Review on Role of Soil and Water Conservation Measures on Soil Physico Chemical Properties and Its Implication to Climate Change Mitigation and Adaptation

Soil degradation is a global threat. Developing countries are more severely affected by soil degradation than developed countries. Ethiopia, one of the developing countries in eastern Africa, is highly threatened by soil degradation problems. The soil degradation problem has had serious consequences in Ethiopia such as occurrence of persistent food insecurity, economic losses and various environmental hazards such as recurrent drought and increases vulnerability of people to the adverse effects of climate variability and change. To overcome these problems, integrated soil and water conservation has been implemented in the country. Thus many studies have been conducted with regards to soil and water conservation which influences soil physical and chemical properties and on climate change adaption and mitigation. Therefore, this review papers was aimed to assess role of soil and water conservation on soil physical and chemical properties and it’s implication to climate change adaptation and mitigation. Different studies showed that soil physical and chemical properties such as Soil texture (sand, silt, clay), Moisture volume (%), FC (%), PWP (%), AWC (%), OC, TN, pH, EC, CEC Ava_P, and Ava_ K, (EB: K, Ca, and Mg) were improved in different agro ecologies. In addition to this, soil treated with soil and water conservation stored more organic carbon as compared to non treated soil. Soil and water conservation also contributed to improve yield and yield component of crops in areas where serious soil erosion and also in moisture deficit areas. Thus, the studies have been showed that the people’s ability to adopted in climate change. Therefore soil and water conservation could improve soil physical and chemical properties, climate change mitigation and adaptation in the country.

Soil organic carbon (SOC) plays a vital role within the overall carbon cycle (Van Oost et al., 2007). Central to the present concept, soil sequesters averagely three hundred times of carbon created by industrial burning of fossil fuels (Lal, 2005). So any slight changes to SOC can have a negative effect on provision of system services. Being preferentially found on the surfaces, SOC has comparatively lower density, making it easier for it to be carried off by runoff. Several studies have shown that the typical loss of SOC by water erosion annually is 1-5 pentagram of carbon that is consequently deposited at the lower areas of a catchment (Berhe, 2012). SOC includes organic compounds (i'e., plant, animal and microbial residues in any stage of decay) that are highly enriched in carbon (Lal, 2008). Consequently the role of SOC is important in edaphic factors like physical, chemical and biological properties of soil. Thus soil organic matter (SOM) determines soil quality, physical properties, crop nutrition and the link between these (Bergmann 1992;Loveland and Webb 2003). The soil physical properties affected by soil OM include aggregate stability, infiltration, water-holding capacity, soil workability, bulk density, aeration and water movement (Bergmann 1992;Loveland and Webb 2003). Soil carbon is affected on a spatial and temporal scale by climatical, edaphic, biotic and lithological factors which influence the balance between the gains and losses of soil carbon (Kurgat, 2011). Girmay et al. (2008) estimated the historic SOC loss from rangelands and croplands of Ethiopia over the last 50 years to be 230-670 TgC (Tg = teragram = 1012 g = million metric ton). Therefore, soil degradation will continue to be a serious threat to the finite land resource if prudent land use and effective soil management strategies to increase SOC concentration and restore soil quality are not implemented.
Sustainable soil management technologies (e.g., appropriate soil and water conservation (SWC) measures, afforestation of degraded soils, water harvesting, crop rotations, crop residue mulching along with cover cropping, agroforestry, and integrated nutrient management) can enhance the SOC stock, reduce soil degradation, and decrease soil's vulnerability to climate change. In addition, judicious soil management can increase people's capacity to adapt and mitigate climate change through C sequestration and greenhouse gas (GHG) emissions reduction (Lal, 2004;Vagen et al., 2005;TerrAfrica, 2009). World croplands can sequester 0.02-0.76MgCha _1 year _1 by adopting recommended management practices (Lal, 2001). Girmay et al. (2008) estimated the historical SOC sequestration potential of croplands in Ethiopia through adapting soil restorative measures at 215 -638 Tg C over a period of 50 years. Soil C sequestration can improve soil quality; restore degraded ecosystems, and increase agronomic/biomass productivity (Lal et al., 2003). Thus, C sequestration is often termed as a win-win or no-regrets strategy (Lal et al., 2003;Girmay et al., 2008). However, it requires the selection and implementation of technologies that are appropriate to specific soils and eco regions (Girmay et al., 2008).
Therefore the objective of this review is to assess role of soil and water conservation on soil physical and chemical properties and it's implication to climate change adaptation and mitigation in Ethiopia.

