Marked Spatial Variability in Acidity Characteristics of Purple Soil at Field Scale Induced by Citrus Plantation

Abstract

Purple soil, predominantly found in the Sichuan Basin of China with a favorable climate, is renowned for its fertility, making it an ideal soil for citrus cultivation. To investigate the effect of citrus plantation on the acidification characteristics of purple soil, we selected one field where citrus trees coexist with varying ages of 3, 10, and 50 years. The soil is a neutral purple soil developed from Jurassic Shaximiao Formation mudstone. A total of 138 soil samples were collected at different depths (0–20, 20–40, and 40–60 cm) beneath the canopies of these citrus trees for physicochemical property analysis. Our results indicate that citrus cultivation caused significant spatial variability in the purple soil acidity within the same field. The pH values of these soils varied from 3.97 to 7.90. The degree of soil acidification under the citrus canopies adheres to the following order: 10-year-old > 50-year-old > 3-year-old citrus trees. Soil pH values were negatively correlated with the contents of N, P, and K available in the soil, particularly exhibiting a significantly negative correlation with these soil fertility indicators under the canopy of the 10-year-old citrus at p < 0.01, suggesting that the intensive fertilizer application typical in citrus plantations accelerated soil acidification. Additionally, soil acidification was associated with an increase in the exchangeable Al³⁺ (from 0 to 7.03 cmol kg⁻¹) and a decrease in the exchangeable Ca²⁺ (from 25.07 to 6.48 cmol kg⁻¹), exchangeable Mg²⁺ (from1.53 to 0.62 cmol kg⁻¹), base saturation (from 100% to 53.4%), and effective cation exchange capacity (from 24.3 to 13.1 cmol kg⁻¹).The acidification of the purple soil enhanced the extractability of metal elements, increasing the bioavailability of essential plant nutrients, such as Fe, Mn, Cu, Zn, and Ni, as well as enhancing the mobility of harmful heavy metals like Pb and Cd. In conclusion, unlike the widespread acidification observed in Oxisols or Ultisols at the field scale, citrus cultivation caused varying degrees of acidification within purple soil at this scale. This variability in soil acidification at the field scale of purple soil can lead to a series of soil degradation problems and should be given due attention in the management of citrus and similar high-economic-value fruit trees.

1. Introduction

Advances in agricultural practices and intensified human activities have contributed to soil acidification emerging as a pervasive environmental challenge, notably in southwestern China [1,2]. Severe soil acidification leads to a deficiency of essential nutrients required for plant growth, ultimately reducing the crop yield and quality [3]. Purple soils, classified according to the Chinese Soil Genetic Classification (GSGC) taxonomy, are developed from the weathering of Triassic to Cretaceous sedimentary purple mudstone or sandstone. These soils predominate in the Sichuan Basin of China, covering an area of approximately 160,000 km² [4]. Based on pH levels, purple soils are classified into three subgroups under the GSGC taxonomy, which are acidic purple soil (pH < 6.5), neutral purple soil (pH 6.5–7.5), and calcareous purple soil (pH > 7.5) [5]. Among them, neutral purple soil accounts for approximately 80%. Without the buffering effect of carbonates, neutral purple soil is prone to acidification, with pH values dropping below 6.5. Our previous study reported that a portion of neutral purple soil has been acidified, and the proportion of acidic purple soil has increased [6]. More alarmingly, the relatively high content of negative surface charges of purple soil induced a rise in the amount of H⁺ and Al³⁺ absorbed on the surface of purple soil, thereby markedly intensifying the acidification degree of purple soil [7]. In recent decades, the excessive use of chemical fertilizer and acid deposition have significantly lowered the pH levels of purple soils [8]. As the primary agricultural soil in the Sichuan Basin of China, the acidification of purple soils has received significant attention [9]. Zhang et al. [10] reported that after 25 years of continuous urea and NH₄Cl application, the pH values of slightly calcareous purple soil decreased by 0.9 and 2.0 units, with base cation contents dropping by 0.9% and 16%. Wang et al. [11] found that the imbalance in soil base cations, caused by crop harvests and improper nitrogen fertilizer applications, is the main driver of purple soil acidification. These studies collectively suggest that purple soils are experiencing acidification issues, which could result in crop toxicity due to increased aluminum (Al³⁺) concentrations and the deficiency in soil base cations [7,12]. Hence, exploring the dominant characteristics and driving factors for the acidification of purple soil appears to be particularly important for green agricultural development.

