher than that of Cd and Pb in soil, content of Cr was lower than that of Cd in rice. These results indicate that paddy fields with elevation of 30-60 m in the downstream plains had high risk to produce late rice with Cd and Pb content exceeding the food safety standard 0.2 mg kg-1, which may be resulted from the driving force of runoff on soil soluble Cd and Pb from terraces to alluvial plains in river valleys.

Fertilization regime shifts the molecular diversity and chlorine reactivity of soil dissolved organic matter from tropical croplands

Soil-derived dissolved organic matter (SDOM) is an important site-specific disinfection byproduct (DBP) precursor in watersheds. However, it remains unclear how fertilization regime shifts the molecular diversity and chlorine reactivity of SDOM in cropland-impacted watersheds. Here, we analyzed the spectroscopic and molecular-level characteristics of the SDOM from croplands that had different fertilization regimes (i.e., non-fertilization, chemical fertilization, straw return, and chemical fertilization plus straw return) for 5 years and evaluated the chlorine reactivity of the SDOM by determining the 24-h chlorine consumption and specific DBP formation potential (SDBP-FP). The SDOM level decreased by chemical fertilization and was not significantly altered by straw return alone or combined with chemical fertilizer. However, all fertilization regimes elevated the molecular diversity of SDOM by increasing the abundance of protein-, lignin-, and tannin-like compounds. The chlorine reactivity of SDOM was reduced by chemical fertilization, but was significantly increased by straw return. Typically, straw return increased the formation potential of specific trihalomethane and chloral hydrate by 339% and 56% via increasing the aromatics in SDOM, whereas chemical fertilization could effectively decrease about 231% of the increased specific trihalomethane formation potential caused by straw return. This study highlights that fertilization regime can significantly shape the molecular diversity and chlorine reactivity of the SDOM in croplands and that partially replacing chemical fertilizer with crop straw is an advantageous practice for reducing DBP risks in drinking water in cropland-impacted watersheds.

Transformation of sulfidized nanoscale zero-valent iron particles and its effects on microbial communities in soil ecosystems

Sulfidized nanoscale zero-valent iron (S-nZVI) is a promising material for in situ soil remediation. However, its transformation (i.e., aging) and effects on the microbial community in soil ecosystems are largely unknown. In this study, S-nZVI having low (S-nZVI (L)) and high sulfur-doping (S-nZVI (H)) were incubated in soil microcosms and bare nZVI was used as a control. Their aged products were characterized using microspectroscopic analyses and the changes in the corresponding soil microbial community were determined using high-throughput sequencing analyses. The results indicate that severe corrosion of both bare and S-nZVI occurred over 56 days of aging with significant morphological and mineral changes. Magnetite, lepidocrocite, and goethite were detected as the main aged products. In addition, sulfate ions, pyrite, and iron polysulfide were formed in the aged products of S-nZVI. Cr(VI) removal test results indicated that S-nZVI(L) achieved the best results after aging, likely because of the optimal FeS arrangement on its nanoparticle surfaces. The presence of nZVI and S-nZVI increased the abundance of some magnetotactic microorganisms and altered bacterial and fungal community structures and compositions. Moreover, the addition of S-nZVI enriched some bacterial and fungal genera related to sulfur cycling because of the presence of sulfide-bearing material. The findings reveal the transformation of S-nZVI during aging and its effects on microbial communities in soil ecosystems, thereby helping to the evaluation of S-nZVI application in soil remediation.

The effects of green waste compost on soil N, P, K, and organic matter fractions in forestry soils: elemental analysis evaluation

We study the effects of green waste compost on soil fertility to provide a theoretical basis for accurately improving forestry soil quality. This study aims to investigate the effects of green waste compost on soil N, P, K, and soil organic matter (SOM) fractions using elemental and FTIR analyses. Therefore, five fertilization treatments were set up for research, including mineral fertilization (M-fert), green waste compost fertilization (G-fert), standard rate of M-fert plus G-fert (GM-fert), half the standard rate of M-fert plus G-fert (1/2 GM-fert), and a control with no fertilizer addition (N-fert). The results showed that GM-fert treatment significantly increased the content of soil NH₄–N, available phosphorus (AP), available potassium (AK), water soluble organic carbon (WSOC), humus (HE), and humic acid (HA), which were 8.53 ± 0.67, 76.1 ± 5.96, 168 ± 3.42, 0.152 ± 0.01, 5.64 ± 0.15, and 4.69 ± 0.21 mg kg⁻¹, respectively. The content of HA (36.7%, F = 7.55, P = 0.01) was positively correlated with the soil N, P, K, and the HA absorption peak. The relative intensities of the alcohol –OH, aliphatic –CH and carbohydrate C–O peaks showed the largest changes, which were 18.6 ± 0.56%, 13.1 ± 0.33%, and 16.3 ± 0.49%. –CH/C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C (49.8%, F = 12.9, P < 0.01) was also significantly positively correlated with soil N, P, K. In conclusion, green waste compost significantly increased soil N, P, K, and HA in forestry soils, and the –CH/CC of HA was the main factor related to soil nutrients.

