Impact of Treated Swine Wastewater on Elemental Distribution in the Growth of Habanero Pepper Seedlings

Abstract

The growing global demand for food has driven an increase in both swine and agricultural production, although swine wastewater poses a significant environmental risk. This study employed elemental mapping techniques to evaluate the effects of swine wastewater irrigation on the spatial distribution and concentration of essential and non-essential elements, as well as on the morphological responses of habanero pepper (Capsicum chinense Jacq.) seedlings. Six treatments were tested, ranging from 0% to 100% swine wastewater (T1 = 20%, T2 = 40%, T3 = 60%, T4 = 80%, T5 = 100%, and T6 = control with conventional fertilization), using a completely randomized design with five replications. Emergence, elemental distribution, morphology, and seedling quality were evaluated. The highest emergence percentages and rates were observed in the 20% wastewater treatment and the control groups. Diluted wastewater treatments promoted potassium and calcium uptake, which correlated with improved seedling growth and vigor, while irrigation with 100% wastewater led to excessive chlorine and sulfur accumulation, negatively affecting morphology. These results indicate that the controlled dilution of swine wastewater optimizes nutrient availability and seedling development, offering an environmentally sustainable alternative for producing high-quality habanero pepper seedlings. This study provides novel insights into the environmental implications of swine wastewater reuse through elemental mapping, underscoring its potential to support sustainable and regenerative agriculture.

1. Introduction

In Mexico, pork is one of the most widely consumed meats, and the pig farming industry has grown significantly in regions such as Yucatán due to its accessibility and economic impact [1]. However, this expansion has resulted in an increased generation of swine wastewater, with over 6,000,000 m³ produced annually in Yucatán alone. Of this volume, more than 30% is discharged untreated, often onto highly permeable karstic soils, posing significant risks in terms of groundwater contamination and public health concerns [2,3].

These discharges contribute to environmental problems such as organic matter accumulation, increased soil salinity, and elevated concentrations of heavy metals [4,5]. Although these issues pose significant risks, swine wastewater also contains high levels of organic matter and mineral nutrients that could be harnessed as biofertilizers [6]. Numerous studies have demonstrated the potential of liquid manures and organic leachates to enhance crop performance. Positive effects have been reported in tomato, maize, and husk tomato crops [7]. Nevertheless, appropriate management strategies are necessary to balance agronomic benefits with the potential environmental risks.

In Capsicum plants, improved growth has been observed following applications of liquid swine excreta [8]. In maize seedlings, bovine manure leachate positively affected both plant height and fresh weight [9]. The use of swine waste residues, particularly in a liquid effluent form, may offer additional agronomic advantages due to their content of essential nutrients such as phosphorus (P), potassium (K), and calcium (Ca), which are vital for plant growth and development [10]. Furthermore, bovine manure applications have been shown to increase nitrogen (N), P, and K concentrations in tomato plants [11]. Similarly, goat manure leachates have enhanced plant height and leaf number in Capsicum annuum L. [12].

Despite these benefits, limited information is available regarding the specific effects of different concentrations of untreated swine wastewater on the physiology, nutrient uptake, and early development of C. chinense seedlings. Understanding these responses is essential to evaluate the agronomic and environmental feasibility of reusing such effluents for horticultural crops, particularly because the seedling stage is critical for plant establishment, growth, and eventual yield. Additionally, interactions between the chemical components of wastewater and plant physiological processes can directly impact fertilizer efficiency and the risks of phytotoxicity or toxic element accumulation.

In this context, international frameworks and guidelines, including those from the FAO and WHO, emphasize the importance of safe wastewater reuse in agriculture to address global challenges such as water scarcity, nutrient recycling, and food security. The Sustainable Development Goals (particularly SDG 6) call for improved wastewater treatment and reuse to reduce pollution and protect ecosystems [13,14]. While this study centers on the habanero pepper, a high-value crop in southeastern Mexico, its findings contribute to the broader discussion on sustainable wastewater management practices in regions facing similar agricultural and environmental pressures.

Therefore, the objective of this study was to evaluate the impact of swine wastewater on the morphology, elemental concentration, and spatial nutrient distribution in habanero pepper (C. chinense Jacq.) seedlings. This research aims to determine the potential use of swine wastewater as a sustainable fertilization strategy that promotes nutrient recycling while reducing environmental and public health risks.

