Agronomic Evaluation of Compost Formulations Based on Mining Tailings and Microbial Mats from Geothermal Sources

Abstract

This study, conducted in Mexico, evaluates the agricultural potential of three compost formulations BFS1, BFS2, and BFS3 produced from mining tailings and thermophilic microbial mats and collected from geothermal environments. The physicochemical characterization included pH, electrical conductivity (EC), macronutrients (N, P, K, Ca, Mg, and S), micronutrients (Fe, Zn, B, Cu, Mn, Mo, and Ni), organic matter (OM), and the carbon-to-nitrogen (C/N) ratio. All composts exhibited neutral pH values (7.38–7.52), high OM content (38.5–48.4%), and optimal C/N ratios (10.5–13.9), indicating maturity and chemical stability. Nitrogen ranged from 19 to 21 kg·t⁻¹, while potassium and calcium were present in concentrations beneficial for crop development. However, EC values (3.43–3.66 dS/m) and boron levels (>160 ppm) were moderately high, requiring caution in saline soils or with boron-sensitive crops. A semi-quantitative Compost Quality Index (CQI) ranked BFS3 highest due to elevated OM and potassium content, followed by BFS1. BFS2, while rich in nitrogen, scored lower due to excessive boron. One-way ANOVA revealed no significant difference in nitrogen (p > 0.05), but it did reveal significant differences in potassium (p < 0.01) and boron (p < 0.001) among formulations. These results confirm the potential of mining tailings—microbial mat composts are low-cost, nutrient-rich biofertilizers. They are suitable for field crops or as components in nursery substrates, particularly when EC and boron are managed through dilution. This study promotes the circular reuse of geothermal and industrial residues and contributes to sustainable soil restoration practices in mining-affected regions through innovative composting strategies.

1. Introduction

Mexico ranks among the top ten producers worldwide of metals and minerals. The extraction of gold, copper, silver, zinc, and iron accounts for about 85% of the country’s metallurgical production value, mainly concentrated in the states of Sonora, Chihuahua, Hidalgo, Durango, Zacatecas, Guanajuato, and Guerrero. This extensive mining activity has caused a significant rise in waste generation, with 586 tailing dams documented nationwide in various operational statuses (active, inactive, temporary, incomplete, or unknown). Poor management of these tailings has led to environmental problems such as acid mine drainage, structural failures resulting in downstream avalanches, sediment accumulation in riverbeds, airborne dispersion of fine particles, and visual pollution in urban areas [1]. Therefore, thorough risk assessments and ongoing updates to safety protocols are essential to mitigate these hazards. Despite ongoing efforts to enhance mining waste management, results remain insufficient. Although no official estimate exists for the total exposed mining waste in Mexico, it is estimated that a single mining company can produce over 700 tons of tailings daily. This suggests hundreds of millions of tons of unmanaged tailings lacking proper containment or treatment. Simultaneously, the increasing demand for sustainable agricultural practices has driven global interest in organic soil amendments. Composting organic residues is a key approach to improving soil fertility, enhancing microbial biodiversity, and reducing dependence on synthetic fertilizers [2]. In particular, composts derived from local agro-industrial byproducts offer a low-cost, environmentally friendly alternative aligned with circular economy principles [3]. In Mexico, mining tailings and microbial mats from thermal springs represent two underutilized resources with notable biotechnological potential. Microbial mats are complex, stratified communities of microorganisms, primarily bacteria and archaea embedded in a matrix of extracellular polymeric substances (EPS). These mats typically form on the surfaces of sediments in aquatic or geothermal environments, such as hot springs or wastewater ponds. In this study, the microbial mats were obtained from geothermal wastewater systems and are composed mainly of thermophilic cyanobacteria, heterotrophic bacteria, and filamentous microbes. Their high organic matter content and microbial activity make them valuable for composting and nutrient recycling. Mining tailings retain organic residues and minerals, while microbial mats host thermophilic microorganisms that can accelerate organic matter decomposition and nutrient cycling [4]. However, few studies have explored the agronomic potential of composts based on these materials. Composts enriched with microbial mats may present unique characteristics due to their thermophilic, mineral-rich origins. These include high levels of micronutrients (e.g., Fe, Mn, and B), stable organic matter, and favorable C/N ratios, all of which support plant growth and improve soil structure. Nevertheless, some risks such as elevated salinity and boron levels must be evaluated, as they may restrict the application of these composts in sensitive crops or degraded soils. This study evaluates three compost formulations BFS1, BFS2, and BFS3 developed from mixtures of mining tailings and microbial mats. The main objective is to characterize their physicochemical and agronomic properties and assess their suitability as biofertilizers and as components in horticultural and field crop substrates. The findings aim to contribute to the sustainable reuse of agro-industrial residues and promote climate-resilient agricultural systems. The proposed approach involves processing mining tailings residues and microbial mats from thermal environments into compost-based biofertilizers suitable for agricultural applications. These bio inputs contribute organic matter, essential macro- and micronutrients, and microbial diversity that enhance soil health and crop productivity. Due to their thermophilic origin, microbial mats impart biological and biochemical attributes that support compost maturation and nutrient cycling [4]. Compost formulations combining mining tailings and microbial mats are particularly suitable for conditions requiring high levels of organic matter, nitrogen, and trace elements such as iron, zinc, and boron. They can be applied strategically in nutrient-deficient or degraded soils, or blended with inert materials (e.g., coconut fiber, perlite) to develop nursery substrates. However, their moderately high electrical conductivity and boron levels require proper management in saline soils or with boron-sensitive crops. Based on these considerations, this work aims to achieve the following: (1) formulate compost mixtures (BFS1–BFS3) using mining tailings and microbial mats; (2) characterize their physical, chemical, and agronomic properties; (3) evaluate their potential as biofertilizers for field crops and as components in nursery substrates; (4) identify crop-specific application strategies based on nutrient requirements and soil conditions; (5) contribute to the sustainable reuse of agro-industrial waste and the reduction in environmental pollution caused by improper disposal of mining tailings and organic residues.

