Co-application of compost and biochar improves soil properties and Desho grass growth on acidic soils in a tropical environment of Southwestern Ethiopia

Abstract

Organic soil amendments offer promising potential to improve soil properties and plant growths. However, data on these benefits of compost and biochar in a tropical environment are limited. We investigated the impacts of compost (C) and coffee-husk biochar (B) sole- and co-application at various amounts (0%C:0%B, 0%C:100%B, 100%C:0%B, 25%C:75%B, 50%C:50%B, 75%C:25%B and 100%C:100%B) on soil properties and Desho grass growth on acidic soils in a tropical environment. Soil texture, pH, CEC, organic matter (OM), organic carbon (OC), total nitrogen (TN) and available phosphorus (AP) plus plant growth parameters were measured. Both sole- and co-applications of compost and biochar had positive effects on soil properties and plant growth. Compost and biochar co-application at full rates showed the highest soil property and plant growth improvements relative to control followed by co-application at half rates for TN, AP and plant growth and biochar sole-application for pH, OM, OC and CEC. However, the sole-application of compost and biochar showed the lowest increases relative to control in pH, CEC, OM and OC, and in AP and plant growth, respectively. Overall, compost and biochar could be co-applied at full rates on acidic soils in a tropical environment to improve soil property and plant growth.

Keywords:

• available phosphorus
• coffee husk
• dry weight
• number of tillers
• organic carbon
• soil pH

1. Introduction

Currently, there is a growing interest in using compost and biochar either as alternatives or integrations of inorganic fertilizers to improve or maintain soil quality and crop production and to reduce environmental pollutions due to excessive use of inorganic fertilizers. Compost is a rich source of nutrients with a high organic matter content. It improves soil properties (e.g., bulk density, porosity, water conductivity, water retention capacity, cation exchange capacity, organic carbon and nitrogen contents, biological function and mineralization) that can ultimately increase plant growth and crop yield (Bass et al., 2016; Carter et al., 2004; Diacono & Montemurro, 2010; Fischer & Glaser, 2012). It has been reported that compost increases N uptake, foliar N content, turf quality and growth of Kentucky bluegrass (Johnson et al., 2006) and crop yield (Diacono & Montemurro, 2010). Being rich in carbon, biochar is also a good soil amendment, particularly for low potential soils, such as acidic or infertile soils (Ippolito et al., 2012). It improves soil quality, plant growth and crop yield by increasing soil pH, cation exchange capacity, porosity and moisture holding capacity (Duku et al., 2011); retaining nutrients (Paz-Ferreiro et al., 2014); stimulating beneficial soil fungi and microbes (Huang et al., 2023); enhancing fertilizer efficiency (Asai et al., 2009); and improving plant nutrient uptake and water use (Glaser et al., 2002). It has been reported that biochar increases maize grain yield (Major, 2010) and biochar and/or compost increases plant height, stem girth and dry matter yield of maize (Mensah & Frimpong, 2018). Biochar co-applied with N fertilizer on a light soil increased barley seed yield by 30% compared to only N fertilizer (Gathorne-Hardy et al., 2009); and on different soil types under different agro-climatic conditions it increased maize biomass yield by 270–350% compared to NPK fertilizer use (Zhu et al., 2015). Similarly, coffee-husk biochar co-applied with P fertilizer on a tropical Nitisol significantly improved soybean seed yield (Asfaw et al., 2019) and rice-husk biochar co-applied with bio-fertilizers improved soil quality and rice productivity (Danapriatna et al., 2023). Acacia tree biomass-derived biochar co-applied with farmyard or poultry manure and mineral P fertilizer in a semi-arid climate also significantly increased maize productivity and P use efficiency than the sole organic or inorganic P fertilizer (Arif et al., 2021).

Some studies (Agegnehu et al., 2016; Fischer & Glaser, 2012; Liu et al., 2012; Manolikaki & Diamadopoulos, 2019; Schulz & Glaser, 2012) also showed synergetic impacts of compost and biochar co-application on soil properties, plant growth and crop yield. It also reported that the co-application of biochar with inorganic and/or organic fertilizers is a sustainable and environmentally friendly solution to improve soil fertility, plant nutrient availability, plant growth and crop yield (Oladele et al., 2019). Moreover, in the aim of the widespread adoption and integration of biochar with farming operations, formulations that combine biochar with inorganic and/or organic fertilizers are likely to have a high nutrient-use efficiency and to be the most cost-effective treatment (Joseph et al., 2021). Yet, data on the effects of compost and biochar co-application on soil properties, plant growth and crop yield in general and field-based evidence on grasses or pastures under acidic soil and/or tropical environmental conditions in particular are scant. Grasses have a high nutrient requirement, especially N and due to fixation and leaching, acid soils or the soils in the humid tropics are deficient in some major nutrients, e.g., N, P, K and Ca, but high in Al and Fe toxicity. This can be alleviated by biochar and compost co-application.

In Ethiopia, a large amount of coffee husk (780,000–912,000 t) is generated annually and it is not always properly disposed of or utilized (Asfaw et al., 2019; Worku, 2023) except using it as organic mulch or fertilizer by some large coffee farms. However, application of raw coffee husk on soils in the humid tropics may not increase the desired soil properties for cop production due to a high rate of organic matter decomposition. So, conversion of this biomass waste into biochar and using it as soil amendment can reduce the waste volume, improve the physicochemical properties of the soil, enhance the nutrient use efficiency of crops and correct soil acidity (Asfaw et al., 2019). In this regard, Dume et al. (2015) characterized biochar produced from coffee husk at 350°C and 500°C and found a high surface area, organic carbon, total N, available P, cation exchange capacity, alkaline pH and base cation concentrations. This is supported by a recent study (Ngalani et al., 2023, 2023) reporting an increase in soil pH, electrical conductivity, available P and organic carbon, and a drastic decrease in exchangeable Al and Fe following the applications of coffee-husk and cacao-pod biochars on acidic soils in West Cameroon. These findings suggest that coffee-husk biochar is an important soil amendment that could be used in tropical acid soils. However, evidence for its suitability when it is co-applied with organic fertilizers, such as compost, under a tropical environment is rare. Hence, data on the synergetic impact of coffee-husk biochar and compost co-application on soil properties and plant growth is needed for the effective use and integration of coffee-husk biochar in the cropping systems. It is also important to know whether the synergetic impact of biochar co-application with compost vary with the amount of each material in the mixture or not. In order to answer these questions, we investigated the influences of compost and coffee-husk biochar applied alone and co-applied at various proportions on soil properties and Desho grass plant growth on acidic soils in a hot sub-moist tropical environment of Southwestern Ethiopia. Desho grass (Pennisetum pedicellatum Trin.) is a perennial grass, indigenous to Ethiopia, and it is palatable for cattle, sheep and other grazers and suitable for intensive management, multiple cutting and soil conservation. But, the grass requires a high fertilizer input (Asmare, 2016; Leta et al., 2013).

