Contribution of on-farm avocado (Persea americana) tree-based agroforestry practice on selected soil physical and chemical properties of Inguti small watershed, in the highlands of North-Western Ethiopia

ABSTRACT Agroforestry has a significant contribution to soil fertility enhancement as productive and protective value. Particularly, fruit tree-based agroforestry practice plays a significant role in improving productivity as well as soil fertility management. This study aimed to determine the contribution of on-farm avocado fruit tree-based agroforestry practice on selected soil physicochemical properties in the case of Inguti small watershed. Field survey and laboratory analyses were carried out based on selected physicochemical properties. Soil samples were taken from 12 sample avocado trees at 3 radial distances (under, near, and far from tree canopy) and two depths (0–20 cm and 20–40 cm) in four directions. Soil composite samples were taken for laboratory analysis. To investigate its contribution to selected soil’s physicochemical properties texture, bulk density, organic matter and carbon, pH, total nitrogen, available phosphorus, and available potassium were considered and the significant variation among treatments were analyzed. Based on the result of the study, except soil texture and soil pH, all other investigated soil physical (bulk density) and chemical properties (organic matter, organic carbon, total nitrogen, available phosphorus and potassium) were significantly varied for distance from avocado tree and soil depth from the surface with a p-value of bulk density P < 0.000 and 0.001), organic matter (P < 0.000 and 0.005), organic carbon (P < 0.000 and 0.006), pH (P < 0.00 and 0.626ns), total nitrogen (P < 0.000 and 0.006), available phosphorus (P < 0.002 and 0.01) and potassium (P < 0.002 and 0.01) for distance and depth respectively. So, on-farm avocado (Persea americana) has a significant contribution to soil physicochemical properties in the study watershed.