The objective of the research is to evaluate the physicochemical properties and agricultural potential of compost formulations (BFS1, BFS2, and BFS3) derived from mining tailings and microbial mats, with a focus on nutrient content, salinity, and elemental toxicity risks. The goal is to propose feasible uses of these composts in sustainable agriculture, particularly in nutrient-deficient or marginal soils.

2. Results

2.1. Physicochemical Properties of Composting Matrix (M1 to M5)

The preliminary mixtures M1 through M5 exhibited substantial variability in nutrient content, organic matter concentration, and indicators of compost maturity (

Table 1. Physicochemical composition of composting mixture (M1, M2, M3, M4, and M5). Data provided by FERTILAB (SA-1359-044/21), based on dry weight.

Parameter	M1	M2	M3	M4	M5
pH	8.45	8.36	7.79	8.30	8.28
Electrical Conductivity (EC) (dS/m)	1.61	1.82	2.82	0.76	1.33
Organic Matter (OM) (%)	40.9	31.8	67.5	21.4	1.47
C/N Ratio	36.1	33.2	11.1	164.	24.1
Total Nitrogen (kg·t−1)	4.8	6.3	33	0.2	0.4
Phosphorus (P2O5, kg·t−1)	1.4	1.8	2.0	1.6	1.5
Potassium (K2O, kg·t−1)	3.0	4.0	4.0	1.4	1.6
Calcium (Ca, kg·t−1)	6.8	6.4	5.2	47	46
Magnesium (Mg, kg·t−1)	2.6	3.4	2.2	8.2	8.1
Sulfur (S, kg·t−1)	2.2	3.2	6.7	1.4	2.4
Sodium (Na, kg·t−1)	3.2	4.3	7.4	1.9	2.8
Iron (Fe, g·t−1)	11,285	13,442	11,492	14,400	13,352
Zinc (Zn, g·t−1)	36	44	41	36	37
Boron (B, g·t−1)	170	221	381	171	179

). All mixtures displayed alkaline pH values, ranging from 7.79 (M3) to 8.45 (M1), typical of composts derived from mineral-rich substrates such as MT. Electrical conductivity (EC) values varied from 0.76 dS/m (M4) to 2.82 dS/m (M3), with M3 showing the highest salinity. Organic matter content was highest in M3 (67.5%) and lowest in M5 (1.47%). C/N ratios ranged from 11.1 (M3) to 164 (M4), with M3 exhibiting the most balanced value. Total nitrogen content was also highest in M3 (3.51%), followed by M2 (0.63%) and M1 (0.48%).

2.2. Macronutrient and Micronutrient Composition of Composting Matrix (M1 to M5)

Potassium (K₂O) concentrations were highest in M3 and M2, both reaching 4 kg·t⁻¹. Calcium content peaked in M4 (47 kg·t⁻¹) and M5 (46 kg·t⁻¹), reflecting the mineral-rich nature of the mining tailings. Magnesium concentrations were also elevated in M4 (0.82%) and M5 (0.81%). All mixtures exhibited high iron (Fe) concentrations, with M4 presenting the highest value (14,400 ppm), followed by M2 (13,442 ppm) and M5 (13,352 ppm) (Table 1). Boron (B) levels exceeded 170 ppm in all mixtures, with M3 reaching the highest concentration at 381 ppm. Regarding C/N ratios, M3 (11.1) and M2 (33.2) were within or close to compost stability thresholds, while M1 (36.1), M4 (164), and M5 (24.1) surpassed acceptable limits.

2.3. Physicochemical Properties of Compost Formulations (BFS1, BFS2, BFS3)

The physicochemical characterization of compost formulations BFS1, BFS2, and BFS3 revealed indicators consistent with compost maturity and agronomic applicability (

Table 2. Physicochemical composition of compost samples BFS1, BFS2, and BFS3. Data provided by FERTILAB (SA-1359-044/21), based on dry weight.

Parameter	BFS1	BFS2	BFS3
pH	7.52	7.38	7.39
Electrical Conductivity (EC) (dS/m)	3.47	3.66	3.43
Moisture (%)	3.11	2.65	3.17
Organic Matter (OM) (%)	39.3	38.5	48.4
Organic Carbon (OC) (%)	22.8	22.3	28.1
C/N Ratio	11.8	10.5	13.9
Total Nitrogen (kg·t−1)	19	21	20
Phosphorus (P2O5) (kg·t−1)	2	2	2
Potassium (K2O) (kg·t−1)	4	4	5
Calcium (Ca) (kg·t−1)	12	13	14
Magnesium (Mg) (kg·t−1)	2	6	8
Sulfur (S) (kg·t−1	6	3	1
Sodium (Na) (kg·t−1)	7	2	0
Iron (Fe) (g·t−1))	17,125	18,682	19,542
Zinc (Zn) (g·t−1)	69	54	56
Boron (B) (g·t−1)	162	205	211

). All three composts exhibited neutral pH values, ranging from 7.38 to 7.52. Electrical conductivity (EC) levels were moderate, with values between 3.43 and 3.66 dS/m. Organic matter (OM) content was elevated across formulations: 38.5% in BFS1, 41.3% in BFS2, and 48.4% in BFS3. C/N ratios fell within the maturity threshold, ranging from 10.5 (BFS2) to 13.9 (BFS3), indicating stable composts unlikely to cause nitrogen immobilization when applied to soil.

