Designing relevant biochars as soil amendments using lignocellulosic-based and manure-based feedstocks

Purpose

Biochars are a by-product of the biofuel processing of lignocellulosic and manure feedstocks. Because biochars contain an assemblage of organic and inorganic compounds, they can be used as an amendment for C sequestration and soil quality improvement. However, not all biochars are viable soil amendments; this is because their physical and chemical properties vary due to feedstock elemental composition, biofuel processing, and particle size differences. Biochar could deliver a more effective service as a soil amendment if its chemistry was designed ex ante with characteristics that target specific soil quality issues. In this study, we demonstrate how biochars can be designed with relevant properties as successful soil amendments through feedstock selection, pyrolysis conditions, and particle size choices.

Materials and methods

Biochars were produced by pyrolysis of parent lignocellulosic feedstock sources—peanut hull (PH; Archis hypogaea), pecan shell (PS; Carya illinoensis), switchgrass (SG; Panicum virgatum), pine chips (PC; Pinus taeda), hardwood wastes (wood), and poultry litter manure (PL; Gallus domesticus), as well as blends of these feedstocks at temperatures ranging from 250 to 700 °C. Additionally, blended feedstocks were made into pellets (>2 mm) prior to pyrolysis at 350 °C. Dust-sized (<0.42 mm) biochar was obtained through grinding of pelletized biochars. After chemical characterization, the biochars were evaluated as fertility amendments in a Norfolk soil (fine-loamy, kaolinitic, thermic, Typic Kandiudult) during two different pot incubation experiments.

Results and discussion

PL biochars were alkaline and enriched in N and P, whereas biochar from lignocellulosic feedstocks exhibited mixed pH and nutrient contents. Blending PL with PC resulted in lower biochar pH values and nutrient contents. In pot experiment 1, most biochars significantly (P?<?0.05) raised soil pH, soil organic carbon, cation exchange capacity, and Mehlich 1 extractable P and K. PL biochar added at 20 g?kg^?1 resulted in excessive soil P concentrations (393 to 714 mg?kg^?1) and leachate enriched with dissolved phosphorus (DP, 22 to 70 mg?L^?1). In pot experiment 2, blended and pelletized PL with PC feedstock reduced soil pH and extractable soil P and K concentrations compared to pot experiment 1. Water leachate DP concentrations were significantly (P?<?0.05) reduced by pelletized biochar blends.

Conclusions

Short-term laboratory pot experiments revealed that biochars can have different impacts at modifying soil quality characteristics. Keying on these results allowed for creating designer biochars to address specific soil quality limitations. In the process of manufacturing designer biochars, first, it is important to know what soil quality characteristics are in need of change. Second, choices between feedstocks, blends of these feedstocks, and their accompanying particle sizes can be made prior to pyrolysis to create biochars tailored for addressing specific soil quality improvements. Utilization of these principles should allow for effective service of the designed biochar as a soil amendment while minimizing unwanted ex facto soil quality changes and environmental effects.     

Retention and release of diethyl phthalate in biochar-amended vegetable garden soils

Purpose

Diethyl phthalate (DEP) is one of the most commonly used plasticizers as well as a soil contaminant. Using biochar to remediate soils contaminated with DEP can potentially reduce the bioavailability of DEP and improve soil properties. Therefore, a laboratory study was conducted to evaluate the effect of biochar on soil adsorption and desorption of DEP.

Materials and methods

Two surface soils (0–20 cm) with contrasting organic carbon (OC) contents were collected from a vegetable garden. Biochars were derived from bamboo (BB) and rice straw (SB) that were pyrolyzed at 350 and 650 °C. Biochars were added to two types of soil at rates of 0.1 and 0.5 % (w/w). A batch equilibration method was used to measure DEP adsorption-desorption in biochar treated and untreated soils at 25 °C. The adsorption and desorption isotherms of DEP in the soils with or without biochar were evaluated using the Freundlich model.

Results and discussion

The biochar treatments significantly enhanced the soil adsorption of DEP. Compared to the untreated low organic matter soil, the soils treated with 0.5 % 650BB increased the adsorption by more than 19,000 times. For the straw biochar treated soils, the increase of DEP adsorption followed the order 350SB?>?650SB. However, for the bamboo biochars, the order was 650BB?>?350BB. Bamboo biochars were more effective than the straw biochars in improving soils’ adsorption capacity and reducing the desorption ability of DEP.

