Previous Article in Journal
Effects of Nitrogen and Phosphorus Supplementation on Responses of Trembling Aspen and White Spruce Seedlings in Reclamation Soils Amended by Non-Segregating Oil Sands Tailings
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Soil Systems
Volume 9
Issue 3
10.3390/soilsystems9030091
soilsystems-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Change Characteristics and Driving Factors of Molybdenum Content in Purple Soil from Southwestern China

by
Xueqin Li
1,2,
Tao Zhou
1,2,
Chunpei Li
1,2,
Xuan Wang
1,2,
Limei Deng
1,2,
Rongyang Cui
1,
Xiaolin Sun
1,2 and
Gangcai Liu
1,*
1
Key Laboratory of Mountain Surface Processes & Ecological Regulation, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*
Author to whom correspondence should be addressed.
Soil Syst. 2025, 9(3), 91; https://doi.org/10.3390/soilsystems9030091
Submission received: 22 April 2025 / Revised: 11 August 2025 / Accepted: 11 August 2025 / Published: 13 August 2025
Browse Figures
Versions Notes

Abstract

Molybdenum (Mo) is an important trace nutrient element in the soil and plays a significant role in maintaining plant growth. However, there are scarce studies on soil Mo content change and its driving factors based on historical soil samples. This paper studied the characteristics of Mo content in three different parent rock types (PRTs) and different eras. The findings indicated that the available Mo (AMo) and total Mo (TMo) in the purple soil were 0.087–0.131 mg/kg and 0.488–0.903 mg/kg, respectively, which were considered deficient. The TMo of J3p was higher than those of J2s and K2j, but the AMo was slightly lower than those of K2j and J2s. Compared with the old samples, the AMo of K2j, J2s and J3p has increased by 35.58%, 120% and 30.86%, respectively, and their TMo has increased by 29.37%, 25.21% and 11.97%, respectively. Our studies showed that PRTs directly impacted AMo, and indirectly influenced TMo and AMo through soil pH and organic matter. Organic matter and pH positively affected TMo, while pH negatively affected AMo. Overall, soil molybdenum content in the study area was generally insufficient, and local governments should comprehensively consider the molybdenum content and its main constraints for scientific fertilisation.

Graphical Abstract

1. Introduction

Research on soil micronutrient distribution, bioavailability, and driving mechanisms is a popular topic in soil science, ecology, geochemistry, and other disciplines [1,2,3]. Soil is the main source of nutrients required by plants, and molybdenum (Mo) is an essential trace element for plants. Mo is used in the form of the molybdopterin cofactor (MoCo), which binds to the active sites of Mo-dependent enzymes. Mo is not only important in higher plants for MoCo in rhizobial nitrogenase of legume–diazotroph symbioses [4,5,6], but also in other enzymes such as nitrate reductase important for nitrate assimilation in all plants including crop plants, aside of other plant enzymes such as sulfite oxidase (sulfite detox), xanthine dehydrogenase and aldehyde oxidase (purine metabolism, ABA synthesis) [7,8]. Appropriate amounts of molybdenum in the soil can increase the yield and quality of agricultural products as well as enhance the resistance of crops [9,10]. Mo deficiency in the soil caused a decrease in the yield, quality, and Mo content of wheat, soybeans, and seeds [11].
The Mo concentration in topsoil varies widely, ranging from 0.0 to 44.1 mg/kg in the United States [12], 0.026 to 14 mg/kg in European agricultural soils [13] and 0 to 1012 mg/kg in China [14]. Soils with available Mo content of less than 0.15 mg/kg are identified as Mo-deficient in China. Many scholars have conducted extensive research on the factors affecting the total and available Mo content in soil [15,16]. Soil Mo concentrations are affected by factors such as parent material, terrain, soil type, land use, and soil physical and chemical properties [17,18,19]. The parent material directly determines the mineral and particle compositions of the soil, thereby affecting the distribution of soil elements [20]. The Mo concentration in Torridonian black shale soil in Scotland is as high as 232 mg/kg [21], while the average Mo concentration in the Beibu Gulf region, where glutenites and clasolites are the main parent materials, is only 1.1 mg/kg [22]. Different land-use types have distinct physical and chemical soil environments, which affect the environmental availability of elements in the soil. For instance, an earlier study found that Mo concentrations were higher in dry land than in paddy fields [23]. Topography can redistribute soil, water, and energy, thereby indirectly affecting the distribution of soil elements. For example, the humidity index varied between low altitudes and high terrain [24]. Factors controlling soil Mo varied across regions [25]. A study on soil Mo concentrations in the Longitudinal Range–Gorge Region of Linshui County, Sichuan Province, China, found that the Mo content in the topsoil was greatly affected by parent material, altitude, and soil type. The synergistic effects of the driving factors affecting soil Mo were revealed using a geographical detector model [14]. Soil properties at the microscale also profoundly affect the concentration of Mo. Researchers have found that the total Mo content of soil was mainly governed by factors such as soil-forming parent material, soil pH, and organic matter [26], whereas available Mo was mainly affected by soil type, pH, and other factors [27]. Moreover, the soil Mo content and pH were important factors affecting the Mo content in crops [28].
Purple soil (Regosols in the taxonomy of the Food and Agriculture Organization of the United Nations (FAO), and Entisols in U.S. taxonomy) is one of the major soil types in China and is mainly found in the upper reaches of the Yangtze River, accounting for approximately 21.7% of its area [29]. The upper reaches of the Yangtze River fall under China’s important ecological security strategy area: the Loess Plateau–Sichuan–Yunnan Ecological Barrier. Therefore, purple soil plays an important role in China’s ecological security strategies. The purple soil zone is an important food-producing area in the country and plays a significant role in food security. Purple soil evolved from purple or purplish-red sandstone and shale formed during the Jurassic and Cretaceous periods. Most purple soil is rich in calcium (calcium carbonate) and nutrients, such as phosphorus (P) and potassium (K), making it fertile. Purple soils can be divided into three main subclasses: acidic, neutral, and calcareous. Purple soils are characterised by thin soil layers (usually less than 50 cm and rarely more than 1 m), low soil organic matter (SOM), and rapid nutrient loss. These features render the current understanding of Mo variation in purple soil ambiguous. Current knowledge of the main factors governing the content of the beneficial element Mo in soils varies, and there is a lack of systematic research on the soil Mo content and its drivers across different parent rock types (PRTs) of purple soils.
In this study, we collected topsoil samples of purple soils in the spring of 2024 from three bedrock types, the Jiaguan, Shaximiao, and Penglaizhen formations, according to the data provided by the Second Soil Census of China. We analysed the differences in the total and available Mo contents and physicochemical properties of soils across different PRTs, as well as the different periods of the soil sample (from the 1980s to 2024). We also analysed the driving factors, identified the key factors, and constructed a structural equation model (SEM) to determine how these factors directly and indirectly influenced soil Mo content. The aim of this study was to provide a scientific basis for the fertilisation of local soils and the improvement of soil Mo availability.

