Assessment of biochar and hydrochar as minor to major constituents of growing media for containerized tomato production

Suitability of coconut bran and biochar as a composite substrate for lettuce cultivation in aquaponic systems

Growth substrates are essential for aquaponic systems and play an important role in vegetable growth and water quality. In this study, we explored an innovative combination of coconut bran and coconut shell biochar (CSB) as a composite growth substrate for lettuce cultivation in aquaponic systems. The study included the control (100 % coconut bran as the growth substrate) and treatment groups (T1-T5; containing 10 %, 20 %, 30 %, 40 %, and 50 % CSB as the growth substrate, respectively). The substrate properties; lettuce growth performance; and soil enzyme activity, nitrogen content, and abundance of microbial communities in the substrate were analyzed to determine the optimal substrate. Our findings indicated that CSB incorporation significantly altered the properties of the substrate, resulting in increased dry and bulk densities, pH, and water-holding capacity, and decreased electrical conductivity, water-absorption capacity, and porosity. Furthermore, the fresh weight of lettuce was notably increased in the treatment groups. The activities of fluorescein diacetate hydrolase, urease, nitrate reductase, and hydroxylamine reductase initially increased and further decreased, reaching the maximum in the T3 group. Conversely, the activity of nitrite reductase and contents of available nitrogen, nitrate-nitrogen, and ammonium-nitrogen in the substrates initially decreased and further increased, with the minimum values observed in the T3 group. The microbial sequencing results indicated that CSB incorporation significantly increased the microbial diversity and relative abundance of microorganisms associated with nitrogen transformation. Moreover, 30 % CSB incorporation exhibited the greatest effect on lettuce growth, with a 34.5 % and 31.6 % increase in fresh weight compared to the control during the growth and harvest periods, respectively. This study indicated the enormous potential of biochar in the research and development of green technologies for substrate amendment in aquaponic systems.

Cattle manure hydrochar posed a higher efficiency in elevating tomato productivity and decreasing greenhouse gas emissions than plant straw hydrochar in a coastal soil

Rehabilitation of degraded soil health using high-performance and sustainable measures are urgently required for restoring soil primary productivity and mitigating greenhouse gas (GHG) emission of coastal ecosystems. However, the effect of livestock manure derived hydrochar on GHG emission and plant productivity in the coastal salt-affected soils, one of blue carbon (C) ecosystems, was poorly understood. Therefore, a cattle manure hydrochar (CHC) produced at 220 °C was prepared to explore its effects and mechanisms on CH4 and N2O emissions and tomato growth and fruit quality in a coastal soil in comparison with corresponding hydrochars derived from plant straws, i.e., sesbania straw hydrochars (SHC) and reed straw hydrochars (RHC) using a 63-day soil column experiment. The results showed that CHC posed a greater efficiency in reducing the global warming potential (GWP, 54.6 % (36.7 g/m2) vs. 45.5-45.6 % (22.2-30.6 g/m2)) than those of RHC and SHC. For the plant growth, three hydrochars at 3 % (w/w) significantly increased dry biomass of tomato shoot and fruit by 12.4-49.5 % and 48.6-165 %, respectively. Moreover, CHC showed the highest promotion effect on shoot and fruit dry biomass of tomato, followed by SHC ≈ RHC. Application of SHC, CHC and RHC significantly elevated the tomato sweetness compared with CK, with the order of CHC (54.4 %) > RHC (35.6 %) > SHC (22.1 %). Structural equation models revealed that CHC-depressed denitrification and methanogen mainly contributed to decreased GHG emissions. Increased soil phosphorus availability due to labile phosphorus supply from CHC dominantly accounted for elevated tomato growth and fruit production. Comparably, SHC-altered soil properties (e.g., decreased pH and increased total carbon content) determined variations of GHG emission and tomato growth. The findings provide the high-performance strategies to enhance soil primary productivity and mitigate GHG emissions in the blue C ecosystems.

Effect of Biochar Type, Concentration and Washing Conditions on the Germination Parameters of Three Model Crops

Biochar has been recognized as a promising and efficient material for soil amendment. However, its effects on seed germination are variable due to its alkaline pH and/or the presence of phytotoxic substances. In this study, two types of biochar (B1 and B2) were mixed with soil at different concentrations (0%, 5%, 10%, 25%, 50% and 100%, w:w), and both the solid and liquid fractions of these mixtures were tested on the germination of basil, lettuce and tomato seeds. Furthermore, solid fractions subjected to a pre-washing treatment (B1_W and B2_W) were also investigated for their effects on seed germination. Three germination parameters were then measured: seed germination number (GN), radicle length (RL) and germination index (GI). Biochar B2_W at 10% increased both RL and GI in basil by 50% and 70%, respectively, while B1 at 25% increased these parameters in tomato by 25%. No effects or negative effects were recorded for lettuce. Liquid fractions (L1 and L2) generally hampered seed germination, suggesting the presence of potentially water-soluble phytotoxic compounds in biochar. These results point to biochar as a suitable component for germination substrates and highlight that germination tests are critical to select the best performing biochar according to the target crop.