2. Materials and methods

2.1. Description of the study area

A field experiment was carried out in the 2019 main cropping season at Jimma University, Institute of Technology, in Southwestern Ethiopia (7°41’N, 36°48’ E, 1850 m asl). The long-term mean total annual rainfall of the area is 1544.5 mm with 67% mean monthly relative humidity, and 27.6 and 11.7°C mean daily maximum and minimum temperature, respectively (Gemeda et al., 2021; Worku & Astatkie, 2011). The soil texture of the site is clayey with a pH of 4.2, which is extremely acidic. Nitosols is the dominant soil type in Southwestern Ethiopia, and it covers 12% of the land surface of Ethiopia. Southwestern Ethiopia is the largest coffee producer in Ethiopia, both in terms of cultivation area and production volume of coffee. In this region, a coffee-based livestock mixed farming system is dominant over other farming systems. Natural pastures, crop residues, stubble grazing and roadside grasses, in that order, are the main feed resources. Free grazing is the most predominant feeding system, and there is no forage cultivation in the region. However, farmers often experience feed scarcity, especially in the dry season and feed shortage is one of the major constraints to livestock production in the region (Duguma & Janssens, 2021).

2.2. Preparation and chemical properties of compost and biochar

The compost and biochar used in this study were prepared from herbaceous plants and chicken manure, and coffee husk, respectively, at Jimma University College of Agriculture and Veterinary Medicine (JUCAVM). Following the composting methods described in Misra et al. (2003), the compost was prepared by filling the herbaceous plants (after properly chopping and mixing) and poultry manure into a shaded composting pit in the alternate layers, A thin layer of topsoil was also spread over each manure layer as source of decomposers (e.g., microorganisms and earthworms) and sticks were staked in the pile in order to reduce temperature and moisture in the pile by removing the sticks when it was too hot and wet. Moreover, the pile was watered to keep its moisture content at optimum level (ca. 50–60% and 30% at the beginning and end of composing, respectievly) and coved with banana and enset leaves to protect it from rain and raise its temperature. To ensure adequate ventilation and uniform decomposition, the pile was also turned and mixed every month and a complete decomposition was obtained four months after piling up. The biochar was prepared using a pyrolysis unit at 350°C pyrolysis temperature and 3 h residence time, and watering for cooling. The prepared biochar was grinded and sieved through a 0.25 mm square-mesh sieve (Dume et al., 2015).

The compost had a pH of 7.2, a cation exchange capacity of 25.0 cmol(+) kg⁻¹ and a C/N ratio of 4.7. Its organic matter, organic carbon, total N and available P contents were 7.3%, 4.2%, 0.9% and 103.7 ppm, respectively. The biochar had a pH of 9.6, a cation exchange capacity of 64.8 cmol(+) kg⁻¹ and a C/N ratio of 11. 9. The organic matter, organic carbon, total N and available P contents of the biochar were 28.4%, 16.5%, 1.4% and 9.8 ppm, respectively.

2.3. Treatments and experimental design

The treatments consisted of different proportions of the combination of the full rates of compost (4.5 t ha⁻¹) and biochar (5 t ha⁻¹) for the study area: 0 t ha⁻¹ compost (C) and 0 t ha⁻¹ biochar (B) (0%C:0%B, control), 4.5 t ha⁻¹ compost and 0 t ha⁻¹ biochar (100%C:0%B), 0 t ha⁻¹ compost and 5 t ha⁻¹ biochar (0%C:100%B), 3.375 t ha⁻¹ compost and 1.25 t ha⁻¹ biochar (75%C:25%B), 2.25 t ha⁻¹ compost and 2.5 t ha⁻¹ biochar (50%C:50%B), 1.125 t ha⁻¹ compost and 3.75 t ha⁻¹ biochar (25%C:75%B) and 4.5 t ha⁻¹ and 5 t ha⁻¹ (100%C:100%B). These treatments were arranged in a randomized complete block design (RCBD) with three replications (blocks). Each experimental unit had an area of 3 m × 4 m with 1 and 1.5 m pathways between plots and blocks, respectively.

After thoroughly mixing with each other, each proportion of compost and biochar was added and thoroughly mixed with the topsoil of each of the respective plots. Two weeks after compost application, Desho grass root splits (suckers) of uniform size were transplanted with inter- and intra-row spacing of 50 and 25 cm, respectively. Each plot contained eight rows of grass, each with 12 plants. Weeds were controlled with frequent hand weeding throughout the experimental period. The accession of the Desho grass used in the study is named Kulumsa DZF # 592 and obtained from the Holota Agricultural Research Center, Ethiopia.

2.4. Response measurements

The soil characteristics of the experimental site before and after the experiment, and the growth of Desho grass after 60, 90 and 120 days of growth (transplanting) and after 30 days of regrowth (30 days after the first harvest) were measured.

2.4.1. Soil analysis

The composite soil samples were collected from nine spots of the entire experimental field and from five spots of each experimental unit before and after the experiment, respectively. In both experimental field and unit, the soil samples were taken in a diagonal pattern and from 0 to 30 cm soil depth by using an auger. After air-drying, each soil sample was crushed by using a mortar and a pestle and sieved by using a 2 mm sieve in order to remove particles greater than 2 mm, such as gravels and stones, and analyzed for soil texture, pH, organic matter (OM), organic carbon (OC), cation exchange capacity (CEC), total nitrogen (TN) and available phosphorus (AP) at the Soil Science Laboratory of JUCAVM.