Conclusions

Adding biochar to soil can significantly enhance soil’s adsorption capacity on DEP. The 650BB amended soil showed the highest adsorption capacity for DEP. The native soil OC contents had significant effects on the soils’ sorption capacity treated with 650BB, whereas they had negligible effects on the other biochar treatments. The sorption capacity was affected by many factors such as the feedstock materials and pyrolysis temperature of biochars, the pH value of biochar, and the soil organic carbon levels.     

Corn growth and nitrogen nutrition after additions of biochars with varying properties to a temperate soil

The effects of biochar properties on crop growth are little understood. Therefore, biochar was produced from eight feedstocks and pyrolyzed at four temperatures (300°C, 400°C, 500°C, 600°C) using slow pyrolysis. Corn was grown for 46 days in a greenhouse pot trial on a temperate and moderately fertile Alfisol amended with the biochar at application rates of 0.0%, 0.2%, 0.5%, 2.0%, and 7.0% (w/w) (equivalent to 0.0, 2.6, 6.5, 26, and 91 t biochar ha⁻¹) and full recommended fertilization. Animal manure biochars increased biomass by up to 43% and corn stover biochar by up to 30%, while food waste biochar decreased biomass by up to 92% in relation to similarly fertilized controls (all P < 0.05). Increasing the pyrolysis temperature from 300°C to 600°C decreased the negative effect of food waste as well as paper sludge biochars. On average, plant growth was the highest with additions of biochar produced at a pyrolysis temperature of 500°C (P < 0.05), but feedstock type caused eight times more variation in growth than pyrolysis temperature. Biochar application rates above 2.0% (w/w) (equivalent to 26 t ha⁻¹) did generally not improve corn growth and rather decreased growth when biochars produced from dairy manure, paper sludge, or food waste were applied. Crop N uptake was 15% greater than the fully fertilized control (P < 0.05, average at 300°C) at a biochar application rate of 0.2% but decreased with greater application to 16% below the N uptake of the control at an application rate of 7%. Volatile matter or ash content in biochar did not correlate with crop growth or N uptake (P > 0.05), and greater pH had only a weak positive relationship with growth at intermediate application rates. Greater nutrient contents (N, P, K, Mg) improved growth at low application rates of 0.2% and 0.5%, but Na reduced growth at high application rates of 2.0% and 7.0% in the studied fertile Alfisol.     

Effects of biochars derived from different feedstocks and pyrolysis temperatures on soil physical and hydraulic properties

Purpose

Biochar addition to soils potentially affects various soil properties, and these effects are dependent on biochars derived from different feedstock materials and pyrolysis processes. The objective of this study was to investigate the effects of amendment of different biochars on soil physical and hydraulic properties.

Materials and methods

Biochars were produced with dairy manure and woodchip at temperatures of 300, 500, and 700 °C, respectively. Each biochar was mixed at 5 % (w/w) with a forest soil, and the mixture was incubated for 180 days, during which soil physical and hydraulic properties were measured.

Results and discussion

Results showed that the biochar addition significantly enhanced the formation of soil macroaggregates at the early incubation time. The biochar application significantly reduced soil bulk density, increased the amount of soil organic matter, and stimulated microbial activity at the early incubation stage. Saturated hydraulic conductivities of the soil with biochars, especially produced at high pyrolysis temperature, were higher than those without biochars on the sampling days. The treatments with woodchip biochars resulted in higher saturated hydraulic conductivities than the dairy manure biochar treatments. Biochar applications improved water retention capacity, with stronger effects by biochars produced at higher pyrolysis temperatures. At the same suction, the soil with woodchip biochars possessed higher water content than that with the dairy manure biochars.

Conclusions

Biochar addition significantly affected the soil physical and hydraulic properties. The effects were different with biochars derived from different feedstock materials and pyrolysis temperatures.     

