Effect of land-use systems on soil chemical properties
Soil Organic C, Total N, C: N ratio and Available P
Organic matter has an important influence on soil chemical properties, soil fertility status, plant nutrition, and biological activity in the soil (Brady and Weil, 2002). Among the different land-use systems, the organic carbon concentration levels were observed in the order of natural forest > G. robusta > grassland > P. patula > C. lusitanica > grazing land > E. globulus > cropland. The overall maximum mean value of the organic carbon concentration was observed beneath the natural forest (3.49% DM), and Grevillea robusta plantation forest (3.14% DM) statistical without significant differences between them, but significant difference from the rest six land-use systems. This probably due to the higher accumulation of soil organic matters (SOM) by adding litters to soils from different heterogeneity plant species with the high rate of biomass production, and better carbon nutrient release or mineralize to the soils through decomposition. Similar results have been reported by Michelsen et. al. (1996), Lemeniha et. al. (2004), Lemma et. al. (2006) that the higher soil organic carbon concentrations observed in the natural forest than different exotic tree plantation forests in different regions of Ethiopia.
Among the land-use systems in the study area, the minimum value of organic carbon concentration was observed in cropland (1.47% DM) and followed by Eucalyptus globulus plantation forest (2.03% DM). The difference could be attributed due to the addition of lower organic matter to the soil via litter inputs and fractions of litter types by losses of organic matter through harvesting of woody and crops biomass from sites, continuous cultivation that aggravates organic matter oxidation lead to losing carbon from the soil in CO2 form. The results were in agreement with the findings of Negassa (2001), Dhaliwal and Singh (2003), Lemeniha et. al. (2004), Duguma et.al. (2010), Sebhatleab (2014), Mengiste et. al. (2015), and Tellen et.al. (2018) who’s reported less organic carbon in the cultivated land and Eucalyptus plantation forest soils than others land-use systems.
The total nitrogen concentration levels distribution trend horizontal as a function of land-use systems were registered in the order of natural forest > G. robusta > grassland > P. patula > C. lusitanica > grazing land > E. globulus > cropland. The highest mean value of total nitrogen content was observed in natural forest (0.31% DM) and followed by the Grevillea robusta plantation forest (0.26% DM), statistically with significant differences between them. This may be attributed to the long–term accumulation of above and below-ground organic matter inputs from litterfall, root turn over mineralization by actions of soil microbes, and N fixation by symbiotic in leguminous plant species diversity in natural forest and other soil microorganisms. This argument is supported by the significant strong positive correlation (r = 0.947) between the total nitrogen and organic carbon. Similar studies were also reported as the higher total nitrogen content was observed in the natural forest than other land-use systems in different areas (Michelsen et. al. 1996, Lemeniha et. al. 2004, Lemma et. al. 2006, Sebhatleab 2014, Tellen et.al. 2018). On the contrary, the lowest mean value of total nitrogen content was observed in cropland (0.130% DM) and followed by Eucalyptus globulus plantation forest (0.173% DM), statistically without significant differences between them. The reasons for the lower total nitrogen contents may be attributed due to the loss of nitrogen either to the atmosphere by evaporation or leached down through the soil by water. Because there is no real store of available nitrates in the soil as nitrates are released from organic matter breakdown or fertilizers unless they are used by plants. Similar studies reported that lower total nitrogen concentrations were observed under agricultural cropland, and Eucalyptus plantation forest than the other land-use systems in different regions of Ethiopia (Michelsen et. al. 1996, Lemeniha et. al. 2004, Duguma et.al. 2010).
The overall mean value of the C: N ratio in all land-use systems were statistically not significant differences between them. The C/N ratio was significantly narrowed from 11.38 in the natural forest soils to 12.08 in the Pinus patula plantation forest soils. The lowest value of the C: N ratio was observed in the natural forest. This may be attributed by the factor of increase nitrogen nutrient inputs with a high rate of nitrogen fixation by different leguminous plant species diversity having a relatively higher protein and nutrient contents in the natural forest that leads to the lower C: N ratio, due to their fast decomposition rate and release of nitrogen to the soil. Faster decomposition of leaf litter enhances the transfer of fresh carbon to mineral soil (Polglase et al., 2000). Similarly, different workers have been reported the lower C: N ratio under different native plant species diversity (Yadessa, 1997; Hailu 2000, and Yadesa, et.al. 2010). The highest value of the C: N ratio (12.08) was observed in the Pinus patula plantation forest. This may be attributed due to lower nitrogen content in the litters of organic matters which results in a higher C: N ratio, and slow decomposition rate. Plant residue with a low C/N ratio (high nitrogen content) decompose more quickly than plant residue with a high C/N ratio (high carbon content) and do not increase soil organic matter accumulation levels as quickly (Janssen 1996, Bengtsson et al. 2003; Springob and Kirchmann 2003).
