The soil laboratory results for the 30 samples were statistically summarized in Table 1. The result showed that soil physical & chemical properties had variation among land use/ covers and agro ecologies. Overall average of soil acidity level/ pH, OC, TN, Ca2+, Mg2+, K+ and CEC in the soils of cultivated land were very low compared to the soils under natural forest and grazing lands in both agro ecologies. Whereas, soils under natural forest had low level of Av.P compared to cultivated and grazing lands. On the other hand, in the Dega agro ecology, soils were high in clay content but low OC and CS as compared to High Dega of the study watershed. In the High Dega agro ecology, soil pH, available P and BD were low while the proportion of Ca2+ and CEC nutrients were slightly higher compared to the soils of Dega agro ecology (Table 1).
Table 1
Mean of soil properties per Land use/ cover and agro ecology and overall statistical summary
Agro ecology
|
Dega
|
High Dega
|
Over all Mean
|
Land use/ cover type
|
Natural F
|
Grazing land
|
Cultivated land
|
Mean
|
Natural F
|
Grazing land
|
Cultivated land
|
Mean
|
|
Natural F
|
Grazing land
|
Cultivated land
|
Soil Physical and chemical Properties
|
PH
|
7.0
|
5.8
|
5.3
|
6.03
|
6.8
|
5.7
|
4.9
|
5.80
|
6.93
|
5.71
|
5.12
|
OC%
|
6.7
|
3.1
|
1.4
|
3.71
|
9.3
|
7.2
|
3.3
|
6.60
|
8.00
|
5.16
|
2.31
|
CS t/ha
|
216.38
|
144.82
|
99.92
|
153.70
|
160.28
|
93.88
|
45.59
|
99.92
|
188.32
|
119.35
|
72.75
|
TN%
|
0.4
|
0.3
|
0.1
|
0.25
|
0.5
|
0.5
|
0.2
|
0.41
|
0.47
|
0.36
|
0.17
|
Av. P(ppm)
|
3.3
|
6.2
|
9.0
|
6.17
|
4.4
|
4.6
|
5.2
|
4.74
|
3.88
|
5.42
|
7.07
|
Clay %
|
24.4
|
43.2
|
59.2
|
42.23
|
26.2
|
23.4
|
39.4
|
29.67
|
25.30
|
33.30
|
49.30
|
Silt%
|
32.6
|
29.0
|
26.0
|
29.20
|
35.6
|
41.8
|
35.2
|
37.53
|
34.10
|
35.40
|
30.60
|
Sand %
|
43.0
|
27.8
|
14.8
|
28.53
|
38.2
|
34.8
|
25.4
|
32.80
|
40.60
|
31.30
|
20.10
|
BD (g/cm3)
|
0.9
|
1.0
|
1.1
|
1.02
|
0.9
|
0.7
|
1.1
|
0.88
|
0.89
|
0.86
|
1.10
|
Ca2+(mEq100g−1)
|
16.5
|
11.9
|
11.5
|
13.29
|
18.7
|
13.1
|
12.3
|
14.72
|
17.60
|
12.53
|
11.89
|
Mg2+(mEq100g−1)
|
12.1
|
2.4
|
1.8
|
5.46
|
11.0
|
2.4
|
2.4
|
5.29
|
11.58
|
2.42
|
2.12
|
Na+(mEq100g−1)
|
0.6
|
0.7
|
0.6
|
0.63
|
0.6
|
0.6
|
0.7
|
0.61
|
0.61
|
0.61
|
0.64
|
K+(mEq100g−1)
|
0.5
|
1.3
|
0.8
|
0.87
|
0.5
|
1.3
|
0.5
|
0.78
|
0.52
|
1.28
|
0.66
|
CEC(mEq100g−1)
|
29.7
|
16.3
|
14.7
|
20.25
|
30.9
|
17.4
|
15.9
|
21.38
|
30.30
|
16.84
|
15.31
|
Soil Texture
Variation in soil texture distribution was observed along land use/ cover types and agro ecological zones. Accordingly, the average sand fraction of soils of natural forestland was high (40.6%) whereas it was low on soils of cultivated and grazing lands (20.1% and 31.3% respectively Table 1). In the reverse clay fraction on the soils of cultivated land was > grazing land > natural forests. This finding was contradicting with the finding of Bewket & Stroosnijder (2003) and Kebede & Raju (2011) which find out highest clay fraction in the forested plots but lowest in both cultivated and grazing fields and vice- versa for sand content of the sample soils. This variation may come from the difference in the density of forest in the two study areas to prevent removal of clay fractions to the down soil profile and slope. The other likely factor for such distinctions could be the process of plowing, clearing and relative planeness of farming fields (Biro et al., 2013). According to Warra et al. (2015) the highest concentration of clay fraction on cultivated land may be attributed to ploughing accentuating weathering, making cultivated lands richer in finer materials. Moreover, the mean percentages of clay fraction in the soils of Dega agro ecological zone were greater than that of High Dega soils. Sand and silt contents were lower in soils of Dega than High Dega agro ecological zone.