Introduction
In most developing countries including Ethiopia, their economy mostly depends on agricultural activities.However, agricultural production is influenced by soil fertility degradation due to the unwise use of land resources and changes in land use (Jemal, 2021).Declining land productivity because of land degradation is a serious problem in most developing countries.This is mainly true for East African countries including Ethiopia, where land degradation is a common feature of farming systems, requiring high external inputs (fertilizers) (Jemal, 2021;Wolle et al., 2021).Different researchers reported more than 50% of cultivable lands in Ethiopia, particularly in highlands, are degraded from moderate to severe level (Wolle et al., 2021).The main causes of land degradation include deforestation, soil erosion, and inappropriate land-use systems as a result of increasing population.
Soil fertility is a dynamic natural property that can change under the influence of natural and humaninduced factors.In agriculture, depletion can be due to excessively intense cultivation and inadequate soil management.Topsoil depletion occurs when the nutrientrich organic topsoil, which takes hundreds to thousands of years to build up under natural conditions, is eroded or depleted of its organic material (Denis, 2017).As the human population continues to increase, human disturbance on the earth's ecosystem to produce food and fiber could place greater demand on soils to supply essential nutrients.Continuous cropping for enhanced yield removes substantial amounts of nutrients from the soil.Imbalanced and inadequate use of chemical fertilizers, improper irrigation, and various cultural practices also deplete the soil quality rapidly (Leone et al., 2017).
As a result of declining land production due to land degradation, soil fertility maintenance is a major concern in tropical Africa including Ethiopia, where most populations are depending on agricultural activities with the rapid population increase.Instead of using chemical fertilizers, soil fertility management practices by the use of indigenous trees and locally available resources are essential in terms of cost and ecological benefit (Wolle et al., 2021).Therefore, agroforestry is alternative farming system that have both ecological and economic benefits.Agroforestry is a land-use system and practice in which forest trees, livestock, and arable land (for crops) are integrated on the same unit of land and managed to give yield on a sustainable basis either simultaneously or sequentially (Amare et al., 2018).It is a practice that is economically sound and culturally compatible (Amare et al., 2018;Emiru, 2016;Jiru, 2019;Sahilu, 2017).Agroforestry involves the cultivation and use of trees in farming systems and is a practical and low-cost means of implementing many forms of integrated land management, especially for small-scale producers (Adane et al., 2019).
Literature shows that tree-based agroforestry systems significantly contribute to alleviating poverty.Because mixing trees with annual crops is an option for diversifying production and increasing the productivity of land, which in turn helps the farmer overcome crop failure due to climate change (Linger, 2014).Furthermore, tree-based agroforestry practices and a home garden are more profitable than monocropping (Linger, 2014).Scattered trees on farm lands and pasture lands, boundary plantation, home garden, live fence and woodlot are commonly used practiced agroforestry (Agidie et al., 2014).Agro-forestry is highly effective be to insure farming practice through enhancement of soil fertility, plant nutrients (N, P, K, Mg, Ca, S), organic matter, soil carbon sequestration, retention of soil moisture, regulate soil temperature and climate change (Abbadiko & Mulugeta, 2017).On-farm trees such as fruit trees mixed with crops are known to contribute to biophysical and economical sustainability (Kassa et al., 2014).Nitrogen-fixing trees under agroforestry significantly increased nutrient pool, organic biomass, and activities of organisms in the soil (Mulugeta, 2014).This would not only be beneficial to the soil but would also be cheaper for resource-poor farmers and provide fodder or firewood (Kumar et al., 2020;Mulugeta, 2014).
North Mecha District has potential for agricultural production as well as agroforestry.However, the increment in population number in the district causes a small plot of land per household.Based on the district data the average cultivated land per household is less than a hectare and most households have no land for agricultural practice.In addition to the small land size, the area is exposed to over-cultivation and soil erosion (North Mecha District Agriculture Office unpublished annual report, 2021).In the district, there is high soil erosion and fertility decline due to over-cultivation and deforestation.According to Gelagay and Minale (2016), the annual soil loss is 0-247 t ha −1 year −1 with a mean annual soil loss of 47 t ha −1 year −1 in the Koga watershed which is found in the district.In addition to soil erosion, soil fertility decline due to cultivation is the main problem in the study area.According to Addis (2021), a study conducted in North Mecha woreda on soil physical and chemical properties, the soil has low pH value, very low soil organic matter, total nitrogen, available phosphorus.So, lack of soil nutrients (nitrogen and phosphorus) is the main problem in the study area.
Currently, the district designed different mechanisms for soil conservation and fertility management.From the different mechanisms, agroforestry practice becomes essential in the district for soil management as well as for income generation.Eucalyptus tree, fruitbased agroforestry, and home garden agroforestry are some of the agroforestry practices in the district.Particularly following the Koga irrigation project, the community practiced fruit-based agroforestry (Avocado) for the last few years, which expected to play dual roles in soil fertility management and in improving the livelihoods of smallholder households even for the national economy as an export (North Mecha District Agriculture Office unpublished annual report, 2021).Different researches were conducted on the issue of Agroforestry.For instance, Alebachew (2012) studied traditional agroforestry practice threats and opportunities in the highlands of the Oromia region; Adane et al. (2019) studied the contribution of a fruit tree-based agroforestry system for household income to smallholder farmers in Dale District, Sidama Zone; Emiru (2016) studied the role of agroforestry practice for sustaining the rural livelihood in the case of Borecha woreda, Illubabor zone.The researchers see the contribution of fruit tree agroforestry from a socioeconomic perspective (Adane et al., 2019;Do et al., 2020).Currently, in North Mecha district, most farmers are practiced fruit tree-based agroforestry practices in commercial style (home garden, Alley cropping, and woodlot) for income generation.Scattered trees on cultivated land for multiple purposes are common in the study watershed.Particularly, following Koga irrigation, the local farmers practiced avocado fruit cultivation combined with crop cultivation since 2016.Currently, in the study watershed crop land combined with avocado fruit trees are highly practiced and increased from time to time by the local farmers.According to local administrative authority, more than 42 hectares (8%) of crop land areas are covered with avocado fruit tree with annual crops in the study watershed.This practice contributes a dual role for the local community for income generation as well as land management practice.However, there is no evidence or very little is known concerning the contribution of avocado tree-based agroforestry on soil properties in the study area.So, the present study aimed to assess the contribution of onfarm avocado (Persea americana) tree-based agroforestry practice to soil fertility (selected soil physical and chemical properties) in the case of Inguti small watershed, North-western Ethiopian highlands.