METHODOLOGY OF REVIEW
To the success of this work, different sources such as journals, proceedings, thesis works and reports related to soil and water conservation and climate change adaptation and mitigation have been used. Several studies recognized that, physical SWC showed a significant difference on chemical properties of soil between conserved and non conserved plot of land. According to the study conducted by (Abay et al., 2016) the graded stone bunds have shown significant improvement in chemical soil properties such as soil OM, TN, pH, and CEC. Moreover, the high OM content of farm plots with SWC practices affect more positively the soil properties as compared to the non conserved farm plots. Also, variation was also significant along slope gradient for some chemicals properties. Worku, H (2017) indicated that, physical SWC (stone faced soil bund and soil bund) is promising in protecting the cultivated land from erosion and the associated nutrient depletion. With regard to analysis of soil characteristics in treated and untreated plots, SOC and total N were higher while BD was lower under the conserved farm. Yonas et al., 2017 also reported that, that the effectiveness of soil and water conservation improves significantly the soil chemical properties (soil pH, K + , available P, SOC, TN, and CEC) than in the adjacent without SWC treatment. This indicates the positive impacts of SWC practices in improving the nutrient status. OM, TN, pH, CEC Ava_P, and EB also significantly improved by biological SWC. However, Kebede et al., 2011 reported that, less SOC, Pavai, pH is measured from conserved plot of land. These perhaps due to: the difference in the past land degradation resulting from continuous cultivation, extractive plant harvest and soil erosion. Alemayehu, A 2007 also confirm in Anjeni watershed that Pavail on non terraced land was higher than the terraced. The significantly low soil pH in level stone bund and soil bund compared to the respective adjacentnonterraced cropland were probably due to loss of relatively more basic cation resulted from erosion before the structures built and did not restore yet after the structures. Under a continuous cropping system soil acidity increases due to the gradual replacement of basic cations by aluminum (Zougmore et al., 2002).

EFFECT OF SOIL AND WATER CONSERVATION ON PROPERTIES OF SOIL 3.1 Effect of Soil and Water Conservation on Chemical Properties of Soil
SWC structures are practically used as support for agronomic and soil management (Morgan, 2005) and considered as the first defense line. Thus, they alone are less likely to improve soil properties significantly under similar management to non terraced. Zougmore et al., 2009 has reported that combining stone rows barriers to run-off with the application of compost was significantly controlled erosion and reduced organic C and nutrient losses than compost or stone row alone.

Effect of Soil and Water Conservation on Physical Properties of Soil
Various studies conducted to evaluate the structural and biological soil and water conservation and on physical soil properties. According to those studies, the percentage of clay content of soil increases with a soil treated with SWC structure, and decreasing sand particles of soil. The decrease in soil BD due to SWC practices at would result in greater water infiltration rates which in turn minimize runoff velocity, thus, sediments and organic matter removal. As a consequence OM accumulation improves a soil physical structure which promotes crop root abundance, crop stand, crop production and better crop residues at the conserved field plot. The land that treated with SWC measures improves the soil moisture content which is a key factor affecting agricultural production in water limited environments. According to (Abay, et al) Bulk density and moisture content of treated soil is increased. Similar results also reported by (worku et al., 2012) and increased the percentage of clay contents was observed. This result also confirms the presence of higher clay fraction of conserved soil due to deposition from the upper slope (Regina et al., 2004). Soil moisture shows significance variation between treated and non treated land. SOM is positively correlated with MC while it is inversely correlated with soil BD. The recorded percentage of sand is lower for soil  Vol.9, No.11, 2019 30 treated with SWC while higher percentage of clay for treated soil. Those results confirm the findings by Lemma et al. (2015).These may be due to soil particles resistance to detachment, and susceptibility to transportation. Gebremichael et al. (2005) reported that selective removal of soil particles to steeper slopes leave behind coarser materials (sand, gravel and stones), while the transported material is deposited as the slope steepness decreases. Sandy soils are less cohesive than clayey soils and thus aggregates with high sand content are more easily detached; silty soils derived from loess parent material are the most erodible type of soil (Blanco and Lal, 2008). Integrated application of manure and soil bund also improves soil bulk density (Yihenew et al., 2015) There is an improvement in hydrological properties in soils of the conserved than those in the non-conserved land (Daniel et al., 2015). The volumetric moisture content, field capacity, permanent wilting point and available water content of soils of the conserved land is higher than the non conserved land. A study by World Neighbors (2000) in Guatemala, Honduras, and Nicaragua reported a 3-15% increase in AWC by the adoption of ecologically sound SWC practices. Improvement in AWC is important because such soils buffer water during periods of water deficit and could significantly improve agronomic productivity of rainfed agriculture. However, the agronomic and economic performances of SWC measures in tropical regions are highly dependent on the amount and distribution of precipitation (Benites & Ofori, 1993). Daniel et al., 2015 reported that, the highest FC and AWC, and also the lowest PWP is recorded from a soil treated with SWC. These trends suggest a positive impact of SWC measures on MC, FC, PWP and AWC.
The highest quantity of clay fractions is recorded under lands treated with elephant grass and sesbania whereas the lowest was in the adjacent degraded grazing land. A similar amount of clay fraction was found at lands treated with elephant grass and sesbania. This indicates elephant grass and sesbania have equal potential to minimize rates of erosion, keep clay materials in its original place, and capture eroded clay materials. The highest value of bulk density is observed at degraded grazing land and lowest at land treated with elephant grass and sesbania. Further, elephant grass and sesbania had similar effects on soil bulk density. Perhaps, the achieved soil bulk density improvement is due to organic matter addition from the plants, reduction of physical soil loss, and exclusion of grazing practices and human interference.

