A Comparative Study on Hydrothermal Liquefied Biochar as Biofertilizer for Increased Plant Yield

Biofertilizer plays a major role in current agricultural practices because of toxic effects produced by the chemical fertilizer over the soil, promoting soil degradation and the plant yield. The chemical fertilizer normally increases the yield at initial application and later it produces toxic effect towards the soil fertility and reduces the yield as the year passes. In order to overcome this issue, old way of eco-friendly agricultural practices are coming back in to this current ages in order to prevent the soil fertility. In olden days cow dung played a major role as a fertilizer, but due to lesser cow dung availability in modern days, different methods for producing biofertilizers are emerging in present technology. The usage of biofertilizer also decrease the erosion of fertile soil, protect the soil from the environmental problem created in the aquatic ecosystem like algal bloom due to erosion of high nutrients from the agricultural land during heavy rainfall (Sunardi et al., 2022). Biofertilizer has a major importance in plant yield, preventing the fertility of the soil and enrichment of the soil nutritive value as year progress. A review on efficiency of the biofertilizers in improving the fruit productivity and novel tool for agriculture which  was illustrated by Mosa et al., (2014) and Boraste et al., (2009). Hazarika and Ansari, (2007) demonstrated the effect of biofertilizer on fruit yield and reported that, about 20% - 25% yield increased when compared to the chemical fertilizers. Irin and Biswas (2023) improved the crop efficiency by incorporating green manure that slowly releases the nutrients into the soil which last long. On holding many advantage on biofertilizer, this also face the constrain created by environmental factors which pulls down its survival in the soil as mentioned by Ganesh et al., (2023). This can be over ruined by incorporating appropriate substance to the biofertilizer.
         
The word “reuse” plays a major role in producing biofertilizer from waste. One such waste management method is Hydrothermal liquefaction process, which is currently used by majority of the researchers in managing the wet waste efficiently and producing value added products that are considered as non-renewable source such as bio oil that can be blended along with commercial fossil fuels to pull down the cost of the existing petrol and diesel. One of the byproduct of hydrothermal liquefaction process is biochar that again enters the environment as waste without any utilization. Such biochar obtained as byproduct from hydrothermal liquefaction process is rich in carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), aluminium (Al) and iron (Fe) which are essential for plant and growth in terms of water retention, aerobic respiration, capturing sunlight energy, helps in metabolism, chlorophyll synthesis, activate tolerance and abiotic stress gene, improves defense response, activate enzyme involved in ATP production and helps in seed production (Ammal et al., 2020). Best route of producing the biochar is hydrothermal carbonization on comparing with other existing thermal treatments in terms of cost, temperature and pressure. The high temperature also reduces the nutrient content of biochar which may pull down its application as biofertilizer (Arun et al., 2020). The biochar produced out of hydrothermal liquefaction process are more advantageous in terms of water holding capacity for long period of time, reduced water evaporation from the field, supply nutrients, reduced pest activity on the plant and reduced disease caused by the organism present on the soil, etc (Bakhat et al., 2021). To commercialize and replace the existing chemical and commercial biofertilizer by biochar, few significant nutrients has to be supplied or a mixture of commercial biofertilizer and biochar can be used on the agricultural field (Oladele et al., 2019) or pulverized manure to improve the yield (Mohan and Jayan, 2024) or engineered biochar (Akhil et al., 2021) or combination of conventional biofertilizer with nanofertilizer (Tamilarasan and Raja, 2024) earthworm proliferated farm waste as manure (Cerci et al., 2020) can be used to enhance the yield and the usage of biochar pulls down the soil pollution and frame way towards the sustainable agricultural goal (Danapriatna et al., 2023). Charcoal based biofertilizer has increased the yield of chickpea as reported by Sharma and Nagpal, (2021). Such biochar hold 99% of efficiency to degrade naturally thereby promoting sustainable environment (Gopinath et al., 2020). Therefore the present study aimed to determine the efficiency of the biochar on the agricultural field against the chemical fertilizer usage with three different plant such as tomato, chilly and lady’s finger.