Description of the study area
The geographic location of Inguti small watershed extends geographically from 11° 25' 00''N to 11° 28' 00'N Latitude and 37° 1' 48''E to 37° 8' 00''E longitude which is found in North Mecha district.The total area of the Watershed is 530 ha (Figure 1).The area is found in Inguti kebele (smallest administrative unit in Ethiopia).
The temperature of the area varies from January to December.The mean monthly temperature of the area showed that the highest mean monthly maximum temperature occurs in March and the lowest in July with mean annual temperature of 24°C (National Meteorology Agency, 2020).While the mean annual rainfall is 1270 mm with the main rainy season from May to October (National Meteorology Agency, 2020).The topography of the area is low-lying plains with small ups and downs.The altitude ranges from 1918 to 2081 m.a.s.l.Chromic vertisols and chromic luvisols are the dominant soil type in the study watershed (Source: ANRS Agriculture Bureau Digital Soil Data, 2007).
The total population of the watershed is about 4054 (2068 male and 1986 female) (North Mecha administration administrative data, 2021).The majority of the population is engaged in agriculture.Crop production is produced in the area both by using rain-fed and irrigation systems.Livestock production has also a significant contribution to improving households' income and alternative options for households' strategies to cope with shocks, stress, and combat food insecurity (North Mecha administration administrative data, 2021).

Sampling techniques, sampling design, and variables
For this study, a non-probability sampling technique was used.First, Inguty small watershed (530 hectare) from the existing watersheds in the North Mecha was selected purposively for the study due to the existence of fruit tree-based agroforestry practice, particularly avocado (Persea americana).From the total area of the watershed 42 hectares of the area covered with fruit tree based agroforestry practice (Persea americana), which have a dual purpose for income generation for the local community, as well as for soil conservation.Secondly, the sample fruit tree (avocado/Persea americana)) for experimental treatments was also selected purposively.Trees, which are 6 years old from the existing trees in the study area were selected based on the age (oldest than others).Based on this only 12 avocado (Persea americana) trees which are found on individual farm lands with the same age under similar management were selected purposively due to budget constraints.
To investigate the contribution of on-farm avocado (Persea americana) tree to soil fertility, the RCBD (Randomized Completely Block Design) experimental design was employed.The plot land around the trees were categorized into 3 treatments based on the distance from the fruit tree (under tree canopy (0.5 m), near tree canopy (0.5-1 m) and far from tree canopy (out of 15 m radius), and 2 soil depths (surface and subsurface); the treatments were taken from each block in 4 directions.Based on these 3 treatments (samples 0.5 m, 0.5-1 m, and far to canopy as a control), for 2 soil depths (0-20 cm and 20-40 cm), and 12 replications (avocado trees with the same age under similar management) were taken for the study.So, in this study 18 treatments (3×3x2) were examined, i.e. 3 treatments with 3 replications (composite sample of 4 trees) with 2 depths.Based on this, composite soil samples from 4 directions at each treatment in the study area were taken for a laboratory soil analysis.

Independent variable
On-farm Avocado (Persea americana) tree-based agroforestry along distance and depth.

Dependent variables
The dependent variables assessed in this study were purposively selected soil physical and chemical properties/or nutrients.So, some selected soil chemical and physical properties such as total nitrogen, available phosphorus, available potassium, organic matter, organic carbon, bulk density, pH, and texture were investigated (Table 1).

Soil Texture
Soil texture affects different physical and chemical properties of soils.It determines the infiltration and retention of water, soil aeration, absorption of nutrients, microbial activities, tillage, and irrigation practices (Crooks, 2018;Gasha, 2019).It also reflects other soil features such as types of parent material, homogeneity, heterogeneity, and intensity of weathering of soil material or age of soil (Wilke, 2005;Yenehun et al., 2021).Soil texture is one of the intrinsic soil physical properties that are less affected by management and which determine the increase in stickiness or ability to mound as the moisture content increases depending on the content of silt and clay fraction, the degree to which the clay particles are bound together into stable granules and the OM content of the soil (Khadka et al., 2019).

Bulk Density
Soil bulk density is defined as the ratio of a mass of dry soil to its field volume and is usually expressed in terms of grams per cubic centimeter (g/cm3).Soil bulk density is a basic dynamic soil property that is influenced by various physical and chemical properties (Khadka et al., 2019).Bulk density is an indicator of the amount of pore space available within individual soil layers or horizons, as it is inversely proportional to pore space (Cadisch, 2014) and it is also an important indicator of soil physical properties, and it affects soil fertility and crop productivity (Bezabih et al., 2016;Yu-Song et al., 2016).

