Abstract

Highlights

  • Fermented organic sheep manure and fermented organic soybean manure primarily transformed Ws-F into Res-F and Or-F, respectively.
  • AE-F can be anchored to the soil DOM through physicochemical reactions.
  • Oxygen-containing functional groups exhibited stronger reactivity and higher affinity for F compared with non-oxygen-containing functional groups.
Fluoride (F) pollution in soils, which can be caused by long-term application of phosphate fertilizers, has serious socio-economic and ecological impacts. Therefore, it is important to find economically viable and environmentally friendly remediation technologies that mitigating the potential hazards of F-contaminated soil while maintaining crop yields. In this study, a soil incubation experiment was carried out to investigate the effects of two different organic manures at various amounts on soil F content and soil properties. We demonstrated that organic manure application at a rate of 5% was optimal to control the content of available fluoride (AE-F) in soil. Organic manure decreased the AE-F content and increased the residual or organically bound content. Furthermore, organic manure application also increased the contents of carbohydrates, proteins, and amides, which can anchor AE-F to the dissolved organic matter (DOM) in soil through physicochemical reactions, thereby reducing the migration of F. Moreover, after organic manure application, complex humus compounds were formed through microbial action, thus reducing the bioavailability of soil F. In summary, the application of organic manures can reduce the accumulation of soil AE-F, thereby reducing the absorption of F by crops in F-contaminated soil and alleviating the potential risk of F contamination.

Excerpts:

Introduction

Fluorine, which is the 13th most abundant element on Earth, is a halogen element and widely distributed in rocks, soil, water, air, plants, and animals, with a stock of 0.078% (Ghosh et al. 2013). The average total fluoride (TF) content in the world’s soil fluctuates between 20 and 700 mg·kg-1, with a median of 200 mg·kg-1 (Wang et al. 2022b). China has a relatively high F content, with 95% of soils having a TF content of 190–1010 mg·kg-1 and an average soil TF content of 480 mg·kg-1, which far exceeds the global average (Chen 2010).
Within a relatively narrow concentration range, F can have both beneficial and adverse effects (Malinowska et al. 2008). Moderate amounts of F in the human body can prevent tooth decay and promote bone formation (Yin et al. 2021), but excessive amounts of F can lead to dental fluorosis and skeletal fluorosis, as well as damage to the liver, kidneys, and nervous system (Berger et al. 2016; Kabir et al. 2020; Wu et al. 2022). According to the guidelines published by WHO (2002), an F intake of >6 mg·d-1 may increase the potential risk of skeletal toxicity, and an intake of >14 mg·d-1 may lead to a significant risk of fractures. Therefore, F pollution has serious socio-economic impacts, posing a grave threat to human health, the environment, and socio-economic development.
Although the soil TF content in China is relatively high, there is still a stream of F entering the soil every year. Common natural sources include the weathering of F-containing rock minerals, volcanic eruptions, the release of marine aerosols, and biomass burning (Linfeng et al. 2021;Mamta and Shweta 2015). Human activities that lead to soil F accumulation include the use of coal for electricity generation; brickmaking; the production of ceramics and glass; aluminum electrolysis; phosphate fertilizer production; the use of fluoride-based pesticides, phosphates, tobacco agents, and insecticides; and the disposal of sewage sludge. Additionally, using fluorinated pollutants to irrigate farmland can also result in the accumulation of large amounts of F in soil (Kumar et al. 2020).
Current methods for remediating F-contaminated soil include electrokinetic remediation, phytostabilization, and the application of chemical amendments. These chemical amendments primarily involve the use of materials such as hydroxyapatite (NHAP), aluminum humate (HAA), wattle humus biosorbent, and biochar (Khan et al. 2022; Alkurdi et al. 2020; Gan et al. 2021; X. Huang et al. 2020; Angelin et al. 2021; Golezani and Abriz 2019; Fan et al. 2022). However, traditional approaches are often associated with challenges, including high costs and pH dependency. In this context, the present study introduces the novel application of organic manures as a sustainable, cost-effective strategy for reducing F bioavailability in contaminated soil. Unlike conventional remediation techniques, organic manures not only mitigate F contamination but also enhance soil health and structure through natural physicochemical processes.
Organic manures, as total nutrient fertilizers and soil amendments, can significantly increase the nutrient content of the soil and improve soil structure. It is well known that soil F solubility is influenced by pH, organic matter, and clay (Zhang et al. 2013), and that using organic manures can increase soil organic matter (SOM) content and change soil properties. Organic matter has a substantial impact on the F bioavailability because SOM contains a large number of active functional groups, which can reduce the bioavailability of pollutants in soil through adsorption, complexation, and chelation. Moreover, the use of organic manures not only decreases reliance on chemical fertilizers and mitigates the depletion of fossil resources, but also minimizes the environmental risks associated with chemical fertilizer usage. In addition, it addresses issues related to agricultural waste management (Yan et al., 2019b).
As SOM degrades, it releases dissolved organic matter (DOM), which, being the water-soluble fraction of SOM, is more prone to migration and transformation in the soil environment. It is a key factor affecting the mobility and availability of pollutants (Kaiser et al. 2002). DOM is also the most active part of the soil carbon pool and an important energy source for soil microorganisms (Zhang et al. 2022). Compared with SOM, DOM is more sensitive to changes in soil physicochemical properties and biological activity, making it an important indicator of short-term changes in soil quality (Gao et al. 2017). It is difficult to directly quantify soil DOM; however, as it has a strong correlation with dissolved organic carbon (DOC), soil DOC is usually used to indirectly reflect soil DOM.
Consequently, changes in DOM can serve as a sensitive indicator of the effectiveness of remediation strategies, such as organic matter amendments, on soil quality. Thus, the application of organic manures holds significant potential for mitigating fluoride contamination in soils by effectively reducing its bioavailability.
This study hypothesizes that the application of organic manures will reduce the F bioavailability in contaminated soil by transforming water-soluble fluoride (Ws-F) into less mobile, less bioavailable forms such as organically bound fluoride (Or-F) and residual fluoride (Res-F). We further hypothesize that higher rates of organic manure application will enhance this transformation, mediated by interactions between F and DOM.
The aims of this study were to (1) investigate the effects of different fermented organic manures on the transformation and bioavailability of F components in soil contaminated with exogenous fluoride; (2) explore the impact of organic manures on soil properties, such as pH, SOM, and DOM; and (3) analyze the potential mechanisms by which organic manures inhibit the accumulation of soil Ws-F. To achieve these objectives, we used arid agricultural soil from Fuping County, Weinan City, Shaanxi Province, in soil incubation experiments and analyzed the soil using UV-Vis, 3D fluorescence spectroscopy, and Fourier transform infrared spectroscopy. This study provides insights for comprehending the potential mechanisms by which organic manures control the accumulation of Ws-F in F-contaminated soil, which could lead to new ideas for soil protection.