Soil Collection and sample collection
 
The soil for field test was collected from the location near Aarupadai Veedu Institute of Technology, Paiyanoor, Chennai. The entire study was carried out between  2020 to 2023. The nature of the soil was loamy - a mixture of sand, clay and red soil. Loamy soil promotes the growth by allowing the roots to spread easy due to the granulated nature. This soil also holds the water content better than the usual red soil.
       
Sample for hydrothermal liquefaction process was collected from the nearby areas. The first biomass - sewage water was collected from the sewage treatment plant of Aarupadai Veedu Institute of Technology, Paiyanoor, Chennai and stored it overnight without disturbance.The solid substances present in the collected water gets settled down due to gravitational force which was separated from the top liquid phase and taken further for hydrothermal liquefaction process. Second biomass - citrus fruit peel waste was collected from the canteen of Aarupadai Veedu Institute of Technology, Paiyanoor, Chennai.The collected peel was segregated and three peel were separated namely lemon, orange and sweet lime. All the peels were grinded and mixed in 1:1:1 ratio and made ready to undergo hydrothermal liquefaction process. Third biomass was food waste collected from Aarupadai Veedu Institute of Technology mess hall and directly used for thermal liquefaction process. The fourth biomass was groundnut shell collected after domestic usage and grounded into coarse powder before entering thermal treatment. Fifth biomass was rice husk collected from Padalam rice mill from Chengalpattu district. The collected husk underwent thermal processing. Last biomass was mixture of all six biomass mixed in equal ratio (1:1:1:1:1) and exposed to liquefaction process.
 
Biomass and soil characterization
 
The collected biomass underwent biochemical analysis to determine the parameters that plays a vital role in determining the yield of bio oil and biochar from hydrothermal liquefaction process. The parameters analyzed include total solid%, cellulose%, volatile solid%, nitrogen%, phosphorous%, potassium%, moisture content and pH. The amount of cellulose, volatile solids and pH plays a major role in thermal liquefaction process determining the yield from the process.
       
The soil collected for field test underwent biochemical method to identify the presence of organic matter, carbon, nitrogen, potassium, phosphorous, carbon:nitrogen ratio and pH. These parameters determine the plant growth partially and act as a control in determining the plant growth yield by chemical fertilizer, commercial biofertilizer and biochar.
 
Biochar from hydrothermal liquefaction process
 
The hydrothermal liquefaction process was carried out for sewage sludge, citrus fruit peel, food waste, groundnut shell, rice husk and mixed biomass by the thermal autoclave reactor of capacity 5 l, with the operating temperature of 350^oC with a heating rate of 10^oC/min. The optimization of reactor was carried out with different temperature ranging from 200^oC, 220^oC, 240^oC, 260^oC, 280^oC and 300^oC, different concentration of 20 g/l, 40 g/l, 60 g/l, 80 g/l and 100 g/l, different time 30 min, 60 min and 90 min. The optimum parameter was found to be 260^oC temperature, 20 g/l concentration and 60 min reaction time. The hydrothermal liquefaction process produces 3 different products- Bio oil, biochar and liquid phase. The bio oil produced can be blended with the fossil fuel and used for transportation purpose. In this study the biochar produced was used as a fertilizer to substitute the usage of the existing fertilizer in the market. All the biochar from different thermal liquefaction process was mixed in equal ratio and taken further for field test.
 
Biochar characterization
 
The biochar from the thermal liquefaction process was characterized by Field Emission Scanning Electron Microscopy (FESEM) with Energy dispersive spectroscopy (EDS) analysis to determine the concentration of various parameters present along with the structural representation. FESEM was used to obtain the structural information at the nuclear envelope of the molecule. Here electron beam was focused towards the sample in which the atoms on the surface of the material get excited and produce signal that contain the composition and surface structural topographical information with the minimum resolution of 1 nm. Secondary electron imaging emits the electrons that are very close to the surface of the materials that gives exact surface information which can be imaged. EDS determines the parameters present in the biochar and quantify the same.
 