Soil texture was determined by a mechanical analysis using a pipette method after removing soil organic matter and carbonates using hydrogen peroxide (30% H₂O₂) and HCl (1 M), respectively, and then dispersing the soils with sodium hexametaphosphate solution (4% (NaPO₃)₆). Sand was separated from silt and clay with a 50 µm sieve, and the silt and clay fractions were determined by the pipette method (SRDP, 2014; van Reeuijk, 2002). After calculating the percentage of each soil particle in dry weight basis, soil samples were categorized into a particular textural class by using a textural triangular method. The pH of the soil was measured by using a glass electrode pH meter in a 1:2.5 soil to distilled water suspension. The OC content was determined according to a wet digestion method following a procedure described by Walkley and Black (1934) and the OM content was determined by multiplying the OC content by a factor of 1.724. CEC was determined by an ammonium acetate method after percolating the soils with ammonium acetate (1 N NH₄C₂H3O₂, pH 7.0) and washing out the excess ammonium acetate with alcohol (ethanol, ca. 80%, pH 7.0) followed by percolating with sodium chloride (10% NaCl) (SRDP, 2014; van Reeuijk, 2002). The TN content was determined by using a Kjeldahl method after oxidizing the OM in 0.1 N H₂SO₄ as described by Black (1965). The AP content was determined following a Bray I method by extracting it with 0.03 M NH₄F and 0.025 M HCL solution and determining the phosphate in the extract colorimetrically with a blue ammonium molybdate method with ascorbic acid (0.1 M) as a reducing agent (van Reeuijk, 2002). The pH and the OM, OC, CEC, TN and AP contents of compost and biochar were also determined like a soil. Each characteristic was analyzed in two replicates. Finally, the changes in soil properties after the experiment were determined for each treatment by calculating the difference of each soil parameter between before and after the experiment.

2.4.2. Growth measurement

Growth variables (plant height, number of tillers per plant, number of leaves per plant and leaf length) were determined after 60, 90 and 120 days of a growing period from six plants that have been randomly sampled from the two middle rows of each plot. Leaf to stem ratio in dry weight basis was determined after 120 days of growing from six sample plants.

Plant height was determined by measuring the height of the main shoot of each sample plant from its base to its last leaf. The numbers of tillers and leaves per plant were determined by counting the number of visible tillers and leaves of each of the six sample plants from each plot, respectively. Leaf length was also determined by measuring the length of each leaf of the six sample plants from the base to the tip of the leaf and the mean was calculated for each sample plant. The leaf to stem ratio was determined by dividing leaf dry weight by stem dry weight of the six sample plants from each plot. To measure this variable, the sample plants were cut at 8 cm above the ground and separated into leaves and stems and thereafter, oven dried at 65°C for 72 h. For each of these growth variables, the mean of the six sample plants was calculated and used for statistical analysis. Moreover, the changes in mean plant height, number of tillers per plant, number of leaves per plant and leaf length between the three measurement times (i.e., after 60, 90 and 120 days of growing) were calculated. This can be considered as a mean monthly growth rate of the grass in plant height, number of tillers per plant, number of leaves per plant and leaf length.

2.4.3. Dry matter measurement

Dry matters of Desho grass were determined after 120 days of growing and after 30 days of regrowth or the first harvest by cutting the entire two middle rows of each plot at 8 cm above the ground. Harvesting after 120 days of growing is considered an optimal harvesting stage of Desho grass for forage (Asmare, 2016; Leta et al., 2013). Soon after harvesting of the two middle rows in each plot at the two harvesting times, the fresh weight of the grass from each plot was measured and afterward, a 300 g of fresh grass sample was taken from each plot and dried in the oven at 65°C for 72 h in order to get a constant weight. From these data, the dry matter of the grass per plot was first determined by calculating the oven-dried weight to the fresh weight ratio of the sample and second by converting the fresh weight per plot into dry weight per plot by multiplying the fresh weight per plot with this ratio. Finally, data of dry matter per plot were converted into tons per ha data.

2.5. Statistical analysis

Data of all variables were subjected to ANOVA using the GLM procedure of SAS 9.3 following a RCBD. For each response variable, the validity of model assumptions (normal distribution and constant variance assumptions on the error terms) was verified by examining the residuals. For significant effects (p < 0.05), the multiple means comparison was carried out by using Tukey’s test at 5% significant level. Pearson’s correlation analysis was also carried to determine the association between different growth-attributes of Desho grass using the Corr procedure of SAS.

3. Results

3.1. Soil properties

The soil texture of the trial site before the experiment was clay loam (USDA classification), with 38% sand, 30% silt and 32% clay; the pH was 4.2 (extremely acidic); and the content of OM, OC, TN, AP and CEC was 5.2%, 3.0%, 0.26%, 50.3 ppm and 26.0 cmol(+) kg⁻¹, respectively. The C/N ratio of the soil was 11.5. The OM, OC and TN contents of the soil of the trial site were lower than that of the biochar and compost used for the study, which were alkaline and neutral with a pH of 9.6 and 7.2, respectively. The OM and OC contents of the soil were approximately 5.5 and 1.4 times lower than that of the biochar (28.4 and 16.5%) and compost (7.3 and 4.2%,), respectively. The TN content of the soil was nearly 5.4 and 3.5 times lower than that of the biochar (1.4%) and compost (0.9%), respectively. The AP content of the soil was about 2.1 times lower than that of the compost (103.7 ppm), but, it was nearly 5.1 times higher than that of the biochar (9.8 ppm). However, the CEC and C/N ratio of the soil were almost similar with that of the compost (25.0 cmol(+) kg⁻¹) and biochar (11.9), respectively. On the other hand, the OM and OC contents, TN content, and CEC and C/N ratio of the biochar, respectively, were about 3.9, 1.6 and 2.5 times higher than that of the compost while the AP content of the biochar was about 10.6 times lower than that of the compost.

