1 Introduction

Teff (Eragrostis teff) is an ancient and highly resilient tropical cereal crop that originated in the northern Ethiopian highlands [1]. It holds a prominent position in the agricultural landscape, constituting 12% of the total food expenditures and standing as the most crucial cereal crop nationwide [2]. Teff cultivation covers a substantial portion of Ethiopia's cultivated land, with approximately 27.02% of the area dedicated to its growth. This significant surge in teff cultivation has turned it into a national obsession, captivating the interest of around 6.3 million farmers [3]. Although teff production offers numerous benefits, smallholder farmers in Ethiopia encounter various challenges that include soil fertility depletion, soil erosion, limited utilization of external inputs, and severe soil acidity [4,5,6].

Inorganic fertilizers have long been employed to address soil nutrient deficiencies and related fertility challenges [7]. However, their widespread use poses environmental risks [8, 9] and negatively impacts beneficial soil organisms crucial for maintaining soil health [10, 11]. Additionally, their high costs render them inaccessible to many economically disadvantaged smallholder farmers [12]. Consequently, the actual yield potential of various food crops, such as teff, a key cereal crop in Ethiopia, falls below average [13]. Hence, there is an urgent necessity to explore environmentally friendly approaches that can enhance soil fertility and crop production.

The utilization of organic amendments such as biochar-based organic fertilizers proves to be highly promising and sustainable in improving soil fertility and crop production. Studies have shown the effectiveness of incorporating organic fertilizers into agricultural soils, leading to notable improvements in soil quality and increased crop yield [14,15,16,17]. Biochar-based organic fertilizers derived from plant or animal sources have a positive impact on soil physicochemical conditions because of their rich organic matter content and ability to balance nutrient levels [18]. Compost and biochar are two examples of organic fertilizers that can be used individually or combined to effectively improve soil fertility [19,20,21]. Research has indicated that the combined application of biochar and compost yields even more positive synergistic effects on soil nutrient contents [14, 19], water holding capacity [22, 23], soil microbial activities, and crop yield [24, 25]. Co-composted biochar, which involves mixing biochar with composting feedstocks during the composting process, has also shown promising results as an organic fertilizer compared to sole biochar or compost [14]. Despite those evident benefits, ongoing uncertainties persist concerning the effects of biochar and biochar-based organic fertilizers on soil fertility and crop production. These uncertainties arise from a variety of factors, including the source of feedstock for biochar production [26], pyrolysis conditions, application rates, biochar mixing ratios, and the diverse characteristics of soil, crop types, and local climates in specific areas [14, 26,27,28]. The existing literature thus presents inconsistent findings, ranging from negative effects with up to a 40% decrease in yield [29] to no significant effects [30] and positive effects with up to a 243% increase in yield [22]. Hence, in-depth research is essential to fully unlock the potential of biochar, and their synergistic interactions with other organic amendments in enhancing soil fertility and fostering plant growth. These investigations will not only advance our scientific knowledge but also pave the way for more efficient and environmentally friendly agricultural practices.

In the Northwestern region of Ethiopia, where this study was conducted, smallholder farmers face significant challenges with soil acidity, which adversely impacts soil health and crop productivity, thereby exacerbating food insecurity and poverty within the region [31,32,33]. While the immediate response has been the use of inorganic fertilizers and lime, these remedies prove ineffective due to their nature and high costs [34]. Utilizing biochar derived from uncommercial charcoal or charcoal left over henceforth referred to as biochar, could emerge as a promising, cost-effective, and environmentally friendly alternative to address soil acidity and enhance crop production in the study area. Smallholder farmers have been actively engaged in Acacia decurrens plantation for charcoal production, resulting in notable improvements in their livelihoods [31, 35, 36], and uses traditionally the leftover uncommercial charcoal for soil fertility management.

In the study area, an estimated annual production of 108,284 tons of charcoal and 4,331.3 tons of uncommercial charcoal/biochar is observed [37]. Ethiopia ranked as the world's third-largest charcoal producer, generates approximately 4.4 million tons of charcoal annually [38], suggesting a 4% contribution of non-commercial charcoal (100,000 tons of biochar) [39]. Nonetheless, a dearth of scientific research exists in our country concerning the potential advantages of Acacia decurrens biochar and its synergistic application for sustainable soil acidity amendment and crop production. The role of biochar in amending acidic soil and enhancing crop production remains largely unexplored. Previous studies on Acacia decurrens plantations were focused on improvements of traditional charcoal production techniques [37], livelihood benefits [31, 40, 41], land use land cover change [40, 42, 43], effects of in situ charcoal production on soil properties [44], and evaluation of soil properties within a rotational system of teff-Acacia decurrens-charcoal production [45]. To the author's knowledge, no previous studies have explored the evaluation of biochar-based organic fertilizer produced from uncommercial charcoal leftover combined with locally available organic residues. This study marks a unique endeavor within the country and existing literature, introducing a novel approach where non-commercial charcoal leftovers are blended with locally sourced organic residues for sustainable acidic soil amendments and crop production. Therefore, this study was initiated with the objective of evaluating the effects of co-composted Acacia decurrens biochar with chicken litter, and cow dung manure in various mixing ratios and application rates on soil physicochemical properties and teff crop yield in acidic soils in the highlands of Northwestern, Ethiopia. This presents a significant opportunity for smallholder farmers to enhance soil fertility and crop yield using biochar-based indigenous fertilizers. Moreover, it can also contribute to the broader field of agricultural research and soil management, potentially leading to the development of more sustainable and efficient farming practices.

