Fanyaaju stabilization and productivity of Pennisetum Pedicellatum in response to planting position and spacing at Hawassa Lake watershed, Southern Ethiopia

Abstract

The efficiency of physical soil conservation structures could be improved through properly managed biological conservation strategies. Even though the physical soil and water conservation structures were implemented in many parts of Ethiopia; the structures were not stabilized well. The experiment was carried out in Hawassa Lake watershed by considering two planting positions (burm and embankment) and four spacings (5, 10, 15, and 20 cm) with four replications from 2018 to 2020 in RCBD design with a factorial combination of treatments. Results showed that desho grass planted on the embankment of fanyaaju with 20 cm space provided soil aggregate stability values of 31.65, 20.53,15.70, 11.74, and 18.70% at sieve sizes of >2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm, respectively. These values were significantly higher than others. The lower soil aggregate stability was recorded on desho grass planted at the end of the burm with 5 cm planting space. Desho grass planted on embankments with 15 cm and 20 cm also registered higher soil moisture values of 29.32 and 23.9%, respectively. The highest grass biomass yield of 1755.1 g was recorded on the embankment + PS of 15 cm. Thus, the desho plants maintained on the fanyaaju embankment with 20 cm space showed prolific root and stem growth and stabilized the physical structure. The marginal rate of return produced on 20 cm + Emb was 62.92%, showing economic viability over other treatments. Therefore, the study strongly recommends planting desho grass on the embankment of fanyaaju with 20 cm planting space as the best option for bund stabilization and productivity.

Keywords:

• burm
• embankment
• soil aggregate
• soil and water conservation
• stabilizer

PUBLIC INTEREST STATEMENT

Land degradation is a global problem affecting environmental and socio-economic conditions. The problem is severe in developing countries like Ethiopia. To solve the problem many soil and water conservation measures are employed in the country. Soil and water conservation is a key system to reduce land degradation risk. This has helped to maintain soil nutrients and reduce soil degradation. The practices are advantageous when physical and biological conservation is integrated. However, this practice was not implemented appropriately in the study area. Furthermore, the established physical soil and water conservation structures, especially fanyaaju, were not stabilized by appropriate desho planting position and spacing. Planting. Due to this, the soil and water conservation work could not achieve the intended objectives of land management. This paper investigates the appropriate desho planting position and spacing. Desho planting on the embankment of fanyaaju with 20 cm spacing was found economically viable and the best option to stabilize fanyaaju. The research result is important to achieve sustainable environmental and socio-economic development.

1. Introduction

Land degradation is the permanent decline in the land capacity to produce yields useful to local livelihoods (Scoones & Toulmin, 1999). About 24% of the world’s land has been affected by land degradation and more than 1.5 billion people live in those degraded land areas (IFPRI, 2012). In developing countries, it has been a serious concern for its negative implications for the rural household’s income and their environment. Its influence was reduced with crop yield decline and social stress (Greenland et al., 1994).

In Ethiopia, soil productivity has been declining as a result of soil erosion, nutrient depletion, and organic matter depletion (Hadera & Gebrekidan, 2013). According to a study by Hurni et al. (2008) about 42 t ha^{− 1}yr⁻¹ of fertile soil containing essential plant nutrients are lost from cultivated lands of Ethiopia due to poor soil and water conservation (SWC) practice. Unwise soil and land use patterns can have a negatively intense impact on soil fertility (Adugna et al., 2021). Predominantly, the conversion of natural ecosystems to farmlands has resulted in a decline in soil’s physical, chemical, and biological qualities. The habit of a more intensive farming system and tillage on steep sloppy lands is declining soil fertility and the high occurrence of soil loss due to erosion (Shiferaw & Holden, 1998)

