Cover Single.cdr BIOTROPIA Vol. 27 No. 2, 2020: 171 - 178 DOI: 10.11598/btb.2020.27.2.1203 SOIL PROPERTIES BELOW EXOTIC TREE PLANTATIONS AT THE SAITHONG SILVICULTURAL RESEARCH STATION IN PRACHUAP KHIRI KHAN PROVINCE, THAILAND** KANOKWAN URAIRAK1*, ROONGREANG POOLSIRP AND SAN KAITPRANEET2 1Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand department of Silviculture, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand Received 14January 2019 / Accepted 29 March 2019 ABSTRACT Often, exotic trees grow very fast in their new environments and sometimes even faster than the indigenous ones. These exotics seem to be tougher and live through harsher conditions. They may be drought resistant, and they can survive long periods with little to no rainfall. These trees are fast growing and are resistant to pests and diseases which tend to damage the locally growing trees. Moreover, these trees can help improve soil physical and chemical properties. Hence, the objective of this study was to compare the physical and chemical properties of soil under different tree plantations, namely Fucalyptus urophylla, Acacia crassicapa and Acacia aulacocarpa at the Saithong Silvicultural Research Station. Three soil composite samples were collected from soil depths of 0 - 10, 10 - 30, and 30 - 50 cm, in the three stands aged 27-year old and planted at a spacing of 2 x 2 m. Soil type in these stands was sandy loam. Soil moisture percentage (12.85%) was significantly the highest in the E. urophylla plantation. Phosphorus (18.2 mg/kg DW), magnesium (6.98 cmolc/kg DW), and potassium (6.98 cmok/kg DW) contents were also the greatest in the E. urophylla plantation. Organic matter content was the highest in the A. crassicapa plantation (7.59%), while nitrogen (0.04%) and calcium (36.17cmolc/kg DW) contents were the highest in the A. aulacocarpa plantation. Significant differences were also observed in the soil bulk density and porosity attributable to structure, tillage, cropping practices, soil depth and compaction. Characterization of soil pore system is equally important in understanding soil physical and parent material composition, which has a direct impact on soil chemistry and fertility. Parent material rich in soluble ions (calcium, magnesium, potassium and sodium) is desirable, as these chemicals are easily dissolved in water and readily available for plants. For soils with poor nutrient content found in warm and humid tropical climates with low water supply, E. urophylla, A. crassicapa and A. aulacocapa have the potential to enhance the aboveground stand production. Undoubtedly, when planted, they can help restore the soil nutrients faster and therefore, can be used in forest plantations or in agroforestry farms. Keywords: exotic tree plantation, Saithong Silvicultural Research Station, soil properties INTRODUCTION Soil properties are important tree growth factors and as such, maintaining soil quality is paramount for tree health. Soil degradation, on the other hand, involves the destruction of soil structure through loss of organic matter, due to factors related to topography, climate, and poor soil management (Ozdemir 1993; Haynes 2000) ^Corresponding author, e-mail: Kanokwan.urai@ku.th; K_Urairak@hotmail.com **This paper was presented at the 3rd International Conference on Tropical Biology 2018, 20-21 September 2018, Bogor, West Java, Indonesia Often, exotic tree species possess more useful attributes than the native species, such as, a faster growth rate, fewer pests and reduced competition, all leading to a higher economic value (Kalinganire 1996; Tavares et al. 1999; Lott et al. 2000; Takaoka 2008; Anglaaere et al. 2011; Tefera et al. 2014). Acacia aulacocarpa and Acacia crassicarpa not only have shown potential for reclaiming deforested areas, but also for shelterbelts, coastal sand dunes fixation, soil improvement and land rehabilitation on a wide range of degraded sites, as a result of their rapid growth, ability to suppress weeds and abundant Skhiyobium nodulation (Doran & Gunn 1987). 