2. Materials and Methods

2.1. Sample Collection and Preservation

In the 1980s, China conducted a second national soil census and collection of samples, many of which have been preserved to the present day (referred to as old soil samples), including the purple soil used in this study. We selected three typical old samples (five samples each): acidic (Jiaguan Group (K2j) sandstone developed into red and purple mud) purple soil, neutral (gray/brown/purple mud developed on the parent material of purple sand shale in the Jurassic Shaximiao Group (J2s)) purple soil and calcareous (a lithologic soil without distinct pedogenic horizons (orthent)—which is derived from the mudstone of the Penglaizhen group (J3p)) purple soil. It is difficult for old samples to be retained for 40 years, so even though the number of old samples is not enough, we still believe that they reflect the original nutrient status of typical purple soil in the 1980s, which can provide a background value for comparison in future soil research. These old samples are preserved at the Yanting Agro-Ecological Station of the Chinese Academy of Sciences (105°27′ E, 31°16′ N).
According to site information from the Second National Soil Census in the 1980s, we resampled 15 soil samples corresponding to the old sample sites in the spring of 2024 and collected another 10 new samples from new sites (all these samples are referred to as new soil samples in this study). The sampling sites were mainly located in Yaan City, Leshan City, Neijiang City, Ziyang City, Nanchong City, and Mianyang City in Sichuan Province, China (Figure 1). In addition, the three parent rock types are codenamed K2j (Jiaguan Formation), J2s (Shaximiao Formation), and J3p (Penglaizhen Formation) according to stratigraphy and will be referred to as such (Figure 1). Simultaneously, we recorded information such as the latitude, longitude, elevation, land-use type, and crops grown at each sampling site. Using a stainless-steel pointed shovel, we dug soil from the 0–20 cm layer using the plum-shaped five-point sampling method and mixed it evenly to form a composite sample. The total Mo content, available Mo content, and basic soil properties of the new and old samples were determined.

2.2. Determination of Soil Chemical Properties

Soil samples were brought back to the laboratory, air-dried, ground through a 2 mm sieve, and stored for determination. The old samples described above from the 1980s and the new samples collected in 2024 were determined to have the same soil nutrient contents by the same soil agrochemical analysis method. The soil pH was determined using the glass electrode method and a soil-to-water ratio of 1:2.5 (w/v). Soil organic carbon and total nitrogen were determined using an elemental analyser (Vario TOC cube, Elementar, Langselbold, Germany). Soil total P and total K were determined using the sodium hydroxide alkali fusion-Mo-antimony colourimetric method and the sodium hydroxide alkali fusion-flame photometric method, respectively (GB 7852-87 [30] and GB 7854-87 [31], respectively). The soil available nitrogen content was determined using the alkali-dissolved diffusion method (LY/T 1228-2015 [32]). Soil available phosphorus was determined using the sodium bicarbonate leaching method (GB 12297-1990 [33]). Soil available potassium was determined using the ammonium acetate leaching-flame photometric method (NY/T 889-2004 [34]).

2.3. Determination of Mo Content in Soil

Both the old samples from the 1980s and the new samples from 2024 were determined simultaneously by the same method for total and available molybdenum content in soil. The total molybdenum content of the soil is determined according to the standard method ‘Soil and Sediment—Determination of 19 total metal elements—Inductively coupled plasma mass spectrometry’ (HJ 1315-2023 [35]). After digestion of soil samples, the total Mo content in soil was determined using inductively coupled plasma mass spectrometry (ICP–MS). The limit of detection (LOD) was 0.1 mg/kg, and the limit of quantification (LOQ) was 0.4 mg/kg. According to ‘Soil Testing–Part 9: Method for determination of soil available molybdenum’ (NY/T 1121.9-2023 [36]), the content of available Mo in soil was determined using inductively coupled plasma mass spectrometry (ICP–MS) after the soil sample was extracted with oxalate–ammonium oxalate solution. The limit of detection (LOD) was 0.002 mg/kg, and the limit of quantification (LOQ) was 0.008 mg/kg. The certified reference material for total molybdenum in soil is numbered GSS 14, and the certified reference material for available molybdenum in soil is numbered GBW(E)070333; the blank is a parallel duplicate sample, and the minimum control sample is 5%; the number of duplicate samples extracted is 5% of the total samples. Furthermore, we assessed the efficacy of Mo by calculating its effectiveness, defined as the ratio of available Mo to total Mo content. This metric was utilised to evaluate the availability of Mo in the soil and identify influencing factors [37].

2.4. Statistical Analysis

Statistical analysis was performed using SPSS 26, with the significance of Spearman’s correlation coefficient and statistical significance set at p < 0.05 and p < 0.01, respectively. Before conducting significance analysis, the Shapiro–Wilk test was used to verify the normal distribution of the data, and a non-parametric test was performed on data that did not follow a normal distribution. All figures were produced using Origin 2021 software and RStudio (R version 4.1.3). Redundancy analysis (RDA) was performed using Canoco 5. SEM was conducted with SmartPLS 4.

3. Results

3.1. Characteristics of Soil Mo Content

3.1.1. Characteristics of Different Parent Rock Types in Soil Mo Content

There were no significant differences in available Mo (AMo) among the parent rock types, but the total Mo (TMo) content varied among the parent rock types (Figure 2a,b). The TMo of J3p (0.903 mg/kg) was significantly higher than that of K2j (0.703 mg/kg), and the TMo of K2j was significantly higher than that of J2s (0.547 mg/kg) (p < 0.05). The soil Mo availability in K2j (19.01%) was significantly higher than that in J3p (11.48%) (Figure 2c). These findings suggested that although J3p has high TMo content, its AMo content is relatively insufficient, leading to low Mo availability. This indicated that TMo did not completely restrict the AMo content. Overall, the AMo content under different parent rock types ranged from 0.087 to 0.131 mg/kg, reflecting relatively low levels [38]. The TMo content of the soil ranged from 0.547 to 0.903 mg/kg, which was lower than the average content of soil TMo in China (1.7 mg/kg) [39].

3.1.2. Characteristics of Temporal Variation in Soil Mo Content

Over the past 40 years, the AMo and TMo contents in the three typical purple soils have increased (Figure 3). We found that the AMo in J2s increased from 0.051 to 0.111 mg/kg, representing an increase of 120%. For K2j, AMo increased from 0.104 to 0.141 mg/kg, reflecting an increase of 35.58%. The AMo increased from 0.08 to 0.106 mg/kg in J3p, corresponding to an increase of 30.86% (Figure 3a). The increases of the TMo content for K2j, J2s, and J3p from the 1980s to 2024 are 29.37%, 25.21%, and 11.97%, respectively (Figure 3b). The increases of the availability of Mo for K2j, J2s, and J3p from the 1980s to 2024 are 10.3%, 68.01%, and 4.6%, respectively (Figure 3c). Significant differences in AMo, TMo, and availability of Mo contents were observed exclusively in J2s between the old and new samples (Figure 3a–c).

3.2. Driving Factors of Soil Mo Content Change

The soil nutrient contents under different parent rock types are shown in Table S1. We first conducted a correlation analysis between the soil properties and Mo content to clarify the influencing factors (Figure 4). Subsequently, we identified dominant factors using RDA (Figure 5). Finally, we used SEM to analyse the direct and indirect effects of the driving factors on soil Mo content (Figure 6).
In Figure 4, the parent rock type, pH, soil organic matter, and soil total nitrogen were the best explanatory variables for soil total molybdenum content. pH, soil organic matter, and soil total nitrogen were the most suitable explanatory variables for soil available molybdenum. pH was the best explanatory variable for the availability of molybdenum. Except for soil organic matter and soil total nitrogen, soil nutrient contents were highly correlated with parent rock types.
Redundancy analysis (RDA) results showed that the content of TMo was positively correlated with soil organic matter, total nitrogen content, available nitrogen, and parent rock type (Figure 5). A significant positive correlation existed between the content of available molybdenum and soil organic matter, available nitrogen, and available phosphorus. Available molybdenum content was negatively correlated with pH, total potassium, and available potassium. Availability of molybdenum was positively correlated with available phosphorus and available nitrogen, and negatively correlated with pH, total phosphorus, total potassium, and parent rock type (Figure 5).
The SEM offered a good fit for data (χ2 = 47.13, GOF = 0.46, NFI = 0.82), which also accounted for 63% of the variations of Amo, and 66% of the variations of TMo (Figure 6). The model showed that not only could parent rock type directly and positively affect AMo, but also could indirectly affect AMo and TMo through directly and positively affecting pH. Soil pH directly and negatively affected AMo, and directly and positively affected TMo. Soil organic matter directly and positively affects AMo, and indirectly affects TMo through directly and positively affecting available nitrogen. Total phosphorus had a negative effect on available nitrogen and a positive effect on available phosphorus. AMo directly and positively affected TMo (Figure 6). Overall, different parent rock types affect soil pH and carbon, nitrogen, and phosphorus nutrient content, and ultimately lead to differences in soil available molybdenum and total molybdenum content.