2 Material and methods

2.1 Study site

The study was carried out at Injibara University's teaching and research farm, located in an arable field within the Banja District of North West Ethiopia (latitude 10° 50’N, longitude 37° 00’ E) (Fig. 1). The average minimum temperature and maximum temperature of the study area are 9.4 °C and 26 °C, respectively. The region receives an annual rainfall distribution of 800 to 2700 mm, with the primary wet season occurring from June to September, followed by a less pronounced wet period extending until November. The increased rainfall distribution in the study area results in the leaching and depletion of vital soil nutrients, leading to widespread soil acidity. Topographically speaking, the study area is relatively flat and fertile, whose elevations vary from 800 to 2700 m above sea level and characterized by high rainfall.

Fig. 1
figure 1

Source own, 2023

Map of the study area.

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A large proportion of rural households follow mixed subsistence agricultural farming systems. Tef (Eragrostis tef Zucc.), barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), and potato (Solanum tuberosum L.) are the main crops [31, 46]. Approximately 34.02% of the area is covered by forests, consisting of 76,554 hectares of plantations and 277,842 hectares of natural forests [33, 40]. Recently, there has been a notable shift towards Acacia decurrens plantation agroforestry systems, primarily for traditional charcoal production [33]. The predominant soil type in the study area is Acrisoil which has shallow to moderately deep sandy clay loam textures [47] and [48]. Acrisols undergo extensive weathering processes, leading to the formation of distinct horizons with varying levels of mineral and organic matter. Acrisols in the study area tend to have low inherent fertility due to the leaching of nutrients caused by high rainfall and acidic conditions (Table 1). However, they can be productive with proper management practices. It is characterized as acid soil with a pH (4.61), low organic carbon content (1.5%), low level of total nitrogen (0.18%), and with available phosphorus content of 10.77 mg/kg (Table 1). The study area's high soil acidity requires prompt adoption of innovative and eco-friendly methods to boost crop production for smallholder farmers. The left over uncommercial charcoal/biochar in the study area could serve as a valuable resource for addressing soil acidity issues and enhancing crop yields.

Table 1 Initial characteristics of the topsoil (0–15 cm), compost, and biochar based indigenous fertilizers
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2.2 Biochar-based fertilizer production

Biochar production from Acacia decurrens was achieved using the Mark V portable steel kiln. The control of airflow and temperature during pyrolysis in MRV portable steel biochar-making kilns was facilitated through specific design features and operational procedures equipped with adjustable vents and dampers that allow for the regulation of airflow and temperature. To control airflow, we adjusted the openings of the vents or dampers to either increase or decrease the amount of oxygen entering the kiln. This adjustment affects the combustion rate and, consequently, the temperature inside the kiln. Temperature measurements were obtained using thermocouples placed at strategic locations within the kiln chamber. These sensors provide real-time temperature readings, allowing us to monitor the pyrolysis process closely and the current biochar was produced on average at 450 °C. Then the biochar was co-composted with locally available organic amendments to increase both the benefits of biochar and other organic amendments.

Co-composting experiments were made using different ratios of Acacia decurrens biochar (B), chicken litter (CL), and cow dung manure (CM). Four distinct biochar-based indigenous fertilizers (BIF) were established, with weight ratios of biochar: litter: manure set at 100:0:0, 25:75:0, 15:85:0, and 25:0:75. These BIF were composted in composting heaps having a size of 1.5 m in length, 1.5 m in width, and 1 m in depth following the method described by [14]. Compost (C) was also produced by utilizing locally available materials such as dry grass, green leaves, vegetable scraps, and soil bedding as a control. The heap was diligently turned every 3 weeks to ensure proper aeration and decomposition. The BIFs underwent composting for 6 months until they reached the desired level of maturity.

2.2.1 Experimental design and treatment

To set up the experiment, a treatment combination that encompassed various inputs was designed: negative control, recommended inorganic fertilizer (IF), compost (C), Acacia decurrens biochar (AB), 15% biochar + 85% chicken litter (BCLL), and 25% biochar + 75% chicken litter (BCLH), as well as 25% biochar + 75% cow manure (BCM). These treatments were applied at rates of 0, 5, 10, and 20 t ha−1, using an experimental plot size of 1.6 m × 2 m. The treatments were applied in a randomized complete block design (RCBD) with three replications. The dega teff variety was chosen as the test crop for this study. To minimize the impact of edge effects, a separation distance of 0.5 m was maintained between plots, while each replicate was positioned 1 m apart. The incorporation of the biochar-based indigenous fertilizers (BIF) into the soil occurred 21 days before planting, and the seeds of the dega teff variety were row planted (Fig. 2), at the recommended rate of 15 kg ha−1 during the 2022/2023 rainfed season. All other agronomic practices were uniformly applied according to the recommended guidelines for all treatments [49].

Fig. 2
figure 2

Biochar based organic fertilizer application on field trials and teff grown after amendments. Biochar application (a); biochar mixing (b); teff sawing (c); teff grown (d). Photos: © Ewunetu T 2022.

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2.2.2 Determination of soil, compost, and BIFs physicochemical properties

Soil sample from topsoil (0–15 cm) was collected using a soil auger before and after biochar-based organic fertilizer application and analyzed for important soil chemical properties (pH, OC, TN, Ave.P, CEC) and physical properties such as texture, BD, moisture, and porosity. Moreover, compost and BIFs were taken from the respective treatments and analyzed for important chemical properties (pH, OC, TN, Av. P, and CEC) and physical properties such as BD, and moisture. Soil samples were air-dried, crushed by using mortar and pestle, and passed through a 2 mm square-mesh sieve except for total N and OC which were passed through a 0.5 mm sieve [50, 51].

Soil texture was determined using the hydrometer method [52]. Bulk density was determined using the undisturbed core sampling method as described in [53]. The weight difference between wet and dry soil samples was divided by that of the dry sample and then multiplied by 100 to yield the soil moisture content [54]. Soil porosity was calculated from soil bulk density value with an assumed particle density of 2.65 g/cm3.