As stated by Warra et al. (2015) and Agegnehu et al. (2019) on the downward surface elevation, increment of clay size fraction was associated with selective removal of finer and lighter materials from higher to lower elevation, as clay requires lower velocity to be transported than silt and sand particles. Thus the main effects of factors, land use/ covers and agro ecologies, were statistically significant for clay (P < 0.05, Table 2). The effects of ago ecology was significant (P < 0.01) on clay and silt, while the effect of land use/ cover was significant on clay & sand.
Table 2
Interaction effects of land use/ cover and Agro ecology on Major soil properties
Source
|
Dependent Variable
|
F
|
Sig.
|
|
F
|
Sig.
|
|
F
|
Sig.
|
Agro ecology
|
PH
|
2.335
|
.140
|
Land use/ Cover
|
51.037
|
.000
|
Agro ecology *
Land use/ Cover
|
.257
|
.776
|
OC
|
6.109
|
.021
|
|
7.916
|
.002
|
|
.305
|
.740
|
CS
|
4.341
|
.048
|
|
6.765
|
.005
|
|
.003
|
.997
|
TN
|
3.924
|
.059
|
|
5.046
|
.015
|
|
.164
|
.850
|
Av.P
|
.704
|
.410
|
|
1.169
|
.328
|
|
.704
|
.504
|
Clay
|
10.584
|
.003
|
|
13.274
|
.000
|
|
3.456
|
.048
|
Silt
|
15.944
|
.001
|
|
1.887
|
.173
|
|
1.881
|
.174
|
Sand
|
2.199
|
.151
|
|
16.967
|
.000
|
|
2.612
|
.094
|
BD
|
3.077
|
.092
|
|
3.440
|
.049
|
|
1.237
|
.308
|
Ca2+
|
6.800
|
.015
|
|
43.654
|
.000
|
|
.609
|
.552
|
Mg2+
|
.487
|
.492
|
|
601.481
|
.000
|
|
3.761
|
.038
|
Na+
|
.186
|
.670
|
|
.259
|
.774
|
|
.662
|
.525
|
K+
|
.900
|
.352
|
|
23.799
|
.000
|
|
.725
|
.495
|
CEC
|
3.414
|
.077
|
|
239.104
|
.000
|
|
.002
|
.998
|
Bulk Density (BD)
In both agro ecological zones, soil samples taken from the lands under cultivation have shown high BD due to high compaction compared to grazing and natural forest lands (Table 1). In the study watershed cultivated lands have high BD (mean = 1.096 g/cm3) than natural forest land (mean = 0.892 g/cm3) followed by grazing land (mean = 0.856 g/cm3). On the other hand there was variation in average BD among the soils in the two agro ecological zones (mean = 1.02 g/cm3 & 0.95 g/cm3 for the soils of Dega and High Dega respectivel). The MANOVA result in Table 2 revealed that the effect of land use/ cover was statistically significant for soil BD (F = 3.004, P = 0.049), whereas the effect of agro ecology was found insignificant. The LSD post hoc test suggested that there was variation in average BD of cultivated land was significantly different from natural forest land (P < 0.05) and grazing land (P = 0.02), Table 3. The increase in BD due to compaction in cultivated land was attributed to intensive cultivation (Reicosky & Forcella, 1998). This finding was in agreement with Mulugeta (2004) that confirmed forest and grass lands have a lower BD than farm land due to soil organic matter concentrations difference and cultivation activities. Selassie & Ayanna (2013) reported that progressive increase in BD was triggered by deforestation and continues cultivation in top plow layers that lead to decline in soil organic matter and compaction from the tillage. Thus the soil BD and SOM have inverse relationships, in turn can affect the aggregate stability of soil and the movement of water and nutrients through it (Kebede & Raju, 2011; Gardner et al., 1999).