4. Discussion

We first have to admit that the number of old (1980s) samples in this study is small, because the number of preserved soil samples from the second soil census in China is indeed very limited. Nevertheless, we still believe that the old sample is very precious, because it reflects the most original situation of the nutrient content of purple soil in the southwest of the last century, in the 1980s, and provides a background value for the change in purple soil characteristics in the future. Earlier studies have investigated soil Mo content in specific areas [3,16], but there has been no long-term comparison of soil Mo content in the same area to explore the changes and possible influencing factors, largely because of the absence of historical samples.

4.1. Effects of Parent Rock Type on Soil Mo Contents

The results showed that the total Mo and available Mo of the three purple soils in the selected study area were lower than 1.7 mg/kg and 0.15 mg/kg, respectively, which was in the deficiency level. This finding may be due to the source and form of Mo in soils [22]. The soils developed from diorite, granite, and rhyolite had a much higher Mo content than the three purple soils discussed in this study (Table 1). The differences in molybdenum content under different parent rock types are mainly caused by the magma origin and mineral composition. Plagioclase is formed by the crystallization of basic magma. This magma usually originates from partial melting of the mantle and is rich in siderophile elements [40]. Molybdenum, as a siderophile element, is relatively enriched in basic magma. Diorite is mainly composed of minerals such as orthoclase and plagioclase. These minerals have certain adsorption and enrichment capabilities for molybdenum, resulting in a relatively concentrated distribution of molybdenum in diorite. Rhyolite is mainly composed of minerals such as quartz, potassium feldspar, and plagioclase. These minerals have relatively weak adsorption and enrichment capabilities for molybdenum, resulting in a low molybdenum content in rhyolite. Sandstone is a typical sedimentary rock, mainly composed of detrital minerals such as quartz and feldspar, as well as clay minerals [41]. Tuff is formed by the accumulation of volcanic debris, mainly consisting of volcanic glass and mineral debris. The mineral components in these rocks have limited adsorption and enrichment capabilities for molybdenum, and during the sedimentation and diagenesis processes, molybdenum is prone to leaching and migration, leading to a low total molybdenum content. Approximately 70% of diorite consists of plagioclase, which is rich in Mo. Some biotite in diorite also contains a certain amount of Mo, resulting in the soil developed on diorite having the highest total Mo content. Tuff, granite, and rhyolite all contain a certain amount of plagioclase and biotite; therefore, the total Mo content of the soil developed from these three types of rocks is also high [42]. The purple soils of the Jiaguan Formation, Shaximiao Formation, and Penglaizhen Formation were formed from remnants of mudstone or shale weathering and slope sediments. They contain a large amount of quartz and undergo a high degree of leaching during formation; therefore, their Mo content is relatively low. Moreover, molybdate ions formed by the weathering of the parent material are adsorbed or wrapped by iron and manganese oxides, resulting in a decrease in available Mo [39]. The parent rock directly influenced the total Mo content in the soil. In this study, the parent materials of the purple soils varied according to formation. In the Jiaguan Formation (acidic soil), the parent material consists of slope sediments derived from thick red-purple sandstone and thin shale weathering. The Shaximiao Formation (neutral soil) is characterised by residuum from thick mudstone weathering, whereas the Penglaizhen Formation (calcareous soil) contains parent material from the residuum of purple mudshale weathering. These differences in the parent material resulted in variations in soil nutrient content and fertility. In general, calcareous purple soils exhibit relatively higher nutrient levels and fertility than acidic and neutral purple soils [43] (Table S1). Neutral purple soils have a high fertility level but may have slightly insufficient organic matter, nitrogen, and phosphorus. Acidic purple soils, characterised by low pH, often require amendments to enhance fertility and increase active carbon components [44]. Consequently, the total Mo content is typically high in calcareous soils but low in acidic and neutral soils.

4.2. Effects of pH on Soil Mo Contents

The available Mo in the soil includes water-soluble Mo (in very small amounts) and exchangeable Mo, which consists of Mo adsorbed on the surfaces of soil minerals and soil colloids in the form of MoO42− ions. The bonding of MoO42− with clay minerals is relatively weak and can be replaced. As soil pH increases from 3 to 6, the adsorption of MoO42− increases. However, above pH 6, this adsorption rapidly decreases, and is almost negligible at pH levels above 8 [45]. In this study, the available Mo content in acidic soil was slightly higher than that in neutral and calcareous soils, both in the new and old samples. Correspondingly, the availability of Mo was higher in the acidic purple soil (Figure 2). This finding was consistent with the results of a previous study [45]. Areas with high pH generally have reduced solubility of soil trace elements, resulting in a decrease in the content of available Mo [46,47]. Soil acidification is characterised by a reduction in the acid neutralisation capacity (ANC) or an increase in the base neutralisation capacity (BNC), which leads to increased soil acidity, as evidenced by a decrease in soil pH [48]. Soil pH decreased by an average of about 0.5 units nationwide [49]. Our results showed that the pH of neutral purple soil and calcareous purple soil decreased by 0.78 and 0.47 units, respectively, in the past 40 years (Table 2). The decrease in soil pH in the study area may be due to soil acidification caused by the excessive application of nitrogen fertiliser and acid rain during long-term agricultural practices [50]. According to the relationship between soil pH and total and available Mo content, a significant negative correlation was revealed between the available Mo and pH, with the available Mo content decreasing as the pH increased. In contrast, the relationship between total Mo content and pH was described by a polynomial fit. Initially, the total Mo content decreased with increasing pH but began to rise again once the pH surpassed 6.5 (Figure 7). Soil acidification adversely affects fertility, biological activity, and plant productivity. For example, it can alter soil structural stability, ultimately affecting soil porosity and water-holding capacity [51]. The nutrient retention capacity of soil is closely linked to its cation and anion exchange capacities, which are influenced by soil pH [52]. Optimal nutrient availability for most plants occurs within a pH range of 6.5–7.5 [53]. Molybdenum is present in moderately alkaline soils. In contrast, acidic soil conditions often lead to poor root growth because of reduced nutrient availability, which is essential for plant development [51]. Therefore, in view of the unique importance of Mo in the nitrogen cycle and plant growth [5,10], more attention should be paid to the response mechanisms of Mo content to soil acidification in the future.

