Effects of ‘ ‘ Guie ’ ’ on Soil Organic Carbon and Other Soil Properties : A Traditional Soil Fertility Management Practice in the Central Highlands of Ethiopia

A traditional soil fertility management practice through soil burning, locally called ‘‘guie’’ is practiced in the central highlands of Ethiopia. The study was conducted to investigate the effect of ‘‘guie’’ on physico-chemical properties of the soil and its long term impact on soil organic carbon (SOC). Two sets of soil samples were collected from the field. The first set was from central part of the heaps of the burnt soil. The second set was from adjacent cultivated fields used with and without ‘‘guie’’ for many years. Collected samples were analysed following standard laboratory procedures. Complete soil burning showed a significant decrease in SOC, total nitrogen, cation exchange capacity (CEC), exchangeable calcium, magnesium, sodium, available iron and clay while it significantly increased available phosphorus, manganese and copper, exchangeable potassium and sand. A significant difference (p<0.001) in SOC was also obtained for the second sample set. The highest (7.14%) and the smallest (1.54%) mean SOC was obtained from cultivated land where ‘‘guie’’ integrated with fallowing and without ‘‘guie’’ and fallowing, respectively. This indicates that fallowing could help the soil to accumulate SOC and if it coupled with the partial soil burning practice, accumulation of stable organic carbon in the soil could be enhanced. Intensive cultivation exposes the soil for erosion accompanied by active mineralization of the organic matter. Thus, results of the present study have an implication that fallowing integrated with ‘‘guie’’ is more sustainable to reserve OC in the soil system than the continuous cultivation.


Introduction
Ethiopia is one of the most populated countries in the Sub-Saharan Africa.Agriculture is the main stay for the majority of the society and contributes more than 40% of the GDP.The agricultural productivity has been heavily dependent on the natural fertility status of the soil.However, the natural potential of the soil to supply nutrients has been significantly reduced mainly due to erosion and depletion.The use of chemical fertilizers to improve the productivity of agriculture is limited because of the poor economic backgrounds of the vast majority of the society to afford the ever increasing cost of fertilizers.Therefore, efforts towards integrated soil fertility management practices may help to improve crop productivity that may lead to sustainable agriculture.For example long term investment on soil and water conservation (SWC) in the highlands of Ethiopia improved soil fertility (Adgo et al., 2013;Alemu et al., 2013;Amare et al., 2013).Ethiopian farmers also use various cultural soil fertility improvement practices such as inter cropping, crop rotation, fallowing and farmyard manure.Farmers in the central highlands of Ethiopia have an old and a unique traditional soil fertility management practice through soil burning locally called ''guie''.Farmers use soil burning (''guie'') to improve crop yield.For ''guie'' preparation, the land is left fallowed for more than 5 years.At the end of the fallow period, the land is ploughed shallowly (~10 cm) in October while the soil is wet.Consequently, the soil gets dry and repeatedly ploughed so that clods of soil associated with a mass of roots (sods) separate easily from other parts of the soil.By collecting these sod associated soils, together heaps are made.With a size of about 80 cm in diameter and 60 cm in height around 900 heaps are made per hectare (Abebe, 1981).The heap is ignited by putting smaller dried

Soil Preparations and Analysis
The samples were air dried under shade, ground using pestle & mortar and sieved to pass through 2 mm sieve.Soil pH was determined in a 1:2.5 soil to water suspension following the procedure outlined by Sertsu and Bekele (2000).The organic carbon content was determined by wet digestion method using the Walkley and Black procedure (Nelson & Sommers, 1982).The total nitrogen content was determined using the Kjeldahl method (Bremner & Mulvaney, 1982) while the available phosphorus was determined following the Olsen procedure (Olsen & Sommer, 1982).The cation exchange capacity (CEC) was determined after extraction of the samples with 1N ammonium acetate at pH-7 and the aliquots were measured by atomic absorption for Ca 2+ , Cu 2+ , Fe 2+ , Mg 2+ and Mn 2+ while prepared aliquots were measured by flame photometer for the determination of K + and Na + following the procedures described by Sertsu and Bekele (2000).Soil texture was analysed using hydrometer method (Sertsu & Bekele, 2000).