Table 3
LSD post hoc Multiple Comparison test of Soil Properties among Land use/ covers
Dependent Variable
|
(I) Interaction
|
(J) Interaction
|
Mean Difference (I-J)
|
Sig.
|
pH
|
Forest
|
Grazing
|
1.220*
|
.000
|
|
Cultivated
|
1.810*
|
.000
|
Grazing
|
Cultivated
|
.590*
|
.004
|
OC
|
Forest
|
Grazing
|
2.846
|
.058
|
Cultivated
|
5.694*
|
.001
|
Cultivated
|
Grazing
|
-2.848
|
.058
|
CS
|
Forest
|
Grazing
|
68.978541*
|
.039
|
|
|
Cultivated
|
115.575231*
|
.001
|
|
Grazing
|
Cultivated
|
46.596690
|
.154
|
TN
|
Forest
|
Grazing
|
.112
|
.257
|
Cultivated
|
.303*
|
.004
|
Cultivated
|
Grazing
|
− .191
|
.059
|
Av. P
|
Forest
|
Grazing
|
-1.549
|
.466
|
Cultivated
|
-3.198
|
.139
|
Grazing
|
Cultivated
|
-1.649
|
.438
|
BD
|
Forest
|
Grazing
|
.036
|
.718
|
Cultivated
|
− .204*
|
.050
|
Grazing
|
Cultivated
|
− .240*
|
.023
|
Ca2+(mEq100g-1)
|
Forest
|
Grazing
|
5.0620*
|
.000
|
|
|
Cultivated
|
5.7050*
|
.000
|
|
Grazing
|
Cultivated
|
.6430
|
.346
|
Mg2+(mEq100g-1)
|
Forest
|
Grazing
|
9.1620*
|
.000
|
|
|
Cultivated
|
9.4590*
|
.000
|
|
Cultivated
|
Grazing
|
− .2970
|
.348
|
Na+(mEq100g-1)
|
Forest
|
Grazing
|
− .0070
|
.896
|
|
|
Cultivated
|
− .0360
|
.504
|
|
Grazing
|
Cultivated
|
− .0290
|
.590
|
K+(mEq100g-1)
|
Forest
|
Grazing
|
− .7580*
|
.000
|
|
|
Cultivated
|
− .1380
|
.250
|
|
Cultivated
|
Grazing
|
− .6200*
|
.000
|
CEC (mEq100g-1)
|
Forest
|
Grazing
|
13.4590*
|
.000
|
|
|
Cultivated
|
14.9900*
|
.000
|
|
Grazing
|
Cultivated
|
1.5310
|
.054
|
Based on observed means. *.The mean difference is significant at the .05 level
|
Soil pH
Acidity of soils (pH values) varies significantly among land use/ cover classes in Rib watershed. As shown in Table 2, the main effect of land use/ cover was statistically significant for soil pH (F = 51.037, P = .000). The LSD post hoc test of MANOVA shows that mean soil pH was significantly different at p < 0.01 (Table 3) between cultivated and natural forest & grazing lands. Soil pH was slightly higher for soils of natural forestlands (mean = 6.93) as compared to cultivated (mean = 5.71) and grazing lands (mean = 5.12), Table 1. This indicated that soils of natural forests were slightly neutral in both Dega and High Dega agro ecological zones (7.0 and 6.8 respectively). Thus, soils in cultivated and grazing lands were more acidic than those of natural forestland soils (Agegnehu et al., 2019). On the other hand despite the variation in pH level of soils of the two agro ecologies, soils of the study watershed can be generally characterized as moderately acidic, pH ranging from 5.6 to 6.5 (Agegnehu et al., 2019). The reduction in soil pH is attributed to the ploughing processes of cultivated fields (Biro, 2013). The conversion of forestland into cultivated land has lead to a drop in organic matter which in turn leads to lower pH (Khresat et al., 2008). These findings were in line with the study by Biro et al. (2013) in Northern part of Gdarif region of Sudan. Another study by Kidanemariam et al. (2012) revealed that lower pH values of cultivated and grazing land soils can be attributed to the removal of basic cations by plants, which causes continuous cultivation with little nutrient returns to the soil, erosion and overgrazing on grazing lands. Another reason for the increase of soil acidity on cultivated lands was intensive farming over a number of years with nitrogen fertilizers (Abate et al., 2013). The finding of this study was in agreement with other studies that find out soil acidity issues is becoming critical in Northwestern highlands of Ethiopia (Genanew et al., 2012; Haile et al., 2009 and Melese et al.,2016). These studies confirmed that acidic soils have poor chemical and biological properties and can affect crop production and productivity of the land.
