Biochar and Other Organic Amendments Improve the Physicochemical Properties of Soil in Highly Degraded Habitat

Land degradation is an endless challenge in the world. Thus, rehabilitation with organic amendments (OAs) is an urgent priority issue. The purpose of this study is to assess the effect of biochar and other OAs application on soil physicochemical properties and growth parameters of cover crops in greenhouse. Biochar, compost and manure were used as OAs. Soil samples were collected from nine random corners of 30 cm depth and composited. In each experiment, five treatments were considered (biochar, compost, manure, mixed and control) at 1:1 ratio of OAs and soil in a pot, with completely randomized design arrangement in triplicate. The field experiment was made on completely randomized block design and each block contained five 41 x 4 m plots assigned at random within the block and separated by 1 m walkways. OAs additions increased soil pH (5.69-8.13), cation exchange capacity (43.78-49.98 cmolc/kg), organic carbon (1.41-2.46%), organic matter (2.43-3.91%), total nitrogen (0.13-0.76%), available P (18.89-28.53 ppm) and (iron, Fe, manganese, Mn, copper, Cu and zinc, Zn) in comparison to non-treated soil. Tripartite treatments had the largest effect on the biomass of cover crops with 3.43 g fivefold of the control (0.7 g) in alfalfa and 4.54 g twofold of the control (2.07 g) in grass pea p ≤ 0.05. Both in field and greenhouse experiments combination of biochar and other OAs showed a better soil fertility increment and plant growth parameters. The study concluded that there is a synergistic effect in OAs on the soil fertility restoration and plant growth performance.


I. INTRODUCTION
Soil fertility is the vital ecological elements for the survival of biota and environmental services to play role in the conservation of biodiversity [1]. Global land assets are severely threatened due to degradation and unsustainable land use practices that need urgent calls to curb the increasing land deterioration. Rehabilitating degraded land is highly essential for regaining ecosystem services such as biodiversity renovation that ensures perpetuation of future generations [2]. The aggravation of land degradation (LD) and restoration to its original state is one of the pronounced and growing concern in the world [3].
LD is a single largest 21 st  productivity [4]. It takes place in all parts of the terrestrial world and leads to a reduction in productivity and quality of the land [5]. Degraded lands are the center of attention as the world demands for food, feed and fuel, whilst the agricultural land base needed for production is shrinking [6]. It has been estimated that LD is severely affecting ~1.5 billion humans and ~12.2 billion hectares of total lands [7]. Soil fertility degradation is certainly a constraint in Sub-Sahara African countries [8]. Highland places are characterized by a high population, high rainfall, sloppy and fragile ecology. It is estimated that ~1 billion tons of topsoil is lost annually in Ethiopia due to soil erosion [9]. In Ethiopia, the annual costs of LD associated to soil erosion and nutrients loss from agricultural and grazing lands are estimated at about $106 million (about 3% of agricultural GDP) from soil and nutrient losses [10,11]. All these translate to an annual total loss of about $139 million (about 4% of GDP).
Land restoration efforts is underway in Ethiopia since 1970s and dedicated to restoring 15 million hectares in 2030 [12]. Degraded soils regularly contain lower OM, nutrients and microbial activity and unsuccessful for plant establishment [13]. Maintenance and enhancement of the quality of degraded lands are dependent on the improvement of physical, chemical and biological properties [14]. OAs application to degraded soils improves deficiencies in nutrients and OM, alters soil porosity and microbial biomass to recover plant establishment [15,16]. Manure, biochar or compost can have a positive effect on soil microbial communities, nutrient supply for sustainable ecosystems' functions Kennedy and Smith [17] that ultimately contribute much to the restoration of soil fertility of a given habitat [18].
There is increasing interest in amending degraded soils with biochar, compost and manure receiving attention in restoring disturbed soils [19]. The effect of biochar and other OAs on soil properties, growth and activity of soil biota has not been sufficiently evaluated with field trials. To overcome problem of LD and deforestation Ethiopian government have attempted to implement different conservation activities. Those are physical structure (terraces, soil bund, water ways and check dams). There is also plantation through trial and error which is not suitable to the environmental conditions. Hence, the results are very scant due to poor management practices and implementation is not well studied as for rehabilitation. It is hypothesized that, biochar and other OAs on degraded habitat, are an inexpensive solution to increase soil function and accelerate re-vegetation. Therefore, the purposes of this study was to Biochar and Other Organic Amendments Improve the Physicochemical Properties of Soil in Highly Degraded Habitat Alemayehu Getahun, Diriba Muleta, Fassil Assefa, Solomon Kiros, and Mariangela Hungria evaluate the effect of different OAs on the physicochemical properties of degraded soil and to assess also the growth of alfalfa (Medicago sativa) and grass pea (Lathyrus sativus) on organically amended soils under greenhouse conditions.

