Soil physical and chemical properties across the natural vegetation
The soil properties are changing in an area due to the dynamic interactions between microclimatic conditions, vegetation types, and altitude [27, 28]. This follows from the pedogenetic factors that dictate soil formation including topographic factors, climate, biota along with parent materials operating over time as elaborated by Jenny . The soil textural differences across the vegetation types might be due to the natural variations in the rate of weathering and some micro-topographical differences such as percentage slope instead of land management practices [30, 31].
The sand content of the soil in ericaceous land was significantly (p < 0.05) higher, compared to other vegetation types which might be due to the higher slope gradient (> 30%) under this land cover type that results in the erosional removal of fine particles down the slope. Consequently, the top soil that constitutes fine soil particles, such as silt and clay, could easily be eroded by the rain and leads for the high proportion of sand content in the area overtime. This is consistent with the high clay content in the moist forest that is located in the lower slope gradient positions. The highest value of clay in moist forest could be due to the high conversion rate of litter in the soil and the lowest value of clay content in the ericaceous land could be due to lower conversion rate of litter in the soil. However, the overall average clay content of the BMNP was less than 20%. Across all land cover type the soil BD was higher in the moist forest and lower in the ericaceous land. This result was consistent with the report by Yimer  that described ericaceous vegetation constituted the least BD compared to other vegetation types.
The lower level of pH in the grassland and afro-alpine land might be due to the high amount of rainfall and low level of temperature and that resulted for high moisture content in the soil. These areas are waterlogged and their watershed is characterized by flat, swampy areas, and many small shallow lakes, that are crucial for stream and river flow regulation into the lowlands, are situated . The mean annual rainfall in these areas ranged from 1000 to 1400 mm and the mean annual minimum and maximum temperatures are 2.4 °C and 15.5 °C, respectively [33, 34]. The higher water holding capacity of the soil in those areas make cations easily solubilized and contributed for high pH level. Overall, the soils investigated in this study were characterized as strongly acid to moderately acid and its pH was ranged from 5.21–5.97 .
The SOC content showed significant variation with the change in vegetation types. The significantly higher content of SOC in ericaceous land, woodland and moist forest might be related to the higher decomposition rate of organic matter under higher temperatures and increased vegetation abundance that produced lots of leaf litter available for decomposition under this land cover type . Recurrent fire in the ericaceous land could be the other reason for the higher amount of SOC in this land cover type since the burning of above ground biomass can add more carbon content into the soil . Conversely, the reduced amount of SOC in the afro-alpine vegetation could be due to its lower temperature owing to its location in the higher altitude and the resulted lower decomposition rate of organic matters by the reduced bacterial activities . Moreover, this vegetation type is covered with small size plants, mainly herbs and shrubs; as a result, less amount of organic matter is produced and added into the soil.
The strong and positive correlation (r = 0.75, p < 0.05) between the amount of TN and SOC in the BMNP strengthen the fact that most nitrogen forms are a part of the soil organic matter . A C/N ratio above 12–14 is often considered indicative of a shortage of nitrogen in the soil . However, this ratio in the study area was below the range and it indicates the high amount of nitrogen in the soil of the study area. Conversely, the amount of AP was varied significantly across the vegetation type and it was higher in the woodland, ericaceous land, moist forest and coffee forest which are located in the lower and middle altitudinal ranges, compared to Afro-alpine and grassland, which are located in the higher altitude. The significant variation in the amount of AP across the vegetation types at different altitudinal ranges could be due to the geology of the area and the nature of the soil [39, 40]. Weinert and Mazurek  suggested that the higher content of phosphorus in the soil could be related to the faster mineralization and mobilization of phosphorus. The downslope leaching from the higher altitudes could also be the other reason for the higher content of AP in the lower altitude vegetation types [32, 42]. The significantly lower level of AP in the grassland and Afro-alpine vegetation could be due to the problem of P-fixation as a result of lower pH level as well as due to the lower rate of organic matter decomposition. When the soil acidity increases, phosphorus availability decreases as it got fixed by the iron and aluminum oxides that are high in the soil solutions with strongly acidic reaction . Soils with high clay content may have the ability to neutralize the acid-extracting solution and thus reduce the amounts of extractable phosphorus . Phosphorus fixation tends to be higher and ease of phosphorus release tends to be lower in soils with higher clay contents .
The CEC of the soil in the study area was significantly higher in those vegetation types shown high level of SOC, such as ericaceous land, moist forest and woodland. Certain soil minerals, particularly clay in combination with organic matter, determine the soil CEC by attracting and holding oppositely charged ions . Similar research reports were also made by Tegene , Eshetu et al.,  and Yimer,  in the Ethiopian highlands. The higher level of soil pH in the ericaceous land, moist and coffee forest could be drawn from the relatively high amount of SOC and CEC as well as the high rate of organic matter decomposition in this vegetation types. Moreover, both moist and coffee forest are situated in the lower altitude and received relatively lower amount of rainfall (from 600–1000 mm) and had higher mean annual maximum temperature (29.1 °C) compared to the middle and higher altitudes [33, 34].