Soil organic Carbon (OC)
Table 1 has shown mean difference of OC content among the three land use/ covers and agro ecology. The average OC was 8.00%, 5.16% and 2.31% for soils of natural forest, grazing and cultivated lands respectively. Forest soils are one of the major carbon sinks on earth, because of the high amount of organic matter stored in forest soils (Tesfaye et al., 2018). Above all, forest soils in the first 1 m depth have held 11% of soil carbon worldwide (Negi et al., 2013). Moreover, the overall mean of OC was 3.71% in the soil of Dega and 6.60% in the soils of High Dega agro ecology. This finding was in line with the findings of Kidanemariam et al. (2012) and Warra et al. (2015) who confirmed that OC showed increasing trend with elevation in all land use/ cover types. The LSD post hoc test in MANOVA (Table 3) suggested that OC was significantly differed between natural forest, grazing lands and cultivated lands. From this finding it was possible to conclude that land use/ cover changes can affect OC concentration of soils under different use and cover (Warra et al., 2015). Continuous cultivation, removal of crop residuals, lack of crop rotation, inadequate agricultural fertilizers use and poor soil management practices were among the major factors that lead to OC deterioration in the soils (Kidanemariam et al., 2012).
Soil Organic Carbon Stock (CS)
Soil carbon stock of the three land use/ covers was calculated based on OC content and soil BD. Variation was found in the distribution of CS among land use/ covers and agro- ecological zones owing to the existing difference in OC and BD. Thereupon, the soils of natural forests in both Dega and High Dega agro- ecological zones retained higher mean carbon stock followed by grazing and cultivated lands (Table 1) (Sebhatleab, 2014). Overall mean of natural forests’ CS was more than two fold higher than cultivated lands. It was confirmed that forests sequester and stores more carbon than any terrestrial ecosystem and act as sources as well as sink of CO2 (Jandl et al., 2007; Tesfaye et al., 2018)
The interaction among the three land use/ cover classes illustrated that mean CS of natural forest was significantly deferent from cultivated and grazing lands (p = 0.04, & P = 0.001 respectively). But there was no significant mean difference between soils of grazing and cultivated lands (Table 3). This finding was consistent with the study by Solomon et al. (2018) which revealed that CS was highest in forestlands and lowest in cultivated lands. The study conducted in Chilimo, a dry Afromontana forest in Ethiopia, found out higher mean carbon stock in natural forest than any other land cover types (Tesfaye et al., 2018).
The higher soil organic carbon stock recorded in the dense forest was mainly because of the biomass inputs and low rate of litter decay. In contrast the report by Guo and Gifford (59) indicated that faster decomposition of grass roots and contribution of higher organic matter to soil exhibited higher soil organic carbon stock. This may be true in areas with conservation intervention (closure) in which free grazing was minimized (Terefe, 2020). However, in most grazing areas including Rib watershed the conversion of vegetation covers into grazing and cultivated lands had affected soil chemical properties; for instance increasing acidification and compactions in turn retarding vegetation growth and soil organic carbon accumulation.