In China, citrus orchards are predominantly situated in southern regions, with the Sichuan Basin being an ideal location for citrus cultivation due to its favorable climate and fertile soil [13,14,15]. High yields in citrus production are largely driven by intensive fertilizer use, a common management practice among smallholder growers who dominate in China [16,17,18]. To maximize productivity, growers often apply extraordinarily large amounts of chemical fertilizers to fruit trees as compared to cereal crops in the same regions. Soil acidification driven by fertilization is at least 10 to 100 times more severe than that by other anthropogenic activities (e.g., acid rain) [19]. Studies have shown that from the 1980s to the 2000s soil acidification was more pronounced in cash crop systems (such as vegetables, fruit trees, and tea), where pH levels decreased by 0.30 to 0.80 units, compared to cereal systems (such as rice, wheat, maize, and cotton), where the decrease was between 0.13 and 0.76 units. Growing citrus trees may render soils more susceptible to acidification than growing cereal crops, primarily due to higher chemical fertilizer inputs [20,21]. Therefore, it is essential to focus more on how fruit tree plantations, such as citrus, affect the acidification process of purple soil.

Unlike cereal fertilization practices, citrus farming employs a distinct approach by applying fertilizers specifically around the canopy drip line [22]. This targeted and intensive use of fertilizers can lead to unintended consequences, such as soil acidification in the immediate vicinity of the citrus tree’s drip line. Consequently, this results in spatial variability in soil acidity at the field scale due to citrus plantations. Purple soil, predominantly found in hilly and low mountain areas, is more suitable for fruit cultivation due to the low profits of grain crop cultivation [23]. However, how citrus plantations affect the acidification process of purple soil remains unclear. Given that neutral purple soil is particularly susceptible to acidification [6], we hypothesized that citrus plantations might be led to the marked spatial variability of soil acidity in neutral purple soil at the field scale. To better understand the impact of citrus plantations on the acidification process of purple soil, we selected a field of this soil type planted with citrus for different durations (3 years, 10 years, and 50 years) to conduct the sampling and analysis. This study provides a better understanding for the green and sustainable development of the citrus industry in the Sichuan Basin of China.

2. Materials and Methods

2.1. Survey and Soil Collection

The study field is located in Yubei District, Chongqing, in the southeastern part of the Sichuan Basin, China (106.493605° E, 29.483937° N) (Figure 1). The soils in this area are classified as purple soils, which have developed from the purple mudstone or sandstone of the Jurassic Shaximiao Formation (J₂s). Typically, these soils exhibit neutral pH values and can be categorized as Inceptisols according to the USDA Taxonomy and Cambisols under the FAO Taxonomy [7]. Three planting durations existed in this field. To better understand the impact of citrus plantation on the acidification process of purple soil, soil samples were collected beneath the canopies of one citrus tree per planting duration. The selected plot has an area of approximately 100 m². The canopy radii of these citrus trees with ages of 3 years, 10 years, and 50 years are 1.0 m, 1.6 m, and 3.0 m, respectively.

All soil samples were collected in July 2023. For each sample tree, we extended a ruler from the trunk to the drip line and then divided the sample locations along this ruler at 20 cm (the 3-year-old and 10-year-old citrus trees) or 30 cm intervals (the 50-year-old citrus trees). At each designated point, soil samples were collected from distinct layers (0–20 cm, 20–40 cm, and 40–60 cm) using a soil auger with an internal diameter of 5 cm. Then, the ruler line was rotated 90 degrees with the trunk as the origin, and the above operation was repeated to complete the second set of sampling. A total of 30, 48, and 60 soil samples were collected from the 3-year-old, 10-year-old, and 50-year-old citrus trees. Prior to further analytical procedures, all soil samples were air dried to a constant weight in a ventilated room (with a temperature of about 22 °C) and then passed through 2 mm sieves after roots and visible organic debris were removed.