Green waste compost significantly increased soil N, P, K, and HE fractions, and the –CH/CC components of the HA structures made the biggest contribution to soil N, P, K in forestry soil.

Interactions of iron-based nanoparticles with soil dissolved organic matter: adsorption, aging, and effects on hexavalent chromium removal

The interactions and mechanisms between soil dissolved organic matter (DOM) and three types of iron-based nanoparticles (NPs), i.e., nanoscale zero-valent iron (nZVI) particles, Fe₂O₃ NPs, and Fe₃O₄ NPs, were investigated in short-term exposure experiments. The adsorption results showed that soil DOM was rapidly adsorbed on the surface of the iron-based NPs with the adsorption rate varying according to Fe₃O₄ > Fe₂O₃ > nZVI. Spectral analysis results revealed that aromatic DOM fractions with high-molecular-weights were preferentially adsorbed. The binding mechanism was determined as hydrogen bonding and ligand exchange via Fourier transform infrared spectroscopy (FT-IR) analysis. Scanning electron microscopy, FT-IR, X-ray photoelectron spectroscopy, and X-ray diffraction were used to identify the corrosion products of the three iron-based NPs at the adsorption equilibrium. The results suggest that Fe₃O₄ and/or γ-Fe₂O₃ and α-FeOOH were the main corrosion products of nZVIs and α-FeOOH was obtained as an aged product of Fe₃O₄ NPs. Results of Cr(VI) removal tests suggest that the aged nZVI achieved 79.87% of Cr(VI) removal and the Cr(VI) removal efficiency was significantly improved by coating DOM onto Fe₂O₃ NPs. The overall data indicate the fate and transformation of iron-based NPs and the enhancement for Cr(VI) removal after interactions between DOM and NPs.

Regulation of soil aggregate size under different fertilizations on dissolved organic matter, cellobiose hydrolyzing microbial community and their roles in organic matter mineralization

The mineralization of soil organic matter is closely related to climate change. Labile organic matter and microbial community are vital intrinsic factors in controlling the mineralization of soil organic matter. Regulation of soil aggregate size on dissolved organic matter (DOM), the cellobiose hydrolyzing microbial community, and their roles in organic matter mineralization remains unclear. The mineralization of organic matter in large macroaggregates (LMA, >2 mm), small macroaggregates (SMA, 0.25-2 mm), and microaggregates (MI, <0.25 mm) from an Ultisol treated with long-term non-fertilizers (Ck), chemical fertilizers (NPK) and animal manure (AM) was observed in this study. The concentration and structure of DOM, activity of β-glucosidase, and the abundance, diversity, and community composition of GH1 (glycoside hydrolase family 1) microbial β-glucosidase encoding genes were investigated. The cumulative CO₂-C emissions occurred in the order LMA < SMA < MI in each fertilization treatment and followed the sequence Ck < NPK < AM in each size of aggregate. The concentration of DOM in the soil aggregates increased as the aggregate size decreased, while the structural complexity of DOM followed the opposite trend. The activity of β-glucosidase in the smaller aggregates was higher than that in the larger aggregates, and the abundance and diversity of the GH1 microbial β-glucosidase genes generally echoed the same trend. The dominant microbial classes harboring GH1 β-glucosidase genes in the soil aggregates were Actinobacteria, Alphaproteobacteria, Gammaproteobacteria, Flavobacteria, Eurotiomycetes, and Sordariomycetes. The relative abundance of Actinobacteria, Sordariomycetes, and Eurotiomycetes revealed significant differences among the aggregates. Redundancy analysis confirmed that microbial GH1 β-glucosidase community in the soil aggregates was primarily regulated by DOM concentration and pH. Structural equation modelling revealed that soil aggregates mainly regulated the β-glucosidase activity and DOM concentration and then the abundance and diversity of the GH1 microbial β-glucosidase genes in controlling organic matter mineralization.

Distinct Root Microbial Communities in Nature Farming Rice Harbor Bacterial Strains With Plant Growth-Promoting Traits

A nature farming system is an ecological farming practice that entails cultivating crops without using chemical fertilizers and pesticides. To understand the diversity and functions of root microbiomes associated with nature farming systems, we compared the root microbial community of rice under nature farming conditions with those under conventional farming conditions. High-throughput amplicon analysis demonstrated a higher abundance and greater diversity of the root microbiome under unfertilized nature farming conditions than under conventional conditions. The application of chemical fertilizers reduced the microbial diversity and abundance of some beneficial taxa important for plant growth and health. Subsequently, we isolated and identified 46 endo- and epiphytic bacteria from rice roots grown under nature farming conditions and examined their plant growth-promoting activity. Six potential isolates were selected for plant growth assessment in insoluble P- and K-containing media. Most of the isolates promoted rice growth, and Pseudomonas koreensis AEPR1 was able to enhance rice growth significantly in both insoluble P- and K-containing media. Our data indicated that nature farming systems create a distinct root microbiome that is comparatively more diverse and supports plant growth under low-input cultivation practices than under conventional practices. The potential isolates could be exploited as sources with potential applications in sustainable agriculture.