Phosphorus (P) is an essential element classified as a macronutrient because of the relatively large amounts of P required by plants. In the study area, the concentration of available phosphorus levels was found in the following order: natural forest > P. patula > grassland > grazing land > cropland > G. robusta > E. globulus > C. lusitanica land-use systems. The available phosphorus concentration mean values ranged between 2.45 and 31.52 mg/kg of soil. The overall available phosphorus concentration in natural forest (31.52 mg/kg of soil) was found to be higher than other land-use systems. This might be attributed due to a combination of low nutrient demand by natural forest woody plants as compared with different exotic trees species plantation forests, and better phosphorus nutrient release or mineralize to the soils by different heterogeneity plant species diversity during organic matters decomposition. This argument is supported by the analyzed simple linear correlation relationships between available phosphorus was positively correlated with the soil organic carbon (r = 0.547). This result is in agreement with the findings of Michelsen et. al. (1996), Nsabimana, et.al. (2008), Sebhatleab (2014) whose observe the higher concentration of available phosphorus in the natural forest than others land-use systems. The lower mean values of available phosphorus concentration were recorded in Cupressus lusitanica (2.45), and Eucalyptus globulus(3.12) plantation forests statistical without significant differences between them. This may be attributed due to fast-growing tree plantations species were associated with a more intense uptake of nutrients from the soil than slow-growing forest species, and the loss of organic matters by rotational harvesting and took away from the site. Michelsen et al. (1993); and Lisanework & Michelsen (1994) were noted that phosphorus (P) concentration limiting plant growth and leaf litter decomposition under C. lusitanica and E. globulus plantations in Ethiopia.
Soil pH, EC (ds/m), and CEC
The present study results showed that soil pH significantly varied with land-use systems. The overall mean value of soil pH level distribution in different land-use systems was observed in the order of grassland > natural forest > grazing land > P. patula > G. robusta > E. globulus > C. lusitanica > cropland. Among the land-use systems, the maximum mean value of pH (6.49) was observed in grassland and followed by natural forest (6.20) without significant difference between them. This might be attributable to the higher litter deposition from the aboveground and below ground which gone under decomposition by the action of micro-organisms and subsequent mineralize to releases basic cations to the soil in both land-use systems. This argument is agrees with the simple positive correlation relationships between soil pH with soil organic carbon (r = 0.432), cation exchange capacity (CEC) (r = 0.649), exchangeable cations of Ca(r = 0.592), Mg(r = 0.261), and K(r = 0.325) in the soils. Similar studies results were reported by Michelsen et al. (1993), Michelsen et. al. (1996), and Nsabimana et al., (2008), for soils of higher pH level under natural forest than others land-use systems. Similarly, the higher soil pH under grassland than other land-use systems was reported by Kaur and Toor (2012).
The lower mean values of soil pH were recorded in cropland, Cupressus lusitanica plantation, and Eucalyptus globulus plantation forests in the orders of 5.38, 5.41, and 5.48, respectively, statistical without significant differences between them. The main reasons for the lowest value of soil pH in the cropland probably due to poorly managed cultivation; use of chemical fertilizers including urea, DAP, and potash, inappropriate use of ammonium-based fertilizers, intensive use of herbicides such as roundup which contain high amounts of cations that helps to neutralize the negative charges i.e. a higher concentration of H+ (lower pH) will neutralize the negative charge in soils, and soil erosion. Balesdent et al (2000) and Tejada and Gonzalez (2009) have found that cultivation on farmlands led to the soil acidity increase. Other researchers have also observed the lower soil pH in cultivated farmland than other land-use systems (Emiru and Gebrekidan 2013, Tellen, et.al. 2018). Similarly, the decrease in soil pH under the Cupressus lusitanica and Eucalyptus globulus plantation forests could be due to the fast-growing exotic tree plantation forests were acidify the soil in nature by accumulating basic cations in the forest biomass, increasing production of organic acids from decomposing litter and by increasing cation leaching. The fast-growing exotic tree species forest that consumes high water for biomass production may increase solute concentrations and the mineralization of organic sulfur in the soil which leads to decrease soil pH. Similar studies were also reported that a decline in soil pH under the fast-growing exotic trees plantation forests (Jobbágy and Jackson, 2003; Mishra et al., 2003, Sanchez et al., 2003, Nsabimana et al. 2008, Ashagire et. al. 2008, and Tellen, et.al., 2018).
