High nitrogen addition after the application of sewage sludge compost decreased the bioavailability of heavy metals in soil

Intensive ammonium fertilizer addition activates iron and carbon conversion coupled cadmium redistribution in a paddy soil under gradient redox conditions

While over-fertilization and nitrogen deposition can lead to the enrichment of nitrogen in soil, its effects on heavy metal fractions under gradient moisture conditions remains unclear. Here, the effect of intensive ammonium (NH4+) addition on the conversion and interaction of cadmium (Cd), iron (Fe) and carbon (C) was studied. At relatively low (30-80 %) water hold capacity (WHC) NH4+ application increased the carbonate bound Cd fraction (F2Cd), while at relatively high (80-100 %) WHC NH4+ application increased the organic matter bound Cd fraction (F4Cd). Iron‑manganese oxide bound Cd fractions (F3Cd) and oxalate-Fe decreased, but DCB-Fe increased in NH4+ treatments, indicating that amorphous Fe was the main carrier of F3Cd. The variations in F1Cd and F4Cd observed under the 100-30-100 % WHC treatment were similar to those observed under low moisture conditions (30-60 % WHC). The C=O/C-H ratio of organic matter in soil decreased under the 30-60 % WHC treatment, but increased under the 80-100 % WHC treatment, which was the dominant factor influencing F4Cd changes. The conversion of NH4+ declined with increasing soil moisture content, and the impact on oxalate-Fe was greater at 30-60 % WHC than at 80-100 % WHC. Correspondingly, genetic analysis showed the effect of NH4+ on Fe and C metabolism at 30-60 % WHC was greater than at 80-100 % WHC. Specifically, NH4+ treatment enhanced the expression of genes encoding extracellular Fe complexation (siderophore) at 30-80 % WHC, while inhibiting genes encoding Fe transmembrane transport at 30-60 % WHC, indicating that siderophores simultaneously facilitated Cd detoxification and Fe complexation. Furthermore, biosynthesis of sesquiterpenoid, steroid, butirosin and neomycin was significantly correlated with F4Cd, while glycosaminoglycan degradation metabolism and assimilatory nitrate reduction was significantly correlated with F2Cd. Overall, this study gives a more comprehensive insight into the effect of NH4+ on activated Fe and C conversion on soil Cd redistribution under gradient moisture conditions.

Differences in soil bacterial community structure during the remediation of Cd-polluted cotton fields by biochar and biofertilizer in Xinjiang, China

Introduction

Heavy metal pollution is a major worldwide environmental problem. Many remediation techniques have been developed, these techniques have different performance in different environments.

Methods

In this study, soil sampling was conducted in multiple cotton fields in Xinjiang, China, and found that cadmium (Cd) was the most abundant soil heavy metal. Then, to find the most suitable technique for the remediation of Cd pollution in cotton fields, a two-year study was conducted to explore the effects of cotton straw-derived biochar (BC, 3%) and Bacillus-based biofertilizer (BF, 1.5%) on cotton Cd uptake and transport and soil microbial community structure under Cd exposure conditions (soil Cd contents: 1, 2, and 4 mg·kg-1).

Results

The results showed that the bioaccumulation coefficients (Cd content of cotton organs / soil available Cd content) of cotton roots, stems, leaves, and buds/bolls reduced by 15.93%, 14.41%, 23.53%, and 20.68%, respectively after the application of BC, and reduced by 16.83%, 17.15%, 22.21%, and 26.25%, respectively after the application of BF, compared with the control (no BC and BF). Besides, the application of BC and BF reduced the transport of Cd from soil to root system, and enhanced the diversity of soil bacterial communities (dominant species: Alphaproteobacteria and Actinobacteria) and the metabolic functions related to amino acid synthesis. It was worth noting that the differential species for BF group vs BC group including Alphaproteobacteria, Gemmatimonadetes, Bacilli, and Vicinamibacteria were associated with the enrichment and transport of Cd, especially the transport of Cd from cotton roots to stems.

Discussion

Therefore, the application of BC and BF changed the soil bacterial diversity in Cd-polluted cotton field, and then promoted the transport of Cd in cotton, ultimately improving soil quality. This study will provide a reference for the selection of soil heavy metal pollution remediation techniques in Xinjiang, China.