Along with the soil sampling, we also investigated the fertilization practices for citrus trees in this orchard. Typically, these citrus trees are fertilized once in March and again in July annually. Chemical compound fertilizers, rather than organic ones, are predominantly used in the orchard. The N–P₂O₅–K₂O ratio of fertilizer is 22-9-9 in spring, while 15-15-15 in summer. After digging the dressing ditches along the drip line, an aliquot of 1 kg and 2 kg of fertilizer was applied to the ditches each time for trees less than 5 years old and over 5 years old. The ditches were dug in another position of the drip line before the next fertilization. Since the chemical fertilizers were applied along the drip line, the annual application rates of fertilizer per meter of drip line were 0.32, 0.40, and 0.21 kg for the 3-year-old, 10-year-old, and 50-year-old citrus tree, respectively. Consequently, the annual soil fertilizer application tends to be relatively higher under citrus trees of 10-years-old compared to those of 3-years-old and 50-years-old.

2.2. Soil Analysis

The properties of soil samples were analyzed in the laboratory based on the description provided by Lu [24]. Soil pH was measured using a potentiometric method (DDS-307, Fangzhou, Guangzhou, China) with a soil-to-water ratio of 1:2.5. Soil organic matter (SOM) was measured by the K₂Cr₂O₇-H₂SO₄ oxidation method. Soil available nitrogen (AN) was determined using the alkali hydrolysis diffusion method. Soil available phosphorus (AP) was extracted using NaHCO₃ and measured by molybdenum–blue colorimetry. Soil available potassium (AK) was extracted using NH₄OAc and measured by flame photometry (AP1401, Aopu, Meizhou, China). Soil exchangeable acidity, H⁺, and Al³⁺ were quantified using the KCl leaching–neutral titration method. Soil exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) were extracted with 1.0 mol L⁻¹ NH₄OAc. The Ca²⁺ and Mg²⁺ concentrations were measured by atomic absorption spectrometry (Z-5000, Hitachi, Tokyo, Japan), and K⁺ and Na⁺ concentrations were measured using flame photometry. Effective cation exchange capacity (ECEC) was calculated as the sum of exchangeable acidity and exchangeable base cations. Base saturation (BS) was computed as the percentage of non-acid saturation, derived from the ratio of exchangeable base cations to ECEC. Soil available metals (Fe, Mn, Cu, Zn, Pb, Cd, and Ni) were measured by atomic absorption spectrophotometry (Z-5000, Hitachi, Tokyo, Japan) equipped with flame and graphite furnace atomizers after DTPA (diethylenetriamine pentaacetic acid) extraction. Soil crystal mineral composition was identified using X-ray diffraction (XRD, XD-3, Persee, Beijing, China).

2.3. Data Statistics and Analysis

The Origin 2021 and SPSS 23.0 software were used for data analysis. A one-way analysis of variance (ANOVA) with the Tukey test was used to test the significant difference among variables. Correlation analysis was conducted using the Pearson correlation coefficient method. In this article, 3a represents the 3-year-old citrus tree, 10a represents the 10-year-old citrus tree, and 50a represents the 50-year-old citrus tree.