Effects of heating and autoclaving on sorption and desorption of phosphorus in some forest soils

A number of biological and chemical processes may affect soil phosphorus availability when forest fires occur, partly as a result of heating. We describe here a laboratory experiment to study the effects of soil heating on changes in sorption and desorption of P. Autoclaving was also included as an additional treatment of moist heating under pressure. Five forest soils (two Podzols, one Arenosol, one Luvisol and one Alisol) were heated to 60°C, 120°C and 250°C or autoclaved for 30 min. They were repeatedly extracted with Bray I and analysed for inorganic and organic P fractions. The desorbed P data were fitted to an asymptotic exponential equation to obtain the desorption rate and capacity parameters. Podzol and Arenosol soils showed a quick P desorption after heating, while Luvisol and Alisol soils showed a slow desorption rate. The immediate increase in available P that occurred after heating or autoclaving originated mostly from solubilisation of microbial metabolites and soil organic components. Autoclaving decreased P sorption capacity in all soils, but the effects of heating on P sorption differed among soils. Except for one of the soils, the low P-fixing soils (Podzol and Arenosol) showed a decrease in P sorption when heated to high temperatures, whereas the high P-fixing soils (Luvisol and Alisol) showed little changes after heating. Fire intensity and soil characteristics are important factors determining short-term and long-term soil P dynamics.     

Effect of coffee husk and cocoa pods biochar on phosphorus fixation and release processes in acid soils from West Cameroon

Acid soil in West Cameroon has limited phosphorus (P) availability which limits plant growth. This is mainly because of low pH, high levels of exchangeable aluminium (Al) and iron (Fe) and fixation of P. In this study, acid soils, sampled in Bafang, were amended with biochar produced from coffee husks (CH) and cocoa pod husks (CP) at two different temperatures (350 and 550 °C) in other to evaluate the effect on the physicochemical properties of the acid soil and the effect on P sorption and desorption. The soil was amended with biochar at a rate of 0, 20, 40 and 80 g/kg and incubated for 7 and 60 days. Physicochemical properties of all soil–biochar samples were determined followed by sorption experiments and data fitted in the Langmuir and Freundlich isotherm models in other to evaluate soil P sorption capacity and its affinity to soil amended with biochar. Moreover, desorption studies were done to evaluate the availability of P in soil amended with biochar after sorption. The outcomes of this study reveal an increase in soil pH, electrical conductivity (EC), available P, soil organic carbon and a drastic decrease in exchangeable Al and Fe. The point of zero charge of biochar-amended soil was higher than the control and increased with amendment rate. The experimental data of the sorption of P on soils and soil–biochar samples fits into Langmuir and Freundlich models (R² > 0.9) suggesting that the P adsorption is controlled by both model mechanisms. Soil–biochar mixture results in a decrease in the sorption capacity as compared with the control and the decrease was predominant with increasing amendment rate. At amendment rates of 20, 40 and 80 g/kg after 7 days of incubation, $Q_{\max }$ for SCH350 were 2267, 2048 and 1823 mg/kg which increased to 2407, 2112 and 1990 mg/kg after 60 days of incubation. This tendency was observed for all biochar inputs with respect to the increase in incubation days. Furthermore, desorption of P from soil–biochar mixtures was enhanced with biochar added at greater rate and produced at higher temperature. The desorption percentage was increased by more than around 10% for all biochar types from 20 mg/kg to 80 mg/kg amendment. Thus, biochar addition to acid soils reduces P fixation to acid soil and improves P desorption to soil solution, thereby providing more available P in the soil solution and better conditions for plant growth.     

Biochar-induced changes in soil properties affected immobilization/mobilization of metals/metalloids in contaminated soils

Purpose

Remediation of metal contaminated soil with biochar is attracting extensive interest in recent years. Understanding the significance of variable biochar properties and soil types helps elucidating the meticulous roles of biochar in immobilizing/mobilizing metals/metalloids in contaminated soils.

Materials and methods

Six biochars were produced from widely available agricultural wastes (i.e., soybean stover, peanut shells and pine needles) at two pyrolysis temperatures of 300 and 700 °C, respectively. The Pb-, Cu-, and Sb-contaminated shooting range soils and Pb-, Zn-, and As-contaminated agricultural soils were amended with the produced biochars. The mobility of metals/metalloids was assessed by the standard batch leaching test, principal component analysis and speciation modeling.