Dual Regulatory Role of Penicillium oxalicum SL2 in Soil: Phosphorus Solubilization and Pb Stabilization

The mechanisms of the P. oxalicum SL2-mediated microbial community on phosphorus solubilization and Pb stabilization were investigated through a 90-day soil experiment. In the treatments inoculated with P. oxalicum SL2, the amount of P. oxalicum SL2-GFP remained at 77.8%-138.6% of the initial inoculation amount after 90 days, and the available phosphorus (AP) content increased 21.7%-40.8% while EDTA-Pb decreased 29.9%-43.2% compared with CK treatment. SEM-EDS results showed that P. oxalicum SL2 changed the agglomeration degree of microaggregates and promoted the combination of Pb with C and O elements. These phenomena were enhanced when applied with Ca3(PO4)2. Microbial community analysis showed that P. oxalicum SL2 improved soil microbial activity, in which the fungi absolute abundance increased about 15 times within 90 days. Correlation analyses and a partial least-squares path model showed that the activation of Penicillium, Ascobolus, Humicola, and Spizellomyces in a fungal community increased the content of oxalate and AP, which directly decreased EDTA-Pb content, while the change of Bacillus, Ramlibacter, Gemmatimonas, and Candidatus Solibacter in the bacterial community regulated Fe/Mn/S/N cycle-related functions, thus promoting the conversion of Pb to oxidizable state. Our findings highlight that P. oxalicum SL2 enhanced the microbial-induced phosphate precipitation process by activating soil microbial communities and regulating their ecological functions.

Insight into the pathways of biochar/smectite-induced humification during chicken manure composting

As representative organic and inorganic additives, both biochar and smectite exhibit an excellent capacity to improve humification efficiency during composting. Nevertheless, the mechanisms underlying biochar/smectite-induced compost humification have still not been fully explored from the perspective of overall organic substances. In this study, three composting treatments were performed as follows: 10 % biochar-amended composting, 10 % smectite-amended composting and natural composting without any additive. UV-visible parameters and synchronous hetero two-dimensional correlation spectra showed that biochar accelerated dissolved organic matter (DOM) complications, unsaturation and aromatization. For example, biochar promoted the C2 and simple C3 peaks to convert into a sophisticated C3/360 peak. However, the effect of smectite was negligible in complicating the DOM structure. Both biochar and smectite displayed an invigorating role in promoting humic substance (HS) formation. The strengthened relations between bacterial richness and physicochemical indicators and HS fractions might contribute to the positive action of biochar/smectite on HS synthesis. Network analysis showed that both bacterial functional omnipotence and specialization in response to the addition of catalysts may contribute to compost humification. The chemical pathway involved in DOM humification was intensified by enhancing the role of pH in biochar composting and weakening the degradation of unsaturated aromatic compounds of DOM with smectite addition. These findings benefit the practical application of biochar/smectite in promoting composting efficiency.

Unlocking fertilization potential of anaerobically digested sewage sludge centrate for protein-rich rice cultivation with composted sludge amendment

The use of composted sewage sludge (CSS) and centrate as alternatives to synthetic fertilizers in rice cultivation holds great promise. This study aims to determine the effects of varying doses and timings of centrate derived from anaerobically digested sewage sludge on rice yield, nutrient quality, and soil fertility when applied as a topdressing to rice fields fertilized with CSS. At the panicle initiation (PI) stage, 100, 300, and 500 kg N ha-1 of centrate topdressing (CT100, CT300, and CT500, respectively) was applied. In addition, different topdressing timings at a total dose of 500 kg N ha-1 were evaluated, including a two-split application (40% at active tillering (AT) and 60% at PI; CT500S2) and a three-split application (40% at AT + 40% at PI + 20% at heading; CT500S3). At a rate of 160 kg N ha-1, CSS was used as a base fertilizer in all treatments. A control treatment received synthetic fertilizers at a rate of 160 kg N ha-1 as a base application and 100 kg N ha-1 as a topdressing. Results showed that CSS-treated rice plants exhibited a lower N status and leaf chlorophyll content during the vegetative growth stage; however, the split application of centrate topdressing improved plant N status, resulting in an increase in biomass and grain yield. Centrate and CSS tended to increase the mineral content of rice; nevertheless, a significant accumulation of As in grains raised concerns about food safety. Combining CSS and centrate has the potential to increase rice production, improve grain nutritional value, and decrease reliance on synthetic fertilizers. However, it is essential to optimize this fertilization, mitigate environmental risks, and ensure food safety by employing appropriate fertilization dosing and timing as well as appropriate field management strategies.