3. Results and Discussion

3.1. Variation in Soil Acidity After Planting Citrus for Different Years

The heatmap in Figure 2 illustrates the spatial variation in the soil pH under citrus canopies across different planting years. The pH value of 138 soil samples ranged from 3.97 to 7.90. Specifically, the soil pH ranged from 6.99 to 7.85 beneath the 3-year-old citrus canopy, 3.97 to 7.28 beneath the 10-year-old citrus canopy, and 5.06 to 7.90 beneath the 50-year-old citrus canopy. At depths of 0–20 cm, 20–40 cm, and 40–60 cm the soil pH values for the 3-year-old citrus tree were 7.39 ± 0.17, 7.44 ± 0.18, and 7.51 ± 0.13, respectively. For the 10-year-old citrus tree, the corresponding pH values were 5.61 ± 0.55, 5.64 ± 0.85, and 5.98 ± 0.77. For the 50-year-old citrus tree, the pH values were 6.95 ± 0.39, 7.25 ± 0.37, and 7.29 ± 0.28 (Figure 3). The maximum difference in the soil pH is approximately 4.0 within an area of about 100 m². These results demonstrate a marked field-scale spatial heterogeneity in the soil pH induced by citrus cultivation. Overall, the degree of soil acidification under citrus canopies adheres to the following order: 10-year-old > 50-year-old > 3-year-old citrus trees.

As the soil pH drops, Al³⁺ is released from the soil minerals and adsorbed onto the cation-exchange sites of the soil colloids. Concomitantly, the hydrolysis of Al³⁺ further lowers the pH of the soil solution [25]. In this study, the associations of the soil pH with the exchangeable acidity and exchangeable Al³⁺ in purple soils are illustrated in Figure 4. The acidification of purple soils resulted in an obvious increase in the exchangeable acidity and exchangeable Al³⁺ at pH values below 5.5. When the soil pH dropped to below 5.0, the exchangeable Al³⁺ became the predominant component of exchangeable acidity. The measured maximum values for the soil exchangeable acidity and exchangeable Al³⁺ were 7.03 and 6.50 cmol kg⁻¹, respectively. It is well known that Al³⁺ is highly toxic to most organisms and is responsible for much of the deleterious impact of soil acidity on plants [26]. In acidic soils, Al toxicity inhibits root development and reduces crop yields [27,28]. For instance, Butchee et al. [29] found that soil acidity reduced the grain sorghum yield by 10% at a soil pH of 5.42. Exchangeable Al³⁺ levels above 18 mg kg⁻¹ resulted in yield reductions of 10% or greater. Baquy [30] reported that the critical soil pH and exchangeable Al³⁺ of the Ultisol for wheat were 5.29 and 0.56 cmol kg⁻¹, respectively. Although these critical values vary depending on soil and plant types, some acidified purple soils in this study exhibited exchangeable Al³⁺ concentrations surpassing critical toxicity levels. Consequently, the acidification of purple soils could lead to significant Al³⁺ toxicity, posing a serious threat to plant health and productivity. Therefore, greater attention should be paid to the acidification of purple soils to mitigate these adverse effects.

3.2. Correlation Between Soil pH and Soil Fertility Indicators

In this study, citrus plantation caused a large spatial variation in soil acidity in purple soils. This is primarily due to the targeted and intensive use of fertilizer. This hypothesis can be validated through an examination of the relationship between pH and soil fertility indicators. Since the application of chemical fertilizer directly influences the content of AN, AP, and AK in the soil [31,32]. The correlation between soil pH and four fertility indicators (SOM, AN, AP, and AK) is shown in Figure 5. For all soil samples (Figure 5a), there was a negative correlation between the soil pH and the AN, AP, and AK. Notably, the correlation with AP was significant at p < 0.05, and the correlation with AK was highly significant at p < 0.01. This suggests that soils with higher levels of AN, AP, and AK tend to have lower pH values. More specifically, under the canopy of the 3-year-old citrus tree (Figure 5b), no significant correlation was observed between the soil pH and the three available nutrients. Under the canopy of the 10-year-old citrus tree (Figure 5c), the soil pH showed a highly significant correlation with the three available nutrients at p < 0.01. Under the canopy of the 50-year-old citrus tree (Figure 5d), the soil pH was negatively correlated with the three available nutrients, with the correlations for AP and AK being significant at p < 0.05 and p < 0.01, respectively. The correlation between the soil pH and available nutrients under the canopies of citrus trees with different planting years followed the order of 10 year > 50 year > 3 year, which aligns with the spatial variation in the soil acidity. This is because the annual soil fertilizer application tends to be relatively higher under citrus trees of 10-years-old compared to those of 3-years-old and 50-years-old. Therefore, fertilization practices are the primary factor responsible for the acidification of purple soil under citrus trees of varying ages. Acid purple soils are mainly in the pH buffering stage of cation exchange. The degree of the acidification of purple soil is determined by the adsorption balance between acid (H⁺ and Al³⁺) and base (K⁺, Na⁺, Ca²⁺, and Mg²⁺) cations on soil colloid surfaces [33]. The overuse of chemical fertilizers results in a greater adsorption of acid cations on soil colloid surfaces, thereby exacerbating soil acidification.