Results and discussion

The changes in soil properties were correlated to feedstock types and pyrolysis temperatures of biochars based on the principal component analysis. Biochars produced at 300 °C were more efficient in decreasing Pb and Cu mobility (>93 %) in alkaline shooting range soil via surface complexation with carboxyl groups and Fe-/Al-minerals of biochars as well as metal-phosphates precipitation. By contrast, biochars produced at 700 °C outperformed their counterparts in decreasing Pb and Zn mobility (100 %) in acidic agricultural soil by metal-hydroxides precipitation due to biochar-induced pH increase. However, Sb and As mobility in both soils was unfavorably increased by biochar amendment, possibly due to the enhanced electrostatic repulsion and competition with phosphate.

Conclusions

It is noteworthy that the application of biochars is not equally effective in immobilizing metals or mobilizing metalloids in different soils. We should apply biochar to multi-metal contaminated soil with great caution and tailor biochar production for achieving desired outcome and avoiding adverse impact on soil ecosystem.

     

Phosphorus availability and grass growth in biochar-modified acid soil: A study excluding the effects of soil pH

The low efficiency of phosphorus fertilization in weathered soils can limit plant development. The application of biochars in these areas has been seen as an important way to increase the efficiency of phosphorus fertilization and to promote better plant growth. However, biochars are alkaline materials that can increase soil pH and thus change the nutrient dynamics, which has been often ignored in studies of this nature. Here, all treatments had their pH standardized at 6.1 to eliminate the influence of pH on biochar application responses. The main goal of this study was to evaluate the real potential of coffee straw and eucalyptus bark biochars, produced under different pyrolysis temperatures, in the optimization of phosphorus fertilization and the development of Brachiaria brizantha. A greenhouse experiment was set up in a 2 × 2 × 5 factorial scheme, conducted for 120 days. The biochars, prepared from coffee straw and eucalyptus bark at 350 and 600°C, were applied at five rates in a Red-Yellow Oxisol. The application of biochars may reduce the demand for nutrients and correctives, optimize phosphorus fertilization and improve the development of Brachiaria brizantha, but this ability depends on the raw material and the pyrolysis temperature used in its production. All analysed biochars can contribute to higher tillering and dry matter production, but only coffee straw biochars and eucalyptus bark biochar produced at 350°C were efficient in the optimization of phosphorus fertilization until 120 days of cultivation of Brachiaria brizantha.     

Effect of Organic Residues and Their Derived Biochars on the Zinc and Copper Chemical Fractions and Some Chemical Properties of a Calcareous Soil

ABSTRACT

Addition of more resistant organic materials, such as biochars, to soils not only enhances soil C sequestration but also can also benefit soil fertility. The aim of this study was to investigate the effect of two organic materials (sheep manure and vermicompost) and their biochars produced at two pyrolysis temperatures (300 and 500°C) applied at 5% (w/w) on the chemical fractions of Zn and Cu and some chemical characteristics of an unpolluted, light textured calcareous soil. Addition of the raw organic materials and their-derived biochars significantly enhanced plant available K, P, and Zn but significantly decreased plant available Cu in the soil. Sheep manure biochar produced at 300°C was most effective at increasing plant available P (13-fold) and K (1.9 fold) likely due to formation of more soluble forms of P and K compared to raw material or biochar produced at higher temperature (500°C). Whereas, raw vermicompost and sheep manure were most effective at enhancing plant available Zn, by increasing water soluble and exchangeable Zn fraction likely due to organic complexation. All amendments, especially biochars produced at 300°C reduced water soluble and exchangeable Cu mainly attributed to increased soil P availability. The results of this study showed that in the short-term, addition of the low-temperature biochars was best for enhancing soil P and K availability, but concomitantly reduced Cu availability the most, whereas, addition of the raw organic materials was better for enhancing Zn availability compared to the biochars.     

