Growth response of Tabernaemontana pandacaqui Poir. in soil from a former mining site in Marinduque, Philippines

Author info »

Introduction

Mineral resources are one of the Philippines’ acknowledged rich assetPhilippines’ acknowledged rich assets that significantly contribute to the country’s economic development. Marinduque is among the mineral-rich provinces with large copper deposits discovered by mining prospectors in the late 1960s. It became the home to one of the most significant mining operations in the country. However, a disastrous mine tailing accident in 1996 affected the provincial island (Gregory, 2004)).  Republic Act 7942 or the Philippine Mining Act of 1995 states that “Contractors and permittees shall technically and biologically rehabilitate the excavated, mined-out tailings covered, and disturbed areas to the condition of environmental safety, as may be provided in the implementing rules and regulations of this Act”. In this regard, the Department of Environment and Natural Resources (DENR) requires responsible rehabilitation of mined areas (Teves, 2018). At least 22 inactive and abandoned mine sites need a plan for rehabilitation like the case of an inactive copper mining site in Mogpog, Marinduque. The government and mining companies are making efforts to restore mined-out areas and convert areas as productive assets (Felix, 2004). The mountain situated in the area was flattened and replaced with a mountain of dogged unprocessed copper-rich soil that reached adjacent rice fields, agricultural areas, and mangrove forest. The mining activities led to several environmental problems, such as deforestation, leaving only less than 30% of the forest cover in 2001 (Medianista & Labay, 2017). Such events caught the Philippine government’s attention and made them prioritize the rehabilitation and remediation of the inactive mine sites to enhance its current state. Thus, improving biodiversity can help in natural ecological processes and human consumption (Coracero & Malabrigo, 2020).

Phytoremediation is an emerging biological approach in the cleanup of contaminated sites. According to Sigurdur Greipsson (2011), phytoremediation is a recently developed technology that offers a cost-effective solution by using plants, and associated soil microbes, to reduce the content, or toxic effects, of contaminants in the environment. It is a remediation technique that takes advantage of the biological processes of plants. Plants alter the soil through theshoot and root growth, water and mineral acquisition, senescence, and eventual decay. The following series of actions directly or indirectly absorb, sequester, and/or degrade contaminants. Planting and growing compatible species either remove the pollutant or physically or chemically convert contaminants into a component that no longer poses a threat to the surrounding environment (Cunningham & Ow, 1996).

In the Philippines, DENR uses phytoremediation species such as Acacia auriculiformis Benth., Acacia mangium Willd., Pterocarpus indicus Willd., Casuarina equisitifolia Forst., and Chrysopogon zizanioides (L.) Roberty. The Philippine governmentPhilippine government supported the department’s initiative supported the department’s initiative, thus issuing Executive Order 270 entitled “National Policy Agenda in Revitalizing Mining in the Philippines” on January 16, 2001 and stated in Section 2-I, that “Remediation and Rehabilitation of abandoned mines shall be accorded top priority to address the negative impacts of past mining projects.” (DENR, 2014). One of major steps under the executive order was the formation of the Bioremediation Research Team through National Academy for Science and Technology to conduct studies and research to develop technologies that can help in the remediation of toxic mining wastes (Raymundo et al., 2006).

The DENR initiated to plant A. auriculiformis, also known as earleaf acacia, in the area in 2009. According to the ERDB Director Dr. Henry A. Adornado, 42 ha of mined-out areas were successfully planted. It was found that A. auriculiformis and other species that were planted for the phytoremediation, such as Pterocarpus indicus Willd. were most efficient in the absorption of metals (Sarian, 2017).  There were no records on the number of individuals of these species the DENR have planted throughout the years. Nevertheless, patches of A. auriculiformis can be observed and have dominated the area.  Earleaf Acacia is a fast-growing introduced species that can grow on poor quality soil. Earleaf acacia has dense roots and tolerates seasonally waterlogged soil and produces a heavy leaf litter, making it suitable for rehabilitation of degraded land (Contu, 2012). These characteristics make the species compatible for stabilizing eroded land. It is popular in stabilizing and revegetating mined-out areas because A. auriculiformis can grow in infertile soil and tolerate both highly acidic and alkaline soils capable of nitrogen fixing (Orwa et al., 2009).