3.3. Effects of Soil Acidification on Exchangeable Base Cations

It is well established that a balance exists between cations that cause and do not cause acidification on soil colloid surfaces [34]. Soils become acidic when H⁺ ions in the soil solution displace base cations (K⁺, Na⁺, Ca²⁺, and Mg²⁺) from these surfaces [35]. Consequently, the acidification process signifies the depletion of base cations in the soil. The correlation analysis between the exchangeable acidity and exchangeable base cations in all acidic soils in this work is presented in Figure 6. The results indicated a positive correlation between the three acidity indicators (exchangeable acidity, H⁺, and Al³⁺) and exchangeable K⁺. Notably, the correlation between exchangeable H⁺ and exchangeable K⁺ was statistically significant. This observation can be attributed to the fact that, although soil acidification typically reduces exchangeable K⁺, the excessive application of potassium-containing compound fertilizers exacerbated soil acidification. Given the relatively low proportion of exchangeable Na⁺ among the four base cations, no clear correlation was observed between the soil exchangeable acidity and exchangeable Na⁺. However, the soil exchangeable acidity, H⁺, and Al³⁺ showed a highly significant negative correlation with the exchangeable Ca²⁺ and Mg²⁺. As the soil pH decreased and the soil acidification intensified, the exchangeable Ca²⁺ and exchangeable Mg²⁺ in the soil progressively diminished (Figure 7). After the purple soils underwent acidification (pH < 6.5), the contents of exchangeable Ca²⁺ decreased from 25.07 to 6.48, representing a reduction of 74.1%. Additionally, the content of exchangeable Mg²⁺ decreased from 1.53 to 0.62 cmol kg⁻¹, indicating a reduction of 59.5%.

During the soil acidification process, base cations adsorbed on the surface of soil colloids were progressively displaced by H⁺ and Al³⁺, resulting in a gradual decrease in the BS (Figure 8). Consequently, there was a highly significant and negative correlation between the exchangeable acidity and BS (Figure 6). In this study, due to soil acidification, the BS of purple soils decreased from 100% to 53.4%. Additionally, the exchangeable acidity was also strongly negatively correlated with the ECEC. The decrease in the soil pH resulted in a reduction in the ECEC, ranging from 24.3 to 13.1 cmol kg⁻¹ (Figure 7), indicating that the acidification of the purple soil diminished the amount of surface negative charges. The reduction in surface negative charges can be attributed to the decrease in the constant charge caused by the decomposition of silicate clay minerals and the decrease in the variable negative charge derived from the protonation of functional groups on soil colloids. To elucidate the impact of soil acidification on the mineral composition of purple soil, samples from soils with three different pH levels (4.05, 6.05, and 7.60) were analyzed (Figure 8). The results showed that soil acidification did not obviously alter the mineral composition of purple soil. The primary minerals consistently found in purple soils across varying pH levels include montmorillonite, mica, kaolinite, albite, microcline, quartz, and hematite. Base cations released from the weathering of silicate minerals play a crucial role in alleviating the acidification and even ameliorating the acidity of purple soil. And this could be the reason for the pH rebound observed in purple soil after 50 years of citrus growth in this study. The abundant clay mineral composition helps slow down the further acidification of the purple soil. However, the acidification with spatial heterogeneity due to citrus cultivation can still lead to a decrease in the exchangeable base cations, BS, and ECEC.