Soil pH
Soil reaction is the degree of soil acidity or alkalinity, which is caused by the exactly chemical, mineralogical, and/or biological environment.Soil reaction affects nutrient availability and toxicity, microbial activity, and root growth.Even though there are plants that flourish in acid or alkaline media, most crops perform best in slightly acidic soil to neutral (pH 6.0-7.0)(Crooks, 2018).The values of pH less than 5.5 may lead to aluminum toxicity, therefore unavailability of phosphorus and some of the soil micronutrients such as molybdenum, and reduced biological activity (Gasha, 2019;Yeshaneh, 2015;Yu-Song et al., 2016).

Organic Matter and carbon
Soil organic matter is the parts of the soil that consists of plant or animal in various stages of breakdown (decomposition).Most productive agricultural soils have between 3 and 6% organic matter.SOM is composed mainly of carbon, hydrogen and oxygen, and has small amounts of other elements, such as nitrogen, phosphorus, sulfur, potassium, calcium and magnesium contained in organic residues.Soil OM arises from the remains of green plants, animal residues, and excreta that are deposited on the surface and mixed to a variable extent with the mineral component (Hoskins, 1997;Kassa et al., 2014;Singh et al., 2016).Soil organic matter (OM) is a dynamic and large pool of carbon that is subject to change to changes in management practices as a result of varying land uses.Organic matter plays a vital role in regulating the flow and supply of plant nutrients and water flow and determining the physical attributes of soil.Organic matter has an important influence on soil's physical and chemical characteristics, soil fertility status, plant nutrition, and biological activity in the soil (Akinde et al., 2020;Crooks, 2018).

Total Nitrogen
Soil total nitrogen is naturally used as an important index for soil quality evaluation and reflects the soil's Nitrogen status (Belachew & Abera, 2010).Because of the high nitrogen requirements of plants and the low level of available N in virtually all types of soils, it is considered the most important and dynamic nutrient element in managed ecosystems.Soil total N composed of inorganic (NH4+, NO3-and NO2 -) and organic forms (OM) are subject to change due to various factors (van Beek et al., 2016).

Available Phosphorous
Phosphorus is known as the master key to agriculture because the lack of available P in the soil restricts the growth of both cultivated and noncultivated plants.Following N, P has a more widespread influence on both natural and agricultural ecosystems than any other essential elements.In most natural ecosystems, such as forests and grasslands P uptake by plants is constrained by both the low total amount of the element in the soil and by the very low solubility of the scarce quantity that is present (Singh et al., 2016).

Available Potassium
Potassium is the third most important essential element next to N and P that restrict plant productivity.Its behavior in the soil is influenced primarily by soils of cation exchange capacity and mineral weathering rather than by microbiological.The difference in the allocation of K depends on the mineral present, particle size distribution, degree of weathering, soil management practices, climatic conditions, degree of soil development, the intensity of cultivation, and the parent material from which the soil is formed.The better the proportion of clay minerals high in K, the better will be the potential K availability in soils (Singh et al., 2016).

Data sources
Both primary and secondary data sources were used for the study.The primary data were collected from sample soil taken from treatment fields and secondary data were reviewed from both published and unpublished literatures.

Field survey on soil sample and laboratory analysis
To assess the contribution of on-farm Avocado fruit treebased agroforestry practice to soil fertility, soil samples from the field were taken (Figure 2).The soil samples were taken from the avocado cultivation field based on a determined radius from the tree, and at different soil depths.Before taking soil samples from the field, any other material on the soil surface including plant and grass litter were removed.Then, in each plot soil samples were collected in four corners and at the center in two different soil depths, i.e. 0-20 cm and 20-40 cm.Composite soils from 4 directions and were prepared for a soil laboratory experiment.And then, the sample soils were taken to ADSWE (Amhara Design and Supervision Workers Enterprise) for laboratory analysis.