The results of soil chemical properties of the study site after the experiment and their changes compared to before the experiment for each proportion of compost and biochar combined application are presented in Tables 1 and 2. As indicated in Table 1, the combined application of compost and biochar significantly affected all soil properties considered in the study. The pH, CEC, and the OM, OC, TN and AP contents were significantly (p < 0.05) higher for 100%C:100%B, but lower for 0%C:0%B than for all other proportions. The pH, CEC and the OM and OC contents were also much (p < 0.05) higher for 0%C:100%B than for the remaining treatments other than 100%C:100%B. Likewise, the TN and AP contents were much (p < 0.05) higher for 50%C:50%B, and 50%C:50%B and 0%C:100%B, respectively, than for the others, except for 75%C:25%B and 100%C:100%B. The OM, OC and CEC contents significantly varied between each treatment, other than between 25%C:75%B and 50%C:50%B for CEC. Yet, the pH, TN and AP did not significantly vary between some treatments (Table 1). When we compare compost and biochar application alone (i.e., 100%C:0%B and 0%C:100%B), the AP content was much higher for the former, whereas pH, OM, OC and CEC were much higher for the latter. But, TN did not significantly vary between the two treatments (Table 1).

Table 1. Means and standard errors of soil chemical properties for the different proportions of the combined application of compost (C) and biochar (B)

Compost to biochar proportion	pH	OM (%)	OC (%)	CEC (cmol(+) kg^−1)	TN (%)	AP (ppm)
0%C:0%B	4.4 ± 0.01 f	5.4 ± 0.02 g	3.1 ± 0.01 g	28.0 ± 0.75 f	0.27 ± 0.01 e	51.0 ± 0.04 e
100%C:0%B	5.7 ± 0.02 e	5.7 ± 0.01 f	3.3 ± 0.01 f	31.1 ± 0.70 e	0.36 ± 0.01 cd	54.7 ± 0.11 b
0%C:100%B	6.4 ± 0.02 b	8.4 ± 0.03 b	4.9 ± 0.01 b	43.9 ± 0.58 b	0.34 ± 0.01 d	52.3 ± 0.03 d
75%C:25%B	5.8 ± 0.07 de	6.6 ± 0.03 e	3.8 ± 0.02 e	33.5 ± 0.44 d	0.40 ± 0.01 bc	53.2 ± 0.11 c
50%C:50%B	5.9 ± 0.03 cd	7.6 ± 0.03 d	4.5 ± 0.02 d	40.2 ± 0.41 c	0.43 ± 0.01 b	54.2 ± 0.44 b
25%C:75%B	6.0 ± 0.02 c	8.0 ± 0.03 c	4.6 ± 0.02 c	40.2 ± 0.41 c	0.32 ± 0.01 d	52.9 ± 0.03 cd
100%C:100%B	6.8 ± 0.01 a	9.4 ± 0.05 a	5.4 ± 0.03 a	49.2 ± 0.23 a	0.50 ± 0.01 a	56.8 ± 0.08 a
P-value	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001

Different letters in a column indicate significant differences among means, using the Tukey’s test, at p < 0.05.

Abbreviations: OM, organic matter; OC, organic carbon; TN, total nitrogen; AP, available phosphorus; CEC, cation exchange capacity.

Table 2. Changes of soil chemical properties during the experiment period (after the experiment) for the different proportions of the combined application of compost (C) and biochar (B)

Compost to biochar proportion	pH (%)	OM (%)	OC (%)	CEC (%)	TN (%)	AP (%)
0%C:0%B	5.3	4.0	4.0	6.0	3.8	1.5
100%C:0%B	36.5	10.4	10.3	18.0	38.5	8.6
0%C:100%B	53.9	61.7	61.2	66.4	30.8	3.8
75%C:25%B	39.6	26.5	26.2	26.8	53.8	5.8
50%C:50%B	42.2	49.0	48.7	52.4	65.4	7.8
25%C:75%B	45.0	53.1	53.0	52.1	23.1	5.0
100%C:100%B	63.0	80.4	80.1	86.5	92.3	12.9

Abbreviations: OM, organic matter; OC, organic carbon; TN, total nitrogen; AP, available phosphorus; CEC, cation exchange capacity.

As indicated in Table 2, the soil pH, CEC and OM, OC, TN and AP contents of the experimental site increased for each treatment. However, the increases were much higher for compost and biochar application than for the no compost and biochar application (0%C:0%B or control). Particularly, the increase was much higher for 100%C:100%B than for all remaining combinations of compost and biochar. Next to 100%C:100%B, the increases in pH, OM, OC and CEC were higher for 0%C:100%B and that in TN and AP was higher for 50%C:50%B and 100%C:0%B, respectively than for the other treatments. In general, the increases in pH, OM, OC and CEC decreased as the proportion of compost application increased from 0% to 100% and that of biochar decreased from 100 to 0% (Table 2). The soil texture also changed from clay loam to loam for the combined application of compost and biochar regardless of their different proportions, but unchanged for 0%C:0%B (control).

Overall, all studied soil chemical properties were higher for 100%C:100%B, but lower for 0%C:0%B than all other compost-biochar combined applications. Next to 100%C:100%B, pH, OM, OC and CEC were higher for 0%C:100%B than for the other combined applications of compost and biochar, but TN and AP were higher for 50%C:50%B, and 50%C:50%B and 100%C:0%B, respectively. Moreover, pH, OM, OC and CEC showed a trend of reduction as the proportion of compost application increased from 0% to 100% and that of biochar application decreased from 100% to 0% (Table 1). Similar results were observed for the increases of these soil variables of the experimental site between before and after the experiment (Table 2).

3.2. Plant growth

3.2.1. Plant height, tiller number, leaf number and leaf length

Results of plant height, number of tillers per plant, number of leaves per plant and leaf length of Desho grass after 60, 90 and 120 days of growing, and their increases between (1) 60 and 90 days, and (2) 90 and 120 days of growing for the different proportions of compost and biochar combined application are given in Figures 1 and 2 and in Table 3, respectively.

Figure 1. Means of plant height (A) and number of tillers per plant (B) of Desho grass after 60, 90 and 120 days of growing for the different proportions of the combined application of compost (C) and biochar (B). Error bars indicate standard errors of the means. Different letters above the histograms of each growing period indicate significant differences among means, using Tukey’s test, at p < 0.05.

Figure 2. Means of number of leaves per plant (A) and leaf length (B) of Desho grass after 60, 90 and 120 days of growing for the different proportions of the combined application of compost (C) and biochar (B). Error bars indicate standard errors of the means. Different letters above the histograms of each growing period indicate significant differences among means, using Tukey’s test, at p < 0.05.

