Cause and cost of onsite nutrient loss replacement in the highlands of Ethiopia and

Soil erosion and nutrient depletion has been a major national agenda and remains an important issue in the highlands of Ethiopia. In this review it was found that onsite nutrients are lost in agricultural land in the form of soil erosion, crop residue removal, harvested products, gaseous and leaching losses. Most of the essential plant nutrients are found on the top surface soil and hence the top layer of the soil is subjected to soil erosion and most onsite nutrients have been lost. Similarly, crop residue removal is a common activity in the highland of Ethiopia and causes a continuing onsite nutrient loss problem. Hence, soil nutrient depletion is becoming a major challenge for agricultural production. Moreover, nutrient loss from agricultural land became an economic loss to the farmers by both reducing crop yield and increasing the replacement cost of nutrient loss. Practicing proper soil and water conservation measures had a positive impact on the reduction of onsite nutrient losses and consequently increased crop productivity. However, limited studies have been reported on the impact of soil and water conservation practices on nutrient loss in Ethiopia. As a result, further studies need to conduct on the implication of soil and water conservation measures on a nutrient loss management. Review article Cause and cost of onsite nutrient loss replacement in the highlands of Ethiopia and implication of Soil and Water Conservation measures Kefyialew Tilahun Ejegue* Department of Natural Resource Management, College of Agriculture and Natural Resource, Mekdela Amba University, P.O. Box 32, Tulu Awuliya, Ethiopia Received: 16 March, 2021 Accepted: 08 April, 2021 Published: 10 April, 2021 *Corresponding author: Kefyialew Tilahun Ejegue, Department of Natural Resource Management, College of Agriculture and Natural Resource, Mekdela Amba University, P.O. Box 32, Tulu Awuliya, Ethiopia, Tel: +251927669020; E-mail:


Introduction
Soil erosion is among the most challenging and continuous environmental problems in highland parts of Ethiopia [1][2][3]. Particularly, the Blue Nile basin of the country loses fertile soils with a rate of 131 million ton per year [4]. Soil erosion and nutrient depletion have been a major national agenda and remain an important issue in Ethiopia [1,5]. Ethiopia is reported to have the highest rates of soil nutrient depletion through soil erosion in sub-Saharan Africa [6]. At the national level, full nutrient balance results indicate a depletion rate of 122 kg N ha-1 yr-1, 13 kg P ha-1 yr-1 and 82 kg K ha-1 yr-1) [7]. Depletion rates were highest in the relative intensive farming systems in mountainous areas located in the central and southern parts of Ethiopia [8]. Soil erosion induced by water had an impact on national food supply [9], deteriorate soil fertility and reduce agricultural productivity [10,11], environmental sustainability [12], downstream fl ooding and reservoir sedimentation [13,14] and loss of valuable plant nutrients [7,15,16]. As a result, nutrient loss from agricultural land causes an economic loss to the farmers by both reducing crop yield and increasing the replacement cost of nutrient loss [17,18]. Moreover, low intrinsic soil fertility, negative nutrient balance, limited replenishment of removed nutrients, and high erosion rates cause soil fertility decline and become a major threat to current and future food production [19,20].
Various studies at watershed scale in northwestern highlands of Ethiopia have been reported that, the rate of soil