Augmenting soil water storage using uncharred switchgrass and pyrolyzed biochars

Biochar is an amendment that can augment soil water storage; however, its projected cost per ton could be financially limiting at field application scales. It may be more monetarily convenient if an alternate amendment was available that could deliver similar soil enhancements. We compared two switchgrass biochars pyrolyzed at 250 and 500 °C with raw switchgrass (uncharred) on moisture storage and bulk density changes in a Norfolk loamy sand (fine‐loamy, kaolinitic, thermic Typic Kandiudult). Amendments were mixed into triplicate pots at 20 g/kg along with untreated controls. Soils were laboratory incubated at 10% moisture content (w/w) for 118 days, and the pots were irrigated three times with 1.3 pore volumes of deionized water every 30 days. Soil bulk densities were recorded before each irrigation event. Assessment of alterations in soil water storage was examined through cumulative water evaporative losses from incubation day 0 to day 33 and by monitoring soil water contents for 13 consecutive days past each irrigation event. Rankings of soil water evaporative losses were as follows: uncharred switchgrass ≤ switchgrass (500 °C) ≤ switchgrass (250 °C) < control. After the first irrigation event, uncharred switchgrass amendment significantly increased moisture storage compared with soil treated with biochar and the control. While all amendments increased water storage relative to the control, uncharred switchgrass delivered equivalent, if not slightly better, moisture storage improvements compared with the two switchgrass biochars. Uncharred switchgrass would likely not be as effective over the long term (years to decades) as pyrolyzed biochars, due to greater degradation of uncharred material.     

Positive and negative carbon mineralization priming effects among a variety of biochar-amended soils

Pyrogenic carbon (biochar) amendment is increasingly discussed as a method to increase soil fertility while sequestering atmospheric carbon (C). However, both increased and decreased C mineralization has been observed following biochar additions to soils. In an effort to better understand the interaction of pyrogenic C and soil organic matter (OM), a range of Florida soils were incubated with a range of laboratory-produced biochars and CO₂ evolution was measured over more than one year. More C was released from biochar-amended than from non-amended soils and cumulative mineralized C generally increased with decreasing biomass combustion temperature and from hardwood to grass biochars, similar to the pattern of biochar lability previously determined from separate incubations of biochar alone.The interactive effects of biochar addition to soil on CO₂ evolution (priming) were evaluated by comparing the additive CO₂ release expected from separate incubations of soil and biochar with that actually measured from corresponding biochar and soil mixtures. Priming direction (positive or negative for C mineralization stimulation or suppression, respectively) and magnitude varied with soil and biochar type, ranging from −52 to 89% at the end of 1 year. In general, C mineralization was greater than expected (positive priming) for soils combined with biochars produced at low temperatures (250 and 400 °C) and from grasses, particularly during the early incubation stage (first 90 d) and in soils of lower organic C content. It contrast, C mineralization was generally less than expected (negative priming) for soils combined with biochars produced at high temperatures (525 and 650 °C) and from hard woods, particularly during the later incubation stage (250-500 d). Measurements of the stable isotopic signature of respired CO₂ indicated that, for grass biochars at least, it was predominantly pyrogenic C mineralization that was stimulated during early incubation and soil C mineralization that was suppressed during later incubation stages. It is hypothesized that the presence of soil OM stimulated the co-mineralization of the more labile components of biochar over the short term. The data strongly suggests, however, that over the long term, biochar-soil interaction will enhance soil C storage via the processes of OM sorption to biochar and physical protection.     

Influence of Pyrolysis Temperatures on FTIR Analysis,Nutrient Bioavailability,and Agricultural use of Poultry Manure Biochars

This study was conducted to investigate the effect of pyrolysis temperature on chemical properties of poultry manure (PM) biochar over the range of 200–500°C. Chemical properties of biochar produced at 200°C were almost the same as PM, but significant changes were observed in higher-temperature-produced biochars. According to elemental and fourier transformation infrared analyses, the degree of carbonization in biochar was accelerated with increasing pyrolysis temperature. Biochar yield decreased, while its pH, cation exchange capacity, and P, K, Fe, Mn, Zn, and Cu contents increased with increasing pyrolysis temperature. The biochar produced at 400°C or 500°C was highly alkaline. Also, due to high electrical conductivity, these types of biochars may not be suitable for salt-sensitive crops. It was concluded that the pyrolysis temperature of more than 300°C reduces the quality of PM biochar for use in calcareous soils, although it may be suitable for acidic soils or environmental application.     