This study aims to assess the effects of A. auriculiformis to the growth of Philippine native species in Mogpog, Marinduque, such as Tabernaemontana pandacaqui (Pandakaki). Specifically, the study aims to assess and mass produce endomycorrhizal fungi of plants in the area and determine the growth behavior and response of Pandakaki with Earleaf Acacia, endomycorrhizas, and soil amendments (inclusion of biofertilizers).

Methodology

Study site

Inventory and samples were gathered in Brgy. Capayang, Mogpog, Mariduque and is located at 13°30'16"N 121°51'42"E (Figure 1a). Post mine areas have soils contaminated with acid mine drainage (Medianista & Labay, 2017).

Figure 1: Satellite image of mining area (a) (Google Earth, 2019) and the study site (b) in Brgy. Capayang, Mogpog, Marinduque.

The province was known for its rich mineral deposits such as gold, copper, and iron. During the 1960s up to the late 1990s, mining activities were operated on-site in open-pit mining. All mining activities on the island were ceased when the 1.5-3 million cubic meters of sulfidic tailings slurry from the Tapian Pit storage area into the Makulapnit River, Boac River, and eventually the ocean west of the island (Plumlee et al., 2000). The mining caused several environmental issues brought about by improper mining activities, such as mine tailings being dumped in rice fields, mangroves, agricultural lands, and forest lands.

The mine site in Brgy. Capayang, Mogpog, Mariduque is a copper-rich soil.  Soil analysis from the study of Tulod et al. (2012) showed a high concentration of copper and low content of other toxic contaminants (Table 1). The soil in the area was a loam type soil with 62% sand, 36% silt, and 2% clay and highly acidic (pH = 4.4) with deficient nitrogen (100 ppm), potassium (101.4 ppm), phosphorus (81 ppm), OM content (0.24%), and CEC (17.7 cmol/kg).

Table 1. Heavy metals present in the soil of inactive/abandoned mining sites (Tulod et al., 2012).

The growth of vegetation in this kind of condition is nearly unattainable. Excess levels in minerals in soil lead to abnormality in plant growth. The ideal pH for plant growth ranges from 5.5 to 7.0, although plants have adapted to withstand pH beyond this range. It is significant in many chemical processes in the soil, such as plant nutrient availability (The Mosaic Company, 2020).  To address the condition in the inactive/abandoned mining site, the DENR initiated the site’s rehabilitation, and one of their actions was planting A. auriculiformis. The area is now covered with clumps of A. auriciformis, and other plant species are starting to thrive in the copper-rich soil (Figure 1b).

Selection of species for the experiment

The vegetation composition of the area was considered in selecting the appropriate species to test. Thus, vegetation assessment was conducted in the area. Three types of ecosystems were surveyed: the natural forest, agroforestry ecosystem, and the Earleaf Acacia stand. Acacia auriculiformis A. Cunn. ex Benth. and Tabernaemontana pandacaqui Poir.were found to be the most populous for trees and understorey species, respectively. A. auriculiformis is an introduced species used in the rehabilitation and phytoremediation, while T. pandacaqui is a Philippine native species thriving in the area. The effect of their coexistence in an ecosystem is an interesting aspect to study. Therefore, these two species were used as the subjects of the experiment.