ROLE OF SWC TO MITIGATE CLIMATE CHANGE.
There is much concern that the increasing concentration of greenhouse gases in general, and carbon dioxide in particular contributes to global warming by trapping long-wave radiation reflected from the earth's surface. Over the past 150 years, the amount of carbon in the atmosphere has increased by 30%. Most scientists believe that there is a direct relationship between increased levels of carbon dioxide in the atmosphere and rising global temperature (Stavins and Richards, 2005).
One proposed method to reduce atmospheric carbon dioxide is to increase the global storage of carbon in soils. Though, soil carbon storage is a win-win strategy. It mitigates climate change by offsetting anthropogenic emissions; improves the environment, especially the quality of natural waters; enhances soil quality; improves agronomic productivity; and advances food security (FAO, 2005;Lal, 2009;Adesodun and Odejim, 2010;Kumar et al., 2009). Soils store 1502 Gt carbon (Jobbagy and Jackson, 2000), an amount that is two times greater than the amount found in the atmospheric carbon pool (Battle et al., 2000;Lal, 2004). In addition to carbon storage, the turnover time of organic carbon is important in understanding the role of soils in the global carbon cycle. Thus, soil carbon sequestration through changes in land use and management is one of the important strategies to mitigate the global greenhouse effect. Important land uses and practices with the potential to sequester soil organic carbon include conversion of cropland to pastoral and forest lands, conventional tillage to conservation tillage or no-tillage, and no manure use to regular addition of manure. However, food security needs for the world teeming population make conversion of cropland to forestland unsustainable. Therefore, increased food demands call for management of croplands to ensure food security and at the same time enhanced soil organic carbon sink within the soil to minimized atmospheric emission of CO2 (Adesodun and Odejimi, 2010). In this case, afforestation and conservation programs have been made in the last three decades (Badege, 2001). In addition to this there was a huge areal closure activity in the country for the purpose of rehabilitating degraded lands which have their own role in increasing soil carbon stock.
According to the study conducted in Anjeni watershed, higher soil organic carbon concentration and soil organic carbon stock recorded from a soil treated with SWC than non conserved soil (Daniel et al., 2015). The soil and water conservation system reduce surface runoff and soil loss, retain water that enhances crop growth and contributes to SOC input. Thus several studies confirm that, SOM and SOC are increased because of conservation measures. However, Kebede et al., 2011 disagree with these results that, the concentration of SOC is decreased from conserved lands (treated with level soil bund and stone bunds). Thus he suggests that, SOC is less because of intensive tillage, continuous cropping, removal of crop residues, and low organic carbon input in croplands.