2. Materials and Methods

2.1. Study Site

The experiment was carried out at the Faculty of Engineering of the Autonomous University of Yucatán, specifically within the Environmental Engineering Laboratory. A greenhouse structure enclosed with mesh and covered with plastic sheeting was constructed to cultivate habanero pepper (C. chinense Jacq.) seedlings irrigated with treated swine wastewater.

2.2. Swine Wastewater Collection and Characterization

The swine wastewater used in this study was collected from a pig farm selected based on its size, production scale, location, compliance with regulatory standards, and the presence of an on-site wastewater treatment plant (WWTP). The WWTP system includes a pumping station, solids separator, anaerobic biodigester, and facultative lagoons equipped with aeration and zoning. The wastewater for the experiment was obtained after the final stage of treatment. The chemical oxygen demand (COD) analysis was performed on the same day as sample collection. The remaining samples were stored in a cold room at 4 °C for one day prior to further analysis of the evaluated variables, ensuring consistency of the results. According to Mexican regulations (NOM-001-SEMARNAT-2021 and NOM-003-ECOL-1997), this treated swine wastewater meets the requirements for agricultural reuse. Specifically, key parameters such as the chemical oxygen demand (COD), biological oxygen demand (BOD), suspended solids, and total phosphorus, among others, were within the permissible limits set by these standards, ensuring the safety and suitability of the wastewater for irrigation purposes.

2.3. Experimental Design and Plant Material

The experimental design was a completely randomized design (CRD) with five replicates per treatment. Each replicate consisted of 10 seedlings. Seeds of the habanero pepper (C. chinense Jacq.) were sown individually into polystyrene trays (200 cells) filled with peat-moss-based substrate (Green Forest Mexico). After germination, seedlings were irrigated with different treatments: T1 = 20% wastewater + 80% water; T2 = 40% wastewater + 60% water; T3 = 60% wastewater + 40% water; T4 = 80% wastewater + 20% water; T5 = 100% wastewater; and T6 = control (100% water, grown with conventional fertilization). Irrigation was performed three times per week over a period of 40 days. The control group received a conventional nutrient solution with 190 mg L⁻¹ of NPK (Poly-Feed 19-19-19, Haifa Chemicals, Mexico City, Mexico). The experiment was conducted during the spring–summer season. Average environmental conditions were 32 ± 3 °C daily temperature and 60% relative humidity. The seedlings were grown under natural light conditions, with an approximate photoperiod of 13 h of light and 11 h of darkness, which is typical for this time of year in the region.

2.4. Evaluated Variables

In the swine wastewater and its dilutions according to each treatment, physicochemical parameters were determined. The pH and electrical conductivity (EC) were measured using a pH meter and conductivity meter, respectively. The chemical oxygen demand (COD) of suspended solids (SSs) was determined according to the method by Knechtel [15]. Total Kjeldahl nitrogen (TKN) was determined by the Kjeldahl method. Total phosphorus (TP) was measured by direct colorimetric analysis in accordance with the current regulations. For the seedlings, physiological variables such as seedling emergence and the emergence rate were evaluated, as well as morphological and growth variables, including the elemental distribution and concentration, as well as seedling quality indices.

2.5. Emergence Test and Emergence Rate Index

Seedling emergence and the emergence rates were evaluated in polystyrene trays filled with the germination substrate. One seed was placed in each cell, and a seedling was considered to have emerged when the hypocotyl hook was visible. The count was conducted daily for a period of seven days. The variables were calculated according to the method outlined by Hernández-Pinto [16].

2.6. Evaluation of the Morphological Traits of the Seedlings

Seedling height was measured from the base to the tip of the apical shoot using a tape measure. At 40 days after emergence, seedlings were harvested and separated into roots and shoots (leaves + stem). Each section was oven dried at 70 °C for 3 days and then weighed to determine the dry biomass. Based on these data, the Shoot-to-Root Ratio (SRR) and the Dickson Quality Index (DQI) were calculated, following the method described by Dickson [17]. The DQI was calculated using the total dry biomass of the seedlings and is based on the relationship between height, stem diameter, and biomass distribution, providing a comprehensive measure of seedling morphological quality.

