Effect of Rhamnus prinoide (Gesho) Intercropping in Wheat Field on Soil Nutrient and Moisture in the Drylands of North Ethiopia

Rhamnus prinoide (Gesho) intercropping is a common practice with significant socioeconomic and ecological importance in Ganta-afoshum district. However, this practice is not well explored so far, especially from the perspective of its impact on sustainable land management while land degradation and low productivity is a challenge in the district. The objective of this study was to examine the effect of R. prinoide intercropping on soil nutrient and moisture. The study was conducted in Ganta-afoshum district, eastern zone of Tigray regional state where smallholder farmers practice R. prinoide intercropping. Soil samples were taken from farmland with dense, medium and sparse density of R. prinoide intercropping and from farmland without trees as a control for analysis. The density of the trees significantly affected the soil bulk density (P<0.05). Soil moisture content significantly increased with increasing soil depth and by the interaction effects of soil depth and density (P<0.05). Soil nitrogen concentration significantly influenced by the density and soil depth (P<0.05). Soil phosphorus and potassium concentration significantly affected by soil depth (P<0.05). Lower mean soil pH values were observed for soils under the farm with dense trees as compared to soils in open farm. Overall, the enhancing and exploitative effects of R. prinoide intercropping on soil nutrient and moisture is minimal. This could be the reason for being practiced and maintained by most farmers.


INTRODUCTION
Increasing human population has caused pronounced reduction in tree cover although trees remain an important element of most human-dominated landscapes throughout the tropics (Schroth and Sincliar, 2003). It is providing a wide range of important products and services, while underpinning the sustainability of the farming systems (Cooper et al., 1996). Maintaining soil fertility is the main aspect of sustainable land use. Low and declining soil fertility is recognized by many tropical farmers as the major constraint to agricultural production (Smaling et al., 1997). Trees can improve the nutrient balance of a site and nutrient cycling in a system  and water-use efficiency in drylands (UNEP, 1992). When trees are present © CNCS, Mekelle University 127 ISSN: 2220-184X in crop fields they have a profound influence on majority components of water balance (Teixeira et al., 2003).
Strategic placing of trees in the landscape may prevent, enhance or direct flows of soil, water, nutrients, fire and organisms across the landscapes . The tools that are available to the farmer for optimizing water balance, nutrient cycles and positive interaction are the selection of plant (and animal) species, their spatial and temporal arrangement (system design), and their management (Teixeira et al., 2003;. Intercropping is an efficient farming system for better resource utilization and improving productivity (Vandermeer, 1992). R. prinoide intercropping is a common practice in north Ethiopia. Rhamnus prinoides L'Herit, which belongs to the family of Rhamnaceae (Kokwaro, 1993), is a wide spread plant species in east, central and south African countries (Orwa et al., 2009). Its common name is dogwood and local name is "gesho". The African dogwood, R. prinoides (Rhamnaceae) is a dense shrub or a tree that grows up to 9m high (Berhanu and Teshome, 1995), common in medium to high altitudes, and thrives in moist humus soils (Orwa et al., 2009).
R. prinoide trees are cultivated to control soil erosion, as a hedge or wind breaker, for shading and as an ornamental plant (Orwa et al., 2009). In Ethiopia, R. prinoides (gesho) is found growing in the wild and cultivated lands usually at altitudes of 1500-2500m (Berhanu and Teshome, 1995). In north Ethiopia, it is common especially in central and eastern parts of the Tigray regional state mainly in the Ahforom and Ganta-afeshum districts. R. prinoide intercropping is a dominant practice mainly for its commercial purposes and home consumption i.e. to make local drinks called Tella and Tej, similar to beer. It is also used by the local people as traditional medicine to treat different diseases such as tonsillitis (Birhanu, 2013), uvulitis (Andemariam, 2010), and hepatitis (d' Avigdor et al., 2014). Above all, the number of R.
prinoides plants owned by the farmer is also used as an indicator of wealth and social status by the community.
Although, R. prinoide intercropping is a common practice in many areas in Tigray, north Ethiopia, it is not well explored and appreciated so far, especially from the perspective of sustainable land management. Hence, a study was conducted in Ganta-afoshum districts, eastern Tigray. Land degradation, nutrient deficiency and moisture stress are the production constraints in the area and land holding is small in the region ranging from 0.25 to 0.5ha (BoARD, 2010 the effort to assist farmers to improve productivity of R. prinoide intercropping based on scientific evidence is minimal. R. prinoide intercropping might have an impact on the status of soil nutrient of farmland through its above and below ground litter falls and its eco-physiological traits which requires examination for appropriate management. R. prinoide characterized as a species of humid or moist areas (Orwa et al., 2009) which is in contrary to the study area where moisture is the production constraint that requires examination on its impact on soil moisture in the farm land. Present paper examines the effect of R. prinoide intercropping in wheat field at varied density (dense, medium and sparse) on soil nutrient and soil moisture.