Description of the Ethiopian highlands
The highlands are extremely heterogeneous, with steep escarpments (Figure 1). The highland parts of Ethiopian which cover the major portion of the country (40% of the total area of the country), characterized by high human and livestock population pressure, high land fragmentation, rapid expansion of agricultural land, extensive cultivation eventually resulted in high land degradation [36]. The highlands are known as "the roof of Africa" (in Africa the majority of land over 3000m is found in Ethiopia) and reach 4533m at the summit of Ras Dashen in the scenic world heritage Simien Mountains [37]. Most of the sub-Saharan Africa's Afroalpine ecosystem above 3200 m is found in Ethiopia [38]. Ethiopian highlands have been designated hotspots for large number of rare land plant species [39]. The Ethiopian Highlands are climatically important in trapping moist air that mainly comes from the Indian Ocean, and providing precipitation to the country. Average annual rainfall varies between 600 mm per year in Tigray (the north) and more than 2,000 mm per year in the southwestern highlands [40].
And also in northwestern highlands the mean annual rainfall is 2454 mm (85% during the wet season), and the mean daily temperature ranges from 9.4-25 °C [14]. loss from sheet and rill erosion ranged from 50.31 to 237 t ha -1 y -1 [21][22][23] and as high as 127 to 540 t ha -1 y -1 for gully erosion [24][25][26]. This implies that soil erosion status in Ethiopian highlands exceed the soil loss tolerable limit of Ethopia 18 t ha -1 y -1 [27]. On the other hand, surface runoff leads to onsite nutrient removal and eventual deposition in depressions, or further downstream in valleys, lakes and reservoirs [1,13,15].
In addition, nutrients are temporally lost in agricultural land via crop residue removal, harvested product and leaching, and gaseous [8,28,29]. Particularly, soil erosion is a key determinant of the negative nutrient balances emphasizing the need for improved soil and water conservation measures at farm level and at catchment level [8]. In response to soil degradation challenges, government, NGO and development partners have invested substantial resources in promoting soil and water conservation practices since the mid-1970s and 80s as part of efforts to improve environmental conditions and ensure sustainable agricultural production [30][31][32]. Combating land degradation and investing in the soil and water conservation for future generations is a major development task promoting sustainable land management [10,33]. FFor instance, Addis, et al. [34] reported that soil and water conservation measures in

Soil and water conservation practices in highlands of Ethiopia
Soil and water conservation technologies were implemented on cropland in many highland part of Ethiopia since the 1970s and 1980s drought and famine specially, in Wollo and Tigray [30]. Soil conservation is a need to reduce soil fertility depletion and achieve sustainable land management, which is non-negotiable in developing countries where agriculture is the main source food for a growing population [41].  [13] reported the commonly implemented SWC measures in Ethiopia; such as soil bunds combined with trenches in croplands, which are constructed by mobilizing the community through the free-labor day scheme; soil bunds integrated with Sesbania trees in croplands, where the trees are also being used for animal feed through a cut-and-carry system; and exclosures combined with trenches in degraded steep slopes.

Agricultural practices in the highlands of Ethiopia
Agriculture is the dominant sector and biggest employer of the economically active population in the highlands of Ethiopia (more than 88% of the total population). The livelihood of the community is dependent on mixed farming system i.e. crop production and animal rearing is the main economic activity.
Teff, maize, fi ngermilte, wheat and barley crops and livestock's such as cattle, goats and sheep production were an important source of household consumption and sell [52][53][54]. In addition, acacia decurrens tree-based farming system is commonly experienced by the larger portion of smallholder farmers in the highland agricultural landscape particularly in northwestern Ethiopia highland as the major source of income from charcoal production and crop production [55]. This farming system is considered as one approach of agroforestry that has a positive effect on restoring soil degradation through the improvement of soil chemical and physical properties and reduces soil erosion with its associated problems [56,57].
Crop rotation is the practice of cultivating different sequences of crops on the same plot of land. It can have a major impact on soil health, due to emerging soil ecological interactions and processes that occur with time [6,58]. Farmers in highlands of Ethiopian practices crop rotation with in the cereal to cereal and legumes to cereal crop each year, of which mostly teff to maize, bean to fi ngermilte, maize to teff and fi ngermilte, barely to fi ngermilte, wheat to potato are common cropping change system [58,59].

Landscape characteristics and soil type
The northwestern portion of highlands of Ethiopia which covers the Tigray and Amhara Regions characterized up to 30 % or more in terrain topography. The slopping area of the highland mostly vulnerable to soil erosion due to intensive cultivation and lack proper land management practices [3,14,60]. Information on soil is an essential in sustainable utilization of soil resources and sound land use planning [61].
According to (Food and Agriculture of the United Nations