CLIMATE CHANGE ADAPTATION
Ethiopia is extremely vulnerable to the impacts of climate change (Aklilu et al., 2009). Similarly, Burnett (2013) reported that Ethiopia has been identified as one of the most vulnerable countries to climate variability and change, and is frequently faced with climate-related hazards, commonly drought and floods. Since the early 1980s, the country has suffered seven major droughts five of which have led to famines in addition to dozens of local droughts. Major floods also occurred in different parts of the country in 1988, 1993, 1994, 1996and 2006(World Bank, 2010. Vulnerability is not the same for populations living under different social, economic,political, institutional and environmental conditions. For example, pastoralists in Yabello district tend to bemore vulnerable to climate change than farmers (Oxfam international, 2010). Several studies indicate that soil and water conservation measures used as a climate change adaptation through encouraging crop yield, even in areas where moisture deficit affect crop yields. SWC used to save moisture in the soil and make conducive environment for plants. Therefore it provides a yield advantage.  Eshetu et al., 2016a andEshetu et al., 2016b reported that the adoption ofFanya juu and Fanya chin and soil bund moisture conservation structure improves crop production and increase the yield advantage as Soil moisture/rainfall decrease. Also considered conserving soil moisture recommended with early maturing and drought resistant maize variety or other annual crops to be further scaled up in moisture stress areas. Also Dagnaw et al., 2018 indicated that Open end tied ridge planting on the furrow showed a promising result on maize grain yield and soil moisture conservation as compared to traditional practice. Therefore, tied ridge as in-situ moisture conservation is effective measures in storing 33.7% additional soil water for the next cropping season as compared to traditional practice. It is recommended that, the practice should be demonstrated and scaled up in moisture stress areas of the country. Terraces improve the yield and yield components of maize and wheat (Tadele et al., 2013). The study of SWC also extends to fruit crops which is affected by moisture deficit due to recent climate change occurred. Therefore, Negarim and semicircular micro catchments with mulch increases the yield and yield components of banana in moisture deficit areas (Tadele et al., 2017). Tekle and Wedajo, 2015 reported that critical to use and apply soil moisture conservation practices in the current agricultural production system and in order to use the available in situ water efficiently and effectively to bring improved grain and biomass yield and also improved productivity and production of sorghum in a sustainable manner. Solomon, 2015 recommend that Water conservation techniques at farm level are essential options for the semi arid area for improving yield through better soil water storage. With the current change in global climate, adaptation methods like the use of conservation approaches are to be implemented if the agriculture sector is to continue to meet the ever increasing food demand especially in developing countries like Ethiopia.
Generally, the current condition of Ethiopian climate change has been changing from time to time which affects the crop production, livestock and all natural resources. The recurrent occurrence of drought in the country leads to famine in arid and semi arid areas. In order to adapt to the change, the farmers adopt SWC on their farm lands and also on area enclosure. Thus, according to the study conducted by many authors, application of SWC provides food and feed even if below of its potential.

SUMMARY AND CONCLUSION
Ethiopia, one of the developing countries in eastern Africa, is highly threatened by soil degradation problems. Depletion of soil organic matter (SOM) and nutrients, salinization, and soil erosion by water are the most critical forms of soil degradation. SWC measures, afforestation of degraded soils, water harvesting, crop rotations; crop  Vol.9, No.11, 2019 32 residue mulching along with cover cropping, agroforestry, and integrated nutrient management can enhance the SOC stock, reduce soil degradation, and decrease soil's vulnerability to climate change. In addition, judicious soil management can increase people's capacity to adapt and mitigate climate change through C sequestration and greenhouse gas (GHG) emissions reduction.
In Ethiopia different SWC has been implemented under different land use system to rehabilitate degraded lands, enhance soil productivity, improve micro climate, and improve crop yields under moisture stress areas. In line with this, different researches have been conducted in different agro ecologies to assess the role of SWC on soil physico chemical properties and its role on to climate mitigation adaptation. Among physical SWC, soil bund, stone bund, stone face soil bund, Fanya juu, Fanya chin, terraces, and among biological SWC plantation of elephant grass and sesbania was implemented. Also integrated soil bund with manure demonstrated to improve soil fertility. According to the research conducted in the country, both biological and physical SWC has improved both physical and chemical soil properties. However the integrated SWC was more effective rather than individual conservation measures. According to the research reviewed, a few considerations were given to physical soil properties as affected by SWC. Physical soil properties affect chemical soil properties.
SWC is act as a climate change mitigation strategies through reduction of CO2 emission to the atmosphere which is in particular contributes to global warming. According to the study conducted in the country, higher soil organic carbon concentration and soil organic carbon stock recorded from a soil treated with SWC than non conserved soil. But most studies have been conducted to determine SOM concentration of the soil, thus SWC increases SOM and in directly store more carbon in soil. However many studies disagree with such finds that only SWC cannot significantly increase SOM alone. Because, it needs an integrated approach of biological and physical SWC to enhance SOM Ethiopia is the most vulnerable countries to climate variability and change, and is frequently faced with climate-related hazards, commonly drought and floods which lead to famine. SWC plays a major role on climate adaptation in order to produce yields especially under moisture deficit areas. The most effective SWC to harvest water were tied ridges with mulch, Circular Pitting, Open Ridge, Half Moon terraces, level soil bund, Fanya juu and Fanya chin, Negarim and semi circular structure with mulch were implemented with different commodities and gives yield advantages than un conserved water on the fields. Even if more consideration were given to physical SWC, soil physical and chemical soil properties were improved and indirectly mitigate climate changes in the country. In addition to this, small holder farmers produce crop yield under limited moisture content of soil by harvesting the limited rainfall. Therefore soil and water conservation could be demonstrated and popularized on all land uses