171 BIOTROPIA Vol. 27 No. 2, 2020 Eucalyptus is also one of the most commonly used species for reforestation in commercial plantations, with the added benefit of being an exotic species in study site which can promote the establishment of a stable forest cover in degraded sites (Farias et al. 2016). Eucalyptus spp. and Acacia spp. can grow rapidly in poor soils (Yang et al. 2009; Chen et al. 2011). Planting exotic trees in tropical countries is becoming an increasingly important forestry activity as many tropical countries that depended on wood supply from natural forests are now recognizing the need to establish plantations to augment supplies from the dwindling and unsustainable natural forests. The total area of tropical forest plantations increased from about 6.7 million ha in 1965 to 109 million in 2005. Though most species used for tropical plantations are already fast growing, their growth rate can still be improved substantially through appropriate silvicultural practices, such as site-species matching, site nutrient management, and the use of hybrid species. Sandy soil types are well aerated, thereby, allowing suitable seeds to germinate easily and the roots to penetrate properly. However, such soil types have an inherent disadvantage of being nutrient hungry as nutrients are easily leached away due to excellent soil drainage. Plants that are grown on a sandy loam soil need frequent irrigation and fertilization to maintain a healthy growth. The best way to improve a sandy loam soil is to mix organic matter into the soil. Applying a 2- to 4-inch thick layer of compost or peat moss over the area can significandy improve the ability of a sandy loam soil to hold nutrients. Many eucalyptus plantations are grown on usually nutrient deficit sandy soil types. Hence, attempts were made to plant eucalyptus in semi-arid regions on sandy soil types. In this study, leguminous trees can be planted to improve the soil nitrogen content, this can change the litter quality and soil biology. For a sustained fast-growing exotic tree plantation, it is important to manage sandy soil types in order to increase the soil organic matter content. Silvicultural practices, which help increase the organic matter content in Eucalyptus spp. and Acacia spp. plantations on sandy soil types, are also essential for their sustainability. Moreover, one of the major interests in Acacia is the compounds present in the plant parts that can be used as natural herbicides. Evidence of the role of allelopathy in weed control has been cited in many publications (Chou 1995; Rice 1995; Rizvi & Rizvi 1992; Waller 1987). The introduction of exotic plants may alter the nutrient cycle of the system either direcdy, by modifying the quality and quantity of litter entering the soils beneath, or indirectly, by altering the physico-chemical site properties below their canopy. Moreover, the planting of native species on degraded land are highly inappropriate and of high risk due to drought, insect pests and low soil fertility. As such, exotic species are planted to improve the poor soil condition in such an area. Hence, the objective of this study is to compare the soil physical and chemical properties of three plantations of Eucalyptus urophylla, Acacia crassicarpa and Acacia aulacocarpa on a site with sandy soil types and rain all year round to be an idea for soil develops in the future., to help increase the productivity of exotic tree plantations planted on sandy soil types, as was present in the study area. MATERIALS AND METHODS The study was conducted at the Saithong Silvicultural Research Station, Prachuap Khiri Khan District, Thailand located at 11°17’04” N latitude and 98°50’25” E longitude (Fig. 1). The area receives a mean annual rainfall of 1,175.9 mm, with temperatures ranging from a minimum of 16.3 °C in February