Germination test
 
Biochar (20 g) was taken and added with 20 ml of distilled water and mixed well. The above mixture was kept undisturbed for 20 min. The seeds were washed well and shade dried. The seeds were sowed on the biochar and monitored until the seeds started to germinate. Only distilled water measuring 5 ml was added every day to the fertilizer to maintain the moisture content of the biochar. The entire process was carried out in dark and the test was carried to prove whether the biochar had the ability to germinate the seeds by supplying the nutrients needed for its growth without the use of soil. Only lady’s finger was used since the germination time for this is very less on comparing with other two seeds (chilly and tomato).

Field test
 
Black plant bags were purchased with dimension of 45 cm height, 30 cm width and 30 cm diameter. 36 bags were filled with 30 kg of loamy soil each. 3 bags were grouped and splitted as 4 groups for each palnt. First group served as control, second group served as chemical fertilizer, third group served as commercial biofertilizer and last group as biochar. 10 g of fertilizer and char was mixed alon with the soil in each bag except control and set ready for field test. Seeds of tomato, lady’s finger and chilly plants were purchased and soaked in water for 8 hours. After 8 hours seeds were sown in the bags by splitting it into 4 sets. One set of seeds were sown in the soil (control), next set of seeds was sown in the soil with chemical fertilizer. The third set of seeds was with commercial biofertilizer and final set with biochar respectively for all three types of plants. Each set contains three plants. The efficiency of the biochar was found by comparing the growth of plant under various parameters such as plant height, breadth of the leaves, weight of the fruit/vegetable produced. Ist flowering day, total number of fruits/vegetable produced under regular basis and monthly intervals among the various sets.

Biomass and soil characterization
 
The biomass was characterized that revealed the presence of parameters mentioned in the Table 1. Among all the biomass collected, citrus fruit peel holds the highest cellulose content that played a major role in conversion of biomass into bio oil and biochar. The other parameters like nitrogen, phosphorous, potassium, total solids and volatile solids impacts on biochar and its efficiency towards plant growth. The yield of biochar from the biomass by hydrothermal liquefaction process is influenced by the presence of cellulose, hemicellulose, lignin and carbon content (Zheng et al., 2015). The lignocellulosic substances present in the biomass and carbon content present in the biomass determines the yield percentage of bio oil and biochar produced from hydrothermal liquefaction process (Anastasakis and Ross, 2015).


       
The soil source used for the field trails was loamy soil which was analyzed to know about the nutritive nature and the results were illustrated in Table 2. The pH of the soil in region was found to be around 7.5 which were mild alkaline. Based on the rainfall in the area, the acidity of the soil varied. During high rainfall the soil appeared acidic and during less rainfall, the soil recorded alkaline nature. Other parameters like carbon, phosphorus, potassium, electrical conductivity and nitrogen varied from one place to another (Nisha et al., 2017).


 
Comparitive study of physico chemical parameters of Chemical fertilizer, commercial biofertilizer and biochar
 
Various physico chemical parameters were analyzed such as colour, odour, ash, fat, porosity, moisture content, bulk density, water holding capacity, pH, electrical conductivity, total solid, volatile solid, carbon, C:N ratio, nitrogen, phosphorous and potassium were analyzed during the period of investigation for Chemical fertilizer, biochar and commercial biofertilizer as per standard protocol and readings were registered in Table 3. The colour of the three samples were dark brown and pH registered were 8, 5.6 and 5.5, for chemical fertilizer, biochar and commercial biofertilizer respectively and C:N ratio was 4:1 for biochar and 1:93 for commercial biofertilizer during the period of study.  The water holding capacity was found to be maximum in biochar 280 ml/kg and 260 ml/kg for commercial biofertilizer and the porosity recorded during the period of the study were 88.88%, 85.71% and 31.9% for biochar, commercial fertilizer and chemical fertilizer respectively which has correlated with the degree of water holding capacity. Oladipupo and Ayorinde (2015) analyzed biofertilizer on various parameters like pH, N, P, K concentration, etc to obtain the nutritive value of the produced biofertilizer. Katre (2012) converted the vegetable waste to biofertilizer. The carbon, nitrogen, pH and moisture content were determined for the produced biofertilizer. Igbokwe et al., (2015) manufactured biofertilizer from saw dust with organic waste - urine, sewage sludge, cow dung. On measuring the nutritive value, the saw dust with sewage sludge possessed more nutritive value and suitable for the plant growth.