Soil erosion
Most of the essential plant nutrients are found on the top surface soil and hence the top layer of the soil is subjected to soil erosion and most onsite nutrients have been lost.
Various studies reported that soil erosion in the form of water resulting in the loss of valuable plant nutrients (NPK and SOM) with the eroded soil [1,16,18,[66][67][68]. For instance, in the past Haileslassie [17] reported that the contribution of soil erosion to NPK loss in teff land use was 70%, 80%, and 63 % respectively. This implies that nitrogen phosphorous and  [29] reported NPK nutrient loss due to wheat and hanfets residue removal was 8 kg ha -1 , 2.6 kg ha -1 , and 40.4 kg ha -1 respectively in Werie-Leke districts in the tigray region. Negash, et al. [76] also observed that 59, 13.9, and 79 kg ha -1 yr -1 of N, PK lost respectively due to consumption of crop residue and dug cake for fuel energy (Figure 2). This implies that most of the smallholder farmers of Ethiopia harvested all crop residues for the purpose of fi rewood and animal feed consequently, a high amount of essential plant nutrients are lost out of the system. The removal of crop residue is a common activity by smallholder farmers after harvested the product and a continuing onsite nutrient loss problem due to lack of detailed awareness for farmers about the role of leaving >30% crop residue on soil fertility improvement. This is similar which is reported previously in the FAO [77] in most parts of Ethiopia 85% of total residues are removed from cultivated land being used as domestic energy sources. For instance, Admassu, et al. [19] reported that the average soil loss was lower (16 t ha −1 yr −1 ) in zero tillage with 2 t ha −1 yr −1 crop residue and higher

Harvested product
Soil fertility decline could be the major factor for the apparent lower crop yield in Ethiopia as a result, soil nutrient loss status evaluation of crops before and after harvesting production is important for sustainable production. This

Effectiveness of soil and water conservation measures on soil loss reduction
Human activities associated with soil and water conservation practices such as physical, biological and soil management practices could reduce soil loss in different part of Ethopia (Table 1). Various studies have conducted on the effect of soil and water conservation measures on soil loss in Ethopia highlands [14,15,16,69,60,71]. For instance, Admassu et al. (19) observed that zero/minimum tillage with 2 t ha −1 yr −1 crop residue was reduced soil loss by 47% compared to control treatment in the Humid Highlands of Ethiopia. Grum, et al. [16] also reported tied ridges with straw mulch reduced soil loss by 91 % compared to non-conserved land Gule sub-watershed. In addition, Abrha et al. [69] observed that stone-faced soil bund reduced soil loss by 90% compared with untreated cultivated fi eld in Welkait district Western Zone of Tigray Ethiopia.
These implying that implementation of soil and water conservation measures are effective to reduce the amount of soil loss through creating surface roughness, convey the erosive surface fl ow via increasing infi ltration rate and minimize the removal of top fertile soil out of the catchment through enhance accumulation of sediment behind it. This argument is line with study made by Jemberu, et al. [72] observed that a farm land treated with soil bund increased porosity, infi ltration by 14.2% and 41 % respectively compared to those of untreated farm land in Koga catchment, highlands of Ethiopia. Amdemariam, et al. [73] also reported that a 9-year old soil bund had the highest mean infi ltration rate (0.88 cm hr -1 ) whereas the nonconserved land had the lowest mean infi ltration rate (0.24cm hr -1 ). Furthermore, Tiki, et al. [74] indicated that averagely 45.74 t/ha/yr sediment was accumulated on soil bunds in Goba District in Bale Zone South East Ethiopia. Likiely, Nyssen, et al. [75] also reported that sediment accumulation rate on stone bund is 57t/ha/year. This implies that implementation of proper SWC of soil and water conservation measures are important for soil erosion control through enhancing infi ltration of surface runoff and trapping sediment behind it.

Crop residue removal
Complete crop residue removal and continuous cultivation without fallowing and inadequate organic fertilization of agricultural fi eld were caused for nutrient loss. Kiros, et al. [28]  is because some studies showed that the highest onsite nutrient loss mainly caused by the harvested product. For instance, Hailisalassie, et al. [7] observed that nutrient loss from vegetable and permanent crops was mainly removed via harvested product and crop residue removal. Kiros, et al. [28] also determined nutrient losses of NPK from harvested crops in the May Leba catchment in northern Ethiopia and obtained that the highest NPK nutrient is lost in the range 28.69 to 82.64, 0.12 to 0.32, and 9.37 to 18.02 kgha -1 yr -1 respectively across all the landscapes. This implies that similar to crop residue removal, harvested product also increased nutrient loss and nutrient withdrawal increased in the monocroping farming system. Similarly, Belete [78] observed averagely 59.1 kg ha -1 NPK nutrient loss due to the harvested product of the cereal in North-Western Ethiopia. Likiely, Van Beek, et al. [8] obtained high total N loss in Bure and Dera districts farmland in North-Western Ethiopia. This implies that though it varies in harvested product type each season a lot of nutrients are lost in cultivated land.