to a maximum of 42.9 °C in July, with an annual mean of 28.4 °C. Its soil is primarily of the dark brown to brown (7.5YR4/4) loam to sandy loam (Sadao series) which has moderately fine to medium texture and very strongly acid (field pH 4.5). Using a completely randomized design, three soil samples were collected from soil depths of 0 - 10, 10 - 30, 30 - 50 cm at the E. urophylla, A. crassicarpa and A. aulacocarpa stands, aged 27- year old and planted at a spacing of 2 x 2 m. The soil physical and chemical properties were determined as follows; the bulk density using the core method (Jalota et al. 1998), particle density by the pycnometer method (Jalota et al. 1998) using an air comparison pycnometer, soil texture by the hydrometer method, pH in 1:1 soil: water suspension by a pH meter, organic matter using the Walkley and Black’s rapid titration method 172 Soil properties below exotic tree plantations at Saithong Silvicultural Research Station — Urairak et al. kfiiaii:io]:MGuivi toUl glnrnY?fLi -T*T ?̂nYcnn bi ix 6 col:I m mmm mam [S**, mt fmm WM&SB fcAjBmmbak 0 95 190 380 570 780 AZT 1 Sqmra kKonwtan 1:10,000 Figure 1 Saithong Silviculture Research Station, Prachuap Khiri Khan Province, Thailand (Walkley & Black 1934), total nitrogen using the Dumas or dry combustion method (Jackson 1965) using a CHNS analyzer PerkinElmer series 2400 Series II CHNS/O Elemental Analyzer, the available phosphorus (P) by Bray II method (Bray & Kurtz 1945) using a spectrometer, and the exchangeable potassium (K), calcium (Ca) and magnesium (Mg) in ammonium acetate (NEUOAc) 1 N pH 7.0 by using an atomic absorption spectrometer. Variance analyses of the experimental data were done using the SPSS statistic program at a significance level of p < 0.05. To monitor soil changes induced by tree growth, the soil properties, organic matter accumulation and decomposition, as well as nutrient cycling are measured on several sample plots over the full rotation of a plantation (Lundgren 1978). In this study, soil sampling was carried out in the E. urophylla, A. crassicarpa and A. aulacocarpa plantations aged 27-year old located near each other on the same soil type. RESULTS AND DISCUSSION Soil Physical Properties The three stands were of a sandy loam type and experienced rain all year round. Soil moisture percentage was the highest in E. urophylla plantation (12.85%) (Table 1). The potassium (18.2 mg/kg), Mg (6.98 cmok/kg DW) and P (6.98 cmolc/kg DW) contents were the highest in the E. urophylla plantation. (Table 2). The organic matter content was the highest in A. crassicarpa plantation (7.59%). Calcium (36.17cmolc/kg DW) content was the greatest in A. aulacocarpa plantation (Table 2). These results revealed that leguminous species are instrumental in increasing soil organic matter content, invariably with the presence of the nitrogen-fixing bacteria (Turnbull 1984; Bernhard-Reversat 1993; Bernhard-Reversat et al. 1993; Higa & Higa 2000). However, the leguminous species can also cause soil acidification (Binkley 1992; Yamashita et al. 2008; 173 BIOTROPIA Vol. 27 No. 2, 2020 Table 1 Soil physical properties in the 27-year old E. urophylla, A. crassicarpa and A. aulacocatpa plantations at the Saithong Silvicultural Research Station, Prachuap Khiri Khan Province, Thailand Soil parameters Depth (cm) E. urophylla A.crassicarpa A. aulacocarpa F-value Particle density (g/cm3) 0-10 2.55 2.53 2.58 0.49ns 10-30 2.58 2.53 2.71 2.47ns 30-50 2.66 2.61 2.59 0.48ns Mean 2.60 2.56 2.63 Bulk density (g/cm3) 0-10 0.89b 0.91ab 0.92a 7.36* 10-30 0.89b 0.91a 0.90ab 5.75* 30-50 0.88b 0.91a 0.90a 16.88* Mean 0.89 0.91 0.91 Porosity (%) 0-10 33.84 34.82 34.70 0.70 ns 10-30 33.52 34.93 32.49 2.30 “ 30-50 32.05 33.80 33.90 1.84 ns Mean 33.14 34.52 33.70 Soil moisture (%) 0-10 12.76a 10.28ab 8.63b 7.36* 10-30 12.16a 9.96b 10.52b 5.81* 30-50 13.63a 10.25b 10.83b 17.26* Mean 12.85 10.16 9.99 Sand (%) 0-10 74.89 76.35 74.65 0.10ns 10-30 