2.4. Macronutrient Composition of Compost Formulations (BFS1, BFS2, BFS3)

All three compost formulations (BFS1, BFS2, and BFS3) contained agronomically relevant levels of macronutrients (

Table 3. Macro- and micronutrients of compost samples BFS1, BFS2, and BFS3. Data provided by FERTILAB (SA-1359-044/21), based on dry weight.

Macronurients
Elements	BFS1	BFS2	BFS3
Nitrogen (kg·t−1)	19	21	20
Phosphorus (P2O5) (kg·t−1)	2	2	2
Potassium (K2O) (kg·t−1)	4	4	5
Calcium (Ca) (kg·t−1)	12	13	14
Magnesium (Mg) (kg·t−1)	2	6	8
Sulfur (S,) (kg·t−1)	6	3	1
Sodium (Na) (kg·t−1)	7	2	0
Micronutrients
Iron (Fe) (kg·t−1)	9873	6276	6336
Zinc (Zn) (kg·t−1)	38	36	26
Boron (B) (kg·t−1)	211	359	275
Copper (Cu) (kg·t−1)	11	6	8
Manganese (Mn) (kg·t−1)	320	260	180
Molybdenum (Mo) (kg·t−1)	1	1	1
Nickel (Ni) (kg·t−1)	14	12	8

). Total nitrogen (N) content ranged from 19 to 21 kg·t⁻¹, with BFS2 exhibiting the highest concentration. Phosphorus (P₂O₅) levels were relatively low, ranging from 1.6 to 2.0 kg·t⁻¹ across all formulations. Potassium (K₂O) was most abundant in BFS3, reaching 5.5 kg·t⁻¹. Calcium (Ca) content peaked in BFS1 at 7.2 kg·t⁻¹, reflecting the mineral composition of the tailings used.

2.5. Micronutrients and Environmental Considerations

The compost formulations exhibited elevated concentrations of key micronutrients essential for plant metabolism, including iron (Fe), manganese (Mn), zinc (Zn), and boron (B) (Table 3). Boron levels were notably high, reaching 211 ppm in BFS2 and 275 ppm in BFS3, exceeding commonly accepted agronomic thresholds (20–100 ppm). Iron content was highest in BFS1 (9873 ppm), while Zn, Mn, and Cu levels across all composts remained within safe agronomic limits. No phytotoxic concentrations of heavy metals such as silver (Ag), lead (Pb), or cadmium (Cd) were detected in any sample.

2.6. Comparative Evaluation and Application Potential

The comparative analysis of compost formulations BFS1, BFS2, and BFS3 revealed distinct nutrient profiles that support differentiated agronomic applications (

Table 4. Comparative evaluation and agronomic application potential of compost formulations. N = nitrogen; K = potassium; Ca = calcium; Fe = iron; Mn = manganese; Mg = magnesium; B = boron; OM = organic matter; EC = electrical conductivity; Na = sodium; S = sulfur.

Formulation	Key Nutrient Strengths	Agronomic Advantages	Limitations	Recommended Uses	Suggested Crops	References
BFS1	Moderate N, high Ca, Fe, Mn	Balanced profile; improves soil structure and micronutrient supply	Moderately high Na and EC	General field crops; iron-deficient or calcareous soils	Corn, sorghum, lettuce, spinach	[5,6]
BFS2	Highest N, high B, Mg	Supports N-demanding crops; promotes microbial activity	High B; caution in boron-sensitive crops and saline soils	Leafy vegetables, cereals; avoid in B-sensitive crops	Cabbage, broccoli, maize, alfalfa	[7,8]
BFS3	Highest K, OM, Mg; lowest Na and S	Enhances fruiting; suitable for degraded soils and seedbeds	Moderate B and EC; may require dilution in nurseries	Fruiting crops, nursery substrates (mixed with inert media)	Tomato, chili, watermelon, strawberries, ornamental plants	[9,10]

). BFS1 presented a balanced composition, with nitrogen at 19 kg·t⁻¹, potassium at 4 kg·t⁻¹, and the highest calcium concentration (7.2 kg·t⁻¹). It also exhibited the greatest iron (9873 ppm) and manganese (320 ppm) contents. BFS2 recorded the highest total nitrogen (21 kg·t⁻¹) and boron concentration (359 ppm), along with magnesium at 6 kg·t⁻¹. BFS3 displayed the highest levels of potassium (5.5 kg·t⁻¹), magnesium (8 kg·t⁻¹), and organic matter (48.4%). Sodium and sulfur levels were low in BFS3 (0 and 1 kg·t⁻¹, respectively), compared to moderate values in BFS1 and BFS2.

2.7. Compost Quality Index (CQI)

A semi-quantitative scoring system was developed to assign values from 0 to 10 for each compost formulation, based on six agronomic parameters: total nitrogen (N), potassium (K), organic matter (OM), carbon-to-nitrogen ratio (C/N), electrical conductivity (EC), and boron (B). These parameters were selected for their relevance to soil fertility and compost maturity. Notably, boron and electrical conductivity were assigned a double weight (2 points) compared to other parameters, resulting in a total maximum score of 16. Figure 1 shows the CQI results for BFS1, BFS2, and BFS3, reflecting differences in overall agronomic quality.