3.4. Effects of Soil Acidification on Available Metals

Soil acidification has a profound impact on the soil environment, particularly by enhancing the mobility of metals within the soil [36].

Table 1. Concentrations of available metals at different soil depths under citrus tree canopies with varying planting durations (mg kg⁻¹). Different lowercase letters indicate significant differences among depths for each planting duration at p < 0.05. Data in table are expressed as mean ± standard error.

Years	Depth	Ava. Fe	Ava. Mn	Ava. Cu	Ava. Zn	Ava. Pb	Ava. Cd	Ava. Ni
3	0–20 cm	17.19 ± 9.42 a	19.33 ± 3.72 a	1.21 ± 0.64 a	2.58 ± 0.99 a	0.95 ± 0.20 a	0.07 ± 0.02 a	0.30 ± 0.06 a
	20–40 cm	15.11 ± 11.03 ab	20.32 ± 4.07 a	0.70 ± 0.29 b	1.07 ± 0.42 b	0.76 ± 0.12 b	0.05 ± 0.02 b	0.33 ± 0.10 a
	40–60 cm	8.65 ± 3.33 b	21.85 ± 2.19 a	0.48 ± 0.10 b	0.67 ± 0.37 b	0.71 ± 0.08 b	0.03 ± 0.01 c	0.28 ± 0.02 a
10	0–20 cm	39.38 ± 13.70 a	39.63 ± 4.65 a	1.33 ± 0.49 a	2.40 ± 0.58 a	1.49 ± 0.31 a	0.10 ± 0.02 a	0.83 ± 0.26 a
	20–40 cm	32.00 ± 18.25 ab	40.05 ± 9.68 a	0.68 ± 0.13 b	0.85 ± 0.26 b	1.00 ± 0.32 b	0.04 ± 0.01 b	0.98 ± 0.40 a
	40–60 cm	21.73 ± 14.41 b	35.48 ± 7.79 a	0.52 ± 0.14 b	0.56 ± 0.32 b	0.74 ± 0.31 c	0.02 ± 0.01 c	0.73 ± 0.59 a
50	0–20 cm	21.56 ± 11.80 a	39.50 ± 8.16 a	1.61 ± 0.49 a	3.16 ± 0.80 a	1.15 ± 0.21 a	0.14 ± 0.02 a	0.55 ± 0.28 a
	20–40 cm	12.29 ± 11.34 b	26.05 ± 7.65 b	0.62 ± 0.18 b	0.72 ± 0.29 b	0.64 ± 0.20 b	0.04 ± 0.01 b	0.36 ± 0.36 ab
	40–60 cm	10.31 ± 5.93 b	25.57 ± 13.75 b	0.39 ± 0.16 c	0.57 ± 0.48 b	0.50 ± 0.16 c	0.02 ± 0.02 c	0.25 ± 0.28 b

shows the content of available metals (Fe, Mn, Cu, Zn, Pb, Cd, and Ni) at different soil depths under citrus tree canopies with three cultivation periods. The available metal content was increasing from the deeper to shallower soil layers, which is consistent with the progression of the soil acidification. Li et al. reported similar findings [37] in their study of the effect of soil acidification on metal extractability (Cu, Zn, Pb, and Cd) in old orchards of the Jiaodong Peninsula, China. Decades of horticultural cultivation led to the higher availability of these four heavy metals in topsoil. The correlation analysis between the pH values and available metals across all samples in this work is shown in Figure 9. The results show that the soil pH was negatively correlated with the content of each available metal with the following correlation coefficients: −0.88 ***, −0.87 ***, −0.70 ***, −0.61 ***, −0.22 **, −0.078, and −0.054 for available Ni, Fe, Mn, Pb, Cu, Zn, and Cd, respectively. This indicates a stronger activation effect of the purple soil acidification on the available Fe, Mn, Cu, Pb, and Ni (Figure 10).