73.68 76.35 75.68 0.35ns 30-50 74.23 74.77 74.35 0.01ns Mean 74.27 75.82 74.89 Silt (%) 0-10 13.45 10.67 11.33 0.28ns 10-30 12.00 9.33 10.00 0.27ns 30-50 6.79 6.67 8.67 2.50ns Mean 10.74 8.89 10.00 Clay (%) 0-10 11.65 12.99 14.03 3.05ns 10-30 14.32 14.32 14.32 0.00ns 30-50 18.99 18.56 16.99 0.19ns Mean 14.99 15.29 15.11 Notes: * = significant difference at p < 0.05; Superscripts a and b in the same row indicate significant differences at p < 0.05 using Duncan’s Multiple Range Test; ns = not significant. Kasongo et al. 2009; Koutika et al. 2014). Differences in soil bulk density and porosity was caused by structure, tillage practices before and after harvest, cropping practices, soil depth and soil compaction. Heavy machinery can cause soil compaction, as well as feet stomps caused by animal passage. Characterization of soil system is important for understanding the composition of soil physical and parent materials or drift deposits, as these have a direct impact on soil chemistry and fertility. Parent materials rich in water soluble ions, such as Ca, Mg, K, and sodium (Na), are easily dissolved in water and readily available for absorption by plants. The mean bulk density values were generally higher for A. crassicarpa (0.91 g/cm3) and A. aulacocarpa plantations (0.91 g/cm3) (Table 1). In the upper 0-10 cm of the soil profile, the values indicated a greater soil compaction for the A. aulacocarpa plantation (2.63 g/cm3) compared to the deeper soil layers. Soil in the plantations was mainly of a sandy loam type. At the study sites, the data showed no evidence of textural change as a result of reforestation with E. urophylla, A. crassicarpa and A. aulacocarpa plantations. At the time scale involved, a comparatively stable property like soil texture is unlikely to undergo drastic changes as a result of planting the E. urophylla, A. crassicarpa and A. aulacocarpa trees. The porosity mean was higher in the A. crassicarpa plantation (34.52%) and the mean of soil moisture was higher in the E. urophylla plantation (12.85%) (Tablet). Moisture retention is an important factor governing soil fertility. However, sand is porous and has a low water retention capacity Therefore, soil types in the study sites are of poor quality as they are of the sandy texture. 174 Soil properties below exotic tree plantations at Saithong Silvicultural Research Station — Urairak et at Soil Chemical Properties Table 2 Soil chemical properties in the exotic tree plantations of E. urophylla, A. crassicarpa, and A. aulacocarpa, at the Saithong Silvicultural Research Station, Prachuap Khiri Khan Province Soil parameters Depth (cm) E. urophylla A. crassicarpa A. aulacocarpa F-value pH 0-10 4.20a 3.87b 3.89b 10.52* 10-30 4.64a 4.03b 4.10b 5.89* 30-50 4.71 4.23 4.39 0.08 ns Mean 4.52 4.04 4.13 Organic matter (%) 0-10 0.73 1.18 2.46 1.36 ns 10-30 2.01 2.52 5.83 1.60 ns 30-50 6.69 3.89 6.24 0.76 ns Mean 3.14 7.59 4.84 N (%) 0-10 0.03b 0.04b 0.06a 9.80* 10-30 0.02 0.03 0.03 1.00 ns 30-50 0.02 0.03 0.02 1.50 ns Mean 0.02 0.03 0.04 P (mg/kg) 0-10 2.55 1.59 1.73 2.28 ns 10-30 1.68 1.28 1.25 0.19 ns 30-50 1.24 1.04 0.59 0.95 ns Mean 1.82 1.30 1.19 K (cmolc/kg DW) 0-10 9.05 8.28 9.03 0.25 ns 10-30 5.67 4.89 5.69 1.26 ns 30-50 6.21 6.16 5.94 0.27 ns Mean 6.98 6.44 6.72 Ca (cmolc/kg DW) 0-10 31.57b 36.45b 54.90a 8.04* 10-30 25.82 24.02 27.96 0.31 ns 30-50 29.99 25.15 25.64 0.32 ns Mean 29.13 28.54 36.17 Mg (cmolc/kg DW) 0-10 9.05 8.28 9.03 0.25 ns 10-30 5.67 4.89 5.69 1.26 ns 30-50 6.21 6.16 5.94 0.27 ns Mean 6.98 6.44 6.72 Notes: superscripts a and b, across row, indicate significant differences at p < 0.05 using Duncan’s Multiple Range Test; ns = not significant. Soil in all three plantations was generally acid. The pH values were higher in E. urophylla plantation (pH 4.52) (Table 2), although the difference was not significant at a depth of 30- 50 cm at any sites. Organic matter content was the highest in the A. crassicarpa