Supernatant suspension of a 1:2.5 soil and water combination was used to test the pH using a pH meter [55]. Soil organic carbon was determined by the Walkley and Black method [56]. Available phosphors were determined by using the Olsen extraction method [57]. The total nitrogen and cation exchange capacity was determined by the Kjeldahl [58] and ammonium acetate methods [59], respectively.

2.3 Agronomic data collection

In each experimental plot, ten mother plants were randomly selected from the inner net plot size of 2.4 m2 to measure the number of tillers and the height of the plants at physiological maturity. The panicle length, which represents the distance from the node where the first panicle branches emerge to the tip of the panicle, was determined by averaging measurements from ten pre-tagged mother plants chosen randomly in each plot. Grain yield was calculated by harvesting the crop within the net plot area, excluding border effects (one row from both sides). The harvested grain yield was adjusted to a moisture content of 12.5% before further statistical analysis, following the method [49]. To calculate the straw yield, the grain yield was subtracted from the overall above-ground biomass yield, and the grain yield was measured after threshing.

To assess the economic aspects of different biochar-based indigenous fertilizer treatments, economic data was calculated using the approach outlined by the International Maize and Wheat Improvement Center (CIMMYT) [60]. The average yield was adjusted downward by 10% to account for the difference between the experimental yield and the expected yield of farmers using the same treatment. Marketable teff yield was valued based on the current market price during the study season (1 kg = 65 ETBFootnote 1), as obtained from local markets immediately after harvest. In the partial budget analysis, various parameters such as gross field benefit, total variable cost, net benefit, and marginal rate of return were considered.

To identify potentially profitable treatments, a dominance analysis procedure was conducted. This involved ranking the treatments in ascending order based on their total variable cost, from the lowest to the highest. For each pair of ranked non-dominated treatments, the marginal rate of return was calculated as a percentage. The percent marginal rate of return between any two non-dominated treatments represented the return per unit of investment for crop production. A marginal rate of return above 100% was considered necessary for a treatment to be considered a viable option for farmers [60].

2.4 Statistical data analysis

As part of the statistical analysis, the treatments were subjected to an analysis of variance (ANOVA) to ascertain the presence of any notable variances. In instances where significant differences were detected, mean comparisons were performed using Tukey's honestly significant difference (HSD) tests, with the aid of SAS version 9.4. Significant analysis (p-value) was done within the treatment group as well as in each treatment group compared with the control. Origin Pro 2019b software was used to make graphs of results at the study sites.

3 Results and discussion

3.1 Initial physical and chemical properties of the soil, compost, and biochar-based indigenous fertilizers (BIFs)

Table 1 presents the initial physical and chemical properties of the soil in the study, compost, and various indigenous biochar-based fertilizers. The soil at the experimental site had a sandy clay loamy texture, composed of sand (52%), clay (31%), and silt (19%). It exhibited a high bulk density of 1.22 g/cm3 and very strong acidity, with a pH value of 4.61. Additionally, the soil showed low levels of organic carbon (1.5%), total nitrogen (0.18%), and available phosphorus (10.77 mg/kg), along with a cation exchange capacity of 25 cmol ( +)/kg. Due to its sandy clay loam texture and low organic carbon content, the soil displayed poor water retention capabilities in the plow layer, as evidenced by the low moisture content in Fig. 4.

The higher bulk density could impede root penetration and development, resulting in reduced crop root volumes. Consequently, strategies to manage soil fertility are necessary to enhance soil health and crop productivity. Analysis of chemical properties before amendments revealed that biochar had a higher carbon-to-nitrogen ratio (98.5), pH (8.56), and organic carbon content (65%). It also exhibited moderate values for available phosphorus (250.25 mg/kg), total nitrogen (0.66%), and cation exchange capacity (30 cmol ( +)/kg). Therefore, the physicochemical characteristics of Acacia decurrens biochar suggest its potential utilization as a bulking agent for composting or as an amendment for degraded soil [61,62,63]. This could be applied either independently or in combination with compost or inorganic fertilizer for improved results.

Furthermore, BCLL, BCLH, and BCM exhibited notable advantages over compost and sole biochar. The BIFs produced from BCLL exhibited the highest levels of available phosphorus (625.75 mg/kg), total nitrogen (1.24%), and cation exchange capacity (58.02 cmol ( +)/kg), followed by BIF produced from BCLH, and BCM (Table 1). Sole biochar demonstrated higher soil pH and organic carbon content compared to the other treatments. These findings can be attributed to the liming effect of biochar, which increases soil pH, as well as its higher organic carbon content [21, 64]. These results suggest that the inclusion of biochar in compost mixtures can have positive effects on soil properties such as pH and organic carbon content. Furthermore, specific combinations of biochar and organic waste materials, such as chicken litter and cow dung manure, can enhance the availability of essential nutrients like phosphorus and nitrogen, as well as the cation exchange capacity of the soil [14, 19].

3.2 Soil physical properties

The study results show that the lowest bulk density (0.97 g/cm3) was observed in the treatment containing biochar only at a higher rate (20 t ha−1), while the highest bulk density was recorded in the control group with no amendments (1.21 g/ cm3) (Fig. 3). Compared to the control, the findings revealed a significant (P < 0.001) reduction in bulk density (Fig. 3) and a significant improvement in moisture content (Fig. 4) and soil porosity (Fig. 5) with the application of biochar, co-composted biochar, and compost treatments, especially when applied at higher rates (Figs. 3, 4, 5). Overall, the enhancement of soil physical properties followed the order of sole biochar > BCM > BCLH > BCLL > COM > IF > control treatments. The application of 20 t ha−1 biochar resulted in a 20% decrease compared to the control, 19% compared to IF, and a 13% reduction compared to BCLL and BCM amendments.