It is a fact that effective soil and water conservation will be achieved only when the erosive impact of both raindrops and runoff is prevented and controlled on the land (Taye et al., 2013). SWC practices have positive impacts on soil and crop productivity of cultivated lands; however, their effect is more pronounced when physical SWC practices are integrated with biological SWC practices and at a longer establishment (Tanto & Laekemariam, 2019). Terracing is one of the most promising and most effective long-term alternatives for combating land degradation (Mishra et al., 2020). The construction of terraces in integration with other practices is key to reducing soil erosion and is supportive to improve the shortage of food self-sufficiency (Mishra et al., 2021). The terrace, which is one part of physical soil and water conservation (SWC) practices alone, is not effective in minimizing the impact of raindrops on soil, and soil erosion occurrence could continue even if mechanical structures were constructed although at a reduced rate (Ebabu et al., 2019). Terraces including fanyaaju are small embankments constructed at the outer edge of the terraces to control water flow (Mishra et al., 2013). They are more effective when the practice is combined with vegetative measures (Kagabo et al., 2013).

To reduce soil degradation, restore degraded lands, and increase agricultural productivity, the Ethiopian government initiated different natural resource management activities through soil and water conservation programs with the help of nongovernmental organizations (NGOs) and development agencies (Hawando, 1997; Wolka, 2014). Following the initiation of the program, the regional bureaus of agriculture, district agricultural offices, and other lower-level administrative bodies mobilized farmers to help with the construction of soil and water conservation measures. Soil and water conservation activities are now undertaken as the main part of watershed development practice in Ethiopia especially in southern Ethiopia. Consequently, physical and biological SWC measures have been introduced in numerous main and sub-watersheds parts of the region that are managed by local communities (Adimassu et al., 2018)

According to MoA (2005), physical structure stabilization implies the planting of grass, crops, shrubs, and trees in different combinations to strengthen the resistance and stability of structures such as bunds, trenches, check dams, etc., against raindrops splash effect, runoff, and cattle trampling. At the same time, the surface area occupied by the structure could be productive due to its stabilization. According to Mishra et al. (2022), Interventions such as planting horticultural trees and cultivating broom grass on terrace risers for soil fertility have significantly decreased soil loss from the area. Recently, the Ethiopian government recognized the causes of SWC structure failure and concentrated on stabilizing Physical SWC measures with biological SWC measures, especially Fanyaaju and soil bund with desho grass on agricultural lands (Mitiku et al., 2006). (Smith, 2010) noted that different public and private groups implemented soil and fanyaaju bunds with desho grass across the slope in densely populated highland areas of Ethiopia for sustainable SWC programs. Tanto and Laekemariam (2019) also found that the changes in soil properties and yield become increasing when physical SWC structures are stabilized with desho grass and with increasing age of establishment. Additionally, stabilizing physical SWC structures by planting multi-purpose grasses/plants would also benefit farmers by providing fodder/fuel.

Perennial crops can potentially be a win–win strategy to combat the dual challenges of globally increasing demands on biomass yield and the need for climate change mitigation (Chen et al., 2022). Pennisetum Pedicellatum (Desho) grass is native to tropical zones that are categorized under the Poaceae family and can grow anywhere from 1500 to 2800 m.a.s.l (Smith, 2010). It has a big root growth system and can produce a huge amount of biomass per m² (Ramirez et al., 2010). In Ethiopia, the grass was discovered in the Chencha district, Gamo Gofa zone in 1991 (Welle et al., 2006), then it has been widely used in various agroecological parts of the country (Leta et al., 2013). Desho has a great potential to control water loss effectively and recovers rapidly even under severe drought conditions (Jantawat et al., 2006).