2.8. Analysis of Variance (ANOVA)

A one-way ANOVA was performed to evaluate whether the differences in nutrient concentrations among compost formulations were statistically significant. The analysis was based on triplicate measurements for total nitrogen (N), exchangeable potassium (K₂O), and boron (B). The results indicated no significant differences in nitrogen content among BFS1, BFS2, and BFS3 (p > 0.05), suggesting similar levels of N enrichment. In contrast, potassium concentrations showed statistically significant differences (p < 0.01), with BFS3 having the highest K₂O content. Boron levels exhibited highly significant variation (p < 0.001), with BFS2 presenting the highest concentration (Figure 2).

2.9. Nutrient Profile Visualization

To illustrate the compositional differences among the compost samples, a grouped bar chart was generated (Figure 3), presenting normalized concentrations (0–100) for seven essential nutrients: nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), sulfur (S), and boron (B). The visual analysis shows that BFS2 has the highest levels of nitrogen and boron. BFS3 stands out with elevated potassium and magnesium concentrations, along with notably low sodium and sulfur levels. BFS1 exhibits a more uniform distribution across all nutrients.

2.10. Interpretation and Agronomic Relevance

The integration of the Compost Quality Index (CQI) and nutrient profile visualization enabled a comparative interpretation of compost suitability. BFS3 was identified as the most appropriate for potassium-demanding crops such as tomato, watermelon, and ornamentals, and can be used in nursery substrates at proportions of 20–40% v/v. BFS2, with the highest nitrogen content, was optimal for nitrogen-demanding crops like leafy vegetables and cereals. BFS1, due to its balanced nutrient profile and moderate salinity, was suitable for general field application across diverse crop types.

3. Discussion

3.1. Physicochemical Properties of Composting Matrix (M1 to M5)

The physicochemical profile of the composting matrices underscores the influence of raw material composition, particularly the contribution of microbial mats. M3, enriched with microbial biomass, demonstrated favorable traits such as a low C/N ratio and high organic matter and nitrogen content, suggesting advanced compost maturity and strong nitrogen availability. These characteristics align with previous findings on the role of thermophilic microbial consortia in accelerating organic matter decomposition and nutrient stabilization [11]. In contrast, M4 and M5 showed poor biological stability due to high C/N ratios and low organic matter, indicating the need for extended composting or supplementation with nitrogen-rich materials. Although EC levels in M1, M2, M4, and M5 remained within acceptable limits for general agricultural use, the elevated salinity in M3 suggests the necessity of dilution before applying to salt-sensitive crops or seedbeds. Overall, M3 emerges as the most suitable base for compost formulation due to its balanced nutrient profile and maturity indicators, supporting its role as a core component in BFS variants [7].

3.2. Macronutrient and Micronutrient Composition of Composting Matrix (M1 to M5)

The macro- and micronutrient profiles of the composting matrices underscore the strengths and limitations of each mixture for agronomic use. M3 emerged as the most biologically stable and nutrient-rich option, due to its balanced C/N ratio, high nitrogen content, and substantial organic matter. These characteristics validate its selection as the core ingredient (40–60%) in final compost formulations. M1 and M2, despite moderate nutrient profiles, offer acceptable EC values and nutrient balance, making them suitable for blending. Conversely, M4 and M5, while rich in calcium and magnesium, showed low nitrogen and poor C/N ratios, requiring either additional composting or supplementation to improve biological stability. The elevated boron concentrations, particularly in M3, highlight the need for careful application in boron-sensitive crops, aligning with concerns raised by [8]. Overall, the formulation strategy centering on M3 and adjusting the proportions of M1, M2, and M4 was justified by their respective chemical profiles and intended agronomic applications [12].

3.3. Physicochemical Properties of Compost Formulations (BFS1, BFS2, BFS3)

The neutral to slightly alkaline pH values observed are favorable for microbial activity and nutrient availability, while also contributing to organic matter stabilization and reduced heavy metal solubility, which is particularly relevant given the mineral-rich origin of the raw materials [12]. Although EC values slightly exceed the threshold for germination substrates (<2.5 dS/m), they remain acceptable for field application in well-drained soils and can be mitigated through blending with inert materials, as supported by [13]. The elevated OM levels in all formulations exceed international benchmarks (e.g., [14], supporting their maturity and use as soil amendments. BFS3′s high OM content likely results from a greater proportion of microbial mat biomass, which also contributes to its enhanced porosity and water retention capacity. The favorable C/N ratios further confirm the composts’ maturity and readiness for agronomic use [11]. Compost with a C/N ratio below 15 is generally considered stable and unlikely to cause nitrogen immobilization when applied to soil. Although the composting period was limited to 15 days, the resulting formulations exhibited physicochemical indicators consistent with advanced maturity, including stable pH, C/N ratios below 14, and high organic matter content. This rapid stabilization is attributed to the use of thermophilic microbial mats, which introduce heat-tolerant microbial populations capable of accelerating organic matter decomposition. Similar results have been reported in composts inoculated with thermophilic consortia [9,11]. While enzymatic or microbial activity markers, such as dehydrogenase activity, were not measured in this study, the convergence of multiple maturity indicators suggests effective microbial processing. Nonetheless, future studies should incorporate enzymatic assays to validate microbial dynamics and confirm biological stability.