2.7. Elemental Analysis of Seedlings

The elemental analysis was conducted 40 days after seedling emergence. Five seedlings per treatment were selected and placed on graphite plates. Analyses were carried out using an X-ray microfluorescence system (M4 TORNADO, Bruker, Billerica, MA, USA) operating at 50 kV and a 3 cm working distance. The entire surface of each seedling was scanned to determine the distribution and concentration of K, Ca, P, Cl, and S. These elements were selected due to their critical physiological roles in early plant development: K, Ca, and P are essential macronutrients involved in cell division, elongation, and energy metabolism, while Cl and S, although required in smaller quantities, are vital for photosynthesis, osmotic regulation, and amino acid synthesis. Elements such as nitrogen (N) were excluded because the equipment cannot reliably detect low-atomic-number elements (Z < 11), and magnesium (Mg) yielded inconsistent signals during preliminary testing. The instrument was calibrated using certified multi-element reference standards (Bruker XRF Calibration Standards), and measurements were performed in the spot mode with background and matrix corrections applied.

2.8. Statistical Analysis

Percentage data were transformed using the arcsine square root method. An analysis of variance (ANOVA) was performed for the evaluated variables at a significance level of p ≤ 0.05. When significant differences were found between treatments, the means were compared using Tukey’s test (α = 0.05). All statistical analyses were conducted using Statistica version 7 (StatSoft, Tulsa, OK, USA).

3. Results

3.1. Physicochemical Parameters of Swine Wastewater

Swine wastewater treatments did not show statistically significant differences in pH (p = 0.1781). Statistically significant differences in EC were observed among treatments (p = 0.0199). Regarding electrical conductivity (EC), the control group had the lowest value (878 µS/cm). EC increased with the proportion of swine wastewater, with the T1 treatment group registering 2120 µS/cm and the T5 treatment group (100% wastewater) showing the highest value at 7233 µS/cm—an increase of over 720% compared to the control. A similar trend was observed for chemical oxygen demand (COD); treatments with a higher proportion of wastewater showed the highest values (T5 = 563 and T4 = 485 mg/L), with the organic load in the wastewater influencing the COD concentrations in the treatments. The concentration of total Kjeldahl nitrogen (TKN) increased progressively with the proportion of swine wastewater in the treatments. The highest TKN concentration was observed in the T5 treatment group (100% wastewater), which reached 11.5 mg/L—representing a 98% increase compared to the control group (T6 = 5.8 mg/L). On the other hand, the total phosphorus (TP) results were reversed: the T1 treatment group (11.2 mg/L), with the lowest percentage of wastewater, had higher values compared to the other treatment groups, while those with higher wastewater concentrations, as well as the control group, showed lower values (

Table 1. Physicochemical parameters of swine wastewater and wastewater dilutions in the treatments.

Treatments	pH	EC (µS/cm)	COD (mg/L)	TKN (mg/L)	TP (mg/L)
T1: 20% wastewater + 80% water	7.53 NS	2120 ± 0.11 e	144 ± 0.17 d	136 ± 0.2 e	11.2 ± 0.12 a
T2: 40% wastewater + 60% water	8.55 NS	3437 ± 0.13 d	201 ± 0.22 c	247± 0.3 d	9.80 ± 0.15 a
T3: 60% wastewater + 40% water	8.7 NS	4612 ± 0.17 c	290 ± 0.27 b	422 ± 0.33 c	8.11 ± 0.21 b
T4: 80% wastewater + 20% water	8.91 NS	5700 ± 0.24 ab	485 ± 0.41 a	571 ± 0.52 b	5.34 ± 0.33 c
T5: 100% wastewater	9.20 NS	7233 ± 0.31 a	563 ± 0.55 a	711 ± 0.74 a	6.63 ± 0.38 c
T6: 100% water (control)	7.0 NS	878 ± 0.034 f	5 ± 0.01 e	5.8 ± 0.1 f	6.0 ± 0.09 c
p-value	0.1781	0.0199	0.0398	0.0297	0.0241

pH: hydrogen ion concentration in the water; EC: electrical conductivity; COD: chemical oxygen demand; TKN: total Kjeldahl nitrogen; TP: total phosphorus. The data are means ± SE. Different letters in the same column indicate statistically significant differences between treatments (Tukey’s test, p < 0.05). NS: Not significant.