Even though a lot of efforts have been carried out in the construction of mechanical SWC structures, there is still soil erosion resulting from land degradation. Many Constructed structures were damaged in many parts of Ethiopia including Hawassa Lake Watershed (Mitiku et al., 2006). This is due to poor bund stabilization systems of physical structures. Planting Desho grass on fanyaaju at an appropriate position and space is important to reduce nutrient and water competition between individual grasses, reduce soil moisture evaporation, yield improvement, weed control, reduce completion for sunlight, fast canopy establishment, and improve root growth system that helps to stabilize soil (Mekuriaw et al., 2017).The null hypothesis states that there is no difference in using planting position and spacing on fanyaaju for its stabilization and desho productivity. However, this argument has been refuted time and again across varying slopes, soil texture, agroecology, and socio-economic categories. According to Gessesse (2014), failures in physical SWC structures revealed their requirement for stabilization with biological measures at proper planting position and space. Correspondingly, at Hawassa Lake sub-watershed, certain mechanical SWC structures, especially fanyaaju,were constructed broadly but poorly stabilized by desho grass. Farmers of the study area were not planted a desho grass at proper planting position and spacing, because there was no recommended information on desho grass planting position and spacing on the fanyaaju. Due to this, planted desho grasses were not performed on fanyaaju. Consequently, the constructed fanyaaju were damaged, and the soil erosion risk was increased from past to recent years. So, this study aims to specify the optimum desho grass planting position and spacing on fanyaaju bunds appropriate for bund stabilization and biomass yield.

2. Methodology

2.1. Study area description

The experiment was conducted on the farmland area of the Udo Wotate peasant association (PA) of Hawassa Lake Watershed located in the Hawassa Zuria district as shown in Figure 1. The total population of the Hawassa district is 124,472, of whom 62,774 are men and 61,698 are women (CSA, 2007). Udo Wotate PA is geographically located at the Great Rift Valley of Ethiopia in coordinates 7°5’3.22“N and 38°21’44.95“E with an altitude of 1718 m above sea level. It is located 226 km south of Addis Ababa of Ethiopia the capital city. The study area map was sketched using Arc GIS 10.8 using shape files from the Ethiopian Mapping Agency (ema.gov.et). According to the National Meteorological Agency (NMA) of Ethiopia (2021), the mean annual rainfall was 977.5 mm. The average maximum and minimum temperatures of the area was 27.54°C and 13.24 °C, respectively, as shown in Figure 2. A crop production system is mainly a rain-fed-based and subsistence system. The main annual crops cultivated in the area are maize (Zea mays), haricot bean (Phaseolus vulgaris), and millet (Elusine coracana).

Figure 1. Location of the study area.

Figure 2. Average annual rainfall and temperature of the study area (1990 to 2020).

2.2. Experimental procedure

The experiment was conducted within 3-year period (2018–2020). In the first year of site selection, soil samples collection before the trial and construction of the fanyaaju bund was conducted. The soils were collected at each 3-month interval during rainfall events starting from the third month of fanyaaju structure construction. Similarly, the grass was harvested at 3-month intervals. The grass height data were collected at one-month intervals.

2.2.1. Treatments

2.2.2. Experimental layout

The experiment was laid out using eight treatments and four replicates in a factorial randomized complete block design (RCBD). The length of each plot (treatment) was 10 m, and its width was fixed based on the slope of the land. The distance between the replications (blocks) was 0.5 m.

Fanyaaju bund construction: Fanyaaju bund was used as a mechanical soil and water conservation structure for all treatments as shown in Figure 3. It was constructed during the dry season (January month). It was constructed with an embankment height of 40–60 cm and a burm width of 25 cm.

Figure 3. (a) daily laborers planting desho on fanyaaju at the study area. (b) desho grass that was planted on fanyaaju embankment and the end of burm.

2.3. Statistical data analyses

The experiment was conducted with four field replications. The values presented in the tables are by means of eight treatments within four replicates ± standard errors (mean ± SE). The data set was checked for normality (Shapiro–Wilk test, P > 0.05) and homogeneity of variance (Chi-square Test, P > 0.05) before Analysis of Variance (ANOVA). For fresh, dry biomass and plant height, the data did not meet the requirement for normality. Therefore, the data were square-root transformed and retested for normal distribution with the Shapiro–Wilk test. Afterward, two-factor ANOVA was performed to test the effects of desho grass planting position and spacing on soil aggregate size distribution, soil moisture content, grass height, and grass harvested biomass. Post-hoc tests for multiple comparisons using Least Significant Differences test (LSD test, P < 0.05) were performed on each measured parameter after ANOVA. R software was used to perform ANOVA analyses. The level of significance was defined at P < 0.05 for all statistical analyses, if not mentioned specifically.

