Polypropylene microplastics affect the distribution and bioavailability of cadmium by changing soil components during soil aging

Compared with the widespread and serious heavy metal contamination in soils, microplastic pollution has gained attention only recently. Little is known about how microplastics affect the distribution of heavy metals in soils, especially across soil components level. In this study, a 180-day soil aging experiment and soil density fractionation were performed to investigate the effect of polypropylene (PP) microplastics on the binding behavior of cadmium (Cd) to solid components, i.e. particulate organic matter, organo-mineral complexes (OMC), and mineral. Results showed addition of 2-10% microplastics in soils induced the decomposition of OMC fraction by 10.88-23.10%. Compared to the control, the content of dissolved organic carbon increased, and pH, humic substances, and soil organic matter decreased with microplastics. After 180d of aging, the content of Cd in OMC fraction increased by 17.92%, while microplastics made Cd contents decline by 10.01-19.75%. The impacts strongly depended on the dose and surface characteristic of microplastics. Overall, PP microplastics increased the concentration of bioavailable Cd in soils via decreasing soil retention of Cd by the OMC fraction. These findings based on the solid components level will provide a new perspective for understanding microplastics effects on soil systems and pollutants.

Combined application of chemical fertilizer with green manure increased the stabilization of organic carbon in the organo-mineral complexes of paddy soil

The influence of the combined application of chemical fertilizer with green manure on the stabilization of organic carbon (C) was explored in the organo-mineral complexes of paddy soil. The organo-mineral complexes were isolated from paddy soil treated with no fertilizer, chemical fertilizer alone, and chemical fertilizer combined with increasing amounts of Chinese milk vetch (CMV). The stability (reflected by mineralizable carbon proportion), the content and chemical composition of organic C, the Fe/Al oxides and their associated organic C in the organo-mineral complexes were investigated. The application of chemical fertilizer in combination with CMV significantly improved the stability of organic C in the organo-mineral complexes. The combined application of chemical fertilizer with CMV slightly decreased the proportion of O-alkyl C (easily decomposed) yet somewhat increased the proportions of carbonyl C and aromatic C (difficultly decomposed) and aromaticity index in the organo-mineral complexes. The treatments of chemical fertilizer combined with CMV showed more Fe oxides and Fe/Al-associated organic C and higher proportion of Fe/Al-associated organic C in the total organic C of the organo-mineral complexes. The mineralizable carbon proportion displayed significantly negative correlations with carbonyl C and Fe/Al oxide-associated organic C in the organo-mineral complexes. The Fe/Al oxides were likely to be preferentially bound with the aromatic C and carbonyl C in the organo-mineral complexes. Overall, the combined application of chemical fertilizer with CMV facilitated the association of difficultly decomposed carbon and Fe/Al oxides, which significantly improved the stabilization of organic C in the organo-mineral complexes of paddy soil.

Organo-mineral complexes alter bacterial composition and induce carbon and nitrogen cycling in the rhizosphere

It is widely thought that organo-mineral complexes (OMCs) stabilize organic matter via mineral adsorption. Recent studies have demonstrated that root exudates can activate OMCs, but the influence of OMCs on plant rhizosphere, which is among the most active areas for microbes, has not been thoroughly researched. In this study, a pot experiment using Brassica napus was conducted to investigate the effects of OMCs on plant rhizosphere. The result showed that OMC addition significantly promoted the growth of B. napus compared to the prevalent fertilization (PF, chemical fertilizer + chicken compost) treatment. Specifically, OMC addition increased the relative abundance (RA) of nitrogen-fixing bacteria and the bacterial α-diversity, and the operational taxonomic unit (OTU) group with RA > 0.5% in the OMC-treated rhizosphere was the result of a deterministic assembly process with homogeneous selection. Gene abundance related to nitrogen cycling and the soil chemical analysis demonstrated that the OMC-altered bacterial community induced nitrogen fixation and converted nitrate to ammonium. The upregulated carbon sequestration pathway genes and the increased soil microbial biomass carbon (23.68%) demonstrated that the bacterial-induced carbon storage in the rhizosphere was activated. This study shows that the addition of OMCs can influence the biogeochemical carbon and nitrogen cycling via regulating microorganisms in the rhizosphere. The findings provide fresh insights into the effects of OMCs on the biogeochemical cycling of important elements and suggest a promising strategy for improving soil productivity.