plantation (7.59%) and its level was lower at a depth of 0-10 cm. Total nitrogen content was the highest in the A. aulacocarpa plantation (0.04%) and was maximum at a depth of 0 - 10 cm. In a similar study, soil N improved significantly under the canopy of Acacia sieheriana trees in Songa Pastures, Rwanda (Mugunga & Mugumo 2013). Acacia has high rates of nitrification and high nitrogen availability for plants (Marchante et al. 2008). Soil nitrogen and nitrogen produced from the fallen leaves are tightly associated with ecosystem processes that support aboveground biodiversity, the living biomass above the soil including the stem, stump, branches, bark, seeds and foliage (Clark & Tilman 2008; Hautier et al. 2009; Dickson & Foster 2011). P level was the highest in the E. urophylla plantation (6.98 mg/kg) and it was maximum at a depth of 0-10 cm (Table 2). Nutrient quantities, organic matter and exchangeable cations (Ca, Mg, and K) expressed in cmok/kg DW, were obtained from soil profiles of the E. urophylla, A. crassicarpa, and A. aulacocarpa plantations (Table 2). The nitrogen (0.04%) and Ca (17cmolc/kg DW) contents were the highest in the A. aulacocarpa plantation (Table 2). As indicated by the exchangeable cation analyses, K content was the greatest in the E. urophylla plantation (6.98cmok/kg DW) and its level was maximum at a depth of 0-10 cm. Mg content was the highest in the E. urophylla plantation (6.98 cmok/kg DW) and was 175 BIOTROPIA Vol. 27 No. 2, 2020 maximum at a depth of 0-10 cm, in all the plots. Ca content (36.17 cmolc/kg DW) was highest in the A. aulacocarpa plantation and was maximum at a depth of 0-10 cm. Soil chemical properties were higher at the surface layer of 0 - 10 cm depth compared to those in the subsoils layers at 10 - 30 and 30 - 50 cm depths. This can be attributed to a higher concentration of organic matter in the upper soil layer, a condition that commonly occurs in most tropical soils (Obatolu & Ibiremo 1999). Results of soil analyses on physical properties show that the soil texture remained unchanged under the three vegetation types. Most soils in Thailand, being sandy, tend to have poor mineral absorption, low acidity and low absorption. The presence of exotic tree plantations was expected to improve the sandy soil quality. The introduction of exotic plants could have altered the nutrient cycle of the system either directly, by modifying the quality and quantity of litter entering the soils beneath, or indirectly, by altering the physical-chemical site properties below their canopy. In sandy soils, the ability of a plant's root or trunks in making food is lowered, making their growth slower and inefficient. Generally, the encroachment of exotic plant species poses a major threat to the biodiversity and ecosystem stability, however, little attention is given to the potential impacts of these invasions on soil nutrient cycling. The differences between exotic and native species do not show trends in other components of the nutrient cycles, for example, the size of soil pools of carbon and nitrogen. In some cases, a given species can have different effects on different sites, suggesting that the new composition can positively affect the invaded community and its environment such as the soil type, thereby influencing the direction and magnitude of the impact at an ecosystem-level. Exotic plants can alter the soil nutrient dynamics particularly in terms of biomass and productivity, tissue chemistry, plant morphology and phenology. As such, research may focus on these issues and integrate these in evaluating the impacts of invasive species. Individual species have affected the various components of the carbon and nutrient cycles, including the pools of aboveground and belowground carbon, nitrogen, and other elements; the net primary productivity and plant growth rates; the chemical quality and rates of litter fall; and the nutrient and carbon mineralization