Fig. 3
figure 3

Effects of different biochar-based organic fertilizers on soil bulk density. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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Fig. 4
figure 4

Effects of different biochar-based organic fertilizers on soil moisture contents. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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Fig. 5
figure 5

Effects of different biochar-based organic fertilizers on soil porosity. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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Other studies have also reported similar findings regarding the positive impact of biochar on soil physical properties such as soil bulk density, moisture content, and porosity [19, 64,65,66,67] According to [68], biochar, characterized by its higher C/N ratio and increased stability against decomposition, forms stable organo-mineral compounds on the surface of soil aggregates. This process leads to decreased bulk density and increased soil moisture content. The combination of biochar and chicken litter organic amendments can improve soil structure by increasing aggregation, reducing compaction, and enhancing pore space. This improvement promotes better water infiltration and root penetration [69, 70]. The combination of biochar and cow dung manure can also improve soil texture by increasing organic matter content and enhancing soil aggregation, leading to better soil structure and tilth [14].

In terms of soil moisture and porosity, the highest values were recorded in the soil amended with the higher application rate of sole biochar, while the control treatment had the lowest values (Fig. 4). The moisture content in the biochar-amended soil at a rate of 20 t ha−1 was 16% higher than that of the control soil. Previous studies have also shown that biochar addition improves soil water retention capacity, penetration resistance, and porosity [71]. The large surface area and porous structure of biochar contribute to increased nutrient and soil moisture retention [61, 72]. The differences in soil moisture content between biochar-amended plots and the control can be attributed, at least in part, to the initial differences in bulk density between treatments [19].

In addition, the study demonstrated that the incorporation of a co-composted biochar containing 15% biochar and 85% chicken litter into the soil resulted in a 2–8% increase in soil moisture (Fig. 4) and higher porosity (Fig. 5) compared to the control group. Previous studies by Pandit, Schmidt [22], Adekiya, Agbede [19], and Bashir, Rizwan [23] have also shown similar improvements in soil moisture with the addition of biochar and manure. Likewise, research conducted by Tessfaw [16] demonstrated the immediate impacts of using Khat-derived biochar and co-composted biochar in the composting process, leading to an enhancement in the soil's water-holding capacity. Soil bulk density also exhibited similar improvements when utilizing co-composted biochar composted with chicken litter, biochar mixed with cow manure, and compost at appropriate application rates when compared to the control group and the treatment containing inorganic chemical fertilizer. These findings align with Bass [73] who reported comparable soil bulk density values when different types of biochar, including co-composted biochar and compost, were applied at various trial points.

It is noteworthy that the impact of biochar-based organic fertilizers on soil physical properties varies. The beneficial effects of biochar-based amendments are more pronounced in coarse-textured soils, whereas they are less significant in clay-textured soils [74]. Comparable observations were made in this study regarding the significant effects of applying biochar-based organic fertilizers on soil moisture and soil porosity in a sandy clay loam soil texture (Table 1). This highlights the potential of biochar as a viable solution in regions characterized by limited rainfall and moisture availability. The combination of biochar and chicken litter improves soil aeration by increasing pore space and reducing soil compaction, promoting better root growth and nutrient uptake [69].

3.3 Soil chemical properties

3.3.1 Soil pH

Figure 6 demonstrates significant differences (P < 0.001) in soil pH values among various organic amendments. Notably, the treatment exclusively applying 20 t ha−1 of biochar exhibited the highest pH value (5.59). The application of biochar alone increased soil pH by 0.99 units. This rise in soil pH resulting from the application of biochar can be attributed to its significantly high pH level of 8.56 compared to other treatments (Table 1). These findings are consistent with previous studies [63, 75,76,77,78], which also observed an elevation in soil pH in biochar-amended soil due to the alkaline nature of biochar, resulting from the accumulation of ash content during the pyrolysis process. According to studies by Tessfaw [79] and Dang [64], the alkalinity of biochar offers advantages to acidic soils. It acts as a liming agent, effectively increasing the pH and reducing the levels of exchangeable aluminum.

Fig. 6
figure 6

Effects of different biochar-based organic fertilizers on soil PH. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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In this study, the combination of 25% biochar with either CM manure or CL in composting resulted in a notable improvement in soil pH, with increases ranging from 0.12 to 0.83 units for the former and 0.15 to 0.81 units for the latter. Moreover, no significant differences were observed between the application rates of biochar composted with chicken litter or cow dung manure, nor between the two different biochar composting mixtures. These findings indicate that both approaches of biochar composting effectively enhance soil pH, and their impact on soil pH improvement is comparable, regardless of the specific organic material used for composting. Our results align with previous research, such as the study by Izilan [63], which also demonstrated high pH values in soils amended with biochar-compost mixtures at the highest application rate of 20 t ha−1, further supporting the positive effect of these amendments on soil pH. Additionally, other studies, such as the one conducted by Dang [64], found that biochar, compost, and lime treatments raised soil pH values by approximately 0.5 units compared to the control. Similarly, the research by Ghosh, Ow [80] reported significant increases in soil pH and electrical conductivity (EC) associated with all compost and biochar treatments compared to the control. It is worth noting that biochar and compost may undergo changes in their characteristics due to the compounds generated during their production processes. For instance, when biochar is derived from wood feedstock, it can exhibit a greater capacity for liming compared to biochar obtained from non-wood feedstocks [81]. This highlights the importance of considering the source and nature of biochar and compost materials in their potential effects on soil pH and overall soil health.

On the contrary, the application of IF resulted in lower pH values (4.51), which were 2% lower than the control (4.60). This decrease in soil pH is likely attributed to the acidifying effects of ammonium-based fertilizers. These findings align with previous studies by Izilan [63], which also attributed the increase in soil acidity to the use of nitrogen (N) fertilizers. The author elucidated that this phenomenon is driven by soil microorganisms, which convert ammonia cations into nitrates. As a result, H+ cations are released as a byproduct, leading to intensified soil acidity, while NO3− is susceptible to leaching.