Comparative sorption and leaching study of the herbicides fluometuron and 4-chloro-2-methylphenoxyacetic acid (MCPA) in a soil amended with biochars and other sorbents

Biochar, the solid residual remaining after the thermochemical transformation of biomass for carbon sequestration, has been proposed to be used as a soil amendment, because of its agronomic benefits. The effect of amending soil with six biochars made from different feedstocks on the sorption and leaching of fluometuron and 4-chloro-2-methylphenoxyacetic acid (MCPA) was compared to the effect of other sorbents: an activated carbon, a Ca-rich Arizona montmorillonite modified with hexadecyltrimethylammonium organic cation (SA-HDTMA), and an agricultural organic residue from olive oil production (OOW). Soil was amended at 2% (w/w), and studies were performed following a batch equilibration procedure. Sorption of both herbicides increased in all amended soils, but decreased in soil amended with a biochar produced from macadamia nut shells made with fast pyrolysis. Lower leaching of the herbicides was observed in the soils amended with the biochars with higher surface areas BC5 and BC6 and the organoclay (OCl). Despite the increase in herbicide sorption in soils amended with two hardwood biochars (BC1 and BC3) and OOW, leaching of fluometuron and MCPA was enhanced with the addition of these amendments as compared to the unamended soil. The increased leaching is due to some amendments' soluble organic compounds, which compete or associate with herbicide molecules, enhancing their soil mobility. Thus, the results indicate that not all biochar amendments will increase sorption and decrease leaching of fluometuron and MCPA. Furthermore, the amount and composition of the organic carbon (OC) content of the amendment, especially the soluble part (DOC), can play an important role in the sorption and leaching of these herbicides.     

Effect of temperature on biochar priming effects and its stability in soils

Recent studies have shown both increased (positive priming) and decreased (negative priming) mineralisation of native soil organic carbon (SOC) with biochar addition. However, there is only limited understanding of biochar priming effects and its C mineralisation in contrasting soils at different temperatures, particularly over a longer period. To address this knowledge gap, two wood biochars (450 and 550 °C; δ¹³C −36.4‰) were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol; δ¹³C −17.3 to −28.2‰) at 20, 40 and 60 °C in the laboratory. The proportions of biochar- and soil-derived CO₂–C were quantified using a two-pool C-isotopic model.Both biochars caused mainly positive priming of native SOC (up to +47 mg CO₂–C g⁻¹ SOC) in the Inceptisol and negative priming (up to −22 mg CO₂–C g⁻¹ SOC) in the other soils, which increased with increasing temperature from 20 to 40 °C. In general, positive or no priming occurred during the first few months, which remained positive in the Inceptisol, but shifted to negative priming with time in the other soils. The 550 °C biochar (cf. 450 °C) caused smaller positive priming in the Inceptisol or greater negative priming in the Entisol, Oxisol and Vertisol at 20 and 40 °C. At 60 °C, biochar caused positive priming of native SOC only in the first 6 months in the Inceptisol. Whereas, in the other soils, the native SOC mineralisation was increased (Entisol and Oxisol) and decreased (Vertisol) only after 6 months, relative to the control. At 20 °C, the mean residence time (MRT) of 450 °C and 550 °C biochars in the four soils ranged from 341 to 454 and 732−1061 years, respectively. At 40 and 60 °C, the MRT of both 450 °C biochar (25−134 years) and 550 °C biochar (93−451 years) decreased substantially across the four soils. Our results show that biochar causes positive priming in the clay-poor soil (Inceptisol) and negative priming in the clay-rich soils, particularly with biochar ageing at a higher incubation temperature (e.g. 40 °C) and for a high-temperature (550 °C) biochar. Furthermore, the 550 °C wood biochar has been shown to persist in soil over a century or more even at elevated temperatures (40 or 60 °C).     

Variations in mineral element concentrations of poultry manure biochar obtained at different pyrolysis temperatures,and their effects on crop growth and mineral nutrition

The effect of pyrolysis temperature on the nutritional quality of agricultural biochar is unclear, so better understanding of its properties and how it affects soil nutrient availability and plant growth is needed. Biochars obtained at different pyrolysis temperatures (250, 300, 350 and 400 °C) were characterized by thermogravimetric analyser and Fourier transform infrared spectroscopy. Biochars were applied at a rate of 10 g/kg to find out their effects on the mineral nutrition and growth of lettuce. The experimental results suggested that high biochar temperatures caused oxidation of the mineral elements, breaking of C–C and C–H bonds and removal of aliphatic and peptide groups from the pyrolysed materials. The total concentrations of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), copper (Cu), manganese (Mn) and boron (B) were increased by increasing pyrolysis temperatures, although water‐soluble concentrations of those elements were greatly reduced (with the exception of K and B). Compared to the control, dry weights of lettuce and maize crops were significantly increased by the biochar treatments obtained at 300 and 350 °C. Biochar treatments significantly increased the P and K concentrations of both plants compared to the control, while concentrations of Ca and Mg in lettuce plants were decreased. Iron, Mn and B concentration of the lettuce plants were reduced and Zn concentration of maize increased by the biochar treatments. It was concluded that in terms of an agricultural product, biochars produced at low temperature are better.     