Isolation, characterization, and mass production of endomycorrhizas

Eight (8) abundant wildling species were collected still attached to its soil at the rhizosphere layer. These species were Trema orientalis (L.) Blume, Guioa koelreuteria (Blanco) Merr., Antidesma ghaesembilla Gaertn., Morinda citrifolia L., T. pandacaqui, Pittosporum pentandrum (Blanco) Merr., Alstonia macrophylla Wall. ex G. Don, and A. auriculiformis. For each sample, 25 grams of soil closest to the host plant’s fine roots was subjected under the sieving and centrifugation method as stated in the book of Brunrdett et al. (1996). The soil is then continuously washed with water and sieved in 150 µm and 45µm mesh. After sieving, the remaining soils were placed in centrifuge tubes labeled with the corresponding mesh size, and then water was added. These tubes were placed in a centrifuge for 5 minutes at 2000 revolution per minute (RPM). Next, floating sediments were drawn off. Then, sucrose was poured into the centrifuge tubes. Once again, the tubes were placed in the centrifuge and were left to spin for 2 minutes at 2000 RPM. Following this step, floating sediments were sieved, and soils that settled at the bottom of the centrifuge tubes were discarded. Sieved soils were then placed in a petri dish. All were examined under the microscope and were isolated. The genus and individual count of the spores were recorded. The same procedures were done for each soil sample for the plant species collected. All isolated spores were then mass produced at UPLB BIOTECH laboratory and harvested with Centrosema pubescens Benth. as host plants.

Experimental set-up and observation

The interaction of T. pandacaqui and A. auriculiformis under different soil amendments were observed from January 4, 2018, to April 4, 2018. To have an in-depth observation of the influence of A. auriculiformis to T. pandacaqui, some 50 T. pandacaqui were planted individually in 4 x 8 cm polyethylene bags, and 50 T. pandacaqui with A. auriculiformis were planted together in 4 x 8 cm polyethylene bags (Figure 2). Sterilized soils collected from the old mining site were used as planting media.  Both T. pandacaqui with and without A. auriculiformis were subjected to five soil amendments with ten replicates for each amendment. The following treatments were done, and all arranged based on CRD:

Figure 2: Experimental layout showing T. pandacaqui (a) and set up with A. auriculiformis () *(Note for colored tags: Control (white), Marinduque inoculants (blue), MykoVAM (green), Bio-N (yellow), and combination (red))

a. Control – sterilized soil only

b. Marinduque inoculants – sterilized soil plus the mass-produced endomycorrhizal inoculant

c. MykoVAM – sterilized soil plus MykoVAM biofertilizer (a biofertilizer produced by UPLB Biotechnology which aims to aid in nutrient and water absorption)

d. BIO-N – sterilized soil plus BIO-N biofertilizer (a biofertilizer produced by UPLB Biotechnology which has effective nitrogen supplement for plants)

e. The combination of all treatments – sterilized soil plus massed produced endomycorrhizal inoculant, MykoVAM, and BIO-N.

Initial root, height, and diameter were recorded for all plants. The plant’s height and diameter were measured in the level of the half of the height of polyethylene bag at least once a week. Changes in the diameter, height, and distinguishable physiological differences of the plants were recorded.

Results and Discussion

Endomycorrhizal assessment and mass production

The endomycorrhizal fungi isolated from sampled ecosystem were characteristically microscopic that ranged from 45-150 µm. Their colors were transparent to white, pale to dark yellow, orange to dark orange, or brownish to dark brown. Some spores were double-walled with shapes characteristically spherical, oblongate, ellipsoidal, or irregular. Other endomycorrhizal isolates exhibited hyphal attachments that were connected to fine roots (Table 2 & Figure 3).

Table 2. Morphological description of endomycorrhizal genera isolated from different host plants and vegetative genera.

Figure 3: Samples of (a) Acaulospora, (b) Gigaspora, (c) Glomus, and (d) hyphal attachment of endomycorrhizal isolates from rhizosphere in inactive/abandoned mining site in Brgy. Capayang, Mogpog, Marinduque.

There were only three genera of endomycorrhizal fungi that were analysed in terms of spore density across all plots. Endomycorrhizal spores consisted of 228 Acaulospora (54%), 116 Acaulospora with hyphae (28%), 73 Glomus (17%), and 5 Gigaspora (1%) (Figure 4). Acaulospora was the most prevalent among the AMF isolated.