Recent progress on the phytotoxic effects of hydrochars and toxicity reduction approaches

Hydrothermal carbonization of wet biomasses has been known to produce added-value materials for a wide range of applications. From catalyst substrates, to biofuels and soil amendments, hydrochars have distinct advantages to offer compared to conventional materials. With respect to the agricultural application of hydrochars, both positive and negative results have been reported. The presence of N, P and K in certain hydrochars is appealing and may contribute to the reduction of chemical fertilizer application. However, regardless of biomass, hydrothermal carbonization results in the production of phytotoxic organic compounds. Additionally, hydrochars from sewage sludge often contain heavy metal concentrations which exceed the regulatory limits set for agricultural use. This review critically discusses the phytotoxic aspects of hydrochar and provides an account of the substances commonly responsible for these. Furthermore, phytotoxicity reduction approaches are proposed and compared with each other, in view of field-scale applications.

The Negative Effects of High Rates of Biochar on Violas Can Be Counteracted with Fertilizer

Increasing costs and environmental issues regarding excessive use of peat moss is impacting the horticultural industry. Biochar is a valuable substrate additive that has the potential to reduce the use of peatmoss in greenhouse production. However, its varying effects on ornamentals requires that individual species and cultivars of crops must be evaluated to determine the threshold for benefits. Viola cornuta is a high value ornamental crop; however, information on how different rates of biochar rates affect productivity and physiology of Viola cultivars in container production is not known. To determine if biochar rates could increase the productivity of Viola, we mixed a peat-based substrate with 10, 25, and 50% (w:w) hardwood biochar in two studies on four cultivars. Without fertilizers, 10 and 25% biochar improved plant biomass, growth, root length, and flowering, but 50% biochar was found to have negative effects on plant growth and flowering. Cultivars varied in their response to biochar rates. When fertilizer was applied in the second experiment, biochar rates did not impact growth parameters or flowering. These results suggest that up to 25% biochar can be used in Viola production without detrimental impacts. However, 50% biochar can be used with the addition of fertilizer without negatively affecting plant growth. Biochar can have a short-term impact on the growth characteristics of Viola plants in container production, but fertilization and growing period of Viola may influence these effects. These results indicate that biochar could be the suitable replacement for peat moss, with up to 50% biochar rate in Viola production reducing the environmental and economic burden for peat moss.

Effect of biochar produced from sewage sludge on tomato (Solanum lycopersicum L.) growth, soil chemical properties and heavy metal concentrations

The addition of biochar, as shown in the literature, improves significantly the chemical and physical soil properties and plant growth. This study examined the effect of biochar, compost and the combination of them on growth, nutrient and heavy metal concentrations of tomato. Biochar (BC) was produced from sewage sludge by pyrolysis at the temperature of 300 °C. The pot trials were carried out under an open-side greenhouse for a total of four months and under four treatments. The treatments applied were: Untreated soil (Control); soil with 2% w/w biochar (BC-SS); soil with 2% w/w compost (Compost); a mixture of biochar and compost at a 2% w/w level (BC-SS + Compost). The application of biochar exhibited substantial improvement on several soil properties. Total organic carbon (TOC) of soil increased (67%-85%), as did the nitrate nitrogen (55%) and ammonium nitrogen (145%). Additionally, available phosphorus significantly increased (45.5%-54.5%) by the application of biochar with/without compost. Dry weight of the aboveground (stems) and belowground (roots) plant tissues increased as well, although tomato yield was not increased significantly. Concentration of heavy metals and trace elements in tomato tissues was quite low. Traces of chromium (Cr), nickel (Ni), and cobalt (Co) were found only in roots of those treated, while silicon (Si) was present in the roots and stems. Arsenic (As), molybdenum (Mo) and lead (Pb) were detected in all plant tissues, but their concentrations did not exceed the permissible levels established for vegetables. Furthermore, the concentration of arsenic (As) and lead (Pb) in fruits decreased by the addition of the amendments (12%-65%). In conclusion, the addition of sewage sludge biochar improved soil characteristics and plant growth. Yet, prior to its general use, factors such as the type of biomass, soil, rate of application and crop must always be taken into consideration.

Potential Use of Waste Activated Sludge Hydrothermally Treated as a Renewable Fuel or Activated Carbon Precursor

In this work, dewatered waste activated sludge (DWAS) was subjected to hydrothermal carbonization to obtain hydrochars that can be used as renewable solid fuels or activated carbon precursors. A central composite rotatable design was used to analyze the effect of temperature (140–220 °C) and reaction time (0.5–4 h) on the physicochemical properties of the products. The hydrochars exhibited increased heating values (up to 22.3 MJ/kg) and their air-activation provided carbons with a low BET area (100 m²/g). By contrast, chemical activation with K₂CO₃, KOH, FeCl₃ and ZnCl₂ gave carbons with a well-developed porous network (BET areas of 410–1030 m²/g) and substantial contents in mesopores (0.079–0.271 cm³/g) and micropores (0.136–0.398 cm³/g). The chemically activated carbons had a fairly good potential to adsorb emerging pollutants such as sulfamethoxazole, antipyrine and desipramine from the liquid phase. This was especially the case with KOH-activated hydrochars, which exhibited a maximum adsorption capacity of 412, 198 and 146 mg/g, respectively, for the previous pollutants.