The implications of this study are significant for sustainable agricultural practices. Biochar and biochar compost applications can serve as viable strategies to improve soil pH and thereby enhance nutrient availability and microbial activity [77]. The findings suggest that incorporating biochar and compost into soil amendment practices can be an effective alternative to traditional lime materials for mitigating soil acidity [25]. However, it is crucial to consider the specific soil types and their buffering capacities when planning such amendments.

3.3.2 Soil Organic Carbon (SOC)

In this investigation, SOC content varied between 1.33% and 3.02%. Soil amendment with biochar, biochar compost, and compost significantly (P < 0.001) enhanced SOC content (Fig. 7). In contrast, the application of IF had no noticeable impact on SOC levels compared to the control. The result of the study revealed that the highest SOC content was observed in the treatment involving 20 t ha−1 of sole biochar (3.02%), followed by 20 t ha−1 of BCLH and BCM. It was observed that the application of medium and higher rates of biochar led to a significant increase in SOC levels by 61–127% compared to the control. This study aligns with previous research that reported soil carbon increases following the application of biochar, compost, and their combinations [17, 63, 77, 78]. In agreement with these findings, significant increases in total organic carbon (TOC) were noted in soils treated with grape pomace biochar or a combination of grape pomace biochar and compost, when compared to the control group [80]. The study underscored that the elevated TOC levels could be linked to the initial TOC content of the particular biochars employed, thus emphasizing the crucial role of biochar selection in maximizing the advantages of enhancing soil carbon [67, 78, 82].

Fig. 7
figure 7

Effects of different biochar-based organic fertilizers on soil organic carbon. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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As the proportion of biochar in the composting mixture rises and higher amounts of biochar, biochar compost, and compost are applied, there is a clear and consistent increase in soil organic carbon levels. The increased soil organic carbon found in biochar is primarily attributed to its recalcitrant carbon composition, which effectively withstands microbial decomposition. This unique quality allows biochar to persist in the soil over an extended period, ultimately playing a significant role in long-term carbon sequestration efforts [82, 83]. In this study, we employed biochar derived from woody biomass, characterized by a substantial lignin content. According to Yu and Hu [84], biochars with a high lignin content exhibited decreased CO2 mineralization compared to those derived from feedstocks abundant in cellulose, suggesting a stronger priming effect on the soil. Furthermore, the inclusion of biochar in compost effectively enhanced the carbon content, resulting in a subsequent increase in soil organic matter levels.

The significant increase in SOC content observed in our study demonstrates the positive impact of biochar, biochar compost, and compost amendments on soil organic carbon accumulation. The findings highlight the potential of these amendments for enhancing soil fertility and carbon sequestration in agricultural systems, thus warranting further investigation and consideration in sustainable soil management practices [77, 83]. On the other hand, the lack of significant effect on SOC from inorganic fertilizer application implies that alternative strategies, such as biochar-based organic fertilizer and compost amendments, may be more effective in promoting SOC accumulation in agricultural soils.

3.3.3 Total Nitrogen (TN)

The application of the 20 t ha−1 treatment, consisting of BCLL, resulted in the highest TN content of 0.27%, while the control treatment exhibited the lowest TN content of 0.17% (Fig. 8). Except for the treatment involving 20 t ha-1 (15% B + 85% CL), there were no significant variations observed in TN content among the other treatments. Surprisingly, applying a rate of 10 t ha-1 (15% B + 85% CL) resulted in higher total nitrogen (TN) compared to the treatments using 20 t ha-1 (25% B + 75% CL), 20 t ha-1 (25% B + 75% CM), and 20 t ha-1 of compost (Fig. 8). The increase in TN content appeared to be directly associated with the higher initial TN contents of the treatments employed in the study, as shown in Table 1. The rise in TN content may be also attributed to the beneficial capacity of biochar compost to retain essential nutrients, including nitrogen, within the soil. Chicken litter contains higher nutrients such as nitrogen and phosphorus, which can be prone to leaching or volatilization [85]. When combined with biochar, these nutrients are retained within the soil, reducing the risk of environmental pollution and increasing their availability for plant uptake [85]. Similar to chicken litter, cow dung manure is rich in nitrogen, resulting in increased soil nitrogen levels when combined with biochar [86]. The organic amendment in the compost works synergistically to enhance nutrient accessibility for plants by reducing leaching and increasing the soil's nutrient-holding ability [78]. Biochar alone may not directly increase soil nitrogen levels as it has a low nitrogen content but it can enhance nitrogen retention by reducing leaching and increasing microbial activity, which aids in nitrogen cycling.

Fig. 8
figure 8

Effects of different biochar-based organic fertilizers on soil total nitrogen. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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The combination of biochar and compost can increase soil nitrogen levels due to the addition of nitrogen-rich organic matter from compost [14]. Several previous studies have also explored the effects of biochar and its combined application on soil TN levels. For instance, Schulz [87] reported a significant correlation between the application of composted biochar and increased soil TN levels, which also showed positive associations with plant height and dry matter yield. Similarly, in a pot experiment, Kang [71] found that the application of all types of biochar-based organic fertilizers resulted in elevated soil TN levels compared to the control group. Agegnehu et al. (2016) observed that the application of biochar and compost led to a soil TN increase ranging from 14 to 29%.

Moreover, the utilization of biochar, either on its own or in higher proportions (25%) blended with organic amendments, resulted in a noteworthy rise in soil pH and SOC levels. Nevertheless, these treatments were accompanied by the lowest overall nitrogen content (Fig. 8) when compared to the application of a mixture comprising 15% biochar composted with a blend of CL and CM. The lower TN content observed in the soil amended solely with biochar can be attributed to its carbon-dominated composition, which typically contains a small percentage of nitrogen by weight [14]. Biochar application can affect nitrogen availability through two mechanisms: soil mineral nitrogen immobilization due to the introduction of labile carbon and increased carbon-to-nitrogen ratio, and nitrogen fixation through absorption [88]. Both mechanisms can potentially reduce the availability of plant-available nitrogen derived from plant-based biochar [89].