). The percentage of swine wastewater in the various treatments influenced the physiological characteristics of the seeds and seedlings of the habanero pepper plant.

3.2. Seedling Emergence and the Emergence Rate

Significant differences were found between treatments for the physiological variables of seedling emergence (p = 0.0020). The T1 treatment (97%) and control (98%) groups showed higher emergence values compared to the other treatment groups. Similarly, the emergence rate was highest in the T1 treatment and the control groups (p = 0.0056), with 13.09 and 13.58 seedlings per day, respectively. For both variables, increasing the percentage of swine wastewater negatively affected seed vigor, reducing both the emergence percentage and seedlings per day. However, the 20% wastewater dilution treatment (T1) enhanced seedling emergence, resulting in the highest similarly compared to the control group emergence percentage (

Table 2. Seedling emergence percentage and rate for habanero pepper (C. chinense Jacq.) seedlings irrigated with swine wastewater.

Treatments	Seedling Emergence (%)	Emergence Rate (Seedling/Day)
T1: 20% wastewater + 80% water	97 ± 0.06 a	13.09 ± 0.01 a
T2: 40% wastewater + 60% water	87 ± 0.05 ab	9 ± 0.03 b
T3: 60% wastewater + 40% water	81 ± 0.06 bc	5.1 ± 0.01 c
T4: 80% wastewater + 20% water	78 ± 0.87 bc	3.77 ± 1.04 c
T5: 100% wastewater	70 ± 2.0 c	3.10 ± 1.51 c
T6: 100% water (control)	98 ± 0.03 a	13.58 ± 0.01 a
p-value	0.0020	0.0056

The data are means ± SE. Different letters in the same column indicate statistically significant differences between treatments (Tukey’s test, p < 0.05).

).

3.3. Distribution and Concentration of Essential and Non-Essential Elements in Habanero Pepper Seedlings

The distribution and concentration of essential elements in habanero pepper seedlings were affected by the dilution levels of swine wastewater. K accumulation was particularly influenced, with the T1 treatment and the control (T6) groups showing a greater K presence in actively growing tissues such as stem and root meristems. Conversely, higher concentrations of wastewater were associated with reduced K accumulation in these zones (Figure 1).

Ca, similar to K, was distributed in the growth areas of the seedlings, as it is an element that acts as a metabolic activator and is involved in plant growth. In the treatment groups with 20% and 40% swine wastewater (T1 and T2), a higher distribution was observed in the growth meristems, even higher than in the control (T6) group. The distribution was also found in the root, but in smaller amounts, while the T5 treatment group showed the lowest distribution (Figure 2).

P, being a mobile element, was found in the stem and roots of the seedlings. A similar distribution was observed in most treatment groups (T1, T2, T3, and the control), except for the T4 and T5 treatment groups, which showed lower elemental distribution. This could be due to inhibition of element absorption as a result of a higher concentration of another element, which was observed in the smaller seedlings obtained from these treatment groups (Figure 3).

Cl, despite being a non-essential element for plants, is generally required in low concentrations. The T5, T4, and T3 treatment groups showed a greater distribution of this element. This is attributed to the higher concentrations of Cl present in swine wastewater, which originates from activities such as pen cleaning on pig farms. Consequently, seedlings in treatment groups with higher wastewater dilutions exhibited increased concentrations of the analyzed elements. In contrast, the T1 treatment group, T2 treatment group, and the control group showed lower elemental concentrations (Figure 4).

Elemental analysis showed elevated sulfur levels in treatments groups with higher wastewater concentrations (T5 and T4). Prolonged exposure to wastewater impaired seedling growth and reduced survival due to toxicity. In contrast, the T1 treatment group, T2 treatment group, and control (T6) group exhibited lower sulfur levels, which corresponded to better seedling development (Figure 5).