3.4. Macronutrient Composition of Compost Formulations (BFS1, BFS2, BFS3)

The macronutrient profiles of the composts highlight their potential for targeted agronomic applications. The elevated nitrogen content in BFS2, attributed to its greater proportion of microbial biomass, positions it as a suitable amendment for nitrogen-demanding crops such as maize, leafy greens, and alfalfa [7]. The uniformly low phosphorus content across all formulations is consistent with the inherent limitations of the source materials, specifically the low P levels in microbial mats (173 ppm) and undetectable P in mining tailings. This suggests that supplemental phosphorus fertilization may be necessary, especially for high-demand crops like carrots and tomatoes [15]. The high potassium concentration in BFS3 makes it particularly appropriate for tuber and fruit crops, given potassium’s role in carbohydrate transport, fruit quality, and stress resistance [16]. Meanwhile, the calcium enrichment in BFS1, derived from mineral tailings, enhances its utility in acidic or leached soils, where calcium supports root development and mitigates abiotic stress [17].

3.5. Micronutrients and Environmental Considerations

The elevated boron concentrations in BFS2 and BFS3 present a dual implication: while beneficial for boron-tolerant crops like sunflower and beet, they may pose phytotoxic risks for sensitive species such as beans, citrus, or strawberries [8]. This necessitates crop-specific application strategies and possibly dilution with boron-free substrates. The high Fe content in BFS1 likely stems from the tailings and may be further enhanced by microbial processes associated with thermophilic mats [4], supporting its role in photosynthesis through chlorophyll synthesis. The presence of Zn, Mn, and Cu within safety thresholds ensures micronutrient sufficiency without phytotoxic effects. Importantly, the neutral pH of the composts likely limits metal solubility and mobility, reducing environmental risks [18]. Nonetheless, due to the mining origin of the tailings, trace metals such as Ag, Pb, and Cd may accumulate over time. Although not detected at hazardous levels in this study, periodic monitoring of total and bioavailable metal fractions is recommended, particularly for long-term use or in sensitive agroecosystems. Future studies should also evaluate metal mobility under variable pH and moisture conditions to fully assess environmental safety.

3.6. Comparative Evaluation and Application Potential

The distinct nutrient profiles observed in each compost formulation indicate specific agronomic application potentials based on crop nutrient demands and soil conditions. BFS1, with its balanced macro- and micronutrient content, is suitable for crops grown in calcareous or iron-deficient soils, offering moderate resistance to salinity stress. However, its sodium (7 kg·t⁻¹) and sulfur (6 kg·t⁻¹) levels require consideration for long-term use in salt-sensitive environments. BFS2′s elevated nitrogen content makes it ideal for nitrogen-demanding crops (e.g., leafy greens and cereals), though its high boron level (359 ppm) necessitates caution when applied to boron-sensitive species or saline soils. Magnesium content in BFS2 may improve plant tolerance to abiotic stress [8]. BFS3′s rich potassium, magnesium, and organic matter content positions it as the best candidate for degraded or nutrient-poor soils and fruiting crops, while its low sodium and sulfur make it highly suitable for nursery substrates especially when blended with inert materials like perlite or coconut fiber [9]. All three composts met maturity and stability criteria, but their differences in EC, sodium, and boron levels emphasize the need for crop and site-specific application strategies. For instance, BFS2 and BFS3 may require dilution when used in seedling production, whereas BFS1 can be broadly applied in field conditions. These findings support the use of composts derived from MT and thermophilic MM as sustainable biofertilizers. The synergy between mineral-rich tailings (providing micronutrients like Fe, Zn, Mn) and thermophilic microbial mats (enhancing organic matter degradation and nutrient mineralization) promotes faster compost maturation and broader nutrient profiles. Previous research supports the agronomic potential of such materials, citing enhanced enzymatic activity and soil resilience [5,16]. Nevertheless, future work should investigate microbial diversity and functionality through metagenomic approaches to optimize formulations.

3.7. Compost Quality Index (CQI)

The weighting of boron and EC acknowledges their critical role in plant safety and compost application viability. Boron, although essential in trace amounts, becomes toxic above 100 ppm for sensitive crops like beans and citrus, necessitating careful compost use. Similarly, elevated EC can hinder germination and root development, especially in saline-prone or nursery environments. By integrating these risks into the CQI, the scoring system provides a more realistic and safety-oriented assessment of compost performance. This approach aligns with the recent literature that emphasizes the importance of risk-based compost evaluation frameworks [9,10], enabling more accurate recommendations for field versus nursery applications.

3.8. Analysis of Variance (ANOVA)

The statistical analysis confirms that while nitrogen content is comparable across formulations, potassium and boron levels vary substantially, influencing their agronomic application. The elevated K₂O concentration in BFS3 supports its recommendation for potassium-demanding crops such as tomatoes or potatoes. Conversely, the high boron level in BFS2 reinforces the cautionary guidance against its use on boron-sensitive crops like beans, citrus, or strawberries. These findings are consistent with the patterns observed in the bar chart and contribute to refining compost selection based on crop nutrient requirements and sensitivity thresholds.

3.9. Nutrient Profile Visualization

These visual findings support and complement the quantitative data, providing a clearer understanding of each formulation’s nutrient profile. The high boron and nitrogen content in BFS2 confirms its potential for nitrogen-demanding crops but underscores the risk for boron-sensitive species. The nutrient enrichment in potassium and magnesium in BFS3, coupled with its low salt content, highlights its suitability for crops grown in saline or degraded soils. The balanced nutrient profile of BFS1 supports its use as a general-purpose compost, adaptable across a wide range of soil types and crop systems.

3.10. Interpretation and Agronomic Relevance

Despite their agronomic potential, BFS2 and BFS3 exhibited elevated boron concentrations, posing a phytotoxicity risk to sensitive crops such as beans, lettuce, and citrus. Application of these composts should be restricted to well-drained soils and boron-tolerant crops like sunflower, beet, and maize. For sensitive systems, BFS2 and BFS3 should be blended with inert, boron-free materials (e.g., perlite, coconut fiber, sterilized sand) at 20–30% v/v to reduce boron levels below critical toxicity thresholds (<100 ppm). Field applications should consider soil texture and rainfall timing to minimize boron accumulation. The overall analytical approach supports the development of crop- and site-specific compost application strategies, reinforcing the potential of these materials as tailored biofertilizers for sustainable agriculture.