2.4. Determination of Soil Aggregate Stability (SAS)

To determine soil aggregate stability, the soil samples were collected to a depth of 20 cm from fanyaaju and determined using the wet sieving method. Soil samples collected from the field were air-dried, and 50 g of each was sieved using sieves with mesh sizes in mm of >2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm as shown in Figure 4. The final resistant gravel particles were separated using a sodium hexametaphosphate solution. The method described by (Kemper & Rosenau, 1986) was used to determine the wet aggregate stability of air-dried aggregates. Accordingly, the percentage of wet aggregate stability for each size class was determined by using the following equation:

Figure 4. (a) moisturizing soil using >2mm sieve placed on water-filled dish (b) washed soils arranged at four sized sieves before oven drying at the soil laboratory.

Wet aggregate stability (%) = $\frac{Mr}{Mt}\ast 100$

Where Mr is the mass of resistant aggregates for each size class and M_t is the total mass of wet-sieved soil.

2.5. Determination of soil moisture content, grass height, and biomass

The soils were collected at each 3-month interval during rainfall events; to evaluate the soil moisture content of the treatments by using a core sampler. Wet soil weight was measured at the field using a sensitive balance. To measure dry weight, the collected soil was oven dried at 105°C for 24 h.

The following formula was used for calculating the soil moisture content (Reynolds, 1970).

$SMC=\frac{Ww-wd}{wd}\ast 100$

Where: SMC = Soil moisture content dry base (%), Ww = Weight of the wet soil (g), W_d = Weight of the dry soil (g)

To evaluate the impact of integrated SWC practices on selected soil properties; Representative composite soil samples were taken before and after construction from 20 cm, following the standard soil sampling procedure to evaluate the change in fertility status of the treated plots. Each composite soil sample was subjected to physicochemical analysis. USG soil textural class classification was employed to classify soil texture. The collected data were analyzed using simple descriptive statistics and R software packages.

The grass height was measured each month, and the grass was harvested at 3-month intervals to acquire biomass data. A representative sample of desho grass was harvested from 3 m from each treatment. The average fresh and dry biomasses collected from a 3-m-long area of fanyaaju was employed for analysis. The plant height was measured during the grass maturity stage before harvesting. The fresh weight was measured at the field immediately after harvesting using spring balance, and the dry biomass weight was measured using a sensitive balance after oven drying the grass at 105°C for 24 h.

2.6. Cost benefit analysis

Cost-Benefit Analysis was done using Partial budget analysis following CIMMYT (1988)

$\mathrm{M}\mathrm{R}\mathrm{R}\left(\mathrm{\%}\right)=\left(\frac{netbenefittrt2-netbenefittrt1}{costtrt2-costtrt1}\right)\ast 100$

3. Result and discussion

3.1. Soil properties of the soil before and after the experiment

Variations in soil properties values were recorded before and after the trial (Tables 2 and 3). The average pH (1:2.5), OC (%), OM (%), TN (%), P (ppm), Av.K (mg/kg), and CEC (meq/100 g) of the study area before the trial were 7.11, 1.22, 2.11, 0.11, 8.88, 19.4, and 3.5, respectively, as shown in Table 1. The average composition of clay, silt, and sand was 0.5%, 10.75%, and 88.75%, respectively. This means the experimental site was dominantly categorized under the sandy soil textural class; this is due to the high percentage of sand content.