Selective metal leaching from technosols based on synthetic root exudate composition

This study focused on metal release from technosols induced by synthetic root exudate (SRE). The effect of SRE composition on metal release was studied using six technosols. This was done by treating the technosols with SRE solutions having varying concentrations of low molecular weight organic acids (LMWOAs), namely oxalic, citric, and malic acids. Consequently, the physico-chemical parameters (pH and electric conductivity), Ca, Mg, Fe, Zn, and Cu release (by atomic absorption spectroscopy, AAS), chemical changes (by Fourier transform infrared, FT-IR), and organic parameters (by fluorescence) were investigated. Metal release showed to be dependent on the SRE composition and technosol characteristics. Citric acid selectively released Ca, Mg, Zn, and Cu from technosols in a concentration-dependent manner; oxalic acid showed a significant role in the release of Mg and Fe. Under relatively high LMWOA concentrations, particulate organo-mineral complexes precipitated. Additionally, technosol weathering was seen by the dissolution of humic substances and ferriallophanes, which in turn caused metal release. However, re-precipitation of these phases showed to re-sorb metals, thus underestimating the role of LMWOAs in metal release. Therefore, the selective metal leaching was highly dependent on the SRE composition and LMWOA concentrations on one hand, and on the mineral, organic, and organo-mineral components of the technosols on the other. The understanding of such processes is crucial for proposing and implementing environmental management strategies to reduce metal leaching or for the beneficial re-usage of metals (e.g., for agromining) from technosols.

Long-term influence of maize stover and its derived biochar on soil structure and organo-mineral complexes in Northeast China

The influence of biochar on the soil structure and aggregate stability has been debated in previous studies. To probe the action of biochar on soil aggregates, a 5-year field experiment was implemented in the brown earth soil of northeastern China. We determined the aggregate distribution (> 2000 μm, 250-2000 μm, 53-250 μm, and < 53 μm) and organic carbon (OC) and organo-mineral complex contents both in the topsoil (0-20 cm) and within the soil aggregates. Three treatments were studied as follows: control (basal application of mineral NPK fertilizer), biochar (biochar applied at a rate of 2.625 t ha^-1), and stover (maize stover applied at a rate of 7.5 t ha^-1), and all treatments received the same fertilization. The biochar and stover applications decreased the soil bulk and particle densities significantly (p < 0.05) and enhanced the soil total porosity. Both amendments significantly (p < 0.05) enhanced the total OC, heavy OC fractions, and organo-mineral complex quantities in the bulk soil as well as in all the studied aggregate fractions. Biochar and stover applications promoted the formation of small macroaggregates. A greater amount of organic matter was contained in the macroaggregates, which led to the formation of more organo-mineral complexes, thereby improving soil aggregate stability. However, the different mechanisms underlying the effect of biochar and stover on organo-mineral complexes need further research. Biochar and stover applications are both effective methods of improving the soil structure in Northeast China.

Organo-mineral complexes protect condensed organic matter as revealed by benzene-polycarboxylic acids

Condensed organic matters (COM) with black carbon-like structures are considered as long-term carbon sinks because of their high stability. It is difficult to distinguish COM from general organic matter by conventional chemical analysis, thus the contribution by and interaction mechanisms of organo-mineral complexes in COM stabilization are unclear and generally neglected. Molecular markers related to black carbon-like structures, such as benzene polycarboxylic acids (BPCAs), are promising tools for the qualitative and quantitative analysis of COM. In this study, one natural soil and two cultivated soils with 25 y- or 55 y-tillage activities were collected and the distribution characteristics of BPCAs were detected. All the investigated soils showed similar BPCA distribution pattern, and over 60% of BPCAs were detected in clay fraction. The extractable BPCA contents were substantially increased after mineral removal. The ratios of BPCA contents before and after mineral removal indicate the extent of COM-mineral particle interactions, and our results suggested that up to 73% COM were protected by mineral particles, and more stronger interactions were noted on clay than on silt. The initial cultivation dramatically decreased COM-clay interactions, and this interaction was recovered only slowly after 55-y cultivation. Kaolinite and muscovite are important for COM protection. But a possible negative correlation between BPCAs and reactive iron oxides of the cultivated soils suggested that iron may promote COM degradation when disturbed by tillage activities. This study provided a new angle to study the stabilization of COM and emphasized the importance of organo-mineral complexes for COM stabilization.