Figure 4: The proportion of endomycorrhizal genera of spores isolated from rhizosphere across vegetation formerly mined from Brgy. Capayang, Mogpog, Marinduque.

Based on host plants, species A. auriculiformis had the highest spore count among the few soil samples of various plant species. A total of 101 individual counts of endomycorrhizal fungi were isolated from the rhizosphere of A. auriculiformis (Figure 5). The spores per 75 g of dry soil comprised of Acaulospora (71) and Glomus (30). The number of spores isolated from rhizosphere per 75 g of dry soils of T. pandacaqui comprised of Acaulospora (50), Glomus (16), and Gigaspora (2).

Figure 5: Total number of isolated spores (per 75 g dry soil) from different host plants growing within ecosystems in inactive/abandoned mine

The diversity of endomycorrhizal fungi is significantly influenced by nutrient composition and soil texture varying in capacity and tolerance (Zhao, et al., 2017).  Certain species are drought resistant, tolerant to pathogens, and could endure extreme soil temperature (Aggangan et al., 2015). Acaulospora, Glomus, and Gigaspora are particularly tolerant to acidic and/or alkaline soils (Clark, 1997). These species are resistant to harmful soil conditions like the mine site left in Brgy. Capayang, Mogpog, Marinduque.

According to Lovelock et al. (2003), endomycorrhizal fungi are interdependent with plant roots. Growth rates or productivity could influence the sporulation of fungal symbionts. A greater volume of spores was observed with fast-growing species. A. auriculiformis is a fast-growing species, and its physiological characteristics affected the abundance of endomycorrhizal fungi.

After mass production, the number of isolated spores gathered from the rhizosphere of 75 g dry soil of T. pandacaqui was comprised of 212 spores in 150 µm sieve, and 228 in 45 µm sieve collected and were utilized as soil amendments for the experimental setup. Spores that were isolated were relatively large, have a range of colors from yellow to dark orange and light to dark brown, and the shapes were oblongate and ellipsoidal (Table 3), and the majority were Acaulospora species.

Table 3. Endomycorrhizal fungi (spore/ 25 g dry soil) isolated from rhizosphere of T. pandacaqui with sizes and description.

There were no reports and/or studies found discussing the symbiotic relationship between T. pandacaqui and endomycorrhizae. It is possible that the two have no special interaction, and it only exhibits only typical association of plants and endomycorrhizae. Nevertheless, the isolated spore may be accounted for T. pandacaqui to in such an environment as the copper-rich soil of the former mining site in Mogpog, Marinduque. Aggangan and Cortes (2018) stated that endomycorrhizal fungi enhance plant tolerance to heavy metal contamination and induced Cu retention in the roots of the seedlings. Indigenous endomycorrhizal fungi have the potential in the rehabilitation of mined-out areas in the Philippines.

Growth Response of T. pandacaqui

Growth and survival of T. pandacaqui with A. auriculiformis and amendments were not significant in height and diameter at first month. Development of T. pandacaqui at 60 days after inoculation showed 50% mortality when planted with A. auriculiformis. Growth of T. pandacaqui was improved when applying MykoVAM than all other amendments. At harvest, the growth of T. pandacaqui was better with A. auriculiformis. The height of T. pandacaqui with and without A. auriculiformis was significantly different (p < 0.01). Height measurement of T. pandacaqui with A. auriculiformis in the control was the highest and grew by 21%. And the highest height value of T. pandacaqui without A. auriculiformis was MykoVAM by 11% among all treatments (Figure 6).

Figure 6: Final Height of T. pandacaqui in response to inoculation of amendments when planting with or without A. auriculiformis (Note: MI- refers to Marinduque inoculants; All-means combination of all treatments)

Height increment every 30 days of T. pandacaqui with A. auriculiformis applied with MykoVAM had the highest increment and by the end of the observation, it grew by 22% (Figure 7a). Control and MykoVAM was significantly different (p < 0.05). The response of T. pandacaqui without A. auriculiformis to Bio-N yielded the highest growth increment in height by 14% after 30 days, and from the second month until harvest, control had the highest growth increment by 11% (Figure 7b). Both Bio-N and Control have significant differences (p < 0.05).