A. Description of the Study Area
The study was carried out on degraded soils of Central Highlands of North Shewa Zone, Ethiopia from September 2016 to May 2018. It is located at 9° 45' 57'' N and 38° 42' 06'' E. The soil is characterized as indicated in (Table I). More than 65% of the area is characterized by steep-slopes, a mountainous topography, valleys and gorges. Its altitude extends from 1000 to 3500 m.a.s.l. [20]. The average rainfall of the Woreda is about 1200 mm. Temperature ranges from11.5°C to 35°C with average relative humidity 62%. About 8 percent of land is unusable (abandoned) land.

B. Soil and OAs Collection
Four kilograms of soil samples were taken randomly from nine corners from a depth of 30 cm and stored at room temperature. Mixed well, analyzed following standard procedure [21]. Biochar (coffee husk), compost (recycled floriculture) and manure (farmyard manure) are used as OAs. The biochar was prepared by means of slow pyrolysis of coffee husk in a biochar kiln operating at a temperature of 350°C. The properties of soil samples and OAs are listed in (Table I and II).

C. Soil and OAs Analysis
The composite soil samples and OAs were air dried, ground and sieved over 2 mm mesh used for soil physicochemical analysis. Soil pH and electrical conductivity (EC) were measured in soil: water suspension (1:2.5) ratio [22]. Cation exchange capacity (CEC) was determined by Sodium equivalent by Flame Emission Spectrophotometer (FES) [26]. Ammonium acetate (pH=7) was used to extract the exchangeable cations (Ca, Mg, K, and Na). Exchangeable Ca and Mg were measured by EDTA titrimetric method and exchangeable K and Na by Flame Emission Spectrophotometer [23].
Plant available K was determined by the ammonium acetate (CH3COONH4) method Bashour and Sayegh [24]; whilst available P was extracted by sodium bicarbonate solution as described before ISO. [25]. Soil organic carbon (OC) and total N (TN) content were determined by dry combustion methods based on [25]. Soil organic matter (OM) was calculated by multiplying soil OC by 1.724 assuming average C concentration of OM of 58%, (%OM = %OC x 1.724). Micronutrients (Cu, Fe, Mn, and Zn) were extracted with ammonium bicarbonate di-ethylene tri-amine penta-acetic acid (DTPA), as described before [26].

D. Greenhouse Experiments
The performance of OAs on growth of alfalfa and grass pea was carried out. All experiments were done in triplicate and the pots 3.5 L capacity were arranged in a RCBD in greenhouse, 12 h photoperiod, day 25±2°C and night temperature 17±3°C. The treatments are biochar, compost, manure, biochar + compost, biochar + manure, compost + manure, biochar + compost + manure, and control. All the treatments were applied at 50:50 ratio of OAs and soil per pot. In each pot, 5 surfaces sterilized seeds of the alfalfa and grass pea were sown separately followed by a reduction to three plants 10 days later. Each pot was watered daily with tap water for a period of 30 days. Shoot height (SH), shoot dry weight (SDW), root dry weight (RDW), shoot fresh weight (SFW), nodule number (NN), nodule fresh weight (NFW) and nodule dry weight (NDW) were measured.

E. Statistical Analysis
Mean separation was done using the Duncan multiple grouping of means at 5 % probability level when the ANOVA showed significant effects. The values were presented as means ± standard deviation (SD), where p ≤ 0.05 was considered to be statistically significant. All statistical analyses were performed using SAS software package (version 9.0).

