Biochar, an environmentally friendly soil conditioner, is produced using several thermochemical processes. It has unique characteristics like high surface area, porosity, and surface charges. This paper reviews the fertilizer value of biochar, and its effects on soil properties, and nutrient use efficiency of crops. Biochar serves as an important source of plant nutrients, especially nitrogen in biochar produced from manures and wastes at low temperature (≤ 400 °C). The phosphorus, potassium, and other nutrient contents are higher in manure/waste biochars than those in crop residues and woody biochars. The nutrient contents and pH of biochar are positively correlated with pyrolysis temperature, except for nitrogen content. Biochar improves the nutrient retention capacity of soil, which depends on porosity and surface charge of biochar. Biochar increases nitrogen retention in soil by reducing leaching and gaseous loss, and also increases phosphorus availability by decreasing the leaching process in soil. However, for potassium and other nutrients, biochar shows inconsistent (positive and negative) impacts on soil. After addition of biochar, porosity, aggregate stability, and amount of water held in soil increase and bulk density decreases. Mostly, biochar increases soil pH and, thus, influences nutrient availability for plants. Biochar also alters soil biological properties by increasing microbial populations, enzyme activity, soil respiration, and microbial biomass. Finally, nutrient use efficiency and nutrient uptake improve with the application of biochar to soil. Thus, biochar can be a potential nutrient reservoir for plants and a good amendment to improve soil properties.

Various studies have established that feedstock choice, pyrolysis temperature, and pyrolysis type influence final biochar physicochemical characteristics. However, overarching analyses of pre-biochar creation choices and correlations to biochar characteristics are severely lacking. Thus, the objective of this work was to help researchers, biochar-stakeholders, and practitioners make more well-informed choices in terms of how these three major parameters influence the final biochar product. Utilizing approximately 5400 peer-reviewed journal articles and over 50,800 individual data points, herein we elucidate the selections that influence final biochar physical and chemical properties, total nutrient content, and perhaps more importantly tools one can use to predict biochar’s nutrient availability. Based on the large dataset collected, it appears that pyrolysis type (fast or slow) plays a minor role in biochar physico- (inorganic) chemical characteristics; few differences were evident between production styles. Pyrolysis temperature, however, affects biochar’s longevity, with pyrolysis temperatures > 500 °C generally leading to longer-term (i.e., > 1000 years) half-lives. Greater pyrolysis temperatures also led to biochars containing greater overall C and specific surface area (SSA), which could promote soil physico-chemical improvements. However, based on the collected data, it appears that feedstock selection has the largest influence on biochar properties. Specific surface area is greatest in wood-based biochars, which in combination with pyrolysis temperature could likely promote greater changes in soil physical characteristics over other feedstock-based biochars. Crop- and other grass-based biochars appear to have cation exchange capacities greater than other biochars, which in combination with pyrolysis temperature could potentially lead to longer-term changes in soil nutrient retention. The collected data also suggest that one can reasonably predict the availability of various biochar nutrients (e.g., N, P, K, Ca, Mg, Fe, and Cu) based on feedstock choice and total nutrient content. Results can be used to create designer biochars to help solve environmental issues and supply a variety of plant-available nutrients for crop growth.

Considering the global issue of vegetable wastes generation and its impact on the environment and resources, this study evaluated the conversion of four largely produced vegetable wastes (cauliflower, cabbage, banana peels and corn cob residues) into biochar. Each waste was tested individually and as a combined blend to assess feedstock influences on biochar properties. In addition, various pyrolysis temperatures ranging from 300 °C to 600 °C and two particle size fractions (less than 75 µm, 75–125 µm) were considered. Biochars were characterized for various properties that can influence the biochars’ effectiveness as a soil amendment. It was found that pyrolysis temperature was the most dominant factor on biochar properties, but that individual feedstocks produced biochars with different characteristics. The biochars had characteristics that varied as follows: pH 7.2–11.6, ECE 0.15–1.00 mS cm⁻¹, CEC 17–cmol_c kg⁻¹ and ζ-potential − 0.24 to − 43 mV. Based on optimal values of these parameters from the literature, cauliflower and banana peels were determined to be the best feedstocks, though mixed vegetable waste also produced good characteristics. The optimum temperature for pyrolysis was around 400 °C, but differed slightly (300–500 °C) depending on the distinct feedstock. However, smaller particle size of biochar application was always optimal. Biochar yields were in the range of 20–30% at this temperature range, except for corn cobs which were higher. This study demonstrates that pyrolysis of dried vegetable wastes is a suitable waste valorization approach to produce biochar with good agricultural properties.

This study assembled corn stalk-derived biochar (BC) with layered double hydroxide (LDH) through a rapid coprecipitation of biochar and metal (Ni/Fe/Zn) hydroxide precipitates. A BC and LDH composite (BC-LDH) and modified BC-LDH material after heating (BC-LDH-P) were prepared successfully for atrazine adsorptive removal. The physicochemical properties of the synthesized samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS). Batch experiments were conducted to study the sorption of atrazine onto BC, BC-LDH, and BC-LDH-P from the aqueous solution. The adsorption of atrazine onto BC-LDH and BC-LDH-P were performed following with the pseudo-second-order kinetic model, and the sorption isotherms agreed well with the Freundlich fitting model. The adsorption amounts of the three materials are arranged in descending order is as follows: BC-LDH-P > BC-LDH > BC (143.15 > 123.10 > 95.93 mg g⁻¹, respectively). Due to the high crystallinity of the LDH material and the high binding degree with biochar, Ni/Fe/Zn-LDH biochar composites offer a potential alternative to a carbon-based adsorbent for atrazine removal from aqueous solution.

