Impact of woody plant species on chemical properties of soil in the case of Ethiopia.
A woody plant functional trait that directly affects its fitness and environment is decisive to ensure the success of an Agroforestry practice. Hence, recognizing the woody plant functional traits is very important to boost and sustain the productivity of the system when different plants are sharing common resources, like in Agroforestry system. Therefore, the objective of this paper was to understand how woody plant functional traits contribute to sustainable soil management in Agroforestry system and to give the way forward in the case of Ethiopia. The contribution of woody plant species in improving soil fertility and controlling soil erosion is attributed by litter accumulation rate and the season, decomposability and nutrient content of the litter, root physical and chemical trait, and spread canopy structure functional trait. However, spread canopy structure functional trait is used in coffee based Agroforestry system, while with management in Parkland Agro forestry System. Woody species of Agroforestry system added a significant amount of soil TN, OC, Av.P, K, Na, Ca, and Mg nutrients to the soil. Woody plant species of Agroforestry system and their functional traits are very important to ensure sustainable soil management. Thus, further investigation of the woody plant functional traits especially the compatibility of trees with cops is needed to fully utilize the potential of woody species for sustainable soil management practice.
- woody plant
- functional trait
- soil fertility
- soil erosion
- agroforestry system
Woody plant functional trait, morphological-physiological-phenological characters that measured at an individual level and directly affects its fitness  and environment  is decisive to ensure the success of the Agroforestry practice. Agroforestry is indicated to be a prominent strategy to address land degradation, food security, and climate change challenges in Africa in general and in Ethiopia in particular too . This is due to Agroforestry is a dynamic, ecologically based, natural resource management system that, through the integration of woody plants in farm- and rangeland, diversifies and sustains smallholder production for increased environmental, economic and social benefits .
Land degradation is a common environmental problem in Ethiopia for many years back to date due to the natural capital of the land resource is declining from time to time [5, 6, 7]. The primary reason of land degradation is land exploration for agricultural purpose to feed the ever increasing population [5, 6, 8] and predicted to be continued with the current trend . As a result, soil erosion, droughts, loss of biodiversity and food insecurity are challenging the daily life of the rural population in Ethiopia [10, 11].
Therefore, integrated land use system such as Agroforestry system is very essential to combat land degradation and its consequences like soil degradation to ensure sustainable use of resources . Hence, recognizing the woody plant functional traits are very important to boost and sustain the productivity of the system when different plants are sharing common resources like in Agroforestry system . The canopy feature of woody plants, the height, diameter, specific root length and leaf area are among others refer to the morphological traits while the internal process and chemical composition of the woody plant denotes to physiological traits of the plant . Phenology of woody plant species defines the timing of different phases of life cycle such as leaf shading and re-growing, flowering, fruiting and seed dispersal [15, 16]. Thus, these functional traits of woody plants in Agroforestry system are the core feature in supporting sustainable soil management. Wherein sustainable soil management refers to an optimum level of field soil health and productive capacity to provide ecosystem services such as provision of clean water, hydrologic and nutrient cycling, habitats for microorganisms and mesofauna, carbon sequestration, and climate regulation .
Hence, Agroforestry systems provide different ecosystem services. Different researchers confirmed that Agroforestry systems in Ethiopia endowed with highly diversified woody species [18, 19, 20]. The woody species diversity in Agroforestry system have indispensable role of natural forest conservation  as the farmers use woods from the Agroforestry system than natural forest. Moreover, the Agroforestry practices are central for keeping biodiversity and soil fertility at levels which are similar to the natural forest . Research from Southern Tigray in Ethiopia indicates that Agroforestry practice has decreased soil erosion of the area . However, how woody plants’ functional traits support sustainable soil management has not been explored and reported in detail. Thus, in this review, how woody plants’ functional traits support sustainable soil management in the Agroforestry system are discussed from soil fertility improvement and soil erosion control perspectives.
2. Woody plant functional traits and sustainable soil management
2.1 Above ground woody plant trait
Woody plants improve soil fertility and control soil erosion through their litter, canopy and root systems . Ref.  holds a similar opinion when he states that from litter perspectives, 100% of the respondent from Jabithenan District, North-Western Ethiopia confirmed that Home garden Agroforestry system produce higher litter stock from weeds, grasses, and tree leaves than non-tree system. A similar finding by  reveals that in West Guji Zone, South Ethiopia, farmers noted that tree species that sheds its leave before the onset of rain and can easily decomposed are integrated in to farm land to increase the soil fertility. For example,
Regarding to canopy,  state the shape of the canopy of the woody plants and the size of the leaves are very crucial in minimizing soil erosion rate. Their major findings reveal that in the case of Bonga and Yayu-Hurumu districts, Southwestern Ethiopia, 98.2% of respondents preferred woody plants with thin and small leaves in decreasing the intensity of soil erosion than broader and larger leaves as coffee shade. Additionally, spreading canopy nature of woody plants can reduce the energy of raindrops by intercepting rainfall than narrow one. Hence, protecting the soil surface against the impact of rainfall drops by intercepting runoff .