4.3. Effects of Soil Organic Matter and Other Nutrients on Soil Mo

Soil organic matter can also directly affect nitrogen content and indirectly affect available Mo. Soil organic matter primarily fixes Mo in the soil through complexation and adsorption, leading to a high total Mo content when organic matter is abundant [45]. Mo interacts with organic matter in soil, and Mo can form complexes with organic matter. When organic matter decomposes and mineralises, Mo is released into the soil. Additionally, organic acids and other substances produced during decomposition can enhance the release of Mo by promoting the dissolution of Mo-containing minerals [54]. Therefore, soils with high organic matter content tend to have high available Mo content [55] (Table S1).
The findings demonstrate that the new samples of acidic, neutral, and calcareous soils demonstrated increased contents of soil Mo compared to the old samples (Figure 3). This difference is due to the increase in chemical fertilizer application rate, which enabled the decomposition of SOM and activation of more trace elements [46]. Some studies have pointed out that the total amount of fertilizer in China has increased rapidly over time. From 1980 to 2014, the amount of fertilizer used increased from 12,694,000 tons to 59,959 million tons, an increase of nearly five times [56]. By 2015, it reached a maximum of 60,226 million tons, and since then, although the amount of fertilizer has declined, it still remains at about 50 million tons [57]. The increase in molybdenum (Mo) over the past 40 years may be attributed to the application of phosphorus (P) fertilizers, which often contain small amounts of Mo as a contaminant, even at parts-per-million (ppm) levels [5]. Phosphorus fertilization can elevate soil Mo concentrations, potentially alleviating Mo limitations. For instance, it has been reported that the use of triple superphosphate (TSP) can raise soil Mo levels by approximately 20% after application. In some cases, Mo was deliberately added to superphosphate to address deficiencies in leguminous forage crops, as Mo is readily retained in the soil and can remain effective for many years [58]. However, the presence of trace Mo in P fertilizers has complicated the interpretation of plant responses, as positive effects of P fertilization may, in part, result from unintended Mo supplementation [4]. Meanwhile, organic acids are produced during decomposition and mineralisation processes, enhancing the solubility of trace elements in the soil and increasing the content of available trace elements [59,60]. The mineralisation of organic matter releases significant amounts of N, P, S, and trace elements, which support crop growth; thus, nitrogen content is also closely related to molybdenum content. Additionally, given the close relationship between carbon and nitrogen cycles, the correlations between nitrogen, organic matter, and available trace elements are highly similar [16,61]. The P and Mo anions compete for adsorption sites, with soil generally having a higher adsorption capacity for phosphate ions than for molybdate ions. Consequently, P and Mo are typically negatively correlated, consistent with the negative correlation observed between the available P and total Mo in this study [62,63]. The total Mo content in the soil serves as a reservoir of available Mo. Consequently, there was a positive correlation between total and available Mo. This study found that the total Mo had a direct positive effect on the available Mo content (R = 0.88), consistent with previous research findings [9,64]. Irrigation and pollution also have an impact on the migration and accumulation of molybdenum. For instance, water-saving irrigation methods such as drip irrigation and sprinkler irrigation can distribute the irrigation water more evenly in the soil, reducing the accumulation of molybdenum on the soil surface, thereby decreasing the chance of molybdenum entering plants through the soil [65]. The industrial activities such as mining and smelting of molybdenum are one of the main sources of molybdenum pollution. During these processes, a large amount of molybdenum-containing dust, wastewater, and waste residue are produced. If these pollutants are not properly treated, they will enter the environment, leading to an increase in the content of molybdenum in the soil, water bodies, and air. The application of molybdenum-containing waste residue and the use of molybdenum-containing pesticides will also increase the content of molybdenum in the soil [66]. Furthermore, some agricultural wastes, such as livestock manure, may also contain molybdenum. If these wastes are directly used as fertilizer without any treatment, it will lead to an increase in the molybdenum content in the soil [67]. Over the past 40 years, human farming activities have become increasingly frequent and have exerted a profound influence on the soil. For example, the increase in fertilizer application rate, the change of fertilizer type and the change of management practices have significantly altered the soil environment [68,69,70], thereby affecting the soil nutrient status, including the content of trace elements.
The improvement of molybdenum availability directly affects the absorption and utilization efficiency of nitrogen, sulfur, and other elements by plants; it can also enhance the stress resistance and disease resistance of crops, thereby increasing their yield and quality. Zhang, et al. [71] conducted research and found that the rich-molybdenum water-retaining agent can be used to enhance the water retention capacity of soil in arid areas, improve the growth environment and nutritional status of soybean seedlings, and maximize the yield potential of soybeans in Northeast China. Rana, et al. [72] found that an increase in the availability of molybdenum significantly enhanced the biological yield of crops as well as the concentrations and absorption efficiency of nitrogen, phosphorus, and molybdenum; molybdenum also improved the absorption efficiency of mineral nutrients by enhancing the chemical properties of the soil, especially by increasing the availability of phosphorus. The current situation of low molybdenum content and low molybdenum availability in purple soil can be improved through reasonable soil fertility management strategies. Applying more organic fertilizers can improve soil structure, enhance soil’s water retention and aeration capabilities, increase the content of organic matter in the soil, thereby enhancing soil fertility and the availability of molybdenum elements. At the same time, rational use of chemical fertilizers can quickly replenish the nutrients lacking in the soil, but it is necessary to avoid long-term application, which may lead to soil acidification. Molybdenum fertilizer can be applied to the soil by means of banding or broadcasting. The dosage is usually small, ranging from 0.5 to 2 μg/A [73]. Applying soluble molybdenum sources, such as sodium molybdate or ammonium molybdate, to the leaf surface can more quickly correct molybdenum deficiency symptoms. In areas with molybdenum deficiency, it is common to treat seeds with a small amount of molybdenum fertilizer to ensure that each seed receives an appropriate amount of molybdenum for healthy growth. For acidic soil, applying an appropriate amount of lime can neutralize the pH and increase the availability of molybdenum [74]. Meanwhile, the application of phosphate fertilizer can also release molybdenum through the exchange with the MoO42− sites on the soil surface. Correcting molybdenum deficiency can also ensure that the use efficiency of nitrogen fertilizer is higher, the cost is lower, and the environmental impact is smaller [75].

5. Conclusion

Our findings showed that the soil Mo content was low, with an AMo of 0.087–0.131 mg/kg and TMo of 0.488–0.903 mg/kg in purple soil from Southwest China. The TMo of J3p was higher than those of J2s and K2j, but the AMo was slightly lower than those of K2j and J2s. In the last 40 years, AMo and TMo in the three typical purple soils have shown increasing trends. Parent rock, pH, and soil organic matter were the key driving factors leading to changes in TMo and AMo in purple soil. In summary, our study not only reported the dominant factors affecting molybdenum content in purple soils but also increased our understanding of the 40-year change in molybdenum content and its influencing mechanisms. Understanding the interactions between these macro- and micronutrients can improve predictions of how farmland will respond to global change.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/soilsystems9030091/s1, Table S1: Soil nutrients under different parent rock types.

Author Contributions

X.L.: Writing—review and editing, writing—original draft, visualisation, formal analysis, and data curation. T.Z.: Writing—review and editing, investigation, data curation. C.L.: Writing—review and editing, visualisation, methodology, investigation. X.W.: Writing—review and editing, methodology, investigation, supervision. L.D.: Writing—review and editing, visualisation, investigation, validation. R.C.: Methodology, validation, writing—review and supervision. X.S.: Writing—review and editing, visualisation, validation, and investigation. G.L.: Writing—review and editing, investigation, funding acquisition, conceptualisation, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research and the APC were funded by the Sichuan Natural Science Foundation project and the National Key Research and Development Project (grant number: 2023YFD190003603).

Data Availability Statement

Data will be made available on request.

Acknowledgments

We are extremely grateful for the financial support provided by the Sichuan Natural Science Foundation project and the National Key Research and Development Project for this research (grant number: 2023YFD190003603).

Conflicts of Interest

The authors declare that they have no competing financial interests or personal relationships that may have influenced the work reported in this study.