The swine wastewater treatments significantly influenced the concentration of essential nutrients (K, Ca, and P) in habanero pepper seedlings. K levels peaked in the T1 treatment group (4.5 mg g⁻¹), representing a 44% increase over the control (T6), and this was more than 60% higher than in the highest-concentration treatment group (T5, 1.74 mg g⁻¹) (Figure 6a). Ca was most abundant in the T1 treatment group (1.24 mg g⁻¹), with slightly lower values in the T2 (1.17 mg g⁻¹) and T3 (1.04 mg g⁻¹) treatment groups, and markedly reduced levels in the T4 and T5 (Figure 6b) treatment groups. P showed a similar trend, with the highest concentration in the T1 (0.05 mg g⁻¹) treatment group, followed by the control (T6, 0.04 mg g⁻¹) group, while the T4 and T5 (both 0.03 mg g⁻¹) treatment groups had the lowest concentrations (Figure 6c). These results highlight the importance of wastewater dilution for enhancing nutrient uptake

The highest concentrations of Cl and S were recorded in the T5 (0.44 and 0.07 mg g⁻¹, respectively) and T4 (0.40 and 0.06 mg g⁻¹, respectively) treatment groups, likely associated with the intensive use of additives and disinfectants in swine farming practices, which elevate their levels in wastewater. In contrast, the T1 treatment group, T2 treatment group, and control group exhibited significantly lower Cl and S concentrations (Figure 7). In the T5 treatment group, the elevated levels of these elements induced toxicity symptoms, including seedling mortality.

3.4. Morphological Characteristics in Habanero Pepper Seedlings

The differences (p = 0.0421) in the carbon/nitrogen (C/N) ratio were observed in response to the swine wastewater concentration. The T5 and T4 treatment groups, which received the highest concentrations, showed elevated C/N ratios of 4.2 and 4.07, respectively, suggesting a higher nitrogen content in the leaf tissues. However, these treatments did not correspond to greater morphological development. In terms of seedling height, the greatest values were recorded in the T1 (8.11 cm) treatment group and the control (T6, 7.87 cm) group, while the T5 treatment group exhibited the lowest height (3.31 cm). The T1 treatment group also demonstrated a greater dry biomass in the stem, root, and total plant weight (130.94, 41.83, and 172.77 mg, respectively), outperforming all other treatments, including the control group (

Table 3. Morphological characteristics of habanero pepper seedlings irrigated with swine wastewater and its dilutions.

Treatments	C/N	Height (cm)	Stem Dry Weight (mg)	Root Dry Weight (mg)	Total Dry Weight (mg)	Slenderness Index	Dickson Quality Index
T1: 20% wastewater + 80% water	3.75 ± 0.09 b	8.11 ± 0.06 a	130.94 ± 0.003 a	41.83 ± 0.001 a	172.77 ± 0.003 a	4.26 ± 0.04 a	0.23 ± 0.001 a
T2: 40% wastewater + 60% water	3.90 ± 0.14 b	7.25 ± 0.08 bc	107.63 ± 0.004 b	26.39 ± 0.002 b	134.02± 0.004 b	3.77 ± 0.05 b	0.15 ± 0.005 b
T3: 60% wastewater + 40% water	3.97 ± 0.22 ab	6.68 ± 0.08 c	105.18 ± 0.005 b	30.43 ± 0.003 b	135.63 ± 0.003 b	3.73 ± 0.06 b	0.19 ± 0.06 ab
T4: 80% wastewater + 20% water	4.07 ± 0.30 a	6.46 ± 0.10 c	96 ± 0.007 b	22.87 ± 0.003 b	118.87 ± 0.006 b	3.59 ± 0.08 b	0.017 ± 0.09 b
T5: 100% wastewater	4.20 ± 0.55 a	3.31 ± 0.32 d	58.84 ± 0.006 c	6.85 ± 0.010 c	65.69 ± 0.008 c	2.69 ± 0.24 c	0.05 ± 0.11 c
T6: 100% water (control)	3.40 ± 0.01 c	7.87 ± 0.05 a	112.51 ± 0.002 ab	41.85± 0.001 a	154.36 ± 0.003 ab	3.85 ± 0.05 ab	0.22 ± 0.001 a
p-value	0.0421	0.0362	0.0237	0.0473	0.0197	0.0234	0.0452

C/N: carbon-to-nitrogen ratio. Data are presented as means ± SE. Different letters within the same column indicate statistically significant differences between treatments (Tukey’s test, p < 0.05).

).