Soil laboratory analysis
Samples were collected from selected locations in the field.The samples from the same treatment were thoroughly mixed to obtain a representative sample and analysis of this sample gives average values for the entire treatment area.Soil samples were immediately air-dried at room temperature for three days.
The sample soil laboratory was determined as the particle size distribution of the soils was analyzed according to the procedure outlined by Bouyoucos (1962) with the help of the hydrometer method as cited in Abate et al. (2016).The bulk density of the soil was estimated from undisturbed soil samples which were collected by using a core sampler following the procedures used by Black (1965).Each core sample was oven-dried, and the bulk density was calculated by dividing the mass of the oven-dry soil by the respective volume as it existed naturally under field conditions as cited in Abate et al. (2016).Organic carbon and organic matter were determined by the wet dichromate acid oxidation method.Soil pH was determined in H 2 O using a 1:2.5 soil to water ratio using a digital pH-meter with a glass-calomel combination electrode (Abate et al., 2016).Total N of the soil was determined by the Micro-Kjeldahl digestion, distillation, and titration method in the form of a percent (Motsara & Roy, 2008).Available phosphorus was determined by the Olsen method (Olsen et al., 1954cited in Abate et al. (2016)).
Potassium was measured by a flame photometer (Rowell, 1994) as cited by Abate et al. (2016).

Statistical data analysis methods
The significance difference of selected soil physical and chemical properties among treatments (distance and depth) were analyzed based on ANOVA test with 95% confidence interval using Statistix 8 software.

Contribution of on-farm avocado (Persea americana) on soil physical properties
Based on the recorded data from the sample soils, the contribution of on-farm avocado fruit tree-based agroforestry practice on selected physical properties particularly soil texture and bulk density results at different distance from the tree, and depths were presented as follows.

Soil texture
Based on the results of the study (soil texture), the sand content of samples ranged from 22 to 45% under the tree canopy, 14-35% near the tree canopy, and 20-42% far from the tree, and that of clay contents was 30-58% under the tree, 34-60% near canopy and 46-50% far from the tree canopy, and the silt contents of the sample soil under, near and far from tree canopy were 16-30%, 22-33%, and 24-30% respectively (Table 2).The results of this study revealed that the textural class of the soil was clay loam and clay at both 0-20 and 20-40 cm depths.Based on the analysis of variance of result, the sand contents of the sample soils in the study site among treatments (distance and depth) were not significantly varied (P > 0.50 for distance from the tree and P > 0.49 depth from the surface), the clay content of the sample soils among treatments (a long distance and depth) were also not significantly varied with 95% CI (P > 0.75 and P > 0.92) respectively and the same to clay and sand contents of the sample soil, the silt contents of the soil among treatments (distance from the tree and depth) were not significantly varied with (p > 0.76 and p > 0.31) for distance and depth respectively.This finding agreed with the finding of the research conducted by Kassa et al. (2014), but disagreed with the findings of 'there is a significant variation among treatments' conducted by Ketema & Abayineh (2015).However, the nonsignificant difference in soil texture between surface and sub-surface soils under avocado trees at different distances might be attributed to the similarity in parent material from which the soils had originated and moreover, it might be difficult to change the soil texture in short (6 years) time horizon.However, the clay content is increased and sand and silt contents the soil is decreased with increasing distance from tree canopy.

Soil bulk density
Based on the sample soils' bulk density recorded data, the mean value of soil bulk density indicates that higher values were recorded out of the tree canopy and on the sub-surface with a range of 0.91 g/cm3 to 1.26 and 1.04 g/cm 3 to 1.34 g/cm 3 in the surface and subsurface respectively, and 1.02-1.34g/cm 3 , 0.92-1.21g/cm 3 and 0.91 to 1.15 g/cm 3 for under canopy, near canopy and far from canopy respectively.Based on the analysis of variance, BD was significantly different among treatments both in-depth and distance (P < 00 and P < 0.00 respectively).The BD of the soil showed an increasing tendency from surface to sub-surface and radial distances from the avocado trunk to the control (far from tree canopy (Table 2).The increasing trend of bulk density with soil depth might be attributed to low compaction, and the increase of bulk density as distance increases from the fruit tree might be due to the availability of high soil organic matter under the tree as the study conducted by Manjur et al. (2014).This indicates that bulk density increases as depth increase from the surface due to the increase of soil compaction and bulk density increases as distance increase from the tree canopy due to the availability of organic matter under the tree canopy.Based on the results of the study high bulk density was recorded at depth of 20-40 cm and out of tree canopy (although there is variation among sample trees).Therefore, an on-farm avocado contributes to reduce soil bulk density in the study watershed.So, the finding of this study indicated that BD of the soil showed an increasing tendency from surface to sub surface as the finding by Kassa et al. (2014); Manjur et al. (2014) and increasing bulk density with radial distances from the avocado trunk lined to the findings of Kassa et al. (2014); Ketema & Abayineh (2015); Manjur et al. (2014).Similarly, another researchers also confirmed that bulk density is low under forest land due to high organic matter and high under cultivated land (Bezabih et al., 2016;Defera et al., 2019).