Table 3. Soil properties after the experiment

Composites	Soil properties							Soil texture
	pH (1:2.5)	OC (%)	OM (%)	TN (%)	P (ppm)	Av.K (mg/kg)	CEC (meq/100 g)	% sand	% clay	% silt	Textural class
Composite 1	7.25	2.3	3.96	0.198	12.7	16.7	5	90	2	8	Sandy
Composite 2	7.2	1	1.72	0.086	13.5	28.5	6	92	0	8	sandy
Composite 3	7.15	1.2	2.07	0.103	15.9	35.8	4	89	1	10	sandy
Composite 4	7.05	1.5	2.59	0.129	10.5	20	7	88	0	12	sandy
Average	7.16	1.5	2.59	0.129	13.15	25.25	5.5	89.75	0.75	9.5	sandy

Table 1. Treatments used on the trial

	Treatments
1.	Planting at the end of burm + PS of 5 cm (one row).
	Planting at the end of burm + PS of 10 cm (one row).
3.	Planting at the end of burm + PS of 15 cm (one row).
3.	Planting at the end of burm + PS of 20 cm (one row).
4.	Planting at the top of the embankment + PS of 5 cm (one row).
5.	Planting at the top of the embankment + PS of 10 cm (one row).
6.	Planting at the top of the embankment + PS of 15 cm (one row).
7.	Planting at the top of the embankment + PS of 20 cm(one row)

PS= stabilizers planting space, Emba = Embankment.

According to Table 2, the average soil property values of pH, OC (%), OM (%), TN (%), P(ppm), Av.K (mg/kg), and CEC (meq/100 g) after the trial were 7.16, 1.5%, 2.59%, 0.129%, 13.15 ppm, 25.25 (mg/kg), and 5.5 (meq/100 g), respectively. This result showed that fanyaaju stabilized with desho grass improved soil organic carbon, organic matter, total nitrogen, and phosphorus contents of the soil. However, the percent composition of texture and pH value could not show a notable change due to the implementation of fanyaaju stabilized with desho grass within 3-year period. This is in agreement with the findings of Haregeweyn et al. (2017) that states the impact of soil and water conservation structures requires a long time to show improvement in soil texture and pH value.

Table 2. Soil properties before the experiment

Composites	Soil properties							Soil texture
	pH (1:2.5)	OC (%)	OM (%)	TN (%)	P (ppm)	Av.K (mg/kg)	CEC (meq/100g)	% sand	% clay	% silt	Textural class
Composite 1	7.23	1.95	3.36	0.17	7.5	10.5	3	92	0	8	Sandy
Composite 2	7.13	0.5	0.86	0.04	9.2	20.9	4	90	0	10	sandy
Composite 3	7.1	0.98	1.69	0.09	11.5	30.5	2	87	1	12	sandy
Composite 4	6.98	1.46	2.52	0.13	7.3	15.7	5	86	1	13	sandy
Average	7.11	1.22	2.11	0.11	8.88	19.4	3.5	88.75	0.5	10.75	Sandy

3.2. Soil aggregate stability at different desho grass stabilizer planting spaces and positions

3.2.1. Soil aggregate stability

Statistically significant variation of soil aggregate stability was observed between treatments at p < 0.05 (Table 4). The highest soil aggregate stability value of 31.6 (sieve >2 mm) was recorded on the bund in which the desho was planted on the embankment with 20 cm planting space. For sieve sizes of 1, 0.5, 0.25, and <0.25 mm, the aggregate stability values of 20.53, 15.70, 11.74, and 18.70 mm, respectively, were measured for the same treatment. However, the lowest mean soil aggregate stability value of 15.15 (sieve >2 mm) was recorded on the bund in which the desho was planted on the end of burm with 5 cm planting space. For sieve size of 1, 0.5, 0.25, and <0.25 mm, the aggregate stability values of 8.2, 7.55, 5.59, and 7.64 mm, respectively, was measured for the same treatment. The better aggregate stability due to 20 cm spacing was attributed to favorable space for root growth (stabilization). Thus, higher soil aggregate stability value was recorded from the embankment than at the end of the burm. The stabilizing grass planted on the embankment easily stabilized and compacted the embanked soil. But, the grass stabilizers planted on the end of the embankment required a long time to develop its root and expand to the embankment. The finding was in line with Getahun et al. (2015) and Welle et al. (2006) who states that fanyaaju has been stabilized well using desho grass, then minimized runoff velocity in farmlands of Ethiopia’s highland agro-ecologies.

