The change of agricultural land to other land use types may lead to storage of SOC depending on the type of vegetation. Most agricultural soils have the ecological potential to sequester SOC above their existing levels  . This is because most soils in conventional agricultural ecosystems have lost SOC (also nutrients along with SOC) from their antecedent SOC pool   . Soil disturbance through tillage is a major cause of reduction in the number and stability of soil aggregates, and subsequently, SOC depletion  . However when conventional agricultural land use type is changed to other agricultural land use types, it will lead to change in the quality and quantity of residue inputs. The change in residues depends on the productivity and the management of the vegetation replacing the annual crops  . Depending on residue input and management of the replacement vegetation, there is a large variation in magnitude and length of time that SOC may accumulate. Reduced soil disturbance through tillage decreases the rate of SOC mineralization  and increases SOC storage. SOC storage in agricultural soils has the greatest potential to mitigate climate change in sub Saharan Africa agriculture  , and to increase agricultural productivity  .
Conversion of annual agriculture to plantation forests is shown to increase soil nutrients levels and sequester SOC   . Other studies have also investigated the effect of agricultural management systems such as nutrient management, no till, and mulch tillage on SOC, and found that SOC increased under these systems when compared with conventional agriculture   . This increase is attributed to the improved organic matter input and/or reduced negative impact of tillage in these agricultural managements. Conversion of annual agriculture to perennial grasses results in increased SOC compared to agricultural soils  . These conversions contribute to soil fertility replenishment as well as climate change mitigation. However, few studies have examined the impact of perennial crops, particularly in low input tropical agricultural systems.
In southern Ethiopia’s Wondo Genet Woreda, agricultural land use has been changing over the last three decades. Studies by  showed that the number of farmers growing annual crops such as maize (Zea mays L.) decreased significantly between 1985 and 2002. During the same period, the number of farmers who grew khat (Catha edulis F.) and sugarcane (Saccharum officinarum L.) increased. This trend has continued to the present and resulted in the dominance of khat and sugarcane in the landscape. The presence of simultaneous cultivation of maize along with khat and sugarcane as well as the presence of some coffee (Coffea arabica L.) farms in the area provide an opportunity to study the effect of these perennial crops on soil properties by comparing with strictly maize cultivation. Thus the objective of this study was to examine the effect of perennial land uses on SOC, Soil N and base cations.
2.1. Study Area
Wondo Genet Woreda is located 273 km south of Addis Ababa in southern Ethiopia at 7˚4'30'' and 7˚7'30''N latitude and 38˚33'30'' and 38˚39'0''E longitude (Figure 1). It is one of the most densely populated areas in Ethiopia with over 588 people∙km−2. Most of the agricultural lands are found in the same agro climatic zone, and situated between elevations 1700 and 1900 m a.s.l. At the higher elevations (>2000 m a.s.l.) there are steep grassy slopes, bush cover and remnants of degraded forest. The area has a bimodal rainfall pattern with mean annual rainfall of 1240 mm. The mean annual temperature is 19.5˚C. The soils are Andisols  . The underlying parent materials are of alkali trachytes and basalts, often overlain by volcanic ash deposits from the late tertiary volcanic period  .
The farming system consists of mixtures of annual crops (maize) and perennials (shade coffee, khat and sugarcane). Coffee arabica, which is native to Ethiopia, is the world’s most widely traded tropical agricultural commodity. Shade trees offer several advantages for coffee cultivation, including temperature regulation and weed suppression  . Khat, a mild stimulant, is an evergreen plant that grows mainly in Ethiopia, Yemen and other African countries along the Indian Ocean. Sugarcane is commonly cultivated by many Ethiopian smallholder farmers for household consumption (chewing and sucking the juice) and income generation.
2.2. Soil Sampling
Since land use types were not uniformly distributed, we looked for the sites with maize and at least one adjacent perennial land use. Accordingly, Gotu, Wondo Genet College campus (WGCC), Finance and Chaffee were identified for this study. The selected land uses at each site had similar site properties and land use history. The study sites information, including land use history and present management, is shown in Table 1.
Soil samples were collected in April 2013 from the four corners of a square plot (20 m × 20 m) located randomly in each cropland use at each site  . SOC is generally concentrated in the top 30 cm of the soil and so land use change is generally expected to have the greatest impact on SOC in these upper layers  . Thus, soils were sampled at 0 - 30 cm soil depth using a core sampler (diameter = 7.2 cm). A total of forty soil samples were taken from all the agricultural land uses. Soil cores in the sugarcane were taken from a position that was vertically half way between the top of the ridge and bottom of the furrow. All soil samples were air-dried at room temperature, and then sieved (2 mm mesh).