Figure 7: Height Increment of T. pandacaqui in response to inoculation of amendments when planting (a) with and (b) without A. Auriculiformis.

On the contrary, T. pandacaqui without A. auriculiformis had a superior final diameter measurement and was significantly different from T. pandacaqui with A. auriculiformis (p < 0.05). Both Control and MykoVAM were the top treatments for the diameter (Figure 8). In T. pandacaqui with A. auriculiformis, MykoVAM had the highest increment for 2 months by 6%, and after 90 days, Marinduque inoculant had the highest diameter increment valuing 38% from the initial (Figure 9a). The diameter of T. pandacaqui alone, Bio-N has the highest diameter increment in 30 days by 14%, in 60 days, Marinduque inoculants had the highest increment valuing 34%, and combination of all treatments after 90 days with 60% increment from the initial planting; no significant difference was observed Figure 9b).

Figure 8: The final diameter of T. pandacaqui in response to inoculation of amendments when planting with or without A. auriculiformis (Note: MI- refers to Marinduque inoculants; All- means the combination of all treatments).

Figure 9: Diameter increment of T. pandacaqui in response to inoculation of amendments when planting (a) with and (b) without A. auriculiformis.

Based on the results, the application of MykoVAM has standout among other treatments; a fungi-based bio-fertilizer developed by UPLB- BIOTECH. It contains spores, infected roots, and propagules of endomycorrhizal fungi that are estimated to replace 60%-85% of plants’ chemical fertilizer requirement and absorb water and nutrients, particularly phosphorus. Also, the inoculants aid the plant in the resistance of pathogens in the roots and increase drought and heavy metal tolerance (de la Cruz, 2012). Phosphorus is vital to the growth of new tissue and the division of cells. It provides the plant the ability to resist disease due to stimulating the fast growth and development of plants (Tajer, 2016). Similar to this amendment was Marinduque inoculant. However, it did not perform well like the MykoVAM. It may be possible that the spores that can be found in the Marinduque soil that were collected had reduced due to the long storage before it was isolated.

Moreover, it was also observed that Bio-N provided a positive impact on the growth of T. pandacaqui. Bio-N is a microbial-based fertilizer that converts nitrogen gas into its available form and reduces the nitrogen requirements of crops up to 50%. It was developed by Dr. Mercedes Umali-Garcia and Teofila S. J. Santos of the National Institute of Molecular Biology and Biotechnology (BIOTECH), University of the Philippines Los Baños (UPLB), in 1985 to lessen the cost of fertilizers (Calibo, n.d.).  Nitrogen is a substantial food for plants that promotes the vegetative part’s growth and development and prompts the uptake and utilization of nutrients, including potassium, phosphorous, and controls overall plant growth (Leghari et al., 2016).

15 replicates out of 50 who survived in the experimental setup for T. pandacaqui with A. auriculiformis and only 5 out of 50 were left in the other set up. Length of the roots and number of leaves were higher in T. pandacaqui with A. auriculiformis (Figure 10 and 11). Root colonization of the ten root samples of T. pandacaqui without A. auriculiformis showed 33% of colonization.  However, T. pandacaqui with A. auriculiformis exhibited higher root colonization having 45% infection. Only the hyphae were accounted for root colonization because other parts of the roots were disfigured during the staining.

Figure 10: Root development of T. pandacaqui in response to planting with and without A. auriculiformis (n=20).

Figure 11: Foliage count of T. pandacaqui in response to planting with and without A. auriculiformis (n=20).