Of the seven metals measured, Pb and Cd are usually considered as toxic heavy metals. The soil acidification increased the extractability of both. Nevertheless, China’s risk control standard for the soil contamination of agricultural land (GB 15618-2018) solely establishes limits for the total heavy metal content rather than available forms. Thus, assessing the risk based on the available Pb and Cd levels obtained in this study is rendered unfeasible. The abnormally high level of Fe, Mn, Cu, Zn, and Ni, though essential micronutrients for plant growth, also causes harm to plants [38]. According to the “Grading standards for monitoring indicators of cultivated land quality in major agricultural regions and provinces”, issued by the Cultivated Land Quality Monitoring and Protection Center, Ministry of Agriculture and Rural Affairs of China, the grading standards for the available Fe, Mn, Cu, and Zn in the cultivated land of Chongqing are shown in

Table 2. The grading standards for the available Fe, Mn, Cu, and Zn of the cultivated land in Chongqing, as specified in the “Grading standards for monitoring indicators of cultivated land quality in nine major agricultural regions and provinces” issued by the Cultivated Land Quality Monitoring and Protection Center, Ministry of Agriculture and Rural Affairs of China.

	Upper Limit	Grade 1 (Highest)	Grade 2 (High)	Grade 3 (Medium)	Grade 4 (Low)	Grade 5 (Lowest)
Ava. Fe mg kg−1	-	>20.0	10.0–20.0	5.0–10.0	3.0–5.0	≤3.0
Ava. Mn mg kg−1	-	>30.0	15.0–30.0	5.0–15.0	1.0–5.0	≤1.0
Ava. Cu mg kg−1	15.0	>2.0	1.0–2.0	0.5–1.0	0.2–0.5	≤0.2
Ava. Zn mg kg−1	10.0	>3.0	1.0–3.0	0.5–1.0	0.3–0.5	≤0.3

. It can be seen that the mean values of the available Fe and Mn in the 0–60 cm soil layer with a 10-year cultivation period and in the 0–20 cm soil layer with a 50-year cultivation period are all at Grade 1 level (Table 1 and Table 2). The acidification process of purple soil significantly increased the bioavailability of Fe and Mn. For the three cultivation periods, the contents of available Cu and Zn in the topsoil layer mainly fall into Grade 2, while the contents of available Cu and Zn in the subsoil layer mainly fall into Grade 3. This indicated that the acidification of the topsoil moderately increased the availability of Cu and Zn, but remains well below the upper limit, posing a negligible environmental risk. Therefore, the acidification of purple soil has a dual effect on the activation of metals in the soil. While it increases the bioactivity of heavy metals, like Pb and Cd, it also enhances the bioavailability of essential plant nutrients, such as Fe, Mn, Cu, and Zn.

4. Conclusions

This study confirmed that citrus cultivation over varying durations significantly altered the soil acidity within the same plot of purple soil. Overall, the degree of acidification of the soils under citrus tree canopies adhered to the following order: 10 years > 50 years > 3 years. The variability in the acidification within the same plot was due to the excessive and non-uniform application of chemical fertilizers. Increased soil acidification resulted in higher levels of exchangeable Al³⁺, posing a potential risk of aluminum toxicity to crops. Compared to non-acidified soils within the same plot, the acidified soils exhibited lower levels of the exchangeable Ca²⁺, exchangeable Mg²⁺, BS, and ECEC. Additionally, the acidification of purple soil enhanced the extractability of beneficial metals, particularly Fe, Mn, Cu, Zn, and Ni, while it increased the bioactivity of heavy metals like Pb and Cd. In summary, citrus cultivation can cause the rapid acidification of neutral purple soil over a decade, leading to a series of environmental hazards. To tackle these challenges, it is essential to implement sustainable agricultural practices that can alleviate the acidification of purple soil and safeguard its health in the long term. Specific approaches involve advancing precision fertilization and integrated water-fertilizer technologies, applying adequate lime to regulate soil acidity, and enhancing the use of organic fertilizers.