However,  states that trees such as
Concerning tree phenology of leaf fall and flowering period of woody plants,  indicated that farmers especially women, have limited knowledge. Their finding also reveal that farmers have better knowledge on fruiting time of edible fruit tree species because it is related to their income generation for Lemo District in SNNPR Region.
2.2 Below ground woody plant trait (root trait)
From root morphological trait perspective, uses of mixture of plant species are advised on sloppy areas for soil and water conservation practices . However, research in South Ethiopia, indicates that
However, the belowground functioning of Agroforestry systems is still lacking, because numerous and complex site-specific interactions and trade-offs are at play . For example,  state that the existence of
The positive impact of trees on yield may be attributed by different factors. For example, there is higher Arbuscular Mycorrhizal Fungi under and at the periphery of the
Regarding to water use between trees and crops,  show that there is higher soil infiltrability under single trees than in the open areas indicating a positive impact of trees on soil hydraulic properties influencing groundwater recharge. Further, [50, 51] indicate the occurrence of plant hydrologic niche segregation in the agroforestry system suggesting weak competition for water between the components of the system. In coffee based Agroforestry system,  reported the coffee water uptake is mainly sustained from shallow soil sources (< 15 cm depth), while all shade trees relied on water sources from deeper soil layers (> 15 to 120 cm depth).
Concerning to allellopathic effect, different woody plant species produce different chemicals with allelopathic contents such as benzoic, cinnamic and phenolic acids, which have the potential to inhibit neighboring plants either positively or negatively depending on their concentrations . The potential allelopathic effect of different Agroforestry tree species on Ethiopian main crops was studied in different parts of the country by different authors. For instance, the study conducted by  on the effects of four woody species on seed germination, radicle and seedling growth of four main Ethiopian crops namely;
3. Role of woody plants in soil fertility enhancement
Trees have impressive potential to improve soil fertility and forbid soil erosion in land management like farmland and watershed management [58, 59, 60, 61]. For example, benefits of farmland woody plant species in the case of Northwestern Ethiopia are tremendous and soil fertility enhancement and management role indicates 35.14% among other benefits .
Dispersed trees on smallholder farms enhance soil fertility. For instance, research done in Tigray region reveals that
|Species name||Sample plots||Chemical properties of soil, exchangeable base (Meq/100 g soil)||References|
|Na||K||Ca||Mg||OC %||TN %||AvP (ppm)|
|CN||0.55 ± 0.21a||2.64 ± 1.75a||7.87 ± 1.84a||2.81 ± 0.76a||—||—||—|||
|OP||0.43 ± 0.23a||1.47 ± 0.221b||6.21 ± 1.48a||2.44 ± 0.32a||—||—||—|
|CN||0.69 ± 0.11a||4.42 ± 1.65a||12.41 ± 3.24a||3.39 ± 1.76a||—||—||—|
|OP||0.60 ± 0.28a||1.86 ± 0.89b||12.15 ± 2.45a||3.27 ± 0.92a||—||—||—|
|CN||—||—||—||—||1.73 (0.16)a||0.26 (0.01)a||7.21 (0.20)a|||
|NCN||—||—||—||—||1.28 (0.09)b||0.13 (0.01)b||6.55 (0.19)a|
|FCN||—||—||—||—||1.30 (0.11)b||0.12 (0.01)b||6.02 (0.21)b|
|CN||—||—||—||—||1.02 (0.06)a||0.13 (0.005)a||6.37 (0.28)a|
|NCN||—||—||—||—||0.70 (0.06)b||0.09 (0.005)b||5.78 (0.21)a,b|
|FCN||—||—||—||—||0.65 (0.05)b||0.07 (0.004)c||5.32 (0.17)b|
|LS||0.05 ± 0.00a||1.57 ± 0.10a||25.48 ± 1.33a||3.39 ± 0.17a||2.37 ± 0.11a||0.21 ± 0.01a||3.85 ± 0.31a|||
|LEG||0.042 ± 0.00a||1.41 ± 0.18a||24.14 ± 4.6a||3.20 ± 0.6a||2.17 ± 0.03b||0.185 ± 0.0b||3.52 ± 0.46a,b|
|DGL||0.032 ± 0.0b||1.15 ± 0.18b||17.58 ± 0.8b||2.33 ± 0.11b||1.97 ± 0.15c||0.165 ± 0.0c||2.86 ± 0.47b|
|CN||—||2.27a ± 0.95||—||—||6.49a ± 1.31||0.67a ± 0.15||7.52a ± 1.87|||
|OP||—||0.41b ± 0.32||—||—||2.54b ± 0.65||0.41b ± 0.12||3.81b ± 0.91|