rates. These evidences strongly suggest that when a community undergoes species composition changes due to the invasion and spread of exotic species, subsequent changes in the nutrient cycling processes are expected. CONCLUSION The adoption of exotic trees in agroforestry systems or forest tree plantations can significantly impact the soil nutrient content as a result of their rapid leaf turnover and nutrient release through decomposition leading to significant increases in soil fertility. In this study, the E. urophylla, A. crassicarpa and A. aulacocarpa trees helped improve soil fertility which most likely could enhance the herbage productivity, subsequently improving the agroforestry systems. Integrating these species in agroforestry systems and preserving these trees in arid and semi-arid areas could help maintain and enhance the sustainability of these ecosystems. Eucalyptus spp. and Acacia spp. not only grow on soils with poor quality, but can also grow rapidly and survive under long periods without rainfall. The Acacia and Eucalyptus spp. in the present study played a significant role in improving the soil structure and the availability of essential nutrients. ACKNOWLEDGEMENTS This research was supported by Saithong Silvicultural Research Station for study site and the Laboratory of Forest Soil, Department of Silviculture, Faculty of Forestry, Kasetsart University, Thailand. REFERENCES Anglaaere LCN, Cobbina J, Sinclair FL, McDonald MA. 2011. The effect of land use systems on tree diversity: farmer preference and species composition of cocoa-based agroecosystems in Ghana. Agrofor Syst 81:249-65. Bernhard-Reversat F. 1993. Dynamics of litter and organic matter at the soil-litter interface in fast-growing 176 Soil properties below exotic tree plantations at Saithong Silvicultural Research Station — Urairak et al. tree plantations on sandy ferrallitic soils (Congo). ActaOecol 14:179-95. Bernhard-Reversat F, Diangana D, Tsatsa M. 1993. Biomasse, mineralomasse et productivite en plantation d’ Acacia mangium et A. auriculiformis au Congo. Bois etFoUets des Tropiques 238:35-44. Binkley D. 1992. Mixtures of nitrogen-fixing and non¬ nitrogen-fixing tree species. In: Cannell M, Malcolm D, Robertson P, editors. The Ecology of Mixed-Species Stands of Trees. Oxford (UK): Blackwell Scientific, p. 99-123. Bray RH, Kurtz LT. 1945. Determination of total organic and available form of phosphorus in soil. Soil Sci 59:39-45. Chen D, Zhang C, Wu J, Zhou L, Lin Y, Fu S. 2011. Subtropical plantations are large carbon sinks: Evidence from two monoculture plantations in south China. Agric For Meteorol 151:1214-25. Chou CH. 1995. Allelopathy and sustainable agriculture. In: Inderji, Dakshini KMM, Einhellig FA, editors. Allelopathy: Organisms, Processes and Applications. ACS Series No. 582. Washington DC (US): American Chemical Society, p. 211-23. Clark CM, Tilman D. 2008. Loss of plant species after chronic low-level nitrogen deposition to prairie grasslands. Nature 451:712-5. Dickson TL, Foster BL. 2011. Fertilization decreases plant biodiversity even when light is not limiting. EcolLett 14:380-8. Doran JC, Gunn BV. 1987. Treatments to promote seed germination in Australian acacias. In: Turnbull JW, editor. Australian Acacias in Developing Countries. ACIAR Proceedings No 16. p. 57-63. Farias DJ, Marimon SB, Silva DL, Petter AF, Andrade RF, Morandi SP, Marimon JR. 2016. Survival and growth of native Tachigali vpilgaris and exotic Eucalyptus urophylla x Eucalyptus grandis trees in degraded soil with biochar amendment in southern Amazonia. For Ecol Manage 368:173-82. Hautier Y, Niklaus PA, Hector A. 2009. Competition for light causes plant biodiversity loss after eutrophication. Science 324:636-8. Haynes RJ. 2000. Interactions between soil organic matter status. Cropping history, method of quantification and sample pretreatment and their effects on measured aggregate stability. Biol Fert Soils 