Soil pH
Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units.Soil pH was determined in H 2 O using a 1:2.5 soil-to-water ratio using a digital pH meter with a glass-calomel combination electrode (Abate et al., 2016).The mean pH value of the sample soils in the study area ranged from 4.99 to 5.4 as presented in table (Table 3).According to Flynn (2015), the soils are categorized as moderately acidic.
Based on analysis of variance of soil pH values from samples at different distances and depths, the pH value of sample soils was not significantly different along distances (p > 0.08) as the study conducted by Kassa  Ketema & Abayineh (2015), and also there were no significant difference in depths (p > 0.87) as the study conducted by Kassa et al. (2014); Ketema & Abayineh (2015).But, the pH value of soil was a decreasing tendency from the tree canopy and decreasing tendency from the surface to subsurface although there is no significant difference along the distance and depth.So, the contribution of on-farm avocados to soil pH at the study site is not significant along the distance from the tree canopy and depth from the surface.This is lined with the study conducted by Kassa et al. (2014); Manjur et al. (2014).The finding is also in line with other findings of other researchers, pH is affected by different land uses (Bezabih et al., 2016;Yeshaneh, 2015), and (Kassa et al., 2014;Ketema & Abayineh, 2015), 'there is no significant difference among treatments across the different radial distance from Avocado trees'.

Soil organic carbon and organic matter
Soil organic carbon and matter have an important indicator of soil fertility (Leone et al., 2017).Based on the results of the study conducted on the contribution of on-farm avocado (Persea Americana) fruit tree based agroforestry on the soil organic carbon indicate that the organic carbon contents of the sample soils have a mean value of 1.15% to 2.14% (2.14% under canopy, 1.27% near the canopy, and 1.15% far from tree canopy), and a mean value of 1.73% and 1.31% for 0-20 cm and 20-40 cm depth, respectively (Table 3).According to Oliver et al. (2013), stated that OC greater than 1% indicates moderate and high if greater than 2%.Based on this, the content of organic carbon in the study area was categorized as moderate and high.But high organic carbon was recorded under the tree canopy and near the surface, and they gradually decreased from the tree and the surface.
Based on the results of analysis of variance, OC has a significant variation among treatments with a 95% confidence interval.So, OC is significantly varied across distance and depth with a p-value of (p < 0.00).The content of soil organic carbon along distance from tree canopy and depth were, there was a decreasing tendency from the tree canopy and from surface to subsurface.This might be due to the availability of humus in the soil contributing from the avocado tree.This variation in organic carbon with distance away from the tree canopies were due to the leaf litter fall and decomposition of dead roots from the tree.This finding was in line with the findings of Kassa et al. (2014); Ketema & Abayineh (2015).Similarly, another researcher (Manjur et al., 2014), confirmed that 'Soil Organic carbon on the surface soil (0-20 cm depth) under the canopy of F. albida and C. macrostachyus was higher than outside'.
In addition to organic carbon, organic matter is an important parameter for making soil alive, by improving different physical, biological, and chemical properties organic matter has a vital role in agricultural soil by supplying plant nutrients, improving the soil structure, improving water infiltration and retention, feeding soil microflora and fauna, and the retention and cycling of applied fertilizer.So, based on the results of this study, the mean organic matter was varied from 2% to 3.66%; 3.66% (under the canopy), 2.18% (near the canopy), 2% (far from the canopy) in the study area, and 2.96% and 2.269% in the depth of 0-20 cm and 20-40 cm, respectively (Table 3).According to Flynn (2015) and Khadka et al. (2019), the distribution of organic matter in the study area was ranged from medium to high status, and this finding line with the study of Kassa et al. (2014).Based on the results of analysis of variance, there was a significant difference in depth and distance from the tree canopy.It was higher under the canopies of avocado trees and decrease as the distance from the trees with a p-value of (p < 0.00) and decrease as depth with a p-value of (p < 0.00).So, on-farm avocado has a significant contribution to soil organic matter enhancement in the study area that plays a significant role in soil fertility enhancement.This finding is lined with the research conducted under tree based agroforestry (Defera et al., 2019;Kassa et al., 2014;Ketema & Abayineh, 2015;Manjur et al., 2014).This finding is also agreed with other findings such as the soil organic matter content was much higher in the vegetable land than in other land use types (Bezabih et al., 2016;Wolle et al., 2021;Yeshaneh, 2015;Yu-Song et al., 2016); the organic carbon which is an index of the soil organic matter differs among the different land uses (Ezekiel et al., 2018;Yeshaneh, 2015); the Organic carbon (OC) contents of the soil showed decreasing tendency from surface to subsurface and radial distances from the avocado trunk as studied by Kassa et al. (2014), from other tree types as studied by Wolle et al. (2021)