2.3. Soil Analysis
Soil pH was determined potentiometrically in a 1:2.5 (v/v) soil: water suspension. Dry bulk density was calculated by dividing the oven dry mass at 105˚C of the <2 mm fraction, by the volume of the core. Volumes of gravel were taken into account, but made up <4% in most of the samples. The soil particle size distribution was determined using the hydrometer method  . Exchangeable
Figure 1. Location map of the study area.
bases (Ca2+, Mg2+ and K+) and cation exchange capacity (CEC) were analyzed in 1 M ammonium acetate extract at pH 7. The concentrations of Ca2+, Mg2+ and K+ in the extract were analyzed by atomic absorption spectroscopy (AAS). CEC was estimated titrimetrically by distillation of the displaced ammonia  . SOC was measured by the Walkley-Black method  and soil N was measured by the Kjeldahl method  .
2.4. Calculations of Stocks and Relative Variation of SOC and Soil N
SOC and total soil N concentrations in 0 - 30 cm soil depth were converted to an area basis (mass∙ha−1) according to the following equations   :
where C = the soil organic C concentration (g∙kg−1), z = thickness of the sampled soil layer (m), ρb = bulk density (Mg∙m−3), SOC = the amount of soil organic C stock(Mg∙ha−1), N = the soil nitrogen concentration (g∙kg−1)and soil N = the amount of soil nitrogen stock (Mg∙ha−1).
Table 1. Site information, management and site history of land uses.
Relative coefficient for the variation in SOC and soil N stock across agricultural land uses were computed by taking the maize land use as a reference  . Hence the relative coefficient for SOC and soil N stock expresses how much increased or decreased in relation to the maize land use.
where = relative coefficient of SOC or soil N stock of perennial land use with respect to maize land use; VP = SOC or soil N stock of perennial land use and Vm = SOC or soil N stock of maize.
2.5. Statistical Analysis
All data were analyzed using SPSS 16.0 statistical software. One way analysis of variance (ANOVA) was used to compare the soil parameters (e.g. SOC, soil N, base cations, CEC) for each site separately. Assumption of equality of variance was tested using Leven’s test. Significant results from ANOVA at P < 0.05 were followed by Tukey’s HSD post hoc separation.
3.1. Soil Properties: pH, Bulk Density and Texture
The soil pH of shade coffee was significantly higher (P < 0.01) than that under maize at WGCC while the soil pH of the sugarcane was significantly lower (P < 0.05) than the maize at Finance (Table 2). Bulk density of shade coffee was significantly lower (P < 0.05) than that under maize while bulk density of sugarcane and khat did not differ (P > 0.05) from that under maize (Table 2). The particle size distribution and texture of the soils are shown in Table 2. The soils at Wondo Genet College campus (WGCC) and Gotu had a sandy loam texture and those of Finance and Chaffee a loam texture.
3.2. Soil Carbon
SOC concentration (g∙kg−1) of khat and sugarcane was not significantly different (P > 0.05) from adjacent maize land. Shade coffee contained significantly higher (P < 0.001) SOC concentrations as compared to adjacent maize land in each studied site (Table 3). As expected, the differences in SOC stock (Mg∙ha−1; Table 3) among agricultural land uses were similar to that of SOC concentration (g∙kg−1). Shade coffee had 50.6 Mg∙ha−1 more SOC compared with the adjacent maize at WGCC while it had 54.4 Mg∙ha−1 more compared with that of the adjacent maize at Finance. The relative increase in SOC stock in shade coffee at the Finance site was high (125%) when compared with the increase at WGCC (86%).
In general, when comparing the effect of different perennial crops with annual maize, SOC stock increased in shade coffee in both sites (Figure 2). Though not significant, the relative increase in SOC in sugarcane at Finance was 34% while relative decrease of 14% was observed at Chaffee when compared with annual maize (Figure 2). The relative increase in SOC in Khat at Gotu and Chaffee was 7% and 4%, respectively (Figure 2).
3.3. Soil Nitrogen
Soil N concentration (g∙kg−1) and soil N stock (Mg∙ha−1) in the shade coffee were significantly higher (P < 0.01) than those in the maize. Soil N concentration and
Table 2. Mean (SE) soil pH, BD, particle size distributions and textural classes under annual and perennial agricultural land uses.
*Values for crop types at each site with the same letters in columns are not significantly different.
Table 3. Mean (SE) SOC, soil N and C/N ratio, exchangeable bases and CEC under annual maize and perennial land uses.