Biochar Type and Ratio as a Peat Additive/Partial Peat Replacement in Growing Media for Cabbage Seedling Production

Biochar has been proposed mainly as a soil amendment, positively affecting plant growth/yield, and to a lesser degree for growing media. In this study, four commercial grade biochars (A-forest wood; B-husks and paper fiber; C-bamboo and D-fresh wood screening), mostly wood-based materials, were selected. Initial mixtures of peat (P) with different Biochar type and ratios (0-5-10-15-20%) were selected for cabbage seedling production. Biochar material had high K content and pH ≥ 8.64 which resulted in increased pH of the growing media. Biochar A and C at 20% reduced cabbage seed emergence. Biochar A, B and D maintained or improved plant growth at low ratio (i.e., 5–10%) while all Biochars increased N, K and P content in leaves. Biochars A and D were further examined at 7.5% and 15% with the addition of two doses of minerals (1-fold and 1.5-fold). Biochar A and D, initially stimulated seed emergence when compared to the control. High dose of fertilizer favored plant growth in Biochar A at 7.5% and Biochar D at 15%. Leaf stomatal conductance was decreased at Biochar A+Fert at 7.5% and Chlorophyll b content was decreased at Biochar A+Fert at 15%. The presence of Biochar A increased the antioxidant activity (as assayed by 2,2-diphenyl-1-picrylhydrazyl-DPPH). Lipid peroxidation was higher in plants grown with fertilized peat and Biochar A at 15%, activating antioxidant enzymatic metabolisms. Potassium, phosphorous and copper accumulation and magnesium deficiency in cabbage leaves were related to the Biochar presence. Wooden biochar of beech, spruce and pine species (Biochar A) at 7.5% and fertilized biochar of fruit trees and hedges (Biochar D) were more promising for peat replacement for cabbage seedling production.

Effects of Biochar on Container Substrate Properties and Growth of Plants—A Review

Biochar refers to a processed, carbon-rich material made from biomass. This article provides a brief summary on the effects of biochar on container substrate properties and plant growth. Biochar could be produced through pyrolysis, gasification, and hydrothermal carbonization of various feedstocks. Biochar produced through different production conditions and feedstocks affect its properties and how it performs when incorporated in container substrates. Biochar incorporation affects the physical and chemical properties of container substrates, including bulk density, total porosity, container capacity, nutrient availability, pH, electrical conductivity and cation exchange capacity. Biochar could also affect microbial activities. The effects of biochar incorporation on plant growth in container substrates depend on biochar properties, plant type, percentage of biochar applied and other container substrates components mixed with biochar. A review of the literature on the impact of biochar on container-grown plants without other factors (such as irrigation or fertilization rates) indicated that 77.3% of the studies found that certain percentages of biochar addition in container substrates promoted plant growth, and 50% of the studies revealed that plant growth decreased due to certain percentages of biochar incorporation. Most of the plants tested in these studies were herbaceous plants. More plant species should be tested for a broader assessment of the use of biochar. Toxic substances (heavy metals, polycyclic aromatic hydrocarbons and dioxin) in biochars used in container substrates has rarely been studied. Caution is needed when selecting feedstocks and setting up biochar production conditions, which might cause toxic contaminants in the biochar products that could have negative effects on plant growth.

Poultry manure and sugarcane straw biochars modified with MgCl2 for phosphorus adsorption

Increases in agricultural productivity associated to the crescent use of finite reserves of phosphorus improved the demand for ways to recycle and reuse this nutrient. Biochars, after doping processes, seem to be an alternative to mitigate the large use of P reserves. Sugarcane straw and poultry manure were submerged in an MgCl₂ solution in a 1:10 solid/liquid ratio and subsequently pyrolyzed at 350 and 650 °C producing biochar. Increasing concentrations of P were agitated with biochars in order to obtain the maximum adsorption capacity of P with the aid of Langmuir and Freudelich isotherm. MPAC was extracted, successively, with H₂SO₄ (0.5 mol L^-1), NaHCO₃ (0.5 mol l^-1 a pH 8.5) and H₂O, until no P was detected in the solution. Biochars without the addition of Mg did not have the ability to adsorb P but had this property developed after the doping process. The poultry manure biochar presented higher MPAC (250.8 and 163.6 mg g^-1 of P at 350 and 650 °C, respectively) than that of sugarcane straw (17.7 and 17.6 mg g^-1 of P at 350 and 650 °C, respectively). The pyrolysis temperature changed significantly the MPAC values for the poultry manure biochar, with an increase in the adsorbed P binding energy for both biochars. H₂SO₄ showed the best extraction power, desorbing, with a lower number of extractions, the greater amount of the adsorbed P. These materials doped with Mg and subjected to pyrolysis have characteristics that allow their use in P adsorption from eutrophic and wastewaters and therefore its use as a slow release phosphate fertilizer, indicating to be competitive in quality and quantity with available soluble chemical sources in the market.