Gaseous loss and leaching
Gaseous/volatilization can be an important pathway of N fl uxes in many agricultural production systems [79]. Nitrogen is easily disappeared through volatilization and leaching from the soil surface due to its very mobile nature. For example, in modern agriculture nitrate is the main source of nitrogen for crops; yet, nitrate is also the most mobile form of N and easily loses from the soil through leaching [80]. And also the conventional furrow irrigated agriculture without management of excess water causes leaching of nitrate-nitrogen and other macronutrients [81]. On the other hand, in high and medium altitude areas where rainfall is high, most of the macronutrient (NPK) is lost through leaching making the nutrient unavailable during the critical stages of crop growth [81]. The loss of nutrients not only troubles the farmer on crop reduction, but it has also hazardous impacts on the environment [34]. As a result, an optimum nitrogen fertilizer application rates for the various growth stages of crops is important to minimize the losses of N and nitrogen use effi ciency (NUE) via leaching and volatilization [80]. Nitrogen use effi ciency expressed as grain production per unit of N applied [82]. The application of high cation exchange capacity materials is reduced N leaching and increases plant N uptake in sandy soils [83]. Similarly, Agegnehu, et al. [84] observed that 17-65% fewer fl uxes of N 2 O produced per unit of peanut produced in the organic amended such as compost and composted biochar-compost than the control treatment.

Soil nutrient depletion
Nutrient balances are calculated from the difference between infl ow and outfl ow of respective nutrients from the system [7]. Input fl ows include chemical and organic fertilizer, atmospheric deposition, and sedimentation while output fl ows from the system include removal of nutrients from the soil by harvests product, crop residue, and soil erosion by water, leaching, and volatilization. Currently, soil nutrient depletion is becoming the major challenge for agricultural production for the stakeholder farmers in Ethiopia [5,8]. This is due to the imbalance between input delivery and output to the system in Ethiopia.
The major causes of soil fertility depletion are inadequate fertilizer use, complete removal of crop residues, continuous cropping systems, and climate and soil types, lack of proper cropping systems and soil erosion and continuous cultivation [85,86]. Agegnehu, et al. [84] also stated that ineffi cient use of organic fertilizer contributes to the depletion of scarce fi nancial resources, increased unit production costs, and potential environmental risks. For example, a study made by Belete [78] reported that the average negative nutrient balance of NPK in the cereal lands of the Tigray region was -65 N, -27 P, and -45 K kgha -1 respectively. Similarly, van Beek, et al. [8] also observed an average negative nutrient balance of -23 ± 73 and -7 ± 64 kg ha -1 N and K respectively under diverse agroecological settings in Ethiopia. Moreover, Haileslassie, et al. [7] also observed a negative full nutrient balance of NPK (-122, -13 and -82 kg ha -1 yr -1 ) respectively on smallholders' mixed farming systems in Ethiopia. The negative nutrient balance implies that net negative losses between infl ows and outfl ows. For instance, Aticho, et al. [5] studied in jimma zone Ethiopia showed that N, P, and K added to cropland were much less than nutrients removed out the system through crop residue removal, soil erosion, and harvesting product. On the other hand, soil management practices alone or integrated with chemical fertilizer resulting in positive nutrient balance. For instance, Tadesse, et al. [82] observed that combined application of 15 t ha −1 farmyard manure and 120 kg N ha −1 resulted in the highest N positive balance (214.8 kg ha −1 ) compared to lonely chemical fertilizer (150.1 kg ha −1 ). The observed highest N balance was just due to the higher amount of N input, coupled with the lower uptake of the nutrient by the plant. Similarly, Kraaijvanger and Veldkamp [29] reported a nutrient balance of NPK under control treatment were -39,-11, and -49, while application of 18 to-1 manure resulted in a positive nutrient balance of NPK, was 0,2, 4 and 4.6 kg ha -1 respectively. This implies that the application of combined fertilizer particularly farmyard manure and optimum inorganic fertilizers improved the soil characteristics and consequently reduced the loss of nutrients from the system.