Statistical Data Analysis
The impact of independent variables such as soil burning on the dependent variables (soil properties) was statistically tested.Analysis of variance (ANOVA) was carried out using SAS 9.2 software (SAS, 2003).For variables showing statistically difference between treatments (p<0.05),further analysis of mean separation was carried out using Duncan's Multiple Range Test (DMRT) at 5% probability.

SOC, Total Nitrogen and Available Phosphorus
The effect of soil burning (''guie'') that particularly changed into ash at the centre of the heap was recognized and resulted in a significant change on SOC, total nitrogen and available phosphorus of the soils.Particularly, the ash component is found to have very low in its SOC content and lower in total nitrogen.Compared to the mean SOC content of the surface soils, the ash component of the burnt heaps contained SOC that is as much as less than 1% (Table 1).The difference in SOC content between burnt and un-burnt soil samples was highly significant (p<0.01).With high temperature, soil organic matter decomposed and changed into ashes.Similarly, Mataix-Solera et al. ( 2011) reviewed in detail that soil burning has a deleterious effect on SOC that results in deterioration of aggregate stability of the soil.Similarly, Sertsu & Sánchez (1978) found that SOC was not affected at heating temperatures up to 200 o C while its effect is dramatic with the increasing temperature.
According to their report, SOC was drastically reduced at temperatures above 200 o C and completely destroyed at 400 o C. Finding the temperature equivalent to their findings (200 o C) at field conditions may help to obtain the positive contribution of soil burning with no or less destructive effects.On the other hand, burnt heaps have different intensity of burning forming clear layers with different effects on SOC (Wehrmann & legesse, 1965 cited by Abebe, 1981).They identified a carbonized layer next to the layer where maximum burning temperature exerted (ash layer).According to the report, colour of the carbonized layer is black that might be associated with carbon trapped from the ash layer as well as partial decomposition of the soil organic matter (pyrolysis effect); this could help to store carbon for many years in the soil similar to biochar.However, our research was based on the ash part where complete burning is taking place in the profile of the heaps and hence limited for making a conclusive remark that ''guie'' has a disastrous effect on the long term SOC reserves.
The highest SOC (7.56%) was obtained from un-burnt soil samples associated with sods followed by un-burnt surface soils (7.17%).The higher SOC from un-burnt soils is associated with sods clearly demonstrates why farmers selectively use it for soil burning (''guie'').Similarly farmers use to plough only the topsoil (< 10 cm) for the purpose of preparing the ''guie'', because high SOC at topsoil helps to burn the soil very easily.
Soil burning (''guie'') also resulted in a highly significant (p< 0.01%) decrease on total nitrogen content of the completely burned soils (Table 1).The minimum value of total nitrogen (0.11%) was obtained at the completely burnt soils, whereas the maximum value (0.79%) was obtained at soil samples associated with un-burnt sods, indicating a significant amount of total nitrogen is contributed from the soil organic matter and sharply decreased upon breakdown of the organic matter due to ash making.Decreasing in total nitrogen does not reflect about the state of available nitrogen.For example Murphy et al. (2006) indicated that the concentration of available nitrogen both NH 4 + and NO 3 -increased in the soil solution after forest soil burning.Similarly, Sertsu and Sánchez (1978) reported that available nitrogen increased by soil burning at a temperature of 200 o C. Surface soil@ 7.17a 0.73a 14.34a Probability ** ** ** @ Surface soil is soil sample that was collected at the depth of 0-10 cm but it was neither burnt nor associated with sods, ** Significant at p ≤ 0.01.
The effect of soil burning on the availability of phosphorus was found Available phosphorus from heaps of burnt soil was more than threefold (50.2 ppm) as compared to the un-burnt ones, whereby the difference was significant (p<0.01) as shown in Table 1.The available phosphorus from soils associated with un-burnt sods was also found higher than un-burnt non-sod surface soil samples, which may be related with the soil organic matter and it is contributed majorly from organic phosphorus.The reasons for the increment of available phosphorus from burnt soil are: decomposition of soil organic matter helps to release phosphorus contained within it.Reduction of available iron by burning as shown in this research may also help to reduce fixation of phosphorus.The clay fraction was also found reduced due to soil burning, which improves phosphorus fixation.Abebe (1981) also indicated that soil burning reduces the exchangeable aluminium so that phosphorus fixation is reduced.Galang et al. (2010) found higher available phosphorus from a field after one week of prescribed burning.However, availability of phosphorus could be only for short time as other cations will come into solution due to soil burning and increase phosphorus fixation in turn.Additionally, the volume of the bunt soil (heaps) is very small compared to the un-burnt soil mass at the plough depth, thus its effect will be diluted and leading only short term availability of phosphorus as well as short term yield increment.Moreover, Ketterings et al. (2002) by heating the soil above 300 o C found an increase in surface area of minerals that increases phosphorus fixation.Therefore the state of heat induced availability of phosphorus may not be any more sustainable.That is why an extended fallowing period has been required after one cropping with soil burning.
Under laboratory condition, Sertsu and Sánchez (1978) found an increase in available phosphorus on the Ethiopian highland soils by heating the soils with temperatures ranging from 26 to 400 o C.However, it was drastically decreased above 400 o C. Interestingly, their result shows that available phosphorus was increased from less than 1 ppm to greater than 30 ppm by rising the heating temperature from 26 to 200 o C without a pronounced deleterious effect on other soil properties.By modifying farmers' soil burning practice to about 200 o C at field condition, equivalent crop yields to that of complete soil burning may be achieved and stable SOC can be accumulated in the soil that may help improve the soil health and other ecosystem and climate related functions.