The growth performance of the indigenous species, T. pandacaqui, in terms of root and leaf development was better with A. auriculiformis. The introduced species’ ability to modify soil composition may have aided the growth of T. pandacaqui in the copper-rich soil that came from the former mine site in Brgy. Capayang, Marinduque. A. auriculiformis has a symbiosis in rhizobia and can easily enteran n easily enter relation with new rhizobia species, leading to increased nitrogen content in soil. The association of A. auriculiformis with both ecto- and endo-mycorrhizal fungi may have influenced the percentage of root colonization of T. pandacaqui. A. auriculiformis has a symbiotic relationship with rhizobial bacteria because it provides nitrogen for the species to grow on low nitrogen soils (Duponnois & Planchette, 2003). Treatments with high root colonization were microbial- and fungi-based fertilizers. The species can also significantly raise the soil organic carbon and the cation exchange capacities (Schmerbeck & Naudiyal, 2014). Several countries are using A. auriculiformis. for phytoremediation. It’s popular for revegetating mine spoils due to its rapid growth and regeneration that result in abundant litter fall, improving the physio-chemical properties of soilin abundant litter fall, improving the of soilin abundant litter fall, which improvesin abundant litter fall improving soil's physio-chemical properties. The species’ phyllodes provide good, long-lasting mulch (Orwa et al., 2009).

Conclusion

Acaulospora species were prevalent from sampled rhizosphere across vegetation types compared with Glomus and Gigaspora, but when mass-produced Acaulospora and Glomus prevailed as an inoculant for T. pandacaqui. Further improvement of endomycorrhizal production is necessary to ensure soil inoculant quality for T. pandcaqui and other indigenous plants. The height and diameter of T. pandacaqui were significant with A. auriculiformis brought by its N-fixing property, though host mortality was higher due to competition. Soil inoculants promoted the growth of T. pandacaqui, but MykoVAM performed significantly in height than control. A. auriculiformis could be maintained as a nurse crop for threatened species like T. pandacaqui and consider the potential of local endomycorrhizal isolates to encourage growth and survival.

References

Aggangan, N. S., & Cortes, A. (2018). Screening mined-out indigenous mycorrhizal fungi for the rehabilitation of mine tailing areas in the Philippines. Reforesta, 6, 71-85.

Aggangan, N., Pampolina, N., Cadiz, N., & Raymundo, A. (2015). Assessment of plant diversity and associated mycorrhizal fungi in the mined-out sites of Atlas mines in Toledo city, Cebu for bioremediation. Journal of Environmental Science and Management, 18(1).

Brundrett M., Bougher N., Dell B., Grove T. and Malajczuk N. (1996). Working with Mycorrhizas in Forestry and Agriculture. AClAR Monograph 32. 374 + x p

Calibo, R. (n.d.). Bio-N mixing plant now operates in Lazi.

Clark, R. (1997). Arbuscular mycorrhizal adaptation, spore germination, root colonization,and host plant growth and mineral acquisition at low pH. Plant and Soil, 192(1), 15-22. doi: https://doi.org/10.1023/A:1004218915413.

Contu, S. (2012). Acacia auriculiformis. The IUCN Red List of Threatened Species: https://www.iucnredlist.org/species/19891902/19997222#habitat-ecology.

Coracero, E. & Malabrigo, P. (2020). Carbon storage potential of the tree species along the ultramafic forest in Sitio Dicasalarin, Barangay Zabali, Baler, Aurora, Philippines. AIMS Environmental Science, 7(6), 589-601. https://doi.org/10.3934/environsci.2020037.

Cunningham, S. D., & Ow, D. W. (1996). Promises and prospects of phytoremediation. Plant physiology, 110(3), 715-719.

Dela Cruz, R. (2012). Mykovam: Effective growth enhancer for coconut. Department of Agriculture-Bureau of Agricultural Research (BAR) Chronicle, 13(2).

DENR. (2014). Project Completion Report Annex C: Remediation of Risk. Philippines: National Program Support for Environment and Natural Resources Management Project.