Effects of pyrolysis temperature and residence time on physicochemical properties of different biochar types

Here we selected eight types of feedstocks to assess the effects of pyrolysis temperature (300°C, 400°C, 500°C and 600°C) and residence time (0.5, 1, 2, 4, 8 and 24 h), respectively, on the physicochemical properties. The fixed-carbon content, pH value and amount of basic functional groups in biochars increased as the pyrolysis temperature increased from 300°C to 600°C; the opposite trend was found in the biochar yield, adsorption capacity and amount of acidic functional groups. Increasing the residence time at low pyrolysis temperature (300°C) resulted in a gradual reduction in the biochar yield and progressive increase in the pH and iodine adsorption number of biochars. However, increasing the residence time at high pyrolysis temperature (600°C) had little effect on the biochar yield or pH, while it decreased the iodine adsorption number of biochars. Given the effects of pyrolysis conditions on the pH and iodine adsorption number of biochars, low-ash agricultural wastes (e.g. wheat straw) can be pyrolysed at 300°C, 2 h to produce biochar for improving alkaline soils; high-ash agricultural wastes (e.g. sweet potato vine) and forest litter (e.g. fresh leaves of apricot tree) are preferably pyrolysed at 300°C, 4 h to produce biochar for use in acidic soils.     

Effects of biochars produced from different feedstocks on soil properties and sunflower growth

The use of biochar as a soil amendment is gaining interest to mitigate climate change and improve soil fertility and crop productivity. However, studies to date show a great variability in the results depending on raw materials and pyrolysis conditions, soil characteristics, and plant species. In this study, we evaluated the effects of biochars produced from five agricultural and forestry wastes on the properties of an organic‐C‐poor, slightly acidic, and loamy sand soil and on sunflower (Helianthus annuus L.) growth. The addition of biochar, especially at high application rates, decreased soil bulk density and increased soil field capacity, which should impact positively on plant growth and water economy. Furthermore, biochar addition to soil increased dissolved organic C (wheat‐straw and olive‐tree‐pruning biochars), available P (wheat‐straw biochar), and seed germination, and decreased soil nitrate concentration in all cases. The effects of biochar addition on plant dry biomass were greatly dependent upon the biochar‐application rate and biochar type, mainly associated to its nutrient content due to the low fertility of the soil used. As a result, the addition of ash‐rich biochars (produced from wheat straw and olive‐tree pruning) increased total plant dry biomass. On the other hand, the addition of biochar increased the leaf biomass allocation and decreased the stem biomass allocation. Therefore, biochar can improve soil properties and increase crop production with a consequent benefit to agriculture. However, the use of biochar as an amendment to agricultural soils should take into account its high heterogeneity, particularly in terms of nutrient availability.     

Sugarcane Bagasse Biochar: Preparation,Characterization, and Its Effects on Soil Properties and Zinc Sorption-desorption

ABSTRACT

Application of alkaline biochar has been proposed as an alternative to lime for remediation of acidic soils. However, questions remain as to how the reactions and fate of metals in acidic soils can be affected by biochar amendment. To find out how biochar addition might affect sorption-desorption behavior of zinc (Zn) in acidic soils, a soil with an initial pH value of 4.67 was treated with different levels [0 (control), 1%, 3%, and 6%] of biochar produced from pyrolysis of sugarcane bagasse at 600°C and incubated for 30 days under 80% of water holding capacity. At the end of the incubation period, important soil chemical properties were measured and batch isotherm experiments were performed to determine soil Zn sorption-desorption parameters. The results showed that the biochar-amended soils had higher pH values (up to 2.5 pH units), electrical conductivity (up to 2.66 times), and cation exchange capacities (up to 42%) relative to the un-amended acidic soil. Biochar addition also led to significant enhancements in soil exchangeable calcium, magnesium, sodium, and potassium cations. Both sorption and desorption isotherm experiments revealed the significantly higher capacity of the biochar-amended soils to retain Zn than that of the control. Moreover, the biochar-amended soils exhibited a higher affinity for Zn sorption than did the un-amended acidic one. It can be concluded that biochar derived from sugarcane bagasse could serve as a good amendment material to reclaim acidic soils and to reduce Zn mobility and toxicity in acidic metal-contaminated soils.     