References

  1. Mao, X.; Liu, L.; Cheng, X.; Hu, Q.; Xiao, F.; Ni, W. Geochemistry and spatial distribution of Se element in soils of typical Se-rich areas in Xinmi, Henan province. Geol. Bull. China 2021, 40, 1664–1670. [Google Scholar] [CrossRef]
  2. Baud, A.; Francus, P.; Smol, J.P.; Antoniades, D.; Gregory-Eaves, I. Geochemical changes in Eastern Canadian lake sediment cores spanning the last ∼150 years highlight a relative shift towards increased metals and erosive materials. CATENA 2023, 225, 107012. [Google Scholar] [CrossRef]
  3. Behera, S.K.; Shukla, A.K.; Pachauri, S.P.; Shukla, V.; Sikaniya, Y.; Srivastava, P.C. Spatio-temporal variability of available sulphur and micronutrients (Zn, Fe, Cu, Mn, B and Mo) in soils of a hilly region of northern India. CATENA 2023, 226, 107082. [Google Scholar] [CrossRef]
  4. Barron, A.R.; Wurzburger, N.; Bellenger, J.P.; Wright, S.J.; Kraepiel, A.M.L.; Hedin, L.O. Molybdenum limitation of asymbiotic nitrogen fixation in tropical forest soils. Nat. Geosci. 2009, 2, 42–45. [Google Scholar] [CrossRef]
  5. Wurzburger, N.; Bellenger, J.P.; Kraepiel, A.M.L.; Hedin, L.O. Molybdenum and Phosphorus Interact to Constrain Asymbiotic Nitrogen Fixation in Tropical Forests. PLoS ONE 2012, 7, e33710. [Google Scholar] [CrossRef]
  6. Mendel, R.R. The Molybdenum Cofactor *. J. Biol. Chem. 2013, 288, 13165–13172. [Google Scholar] [CrossRef]
  7. Stiefel, E.I. The biogeochemistry of molybdenum and tungsten. Met. Ions Biol. Syst. 2002, 39, 1–29. [Google Scholar] [CrossRef] [PubMed]
  8. Mendel, R.R.; Hänsch, R. Molybdoenzymes and molybdenum cofactor in plants. J. Exp. Bot. 2002, 53, 1689–1698. [Google Scholar] [CrossRef] [PubMed]
  9. Li, P.; Wu, H.; Guan, H.; Xu, J.; Wang, Y.; Shen, S.; Wang, Y.; Ge, H. Molybdenum distribution characteristics in soils of agricultural land in Huaibei plain of Anhui province and influencing factors. Rock Miner. Anal. 2023, 42, 361–370. [Google Scholar] [CrossRef]
  10. Jean, M.-E.; Phalyvong, K.; Forest-Drolet, J.; Bellenger, J.-P. Molybdenum and phosphorus limitation of asymbiotic nitrogen fixation in forests of Eastern Canada: Influence of vegetative cover and seasonal variability. Soil Biol. Biochem. 2013, 67, 140–146. [Google Scholar] [CrossRef]
  11. Liu, C. Study on Effect of Molybdenum and Zinc Combination on Winter Wheat and Its Mechanism. Ph.D. Thesis, Huazhong Agricultural University, Wuhan, China, 2021. [Google Scholar]
  12. Smith, D.B.; Cannon, W.F.; Woodruff, L.G.; Solano, F.; Ellefsen, K.J. Geochemical and Mineralogical Maps for Soils of the Conterminous United States; Open-File Report 2014-1082; Geological Survey: Reston, VA, USA, 2014; p. 399.
  13. Reimann, C.; Fabian, K.; Birke, M.; Filzmoser, P.; Demetriades, A.; Négrel, P.; Oorts, K.; Matschullat, J.; de Caritat, P.; Albanese, S.; et al. GEMAS: Establishing geochemical background and threshold for 53 chemical elements in European agricultural soil. Appl. Geochem. 2018, 88, 302–318. [Google Scholar] [CrossRef]
  14. Wu, Z.; Hou, Q.; Yang, Z.; Yu, T.; Li, D.; Lin, K.; Ma, X. Identification of factors driving the spatial distribution of molybdenum (Mo) in topsoil in the Longitudinal Range-Gorge Region of Southwestern China using the Geodetector model. Ecotoxicol. Environ. Saf. 2024, 271, 115846. [Google Scholar] [CrossRef]
  15. Hu, R.; Liu, Y.; Tang, C.; Xiao, Y.; Xiang, P.; Rang, Z.; Zhou, Q.; Li, J.; Li, Q. Vertical Distribution of Boron and Molybdenum in Soil and Their Relationship with Organic Matter in Paddy-Tobacco Growing Areas. Chin. Tob. Sci. 2020, 41, 9–15. [Google Scholar] [CrossRef]
  16. Peng, Y.; Yu, X.; Li, Q.; Feng, L.; Zhang, X.; Wang, Y.; Wang, C. Spatial distribution and influencing factors of soil available microelements in high altitude tobacco planting area in Southwest Sichuan. Chin. Tob. Sci. 2018, 39, 39–47. [Google Scholar] [CrossRef]
  17. Han, Z.; Wan, D.; Tian, H.; He, w.; Wang, Z.; Liu, Q. Pollution Assessment of Heavy Metals in Soils and Plants around a Molybdenum Mine in Central China. Pol. J. Environ. Stud. 2018, 28, 123–133. [Google Scholar] [CrossRef]
  18. Imran, M.; Hussain, S.; He, L.; Ashraf, M.F.; Ihtisham, M.; Warraich, E.A.; Tang, X. Molybdenum-Induced Regulation of Antioxidant Defense-Mitigated Cadmium Stress in Aromatic Rice and Improved Crop Growth, Yield, and Quality Traits. Antioxidants 2021, 10, 838. [Google Scholar] [CrossRef]
  19. Frascoli, F.; Hudson-Edwards, K.A. Geochemistry, Mineralogy and Microbiology of Molybdenum in Mining-Affected Environments. Minerals 2018, 8, 42. [Google Scholar] [CrossRef]
  20. Brandely, M.; Coussy, S.; Blanc, D.; Gourdon, R. Assessment of Molybdenum and Antimony speciation in excavated rocks and soils from the Parisian basin using mineralogical and chemical analyses coupled to geochemical modelling. Appl. Geochem. 2022, 136, 105129. [Google Scholar] [CrossRef]
  21. Parnell, J.; Spinks, S.; Andrews, S.; Thayalan, W.; Bowden, S. High Molybdenum availability for evolution in a Mesoproterozoic lacustrine environment. Nat. Commun. 2015, 6, 6996. [Google Scholar] [CrossRef] [PubMed]
  22. Jiang, W.; Yang, Z.; Yu, T.; Hou, Q.; Zhong, C.; Zheng, G.; Yang, Z.; Li, J. Evaluation of the potential effects of soil properties on molybdenum availability in soil and its risk estimation in paddy rice. J. Soils Sediments 2015, 15, 1520–1530. [Google Scholar] [CrossRef]
  23. Liu, B.; Wu, Q. Molybdenum content of soils in Guangxi and the factors affecting it. J. South Agric. 1996, 3, 132–134. [Google Scholar]
  24. Yuan, Y.; Liu, S.; Luo, K.; Liu, Y.; Tang, Y. Study on the Spatial Variation and Driving Factors of Copper, Cobalt, Molybdenum and Zinc in Top Soil of Typical Agricultural Region in Sichuan Basin. Geol. J. China Univ. 2022, 28, 506–515. [Google Scholar] [CrossRef]
  25. Liu, J.; Zhou, L.; Algeo, T.J.; Wang, X.; Wang, Q.; Wang, Y.; Chen, M. Molybdenum isotopic behavior during intense weathering of basalt on Hainan Island, South China. Geochim. Cosmochim. Acta 2020, 287, 180–204. [Google Scholar] [CrossRef]
  26. Yang, P.T.; Wang, S.L. Sorption and speciation of molybdate in soils: Implications for molybdenum mobility and availability. J. Hazard Mater. 2021, 408, 124934. [Google Scholar] [CrossRef]
  27. Wen, X.; Li, L.; Gao, Y.; Lu, X.; Jie, Y.; Xu, Z. Distribution of Soil Available Molybdenum and its Relationship with Soil Factors in the Tobacco Planting Areas of Qujing. Acta Pedol. Sin. 2019, 50, 7. [Google Scholar]
  28. Xia, Y.; Song, Y.; Hou, J.; Zhao, R.; Wang, X. Distribution Law and Influencing Factors of Molybdenum in Soils and Crops in Luoyang, Henan Province. Rock Miner. Anal. 2021, 40, 13. [Google Scholar] [CrossRef]