Total nitrogen
Based on the results of this study, the total nitrogen content of the sample soil ranged from 0.10 to 0.185%, which is 0.18% (under the canopy), 0.18% (near the canopy), and 0.1% (far from the tree canopy) and 0.148% and 0.113% for depths of 0-20 cm and 20-40 cm, respectively.The mean total nitrogen ranged from 0.10 to 0.19%.According to the study conducted by Khadka et al. (2019), the concentration of total nitrogen in the study area varied from medium to high, whereas medium status is common in most treatments.So, high total nitrogen was recorded near the surface and under the tree canopy than other treatments as presented in the table (Table 3).Analysis of variance showed that the total nitrogen was significantly influenced by fruit trees (avocado).Total nitrogen significantly (P < 0.00) decreased with increasing distance from each tree trunk base and significantly different (P < 0.00) in depth from the surface to the subsurface.Therefore, based on the results of the study, the contents of total nitrogen in the soil at different distances has significant variation and onfarm avocado has a significant contribution to soil total nitrogen in the study area.Different researchers such as Kassa et al. (2014) and Ketema & Abayineh (2015) confirmed such findings.Therefore, the highest mean value of the total nitrogen was recorded in the surface layers and under the tree canopy, while the lowest value was observed in subsurface layers and outside the tree canopy (Table 3).This finding agreed with findings conducted on; total nitrogen showed variation due to land uses and conservation differences (Bezabih et al., 2016).Similar studies conducted by Kassa et al. (2014); Ketema & Abayineh (2015) confirmed that total nitrogen has a decreasing tendency from the avocado tree and the surface.In addition to this, another study conducted on other tree types by Manjur et al. (2014) confirmed that the top or surface soils contained much higher N than those in the sub surface soil, and soils under the tree canopies have higher N than soils in the open cultivated land.

Available phosphorus
Based on the sample soil recorded data, the mean available phosphorus was varied from 3.22 to 7.69ppm, i.e. 7.69ppm, 5.285ppm, and 3.228ppm for under canopy, near the canopy, and far from canopy respectively, and 7.09ppm and 3.8ppm for 0-20 cm to 20-40 cm depth respectively (Table 3).The value of sample soil available phosphorus is categorized as deficient to low based on the soil lab result interpretation held by Flynn (2015).Therefore, the results of the analysis of variance indicated that available phosphorus (P in ppm) content was significantly affected by Avocado fruit tree across distance (P < 0.01) and soil depths (P < 0.01) with 95% significant level (Table 3).So, available phosphorus has decreasing tendency from the tree canopy and surface to subsurface.The highest content of available Phosphorus recorded under the canopy is attributed to the accumulation of organic matter.So, the available phosphorus was significantly affected by On-farm avocado in the study area.The result agreed that available phosphorus was significantly affected by different land-use types as studied by Bezabih et al. (2016), and the available phosphorus content decreased with depth and radial distance from the tree trunk and the highest content was recorded under the canopy as studied by Kassa et al. (2014) and Ketema & Abayineh (2015) which is attributed to the accumulation of organic matter (Kassa et al., 2014).Similarly, another study conducted by Manjur et al. (2014) confirmed that available phosphorus in the upper surface and subsurface soil depths was significantly different and the mean available decreased with increasing soil depth and distance from the tree base of F. albida and C. macrostachyus.Therefore, this indicates that on-farm avocado-based agroforestry practice has a significant contribution to soil available phosphorus.