In the present finding, the mean values of electrical conductivity (EC) of soil were found numerical between 1.78 and 3.47 deciSiemen per meter (dS/m) ranges. According to the current finding the mean value of electrical conductivity (EC) of soil levels across land-use systems was found in the order of E. globulus,> cropland > G. robusta > P. patula > C. lusitanica > grassland > grazing land > natural forest. The maximum mean value of electrical conductivity (EC) was observed in the Eucalyptus globulus plantation forest (3.47) and followed by cropland (3.07) without significant differences between them. For the higher value of EC under Eucalyptus globulus plantation forest may be attributed due to the lower soil moisture content that related with soluble salts accumulate in the upper soils rather than leached down, high evaporation rate due to open canopy, and low infiltration rate in combine resulting to dissolved salts are left behind to accumulate in the upper soils layer, salts originate from the disintegration (weathering) of minerals and rocks, soils with an accumulation of exchangeable sodium are often characterized by poor tilth and low permeability making high EC. This argument is supported by the positive correlation between the electrical conductivity (EC) and exchangeable sodium, Na (r = 0.202), and negatively correlated with organic carbon (r = -0.466). This result is in agreement with the findings of Michelsen et al. (1996). However, for the high level of EC in cropland probably due to tillage intensity and land management practice, cropping system and nature, salt accumulation from commercial fertilizers, chemical contamination (from herbicide, insecticide, and fungicide use by farmers), erosion, runoff, animal manures (usually high tunnels), and compost were contributed to raising EC. Similar studies were reported that higher values of EC in cropland soils than others of land-use systems (Dhaliwal and Singh 2003, Gol 2009, Kaur and Toor, 2012).
The minimum mean value of electrical conductivity was observed in the natural forest (1.783) than the other land-use systems. The lower level of EC under natural forest probably attributed due to the higher accumulation of organic matters (litter deposition) that decomposed and release higher exchangeable cations (K, Ca, Mg) to the soils, which lead to reducing the salinity level and lowering the values of electric conductivity in the soils. This explanation is supported with the negative correlation between electrical conductivity and exchangeable cations K(r= -0.400), Ca(r = -0.532), Mg(r = -0.173), and organic carbon (r = -0.466). This finding is in agreement with a similar study report by Michelsen et al. (1996), and Gol (2009) who had reported that the lower mean value of electrical conductivity under natural forest than other land-use systems.
The mean values of CEC in the study area were found between 18.98 and 33.63 meq/100 gm of soil ranges. The overall mean values of soil CEC level distribution in different land-use systems were recorded in the order of natural forest > P. patula > grassland > grazing land > E. globulus > G. robusta > C. lusitanica > cropland. Among the land-use systems the highest concentration of cation exchange capacity (CEC) was registered in natural forest (33.63). This is probably influenced by the high amount of organic matter accumulation, and high soil pH in the natural forest soils that lead to higher CEC. This means the CEC of soils is affected mainly by the amount and degree of decomposition of the organic matter. In general, the higher soil organic matter (SOM) is resulting the higher the CEC. Because most of the CEC is originates from the amount of SOM decomposition rate that entirely pH-dependent. This argument is supported by a significant positive correlation between the soils CEC and OC (r = 0. 644), and pH (r = 0.649). The CEC is positively correlated with pH; therefore, acid soils have a lower potential of CEC. The present result is an agreement with the findings of Michelsen et al. (1996), Dhaliwal and Singh (2003), Lemeniha et al. (2004), Nsabimana et al. 2008, and Tellen and Yerima (2018) were observed the highest cation exchange capacity (CEC) values in the natural forest than others of land-use systems in different areas.
The lowest concentration value of cation exchange capacity (CEC) was observed in agricultural cropland (18.98) and followed by the Cupressus lusitanica plantation forest (21.26), Grevillea robusta plantation forest (23.24), and Eucalyptus globulus plantation forest (25.78). The occurrence variation may be attributed due to the low additions of organic matters or litters deposition in the agricultural cropland which results in low libration of exchange cation nutrients (Ca, Mg, Na, K) to the soil by decomposition. Similar to the present findings, other researchers were reported the lower values of CEC in mechanized farming (MF) by Lemeniha et al. (2004), Selassie et al. (2015), and Molla and Yalew (2018).