  29. Liu, G. Erosion Regularity of Purple Soil and Its Control Technology; Sichuan University Press: Chengdu, China, 2008. [Google Scholar]
  30. GB7852-87; Determination of Total Phosphorus in Forest Soil. Standardization Administration of China: Beijing, China, 1988.
  31. GB7854-87; Determination of Total Potassium in Forest Soils. Standardization Administration of China: Beijing, China, 1987.
  32. LY/T1228-2015; Nitrogen Determination Methods of Forest Soil. State Forestry Administration: Beijing, China, 2015.
  33. GB12297-1990; Method for the Determination of Available Phosphorus in Calcareous Soil. Standardization Administration of China: Beijing, China, 1990.
  34. NY/T889-2004; Determination of Exchangeable Potassium and NON-exchangeable Potassium Content in Soil. Agriculture Industry Standard: Beijing, China, 2005.
  35. HJ1315-2023; Soil And Sediment—Determination of 19 Total Metal Elements—Inductively Coupled Plasma Mass Spectrometry. China Ministry of Ecology and Environment: Beijing, China, 2023.
  36. NY/T 1121.9-2023; Soil Testing-Part 9: Method for Determination of Soil Available Molybdenum. Ministry of Agriculture and Rural Affairs of the People’s Republic of China: Beijing, China, 2023.
  37. Liu, B.; Sha, M.; Xie, C.; Zhou, Q.; Wei, X.; Zhou, F. Geochemical characteristics of soils elenium and influencing factors of selenium bioavailability in riceroot soils in Qingxi area, GanxianCounty, Jiangxi Province. Rock Miner. Anal. 2021, 40, 740–750. [Google Scholar] [CrossRef]
  38. Liu, Z.; Zhu, Q.; Tang, L.; Xu, J.; Yin, C. Geographical distribution of trace elements deficient soils in China. Acta Petrol. Sin. 1982, 19, 209–223. [Google Scholar]
  39. Liu, Z.; Zhu, Q. Contents and distribution of Mo in soils of China. Acta Sci. Circumstantiae 1990, 10, 132–137. [Google Scholar] [CrossRef]
  40. Lingang, X.U.; Lehmann, B. Mo and Mo stable isotope geochemistry:Isotope system, analytical technique and applications to geology. Miner. Depos. 2011, 30, 103–124. [Google Scholar]
  41. Gaopin, H. Some problems on using petrochemistry methods in restoring the metamorphic rocks to their original natures. Acta Petrol. Mineral. Anal. 1984, 3, 224–230. [Google Scholar]
  42. Ye, X.; Guo, Y.; Wang, G.; Ge, H. Investigation and analysis of soil plantations of molybdenum in the Tieguanyin tea Fujian Province. J. Plant Nutr. Fertil. 2011, 17, 1372–1378. [Google Scholar] [CrossRef]
  43. Ma, S.; Kuang, F.; Tang, J.; Wang, Y.; Wu, P.; Zhu, B. Effects of Planting Patterns on Soil Structure and Fertility of Calcareous Purple Soil in Hilly Areas of Central Sichuan Basin. Acta Petrol. Sin. 2021, 58, 935–947. [Google Scholar] [CrossRef]
  44. Ding, X.; Yan, N.; Wang, Z.; Li, Z.; Huang, R.; Wang, Y.; Dai, W.; Gao, M. Effects of Four Amendments on Fertility and Labile Organic Carbon Fractions of Acid Purple Soil. Environ. Sci. 2024, 45, 1655–1664. [Google Scholar]
  45. Zhang, L.; Cai, Z.; Wang, H.; Yu, Z.; Han, T.; Liu, K.; Liu, L.; Huang, J.; Wen, S.; Zhang, H. Distribution characteristics of effective medium and micronutrient element contents in paddy soils of China. Trans. Chin. Soc. Agric. Eng. 2020, 36, 62–70. [Google Scholar] [CrossRef]
  46. Li, S.; Li, Q.; Zhang, H.; Wang, C.; Xie, Y.; Li, B.; Jiang, X. Areas with higher pH generally reduce the solubility of soil trace elements, resulting in a decrease in the content of available molybdenum. Soils 2016, 48, 1215–1222. [Google Scholar] [CrossRef]
  47. Yu, J.; Liu, J.; Wang, J.; Qi, X.; Wang, Y. Effects of soil pH value on the effective content of nutrient elements in typical black soil. Rock Miner. Anal. 2003, 34, 404–408. [Google Scholar] [CrossRef]
  48. Blake, L. Acid Rain Adn Soil Acidification. In Encyclopedia of Soils in the Environmrnt; Academic Press: Cambridge, MA, USA, 2005; pp. 1–11. [Google Scholar]
  49. Guo, J.H.; Liu, X.J.; Zhang, Y.; Shen, J.L.; Han, W.X.; Zhang, W.F.; Christie, P.; Goulding, K.W.T.; Vitousek, P.M.; Zhang, F.S. Significant Acidification in Major Chinese Croplands. Science 2010, 327, 1008–1010. [Google Scholar] [CrossRef]
  50. Zhang, X.; Guo, J.; Vogt, R.D.; Mulder, J.; Wang, Y.; Qian, C.; Wang, J.; Zhang, X. Soil acidification as an additional driver to organic carbon accumulation in major Chinese croplands. Geoderma 2020, 366, 114234. [Google Scholar] [CrossRef]
  51. Shinji, M.; Hideki, S.; Takashi, S.; Ikuo, M.; Ginting, J.K.; Rudy, S.G. Effects of Acid Soils on Plant Growth and Successful Revegetation in the Case of Mine Site. In Soil pH for Nutrient Availability and Crop Performance; Suarau, O., Ed.; IntechOpen: Rijeka, Croatia, 2017; p. Chapter 2. [Google Scholar]
  52. Mccauley, A.; Jones, C.; Jacobsen, J.S. Soil pH and Organic Matter. Nutr. Manag. Modul. 2009, 8, 1–12. [Google Scholar]
  53. Dinesh, R.; Srinivasan, V.; Srambikkal, H.; Muthuswamy, A. Massive phosphorus accumulation in soils: Kerala’s continuing conundrum. Curr. Sci. 2014, 106, 343–344. [Google Scholar]
  54. Mu, G.; Luo, J.; Cai, L.; Jiang, H.; He, M.; Wang, Q.; Wang, S.; Wang, H. Relationship between soil trace elements with organic matter and pH in Jiexi County, Guangdong Province. Chin. J. Agric. Resour. Reg. Plan. 2019, 40, 208–215. [Google Scholar]
  55. Deng, X.; Xie, Z. Distribution and application effect of soil molybdenum in Hubei Province. Soils Fertil. Sci. China 1994, 18–22. [Google Scholar]
  56. Hou, M.; Zhang, L.; Wang, Z.; Yang, D.; Wang, L.; Xiu, W.; Zhao, J. Estimation of Fertilizer Usage from M ain Crops in China. J. Agric. Resour. Environ. 2017, 34, 360–367. [Google Scholar] [CrossRef]
  57. Song, H.; Li, T.; Liu, Y.; Huang, L.; Yang, L. Temporal Variation of Main Grain Crops Yield, Import and Exportand Fertilizer Consumption of China in the Past 20 Years. J. Soil Water Conserv. 2023, 37, 332–339. [Google Scholar] [CrossRef]
  58. Rebafka, F.P.; Ndunguru, B.J.; Marschner, H. Single superphosphate depresses molybdenum uptake and limits yield response to phosphorus in groundnut (Arachis hypogaea L.) grown on an acid sandy soil in Niger, West Africa. Fertil. Res. 1993, 34, 233–242. [Google Scholar] [CrossRef]
  59. Wang, Y.; Liang, H.; Xu, Y.; Dong, J.; Wang, C.; Zhang, J.; Peng, D.; Li, Z.; Bao, Z.; Wang, H. Contents of Available M icroelements in Typical Tobacco--planting Soils of Shandong Province. Soils 2014, 46, 172–177. [Google Scholar] [CrossRef]
  60. Xu, X.; Zhang, S.; Yu, N.; Pu, Y.; Li, Y.; Jia, Y.; Li, T. Soil Available Iron Spatial Distribution and Influencing Factors Analysis Based on GIS in Middle Reaches of Tuojian. S. China J. Agric. Sci. 2012, 25, 5. [Google Scholar] [CrossRef]