Description of Study Area
The study was conducted in the districts of Ganta-afoshum in the Eastern zone of the Tigray region, northern Ethiopia. The area is potential for R. prinoide production and has suitable agroecological and socioeconomic settings for the species to grow. The district has a geographical location of 14° 07' to 14° 39' N and 38° 57' to 39° 18' E. Ganta-afeshum is one of the most densely populated (340 people/km 2 ) (BoARD, 2010) and severely degraded . Communal hillside exclosure is one of the land rehabilitation practices that have been practiced in the district.
The districts characterized by rugged topography with altitude ranging from 1400 to 3200 m with three agro climatic zones: low lands, mid land and high land (BoARD, 2010). The rainfall distribution of the study areas is unimodal. The annual rain fall ranges from 450 to 650 mm, with a mean annual rain fall of 550mm. The annual temperature ranges from 25 to 34°C with a mean annual temperature of 29.5°C. The dominant soil types by texture in the district vary in ranges of clay, loam, sandy loam and silty loam (BoARD, 2010).The major land cover and land use types are woodland, evergreen scrub, open woodland, bush-land, grassland, cropland, bare soil and rock outcrop (Feoli et al., 2002). Sedentary mixed farming is the main livelihood system of rural society in Ganta-afeshum.

Research Design
The study site was divided into three blocks as upper, middle, and lower based on slope gradient. wheat (selected seed) as companion crop for the last five growing years were considered for study and to install measurement plots in order to address its maximum possible effects. Since, R. prinoide intercropping is practiced at different density by different farmer, density of R.
prinoide trees was the treatment unit. The density of R. prinoide intercropping varied due to different planting spacing and vigorousness of individual plants. After preliminary tree inventory on density and canopy cover was done, four treatments were identified. The treatments were dense (2m spacing and/or >65% canopy cover), medium (2-4m spacing and/or 40-65% canopy cover), sparse (>4m spacing and/or 20-40% canopy cover) and open (without trees). In each block a total of 12farm plots (10x10meter) with three scale of density of R. prinoide trees and free of trees (control) at nearby were randomly demarcated in wheat field as measurement plots.
Wheat has been cultivated uniformly in all farm plots with and without trees for the last five and more years. The design that was employed had a 4*2 factorial arrangement of treatments in Undisturbed soil sample was taken using core sampler with 100 cc from all treatments plots at one randomly selected sampling point from top (0-15cm) and lower (15-30cm) soil depth for the examination of soil moisture and bulk density. The total number of the undisturbed soil sample was same with the disturbed soil sample (72).

Soil Analysis and Laboratory Methods
The collected soil samples were first air dried, then ground and sieved to separate the soil fraction<2 mm. Then the soil samples were analyzed for organic carbon content by wet oxidation method of Walkley and Black (Schnitzer 1982); total nitrogen by the Kjeldahl method (Bremner and Mulvaney, 1982). Available P determined by Olsen method (Olsen et al., 1954) while available K by neutral ammonium acetate extraction (Merwin and Peach, 1951 was determined in a 1:2.5 soil to water suspension (Jackson, 1973). Soil moisture content by oven drying at 105°C and bulk density (BD) by weighing oven dried (105°C) soil samples with known volume, g/cm3 or t/m3 (1 g/cm3 = 1000 kg/m3 = 1 t/m3) (Brady and Weil, 2002).

Statistical Analysis
Soil data was subjected to two way analysis of variance (ANOVA) using the General Linear Model (GLM) procedures of SPSS16. Comparison of treatment means performed using Tukey's Significant Difference test at P < 0.05 probability level.