Table 3. The increases of growth variables of Desho grass between 60, 90 and 120 days of growing for the different proportions of the combined application of compost (C) and biochar (B)

Compost to biochar proportion	Plant height (cm)			Number of tillers per plant			Number of leaves per plant			Leaf length (cm)
	60–90 days	90–120 days	Mean	60–90 days	90–120 days	Mean	60–90 days	90–120 days	Mean	60–90 days	90–120 days	Mean
0%C:0%B	24.4	24.5	24.5	28.7	19.9	24.3	4.6	4.6	4.6	6.2	5.4	5.8
100%C:0%B	33.8	25.8	29.8	41.0	22.2	31.6	8.3	10.5	9.4	7.9	16.8	12.4
0%C:100%B	33.9	22.9	28.4	34.0	24.8	29.4	8.5	6.1	7.3	8.7	8.7	8.7
75%C:25%B	35.7	25.7	30.7	45.4	19.0	32.2	9.6	10.2	9.9	14.7	8.1	11.4
50%C:50%B	39.8	21.2	30.5	44.5	22.9	33.7	11.2	15.5	13.4	15.1	9.3	12.2
25%C:75%B	34.4	23.5	29.0	40.4	21.8	31.1	9.5	6.2	7.9	15.1	4.2	9.7
100%C:100%B	47.5	24.0	35.8	48.7	30.0	39.4	14.0	18.1	16.1	16.1	11.1	14.7

As indicated in Figures 1 and 2A, after 60 days of growing, plant height, number of tillers per plant and number of leaves per plant, respectively, were significantly higher for 100%C:100%B, 50%C:50%B and 100%C:100%B, and for 75%C:25%B, 50%C:50%B, 100%C:0%B and 100%C:100%B than for 0%C:0%B. Leaf length was also significantly higher for 100%C:100%B than for 0%C:0%B, 0%C:100%B and 25%C:75%B (Figure 2B). After 90 and 120 days of growing, plant height, number of tillers per plant, number of leaves per plant and leaf length were considerably higher for 100%C:100%B than for all other compost and biochar combinations, excluding number of tillers and leaves per plant for 50%C:50%B after 90 days of growing (Figures 1 and 2). Plant height after 90 and 120 days of growing, number of leaves per plant after 90 days of growing and number of tillers per plant after 120 days of growing were much higher for 50%C:50%B than for 0%C:0%B and 0%C:100%B (Figures 1 and 2A). Number of tillers per plant after 90 days of growing and number of leaves per plant after 120 days of growing were much higher for 75%C:25%B and 50%C:50%B, and 50%C:50%B, respectively than for all other combinations of compost and biochar, except for 100%C:100%B (Figure 1B). Leaf length after 90 and 120 days of growing was significantly higher for 75%C:25%B, 50%C:50%B and 25%C:75%B, and for 50%C:50%B and 100%C:0%B, respectively than for the other combinations of compost and biochar, except for 100%C:100%B (Figure 2B). When we compare compost and biochar application alone (i.e., 100%C:0%B and 0%C:100%B), number of tillers per plant after 90 days of growing and number of leaves per plant and leaf length after 120 days of growing were significantly higher for the former than the latter. But, plant height did not significantly vary between these two treatments across the three measurement times (Figures 1 and 2).

Similarly, as indicated in Table 3, the increases in plant height, number of tillers per plant, number of leaves per plant and leaf length between 60 and 90 days of growing and their mean monthly increases between 60 and 120 days of growing were higher for 100%C:100%B followed by 50%C:50%B and 75%C:25%B than for the remaining combinations of compost and biochar. But, the increases in all four growth variables were lower for 0%C:0%B than for all other combinations of compost and biochar. Moreover, the increases in all growth variables between 60 and 90 days of growing were much higher as compared to the increases between 90 and 120 days of growing, especially for the combined applications of compost and biochar. But, for 0%C:0%B, the increases in plant height, number of leaves per plant and leaf length between 60 and 90 days of growing and between 90 and 120 days of growing were almost similar.

In general, 100%C:100%B produced longer plants with longer leaves and a higher number of tillers and leaves compared to all other combined applications of compost and biochar. However, 0%C:0%B or control produced shorter plants with shorter leaves and a lower of number tillers and leaves. Next to 100%C:100%B, 50%C:50%B produced longer plants with longer leaves and a higher number of tillers and leaves followed by 75%C:25%B (Figures 1 and 2). Similarly, these proportions of compost and biochar provided higher increases in all three growth variables between the three measurement times (60, 90 and 120 days of growing) than other proportions (Table 3). But, the different combined applications of compost and biochar did not show a significant difference in plant height, number of tillers per plant and number of leaves per plant after 60 days of growing (Figures 1 and 2).

3.2.2. Leaf to stem ratio and aboveground dry matter

Results of leaf to stem ratio and dry matters of Desho grass after 120 days of growing and after 30 days of regrowth for the different proportions of compost and biochar combined application are given in Table 4. Leaf to stem ratio and dry matter of the regrowth were much higher for 100%C:100%B than for all other combinations of compost and biochar. Next to 100%C:100%B, these variables were also higher for 50%C:50%B than for the remaining combinations of compost and biochar. Dry matter after 120 days of growing was higher for 100%C:100%B and 50%C:50%B than for all other combinations of compost and biochar. Dry matter after 120 days of growing was also much higher for 75%C:25%B and 100%C:0%B than for 0%C:0%B, 0%C:100%B and 25%C:75%B. When we compare compost and biochar application alone (i.e., 100%C:0%B and 0%C:100%B), leaf to stem ratio and dry matter were significantly higher for the former than for the latter. But, compost and biochar application alone did not significantly differ from 75%C:25%B and 25%C:75%B, respectively, for both variables. Biochar application alone did not also significantly differ from 75%C:25%B for leaf to stem ratio and dry matter of the regrowth (Table 4).