Biochar is a carbon-rich material obtained after thermochemical conversion of biomass under no oxygen environment. The effect of biochar amendment on soil properties, such as water retention, infiltration and desiccation crack potential was studied in the recent years. However, the effect of biochar or feedstock type on these properties is not explicit. This study investigates the effect of two different (in terms of feedstock) types of biochar on the water retention, infiltration and desiccation cracking behavior of compacted silty sand. Water retention characteristics, infiltration rate and the progression of desiccation cracks were measured after compacting soil amended with 5–10% (w/w) biochar produced from water hyacinth (WHB) and mesquite. Measurements were also taken for an unpyrolyzed material coir pith (CP, sourced from coconut husk)-amended soil for comparing the results of biochar-amended soil. The results show that the amendment of 5% to 10% biochar increased the maximum water holding capacity (θ_s), air entry value (AEV) and water content at 1500 kPa (θ₁₅₀₀) of the soil, whereas decreased the infiltration rate and peak crack intensity factor (CIF) of the soil. Moreover, the application of CP increased the infiltration rate. The amendment of WHB showed the highest increment in AEV and θ₁₅₀₀ and the highest decrement in infiltration rate and CIF compared to the other amendments. Based on the results, it is advisable to use the WHB-amended soil in bioengineered structures that could promote the growth of vegetation by higher water retention and could reduce the potential of leachate formation by decreasing water infiltration and desiccation crack potential.

Biochar has been extensively used for the improvement of soil water retention. However, the effects of various biochars were not well determined. The objectives of this study were to investigate the effects of three biochars [biochars made from bamboo (Bambusaceae), rice straw (Oryza sativa), and tobacco stem (Nicotiana L.)] on soil physical properties and the water retention characteristics of red soil at southeast China. The air-dried soil samples were mixed with ratios of 2%, 5%, and 10% (w w⁻¹) BC (bamboo biochar), RC (rice straw biochar), and TC (tobacco biochar), respectively, and evaluated for changes in soil bulk density (BD), soil saturated water content, field capacity, capillary porosity and soil hygroscopic coefficient. The results showed that BD decreased significantly with the application of the three types of biochar, total soil porosity and capillary porosity increased with the increase of the biochar ratio. The soil hygroscopic coefficient, wilting moisture capacity, saturated water content, and field capacity were significantly affected by the application of the three types of biochar. Compared with the other two treatments, the BC showed the best effects on soil water characteristics. BC treatments with addition ratios of 2%, 5%, and 10% significantly decreased BD by 6.55%, 18.03%, and 36.07%, respectively. Moreover, saturated water content and field capacity were increased by BC. BC treatments significantly increased the readily available water by 32.65%, 42.49%, and 50.01%, respectively. However, the increased non-readily available water induced by the high ratio of biochar addition was not easily utilized by plants. Our results suggested that the biochar amendment can improve soil structure, decrease soil BD, boost soil porosity and capillary porosity, and increase soil moisture constant, and 2–5% of BC was recommended in the field condition.

Biochar amendment improves the physical, chemical and biological characteristics of different soil types under different climatic and environmental conditions. In this study, effects of biochar or live pasture plants existing alone or co-existing on selected soil properties of sandy loam soil under humid lowland tropical climatic conditions were investigated. The changes measured in the amended soil, with or without plants, were compared to the unamended and unplanted soils. Biochar amendment with or without pasture improved moisture retention, lowered bulk density, increased pH and kept the electrical conductivity within ranges conducive for pasture growth. Generally, contents of all the nutrients increased following biochar amendment, however pasture establishment without amendment resulted in depletion of available potassium and magnesium. Under all treatment conditions, soil organic carbon and soil organic matter were significantly depleted. Cogon grass is invasive under all land use systems and contributes to greenhouse gas emissions through slash-and-burn. Using biomass from the grass instead of burning would mitigate CO₂ emissions from the tropics.

There is little understanding as to whether the addition of biochar requires less fertilizer to obtain the potential yield. Furthermore, the additional yield ascribed to the non-nutrient effects of biochar is ambiguously quantified. Therefore, this study is aimed to elucidate the influence of biochar application rate and production temperature on (i) marginal agronomic efficiency (AE_LN), (ii) potential yield (Y_opt), (iii) the amount of mineral fertilizer required to obtain the potential yield (F_opt); and (iv) nutrient use efficiency. AE_LN, Y_opt and F_opt were calculated after fitting the yield response at different levels of mineral fertilizer with a second-degree polynomial. Application of biochar reduced marginal agronomic efficiency, implying that the plant utilized the applied nutrient more efficiently without biochar at a low dose of mineral fertilizer. Biochar increased potential yield but required more mineral fertilizer to obtain the optimum yield. The non-nutrient associated effect of biochar reached to 39% and is mainly attributed to its liming effect. The effect of biochar on AE_LN, Y_opt, F_opt, fertilizer use efficiency and soil pH were more pronounced at the higher application rate. Addition of biochar, however, increased soil Mehlich-P and carbon content, irrespective of production temperature and application rate. This study demonstrated that the short-term effect of biochar application on fertilizer utilization should be examined with caution in low-input cropping systems because the biochar effects were dependent on fertilizer level, biochar application rate, production temperature and their interactions. Further manipulative experiments are recommended to identify the mechanisms that explain the non-nutrient effect of biochar on yield.