A. Effects of OAs on Soil Physicochemical Properties
The amendments with biochar, compost, manure alone or in combination had affected the physicochemical properties of soil differently compared to the control (Table III). Soil pH was dramatically increased (8.4) with the application of the selected OAs (p ≤ 0.05) compared to the control (5.68). Among the four amendments, BAS (biochar amended soil) had the greatest impact on pH (8.4) followed by BCMAS (biochar +compost + manure amended soil) (8.15). BAS, CAS (compost amended soil), MAS (manure amended soil), and BCMAS increased the soil pH by 1.72, 1.13, 1.27, and 1.47 units, respectively. The present result indicated that the addition of OAs could increase soil pH. Similar to the current finding, application of biochar from rice husk, sorghum silage, and sawdust increased the soil pH by 0.76, 1.17, and 1.68 units, respectively [27]. Soil pH increase was in the order of BAS > BCMAS >CAS > MAS compared to the control. The increases in soil pH due to sole application of biochar could be attributed to the high pH of the biochar used in the experiment since it contains a large amount of ash with alkaline nature that can enhance the soil pH. Soil pH increase from 7.27 to 7.85 after biochar amendment was reported due to the aforementioned reasons [28]. However, Mukherjee, et al. [29] have conducted a two-year study with 0.5% biochar by weight as a soil amendment and observed no significant increase p ≥ 0.05 in soil pH. This is consistent with the results that OAs application can significantly increase soil pH [30,31]. Decarboxylation of organic anions due to decomposition, complexation of free H + and Al 3+ ions with organic ligands and increased saturation of soil CEC by Ca 2+ , Mg 2+ Na + , and K + added by the wastes are other possible explanations for soil pH changes [32]. Moreover, green waste compost, biochar, and sedge peat application could increase saline soil pH (6.23 to 6.77), and this may be in accordance with a high content of basic cations of these amendments [33,34]. In line to the present outcome, the application of biosolids such as animal manure and compost on acid soils increases the soil pH appreciably from 4.68 to 7.19 [35].
Compared to the control, OAs increased the total Nitrogen (TN) content in the soil. The largest increase was observed in BAS (0.98%) followed by BCMAS (0.86%), CAS (0.66%) and MAS (0.55%) amended plots (Table III). The increased TN content with manure (0.36%), liquid humus (0.31%) and compost (0.31%) amendments were noted in the previous studies [36]. Moreover, there is a TN increase in biochar (1.6 g/kg) and cattle manure (11.6 g/kg) when treated sandy soil compared to control group (0.18 g/kg) [37]. But in the present finding the high TN content was observed in BAS. The increase in TN of the soil after amendments is closely related to the build-up of OM in the soil. Similar effects were shown by Mantovi, et al. [38] in a 12-year experiments with biosolids and compost. Furthermore, it has been reported that addition of manures (i.e., cow, sheep, and poultry) increased net N released by 42, 25, and 43%, respectively over the control [39]. However, the effects of OAs on nutrient availability depend on their chemical composition and decomposition rates [40]. According to Wu, et al. [41], the application of OAs increased the N content is because of the greater N and OC concentration in the amendments.
The available P content in the soil increased in each amended soil and the highest was found in BCMAS (28.53 ppm), whilst the highest available K content was recorded in MAS (2.16 meq/100) after one year amendment (Table III). The results show a significant increase in physicochemical properties in all amended soil samples compared to the control (p ≤ 0.05). The increased P and K contents observed in the soil were also related to the amendments chemical composition. Thus, OAs can apparently supply P and K to the soil as reported by Reynolds, et al. [42], who have evaluated municipal waste compost and observed an increase in the P and K concentrations. The increased in P and K is might be due to the negative charges in the functional groups of the OM that compete with P and K for adsorption sites and complex Fe and Al ions thereby increasing P and K activity in the soil solution [43]. For example, manure, compost and biochar can provide these nutrients to crops on organic farms [44]. The application of OAs may increase P availability, either directly from the decomposition of OM and release of P or indirectly by increasing the amount of soluble organic acids that increase the rate of desorption of phosphate [45]. The increase in available P after OAs might be due to high microbial activity induced by the addition of OAs which increased P cycling [46]. Moreover, P can be adsorbed by soil colloids, and the mixed OAs are able to bind large quantities of macronutrients and thereby to reduce their removal from soil by leaching [47].