Table 4. Means and standard errors of leaf to stem ratio and dry matters of Desho grass after 120 days of growing (120DG) and after 30 days of regrowth (30DR) for the different proportions of the combined application of compost (C) and biochar (B)

Compost to biochar proportion	Leaf:Stem ratio (g/g)	Dry matter (t ha^−1)
		120DG	30DR
0%C:0%B	1.1 ± 0.03 e	9.5 ± 0.2 d	10.2 ± 0.5 f
100%C:0%B	1.5 ± 0.05 bc	16.4 ± 0.4 b	17.5 ± 0.5 c
0%C:100%B	1.3 ± 0.06 d	13.2 ± 0.2 c	15.2 ± 0.4 de
75%C:25%B	1.4 ± 0.03 cd	16.3 ± 0.3 b	17.1 ± 0.3 cd
50%C:50%B	1.6 ± 0.05 b	20.4 ± 0.5 a	21.7 ± 0.2 b
25%C:75%B	1.3 ± 0.03 d	13.9 ± 0.3 c	14.7 ± 0.3 e
100%C:100%B	1.8 ± 0.02 a	21.5 ± 0.4 a	23.9 ± 0.6 a
P-value	<0.0001	<0.0001	<0.0001

Different letters in a column indicate significant differences among means, using the Tukey’s test, at p < 0.05.

Overall, 100%C:100%B produced a higher leaf to stem ratio and dry matter followed by 50%C:50%B compared to the other combined applications of compost and biochar, while 0%C:0%B produced much lower compared to all other combined applications of compost and biochar (Table 4). For all treatments including the control (0%C:0%B), dry matter of the regrowth (the second harvest) was much higher than that of the 120 days after growing (the first harvest); but, the differences between the two harvests were much higher for 100%C:100%B and 0%C:100%B (2.4 and 2.0 t ha⁻¹) than for the others (0.7–1.3 t ha⁻¹) (Table 4).

3.3. Correlation between plant growth variables

The result of the Pearson’s correlation analysis indicated that different plant growth variables (i.e., plant height, number of tillers per plant, number of leaves per plant, leaf length, leaf to stem ratio, aboveground biomass fresh weight and aboveground biomass dry weight) were significantly (p < 0.05) and positively correlated to each other (Table 5).

Table 5. The Pearson’s correlations among growth variables of Desho grass

	PH	NT	NL	LL	LSR	FW	DW
PH	1
NT	0.93**	1
NL	0.92**	0.92**	1
LL	0.93**	0.91**	0.94**	1
LSR	0.92**	0.89**	0.93**	0.94**	1
FW	0.93**	0.93**	0.98**	0.96**	0.95**	1
DW	0.90**	0.93**	0.97**	0.95**	0.94**	0.98**	1

Abbreviations: PH, Plant height; NT, Number of tillers per plant; NL, Number of leaves per plant; LL, Leaf length; LSR, Leaf to stem ratio; FW, Aboveground biomass fresh weight; DW, Aboveground biomass dry weight. Level of significance: **, Significant at p < 0.05.

4. Discussion

4.1. Soil properties

We initially assumed that the co-application of biochar and compost impacts the soil properties and plant growth more than that of biochar or compost alone, and that the impact can vary with the amount of both materials applied in a mixture. In line with these assumptions, our findings showed a positive effect of the coffee-husk biochar and compost applications on the considered soil properties and plant growth with the highest effects being observed after biochar and compost co-applied at their full rates (i.e., 100%C:100%B). Compost and biochar co-applied at their full rates showed the highest increases relative to control (0%C:0%B) in soil pH (54.8%), CEC (76.0%), OM (73.4%) and OC (73.2%), followed by biochar applied alone (0%C:100%B) (46.1, 57.0, 55.5 and 55.4%, respectively). In contrast, compost applied alone (100%C:0%B) showed the lowest increases relative to control for these soil properties (29.7, 11.3, 6.1 and 6.1%, respectively). The other compost and biochar co-applications (25%C:75%B, 50%C:50%B and 75%C:25%B) increased the soil pH, CEC, OM and OC relative to control by 32.2–37.7, 19.7–43.9, 21.6–47.1 and 21.3–47.1%, respectively. These results agree with those of several studies. The individual and co-application of biochar and compost increased the soil pH, CEC, OC, AP and mineral N in two soils with contrasting pH and texture in Ghana (Mensah & Frimpong, 2018) and the soil pH, CEC, OC and water content in soils with an acidic Eutric Nitisol soil type in Ethiopia (Agegnehu et al., 2016). Similarly, Rivelli and Libutti (2022) showed a positive effect of biochar and vermicompost co-application on soil pH, electrical conductivity, and OC and ion (e.g., P₂O₄³⁻, SO₄²⁻, Na⁺, K⁺ and Mg²⁺) contents. Significant increases in water, CEC, K, Ca, NO₃⁻, NH₄⁺ and carbon contents of the soil following biochar, compost and co-composted biochar organic amendments were also reported by Bass et al. (2016). In Arif et al. (2021), biochar co-applied with poultry manure increased soil P and OC contents relative to untreated control by 104 and 203%, respectively. However, Nguyen et al. (2018) reported no significant interactive effects of rice-husk and -straw biochar and cow manure on soil pH, CEC, electrical conductivity, and available P and exchangeable K concentrations in a saline-sodic soil. In this study, both main and interactive effects did not also affect the concentrations of exchangeable Na⁺, Ca²⁺ and Mg²⁺. These results show that the impact of soil organic amendments on soil properties depends on soil type.

The effect of biochar including that from coffee husk on soil pH, widely reported in the literature (Dume et al., 2015; Major et al., 2012), was also confirmed in the current study (Tables 1 and 2). In the present study, biochar applied alone and co-applied with compost increased the pH of the soil from 4.2 to 6.4 and 6.8, respectively (Table 1). This is likely due to the higher ash content in biochar. But, the increase in pH units following organic amendments varies with the initial pH or CEC of the soil, which is very low for a soil with a neutral pH (Rivelli & Libutti, 2022) or a high CEC (Agegnehu et al., 2017). This leads us to suppose that biochar applied alone or in combination with compost on acidic soils, which are characterized by a low pH and CEC, improves the availability of nutrients. On the contrary, biochar application on soils with a neutral or a higher pH can have a limited effect on nutrient availability (Arif et al., 2016).