Soil Exchangeable bases (Na, K, Ca, Mg), and Base saturation percent
The main ions associated with CEC in soils are the exchangeable cations calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+) are generally referred to as the base cations (Rayment and Higginson 1992). As the function of land-use systems, the concentration of exchangeable cations was generally in the order of Ca > Mg > K > Na in all different land-use systems. These agree with the principle stated as the energy of the adsorption sequence of: Ca > Mg > K > Na. The highest concentration of exchangeable cations of Ca, Mg, and K, were observed in the natural forest in the order of 17.13, 5.37, and 3.60 cmol(+)/kg soil, respectively. These probably attributed due to the higher accumulation of soil organic matters (SOM) by adding of woody plant litters and understory herbaceous plant residues to soils from different heterogeneity plant species with the high rate of biomass production and those undergone decompositions, thereby, liberate cations nutrients of Ca, Mg, and K to the soils. These results are in agreement with the findings of Michelsen et.al. (1993), and Tellen, et.al. (2018). Similarly, Nsabimana et al. (2008) were observed the higher concentration of exchangeable cations of Ca, and Mg under mixed native species (MNS) forest than other exotic tree species plantation forests in southern Rwanda.
Additional reasons for the higher concentration of exchangeable cations of Ca, Mg, and K, in the natural forest, maybe due to reduced losses of cations nutrients from the soil by leaching out withholding positively charged ions (cations) by electrostatic force, reduced runoff, and soil erosion via upper surface cover approach with litters, and low cations nutrients demand by natural forest trees species might contribute to the higher of cations nutrients under natural forest. These explanations are supported by the positive correlation between the organic carbon and exchangeable cations in the soils K (r = 0.576), Ca (r = 0.677) and Mg (r = 0.426). The present findings were agreed with Molla and Yalew (2018) and Kokeb et. al. (2015). In general, a similar observation was reported by Michelsen et. al. (1996) concerned with the higher cations nutrients under natural forest than other exotic tree species plantation forests in the highland of Ethiopia.
The highest concentration of Na was observed in the Eucalyptus globulus plantation forest (0.60 cmol(+)/kg soil) than the other land-use systems. The overall mean values of the distribution of exchangeable Na in the area were found in the orders of Eucalyptus globulus > natural forest > Grevillea robusta > grazing land > Pinus patula > grassland > Cupressus lusitanica > cropland. For the highest concentration of the exchangeable Na + in Eucalyptus globulus plantation forest may be due to the effects of high soil compaction that result with high bulk density, and lower soil moisture content could be facilitated to the soluble salts accumulate in the upper soils rather than leached down. Additionally, the accumulation of exchangeable sodium salts is probably originated from the disintegration (weathering) of minerals and rocks. Similar observations were reported by Michelsen et. al. (1993), Michelsen et. al. (1996) that the higher exchangeable sodium under Cupressus lusitanica, Eucalyptus globules, Eucalyptus grandis, and Eucalyptus saligna plantation forests in the highland of Ethiopia.
The lowest concentration of exchangeable cations of Na, K, and Ca, were recorded in cropland in the order of 0.21, 1.20, and 9.93 cmol(+)/kg soil, respectively. However, the minimum mean value of exchangeable Mg was recorded in the Cupressus lusitanica plantation forest (2.91 cmol(+)/kg soil). This could be due to the low addition of organic matters from external factors and the removal of crop residuals from the cropland by harvesting maybe contribute to lower addition of the cations nutrients of Na, K, and Ca to soils. Another explanation for the lowest concentration of exchangeable Na, K, and Ca in cropland probably due to continuous intensive tillage and cropping systems facilitate to lower bulk density, lower CEC, higher porosity, and higher infiltration rate in soils could lead simply these cations nutrients of Na, K, and Ca were leached out down to the soil depth by water. Similarly, Duguma et. al. (2010), and Molla and Yalew (2018) were observed that the highest concentration of exchangeable Ca in cereal farmland than other land-use systems.
Among the land-use systems, the highest mean value of percent base saturation (80.83) was observed in the Cupressus lusitanica plantation forest. This probably due to the amount and nature of clay particles contents and low concentration of CEC, and low pH level in soils could be contributed to the existence of higher percent base saturation (PBS) under Cupressus lusitanica plantation forest than others land-use systems. Because of the amount and type of clay minerals are responsible factors for CEC in that both clay and colloidal organic matters (COM). This argument is supported by a significant negative correlation between the soils PBS and CEC (r = -0.300) and between PBS and pH (r = -0.050). Similarly, Kebede and Charles (2009) were suggested that clay and colloidal organic matters are negatively charged and can act as anions; as a result, these two materials can absorb and hold positively charged ions (cations). These findings are in agreement with a similar study report by Nsabimana, et.al. (2008), who had reported the higher percent base saturation (93.7%) under Cupressus lusitanica plantation forest among others plantation forests in southern Rwanda.