|CN||—||1.05a ± 1.15||—||—||4.51a ± 1.15||0.49a ± 0.09||4.58a ± 0.85|
|OP||—||0.56b ± 0.24||—||—||2.31b ± 0.91||0.42b ± 0.07||2.50b ± 0.41|
|1.5 m distance from the canopy||0.34 (0.03)a||1.33 (0.32)a||42.05 (1.83)a||13.22 (2.29)a||2.03 (0.21)a||0.41 (0.03)a||11.33 (0.6)a|||
|3.5 m distance from the canopy||0.27 (0.09)a,b||1.13 (0.3)a||39.04 (1.7)a||11.21 (2.1)a||1.49 (0.32)b||0.31 (0.04)b||10.03 (0.4)a|
|25 m distance from the canopy||0.24 (0.06)b||0.79 (0.16)b||29.38 (0.79)b||8.70 (0.66)b||1.38 (0.29)b||0.23 (0.03)c||8.73 (0.47)b|
|1.5 m distance from the canopy||0.31 (0.04)a||0.90 (0.14)a||36.94 (8.31)a||10.25 (1.12)a||1.26 (0.25)a||0.14 (0.00)a||9.03 (1.08)a|
|3.5 m distance from the canopy||0.28 (0.07)a,b||0.84 (0.22)a||34.16 (8.8)a||10.81 (0.82)a||1.03 (0.16)b||0.13 (0.04)b||8.71 (0.74)b|
|25 m distance from the canopy||0.26 (0.04)b||0.49 (0.11)b||24.64 (3.54)||9.88 (0.45)b||0.76 (0.09)b||0.08 (0.01)c||8.47 (0.55)b|
Likewise,  states
|Species||Sample plots||Chemical properties of soil, Exchangeable base (Meq/100 g soil)||References|
|Ficus sur||CN||—||2.27a ± 0.95||—||—||6.49a ± 1.31||0.67a ± 0.15||7.52a ± 1.87|||
|Cordia africana||CN||—||1.05b ± 1.15||—||—||4.51b ± 1.15||0.49b ± 0.09||4.58b ± 0.85|
4. Role of woody plants in soil erosion control
Regarding to soil erosion control, biological soil and water conservation measures like tree and shrub planting are used to strengthen physical structures. The strengthened physical structure enabled to stabilize soil along the physical structures and to reduce the speed of surface runoff, henceforth increasing the infiltration rate of soil [70, 71]. Tree species that commonly being planted along soil and water conservation structures such as bunds and trenches namely are
5. Conclusions and recommendations
Woody plants of Agroforestry system improve soil fertility and forbid soil erosion from farmlands/water shade. Therefore, integration of woody plants on farming system based on the functional trait of woody plant is crucial to sustain soil management benefits of woody plants in Agroforestry systems.
Based on this review, the following are recommended to researchers to undertake study and policy makers to design agroforestry system that enable farmers to fully utilize the woody plant species potential in the Agroforestry system from functional trait point of view to achieve sustainable soil management practice in Ethiopia.
5.1 For researchers
Woody plant phenology such as leaf fall and re-growing and flowering seasons should be clearly investigated as per the Agro ecology because tree phenology is differing per Agro-ecology of the country. There is also lack of clear data on the phenology of major agroforestry woody plant species.
Woody plants’ litter decomposability and their chemical compositions should be investigated further. Similarly,  recommend the importance of woody plants’ litter decomposability and their chemical compositions analysis because litter quality is one among various factors which affects soil fertility based on its type and chemical contents.
The root system of woody plants used for soil and water conservation practice should be investigated.
The significance of the use of single species versus multiple species for soil nutrient improvement and soil erosion control should be evaluated.
Tree management practices of Parkland Agroforestry system to increase crop yield.
5.2 For policy makers
Woody plant functional traits should be considered when policy is designed to ensure sustainable soil management benefits of woody plants while introducing Agroforestry technologies.
We would like to express our special gratitude and thanks to Mr. Melkamu Teklu Kisi for his constructive comments and guidance during this work. Our gratitude and thanks also goes to Darko Hrvojic who invites and remind us to send our work to new book project “Biodiversity of Ecosystems” an Open Access book edited by Dr. Levente Hufnagel.
Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.