Existing Dilemma of the Impact of Eucalyptus Species on Crop Yield and Soil Nutrients in Ethiopia: Review Paper

Eucalyptus is an ever green fast-growing tree species which is introduced in Ethiopia during the regime of Emperor Menelik II to solve the scarcity of wood products especially for fuel wood and construction materials. Eucalyptus is widely planted in Ethiopia due to the economic benefit and special nature of the species such as, it needs minimum care, fast growing, good for fuel and construction material and easily propagation. However, there is dilemma the impact of Eucalyptus on crop yield and soil nutrients. Therefore, this review focused on the influence of eucalyptus species on crop productivity and soil nutrients. Several studies reported that planting of Eucalyptus species nearby or alongside of crop growing areas reduces the productivity of crops by up taking more nutrients and water as compared to neighbouring species whereas, another studies reported that higher crop yield and soil nutrients were obtained from crops growing in the clear-felling eucalyptus stands as compared to continuous cultivated crop lands. In conclusion, Eucalyptus has a negative and positive influence on soil nutrients and crop yield depends on the management planning and tending operation of the species. Therefore, to reduce the adverse effect of eucalyptus giving more attention on its management is very vital.

in Eucalyptus plantation are often more than 20% (Jagger and Pender, 2003).
But, despite on its economic benefit, most of the studies shows that the species has adverse effect on the crop yield and soil nutrients when it grows mixed with crop lands. There is evidence that competition for water and nutrients are always exist when we grow two or more plants in the same field. Consequently, production of agricultural crops which is grown nearby Eucalyptus plantation significantly decline due to the competition of available resources like water, light and nutrient, and shading effect (Gindaba, 2003;Chanie et al., 2013). According to Chanie et al., (2013) crop yield and soil nutrient reduction occurred up to 15 m distance from the eucalyptus plantation. On the other hand the work of Desalegn et al., (2014) suggested that the higher crop yield was obtained from the land which was previously cultivated eucalyptus lands as compared to the continuous crop cultivation lands. Similarly, the higher soil chemical properties and organic matter were recorded from the land previously cultivated eucalyptus land as compared to the adjust crop lands (Birru et al., 2013).
Such uncertainty on ecological implications of exotic trees like Eucalyptus species, which have been used for industrial purpose as well as for agro-forestry are often rise questioned since their ecology has not been appropriately studied (Bernhard-Reversat, 1999). Therefore, the overall aim of this review is to show the influence of eucalyptus species on crop yield and soil nutrients in Ethiopia.

Effect of Eucalyptus Plantation on Crop Yield
Several studies reported that Eucalyptus plantation significantly affects crop production by utilizing readily available nutrients for rapid growth, which leads to reduce available minerals for crops (Jagger and Pender, 2003;Gindaba, 2003;Chanie et al., 2013;Khybri et al., 1992;Malik and Sharma, 1990). Seedlings of Eucalyptus are vulnerable to severe water stress unlike the seedlings of different tree species in Ethiopia (Gindaba et al., 2004). This indicates that Eucalyptus trees need more water and compete with neighboring plants for the availability of water in the soil. For example, the total biomass of crops grown nearby Eucalyptus stand was reduced due to the decline of soil nutrients and shading effect of the eucalyptus (Chanie et al., 2013;. In line with this low water availability is occurred in the soil in the tree-crop interaction land as compared to mono cropping system (Kidanu and Stonier, 2004). The nearest crop plants were wilted unlike the farther stands since Eucalyptus competes for moisture even deeper in the soil. Similarly, light availability and intensity had a significant effect on crop biomass as compared to water availability (Kotowskil et al., 2000). Furthermore, studies in Sudan reveled that high crop yield reduction was recorded nearby Eucalyptus stands due to nutrient depletion and production of toxic exudates (allelochemicals) (EI-Amin et al., (2001). Watson, (2000) also stated Eucalyptus leaf extracts have inhibited the germination of several plants. Therefore, Eucalyptus species caused drawbacks rather than improving the performance of the undergrowth vegetation. Moreover, Eucalyptus trees affected the maize plant performance by reducing available p even if the strength of belowground competition can be decreased with fertilization (James, 1999). Lane et al., (2004) found in China described that the expansion of Eucalyptus plantation on lands previously used for crops and occupied by indigenous trees and grass lowers water tables and reduces water availability for irrigation due to soil hydrophobicity (water repellency) and its deep and dense root network. The work of EI-Amin et al., (2001) also suggested that Eucalyptus plantation have significant effect on soil nutrients and crop productivity but the effects depends on the species, type of site and management activities.
On the other hand growing of crops on previously Eucalyptus cultivated stands are produce higher crop yield than the continuous cultivated cropping lands (Desalegn et al., 2014). The higher yield obtained from the previously planted eucalyptus stand is maybe due to the higher nutrient content of the stand. Because, studies indicated that soil chemical properties and organic matter were improved when the land is changed from eucalyptus stand to crop lands (Birru et al., 2013). Similarly, higher soil nutrients and organic carbon were obtained in plantations of Eucalyptus grandis compared with adjacent natural forest (Alem et al., 2010). Generally, it is not recommended growing the mixture of eucalyptus with agricultural crops simultaneously in the same land due to their competition for water and nutrients, and the shade effect. However, based on the above findings using rotation system of crop and Eucalyptus maybe more effective for crop production and economic benefits for small holder farmers.