B. Effects of OAs on CEC and Exchangeable Cations
The CEC of this study was higher in all amended plots than the control and significantly (p ≤ 0.05) different among the amended plots. Soil CEC was found to be higher in BAS (53.55 cmolc/kg) followed by BCMAS (50.82 cmolc/kg) compared to control (43.78 cmoc/kg) (Table III). Earlier studies proved that biochar resulted in higher soil CEC (29 cmolc kg −1 ) than the control (25.6 cmolc kg −1 ) [48]. The reason for high CEC in BAS is due to an increase in the surface area and charge density on the surface. Biochar from woody materials typically enhances the pH, soil water relations and CEC and ultimately results in improved soil fertility [49]. Applications of composts and manures increase soil fertility and the increase is short term [50]. But, application of biochar to infertile soil has provide longlasting improvements in soil fertility [51,52]. Because of the continuous oxidation of surfaces, and the adsorption of organic acids by biochar, CEC is expected to increase further with time [53]. In agreement to the present result, Cheng, et al. [53] have reported that the incubation of biochar during one year raised its CEC from 1.7 to 71 mmol/ kg. Likewise, according to Chan, et al. [54], biochar addition promotes positive changes in soil quality, such as acidity correction, increased CEC, and an improved environment for root growth.
Compared to the control, application of OAs alone or in combination increased the exchangeable Ca, Mg, K, and Na contents in the soil. Compared to the biochar, the compost used had greater proportions of exchangeable Ca and Mg, 33.37 and 16.49 cmolc/kg, respectively. The highest exchangeable K and Na content were found in the combined treatment (0.42, 0.97 cmolc/kg, respectively), while the least was found in the control (0.18 cmolc/kg). Inline to this finding, the compost used had greater proportions of exchangeable Ca and Mg, 162.7 and 22.7 cmolc kg −1 , respectively [55]. According to Fischer and Glaser [56], compost application alone has a liming effect due to its richness in alkaline cations such as Ca, Mg, and K, which are liberated from organic matter due to mineralization. The increase in the exchangeable bases is as a result of the presence of ash in the biochar which helps in the immediate release of mineral nutrients like Ca and K for crop use [57]. The treatment of OAs increased exchangeable Ca 2+ contents of soil, which may increase the replacement of Na + from the exchange sites, thus improving the remediation efficiency of soil. It has been reported that soil structure could be improved by Ca 2+ through the formation of cationic bridges between soil organic matter and clay particles [58]. For soluble salts, the addition of OAs had a significant influence on K + (Table III). In this finding, the highest increase in exchangeable K + was observed in MAS and BCMAS as verified by other investigators Walker and Bernal [59], who observed the treatment of exhausted soil with compost and poultry manure that resulted in significantly increased soil soluble K + and the increase probably related to the application of OAs. Among the OAs, biochar and compost had the lowest value in soil exchangeable Na + and Mg 2+ compared to other amendments. The increase in Ca and Mg with OAs may be due to the release of organic forms of these elements in the organic residues [60]. As reported by Qadir and Oster [61], an increase in Ca concentration in the soil solution results in the replacement of Na by Ca at the cation exchange sites on the soil particles. If the soil turns into a sodium saturated and forms Na-clay, the soil becomes less fertile. The addition of biochar, compost and manure increased Cu, Fe, Zn and Mn concentration, as did the BCMAS. The concentration of micronutrients was found to be in the order of Fe >Mn> Cu > Zn in almost all the amended soil of the study site (Fig. 1). The micronutrient availability is more pronounced in the BCMAS which might be due to their synergistic activities as noted in the previous studies [62]. These trace metals are essential elements to promote plant growth and required for metabolic functions, gene regulation, and reproduction [63]. In other finding, soil amended with biochar, compost and cattle manure significantly increased the availability of these trace elements in the range of 6.1-460 mg/kg [64]. Moreover, both manure and biochar application could increase the concentration of Zn, Mn and Cu (0.5-2.7, 0.8-2.6, 3.5-4.6 and 0.5-2.3, 0.8-2.4 and 3.5-4.8) respectively [65]. There is a significant decreases were observed in plant-available Fe content of soil following the application of manure and biochar (2.5-2 and 2.5-1.7) respectively [65]. The increments are because of manure and biochar acts as a nutrient source, increases in nutrient availability [66,67]. Iron availability decreases have been reported by Lentz and Ippolito [66] after the biochar application to soil.