3.3.4 Available phosphorus

When compared to the control group, the inclusion of 25% co-composted biochar with 75% chicken litter led to a notable increase in available phosphorus, ranging from 51 to 101% (Fig. 9). Similarly, the incorporation of biochar cow dung manure compost resulted in a 61% to 86% increase in available phosphorus, while sole compost improved available phosphorus levels within the range of 47–72%. Furthermore, when 15% biochar was co-composted with 85% chicken litter, the effects were even more significant, resulting in an increase of 85% to 132% in available phosphorus (Fig. 9). These findings are consistent with previous studies, which also reported increased soil available phosphorus with the application of biochar, co-composted biochar, and compost [14, 16, 25]. Additionally, research on post-trial soil demonstrated that biochar-compost mixtures significantly increased available phosphorus, and improved soil cation exchange capacity (CEC), and pH [22]. Other studies also showed that the application of biochar compost amendments increased available phosphorus in various soils [29, 79, 90], with specific biochar-compost combinations leading to higher phosphorus content. Likewise, when animal bone char and lignocellulose agricultural waste were co-pyrolyzed together, along with bio-augmentation, there was a significant enhancement of 133–167% in P solubility observed, especially at lower production temperatures [65].

Fig. 9
figure 9

Effects of different biochar-based organic fertilizers on soil available phosphorus. C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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The observed increase in phosphorus availability with biochar amendments can be attributed to two primary factors. Firstly, organic amendments generally contain higher initial levels of available phosphorus (Table 1). Secondly, the addition of biochar to compost enhances the sorption and retention of phosphorus within the composting material [25]. The same author explained the rise in available phosphorus with biochar-based organic amendments is linked to decreased Fe and Al activities caused by the increased soil. These findings bear significant implications for agricultural systems and soil fertility. The incorporation of co-composted biochar amendments shows remarkable potential in enhancing phosphorus availability in the soil, offering valuable insights for farmers and practitioners seeking to optimize crop productivity. Additionally, the adoption of organic amendments, such as biochar compost, presents sustainable alternatives to environmentally harmful inorganic fertilizers, thus promoting the adoption of eco-friendly farming practices.

3.3.5 Cation Exchange Capacity (CEC)

Organic amendments had a considerable impact on the soil's Cation Exchange Capacity (CEC), as depicted in Fig. 10. Notably, the application of inorganic fertilizer (IF) did not significantly increase the soil's CEC, suggesting that relying solely on inorganic fertilizers may not suffice to enhance CEC levels in the soil. In contrast, biochar cow dung manure (BCM) and compost both showed substantial improvements, with CEC increases ranging from 24 to 53% and 22% to 39%, respectively, compared to the control (Fig. 10). The most significant enhancement in CEC was observed when using biochar chicken litter (BCLL), with an impressive increase ranging from 26 to 87% compared to the other treatments. The chicken litter contains organic matter and nutrients that can increase soil CEC when co-composted with biochar. The combination enhances the soil's ability to hold onto positively charged ions, thus improving nutrient availability for plants [21]. The higher CEC resulting from biochar-chicken litter and cow dung manure co-composting helps retain nutrients in the soil, reducing leaching and nutrient runoff. This contributes to improved soil fertility and sustained crop productivity. In previous research, it was reported that the most substantial increase in CEC was observed in the soil treated with a biochar-based organic amendment [50]. In a similar vein, the study conducted by Tessfaw [16], revealed that the compost enriched with 5% khat-derived biochar exhibited the highest Cation Exchange Capacity compared to the control compost (compost without khat-derived biochar). The author further elucidated that the heightened Cation Exchange Capacity observed in the composts enriched with biochar can be ascribed to the significant occurrence of essential basic cations, which naturally exist in the khat-derived biochar.

Fig. 10
figure 10

Effects of different biochar-based organic fertilizers on cation exchange capacity. CEC cation exchange capacity, C control, IF inorganic fertilizer, BCLL 15% biochar co-composted with 85% chicken litter, BCLH 75% biochar co-composted with 75% chicken litter, BCM 75% biochar co-composted with 75% chicken litter, Error bars mean ± SE

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Biochar, when co-composted with compost, can increase soil Cation Exchange Capacity (CEC) due to its high surface area and porosity. This allows biochar to adsorb cations such as calcium, magnesium, potassium, and ammonium, effectively increasing the soil's capacity to retain and exchange these nutrients. Furthermore, the carbon compounds within the biochar facilitated the retention and release of nutrients, thereby increasing the soil's ability to retain positively charged ions. These results indicate that the co-application of biochar and organic materials rich in nutrients and organic matter can lead to enhanced CEC, contributing to more efficient and sustainable agricultural systems. It is interesting to note that the application of inorganic fertilizer did not lead to a significant increase in the soil's CEC. This result suggests that relying solely on inorganic fertilizers may not be sufficient to improve CEC levels in the soil.

3.4 Plant height, panicle length, and tiller number

The application of biochar-based indigenous fertilizer had a significant (P < 0.001) positive impact on various aspects of tef plant growth, including plant height, panicle length, and tiller number (Table 2; Fig. 11). Plots treated with 20 t ha−1 (BCLL) exhibited the most remarkable growth, with the tallest average plant height (96.53 cm), highest panicle length (45.80 cm), and highest tiller number (25.57). In contrast, the control group had the lowest values for plant height (41.80 cm), panicle length (21.43 cm), and tiller number (10.57). These findings provide valuable insights into the effects of biochar-based indigenous fertilizers on tef plant growth. The significant increase in plant height, ranging from 41 to 108%, upon the application of biochar, indicates its potential to enhance the growth of the plants. Similarly, the use of compost led to plant height enhancements of 64% to 113%, suggesting the beneficial effects of organic amendments on plant growth. The combined utilization of BCLL resulted in even more impressive outcomes, with plant height increases ranging from 107 to 130% (Fig. 11).