  61. Zhang, H.; Zhang, X.; Li, Q.; Wang, C.; Zhou, Z.; Xiang, J.; Yi, M.; Yang, Y. Spatial variability of soil available microelement and its influencing factors in Yibin. Acta Pedol. Sin. 2017, 48, 575–582. [Google Scholar] [CrossRef]
  62. Sun, W.; Selim, H.M. Molybdenum-phosphate retention and transport in soils. Geoderma 2017, 308, 60–68. [Google Scholar] [CrossRef]
  63. Cheng, S.; Zhang, J. Effect of different phosphorus (P) levels on molybdenum (Mo) availability in soil. J. Anhui Agric. Sci. 1989, 17, 37–43. [Google Scholar]
  64. Liu, P.; Yang, Y. Research progress of molybdenum in soil and its plant effects. J. Agro-Environ. Sci. 2001, 20, 280–282. [Google Scholar] [CrossRef]
  65. Wu, S.; Wu, J.; Zhao, K.; Zhang, J.; Li, S.; Huang, M.; Li, Y. Effects of irrigation, tillage, and nitrogen application on soil ecological stoichiometry characteristics in dryland. Acta Ecol. Sin. 2024, 44, 10377–10390. [Google Scholar] [CrossRef]
  66. Cheng, X.; Sun, J.; Jia, X.; Liu, X.; Zhao, Y. Pollution characteristics and health risk assessment of heavy metals in farmland soil around the molybdenum mining area in Luanchuan, Henan Province. Geol. China 2023, 50, 1871–1886. [Google Scholar] [CrossRef]
  67. Li, L.; Hu, C.; Tan, Q.; Sun, X. Responses and tolerance mechanisms of plants to molybdenum pollution in soil:A review. J. Argo-Environ. Sci. 2022, 41, 700–706. [Google Scholar]
  68. Edwin, G.A.; Muthu, N. Soil Fertility, Integrated Management, and Sustainability. In Life on Land; Leal Filho, W., Azul, A.M., Brandli, L., Lange Salvia, A., Wall, T., Eds.; Springer International Publishing: Cham, Switzerland, 2021; pp. 939–951. [Google Scholar]
  69. Shi, T.-S.; Collins, S.L.; Yu, K.; Peñuelas, J.; Sardans, J.; Li, H.; Ye, J.-S. A global meta-analysis on the effects of organic and inorganic fertilization on grasslands and croplands. Nat. Commun. 2024, 15, 3411. [Google Scholar] [CrossRef]
  70. Zheng, X.; Wei, L.; Lv, W.; Zhang, H.; Zhang, Y.; Zhang, H.; Zhang, H.; Zhu, Z.; Ge, T.; Zhang, W. Long-term bioorganic and organic fertilization improved soil quality and multifunctionality under continuous cropping in watermelon. Agric. Ecosyst. Environ. 2024, 359, 108721. [Google Scholar] [CrossRef]
  71. Zhang, Y.; Jing, Q.; Cui, J.; Chen, G.; Zhao, X.; Hou, L. Effects of molybdenum-rich super absorbent polymers on soil water retention and soybean seedling growth in typical soils of Northeast China. Sci. Soil Water Conserv. 2024, 22, 63–69. [Google Scholar] [CrossRef]
  72. Rana, M.S.; Alshehri, D.; Wang, R.L.; Imran, M.; Abdellah, Y.A.Y.; Rahman, F.U.; Alatawy, M.; Ghabban, H.; Abeed, A.H.A.; Hu, C.X. Effect of molybdenum supply on crop performance through rhizosphere soil microbial diversity and metabolite variation. Front. Plant Sci. 2024, 15, 1519540. [Google Scholar] [CrossRef]
  73. Padhi, P.P.; Mishra, A.P. The role of molybdenum in crop production. J. Pharmacogn. Phytochem. 2019, 8, 1400–1403. [Google Scholar]
  74. Graf, D.L. Chemical Equilibria in Soils. Clays Clay Miner. 1979, 28, 319. [Google Scholar] [CrossRef]
  75. Smith, K.S.; Balistrieri, L.S.; Smith, S.M.; Severson, R.C.; Gupta, U.C. Distribution and Mobility of Molybdenum in the Terrestrial Environment; Cambridge University Press: Cambridge, UK, 1997; pp. 23–46. [Google Scholar]
Soilsystems 09 00091 g001
Figure 1. Distribution of sampling sites and sample storage site. The geological map was obtained from https://www.osgeo.cn/map/m02d9 (accessed on 12 July 2024). Notes: In the legend, different color schemes represent different geological layers. The yellow color scheme represents the Tertiary period (Glutenite, Oil shale, purple sand conglomerate and not classified), the green color scheme represents the Cretaceous period (Red sand conglomerate, mudstone, Rea siltstone, pebbly sandstone with mudstone and not classified), and the blue color scheme represents the Jurassic period (Red sandstone, mudstone, Purple mudstone, sandstone, Lower series and middle series parallel, Dolomitic limestone with mud sand, Parallel layer or unclassified). The last blue icon represents Upper Triassic and Jurassic costratification.
Figure 1. Distribution of sampling sites and sample storage site. The geological map was obtained from https://www.osgeo.cn/map/m02d9 (accessed on 12 July 2024). Notes: In the legend, different color schemes represent different geological layers. The yellow color scheme represents the Tertiary period (Glutenite, Oil shale, purple sand conglomerate and not classified), the green color scheme represents the Cretaceous period (Red sand conglomerate, mudstone, Rea siltstone, pebbly sandstone with mudstone and not classified), and the blue color scheme represents the Jurassic period (Red sandstone, mudstone, Purple mudstone, sandstone, Lower series and middle series parallel, Dolomitic limestone with mud sand, Parallel layer or unclassified). The last blue icon represents Upper Triassic and Jurassic costratification.
Soilsystems 09 00091 g001
Soilsystems 09 00091 g002
Figure 2. Soil available molybdenum (AMo), total molybdenum content (TMo), and the availability of Mo among parent rock types (a–c). Note: A one-way analysis of variance (ANOVA) and a non-parametric test were performed on all four parameters, setting a significance test of p < 0.05. a–c in figures represent the size ordering of mean comparisons, “a” indicates the maximum mean value, “b” comes next, and “c” represents the minimum mean value.
Figure 2. Soil available molybdenum (AMo), total molybdenum content (TMo), and the availability of Mo among parent rock types (a–c). Note: A one-way analysis of variance (ANOVA) and a non-parametric test were performed on all four parameters, setting a significance test of p < 0.05. a–c in figures represent the size ordering of mean comparisons, “a” indicates the maximum mean value, “b” comes next, and “c” represents the minimum mean value.
Soilsystems 09 00091 g002
Soilsystems 09 00091 g003
Figure 3. Comparison of available molybdenum (AMo), total molybdenum content (TMo), and the availability of Mo of the 1980s and 2024 samples among parent rock types (a–c). Note: A one-way analysis of variance (ANOVA) and a non-parametric test were performed on all four parameters, setting a significance test of p < 0.05. a and b in figures represent the size ordering of mean comparisons, with a–b representing largest to smallest.