Soil Bulk Density (BD)
The density of the trees significantly brought difference in BD (Table 1). BD was significantly lower on fields with dense R. prinoide intercropping than when density of R. prinoide is medium, sparse, and open. The interaction between density of trees and soil depth on bulk density was not  (Jiregna et al., 2005) while there was higher bulk density outside the canopy of F. thonningii as compared to the canopy zone (Enideg, 2008).
Lower soil BD under the tree species' canopies is presumably due to the effect of litter addition to the soil. This has resulted from organic matter build up in the soil under the canopies relative to levels in soil outside the canopies. Also, the higher concentration of tree roots near the base of the trees may have had the effect of loosening the soil, thereby reducing soil BD. Furthermore, the soil outside the tree canopies dries out more, being exposed to direct solar radiation. This not only accelerates thermally induced soil organic matter decomposition, but results in the shrinking of organic matter and clay colloids, thereby making the soil more compact .

Soil Moisture Content (MC)
Soil moisture content was not significantly affected by varied density of the trees while significantly (P<0.05) affected by soil depth (Table 1)  surface soil as compared to subsurface soils might be due to variation in soil organic matter (SOM) and exposition to evaporation. The relatively better SOM in farms with tree might make the soil to retain water by increasing its surface area and improving the soil structure (Teixeira et al., 2003). It might be also due to the shading effect of the trees. The soil outside the tree canopies might dry out more, being exposed to direct solar radiation whereas the shade provided by the trees would have enhanced the MC under their canopy. Above all, the relatively better soil moisture within farms with R. prinoide intercropping compared to farms free of any tree planting disproved the claims that tree planting within farmland in dry land exacerbates shortage of soil moisture and consequently affect productivity of farms. This indicated that it is possible to improve water recharge (Ilstedt et al., 2016) and best utilize scarce moisture in the dry lands through appropriate inter-planting of suitable species.

Soil Nitrogen (N)
Soil N concentration showed significant variation (P<0.05) among the different density of R.

Soil Organic Carbon (OC)
Soil OC was not significantly affected by both the different density of trees and soil depth and their interactions (P>0.05) ( Table 2). The soil OC of the present study can be categorized under high soil OC range value according to Bhandari and Tripathi, (1979

Soil Carbon to Nitrogen Ratio (C/N)
The soil C/N ratio decreased with decreasing density of trees (

Soil Phosphorus (P)
Soil P concentration was not significantly affected by the density of R. prinoide intercropping and interaction effect of density and soil depth (P>0.05) while it is significantly affected by the soil depth (P<0.05) (  Enideg (2008) also reported the same in a similar study. Tadesse et al. (2000) found available soil P concentration in the surface soils are significantly higher under the trees than in the open field while in the present case the surface soil P values are higher than the subsurface, in line with the present investigation.

Soil pH
The lower soil pH values in farm with dense R. prinoide as compared to open might be due to several mechanisms that release H + ions, such as soil base cation uptake (or depletion) by the tree, decomposition of organic matter to organic acids and CO2, root respiration and nitrification. Rhodes (1997) suggested that increased accumulation of aboveground biomass and associated cation uptake by the tree component of agroforestry systems as possibly one of the causes for decreased pH in soils. According to FAI (1977), soils having pH value in the range 6.50 to 8.70 are considered normal, that do not require treatment, and are optimum for most crops. Whereas, Tegenu et al. (2008, unpublished data)  influence on pH that coincide the present study findings. Whereas, Pandey et al. (2000) and Hailemariam et al. (2010) found soil pH value that differed significantly among density and soil depths under Balanitesa egyptiaca. Rhodes (1997)  might be due to up take by the tree root system while the higher Mg concentration in the surface soil presumably due to litter fall and relatively lower uptake by the trees.

CONCLUSION
R. prinoide (gesho) intercropping is a dominant farming system in the study site with significant role in the livelihood of communities. The soil nutrient analysis from farms with different R.
prinoide densities showed no significant difference for most nutrients. As a result the enhancing and exploitative effect of R. prinoide on some nutrients at the measured soil depth (0-30cm) is minimal. This indicates that incorporating trees with different resource requirement and utilization zone in farm enhance productivity and efficiency of resource utilization. Above all, the relatively better soil moisture within farms with R. prinoide intercropping compared to farms free of any tree planting disproved the claims that tree planting within farmland in dry land exacerbates shortage of soil moisture and consequently affect productivity of farms. This indicated that it is possible to best utilize scarce moisture through appropriate inter-planting of suitable species in the dry lands. Hence, these could be the reasons that R. prinoide intercropping is being practiced by most farmers and maintained through generation in the study areas and nearby.

ACKNOWLEDGMENTS
We would like to thank Mekelle University for the research fund and the farmers for allow us to take soil samples from their farm.