30(4):270-5. Higa AR, Higa RCV. 2000. Indicates de especies para o reflorestamento [Species indications for reforestation]. In: Galvao APM, editor. Reflorestamento de Propriedades Rurais Para Fins Produtivos e Ambientais um Guia Para Agoes Municipais e Regionais. Colombo (LK): Embrapa Florestas. p. 101-24. Jackson ML. 1965. Soil chemical analysis-advance course. Wisconsin (US): Department of Soils, University of Wisconsin. Jalota SK, Khera R, Ghuman BS. 1998. Methods in soil physics. New Delhi (IN): Narosa Publishing House. Kalinganire A. 1996. Performance of Grevillearobusta in plantations and on farms under varying environmental conditions in Rwanda. For Ecol Manage 80:279-85. Kasongo RK, Van Ranst E, Verdoodt A, Kanyankagote P, Baert G. 2009. Impact of Acacia auriculiformis on the chemical fertility of sandy soils on the Bateke plateau, DR Congo. Soil Use Manage 25:21-7. Koutika LS, Epron D, Bouillet JP, Mareschal L. 2014. Changes in N and C concentrations, soil acidity and P availability in tropical mixed acacia and eucalypt plantations on a nutrient-poor sandy soil. Plant Soil 379:205-16. Lott JE, Howard SB, Ong CK, Black CR. 2000. Long¬ term productivity of a Grevillearobusta-based overstorey agroforestry system in semi-arid Kenya. II. Crop growth and system performance. For Ecol Manage 139:187-201. Lundgren B. 1978. Soil conditions and nutrient cycling under natural and plantation forest in Tanzanian Highlands. Rep For Ecol For Soils 31:426. Marchante E, Kjller A, Struwe S, Freitas H. 2008. Short- and long-term impacts of Acacia longifolia invasion on the belowground processes of a Mediterranean coastal dune ecosystem. Appl Soil Ecol 40:210-7. Mugunga CP, Mugumo DT. 2013. Acacia sieberiana effects on soil properties and plant diversity in Songa Pastures, Rwanda. IntJ Biodivers 2013:11. Obatolu CR, Ibiremo OS. 1999. Use of organic materials for raising cocoa seedlings. In: 25th Annual Conference of Soil Science Society of Nigeria. Proceedings: 1999 November 25-21; Benin City, Edo State, Nigeria, pp. 152-6. Ozdemir N. 1993. Effects of admixturing organic residues on structure stability and erodibility of soils. J Faculty Agric 24(l):75-90. Rice EL. 1995. Biological control of weeds and plant diseases: Advances in applied allelopathy. Norman (US): University of Oklahoma Press. Rizvi SJH, Rizvi V, editors. 1992. Allelopathy: Basic and applied aspects. London (UK): Chapman & Hall. Takaoka S. 2008. Long-term growth performance of Cordia africana and Grevillea robusta trees in the Mount Kenya region. Agrofor Syst 72:169-72. Tavares FC, Beer J, Jimenez F, Schroth G, Fonseca C. 1999. Experiencia de agricultores de Costa Rica con la introduction de arbolesmaderables en plantaciones de cafe [Experience of Costa Rican 177 BIOTROPIA Vol. 27 No. 2, 2020 farmers with the introduction of timber trees in coffee plantations]. Agrofor Am 6(23):17-20. Tefera B, Ruelle ML, Asfaw Z, Tsegay BA. 2014. Woody plant diversity in an Afromontane agricultural landscape (Debark District, northern Ethiopia). For Trees Livelihoods 23:261-79. Turnbull JW. 1984. Six phyllodinous acacia species for planting in the humid tropical lowlands. Pesquisa Agropecuaria Brasileira 19:69-73. Walkley A, Black CA. 1934. An examination of Degtjureff method for determining soil organic matter and a proposed modification of the chroma acid titration method. Soil Sci 37:29-35. Waller GR, editor. 1987. Allelochemical: Role in agriculture and forestry. ACS Symposium Series 330. Washington DC (US): American Chemical Society. Yamashita N, Ohta S, Hardjono A. 2008. Soil changes induced by Acacia mangium plantation establishment: Comparison with secondary forest and Imperata cylindrical grassland soils in South Sumatra, Indonesia. For Ecol Manage 254:362-70. Yang L, Liu N, Ren H, Wang J. 2009. Facilitation by two exotic Acacia: Acacia auriculiformis and Acacia mangium as nurse plants in south China. For Ecol Manage 257:1786-93. 178