Soil pH, CEC, Exchangeable Bases and Micronutrients
The effect of soil burning (''guie'') on pH, CEC, exchangeable bases and available micro nutrients was found significant (Table 2).Its effect on pH was significant (p<0.01).It changed the soil pH from less than 5.7 to more than neutral.An increase in pH by soil burning could be due to the increase of cations in the soil solution.The difference in pH between un-burnt soil samples was insignificant.The value of pH obtained in this research is an ideal range for most cultivated crops.However, this value may be diluted upon applying to the field because its smaller volume compared to the volume of un-burnt soil masses.
The effect of soil burning on (''guie'') CEC was strong and negative.CEC from burnt samples was less than by three fold from un-burnt soil samples (Table 2).Decrease in CEC due to soil burning could be related to a decrease in organic compounds and clay content.Because soil organic compounds and clay are the dominant sources for negative electrical charges that absorb the cations.Our finding is in line with the results of Marwa et al. (2009), andSertsu andSánchez (1978).surface soil is soil sample that was collected at the depth of 0-10 cm but it was not burnt and it was not associated with sods, ** significant at p ≤ 0.01, and NS is non-significant at p ≤ 0.05.
Exchangeable bases were affected by soil burning (''guie'' ).Exchangeable potassium was insignificantly increased by soil burning to about 0.10 cmole (+)/kg soil (Table 2).Its increment from burnt soil may be related to the breakdown of minerals that leads to interlayer potassium cations conversion to exchangeable form.Marwa et al. (2009) reported that the possibility of potassium ion to be fixed by a hexagonal silicate layer is very low as compared to other interlayer cations.On other hand exchangeable Ca 2+ , Mg 2+ and Na + were significantly decreased by burning; this could be because of these cations were changed to other oxides.Our finding is in line with the findings of Marwa et al. (2009) where they accounted the decrease in the exchangeable Ca 2+ , Mg 2+ and Na + because of fixation by the hexagonal silicate layer.
The effect of soil burning (''guie'') was significant on the availability of soil micro-nutrients (Table 2).Cu and Mn were increased significantly on the burnt soils while iron was insignificantly decreased.The availability of micronutrients was increased due to decomposition of organic compounds.Our finding with Mn and Fe availability agrees with the findings of Garcia-Marco and Gonzalez-Prieto (2008).

Soil Texture
Soil burning resulted in a change in soil texture (Table 3).The percentage of clay was significantly reduced whereas sand was increased by burning the soil.The finding, with regard to the decrease in clay content due to burning, is in agreement with the report by Abebe (1981), Urlery and Graham (1993), Ketterings et al. (2000), Ketterings et al. (2002), Hubbert et al. (2006), Parlak (2011), but it contradicts with the finding of Stoof et al. (2010).According to Verma and Jayakumar (2012), change in clay fraction to sand sized particle starts at about 400 °C where clay hydration and clay lattice structure start to collapse.Abebe (1981), Urlery and Graham (1993) suggested that soil burning resulted in a change in texture size from clay to sand by fusion of clay particles into sand size.Additionally, Urlery and Graham (1993) suggested that silicon and aluminium oxides and hydroxides released by decomposition used for cementing the clay particles into sand sized particles.Soil samples (0-10 cm) that are not burnt and not having sods, * significant at p≤0.05, whereas NS is non-significant.