Table 4. Mean percentage of soil aggregate stability under selected desho grass planting position and space on fanyaaju

Stabilizer planting space and position	>2mm	1-2mm	0.5-1mm	0.25–0.5mm	<0.25mm
End of burm + PS of 5 cm	15.15 ± 0.04h	8.20 ± 0.05f	7.55 ± 0.05f	5.59 ± 0.07g	7.64 ± 0.28h
End of burm + PS of 10 cm	18.58 ± 0.26g	10.66 ± 0.31e	8.45 ± 0.39e	8.52 ± 0.34e	9.68 ± 0.23g
End of burm + PS of 15 cm	21.64 ± 0.34f	10.45 ± 0.11e	9.37 ± 0.18d	7.45 ± 0.39f	11.37 ± 0.07f
End of burm + PS of 20 cm	22.71 ± 0.23e	12.58 ± 0.21d	9.55 ± 0.10d	8.47 ± 0.30e	13.45 ± 0.11d
Emba + PS of 5 cm	24.48 ± 0.17d	12.84 ± 0.15d	11.63 ± 0.13c	9.36 ± 0.15d	12.52 ± 0.06e
Emba + PS of 10 cm	26.46 ± 0.11c	14.67 ± 0.11c	13.41 ± 0.43b	10.46 ± 0.12c	14.82 ± 0.06c
Emba + PS of 15 cm	27.61 ± 0.25b	17.40 ± 0.09b	13.72 ± 0.29b	9.75 ± 0.26b	15.49 ± 0.33b
Emba + PS of 20 cm	31.65 ± 0.26a	20.53 ± 0.37a	15.70 ± 0.07a	11.74 ± 0.16a	18.70 ± 0.60a

Mean ± S.E; where S.E. is the standard error. PS= stabilizers planting space, Emba = Embankment. Note: - Means represented with different letters are significantly different at p < 0.05.

3.3. Soil moisture content and biomass of desho grass stabilizer planting spacing and positions on Fanyaaju bund

Statistically significant variation of SMC (%), FBM (g), DBM (g), and Ph (cm) was recorded between treatments at p < 0.05 level of probability. This means SMC (%), FBM (g), DBM (g), and ph (cm) values of the desho grass were influenced by stabilizers planting position and planting space on the fanyaaju (table-5).

3.3.1. Soil moisture content (%)

According to the analysis result, the highest percentage of soil moisture value of 29.32 was observed when the desho grass was planted on top of the embankment with a planting space of 15 cm. Following this, better SMC content values of 23.99, 23.82, and 22.26% were observed for embankment planting with 20 cm, 10 cm, and 5 cm planting space, respectively. This implies that planting the desho on the embankment is an appropriate planting method to conserve soil moisture rather than planting at the end of the burm. This finding agrees with the results of Umer et al. (2019) who states that the combined application of soil conservation bund and grass enhanced soil moisture and other soil properties.

3.3.2. Grass height (Ph)

As shown in Table 5, the high grass height (cm) values 88.00, 87.23, and 88.25 cm were recorded on the End of burm + PS of 5 cm, End of burm + PS of 10 cm, and Emba + PS of 5 cm, respectively. The result showed that stabilizers planted on the end of the burm with 5 and 10 cm spacing have statistically high plant height (cm) values than desho planted on the embankment. This is because the soil and the nutrients of burm were less disturbed than the embankment. In addition, Desho planted nearby (especially with 5 cm and 10 cm spacing) had a dense grass population that attained the maximum height but the minimum grass stem thickness on the fanyaaju.