Table 2 Effects of biochar based indigenous fertilizer application on Tef yield components
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Fig. 11
figure 11

Field tef crop grown after BIF amendments

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The study highlights the synergistic effects of biochar and compost, which can lead to substantial improvements in tef plant growth. Co-composting biochar with chicken litter contributes to increased plant height. The rich nutrient content of chicken litter, combined with the enhanced nutrient retention and soil structure provided by biochar, promotes vigorous vegetative growth and taller plants [69]. The findings of our study align with prior research [25, 63, 90] which consistently demonstrates notable increases in plant height when biochar and compost are used as soil amendments. Similarly, in another study combining straw mulch with biochar treatment has demonstrated the potential to enhance various growth parameters of rainfed maize, including plant height, stem diameter, and leaf area index [91]. This enhancement can be attributed to the application of biochar, which effectively improves soil composition, fertility, carbon content, and water retention capacity, thereby enhancing soil efficiency [91]. Mensah [25] also observed heightened plant heights in two maize varieties cultivated in Ghana, attributing this growth boost to the rise in soil pH that likely improved nutrient availability, particularly in soils treated with both biochar and compost. In another experiment conducted by Manolikaki and Diamadopoulos [90] using loam and sandy loam soils, the application of biochar alone and in combination with compost significantly increased both maize plant height and stem diameter. The utilization of straw mulch and biochar has also a positive impact on the dry weight of maize leaves, stems, and roots during various growth stages, including seedling, jointing, tasselling, filling, and maturity [78].

Surprisingly, the application of 10 t ha−1 (BCLL) showed marginally greater plant height and tiller number compared to certain other treatments, such as 20 t ha−1 (BCM), 20 t ha−1 (BCLH), 10 t ha−1 (BCLH), and 20 t ha−1 compost alone (Table 2). However, these differences were not statistically significant. The higher nutrient content in the (BCLL) treatment may have contributed to its positive impact on plant growth compared to the other treatments (Table 1; Figs. 6, 7, 8, 9). The findings presented in Table 3 support the observed results, indicating that a lower ratio (15%) of biochar in composting when combined with other additives, led to higher levels of essential soil nutrients that promote plant growth. Key soil nutrients such as total nitrogen (TN), available phosphorus (Ave.P), and cation exchange capacity (CEC) exhibited more significant effects compared to the mixture containing a higher ratio (25%) of biochar. This suggests that the proportion of biochar co-composting may influence nutrient availability in the soil solution and nutrient uptake by plants.

Table 3 Effects of biochar based indigenous fertilizer application on Tef yield
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Interestingly, the rate of application of treatments demonstrated a linear correlation with the improvement of tef growth components. However, the percentage of biochar co-composting did not consistently correspond to this correlation when increased from 15 to 25% in conjunction with other organic amendments. This indicates that the carbon-to-nitrogen (C: N) ratio and total nitrogen (TN) content in pure biochar may play a role in nutrient availability in the soil solution and nutrient uptake by plants when used in higher composting mixtures. The significant positive impact of biochar-based indigenous fertilizer on tef plant growth has important implications for sustainable agriculture. The combination of biochar and compost holds promise as an effective approach to enhance crop productivity, particularly for tef cultivation.

3.5 Crop yield improvements through biochar-based organic amendments

The use of biochar, compost, and co-composted biochar resulted in a significant (P < 0.001) increase in teff yield and yield component (Table 3). The highest teff grain yield (2.82 t ha−1) and dry biomass yield (16.67 t ha−1) were achieved with the application of 20 t ha−1 of BCLL while the lowest teff grain yield (0.68 t ha−1) and dry biomass yield (7.50 t ha−1) were observed in the control treatment. BCLL, BCLH, BCM, compost, and sole biochar amendments resulted in impressive crop yield boosts ranging from 116 to 315%, 103–299%, 103–268%, 47–179%, and 32–125%, respectively, compared to the control treatment. Notably, all amendments exhibited yield increases falling within the range of 32–315%. A study [90] also showcased significant enhancements in maize's total aboveground dry weight of 75% increase with compost addition, 98–155% enhancements with biochar amendments, and 327–436% boosts with biochar combined with compost mixtures in loam soil. The combined application of biochar and inorganic fertilizer has resulted in increased yields in the maize-soybean intercropping system [92]. The current findings are also consistent with previous studies that reported higher crop yields from chicken litter composted biochar compared to sole biochar and the control treatment [16, 21, 69, 86, 93]. A field study by [14] shows the application of co-composted biochar with an organic source of fertilizer resulted in yield increases of up to 60% in tropical Ferralsol. The study by [86] also reported that biochar application on degraded tropical sandy soil in Nigeria increased cocoyam production by 6–32%, but when used in conjunction with co-composted poultry fertilizers, cocoyam yield increased by 73–130%.

Biochar compost mitigates the high mineralization rate of soil organic matter, phosphorus deficiency, and aluminum toxicity, and significantly improves crop yields in most tropical soils [94]. The impact of biochar alone on crop yield may vary depending on soil and environmental conditions. It can improve yield by enhancing soil structure, water retention, and nutrient availability, but the effect may not be as significant as when combined with organic amendments [21]. The advantages hold particular significance in the case of severely degraded tropical soils, like those prevalent in Ethiopia. The extensive agricultural practices in this region have resulted in a significant depletion of organic matter, making these benefits all the more crucial. Therefore, this study enhances knowledge on biochar-based fertilizers in sustainable agriculture, bolstering food security, and addressing environmental challenges. However, further research is needed to fully comprehend the long-term effects on soil health, optimize co-composting ratios, and fine-tune application strategies to maximize plant growth benefits.