Duponnois, R., & Planchette, C. (2003). A mycorrhiza helper bacterium enhances ectomycorrhizal and endomycorrhizal symbiosis of Australian Acacia species. Mycorrhiza, 13(2), 81-95.

Felix, R. (2004, February 10). DENR eyes rehabilitation of 22 abandoned mines. PhilStar Global: https://www.philstar.com/business/2004/02/10/238345/denr-eyes-rehabilitation-22-abandoned-mines.

Gregory, C. (2004). Marcropper in the Philippines. http://www.umich.edu/~snre492/Jones/marcopper.htm.

Greipsson, S. (2011). Phyotremediation. Nature Education Knowledge, 3(10), 7.

Leghari, S. J., Wahocho, N. A., Laghari, G. M., Laghari, A. H., Bhabhan, G. M., Talpur, K. H., Lashari, A. A. (2016). Role of Nitrogen for Plant Growth and Development: A review. Advances in Environmental Biology, 10(9), 209-219.

Lovelock, C., Andersen, K., & Morton, J. (2003). Arbuscular mycorrhizal communities in tropical forests are affected by host tree species and environment. Oecologia, 135(2), 268-279. doi: 10.1007/s00442-002-1166-3.

Medianista, R., & Labay, P. (2017). Phytosuccession and Phytosociology of Plants in Ino-Capayang Mined-out Area for Possible Phytoremediation Activities in Marinduque. 4th International Conference on Civil, Environment and Waste Management, (pp. 235-239). Manila.

Orwa, C., Mutua, A., Kindt, R., Jamnadass, R., & Anthony, S. (2009). Agroforestree Database:a tree reference and selection guide version 4.0. World Agroforestry: http://www.worldagroforestry.org/treedb/AFTPDFS/Acacia_auriculiformis.PDF.

Plumlee, G. S., Morton, R. A., Boyle, T. P., Medlin, J. H., & Centeno, J. A. (2000). An Overview of Mining-Related Environmental and Human Health Issues, Marinduque Island, Philippines: Observations from a Joint U.S. Geological Survey - Armed Forces Institute of Pathology Reconnaissance Field Evaluation, May 12-19, 2000. U. S. Geological Survey.

Raymundo, A., Aggangan, N., & Pampolina, N. (2006). Bioremediation: can plants and microbes clean up the environment? 2006 FORESPI Symposium: Forest Landscape Restoration and Rehabilitation. Los Baños, Laguna.

Sarian, Z. (2017). Trees Grow Again in Mine-out Fields. http://agriculture.com.ph/2017/01/07/trees-grow-again-in-mined-outfields/.

Schmerbeck, J., & Naudiyal, N. (2014). Acacia auriculiformis. In: Enzyklopädie der Holzgewächse: Handbuch und Atlas der Dendrologie, (pp. 1-12). doi:10.1002/9783527678518.ehg2014002.

Tajer, A. (2016). What’s the function of Phosphorus (P) in plants? Greenway Biotech, Inc.: https://www.greenwaybiotech.com/blogs/news/whats-the-function-of-phosphorus-p-in-plants.

Teves, C. (2018, February 10). DENR raises urgency for mine rehabilitation. Philippine News Agency: https://www.pna.gov.ph/articles/1024785.

The Mosaic Company. (2020). Soil pH. CropNutrition: https://www.cropnutrition.com/efu-soil-ph.

Tulod, A., Castillo, A., Carandang, W., & Pampolina, N. (2012). Growth performance and phytoremediation potential of Pongamia pinnata (L.) Pierre, Samanea saman (Jacq.) Merr. and Vitex parviflora Juss. in copper-contaminated soil amended with zeolite and VAM. Asia Life Sciences, 21(2), 499-522.

Zhao, H., Li, X., Zhang, Z., Zhao, Y., Yang, J., & Zhu, Y. (2017). Species diversity and drivers of arbuscular mycorrhizal fungal communities in a semi-arid mountain in China. PeerJ, 5(e4155). doi:10.7717/peerj.4155.