Table 5. Percentage of soil moisture content and desho grass biomass under selected desho grass planting position and space

Position x Spacing	SMC (%)	Ph (cm)	FBM(g)	DBM(g)
End of burm + PS of 5 cm	16.88 ± 1.29c	88.00 ± 3.92a	1606.3 ± 160.18e	567.2 ± 51.71e
End of burm + PS of 10 cm	21.52 ± 1.29b	88.25 ± 3.92a	3650.0 ± 160.18d	1167.1 ± 51.71d
End of burm + PS of 15 cm	14.78 ± 1.29c	86.50 ± 3.92ab	6900.0 ± 160.18a	1497.9 ± 51.71b
End of burm + PS of 20 cm	15.26 ± 1.29c	68.00 ± 3.92cd	4650.0 ± 160.18c	1338.9 ± 51.71c
Emba + PS of 5 cm	22.26 ± 1.04b	87.23 ± 3.19a	1655.0 ± 130.04e	560.3 ± 41.98e
Emba + PS of 10 cm	23.82 ± 1.52b	67.59 ± 4.62cd	3490.6 ± 188.60d	1171.3 ± 60.89d
Emba + PS of 15 cm	29.32 ± 1.52a	74.93 ± 4.62bc	6523.9 ± 188.60a	1755.1 ± 60.89a
Emba + PS of 20 cm	23.99 ± 1.52b	56.93 ± 4.62d	5408.2 ± 188.60b	1455.7 ± 60.89bc

Source: Own result (2023). Note: - Means represented with different letters are significantly different at p < 0.05. Mean ± S.E. where, S.E. is standard error Emba= embankment, PS= Position, SMC= soil moisture content, FBM= fresh biomass, DBM= Dry biomass, and ph=plant height.

3.3.3. Fresh and dry biomass (g)

The high fresh grass biomass FBM (g) values of 6523.9 and 6900.0 g were observed on Emba + PS of 15 cm and End of burm + PS of 15 cm, respectively. While the lowest values of 1606.3 and 1655.0 g were recorded on End of burm + PS of 5 cm and Emba + PS of 5 cm, respectively. Similarly, higher dry grass biomass in (g) value 1755.1 g was recorded on Emba + PS of 15 cm. However, the lower DBM (g) values of 560.3 and 567.2 g were obtained when desho grass was planted on Emba + PS of 5 cm and End of burm + PS of 5 cm, respectively. This was because desho grass planted on the end of the burm with 15 cm space/Emba + PS of 15 cm experienced lower intra-row competition among plants for moisture and nutrients. The result conveyed that 15 cm and 20 cm desho grass spacings were favorable for biomass growth. Field pictures of desho during its maturity and harvesting is shown in Figure 5.

Figure 5. (a) desho grass that is ready for harvesting at Hawassa Lake watershed. (b) daily laborer harvesting desho grass.

3.4. Cost Benefit and sensitivity analysis

Further economic analysis of agronomically comparable treatments showed that 15 cm + Emba were dominated (D). Hence, the foregoing treatment was excluded from further consideration. However, 20 cm + Emb treatment produced MRRs of 6292, 8389.33, and 5033.6 for baseline, optimistic, and pessimistic scenarios, respectively as shown in Table 6. That means farmers would benefit 62.92, 83.89, and 50.34 cents for every one ETB invested in fanyaaju construction and desho grass planting on the bund. The MRR could increase further if cereals or legumes or perennials are integrated into the conservation system.