Available potassium
Based on the results of this study, the mean potassium contents of the soils of all treatments ranged from 126.5ppm to 186.633ppm, i.e. 126.578ppm (far from canopy), 143.033ppm (near from canopy), and 186.633 ppm (under canopy), and an average of 165.097ppm and 139.067ppm under 0-20 cm and 20-40 cm depth respectively (Table 3).The availability of available potassium in the sample soils is categorized as medium (150ppm-250ppm) under tree canopy but low (<150ppm) near and far from tree canopy (Hodges, 2010).Based on the results of the analysis of variance indicated that available potassium (K in ppm) content was significantly affected by Avocado fruit trees across distance (P < 0.00) and soil depths (P < 0.03) with a 95% significant level (Table 3).So, available potassium has a decreasing tendency from the tree canopy and surface to subsurface.The highest content of available potassium recorded under the canopy is attributed to the accumulation of organic matter.The mean available potassium is significantly decreasing with increasing distance from the tree base of avocado trees and along the depth.This finding agreed with the research finding conducted by Tamayo-Vélez & Osorio (2018) on 'Soil Fertility Improvement by Litter Decomposition and Inoculation with the Fungus Mortierella sp. in Avocado Plantations of Colombia' and they concluded that potassium was increased due to litter decomposition of the avocado tree.This indicates the on-farm avocado has a contribution to soil potassium enhancement in the study area as a result of litter decomposition as studied by Tamayo-Vélez & Osorio (2018).

Conclusion and recommendations
The analysis of this result indicates that on-farm avocado (Persea Americana) has a significant contribution on selected soil physical (bulk density) and chemical properties (organic carbon, organic matter, total nitrogen, available phosphorus and potassium) except soil texture and pH value.Based on the results of the study, the texture of sample soils was categorized under loam, clay-loam, and clay soil texture, and there was no significant variation among treatments across distance and depth.On the other hand, there is a significant variation of soil bulk density among treatments, i.e. there was a decreasing trend from the avocado tree and from surface to subsurface.
Concerning the selected chemical properties of sample soils at different treatments (distance from tree and depth), except pH, there were a significant variation among treatments.The pH value of soil has a decreasing tendency from the tree canopy and a decreasing tendency from the surface to the subsurface, although there is no significant difference to distance and depth.However, soil organic matter and organic carbon, total nitrogen, available phosphorus, and potassium have a significant variation among treatments, in distance from both tree and surface to subsurface.Based on the results, the values of these soil chemical properties (nutrient values) were highest under the tree canopy and near to the surface than other treatments, and become decreased in value as the distance from the tree canopy and from surface to subsurface.
In general, the soils under the tree canopy in the study area were highest in most soil physicochemical properties such as organic matter, organic carbon, pH, total nitrogen, available phosphorus, and potassium as compared to soils near and far from the tree canopy and subsurface soils.So, the results indicate that the on-farm avocado (Persea Americana) contributes to soil fertility by providing soil organic matter and carbons, which play a significant contribution for soil fertility enhancement.In conclusion, based on the finding of this study, on-farm avocado has a significant contribution to soil fertility enhancement in addition to its role in income generation in the study area.
Agroforestry practice has a dual role in improving food and nutrition security through direct income generation and soil fertility management by providing nutrients and soil and water conservation.Agroforestry, in general, fruit tree-based agroforestry in particular is widely practiced in the study area.Therefore, based on the finding of this study, fruit treebased agroforestry (on-farm avocado/Persea Americana) has a significant contribution to selected soil's physical and chemical properties.Based on this the researcher forwarded the following recommendations for further study.
• The local community and other stakeholders should give priority to such types of fruit treebased agroforestry practices, which have a dual contribution to soil fertility enhancement and income generation for the community.• The local people and the development experts should try to expand such types of agroforestry practices through scientific techniques including selecting suitable land for avocado fruit trees.
• The contribution of on-farm avocado (Persea Americana) to other soil chemical properties and its role in economic growth should be considered in the study area.• In addition to this, the temporal variation of soil physical and chemical properties under on-farm avocado (Persea Americana) fruit tree-based agroforestry practice should be studied for next.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 2 .
Figure 2. Persea americana and field data collection.

Table 1 .
Variables of the study

Table 2 .
Effects of distance and depth from avocado tree on selected soil physical properties (mean) ns ---Not significant *---significant with 95% confidence interval.

Table 3 .
Effects of distance and depth from avocado tree on selected soil chemical properties ns ---Not significant *---significant with 95% confidence interval.SUSTAINABLE ENVIRONMENT et al. (2014);