The increase in OC content observed in this and previous studies (Agegnehu et al., 2016; Mensah & Frimpong, 2018; Rivelli & Libutti, 2022), due to the co-application of compost and biochar, can be not only directly seen as a result of large amounts of carbon in the biochar but also as a result of the supply of organic matter from compost. The contribution of compost to the soil carbon content is clearly evidenced by the higher soil carbon content in which biochar was co-applied with compost (Tables 1 and 2). The application of compost likely boosted the soil organic carbon content by providing organic matter in a higher mineralizable form than that of the recalcitrant biochar, as reported by Sarma et al. (2018).

Regarding the TN and AP contents of the soil, the highest increases relative to control (85.2 and 11.2%, respectively) were detected following compost and biochar co-applied at their full rates (100%C:100%B), followed by 50%C:50%B for the former (59.3%) and 100%C:0%B for the latter (7.1%). But, the lowest increases, both in TN and AP (25.9 and 2.4%, respectively), were detected for biochar applied alone (0%C:100%B). Soil addition with 75%C:25%B and 25%C:75%B also increased the TN and AP contents relative to control by 48.1 and 18.5%, and 4.3 and 3.6%, respectively. These findings agree with those of previous studies showing the increase of soil nutrient contents, particularly of P and K (Wang et al., 2021) and P₂O₄³⁻, SO₄²⁻, Na⁺, K⁺ and Mg²⁺ (Rivelli & Libutti, 2022) in the soils amended with a biochar-vermicompost mixture. Manolikaki and Diamadopoulos (2019) also reported similar results. In this study, separate and combined applications of grape-pomace biochar, rice-husk biochar and compost significantly increased the K concentration in maize tissue with the greatest increase being observed for grape-pomace biochar co-applied with compost on a sandy loam soil. This treatment also showed the highest P concentration when it was applied on a loamy soil. In contrast to these studies, Hannet et al. (2021) showed significantly higher soil TN, AP, Ca and Fe contents for compost applied alone on volcanic soil at a rate of 35 t ha⁻¹ than for biochar and compost co-applied at a rate of 10 t ha⁻¹. The rich content of these nutrients in the compost likely accounted for these results, as also reported by other authors (Lazcano & Domínguez, 2011). The assumption is supported by the finding of the recent study (Rivelli & Libutti, 2022) showing higher P₂O₄³⁻, SO₄²⁻, Na⁺, K⁺ and Mg² contents for a soil treated with vermicompost alone than for a soil treated with inorganic fertilizer alone and untreated control. However, the results of these studies generally indicate that the impacts of compost and biochar co-application on nutrient availability for plants vary with soil conditions, and type (feedstock), form and rate of organic amendment, as reported by Chintala et al. (2014) on acidic soil chemical properties. So, these issues should receive special attention in order to use soil organic amendments as a tool for sustainable agriculture.

4.2. Plant growth

Like soil properties, soil supplied with both separate and mixtures of compost and biochar enhanced the growth of Desho grass. The positive effects on plant growth were observed for all study periods, i.e., after 60, 90 and 120 days of growing and 30 days of regrowth, but its extent varied with treatment, plant growth parameter and study period (Figures 1 and 2; Tables 3 and 4). For example, compost and biochar, both applied alone and in various mixtures, increased plant height, number of tillers per plant, number of leaves per plant and leaf length relative to control by 12.7–40.5, 25.5–66.9 and 14.9–44.1%; 10.6–35.2, 15.2–54.9 and 17.8–53.8%; 42.4–83.8, 62.3–139.2 and 52.1–186.5%; and 32.5–114.6, 36.0–151.2 and 42.7–137.8% after 60, 90 and 120 days of growing, respectively. Addition of compost and biochar, both alone and in mixtures, also increased leaf to stem ratio relative to control by 22.6–71.7% and dry matter yield by 37.8–125.8% and 43.2–129.7% after 120 days of growing and 30 days of regrowth, respectively.

In line with these findings, both single and combined application of biochar and compost on two soils of contrasting pH and texture increased plant height, stem girth and dry matter yield of maize (Mensah & Frimpong, 2018). Similarly, addition of grape-pomace biochar on a sandy loam soil and rice-husk biochar plus compost on a loam soil increased the aboveground dry weight of maize (Manolikaki & Diamadopoulos, 2019). Balidakis et al. (2023) also reported similar findings on the aboveground biomass yield of ryegrass. However, the findings of the present study disagree with those of Rivelli and Libutti (2022) showing no substantial effects of biochar from wood chips and vineyard prunings, both applied alone or in mixture with vermicompost from cattle manure and ammonium nitrate, on plant growth and yield of Swiss chard. Our study is also inconsistent with other studies (Libutti & Rivelli, 2021; Libutti et al., 2020) reporting a lower Swiss chard growth following soil supplied with biochar both applied alone and in a mixture with composts. It does not also agree with Bass et al. (2016) showing a lower banana crop yield and no much change in papaya crop yield following biochar, compost and co-composted biochar organic amendments in tropical Australia. Recent studies also reported no significant impact of rice-hush and -straw biochar co-applications with cow manure on rice growth on a saline-sodic soil (Nguyen et al., 2018) and sole corncob biochar application on growth of red pepper in a pot experiment (Ali Jaaf et al., 2022). These studies indicate that the effects of organic soil amendments on plant growth and crop yield vary with organic amendment (e.g., feedstock, production process and application rate), growing condition (e.g., soil and climate) and crop type (e.g., species and variety).