Effects of land use/land cover change on soil physical and chemical properties
Human induced land cover change and habitat fragmentation owing to the expansion of farmlands and settlements significantly affected most of the soil physical and chemical properties in the BMNP. The clay content in the natural vegetation was significantly (p < 0.05) higher compared to the farmlands. The soil BD in the native vegetation, mainly in the moist and coffee forests, was higher, but not significant (p < 0.05), compared to the farmland and this could be due to the effect of high soil organic matter accumulation in the natural vegetation. However, the soil BD was higher in the farmland compared to the grassland, wood land, afro-alpine and ericaceous land. Soil compaction due to farming and livestock grazing could be the reason for the higher BD . Trampling by cattle has been identified as the primary cause of high BD by compacting the soil surface .
The conversion of natural vegetation into farmland tends to decrease the soil pH . In this study, the decrease in the soil pH was more pronounced in the farmland. The strongly acidic soil reaction under farmland is partly explained by the application of acid forming fertilizers such as DAP and urea for cereal cultivation. Moreover, the SOC and TN in the soils of the farmland were significantly (p < 0.05) lower compared to the natural vegetation. This might be due to the lower amount of organic material returned into the soil system, reduced litter decomposition rates, and high rates of soil organic matter oxidation due to tillage in the farmland [51, 52]. The higher amounts of SOC and TN in the natural vegetation were due to the higher accumulation of organic matter, which were resulted from the increased above and below-ground biomass [53, 54].
Conversely, land use changes significantly affected the CEC of the soil in the BMNP. The CEC of the farmland soil were found lower by 40% than the natural forest. Reports from different study revealed 27 to 43% reduction in soil CEC of the agricultural land compared with the natural vegetation [32, 54]. Thus, it is ascertained that the soil quality deterioration in the BMNP was mainly due to the human induced LULCC.
The influence of altitudinal variation on soil physical and chemical properties
One of the leading factors governing soil organic matter accumulation and turnover is the mean annual temperature and precipitation [55, 56]. The mean annual precipitation of the BMNP area varied from 450 to 2400 mm. The amount of temperature in an area is affected by altitude. As a result, temperature decreases with increasing altitude and vice versa. Accordingly, lower decomposition rate has been asserted at higher altitude above 2800 m asl, i.e., ACZ 3, and higher decomposition rate has been recognized at lower altitude from 1600 to 3000 m asl, i.e., ACZ 1, in the study area. Higher temperature facilitates the decomposition rate of soil organic matter while lower temperature retards it. Large size trees were dominant in the ACZ 1 and that resulted for the high litter production. Whereas, herbaceous and shrub species were dominant in the ACZ 3 and that resulted for the lower litter production. Accordingly, the amount of SOC decreases as altitude increases and vice versa. Based on the FAO  assessment, the higher elevation plains of the Bale mountains are relatively infertile well drained Eutric Cambisols, whereas on the gently sloping foothills, below the escarpment, are relatively fertile Eutric nitisols.
In the same vein soil properties showed strong and positive correlation among themselves including soil pH and AP (r = 0.71, p < 0.01); SOC and TN (r = 0.75, p < 0.01); SOC and CEC (r = 0.74, p < 0.01); and TN and CEC (r = 0.67, p < 0.05). When the soil acidity increases, phosphorus availability decreases as it got fixed by iron and aluminum oxides and hydroxides that are high in the soil solutions with strongly acidic reaction . As would be expected, the contents of TN are the mirror image of SOC that depending on the decomposition and mineralization of soil organic matter as evidenced by the high correlation coefficient [32, 40]. The positive and strong correlation between CEC and SOC is expected because soil organic matter and humus are one of the major sources of charge sites on the clay lattice in addition to the silicate mineral sources of negative charge sites that contribute towards the effective CEC. Although not significant, SOC is positively correlated with pH suggesting that the radical groups (e.g., –COO−) fix the H+ in the soil solution and increase the pH which is the negative logarism of the H+ concentration in the soil solution .
Habitat gradient effect on soil physical and chemical properties
Some of the soil physical and chemical properties in the BMNP were significantly (p < 0.05) affected by the edge habitat. Accordingly, the soil sand and silt content as well as BD were higher in the edge habitat, whereas, the amount of the soil chemical properties assessed, including total N, SOC, CEC and AP were higher in the interior habitat. This result was similar with the report made by Zhou et al.,  and Ruwanza . This could be due to the exploitation and conversion of vegetation in the edge habitat through grazing, agriculture, and settlement expansion. The soils in the edge habitat were more compacted and less fertile. This could be due to the human activities, such as trampling by humans and livestock, agricultural activities and settlement expansion, in the edge habitat. As a result, vegetation in the edge habitats are destructed and less amount of litter content are added. Conversely, the soils in the interior 13habitats were porous and more fertile [58, 59]. This could be due to the less human disturbance and high accumulation of litter content in the interior habitat.