C. Effects of OAs on Soil Organic Matter and Organic Carbon
Soil OM significantly (p ≤ 0.05) increased with the treatment of OAs where 2.69% in CAS and 4.86% in BCMAS compared to control 2.43%. The OM content increments were 2.28%, 0.26%, 0.96% and 2.43% in BAS, CAS, MAS and BCMAS, separately. Similarly, OC was 1.63 and 3.075% in CAS and BCMAS, respectively compared to the control (1.405%; Fig. 2). Soil OM contents of < 2.0% is low; 2.1-3.0% is medium and > 3.1% is high as reported by [35]. Our results indicate that OAs significantly increased OM and OC contents as revealed in the earlier studies [30,31,68]. The increase was more evident in BAS. The application of OAs clearly increased the levels of OM compared to non-treated soils which is in agreement with another study [69]. In line to the finding, Tejada and Gonzalez [70] have concluded that the chemical property of the OAs decided the effect of amendments on soil OM and OC. The increase in soil OC has previously attributed to the continuous addition of C [71]. Many studies have shown that an increase in OC with OAs applications [72,73]. The effect of OAs on soil organic carbon depend on the chemical nature of the amendments [70]. Evidently, application of farmyard manure visibly enhanced the soil OC content in various cropping systems [74]. BAS= biochar amended soil. Means with the same letter are not significantly different at p ≤ 0.05 with Duncan grouping using (mean ± SD).

D. Effects of OAs on Plant Biomass under Greenhouse
In our study, the highest nodule number per plant was measured from BCMAS (118, 111/plant) in alfalfa and grass pea compared to the control (50, 58/plant, respectively; (Fig.  3). Moreover, BAS had the higher nodule numbers (91 and 93) both in alfalfa and grass pea compared to CAS (77,69) and MAS (84,88) in both alfalfa and grass pea, respectively. A similar study from greenhouse experiment confirmed that biochar increased nodule biomass and numbers in legume plants [75]. Mia, et al. [76] have stated biochar considerably increased the number of plant root nodules in red clover (Trifolium pratense L.). In contrast, Quilliam, et al. [77] have reported that aged biochar did not increase the number of root nodules in clover (Trifolium repens L.). According to Rondon,et al. [78], the basic reason for the higher root nodule numbers in legumes in the presence of biochar is attributed to increased availability of the trace nutrients such as boron and molybdenum. As shown by Harter, et al. [79], the high C: N ratio of biochar and the formation of anoxic microsites might favor the growth of free-living as well as plant associated nitrogen-fixing microorganisms. Alfalfa and grass pea plants receiving biochar, manure or compost produced more aerial fresh biomass compared to the control. Plants in BCMAS produced 25% and 29% higher fresh biomass in alfalfa and grass pea, respectively than the control. A relatively similar finding (21 and 19%) from fresh biomass of alfalfa was reported in compost and manure-amended soil, respectively [80]. Many organic wastes represent an important source of N, P, Ca, and others such as Zn, Cu, and Mg that are essential to plant growth [81]. Hence, as many scholars demonstrated, organic fertilizer has a great impact on soil fertility which is essential for good plant growth [82]. The OAs application significantly (p ≤ 0.05) increased the biomass of alfalfa and grass pea. The significant increments were shown in response to the combined application of the amendments (Fig. 4, Table IV and V). Among eight amendments, the triple treatments had the largest effect on the biomass, reaching 3.43 g fivefold of the control (0.7 g) in alfalfa and 4.54 g twofold of the control (2.07 g) in grass pea (p ≤ 0.05). The dry weight of the plants significantly increased in soil applied with OAs, which is comparable to the previous studies [83,84]. A similar (1.31, 1.37 and 1.49 folds increase) in rapeseed meal, manure and biocharamended soil was report compared to the control in wheat biomass [85]. This is attributed to the active role of OAs on soil biota (enzyme activities) and chemical properties (pH) that would result in the release of nutrients from OAs. The growing biomass of soil organisms may be caused by OAs themselves and improve plant health through the decomposition of OM, nutrient cycling, and the improvement of the soil structure [86,87]. Members of the genus Lathyrus include food and fodder crops, ornamentals, soil nitrifiers, dune stabilizers, important agricultural weeds, and model organisms for genetic and ecological research [88].   The effect of single and combined OAs on the growth of alfalfa and grass pea were indicated in (Fig. 5) as compared to control treatment. The figures realized that combined application of biochar and other organic amendments showed better growth performance followed by single application as compared to the non-amended soil. The application of biochar and other organic amendments can recover soil fertility of degraded land. This study suggests that the utilization of easily available bio-waste for degraded land restoration to benefit the ecosystem and the community.