Characteristics of organo-mineral complexes in contaminated soils with long-term biochar application

Long-term studies on the environmental effects following biochar additions to soils, while plentiful, are predominantly focused on the soil fertility, whereas few are on the soil organo-mineral complexes. This study examines the changes of organo-mineral complexes in an acidic paddy soil and a saline-alkali soil which were remediated using biochar for approximately 8 years and 3 years, respectively. The results showed that loosely combined humus increased by 30.1% and 25.1% with the application of 40 t ha^-1 biochar in the acidic paddy soil and the saline-alkali soil, respectively. Meanwhile, an increase of cement (Fe-oxides) was the contributor to the rise of the complexes content. Complex iron in the saline-alkali soil were 30% higher than in the acidic paddy soil with the application of 40 t ha^-1 biochar. Fourier Transform Infrared Spectroscopy showed oxygen-containing functional groups on the surface of the biochar separated from the remediated field. X-ray diffraction analysis indicated that both complexation and sedimentation were involved in heavy metal immobilization. It was found that biochar amendment mitigated the effect of acid rain leaching and reduced vertical migration of the Fe/Al-bound complex, which can prevent soil from podzolization and thus improve its fertility.

Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage

Mineral availability for carbon (C) binding is a key regulator of soil C storage, yet little is known about the highly reactive nanomineral assembly in the paddy soil colloids. Here, using high-resolution transmission electron microscopy (HRTEM), solid-state ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we investigated the coordination nature of short-range-ordered (SRO) minerals in water-dispersible colloids that were isolated from the paddy soil under different six-year fertilization regimes. Our results showed that organic fertilization not only promoted the transformation of crystalline minerals to SRO phases in the bulk soils but also increased the concentrations of Fe, Al and Si in the soil colloids compared to chemical fertilization alone, and thus enhanced the accumulation of organic C in both the bulk soils and the soil colloids. The HRTEM images revealed that water-dispersible colloids in all soils, regardless of treatment, were composed of crystalline Fe nanominerals (with some Al/Si) and SRO-Al/Si nanominerals (with some Fe) associated with organic C. Furthermore, the combined results from the ²⁷Al and ²⁹Si NMR spectroscopy and XPS not only confirmed the presence of SRO-Al/Si nanoparticles as Si-rich allophane and phytolith but also demonstrated that organic fertilization promoted the transformation of aluminosilicates to SRO-Al/Si nanominerals in soil colloids. Together, these findings indicate that six-year organic fertilization promotes the formation of SRO minerals (e.g., ferrihydrite, Si-rich allophane and Fe-substituted allophane, as well as phytolith) in soils and modulates the assembly of organo-mineral complexes possibly by driving the biogeochemical cycles of Fe, Al, Si and specific organic ligands, thus contributing to the long-term storage of C in paddy soils.

Organic matter protection by kaolinite over bio-decomposition as suggested by lignin and solvent-extractable lipid molecular markers

The formation of organo-mineral complexes is essential in organic matter (OM) stabilization. However, limited studies have been conducted to systematically examine the mineral influence on the decomposition of plant residuals at a molecular level. In this study, pine needles and chestnut leaves were mixed with kaolinite at the weight ratio of 5:1. The controls were plant tissues without kaolinite. All the samples were incubated in the laboratory for one year. Molecular markers, including lignin-derived phenols (e.g. Vanilly units, syringyl units and cinnamyl units) and solvent-extractable lipids (e.g. n-alkanoic acid, n-alkanols and n-alkanes), were analyzed. The concentrations of lignin-derived phenols and lipid compounds were higher in the presence of kaolinite than without kaolinite. Lower degradation indexes, such as (Ad/Al)_V (ratio of vanillic acid to vanillin) and CPI (carbon preference index of n-alkanoic acid and n-alkanes), were found in the kaolinite system. These results indicate that kaolinite reduced the OM decomposition. The addition of kaolinite also stabilized some carbohydrates from plants. Furthermore, the degradation of OM led to the generation of persistent free radicals, indicated by electron paramagnetic resonance (EPR) signals. The EPR signals were higher with than without kaolinite. We hypothesize that the adsorption of semiquinone or quinone radicals on kaolinite may limit their reaction with other OM moieties and thus extended their lifetimes. In addition to embedding OM in soil aggregates, our results provide direct evidence of another mineral protective mechanism of soil OM.

Protection of extractable lipid and lignin: Differences in undisturbed and cultivated soils detected by molecular markers