The morphological characteristics obtained led to the production of high-quality seedlings, as evidenced by the higher values of the Stem Index and Dickson Quality Index. The 20% swine wastewater treatment group recorded the best values (4.26 and 0.023, respectively), which were statistically similar to the control group (3.85 and 0.022, respectively), but 36% higher compared to the T5 treatment group, which had the lowest indices (2.69 and 0.005, respectively) (Table 3).

4. Discussion

4.1. Swine Wastewater

The pH and EC values are critical parameters influencing nutrient availability and uptake in plants. The swine wastewater and its dilutions used in this study showed variations within these parameters, which can fluctuate depending on factors such as pig age, diet, and the treatment system employed [18]. Treatments with 20% swine wastewater (T1 = 7.5) and the control (T6 = 7.0) maintained a neutral pH, and their EC values were below 3000 µS/cm (Table 1). These conditions favored the balanced availability and absorption of nutrients by the seedlings. According to the Pérez-Gutiérrez [8], these parameters tend to decrease with lower proportions of swine effluents and increase with higher concentrations. Conversely, high pH and EC values (>3000 µS/cm) can reduce nutrient uptake efficiency and disrupt plant–nutrient interactions, ultimately leading to nutritional imbalances that adversely affect plant growth, development, and productivity [19,20]. This was evident in seedlings under treatment T5, where pH and EC values exceeded optimal thresholds (9.2 and 7233 µS/cm, respectively), resulting in reduced growth (Table 1 and Table 3).

Regarding chemical oxygen demand (COD), 100% swine wastewater (T5) exhibited the highest organic load among all treatments. Nevertheless, the observed values were considerably lower than those reported by the Valdez-Vázquez [6], who documented COD levels ranging from 3000 to 9000 mg/L in similar effluents. Such variations may be attributed to differences in farm size, herd density, feeding practices, and waste management strategies [6]. In the T1 treatment group, dilution of the wastewater reduced the organic load by 74%, resulting in a COD value of 144 mg/L, while the control registered <20 mg/L (Table 1), with both values being notably lower than those reported in the literature.

Swine wastewater typically contains high concentrations of total Kjeldahl nitrogen (TKN), as observed in treatment T5 (711 mg/L), which is primarily attributed to the mixture of urine and undigested feed from pigs. Although the values obtained in this study are lower than those reported by the Bautista [3], who found TKN concentrations ranging from 1260 to 2861 mg/L, they still exceed the permissible limits established by the NOM-001-SEMARNAT-2021 standard, which ranges between 15 and 40 mg/L. The 20% dilution of swine wastewater (T1) effectively reduced TKN levels, facilitating the availability of nitrogen for seedling uptake (Table 1).

Regarding total phosphorus (TP), lower concentrations of wastewater in the treatments increased the availability of the element. Treatment T1 recorded the highest value (11.2 mg/L), which is notably higher than the 4.45 mg/L reported for swine effluents by Cárdenas [18]. Greater phosphorus availability contributed to improved seedling growth and vegetative development (Table 1). Several studies have shown that liquid leachates from livestock manure, containing appreciable concentrations of nitrogen, phosphorus, and potassium, enhance growth in tomato and C. annuum (chile xcat’ik) plants [11,21].

4.2. Physiological Responses of Habanero Pepper Seeds

Seeds imbibed with diluted swine wastewater, particularly at 20% concentration (T1), exhibited a significant increase in the emergence rate (97%) and uniformity (13.09 seedlings day⁻¹) compared to other treatments (Table 2). The mineral content present in the diluted wastewater likely facilitated the uptake of key nutrients, such as N, K, Ca, and P, enhancing seed metabolic activity and promoting faster and more-uniform seedling development. It has been reported that the assimilation and accumulation of essential minerals like N, P, K, and iron during early germination stages can influence internal physiological processes, thereby enhancing vigor and seedling emergence [22,23]. However, excessive nutrient concentrations, high organic load, and elevated electrical conductivity (EC) can produce phytotoxic effects, compromising seed performance and viability [19]. Such negative impacts were evident in the T5 treatment group, which, despite its high elemental and organic content, resulted in poor physiological performance across the evaluated variables (Table 2).