Cost of nutrient loss replacement
Nutrient lost from agricultural land imply an economic loss to the farmers by reducing crop yield and increasing the replacement cost of lost nutrient [1,17,18,66,67]. For example, Erkossa, et al. [1] observed that mean 2024 kg ha -1 reduced in the maize yield due to loss of major N and P nutrient in Dapo catchment and equivalent onsite weighted mean replacement cost was estimated to be 372 USD ha -1 yr -1 . Likely, in Mizewa catchment of the Blue Nile basin study made by Taye, et al. [66] observed that yield reduction of maize due to mean N and P nutrient loss was about 463 kg ha -1 and equivalent onsite weighted mean replacement cost estimated to be 200 ha -1 yr -1 . Selassie and Belay [17] also reported weighted mean replacement cost of N and available P for non-conserved cropland in the Harfetay watershed was estimated to be 6.4USD ha -1 yr -1 (Table 2). Moreover, Wudneh, et al. [18] revealed that the net yield of maize reduced by 838.4 kg ha -1 due to loss of major N and P nutrient in Chekorsa catchment (Table 3). This implies that in addition to the sedimentation effect of exported nutrients on dam /reservoir, it had the highest estimated yield reduction and replacement cost for the total N and available P nutrients lost. Nevertheless, as mineral fertilizers are not affordable for smallholder farmers to replace the nutrients lost from their cultivated land, it is essential that practicing organic fertilizer to ensure the long-term sustainability of agricultural systems and to avoid irreversible losses.

Impact of SWC measures on onsite nutrient loss
Practicing proper soil and water conservation measures had a positive impact on the reduction of onsite nutrient losses ( Figure 3). However, limited studies have been reported on the impact of soil and water conservation practices on nutrient loss in Ethiopia [13,16,17]. For instance, a study made by Grum, et al. [16] showed that tied ridges with straw mulch were reduced total N and available P nutrient loss by 88% and 92% compared to control treatment in the Gule sub-watershed in northern Ethiopia. On the other hand, incorporation of effective microorganisms like bacteria and fungi on straw mulch and tied ridge improved total N and available P (Table 4). Selassie and Belay [17] also observed that stone bund reduced SOC, total N, and available P nutrient loss by 80% 78% and 73% compared to control treatment in the Harfetay watershed in northwestern Ethiopia.

Role of integrated soil management practices on land productivity
The application of optimum organic inputs, proper management of crop residues, crop rotation practices are very crucial to soil erosion reduction and replace the exported nutrients out of the system, consequently improving land productivity [59]. Moreover, application of integrated optimum organic and inorganic fertilizer improving soil fertility and crop productivity in highlands of Ethiopia. For example Elka and Laekemariam [87] reported that integrated application of 150 kg ha -1

Conclusion and policy implication
Soil erosion and nutrient depletion has been a major national agenda and remains an important issue in the highland of Ethiopia. The rate of soil loss from sheet and rill erosion in northwestern highlands of Ethiopia raise up to 237 t ha -1 y -1 and in gully erosion raise up to 540 t ha -1 y -1 . The impact of soil erosion and crop residue removal is the main triggering factor for onsite nutrient loss in agricultural land in highland Ethiopia. Hence, soil nutrient depletion is becoming the major challenge for agricultural production for the stakeholder farmers in Ethiopia due to the imbalance between input delivery and output to the system. Soil erosion is a key determinant of the negative nutrient balances emphasizing the need for improved soil and water conservation measures at farm level and at catchment level. Consequently, nutrient loss from agricultural land had an economic loss due to reducing crop yield and increasing the replacement cost of lost nutrients. However, limited studies have been reported on the impact of soil and water conservation measures on nutrient loss management. Hence, lonely or integrated SWC measures are effective in reducing soil erosion and nutrient loss in Ethiopia due to enhancing soil aggregation, infi ltration, and accumulation of sediment behind the SWC measures. As a result, further studies need to conduct on the implication of soil and water conservation measures on a nutrient loss management. Thereby to minimize the nutrient depletion and economic loss, sustainable integrated land management approach like agroforestry practice, application of organic fertilizer, stone-faced soil bund with vegetative measures are a best alternative new approach. Obviously, agroforestry practices like multipurpose tree plantation, alley cropping, intercropping, and organic fertilizer such as compost, locally available animal manures are pillar to minimize nutrient depletion and economic loss via lonely inorganic fertilizer.  Figure 4: Layout of the experimental runoff plots with and without treatment respectively (a&b); Experiment plots with tied ridges + straw mulch and effective microorganisms (c); Soil-moisture measurement by using Trime-PICO64 soilmoisture sensor (d). Source Grum, et al. [16].