 
Biochar characterization
 
The FESEM imaging of biochar gave non uniform pore size with non-uniform structural morphology. According to the study carried out by earlier researchers, crystalinity of the elements present in biochar, the structure, pore distribution of biochar and structure of aromatic substances present in biochar were dependent highly on the reaction temperature of the thermal process (Kim et al., 2012; Kloss et al., 2012). The elemental composition of the obtained biochar was listed below in Table 4. All the elements present in the biochar holds advantage and improves the plant growth in one or the other way thereby proving the efficiency of the biochar towards better plant growth and could act as a good competitor for the existing chemical and biofertilizer in the agricultural market. The carbon, silicate, oxygen present in the biochar contributes towards soil amendment which plays an important role. Other nutrients present such as sodium, calcium, potassium are present in the biochar that improves the soil quality and increase the plant growth (Suarez-Hernandez and Barrera-Zapata, 2017, Qian et al., 2013, Fidel et al., 2017).


 
Germination test
 
The germination test proved that the biochar possess the ability to supply the nutrients needed for the seeds to germinate without the help of soil. Fig 1 gives the proof. Jiang Yang et al., (2015) carried out germination assay that recorded similar results.


 
Field test
 
The field trials were carried out with three different seeds - tomato, lady’s finger and chilly. At regular intervals the chemical fertilizer, biochar and commercial biofertilizers were applied on the field. 1^st week plant growth was captured in Fig 2 and 15 days growth was captured in the Fig 3 for tomato, lady’s finger and chilly. Various parameters like height of the plant, length of the leaves, breadth of the leaves, 1st flowering day, total number of flowers, total number of fruits and weight of the fruit were monitored to check the efficiency of biochar over the chemical fertilizers as described inTable 5, 6 and 7. Young et al., (2004) carried out field trails for water celery. Height and weight of the plant was measured to find the efficiency of the organic biofertilizer and biochar.











Comparative study of yield efficiency
 
From the above field test results the biochar and commercial biofertilizer had higher yield when considering all the parameters. The individual % of efficiency of all the three fertilizers - chemical, Biochar and commercial biofertilizer were illustrated in Table 8. for all the three plants (tomato, lady’s finger and chilly). Among the three plants, tomato plant gave maximum yield with commercial biofertilizer followed by Biochar when compared with other two plants (chilly and lady’s finger).

The Biochar used as biofertlizer had significant effect on the tomato, chilly and lady’s finger plant. But this treatment is not as significant as the commercially available biofertilizer but still could give better results than the chemical fertilizer. Among the three different plant, tomato gave the best result with best yield percentage on 7 different categories such as height of the plant, leaf lengyj, breadth of the leaf, 1st flowering day, total number of flower, fruits/vegetables/spice and weight of harvested fruits/vegetable/spices. The yield % on total number of flowers produced by tomato plant using chemical fertilizer and biochar was 52.48%, using biochar was 59.17%. Yield % of tomato plant interms of total number of fruits/vegetables/spices produced by using chemical fertilizer was 81.13%, 84.84% for biochar and 91.66% for commercial biofertilizer during the period of study. In terms of weight yield % for tomato plant was recorded as 55.8% using chemical fertilizer, 70.79% using biochar and 74.76% using commercial fertilizer. Further to increase the efficiency of the plant yield, the biochar can be treated with agro biofertilizer effluent where all the nutrients in the effluent will be adsorbed by the biochar and expected to give a better result than the existing commercial biofertilizer.

All authors declare that they have no conflicts of interest.