4. Material and Methods

4.1. Characteristics of Mining Tailings

The mining tailings used in this study were collected from a tailings dam located in the state of Guanajuato, Mexico. These residues originated from silver extraction operations conducted in the 1940s using flotation techniques by La Cooperativa Mining Company, situated at coordinates 21.0287° N and 101.2541° W (Figure 4). The mining tailings from La Cooperativa exhibit a particle size distribution of 22.66% gravel, 48.14% sand, and 29.19% clay by weight. According to the Unified Soil Classification System (USCS), the material is classified as ML, which corresponds to very fine inorganic silts and sands, rock flour, fine silty or clayey sands, or clayey silts with low plasticity (

Table 5. Physicochemical characteristics of mining tailings (MTs) and microbial mat (MM).

Sample	Water Absorption (%)	Porosity (%)	Granulometry
			Gravel (%)	Sand (%)	Silt-Clay (%)	Classification (USCS)
La Cooperativa Mining Tailings (MTs)	21.04	0.86	22.66	48.14	29.19	GP: Poorly graded gravels, mixtures of sand and gravel with few or no fines. SM Silty sands, poorly graded sand and silt mixture. ML: Inorganic silts and very fine sands, rock flour, silty or clayey fine sands, or clayey silts with slight plasticity.
Microbial Mat (MM)	176.61	58.95	0.00	39.25	27.31	OS-H: Organic material with high plasticity sand.

) [19]. In terms of chemical composition, the tailings are predominantly composed of silica (SiO₂, 77.18%), with notable concentrations of titanium dioxide (TiO₂, 0.76%) and calcium oxide (CaO, 7.15%). Lesser amounts of sodium oxide (Na₂O, 8.2%), aluminum oxide (Al₂O₃, 8.02%), ferric oxide (Fe₂O₃, 2.67%), and magnesium oxide (MgO, 0.71%) were also detected (

Table 6. Chemical composition of the mining tailings (MTs) and microbial mat (MM).

Elements	Sample	Elements	Sample
	Mining Tailings (MTs)		Microbial Mat (MM)
Al (ppm)	32,766	Al (ppm)	31,700
K (ppm)	14,933	K (ppm)	10,800
Na (ppm)	5200	Na (ppm)	24,000
Si (ppm)	278,666	Si (ppm)	225,000
P (ppm)	0	P (ppm)	173
Ca (ppm)	29,933	Ca (ppm)	21,400
Fe (ppm)	16,833	Fe (ppm)	15,100
Zn (ppm)	49.46	Zn (ppm)	112
Mg (ppm)	4578	Mg (ppm)	3200
Mn (ppm)	965	Mn (ppm)	596
Mo (ppm)	ND	Mo (ppm)	ND
Ni (ppm)	19	Ni (ppm)	29.7
S (ppm)	4620	S (ppm)	5990
Cu (ppm)	34	Cu (ppm)	40
Sn (ppm)	15	Sn (ppm)	12.2
Zn (ppm)	45.2	Zn (ppm)	112
SiO2 (%)	77.18	SiO2 (%)	69.5
Al2O3 (%)	8.02	Al2O3 (%)	5.03
MgO (%)	0.711	MgO (%)	0.73
Na2O (%)	8.2	Na2O (%)	14.8
CaO (%)	7.15	CaO (%)	3.74
Fe2O3 (%)	2.67	Fe2O3 (%)	2.64
TiO2 (%)	0.76	TiO2 (%)	0.39

). Based on the granulometric and mineralogical analysis reported by [20], the tailing material can be classified as quartz-rich sand due to its dominant silica content and particle size distribution typical of sandy sediments.

4.2. Characteristics of the Microbial Mats

Microbial mat samples were obtained from Comanjilla Water Park, also located in Guanajuato, approximately 32 km from the city of Guanajuato. The site is accessible via Federal Highway 45, which connects the cities of Silao and León, followed by a 10 km detour near the town of Los Sauces (Figure 4). Comanjilla is historically recognized for the therapeutic properties of its hot springs and forms part of a geothermal zone that includes at least 25 hydrothermal sources. Some of these springs form small, hot pools with visible deposits of sulfur and mineral salts. The geothermal area covers approximately 1.2 km², with surface water temperatures ranging from 45 °C to 92 °C [21,22]. In the Comanjilla hot springs, microbial mats are found in two distinct layers of vivid coloration, each approximately 5 mm thick. These layers exhibit striking orange and green hues and have a gelatinous texture. For sampling, the microbial mat was collected using a fine mesh net and left to air-dry for fifteen days on a clean surface. Once fully dried, the material was ground to a 320-mesh particle size to ensure homogeneity. The mats demonstrated a high water absorption capacity (176.61%) and considerable porosity (58.95%), consistent with their organic and highly porous nature. Particle size analysis revealed that 27.31% of the material consists of fine fractions (silts and clays). According to the Unified Soil Classification System (USCS), the mats are classified as OS-H, corresponding to organic material with high-plasticity sand, indicative of substantial organic content and plasticity (Table 5) [21,22]. Regarding chemical composition, the microbial mats contain high concentrations of silicon dioxide (SiO₂, 69.5%), sodium oxide (Na₂O, 14.8%), titanium dioxide (TiO₂, 0.39%), and calcium oxide (CaO, 3.74%). Lower concentrations of aluminum oxide (Al₂O₃, 5.03%), ferric oxide (Fe₂O₃, 2.64%), and magnesium oxide (MgO, 0.73%) were also present (Table 6).