4.3. Distribution and Concentration of Essential Elements in Habanero Pepper Seedlings

Swine wastewater concentration significantly affected the distribution of essential nutrients in habanero pepper seedlings. Treatments T1 (20%) and T2 (40%) showed a higher accumulation of K, Ca, and P, particularly in active growth regions such as the apical and root meristems (Figure 1 and Figure 3). These nutrients are involved in key physiological processes, including photosynthesis, protein synthesis, and energy metabolism [24]. Moderate dilution of the effluent improved nutrient availability and uptake, resulting in enhanced morphological development. In contrast, higher wastewater concentrations appeared to hinder efficient absorption, likely due to the excessive organic load.

Previous studies have shown that nutrient-rich liquid leachates from organic manures enhance the distribution and accumulation of essential elements such as N, P, and K in tomato leaves [11], and also improve SPAD readings in pepper plants following bovine manure applications [23]. Similarly, Calero-Hurtado [25] reported improved growth in bean and cotton plants irrigated with leachates derived from vegetable waste and manures. Swine manure, when properly treated, contains valuable nutrients required for plant development and can serve as a viable alternative to costly chemical fertilizers [8].

In this context, the favorable elemental distribution observed in the T1 and T2 treatment groups, especially in growth regions (Figure 1, Figure 2 and Figure 3), underscores the importance of nutrient accessibility during the early developmental stages. Essential elements like N, K, Mg, P, Ca, and Fe are closely linked to photosynthetic activity and energy transference in plant systems [26]. A reduced elemental distribution observed in the T4 and T5 treatment groups limited photosynthetic activity, metabolic processes, and energy transference in the seedlings, which was reflected in diminished vegetative growth (Figure 1, Figure 2 and Figure 3). The excessive organic and nutrient load in undiluted wastewater (T5) appeared to hinder rather than promote elemental distribution, negatively affecting seedling development.

Non-essential elements such as C and S are typically required in minimal amounts. However, treatments with higher swine wastewater concentrations of 80% (T4) and 100% (T5) resulted in the increased accumulation of Cl and S, particularly in the apical meristems (Figure 4 and Figure 5). This elevated presence is likely due to the high concentrations of these elements in the wastewater, stemming from pig-farm management practices. Excess Cl and S can disrupt the plant’s ionic balance, inhibiting the uptake and translocation of essential nutrients. Such imbalances may induce nutrient deficiencies, water stress, or toxicity, ultimately reducing biomass accumulation and plant growth [20]. In contrast, treatment groups with diluted wastewater (T1 and T2) and the control group showed lower Cl and S levels, which likely allowed for more favorable nutrient interactions. Although Cl and S contribute to secondary metabolism and various physiological functions, their excessive accumulation can negatively impact plant development [27].

Dilutions of swine wastewater at 20% (T1) and 40% (T2) resulted in higher concentrations of K, Ca, and P compared to the other treatment groups, even surpassing the control group (T6) (Figure 6). The reduced organic load in these diluted treatments likely facilitated greater nutrient bioavailability and uptake, contributing to enhanced vegetative growth. Adequate levels of nutrients, especially N, P, K, Ca, and Fe, are essential for critical physiological processes such as photosynthesis. Therefore, aligning nutrient concentrations with the crop’s phenological stage is vital [26,28]. Nitrogen and phosphorus, in particular, play central roles in photosynthetic efficiency and energy transfer within plant metabolic pathways [12,24]. Similar findings have been reported in C. annuum L., where organic amendments, such as goat manure leachate, improved plant height and leaf number due to increased nutrient availability [12]. Conversely, excessive concentrations of raw wastewater (T4: 80%, T5: 100%) were associated with lower nutrient uptake and limited plant development, likely due to the high organic load and potential phytotoxic effects (Figure 6).

The higher organic and nutrient load in the T4 and T5 treatment groups did not significantly enhance the concentration of essential elements but did increase the absorption and accumulation of S and Cl in the seedlings (Figure 7). These elevated levels likely interfered with the uptake of other nutrients, limiting overall plant growth. However, both S and Cl are generally required in small quantities by most plant species and their supply is often naturally sufficient through rainfall [29]. In agricultural systems, plants primarily acquire Cl⁻ from soil solutions via anion channels and active transport mechanisms in root cells. Additionally, Cl⁻ is commonly introduced through irrigation water, organic amendments, and various fertilizer formulations, including potassium chloride (KCl), calcium chloride (CaCl₂), and ammonium chloride (NH₄Cl). The contribution of these sources varies depending on the crop species, soil characteristics, and environmental conditions [30]. Cl is essential for photosynthesis and tends to accumulate in the chloroplasts of leaf cells [31], but excessive concentrations can damage cellular structures, impairing photosynthetic efficiency [32]. In this study, seedlings in the T1 and T2 treatment groups, which had lower Cl levels, exhibited better growth. Similarly, the lower sulfur concentrations in these treatment groups corresponded with improved morphological development, while higher concentrations in the T4 and T5 treatment groups negatively affected plant performance. Previous research indicates that sulfur levels can influence biomass production, morphology, yield, and nutritional quality in various crops [33].