Section snippets

Preparation of F-contaminated soil

Soil (0–20 cm) was collected from uncultivated agricultural fields near a long-term experimental area of the Fuping Modern Agricultural Comprehensive Experimental Demonstration Station of the Northwest A&F University in Fuping County, Weinan City, Shaanxi Province. After air-drying and removing plant debris and pebbles, the soil was sieved through a 2.0-mm mesh and then NaF was added at a concentration of 1000 mg F·kg-1 to simulate a relatively high concentration of exogenous fluoride…

Effects of organic manures on soil F forms

Ws-F and exchangeable F (Ex-F) are collectively referred to as AE-F. The application of M1 and M2 accelerated the conversion of Ws-F (Fig.1). On day 1 of the incubation, the Ws-F content in M1 and M2 treatments was substantially lower than that in the control (CK), indicating that organic manures effectively and immediately control Ws-F content in F-contaminated soil. Compared with M1, adding an equal amount of M2 resulted in lower Ws-F. This indicates that M2 better controlled AE-F than M1. …

Organic manures reduced the accumulation of soil AE-F

Soil F exists in several forms, each with varying mobility and bioavailability, affecting its environmental and biological impact. Ws-F is the most mobile and bioavailable (Loganathan et al. 2001). Ex-F is attracted to the positive charges of clay minerals, organic matter, and hydrated oxides (Gan et al. 2021), while Fe/Mn-F is associated with oxides of iron, aluminum, and manganese (Dehbandi et al. 2017). Fluoride also forms complexes with humus and organic acids, creating Or-F (Chen et al. …

Conclusions

Fermented organic sheep manure (M1) and fermented organic soybean manure (M2) both reduced the contents of Ws-F and Ex-F in F-contaminated soil. Higher manure application rates had a greater effect on F, and M2 had a greater effect than M1. Furthermore, two different mechanisms were observed. M1 mainly transformed Ws-F into Res-F, while M2 mainly transformed it into Or-F. For both manures, an application rate of 5% was identified as an optimal strategy for preventing and controlling AE-F …
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