Straw return enhances the risks of metals in soil?

Interactions between organic matter (OM) and metals in soils are important natural mechanisms that can mitigate metal bioaccumulation in terrestrial environments. A primary source of OM in soils is straw return, accounting for more than 65% of OM input. Straw-OM has long been believed to reduce metal bioaccumulation, e.g., by immobilizing metals in soils. However, there is growing evidence that straw return could possibly enhance bioavailability and thus risks (i.e., food safety) of some metals in crops, including Cd, Hg, and As. Poor understanding of straw return-induced increases in metal bioavailability would add uncertainty in assessing or mitigating risks of metals in contaminated farming soils. Here, 863 pieces of literature (2000-2019) that reported the effects of straw return on metal bioavailability and bioaccumulation were reviewed. Mechanisms responsible for the increased metal mobility and bioavailability under straw return are summarized, including the effects of dissolution, complexation, and methylation. Effects of straw return on the physiology and the absorption of metals in plants is also discussed (i.e., physiological effect). These mechanisms are then used to explain the observed increases in the mobility, bioavailability, and bioaccumulation of Cd, Hg, and As under straw amendment. Information summarized in this study highlights the importance to re-consider the current straw return policy, particularly in metal-contaminated farmlands.

Effects of moisture and salinity on soil dissolved organic matter and ecological risk of coastal wetland

Coastal wetland is the transitional area between land and ocean, which has a unique and sensitive ecosystem. In this study, the effects of moisture and salinity on dissolved organic matter (DOM) and adsorption of heavy metal ions (Cr(VI), Cd(II) and Pb(II)) by soil are investigated. Meanwhile, ecological risks for the potential release of N, P and heavy metals are also predicted. UV-Vis spectrophotometry and three-dimensional fluorescence spectroscopy are used to study the content and structural of DOM under different soil moisture and salinity. Soil adsorption of heavy metal ions is determined by inductively coupled plasma (ICP). The results show that soil moisture and salinity have significant effects on the basic physical and chemical properties of soil. DOM content is the highest in medium moisture and high salinity areas. In addition, the content of protein-like substances in DOM is the highest under all treatment conditions. The results also reveal that the increase of DOM promotes Cr(VI) adsorption and inhibits Cd(II) adsorption by soil. When Pb(II) concentration is high (150 mg/L), the increase of DOM inhibits Pb(II) adsorption by soil. The comprehensive ecological risk of heavy metals is the highest under high salinity. The potential release risk of N and P is the lowest at high moisture and low moisture, respectively. Base on above, effects of soil moisture and salinity on the surrounding ecological environment in coastal wetlands have been revealed, which provides a theoretical basis for the protection of coastal wetland ecological environment.

Chitin degradation and the temporary response of bacterial chitinolytic communities to chitin amendment in soil under different fertilization regimes

Chitin amendment is a promising agricultural management strategy to control fungal and nematodal plant diseases and to improve crop yield. Chitin degradation in the soil contributes significantly to carbon and nitrogen cycling in terrestrial ecosystems. However, little is known about chitin degradation and bacterial chitinolytic communities in agricultural soil under different fertilization regimes. Thus, in the present study, a 42-day soil incubation experiment was conducted, in which soil under four fertilization regimes (i.e., no fertilization (CK), chemical fertilizer (CF), pig manure plus 50% chemical fertilizer (PMCF), and rice straw plus 100% chemical fertilizer (SRCF)) were amended or not with chitin or its monomer, N-acetylglucosamine (NAG). Different nitrogen forms and CO₂ and N₂O emission were measured to evaluate chitin degradation and its environmental implications. SRCF soil had the highest CO₂ emission, chitin N mineralization, and fungal abundance. NAG and chitin were enriched to exploit the chitin degraders. High-throughput sequencing analyses reveled that Streptomycetaceae, Oxalobacteraceae, Gemmatimonadaceae, and Acidobacteria were generally increased upon chitin amendment in CK, CF, and PMCF soil, whereas Streptomycetaceae dominated chitin-amended SRCF soil. Herpetosiphonaceae was enriched only in chitin-amended CK soil. LEfSe and network analysis were used to predict chitinolytic and opportunistic species, and revealed that most previously reported chitinolytic bacteria were detected in the present study and new potential chitin degraders, including unidentified_Solibacterales, Gemmatimonadaceae, and Herpetosiphonaceae, were identified. Some members of Firmicutes, Actinobacteria, and Proteobacteria, including Bacillus, and Kitasatospora, were speculated to be opportunistic species. The findings improve our understanding of the effects of chitin degradation on carbon and nitrogen cycling in agricultural soil under different fertilization regimes and help to identify chitinolytic bacteria.