Microbial biomass growth,following incorporation of biochars produced at 350 °C or 700 °C,in a silty-clay loam soil of high and low pH

Biochar has been widely proposed as a soil amendment, with reports of benefits to soil physical, chemical and biological properties. To quantify the changes in soil microbial biomass and to understand the mechanisms involved, two biochars were prepared at 350 °C (BC350) and 700 °C (BC700) from Miscanthus giganteus, a C4 plant, naturally enriched with ¹³C. The biochars were added to soils of about pH 4 and 8, which were both sampled from a soil pH gradient of the same soil type. Isotopic (¹³C) techniques were used to investigate biochar C availability to the biomass. Scanning Electron Microscopy (SEM) was used to observe the microbial colonization, and Attenuated Total Reflectance (ATR) to highlight structural changes at the surface of the biochars. After 90 days incubation, BC350 significantly increased the biomass C concentration relative to the controls in both the low (p < 0.05) and high pH soil (p < 0.01). It declined between day 90 and 180. The same trend occurred with soil microbial ATP. Overall, biomass C and ATP concentrations were closely correlated over all treatments (R² = 0.87). This indicates that neither the biomass C, nor ATP analyses were affected by the biochars, unless, of course, they were both affected in the same way, which is highly unlikely. About 20% of microbial biomass ¹³C was derived from BC350 after 90 days of incubation in both low and high pH soils. However, less than 2% of biomass ¹³C was derived from BC700 in the high pH soil, showing very low biological availability of BC700. After 90 days of incubation, microbial colonization in the charsphere (defined here as the interface between soil and biochar) was more pronounced with the BC350 in the low pH soil. This was consistent with the biomass C and ATP results. The microbial colonization following biochar addition in our study was mainly attributed to biochar C availability and its large surface area. There was a close linear relationship between ¹³CO₂ evolved and biomass ¹³C, suggesting that biochar mineralization is essentially a biological process. The interactions between non-living and living organic C forms, which are vital in terms of soil fertility and the global C cycle, may be favoured in the charsphere, which has unique properties, distinct from both the internal biochar and the bulk soil.     

New trends in biochar pyrolysis and modification strategies: feedstock,pyrolysis conditions,sustainability concerns and implications for soil amendment

As a waste-derived soil amendment with a long history, biochar has received extensive attention for its capability to improve soil fertility/health; remove or immobilize contaminants in soil, water and air; and mitigate climate change. With the aim of producing engineered biochars with excellent performances, new trends in biochar pyrolytic production and modification strategies have emerged. This review critically summarizes novel pyrolysis methods (e.g., microwave-assisted pyrolysis, co-pyrolysis and wet pyrolysis) and modification approaches (e.g., mineral modification, photocatalytic modification, electrochemical modification) with a focus on (a) the mechanisms involved in environmental remediation processes including soil immobilization, contaminant adsorption and catalytic oxidation; (b) effects of feedstock and pyrolysis conditions on physicochemical properties; (c) sustainability considerations in novel modification and pyrolysis strategies; and (d) the feasibility of extrapolating the results from wastewater treatment to soil remediation. It is argued that in order to achieve the maximum net environmental benefits, ‘greener’ modification methods are warranted, and the risks associated with pyrolysis of contaminated feedstock in soil amendment and contaminant sorption can be minimized through various novel approaches (e.g., co-pyrolysis). Furthermore, novel pyrolysis methods can be combined with emerging modification strategies to synthesize more ‘effective’ biochars. Considering the similar aims of modification (e.g., increase surface area, introduce oxygen-containing functional groups, increase aromaticity), the applicability of several novel approaches could in future can be expanded from contaminant adsorption/degradation in aqueous media to soil remediation/fertility improvement.