Table 6. Economic (partial budget analysis) of the trial planting space and position on Fanyaaju

Variables	Treatment
	10cm+Emb	20cm+Emb	15cm+Emb
Average biomass yield (g/3m)	3490.6	5408.2	6523.9
Adjusted yield (kg/ha)	1163.53	1802.73	2174.63
Gross field benefits (ETB/ha)	1163.53	1802.73	2174.63
Cost of labor (ETB/ha)	700	350	800
Cost of seedling (ETB/ha)	400	200	300
The total cost varies	1100	550	1100
Net Benefit (ETB/ha)	63.53	1252.73	1074.63
MRR (%)		62.92	−32.38
Decision		62.92	D
Optimistic scenario	405	412.5	825
MRR (%)		83.89.33	−43.175758
Pessimistic scenario	675	687.5	1375
MRR (%)		50.34	−25.905455

Assumptions VI between fanyaaju=10 m, 10 kg feed = 10 ETB, 60 ETB=1$, Optimistic scenario- Total cost would be reduced by 25% Pessimistic scenario -Total cost would be increased by 25%.

4. Conclusions

Physical soil and water conservation structures were frequently damaged and lost after construction at the Lake Hawassa watershed, Southern Ethiopia. Although low awareness and motivation of farmers remain in a deriving seat, biological soil, and water conservation factors were also missing. Thus, the integration of physical and biological measures of soil and water conservation remains crucial for stabilization. Correspondingly, farmers of the study area were not planting the desho grass at the right planting position and spacing on the fanyaaju. This resulted in the damage of fanyaaju structures. So, this study was conducted to specify the optimum desho grass planting position and spacing appropriate for bund stabilization and biomass yield at Hawassa Lake Sub-Watershed, Southern Ethiopia. The study was carried out by considering two planting positions (burm and embankment) and four spacing (5, 10.15, and 20 cm) with four replications. The results revealed that desho grass planted on the embankment of fanyaaju with 20 cm space provided higher soil aggregate stability values of 31.65, 20.53,15.70, 11.74, and 18.70% at sieve sizes of >2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm, respectively. The lower soil aggregate stability value was registered on desho grass planted at the end of the burm with 5 cm planting space. Likewise, Desho grass planted on embankments with 15 cm and 20 cm also provided higher soil moisture content of 29.32 and 23.9%, respectively. The highest grass biomass yield of 1755.1 g was also recorded from desho grass planted on the embankment with 15 cm plant spacing. This study specifically concluded that the percentage of soil aggregate stability and soil moisture significantly increased with an increase in planting spacing (5–20 cm) on the embankment. However, a low percentage of soil aggregate stability and soil moisture values were recorded on the fanyaaju bund stabilized with the end of the burm planting.

The organic carbon, organic matter, total nitrogen, and phosphorus contents of the soil were improved due to stabilizing the fanyaaju bund with desho grass. But, the percent composition of texture and pH values couldn’t show notable change. Planting desho grass on fanyaaju embankment with 20 cm spacing produced a marginal rate of return (MRR) of 62.92%, showing robust economic viability over other treatments. The economic benefit from the stabilization of physical soil conservation structures could increase further if cereals, legumes, or perennials are integrated into the conservation system. Therefore, farmers and other stakeholders would practically adopt planting desho on the fanyaaju embankment with 20 cm planting space as the best option at the Lake Hawassa watershed and other areas with similar soil texture, slope, and agro-ecologies. Further studies on the desho grass planting system at fanyaaju are recommended to understand soil stability, moisture dynamics, and desho productivity across wider spacing, clay or silt texture, and greater slope gradient.

Acknowledgments

We gratefully acknowledge the Southern Agricultural Research Institute for supporting our study. We also thank the National Meteorology Agency (NMA) and Ethiopian Mapping Agency (EMA) for providing climate and shape files data, respectively.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data used are available from the corresponding author upon reasonable request.

Additional information

Funding

The author(s) reported that there is no funding associated with the work featured in this article.

Notes on contributors

Zemede Amado Kelbore

Zemede Amado is a soil and water conservation researcher at Southern Agricultural Research Institute (SARI) (Hawassa Agricultural Research Center, Ethiopia). He holds an MSc degree in Watershed Management (Soil and Water Conservation). Zemede Amado has conducted several research activities related to physical and integrated soil and water conservation. He also has experienced in collaborative research with international and national projects. He has an interest to conduct research on the carbon contents of soil and vegetation.