Are Chokeberry Products Safe for Health? Evaluation of the Content of Contaminants and Health Risk

The health-promoting properties of chokeberry fruit have been confirmed in numerous scientific studies. It has been shown that the consumption of these fruits, due to the high content of bioactive compounds, has beneficial effects in neurodegenerative diseases, in addition to having hypolipemic, hypotensive, hypoglycemic, and anti-inflammatory properties. However, different conditions and methods of fruit cultivation, as well as methods of juice and fiber production, may result in a high content of toxic substances, which reduce the health value of chokeberry products. Many substances are environmental pollutants. In this study, for the first time, we examined the content of toxic elements (As, Hg, Cd, Pb), nitrates, and nitrites in all chokeberry juices (organic, conventional, from concentrate, and not from fruit concentrate) without additives and in all chokeberry fibers available in Poland. In addition, risk indicators of adverse health effects were calculated. The median content of the contaminants tested in juices was 0.461 µg/kg for As, 1.170 µg/kg for Cd, 0.427 µg/kg for Hg, 1.404 µg/kg for Pb, 4.892 mg/kg for NO2-, and 41.788 mg/kg for NO3-. These values did not exceed the permissible standards for the calculated indicators. There were also no statistically significant differences in the content of Cd, Hg, and Pb, as well as nitrates (III) and nitrates (V), in the tested juices depending on the method of cultivation and juice production. However, statistically significant differences in As content were found between juices from conventional and organic cultivation (1.032 µg/kg vs. 0.458 µg/kg) and juices from concentrate and not from concentrate (1.164 µg/kg vs. 0.460 µg/kg). There were no statistically significant differences with respect to impurities in fibers. It is shown that the consumption of chokeberry juice and fiber in the amount normally consumed does not pose a health risk associated with the intake of toxic substances; in the case of long-term fiber consumption, the Pb content should be monitored. In particular, organic juices and those not from fruit concentrate are recommended due to the lower As content.

Driving effects and transfer prediction of heavy metal(loid)s in contaminated courtyard gardens using redundancy analysis and multilayer perceptron

The distribution and migration of heavy metal(loid)s in the soil-vegetable systems of courtyard gardens near mining areas have rarely been investigated, leading to potential food safety risks for residents. Moreover, the existing research is mainly focused on the total content of heavy metal(loid)s (tMetals) rather than the bioavailable contents (aMetals). In this study, 26 and 28 pairs of soil and vegetable samples were collected from the courtyard gardens near the Realgar mine in Baiyun Town and the lead-zinc (Pb-Zn) mine in Shuikoushan Town, respectively. The tMetal and aMetal of cadmium (Cd), mercury (Hg), arsenic (As), Pb, chromium (Cr), nickel (Ni), copper (Cu), Zn, manganese (Mn), iron (Fe), and calcium (Ca) in the samples were analyzed in this study. The results showed that courtyard gardens were polluted by various heavy metal(loid)s at varying degrees. The bioavailabilities of different metals varied significantly, among which Cd has the highest bioavailability (> 30%). In the transfer process of heavy metal(loid)s, the transfer rate (Tf) was ranked as soil-roots (1.50) > stems-leaves (1.07) > roots-stems (0.46) > stems-fruits (0.33). Redundancy analysis was used to evaluate the driving effects, and the results revealed that aCa, aZn, and aFe in soil could inhibit the absorption of aCd by plant roots. Soil organic matter was the inhibiting factor regarding the transfer of aAs and aCu, whereas it was also the promoting factor for transferring aPb, aNi, and aCr. Furthermore, the multilayer perceptron (MLP) could effectively predict the Tf of heavy metal(loid)s based on the aMetal. The R² values of the MLP were ranked as follows: 0.91 for As, 0.88 for Zn, 0.85 for Hg, 0.83 for Cu, 0.79 for Cr, 0.66 for Cd, 0.65 for Pb, and 0.52 for Ni. This study emphasizes the aMetal-based ecological characteristics and prediction ability. The study results are significant for guiding residents to strategize appropriate crop planting and ensure the safe production and consumption of vegetables.