The lowest mean value of percent base saturation (70.91) was recorded in the Eucalyptus globulus plantation forest. This probably due to the lower addition of organic matters that undergone decomposition and liberate low cations nutrients of Na, K, Ca, and Mg to soils that lead to lower percent base saturation under Eucalyptus globulus plantation forest than the others land-use systems. This explanation is supported by the positive correlation between percent base saturation and organic carbon (r = 0.262), and a negative correlation between PBS and pH (r = − 0.050). The percent base saturation (%BS) levels were distributed in similar values without significant difference among the LUS of Grevillea robusta plantation, cropland, Pinus patula plantation, grazing land, grassland in the order of 79.78, 77.80, 76.35, 74.53, and 73.32, respectively. The variation of the percent base saturation (PBS) means values among different land-use systems may be due to the variation of amount, and nature of organic matters addition, and degree of decomposition to release cation nutrients to soils. Similar observations were reported by Lemeniha et. al. (2004), Nsabimana, et.al. (2008), and Duguma et.al., (2010), who reported different percent base saturation (%BS) levels under different land-use systems.
Effect of soil depth on soil chemical properties
Concerning to vertical gradient of soil depth the higher mean values of organic carbon (2.89%DM), total nitrogen (0.25%DM), available phosphorus (16.17 mg/kg of soil) concentration were recorded in the upper part of the soil layer at 0–20 cm depth than in the lower subsoil layer at 20–40 cm depth. The organic carbon, total nitrogen, and available phosphorus concentration levels distribution trend across soil depths were decreased gradually from the topsoil layer to the subsoil layer in all land-use systems. This explanation may be due to the higher organic matter inputs to the topsoil layer from plant litters, crop residues, commercial fertilizers, and animal waste materials than the subsoil layer. This is in agreement with previous studies (Michelsen et. al. 1993, 1996, Yadessa and Itanna 2001; Hailu 2000; Lemeniha et. al., 2004, Yadesa et.al., 2010, Kaur and Toor, 2012, Sugihara 2014).
The study results indicated that the lower overall mean values of the C: N ratio (11.64), pH (5.80), and electrical conductivity (EC) (2.22) were observed at topsoil layers than that of the subsoil layers. For the lower value of C: N ratio levels at topsoil is probably due to the highly decomposed organic matter releases higher N- levels on the topsoil than the subsoil layers, thereby, the lowering C: N ratio occurred at the topsoil layer. Similar to this finding, Yadessa and Itanna (2001)and Hailu (2000) also found a lower C/N ratio at the topsoil than in the subsoil layers under different native woody plant species diversity on farmland. The lower values of EC at topsoil are probably due to the addition and accumulation of organic matters at the topsoil surface than subsoil surface that liberates exchangeable cations, thereby, reducing soil EC at the topsoil layer. On the other hand, the pH levels distribution trend in soil was increased gradually from topsoil to subsoil layers in Eucalyptus globulus plantation, Cupressus lusitanica plantation, Pinus patula plantation forests, grazing land, and cropland of land-use systems in the study area. The result of the higher value of soil pH at subsoil than topsoil layers probably due to the leaching of more soluble soil minerals and basic cations from topsoil to the subsoil layer. Similarly, Michelsen et al. (1996) and Zewdie & Olsson, (2008) was reported that the increment of pH values from topsoil to subsoil layers in Eucalyptus globulus and, Pinus patula plantation forest land-use systems in the Ethiopian Highlands.
With regarding a vertical gradient of soil depth the overall higher mean value of exchangeable cations of Na, K, Ca, Mg, and percent base saturation (%BS) concentrations were observed at the topsoil layers than the subsoil layer with significance difference. The distribution of exchangeable cations of Na, K, Ca, Mg, and percent base saturation (%BS) concentration were decreased vertical from the topsoil layer to the subsoil layer in all land-use systems. These probably due to the effect of higher organic matters depositions or accumulation from litters of woody, herbaceous residuals, animal manures, and crops residuals on farmlands that undergone decomposition and mineralized cations nutrients of Na, K, Ca, and Mg to soils then CEC play the roles to retain the released cations at topsoil from the decomposed organic matter rather than translocating them to the subsoil layer. Cations, such as K+, Na+, and Ca2+, can be adsorbed onto soil or organic colloids, making the cations available for plant uptake by preventing cation leaching from the system (Brady and Weil 2007). Similar studies results have been reported by Michelsen et. al., (1996), Lemeniha et al. (2004), Zewdie and Olsson (2008) that the majority of exchangeable cations nutrient concentrations were declined as soil depth increase, except exchangeable Na in some land-use systems in different areas.