In our study, the improvement in teff yield from composted biochar with chicken litter and cow dung manure can be attributed to reduce nutrient leaching and increased nutrient holding capacity of the soil, as well as improved nutrient use efficiency resulting from the combined biochar application [17, 91, 94]. Furthermore, a significant improvement in teff yield from biochar composted with chicken litter and cow dung manure as well as compost aligned with the initial soil condition, which had low nutrient contents such as organic carbon, nitrogen, and available phosphorus (Table 1), as well as the subsequent improvement in physical (Fig. 4) and soil chemical properties after amendment (Fig. 5). Biochar can help buffer soil pH, while chicken litter and cow dung manure provide nutrients and organic matter, collectively contributing to maintaining optimal soil pH levels for plant growth. The superior performance of 15% biochar co-composted with 85% chicken litter in enhancing teff growth and nutrient status can be attributed to its significantly (P < 0.001) higher total nitrogen (TN), available phosphorus (P), and cation exchange capacity (CEC) compared to the other amendments. This can be also attributed to the beneficial effects of biochar when combined with composting, as its low density and porosity improve aeration, water infiltration, and water retention, thereby capturing essential nutrients for plant growth [93].

The positive effects of biochar application on plant growth, particularly in tropical soils, are most pronounced when combined with organic or inorganic fertilizers, as it help retain nutrients [16, 19, 91, 92, 95]. While biochar itself is deficient in essential plant nutrients and has a high C: N ratio, its significant effects are observed when it is combined with other organic amendments [21]. In this study, significant positive effects were observed for straw yield when using the same treatment, with 20 t ha−1 of co-composted biochar (15% B + 85% CL), resulting in an impressive 90% increase. It is important to note that the yield values in our study are higher compared to some previous studies [51, 96, 97], but lower compared to others [49, 76]. The variation in grain yield among studies can be attributed to differences in soil characteristics, climate, crop variety, nature of the amendment used, and management practices across different locations [98].

3.5.1 Economic profitability of the different biochar-based organic fertilizers

A partial budget analysis was carried out to evaluate the financial advantages of adopting a modification in farming methods. To account for the disparities between the actual yields obtained from experimental plots and the anticipated yields from farmers utilizing the same approach, the average yield from on-farm experimental plots was decreased by 10% [96]. The findings of the partial budget analysis for various treatments revealed that the application of 10 t ha−1 of BCLL resulted in the highest marginal rate of return (358.41) compared to the other treatments (Table 4).

Table 4 Partial budget analysis of the different biochar-based indigenous organic fertilizers
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Moreover, the findings of the study reveal that utilizing 20 t ha−1 of co-composted biochar, specifically with BCLL, resulted in the highest yield (2.82 ± 0.03 t ha−1) and benefit (5093.9 USD ha−1). Nevertheless, despite these favorable outcomes, the economic profitability of this approach was comparatively lower than when applying the treatment at 10 t ha−1 (4924.2 USD ha−1). Based on these findings, it is recommended that farmers in the study area and similar sub-tropical highlands opt for the application of 10 t ha−1 of co-composted biochar, consisting of BCLL. This particular combination offers a favorable balance in achieving optimum teff yield while also providing a high marginal rate of return that exceeds the proposed minimum acceptable rate. By adopting this modified farming method, farmers can potentially enhance their financial performance and improve their overall profitability. The findings of the partial budget analysis indicate that certain biochar-based treatments offer favorable economic returns, making them attractive options for smallholder farmers. This can have implications for rural livelihood improvement and poverty reduction.

Consistent with this research, prior studies have demonstrated that biochar compost not only demonstrates its positive influence on environmental sustainability and cost-effectiveness but also offers prospects for enhancing food crop production, safeguarding the environment, mitigating poverty, and contributing to the attainment of sustainable development goals established for the millennium [20]. Furthermore, Sharma [70] has shown that the combination of biochar and compost in a subtropical Inceptisol yielded higher net profits compared to using compost or biochar independently. The same study underscored the remarkable benefits of integrating biochar with farmyard manure and vermicomposting, resulting in improvements in soil hydro-physical properties, water use efficiency, financial returns, and knolkhol yield. A study conducted in Hau Giang Province, Vietnam [64], observed that the utilization of biochar and compost treatments led to an extra income of USD 3,146 and USD 3,096, respectively, in comparison to the control treatment.

4 Conclusion

In conclusion, the application of biochar, co-composted biochar, and compost has demonstrated numerous advantages in mitigating soil acidity and improving soil physicochemical properties and crop production. Soil amendment with co-composting of 15% of biochar with 85% of chicken litter applied at (20 t ha−1) rates significantly amplifies the positive outcomes on soil fertility and teff yield. The proportion of biochar combined with chicken litter and cow dung manure directly impacts teff yield, with lower percentages of biochar resulting in higher yields. Furthermore, the synergistic impact of biochar co-composting is more pronounced when combined with chicken litter compared to cow dung manure. In every instance, as the treatment rates increased, both the enhancement of soil physicochemical properties and crop yield saw significant improvements.

An economic analysis has proven that treating the soil with 10 t ha−1 of co-composted biochar, consisting of 15% biochar and 85% chicken litter, generates a higher marginal rate of return, making it financially viable for farmers. Therefore, we highly recommend this specific treatment for farmers in the study area and similar subtropical highlands. This has implications for scaling up the intervention to benefit a larger population of farmers facing similar soil fertility challenges. It is crucial to enhance farmers' knowledge and provide training on these innovative soil fertility management techniques to harness potential synergies and effectively address soil fertility challenges.