Effects of Eucalyptus Plantation on Soil Nutrient
Many studies have been made in different parts of the world concerning the effects of Eucalyptus plantation on soil and other related environmental conditions, most of the studies focused on soil property changes under the plantations (Janice, 2016;Chanie et al., 2013;Palmberg, 2002;Jagger and pender, 2003;Sunder, 1993;Poore and Friss, 1985). These Studies reported that Eucalyptus has a significant effect on soil nutrients like total Nitrogen, available phosphorus (avail. P), exchangeable calcium (exch. Ca), moisture content (MC). Similarly, the soil pH as well as base saturation exhibited a clear decrease under Eucalyptus compared to adjacent natural forest and other exotic tree species (Lemenih, 2004). In recent years, soil degradation in Ethiopia has serious problem many soils are poor in nutrient and suitable for erosion. The main causes for soil degradation are poor agricultural Journal of Natural Sciences Research www.iiste.org ISSN 2224-3186 (Paper) ISSN 2225-0921 (Online) Vol.11, No.17, 2020 practices, deforestation and overgrazing, but also the poorly planned and management of fast-growing tree plantations may leads to soil quality decline (Lemma, 2006). Fast growing tree species are required more nutrient for growth compared to slow growing species. Eucalyptus is fast growing tree uptakes more nutrient from the soil (Zegeye, 2010; Cole and Rapp, 1981). The work of Heilman & Norby, (1997) and Zerfu, (2002) revealed that higher soil nutrient removal were occurred during the harvesting time because Eucalyptus is fast growing and has short harvesting rotation time. Further, Turner & Lambert, (2000) observed as soil nutrient is decreased in eucalyptus cultivation land as compared to the adjacent native forests in south-eastern Brazil. Since Eucalyptus species are fast growing, and deep and dense rooted, the reducing and drying status of previously functional nearby water stores in the agricultural land is as a result of its greatest water sucking ability as well soil hydrophobicity and poor undergrowth leading the reduction of infiltration capacity and water table.
However, recent studies reported that Eucalyptus plantation has enhanced the soil chemical properties and soil organic matter as compared to the adjacent crop production lands. For example, the work of (Birru et al., 2013) suggested that soil chemical properties and organic matters were better in the previously cultivated eucalyptus land as compared to continuously cultivation crop lands. Therefore, changing eucalyptus land to crop land is possible without affecting the soil properties and crop productivity. In line with this rotating crop lands with Eucalypts can improve soil productivity via translocation of nutrients from deeper horizons to the soil surface (Tchienkoua and Zech, 2004). Further, higher soil nutrients and organic carbon were obtained in plantations of Eucalyptus grandis compared with adjacent natural forest (Alem et al., 2010). Based on the results improving the management operation of eucalyptus stands is very important to increase the positive feedback of the species for soil nutrients.

Allelophatic Effects of Eucalyptus
Allelopathy is a biological process of plants which is producing one or more chemicals hat influence the germination, growth, survival, and reproduction of other plants. Eucalypts trees have allelophatic and inhibiting effect on undergrowth plant regeneration and growth by secreting different types of chemicals, from their leaves, stems and roots such as, phenolics terpenoids (Ahmed et al., 2008;Zhang, 2009;Lisanework and Michelsen, 1993;Poore and Fries, 1985). For instance, EI-Amin et al., (2001) in Sudan reported that Eucalyptus caused crop yield reduction due to nutrient depletion and production of toxic exudates (allelochemicals). Similarly, the above and underground extracts of Eucalyptus camaldulensis could affect germination and early seedling growth of undergrowth plants and crops (Saberi et al. 2013). Moreover, the leaf area, plant height, and leaf chlorophyll of Eucalyptus species significantly repressed the growth of under growth plants (Ahmed et al., 2008;Zhang, 2009;Lisanework and Michelsen, 1993;Poore and Fries, 1985). Molina et al., (1991) reported that, Eucalyptus releases toxic allelo-chemicals into the soil system mainly through litter decomposition products. In this piece of evidence, Eucalyptus plantations affects crop production by releasing toxic allelo-chemical.

Conclusion
The wider existing evidence of Eucalyptus species has a negative effect on crop yield where it grows nearby crop lands by competing resource such as soil nutrient, water and shade effect of the trees. Similarly, it depletes soil minerals required by agricultural crops, the capacity of water in the area and competes with agricultural crops; it reduces ground flora resulting unsuitable for soil erosion control; Thus, there is an irritation that the potential ecosystem will be exhausted in the future because of the described worse environmental modification. But when we managed the species properly it has high economically and ecologically benefit to the people. Because recent studies indicated that planting of crops in clear-felling eucalyptus stands gave higher crop yield as compared to the adjust crop lands. In this regard, Eucalyptus is good to rehabilitate crop lands to reuse for annual crop production and more profitable for the small holder farmers but the species is not appropriate for mixed panting system with annual crops. Therefore, keeping the maximum distance of eucalyptus and crops is good for better production. Furthermore, Ethiopia has large area that is suitable for crop and tree production so by developing land use policy we can get a lot of benefit through planting Eucalyptus species in appropriate site. It is known Eucalyptus plantation has offered both negative and positive opinions from around the world in general and in Ethiopia too. Consequently, giving more attention in its management is very important for sustainable production of crops and eucalyptus trees. Finally, still there is uncertainty the influence of Eucalyptus species on crop yield and soil nutrients. Therefore, further studies needed to reaching the comprehensive understanding about the species.