Figure 3. Comparison of available molybdenum (AMo), total molybdenum content (TMo), and the availability of Mo of the 1980s and 2024 samples among parent rock types (a–c). Note: A one-way analysis of variance (ANOVA) and a non-parametric test were performed on all four parameters, setting a significance test of p < 0.05. a and b in figures represent the size ordering of mean comparisons, with a–b representing largest to smallest.
Soilsystems 09 00091 g003
Soilsystems 09 00091 g004
Figure 4. Correlations between environmental variables and soil available molybdenum (AMo), total molybdenum (TMo), and availability of molybdenum (MoA). Note: Edge width corresponds to the Mantel’s r statistics for the corresponding distance correlations, and edge colour denotes the statistical significance. The positive and negative relationships between the two variables are represented by dark red and light blue, respectively. The deeper the colour, the stronger the relationship. ns indicates no significant difference; * indicates p  <  0.05; ** indicates p  <  0.01; *** indicates p  <  0.001. PRT—parent rock type; SOM—soil organic matter; TN—total nitrogen; TP—total phosphorus; TK—total potassium; AN—available nitrogen; AP—available phosphorus; AK—available potassium.
Figure 4. Correlations between environmental variables and soil available molybdenum (AMo), total molybdenum (TMo), and availability of molybdenum (MoA). Note: Edge width corresponds to the Mantel’s r statistics for the corresponding distance correlations, and edge colour denotes the statistical significance. The positive and negative relationships between the two variables are represented by dark red and light blue, respectively. The deeper the colour, the stronger the relationship. ns indicates no significant difference; * indicates p  <  0.05; ** indicates p  <  0.01; *** indicates p  <  0.001. PRT—parent rock type; SOM—soil organic matter; TN—total nitrogen; TP—total phosphorus; TK—total potassium; AN—available nitrogen; AP—available phosphorus; AK—available potassium.
Soilsystems 09 00091 g004
Soilsystems 09 00091 g005
Figure 5. RDA of soil available (AMo), total molybdenum (TMo), and availability of Mo (MoA) with environmental factors. Note: K2j, J2s, and J3p represent the parent rock types of the Jiaguan Formation, Shaximiao Formation, and Penglaizhen Formation, respectively. The blue arrow represents the molybdenum content, and the red arrow represents the influencing factor of the molybdenum content.
Figure 5. RDA of soil available (AMo), total molybdenum (TMo), and availability of Mo (MoA) with environmental factors. Note: K2j, J2s, and J3p represent the parent rock types of the Jiaguan Formation, Shaximiao Formation, and Penglaizhen Formation, respectively. The blue arrow represents the molybdenum content, and the red arrow represents the influencing factor of the molybdenum content.
Soilsystems 09 00091 g005
Soilsystems 09 00091 g006
Figure 6. Structural equation modelling (SEM) of soil total and available molybdenum contents in relation to environmental factors. The red and blue arrows indicate the positive and negative relationships, respectively, and the numbers on the arrows are standardised path coefficients in the SEM. The solid and dashed lines indicate significant and non-significant paths, respectively (significance levels are ** p < 0.01, and *** p < 0.001).
Figure 6. Structural equation modelling (SEM) of soil total and available molybdenum contents in relation to environmental factors. The red and blue arrows indicate the positive and negative relationships, respectively, and the numbers on the arrows are standardised path coefficients in the SEM. The solid and dashed lines indicate significant and non-significant paths, respectively (significance levels are ** p < 0.01, and *** p < 0.001).
Soilsystems 09 00091 g006
Soilsystems 09 00091 g007
Figure 7. Fitting relationship among soil molybdenum content in purple soil. (a) represents the fitting of available molybdenum (AMo) and pH, and (b) represents the fitting of total molybdenum (TMo) and pH.
Figure 7. Fitting relationship among soil molybdenum content in purple soil. (a) represents the fitting of available molybdenum (AMo) and pH, and (b) represents the fitting of total molybdenum (TMo) and pH.
Soilsystems 09 00091 g007
Table 1. Comparison of total molybdenum and available molybdenum contents in soils of different parent rock types. The bold font is the molybdenum content of this study, and the non-bold font is the data obtained in the cited references [42].
Table 1. Comparison of total molybdenum and available molybdenum contents in soils of different parent rock types. The bold font is the molybdenum content of this study, and the non-bold font is the data obtained in the cited references [42].
Parent Rock TypeTMo (mg/kg)Amo (mg/kg)
Parafoliate (K2j)0.74 ± 0.200.14 ± 0.03
Mudstone (J2s)0.59 ± 0.030.11 ± 0.02
Shale (J3p)0.88 ± 0.100.07 ± 0.01
Rhyolite2.66 ± 2.460.15 ± 0.13
Diorite5.10 ± 6.050.29 ± 0.35
Sandstone0.84 ± 0.690.05 ± 0.03
Metamorphic rock1.79 ± 0.650.09 ± 0.06
Tuff2.38 ± 1.760.19 ± 0.12
Granite2.34 ± 3.400.18 ± 0.25
Table 2. Soil pH in the 1980s and 2024 under different parent rock types in purple soil. a, b and c in table represent the size ordering of mean comparisons, with a–c representing largest to smallest.
Table 2. Soil pH in the 1980s and 2024 under different parent rock types in purple soil. a, b and c in table represent the size ordering of mean comparisons, with a–c representing largest to smallest.
Group1980s2024Changes
K2j4.82 ± 0.22 c4.85 ± 0.15 c+0.03
J2s7.08 ± 0.09 b6.30 ± 0.24 b−0.78
J3p8.15 ± 0.1 a 7.68 ± 0.26 a−0.47
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Li, X.; Zhou, T.; Li, C.; Wang, X.; Deng, L.; Cui, R.; Sun, X.; Liu, G. Change Characteristics and Driving Factors of Molybdenum Content in Purple Soil from Southwestern China. Soil Syst. 2025, 9, 91. https://doi.org/10.3390/soilsystems9030091

AMA Style

Li X, Zhou T, Li C, Wang X, Deng L, Cui R, Sun X, Liu G. Change Characteristics and Driving Factors of Molybdenum Content in Purple Soil from Southwestern China. Soil Systems. 2025; 9(3):91. https://doi.org/10.3390/soilsystems9030091

Chicago/Turabian Style

Li, Xueqin, Tao Zhou, Chunpei Li, Xuan Wang, Limei Deng, Rongyang Cui, Xiaolin Sun, and Gangcai Liu. 2025. "Change Characteristics and Driving Factors of Molybdenum Content in Purple Soil from Southwestern China" Soil Systems 9, no. 3: 91. https://doi.org/10.3390/soilsystems9030091

APA Style

Li, X., Zhou, T., Li, C., Wang, X., Deng, L., Cui, R., Sun, X., & Liu, G. (2025). Change Characteristics and Driving Factors of Molybdenum Content in Purple Soil from Southwestern China. Soil Systems, 9(3), 91. https://doi.org/10.3390/soilsystems9030091

Article Metrics

Back to TopTop