4.4. Morphological Responses of Habanero Chile Seedlings Irrigated with Pig Wastewater

Pig wastewater negatively affected the carbon-to-nitrogen (C/N) ratio in the treatments, primarily due to its high nitrogen content, which led to increased nitrogen uptake by the seedlings. Moderate nitrogen levels can promote growth, but excess nitrogen causes nutritional imbalances [31]. This effect was evident in the T5 (100% wastewater) treatment group, where seedlings exhibited ionic imbalances that impacted their morphology (Table 3). Treatment groups with diluted wastewater and the control group showed a narrower C/N range of 3.4–3.7, which correlated with a greater height and biomass accumulation (Table 3). Despite the low C/N ratio, nitrogen availability enhanced protein synthesis and other growth-related processes [33].

In the T1 (20% wastewater) treatment group, the lower wastewater concentration improved the seedling height and dry biomass in the stems, roots, and total biomass (Table 3). Favorable physicochemical properties of the wastewater supported elemental interactions that promoted growth (Table 1 and Table 3). The reuse of wastewater in agriculture can provide essential nutrients for crop development. In this study, nutrient content in swine wastewater was a key factor in the growth of habanero pepper seedlings. However, at higher concentrations, the excess organic matter, minerals, and metals negatively affected root systems, inhibiting nutrient uptake and reducing plant development, health, or overall system quality [34].

The use of liquid manure and leachates in agriculture has proven beneficial for seedling production in nurseries and serves as an alternative cultivation method [35]. In pepper plants, the application of goat manure leachate enhanced plant height and leaf number, attributed to its high nutrient content [12]. Similarly, bovine manure leachates improved growth in bean (Phaseolus vulgaris L.) and cotton (Gossypium hirsutum) plants [25,35].

Seedling growth and development parameters, such as the Seedling Survival Rate (SSR) and the Dickson Quality Index (DQI), were fundamental indicators of seedling quality. The treatment group with 20% pig wastewater (T1) yielded the best morphological response, surpassing other treatment groups including the control group (T6) (Table 3). These findings align with previous research on bovine and ovine manure leachates applied to C. annuum L. and coffee seedlings, where the SRR ranged from 4.0 to 4.7 and the DQI from 0.18 to 0.40 [36]. Therefore, seedlings irrigated with diluted pig wastewater represent a viable alternative, exhibiting enhanced vigor and survival, which can improve transplantation success in the field [37].

Although this study focused on elemental distribution and accumulation in seedlings, it is important to acknowledge that treated wastewater can introduce potentially harmful contaminants that may negatively affect soil health, crop performance, and groundwater quality [20,38]. Of particular concern are heavy metals, which can persist in the soil long after irrigation, interacting with soil biota and being taken up by plants. These elements can induce physiological and biochemical disturbances in vascular tissues, and their accumulation poses serious environmental and human health risks [39,40].

While the present experiment was limited to a short exposure period during early seedling development, the elemental patterns observed suggest the possibility of long-term accumulation within agroecosystems. Future research should evaluate the cumulative effects of wastewater irrigation on soil properties, microbial communities, and crop productivity, and the potential risks to food safety.

5. Conclusions

This study demonstrated that swine wastewater significantly influences the elemental distribution and morphological development of Capsicum chinense seedlings. A 20% dilution improved nutrient uptake (K, Ca, P), enhanced seedling emergence and uniformity, and supported better growth, while undiluted wastewater (100%) led to physiological stress and reduced quality. These results suggest that applying 20% diluted swine wastewater in seedling production systems is a viable, sustainable alternative that reduces dependency on synthetic inputs, enhances nutrient recycling, and promotes environmentally responsible nursery practices.