The decrease in plant growth with biochar application is mainly attributed to a reduced nutrient availability, chiefly of N (Ghezzehei et al., 2014). This is due to the usually high biochar C/N value, which could lead to immobilization of N (Wang et al., 2019), particularly of NO₃⁻-N (Manolikaki & Diamadopoulos, 2019). With these conditions, the N availability for plant uptake is limited and plant growth and yield are reduced. Conversely, the usually high biochar pH and CEC values together with a high content of plant-available nutrients in compost (NO₃⁻, PO₄^3-, Ca and K) (Agegnehu et al., 2017; Dume et al., 2015; Joshi et al., 2015) can improve nutrient availability for plant uptake, particularly in the acidic soils amended with biochar and compost. As per a review by Agegnehu et al. (2017), soil CEC and OC are strong predictors of yield response, but not biochar physical parameters including pH, carbon content or temperature of pyrolysis. In this regard, the increase in plant growth observed in the present study after co-application of biochar and compost is due to its negative impact on the soil C/N ratio along with its substantial positive impact on other soil properties, such as pH, CEC, and TN and AP contents. The average C/N ratio was reduced relative to control by 6.2, 10.2, 18.5 and 20.5% after soil supplied with 100%C:100%B, 50%C:50%B, 75%C:25%B and 100%C:0%B, respectively. But, it was increased by 24.1 and 23.4% following soil supplied with 0%C:100%B (biochar alone) and 25%C:75%B, respectively. However, it is interesting to note that the C/N ratios of compost (4.7) and biochar (11.8), used in this study, and the soils of the trial site treated with different mixtures of compost and biochar (9.3–14.4) were much lower than the optimum value (24.1) that affects immobilization of N. Furthermore, as reported by Bass et al. (2016), improvements of soil properties including nutrient content, following organic amendments, may not always translate to improved plant growth and crop yield.

For all plant growth variables considered in this study, the highest increase in growth relative to control was observed for compost and biochar co-applied at their full rates (100%C:100%B), followed by half rates (50%C:50%B). But, the lowest increase in growth relative to control was observed for the biochar applied alone. This result confirms the synergism between biochar and compost for plant growth, as reported in the literature (Schulz & Glaser, 2012). These authors observed a higher oat plant growth for biochar + compost treatment containing only half biochar and half compost compared to biochar and compost treatments on an infertile sandy soil in a greenhouse experiment. Therefore, the combination of biochar with compost (organic fertilizer) is most promising for the agronomic performance of crops. But, after 60 days of growing, the addition of compost and biochar in various mixtures showed statistically similar effects on plant growth, especially on plant height, tiller number and leaf number (Figure 1 and 2). Moreover, unlike some soil properties (pH, CEC, OM and OC) (Table 1), the addition of biochar alone showed either a similar or a lower plant growth compared with that of compost alone (Figures 1 and 2; Table 4). For example, soil addition with compost alone increased the dry matter relative to control by more than 70% both after 120 days of growing and 30 days of regrowth; but, that of biochar alone increased it by 37.8 and 48.9%, respectively. Similarly, soil addition with vermicompost increased the Swiss chard plant growth by 22% and yield by 116%, in contrast to biochar in a pot experiment (Rivelli & Libutti, 2022). In another pot experiment, compost application on a sandy soil also showed the highest plant growth and crop yield of oat, followed by a biochar and compost mixture while biochar, fertilizer, biochar and fertilizer mixture and control showed the lowest plant growth and crop yield (Schulz & Glaser, 2012).

Based on these results, compost and biochar co-application on acidic soils at full rates can be the best option to increase plant growth and biomass yield of Desho grass and other grass or pasture species in a tropical climate. Compost applied alone can also be a better choice in comparison to biochar applied alone or in mixtures with compost other than at full and half rates. The results also show that the effect of soil addition with compost and biochar, both applied alone and in mixture, on plant growth are dependent on a growth period of the plant. The effect increases as the growth period of the grass prolongs, for example, from 60 to 120 days (Figures 1 and 2; Table 3). This can either be related to a slow nutrient release of the added materials (compost and biochar) to the soil or a slow rise in nutrient absorption capacity of the grass with increasing of growing period (plant age) or both. Anyway, the date on which the higher positive effect of compost and biochar additions on the growth of the grass, observed in this study, corresponds with the optimum forage harvesting date reported in the literature for Desho grass, i.e., after 120 days of growing (Asmare, 2016; Leta et al., 2013).

The significant positive correlation among the different plant growth traits of Desho grass observed in this study (Table 5) shows that any agronomic practice or growing condition that positively or negatively affects the growth of one trait of the grass will positively or negatively influence the growth of the other ones and the overall growth or biomass yield of the grass.

5. Conclusions

Our study showed that addition of single or mixtures of compost from herbaceous plants and poultry manure and biochar from coffee husk on acidic soils under hot sub-moist tropical environmental conduction enhanced the soil properties and plant growth. But, the impact on both soil properties and plant growth vary with individual and combined applications, as well as the amount of both materials in combined applications. For instance, compost and biochar co-applied at their full rates showed the highest improvements of soil properties and plant growths followed by biochar applied alone for soil pH, CEC and carbon content, and compost and biochar co-applied at their half rates for soil TN and AP contents and plant growth. While all soil and plant parameters were the lowest for untreated control. This result allows us to hypothesize the possibility of using co-application of compost and (coffee-husk) biochar as (1) a substitute of simultaneous application of chemical fertilizers and acid soil amendments (e.g., liming), with the aim of a more sustainable cropping practice in general, and (2) a more sustainable cultivation practice of growing of Desho grass and other grass or pasture species, particularly in a tropical environment. This result also further confirms (1) the fertilizing value of compost and the soil amending value of (coffee-husk) biochar, and (2) our initial assumptions stating that biochar co-applied with compost impacts soil properties and plant growth more than biochar or compost applied alone and the impact can vary with the amount of each material applied in a mixture.

Based on the results of this study, compost and (coffee-husk) biochar co-application on acidic soils in a tropical environment at a rate of equal proportion (100% compost + 100% biochar or 50% compost + 50% biochar) can be used to improve both the soil properties and plant growth. (Coffee-husk) biochar applied alone and co-applied with compost at ¾ and ¼ of the full rates, respectively, can also be the second and third choices to improve some soil properties, such as pH, OM, OC and CEC. Similarly, compost applied alone can be the third choice to increase the plant growth. However, as the present study was carried out in one growing season with two cuts only, this conclusion needs to be confirmed by further long-term field studies with multiple cuts or harvests.

Authors’ contributions

SS: Conceptualization, Methodology, Investigation, Formal analysis, Writing—Original Draft; MW: Conceptualization, Methodology, Formal analysis, Validation, Supervision, Writing—Original Draft; AB: Methodology, Supervision, Writing—Review and Editing.

Acknowledgements

The field experiment of this study was financed by JUCAVM.

Disclosure statement

Authors have no potential conflicts of interest to declare.

Data availability statement

All data is available within the manuscript and there is no additional data to be available.

Additional information

Funding

This work was supported by JUCAVM [JU-10.2019].