4.3. Composting Matrix (M1 to M5)

To establish a controlled composting matrix, five preliminary mixtures (M1–M5) were formulated using different volumetric ratios of two primary substrates: mining tailings (MTs) from La Cooperativa Mining Company and microbial mats (MMs) collected from the Comanjilla geothermal springs (Figure 5). The objective was to evaluate how varying proportions of organic and mineral inputs affect the physicochemical properties and nutrient availability of the resulting composts. Material selection was based on the contrasting characteristics of each component. The microbial mats were rich in organic matter and nitrogen, exhibited high porosity, and harbored thermophilic microbial communities; meanwhile, the mining tailings contributed essential minerals, particularly calcium, magnesium, and iron oxides.

Each mixture was prepared by homogenizing 1 kg of material according to the volumetric ratios specified in

Table 7. Compost formulation. Combination of two primary substrates: microbial mat biomass and mine tailings. Composting matrix, five premixes (M1 to M5). The chemical composition of these matrices was determined using inductively coupled plasma optical emission spectrometry (ICP-OES) for macro- and micronutrient analysis, the Walkley–Black method for organic matter content, and the Kjeldahl method for total nitrogen quantification.

Composting Matrix	Microbial Mat (MM)	Mining Tailings (MTs) (La Cooperativa Guanajuato)
M1	50%	50%
M2	75%	25%
M3	100%	0%
M4	25%	75%
M5	0%	100%
Compost Formulations	Base Composting matrix
BFS1	60% M3 + 20% M1 + 20% M2 50% M3 + 30% M2 + 20% M4 40% M2 + 30% M1 + 30% M3
BFS2
BFS3

.

No additional organic amendments or bulking agents were included in order to isolate the effects of the two substrates. The mixtures were composted under aerobic conditions for an initial stabilization period of 15 days. During this time, each pile was manually turned once per week to promote oxygen diffusion and maintain uniform microbial activity. Moisture content was monitored and adjusted to 50–60% by weight using distilled water, following standard composting practices [12]. At the end of the composting period, subsamples from each mixture were submitted to FERTILAB (Fertilidad de Suelos S. de R.L., EMA-accredited laboratory SA-1359-044/21) for physicochemical and nutrient analysis. The parameters evaluated included pH, electrical conductivity (EC), organic matter (OM), total nitrogen (N), phosphorus (P₂O₅), potassium (K₂O), calcium (Ca), magnesium (Mg), sodium (Na), sulfur (S), and micronutrients (Fe, Zn, B, Mn, Cu, Mo, Ni). The carbon-to-nitrogen (C/N) ratio was also calculated to assess compost maturity and nitrogen availability. The results from these preliminary mixtures informed the development of the optimized compost formulations BFS1, BFS2, and BFS3, as detailed in Section 4.4.

4.4. Compost Formulation

The compost formulations BFS1, BFS2, and BFS3 were developed using proportional combinations of two primary substrates: MM, collected from thermophilic environments in the Comanjilla region (Guanajuato), and MT, a mineral-rich byproduct derived from historical ore beneficiation processes supplied by La Cooperativa Guanajuato. To establish a controlled composting matrix, five preliminary mixtures (M1 to M5) were prepared using varying volumetric ratios of microbial mat and mining tailings, as detailed in Table 7. Based on the results of physicochemical pre-assays conducted by FERTILAB, three optimized compost formulations were selected:

-

BFS1: 60% M3 + 20% M1 + 20% M2;

-

BFS2: 50% M3 + 30% M2 + 20% M4;

-

BFS3: 40% M2 + 30% M1 + 30% M3.

Each formulation was composted under aerobic conditions for a 15-day period. The composting process was conducted in 10 kg batches, with manual turning once per week to enhance oxygenation and ensure uniform microbial activity. The final compost variants were derived from specific combinations of the base mixtures, strategically designed to optimize nutrient content, microbial activity, and organic matter stability. A higher proportion of MM was favored in the blends to enhance nitrogen availability and accelerate decomposition dynamics. This formulation strategy resulted in three distinct compost products (BFS1, BFS2, and BFS3), each with unique nutrient profiles and stabilization characteristics, suitable for targeted agricultural applications such as field fertilization and nursery substrate development (Table 7).

4.5. Compost Characterization

The physicochemical characterization of compost samples BFS1, BFS2, and BFS3 was conducted by FERTILAB (Fertilidad de Suelos S. de R.L.), an EMA-accredited agricultural laboratory (SA-1359-044/21) based in Mexico. Analyses were performed on samples taken directly from production batches, using certified methodologies for solid organic inputs. FERTILAB applied limits of detection (LDs) and quantification (LQ) for each element, as specified in its official accreditation protocols. All tests were conducted under internal quality control standards. The laboratory certifies that the results are valid only for the samples as received, without any pretreatment or user modification.