Understanding formation of organo-mineral association is crucial for soil organic matter (SOM) stabilization. To remove reactive minerals from un-disturbed natural soil (NS) and two cultivated soils (dry-farming soil, TD, and terrace paddy soil, TP), a 10% HF/1M HCl treatment was applied. The mineral protection of different molecular SOM structures before and after cultivation was compared by using markers for lipid and lignin. The removal of reactive mineral increased the lipid extractability in TD and NS similarly, indicating that the cultivation did not reduce the mineral protection; this is attributable to fertilizer application and amorphous Fe oxide enrichment. In TP, the extent of lipid protection was lower than in TD, demonstrating that the protection depends on the type of cultivation. In contrast to lipids, lignin-derived phenols decreased over 80% after acid treatment. Furthermore, the ratios of acid to aldehyde in vanillyl ((Ad/Al)_V) of TD and TP were much higher than in NS, indicating an increased oxidation of lignin in cultivated soils. During acid treatment, two distinct layers of soil particles were identified: an organic matter (OM)-enriched layer (L_OM), and a non-reactive mineral-enriched layer (L_NR) with hardly detectable OC content. However, up to 50% of lipids were detected in L_NR, indicating that lipids did not selectively interact with reactive mineral particles. In TD and TP, (Ad/Al)_V values were higher in L_OM than in L_NR, indicating a strong interaction of oxidized lignin in L_OM. Therefore, the protection of lignin, especially highly oxidized lignin, can depend more on reactive minerals than lipid. Promoting the formation of organo-mineral complexes is the primary strategy for soil management, especially for highly oxidized OM.

Microscopic and spectroscopic characterization of humic substances from a compost amended copper contaminated soil: main features and their potential effects on Cu immobilization

We characterized humic substances (HS) extracted from a Cu-contaminated soil without compost addition (C) or amended with a wheat straw-based compost (WSC) (H1), co-composted with Fe₂O₃ (H2), or co-composted with an allophane-rich soil (H3). Extracted HS were characterized under electron microscopy (SEM/TEM), energy-dispersive X-ray (X-EDS), and Fourier transform infrared (FTIR) spectroscopy. In addition, HS extracted from WSC (H4) were characterized at pH 4.0 and 8.0 with descriptive purposes. At pH 4.0, globular structures of H4 were observed, some of them aggregating within a large network. Contrariwise, at pH 8.0, long tubular and disaggregated structures prevailed. TEM microscopy suggests organo-mineral interactions at scales of 1 to 200 nm with iron oxide nanoparticles. HS extracted from soil-compost incubations showed interactions at nanoscale with minerals and crystal compounds into the organic matrix of HS. Bands associated to acidic functional groups of HS may suggest potential sorption interactions with transition metals. We conclude that metal ions and pH have an important role controlling the morphology and configuration of HS from WSC. Characterization of H4 extracted from WSC showed that physicochemical protection of HS could be present in composting systems treated with inorganic materials. Finally, the humified fractions obtained from compost-amended soils may have an important effect on metal-retention, supporting their potential use in metal-contaminated soils.

Concept model of the formation process of humic acid-kaolin complexes deduced by trichloroethylene sorption experiments and various characterizations

To explore the interactions between soil organic matter and minerals, humic acid (HA, as organic matter), kaolin (as a mineral component) and Ca(2+) (as metal ions) were used to prepare HA-kaolin and Ca-HA-kaolin complexes. These complexes were used in trichloroethylene (TCE) sorption experiments and various characterizations. Interactions between HA and kaolin during the formation of their complexes were confirmed by the obvious differences between the Qe (experimental sorbed TCE) and Qe_p (predicted sorbed TCE) values of all detected samples. The partition coefficient kd obtained for the different samples indicated that both the organic content (fom) and Ca(2+) could significantly impact the interactions. Based on experimental results and various characterizations, a concept model was developed. In the absence of Ca(2+), HA molecules first patched onto charged sites of kaolin surfaces, filling the pores. Subsequently, as the HA content increased and the first HA layer reached saturation, an outer layer of HA began to form, compressing the inner HA layer. As HA loading continued, the second layer reached saturation, such that an outer-third layer began to form, compressing the inner layers. In the presence of Ca(2+), which not only can promote kaolin self-aggregation but can also boost HA attachment to kaolin, HA molecules were first surrounded by kaolin. Subsequently, first and second layers formed (with inner layer compression) via the same process as described above in the absence of Ca(2+), except that the second layer continued to load rather than reach saturation, within the investigated conditions, because of enhanced HA aggregation caused by Ca(2+).

Crop residue stabilization and application to agricultural and degraded soils: A review

Agricultural activities produce vast amounts of organic residues including straw, unmarketable or culled fruit and vegetables, post-harvest or post-processing wastes, clippings and residuals from forestry or pruning operations, and animal manure. Improper disposal of these materials may produce undesirable environmental (e.g. odors or insect refuges) and health impacts. On the other hand, agricultural residues are of interest to various industries and sectors of the economy due to their energy content (i.e., for combustion), their potential use as feedstock to produce biofuels and/or fine chemicals, or as a soil amendments for polluted or degraded soils when composted. Our objective is review new biotechnologies that could be used to manage these residues for land application and remediation of contaminated and eroded soils. Bibliographic information is complemented through a comprehensive review of the physico-chemical fundamental mechanisms involved in the transformation and stabilization of organic matter by biotic and abiotic soil components.