Figure 2. Relative coefficient (RC) for the variations of SOC and Soil N stock in perennial land use as compared with maize land use at each site.
soil N stock did not differ (P > 0.05) between khat and maize (Table 3). Similarly soil N (g∙kg−1) and soil N (Mg∙ha−1) soils did not significantly differ (P > 0.05) between maize and sugarcane. Shade coffee had 5.7 Mg∙ha−1 higher amount of soil N compared with maize at WGCC and the corresponding figure for Finance site was 4.7 Mg∙ha−1. The relative increase in soil N stock under shade coffee at the Finance site was high (126%) when compared with the increase at WGCC (109%). Though not significant, the relative increase in soil N in sugarcane at Finance was 43% while relative decrease of 14% was observed at Chaffee when compared with annual maize (Figure 2). The relative increase in Soil N in Khat at Gotu was 9% while the relative decrease at Chaffee was 7% (Figure 2). C/N ratio of the perennial agricultural land uses was not significantly different from the annual maize cultivated sites (Table 3).
3.4. Exchangeable Bases and CEC
All perennial land use types did not differ in exchangeable Mg2+ from that in the annual maize except for significantly higher (P < 0.05) exchangeable Mg2+ in the khat at Gotu (Table 3). The perennial land use types did not differ in exchangeable Ca2+ from that of annual maize except for the significantly higher (P < 0.05) exchangeable Ca2+ in the shade coffee and sugarcane at the Finance site. Maize had higher (P < 0.05) exchangeable K+ levels than sugarcane. CEC of soils in the shade coffee was significantly higher (P < 0.01) than that under maize. There was no significant difference in CEC between maize and khat as well as between maize and sugarcane.
4.1. Soil Organic Carbon
In this study SOC for the shaded coffee plantations was higher than the adjacent maize. The SOC concentration of shade coffee was also in the upper margin of the reported SOC content for coffee  . In the study area plant parts are not removed from shade coffee except for the coffee berries (Table 1). Hence the difference in SOC between shade coffee and maize could have originated from litter inputs from coffee plants and shade trees. Large litter productions by trees have been demonstrated by some studies in Ethiopia. Litter fall study in south Ethiopia indicated that coffee trees produced 0.5 Mg∙ha−1∙yr−1 whereas shade trees produced 1.7 to 5.3 Mg∙ha−1∙yr−1  . Another study from south western Ethiopia displayed exotic trees litter input in a range of 8.9 - 10.8 Mg∙ha−1∙yr−1  . A litter fall study in north Ethiopia showed tree litter inputs in the range of 0.3 - 4.25 Mg∙ha−1∙yr−1 in area ex-closures  .
Studies have demonstrated the importance of litter input on SOC buildup. Soil amended with crop residue contained twice the SOC than soil with residue removal over 15 years in Nigeria  . The application of 3 Mg∙ha−1∙yr−1 dry matter of Leucaena leucocephala mulch in Kenyan coffee plantations over a 3-year period lead to a 15% increase in SOC concentration  . A hierarchical analysis of published articles indicated that crop residue retention of 1.5 to 2.5 Mg∙Cha−1∙yr−1 or higher increased SOC by 50% in arid climates while almost doubling in equatorial climates  . High SOC has been reported in shade coffee as compared with un-shaded coffee plantations   Sadeghian et al. 2001 as cited in  . This difference between shade coffee and un-shaded coffee plantations may also corroborate the importance of litter input from shade trees. This assertion was verified by a Costa Rican study that showed the annual shade coffee litter fall, measured as dry weight, was 4.5 Mg∙ha−1∙yr−1 higher than in the un-shaded coffee  .
The reduced soil disturbance in the shade coffee when compared to maize could have also contributed to the SOC storage. The result of a global analysis of 67 long term studies,  showed that most cropping systems changing from conventional tillage to no tillage could result in sequestration of 0.57 ± 0.14 Mg C∙ha−1∙y−1. This is caused by a reduced rate of SOC mineralization  and most likely occurred in the first 10 years after changing the tillage practice  . In Malawi, conversion of conventional tillage in smallholder farms to no till resulted in a 75% increase in SOC and a 77% increase in soil N, just 5 years after conversion  . Increasing SOC storage in the shade coffee system may contribute towards the mitigation of rising greenhouse gas emissions.
The lack of difference in SOC between khat and maize as well as sugarcane and maize maybe explained by the removal of plant parts/residues in khat and sugarcane management  . In khat production, a large percentage of fresh leaves were removed during the 2 to 3 per-year harvests (Table 1) thereby reducing the potential soil organic matter input. With sugarcane, every 18 months, the sugarcane plant is harvested including the tops (Table 1), which are used as animal feed. Sugarcane can produce residues in the range 13 - 20 t∙ha−1  , but removing, instead of retaining residues, hastens SOC decline  . Farmyard manure amendment resulted in greater SOC when compared with the same system without manure  . The application of farmyard manure (FYM) at rates of 4 - 12 t∙ha−1, which comprises application rates in the present study, produced average grain yields in Ethiopia  . Therefore the amount of applied farmyard manure may not be sufficient to influence SOC beyond the level of the annual maize.