The study of Girmay et al., (2012) also reported that alteration of dense forests to cultivated lands brought about 25% reduction in soil organic carbon. Because, forests play a vital role in the natural global carbon cycle by capturing atmospheric carbon through photosynthesis and convert it into forest biomass (Tesfaye et al., 2018).
Table 3 LSD post hoc Multiple Comparison test of Soil Properties among Land use/ covers
Total Nitrogen (TN)
As the effects of land use/ cover changes, the lowest mean value of TN concentration was observed on the soils of cultivated (mean = 0.17%) compared to soils of natural forest (mean = 0.47%) and grazing lands (mean = 0.36%), Table 1. And also the distribution of TN showed variation among the study agro ecologies. Hence the average TN was 0.25% and 0.41% over the Dega and High Dega agro ecological zones respectively. This finding was in agreement with the study by Warra et al. (2015) in Kasso catchment of Bale Mountains. The interaction effects of factors test (Table 3) indicated that land use/ cover and agro ecology have statistically significant effect on TN (F = 5.046, P = 0.015 and F = 3.924, P = 0.059 respectively). As shown by the LSD multiple comparisons post hoc test of MANOVA (Table 3), the difference in TN concentration was statistically significant between the soils of natural forest & cultivated lands and between grazing & cultivated lands. Mulugeta (2004) stated that continuous cultivation and poor management practices coupled with rapid mineralization of organic substances and insufficient organic input application could result in lower TN in cultivated land soils.
Available Phosphorous (Av.P)
The overall mean of Av.P were 1.07 ppm, 5.42 ppm and 3.88 ppm for cultivated, grazing and natural forest lands respectively. In similar findings higher Av.P content was also observed on cultivated fields than forests (Lisanework & Michelsen, 1994; Bewket & Stroosnijder, 2003; Kebede & Raju, 2011). These studies suggested that tree in the forests extract more phosphorous than field crops. Furthermore, Lisanework & Michelsen, (1994) reported that the higher Av.P concentration of cultivated field than forest was that, a high proportion of Av.P pool is retained and immobilized by microbes in the litter layers of forests. Among the soils of different land use/ covers of the two agro ecological zones, cultivated land of the Dega agro ecology was with the highest (mean = 9.0 ppm) of Av.P in the watershed (Table 1). Despite of the variations in the mean value of Av.P of the three land use/ covers and among agro ecological zones, the interaction of all these factors were statistically insignificant (P > 0.05, Table 2).
Exchangeable Cations and CEC
Variation in land use/ cover had greater impacts on exchangeable Mg2+, Ca2+, K+ and CEC than agro ecology (Table 2). The mean exchangeable cations capacity (CEC) of the soils in Rib watershed were 30.30, 16.84 and 15.31 mEq 100 g− 1 for natural forest, gazing and cultivated lands respectively (Table 1). The finding of this study was in agreement with Adugna & Abegaz, 2016) who identified the highest mean value of CEC in the soils of forestlands and lowest in cultivated lands in Northern Wollega. The lowest CEC content of cultivated land was thought to be resulted from less soil organic matter concentration, continuous cultivation, and removal of crop residue coupled with sever soil erosion (Sebhatleab, 2014 and Bezabih et al., 2016). As shown by multivariate test, all exchangeable cations (Mg2+, Ca2+, and K+) of the soils were significantly (P = 0.00) affected by land use/ cover types (Table 2). Thus LSD post hoc test showed that significant difference (P = 0.00)in Ca2+ and Mg+ contents between the soils under natural forest & grazing land; between cultivated and natural forestlands (Table, 3). Significant mean difference (P = 0,00) was also observed in K+ between natural forest & grazing, between cultivated & grazing lands. On the other hand the Na+ of the soil did not differ significantly (P > 0.05, Table 2) among land use/ covers and agro ecological zones. This suggests that absence of any effects that can be linked to land use/ cover changes on Na+ in the watershed. The Pearson’s correlation coefficient matrix confirmed that OC and CEC have positive and strong correlation (Correlation coefficient = 0.624, P < 0.001; Table 4).