Long Term Impacts of Soil Burning ''Guie'' on SOC
Fields where soil burning with alternative fallowing has been exercised for long years were compared to adjacent cultivated field where soil burning has never been exercised.The difference in SOC between these two fields was highly significant (p<0.001)(Table 4).The long term effects of removing and burning surface soils combined with long years of fallowing did not bring a deleterious effect upon SOC.On the contrary the SOC was found extremely low at the adjacent cultivated field where there is no fallowing and no soil burning practices.Two major factors may be accounted for the variations of the SOC under the two studied fields: first, the long years of strict fallowing produces large amount biomass production and part of it transferred to SOC.If this practice is accompanied by ''guie'' (a pyrolysis process), there could be biochar accumulation a stable carbon against decomposition.However, if there is a regulated ''guie'' process (a strict pyrolysis); in addition to the improvement in soil structures and nutrient availability, fallowing accompanied by ''guie'' can be equivalent to biocharing soils for better SOC accumulation in the soil system.The second reason may be related to their differences in the state of soil erosion for the two adjacent fields.Better vegetation cover due to fallowing helps to control soil erosion in the case of the first field, whereas the second field has been intensively cultivated without fallowing that exposes the soil for severe runoff erosion.Based on the results of these field comparisons, soil erosion can have a substantial effect on the removal of SOC with the surface soil.Moreover, the smaller fraction of burned soil applied in the field as compared to the larger volume of non-burnt soil may dilute the negative effects of soil burning on SOC.Abebe, 1981) indicated that burnt heaps have different layers including the carbonization layer where the colour of the soil is completely black may be due to partial decomposition of soil organic matter in this layer as well as carbon trapped from the completely burned layer (inner most layer where the layer dominated by ash).Application of this partially decomposed and trapped carbon may help to store more organic carbon in the soil system for longer time without decomposition, which may act similar functions to Biochar.However, this particular subject needs a detail research to identify the components of the SOC whether it is from the char or from the organic residues.
We compared also SOC obtained from the cultivated field where soil burning (''guie'') integrated with fallowing with that of the adjacent forest and grass lands (Figure 3).SOC from cultivated land was found significantly lower than forest and grasslands at all depths.This also indicated that cultivation is still disastrous on the SOC reserves even under fallowing integrated ''guie'' system as compared to the forest and grass lands.

Conclusion and Recommendation
The effects of soil burning or ''guie'' on soil physical and chemical properties and its long term impact on SOC in was investigated in Andit Tid watershed, Central Highlands of Ethiopia, where the system is currently practiced.The finding of the research showed that unregulated soil burning could reduce the SOC, total nitrogen, CEC, exchangeable bases and the clay content.On the other hand, availability of phosphorus, sand and availability of micronutrients increased.Availability of plant nutrients especially phosphorus is the main yield limiting factors in the study area and upon soil burning its availability was increased.The current practice of the farmers on ''guie'' that is integrated with fallowing showed high SOC (7.14%) compared to adjacent fields (1.54%) with intensive cultivation without fallowing and ''guie''.The deleterious effect of soil burning on SOC was not observed in the field as evidenced by this study.The partial burning effect that leaves chars in the soil would accumulate SOC over years.This could also be due to poor vegetation cover that usually accompanied by a severe soil erosion and poor organic return to the soil with continuous cultivation.Of course, the continuous cultivation can hasten microbial decomposition of soil organic matter that reduces the residence time of SOC in the soil system.Thus, results of the present study have an implication that fallowing integrated with ''guie'' is more sustainable to reserve OC in the soil system than the continuous cultivation.
Further research interventions are important on the residence time of the SOC by fallowing without soil burning, enhancing the biomass production through improved fallowing, and effect of soil burning on soil microorganisms.Moreover, determining the optimum temperature at which availability of plant nutrients such as phosphorus is increased with minimum destruction of other soil parameters including the SOC are critically important.

Figure 3 .
Figure3.SOC as a fuction of different land uses.The forest and grass areas have never been changed to any landuse types in the last several decades and they are found adjacent to the cultivated field where long years of soil burning integrated with fallowing have been used

Table 4 .
SOC (%) in the cultivated fields with and without soil burning in Andit Tid watershed