4.2. Soil Nitrogen
The mean soil N stock was higher for the shade coffee plantations by 5.7 Mg∙ha−1 and 4.7 Mg∙ha−1 as compared to the soils of maize farms. In general, soil N varied between different land uses in the same way as SOC which is expected due to the close association between C and N in the soil  . The coffee plant’s fine roots are found in the first 20 to 30 cm soil depth  , and when complemented with shade trees, the potential loss of N via leaching below the shallow coffee roots can be reduced  . N uplift by shade trees can explain the increase in total N in the shade coffee. An average of 155 - 210 kg∙ha−1 nitrate N has been found in sub soils (50 - 200 cm∙ depth) in western Kenya  , where the maize is unable to access this nitrate pool because of shallow root depth. Thus, N cycling by shade trees may contribute to the higher soil N in surface soil layer under coffee. However, soil N might also have been lost to a greater extent through harvest and through leaching under maize, sugarcane and khat.  found higher N mineralization rates under shade coffee in comparison to an un-shaded coffee plantation. These findings may imply that substrate quality and quantity have increased in shade coffee, which may recycle N to soils under shade coffee via natural litter fall.
Maize residue has poor quality with low N concentration and wide C-to-N ratio  . However  has shown that increased level of N-fertilizer has increased the magnitude of N-mineralization rate of the maize residues. In the present study, large proportions of maize residues were removed while low level N fertilizer (except at WGCC) was applied to the annual maize (Table 1), which may restrict N availability in annual maize soils.
4.3. Relative Variations of SOC and Soil N
The relative variations in SOC and soil N between the shade coffees may have occurred due to different factors. The higher increase in SOC storage at Finance than WGCC site may be attributed to Finance’s finer soil texture. Texture is an important determinant of the SOC storage capacity, as higher clay fractions correspond with higher SOC content. The finer soil particles play a central role in the SOC dynamics by promoting the formation of organic mineral complexes, which stabilize SOC and influence the physical protection of SOC within soil aggregates   . Furthermore, finer soil particles enhance water retention in soils and promote biomass production which increases the litter input to the soil  . Besides, the relatively low SOC in the adjacent maize may have contributed to higher relative increase in the shade coffee at Finance. The potential of SOC sequestration is high in agricultural soils which have lost a significant part of their original SOC  . The relatively higher increase in the shade coffee soil N at the Finance site can be due to the low soil N in Finance’s adjacent control maize, compared to WGCC.
Though the differences in SOC and soil N were not significant between khat and maize as well as sugarcane and maize, the relative coefficients for SOC and soil N in khat and sugarcane differed between sites. The high relative increase in SOC and soil N in the sugarcane at Finance as well as in khat at Gotu compared to those of the Chaffee site may be attributable to the management difference between sites (Table 1). In sugarcane at finance and in khat at Gotu, a higher rate of farmyard manure input compared to that at Chaffee could contribute to the difference. Farmyard manure application enhanced SOC storage  . Moreover at Finance the sugarcane is irrigated and a little amount of fertilizer was applied (Table 1). Fertilization and irrigation can enhance SOC by increasing the amount of aboveground biomass and root biomass  . Even if the management for aboveground biomass was similar, the root biomass can contribute for the SOC difference between the sites. Besides the management, at Finance, the low soil carbon of the adjacent maize land may explain the relative increase in SOC and soil N in the sugarcane.
4.4. Base Cations and CEC
Organic matter contributes significantly to the CEC of a soil and the high CEC of soils under shade coffee as compared with those soils under maize can be explained by the higher level of SOC under the shade coffee land use. In this study, the soil under sugarcane contained lower exchangeable K+ when compared with annual maize land use. Such decrease in exchangeable K could be expected as sugarcane is a large K consumer  .
In general, shade coffee land use stored more SOC as compared to adjacent maize, but khat and sugarcane land uses were less effective in SOC storage. The increased SOC in the shade coffee land use is attributed to increased litter input and reduced soil disturbance under shade coffee. The difference in relative SOC storage between shade coffees in different sites could be due to the difference in soil texture and the low level of SOC in the adjacent maize land use. Among perennials, shade coffee also has the potential to improve soil N stock as well. Soils under shade coffee had higher N stock than soils under adjacent maize, while soils under khat and sugarcanes had no difference from soils under maize. Improved soil N under shade coffee could be attributed to factor such as higher litter input, reduced leaching and N-uplift from the coffee plants and shade trees. Perennial shade coffee, khat and sugarcane land uses have generally shown little effect on the base cation concentrations of the soil over two decades. Hence, this finding has implications for selection of appropriate sustainable agricultural land use and land use suitable for climate change mitigation efforts.
This research was supported by the Ethiopian Government Research Fund to Hawassa University. Water Design Laboratory in Addis Ababa, Ethiopia is acknowledged for soil analysis. I am grateful to Erik Karltun for the comments and Robert Sturtevant for editing the English of the manuscript.