The following physicochemical parameters were evaluated for each compost sample: (1) pH and Electrical Conductivity (EC): Measured in a 1:5 compost-to-distilled water suspension using a multiparameter probe (APERA PC60-Z). (2) Moisture Content: Determined gravimetrically by oven-drying at 105 °C for 24 h. (3) Organic matter (OM) was determined by loss-on-ignition at 550 °C, while organic carbon (OC) was estimated using a comparative approach based on oxidation methods, as described by [23], which provides improved accuracy over traditional titration techniques. (4) Total nitrogen (N) was measured using the combustion-based method proposed by [24], which offers improved precision and safety over traditional Kjeldahl analysis. (5) Available Phosphorus (P₂O₅): Extracted using Olsen’s method and analyzed via spectrophotometry. (6) Exchangeable Potassium (K₂O), Calcium (Ca), Magnesium (Mg), Sodium (Na), and Sulfur (S): Measured by flame photometry and ICP-OES following acid digestion. (7) Micronutrients (Fe, Zn, Cu, Mn, B, Mo, and Ni): Determined using ICP-OES after nitric-perchloric acid digestion, following the USEPA 3050B method. (8) Carbon-to-Nitrogen Ratio (C/N): Calculated as an indicator of compost maturity and nitrogen availability. Values between 10 and 15 were considered indicative of stable, mature compost. (9) Visual attributes (texture, odor, and color), were also evaluated to support maturity assessments.

4.6. Nutrient Content Analysis

Macronutrients and micronutrients were analyzed following microwave-assisted acid digestion, using Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES), in accordance with the accredited internal method MET-PO-13. The nutrient analysis included the following: (1) Macronutrients: Total nitrogen (N), determined by Dumas combustion, and phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and sulfur (S). (2) Micronutrients: Iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), molybdenum (Mo), and nickel (Ni).

All results were reported on a dry weight basis and expressed as either mg/kg or % (w/w), depending on the nutrient. In addition, FERTILAB calculated the nutrient contributions per metric ton of compost (kg·t⁻¹ or g·t⁻¹), taking into account the actual moisture content of each sample.

4.7. Statistical and Comparative Analysis

To enhance the evaluation of compost formulations BFS1, BFS2, and BFS3, a combined statistical and comparative approach was incorporated into the methodology. The objective was to identify significant differences in nutrient content among compost types and to support their agronomic classification. A semi-quantitative Compost Quality Index (CQI) was developed to assess the agronomic value of each compost based on six key parameters: total nitrogen (N), potassium (K), organic matter content (OM), carbon-to-nitrogen ratio (C/N), electrical conductivity (EC), and boron (B). Each parameter was assigned a weight ranging from 1 to 2 points, depending on its agronomic relevance in terms of nutrient availability and compost maturity, following established evaluation frameworks [9,10,12]. These criteria emphasize the role of total nitrogen, organic matter content, C/N ratio, and electrical conductivity as reliable indicators of compost quality and nutrient release potential. The weighted scores were summed to yield a global CQI value on a 16-point scale for each compost formulation. To further explore the compositional differences among the compost formulations, a clustered bar chart of seven normalized macronutrients and micronutrients (N, K, Ca, Mg, Na, S, and B) is presented. This visualization highlights the nutrient enrichment patterns in each compost and supports targeted agronomic recommendations based on specific nutritional profiles. In addition, a one-way Analysis of Variance (ANOVA) was conducted to statistically assess whether BFS1, BFS2, and BFS3 differed significantly in nutrient content. Three replicate values were generated per formulation and nutrient to ensure statistical robustness. The variables selected for the analysis of total nitrogen (N), exchangeable potassium (K₂O), and boron (B) were chosen due to their agronomic relevance and observed variability. ANOVA was performed using Microsoft Excel 365 with the Data Analysis ToolPak enabled. Nutrient values were grouped by formulation, and between-group variance was tested at a 95% confidence level (α = 0.05). A p-value < 0.05 was considered statistically significant, indicating that at least one compost formulation differed from the others for a given nutrient. This statistical framework reinforced the reliability of the nutrient comparisons and validated the functional differentiation revealed through compositional analysis and the CQI.

5. Conclusions

This study demonstrates the potential of compost formulations derived from mining tailings (MTs) and microbial mats (MMs) as sustainable biofertilizers. All three formulations (BFS1, BFS2, and BFS3) exhibited physicochemical characteristics indicative of compost maturity and stability, including neutral pH, high organic matter content, and optimal C/N ratios (10.5–13.9). These composts supplied essential macro- and micronutrients such as nitrogen, potassium, calcium, iron, zinc, and manganese, supporting improvements in soil fertility, plant nutrition, and microbial activity in degraded agroecosystems. Among the formulations, BFS2 presented the highest levels of nitrogen and boron, making it suitable for nitrogen-demanding crops that are tolerant to boron. BFS3, enriched in potassium and organic matter, is especially beneficial for fruiting crops and soil rehabilitation, while BFS1 showed a balanced nutrient profile suitable for general-purpose agricultural applications. However, all formulations showed moderately high EC (3.43–3.66 dS/m) and elevated boron levels (>160 ppm), which may restrict their use in saline soils or with boron-sensitive species. Therefore, targeted application strategies such as dilution with inert materials or post-germination use are recommended, particularly in nursery or sensitive crop systems. These composts can be applied at field rates of 3–6 t/ha or incorporated into nursery substrates at 20–40% v/v with materials like perlite or coconut fiber. The innovative integration of geothermal microbial mats and mineral-rich tailings provides a novel biotechnological pathway for valorizing underutilized residues. This approach aligns with circular economy principles by diverting industrial waste from improper disposal while enhancing organic matter cycling in soil. Based on these results, we hypothesize that the co-composting of microbial mats and mining tailings can enhance nutrient recovery and reduce environmental risks. Future studies should evaluate the performance of these composts under field conditions across diverse soil types and cropping systems. In addition, research on long-term impacts including soil microbial dynamics, enzyme activity, greenhouse gas emissions, and trace element accumulation will be critical to ensure agronomic safety and sustainability. Further exploration of thermophilic microbial communities involved in compost maturation may also open new avenues in biocontrol and soil health management.