Table 4
Pearson’s correlation matrix of soil properties
|
PH
|
OC
|
CS
|
TN
|
Av.P
|
Clay
|
Silt
|
Sand
|
BD
|
Ca2+
|
Mg2+
|
Na+
|
K+
|
CEC
|
PH
|
1
|
|
|
|
|
|
|
|
|
|
|
|
|
|
OC
|
.506**
|
1
|
|
|
|
|
|
|
|
|
|
|
|
|
CS
|
.459*
|
.868*
|
1
|
|
|
|
|
|
|
|
|
|
|
|
TN
|
.422*
|
.902**
|
.639*
|
1
|
|
|
|
|
|
|
|
|
|
|
Av.P
|
− .171
|
− .039
|
.000
|
.001
|
1
|
|
|
|
|
|
|
|
|
|
Clay
|
− .480**
|
− .779**
|
-579**
|
− .780**
|
.047
|
1
|
|
|
|
|
|
|
|
|
Silt
|
.134
|
.604**
|
.454*
|
.585**
|
.158
|
− .747**
|
1
|
|
|
|
|
|
|
|
Sand
|
.576**
|
.697**
|
.514**
|
.710**
|
− .162
|
− .912**
|
.408*
|
1
|
|
|
|
|
|
|
BD
|
− .301
|
− .676**
|
-301
|
− .783**
|
.155
|
.679**
|
− .512**
|
− .616**
|
1
|
|
|
|
|
|
Ca2+
|
.752**
|
.626**
|
.620**
|
.504**
|
− .196
|
− .569**
|
.329
|
.578**
|
− .220
|
1
|
|
|
|
|
Mg2+
|
.841**
|
.518**
|
.534**
|
.425*
|
− .219
|
− .514**
|
.095
|
.647**
|
− .193
|
.829**
|
1
|
|
|
|
Na+
|
− .100
|
− .004
|
.012
|
− .005
|
.017
|
.126
|
− .078
|
− .124
|
− .104
|
− .208
|
− .097
|
1
|
|
|
K+
|
− .273
|
− .200
|
-192
|
− .099
|
.074
|
.138
|
.098
|
− .251
|
.007
|
− .340
|
− .500**
|
− .078
|
1
|
|
CEC
|
.846**
|
.591**
|
.599**
|
.484**
|
− .220
|
− .565**
|
.207
|
.648**
|
− .220
|
.939**
|
.968**
|
− .141
|
− .410*
|
1
|
**.Correlation is significant at the 0.01 level (2-tailed), *Correlation is significant at the 0.05 level (2-tailed) |
Relationships between Selected Soil Properties
According to the Pearson’s correlation coefficient, BD, OC, TN, silt and sand contents were negatively and significantly correlated with one another at P < 0.01. In contrast BD had positive and strong correlation with clay fraction of the soil (P < 0.01, Table 4). The statistical analysis of the study indicated that there was significant correlation between Na+ and any of other soil chemical properties in this study. In Rib watershed soil pH was positively and strongly associated (P < 0.01) with sand fraction, OC, Mg2+, Ca2+ and CEC whereas had weak correlation with TN concentration.
This finding suggested that the soil with high basic cations are less acidic and vice- versa. According to Kisinyo et al. (2014) soil acidity is associated with deficiencies of Av. P, OC, Ca2+, Mg2+ and K+ in the soils. Under acidic soil conditions, there has been a gradual depletion of soil bases (Kidanemariam et al., 2013). Relating to soil pH decline (pH < 5.5) there is probability of high concentration of aluminum, manganese and deficiency of Av. P, total nitrogen, sulfur, and other nutrient to retard crop growth. The overall findings of this study imply that soils with high Mg+ and CEC were less acidic than soils having low contents of the aforementioned cations (Tables 1 and 4). This finding was in harmony with other studies carried out in different parts of Ethiopia (Adugna & Abegaz, 2016; Amare et al., 2013; Asmamaw & Mohammed, 2013). In contrast soil pH was negatively and strongly correlated with clay fraction (P < 0.01). In other words as clay fraction of the soil increased, the acidification are likely to rise. Moreover, K+ has no significant correlation with all other properties except its strong & negative association with Mg2+ (Table 4). From this finding it might be possible to suggest that K+ was available at minimum amount and insignificantly associated with all soil properties except CEC. Similarly there was no significant correlation between Na+ and other properties of the sampled soils in this study.