Jeffrey M. Novak

IMPACT OF BIOCHAR AMENDMENT ON FERTILITY OF A SOUTHEASTERN COASTAL PLAIN SOIL

Agricultural soils in the southeastern U.S. Coastal Plain region have meager soil fertility characteristics because of their sandy textures, acidic pH values, kaolinitic clays, low cation exchange capacities, and diminutive soil organic carbon contents. We hypothesized that biochar additions will help ameliorate some of these fertility problems. The study objectives were to determine the impact of pecan shell-based biochar additions on soil fertility characteristics and water leachate chemistry for a Norfolk loamy sand (fine-loamy, kaolinitic, thermic typic Kandiudults). Soil columns containing 0, 0.5, 1.0, and 2.0% (wt/wt) biochar were incubated at 10% (wt/wt) moisture for 67 days. On days 25 and 67, the columns were leached with 1.2 to 1.4 pore volumes of deionized H2O, and the leachate chemical composition determined. On days 0 and 67, soil samples were collected and analyzed for fertility. The biochar had a pH of 7.6, contained 834.2 and 3.41 g kg−1 of C and N, respectively, and was dominated by aromatic C (58%). After 67 days and two leaching events, biochar additions to the Norfolk soil increased soil pH, soil organic carbon, Ca, K, Mn, and P and decreased exchangeable acidity, S, and Zn. Biochar additions did not significantly increase soil cation exchange capacity. Leachates contained increasing electrical conductivity and K and Na concentrations, but decreasing levels of Ca, P, Mn, and Zn. These effects reflect the addition of elements and the higher sorption capacity of biochar for selective nutrients (especially Ca, P, Zn, and Mn). Biochar additions to the Norfolk soil caused significant fertility improvements.

Biochar: A Synthesis of Its Agronomic Impact beyond Carbon Sequestration

Biochar has been heralded as an amendment to revitalize degraded soils, improve soil carbon sequestration, increase agronomic productivity, and enter into future carbon trading markets. However, scientific and economic technicalties may limit the ability of biochar to consistently deliver on these expectations. Past research has demonstrated that biochar is part of the black carbon continuum with variable properties due to the net result of production (e.g., feedstock and pyrolysis conditions) and postproduction factors (storage or activation). Therefore, biochar is not a single entity but rather spans a wide range of black carbon forms. Biochar is black carbon, but not all black carbon is biochar. Agronomic benefits arising from biochar additions to degraded soils have been emphasized, but negligible and negative agronomic effects have also been reported. Fifty percent of the reviewed studies reported yield increases after black carbon or biochar additions, with the remainder of the studies reporting alarming decreases to no significant differences. Hardwood biochar (black carbon) produced by traditional methods (kilns or soil pits) possessed the most consistent yield increases when added to soils. The universality of this conclusion requires further evaluation due to the highly skewed feedstock preferences within existing studies. With global population expanding while the amount of arable land remains limited, restoring soil quality to nonproductive soils could be key to meeting future global food production, food security, and energy supplies; biochar may play a role in this endeavor. Biochar economics are often marginally viable and are tightly tied to the assumed duration of agronomic benefits. Further research is needed to determine the conditions under which biochar can provide economic and agronomic benefits and to elucidate the fundamental mechanisms responsible for these benefits.

Impact of pyrolysis temperature and manure source on physicochemical characteristics of biochar

While pyrolysis of livestock manures generates nutrient-rich biochars with potential agronomic uses, studies are needed to clarify biochar properties across manure varieties under similar controlled conditions. This paper reports selected physicochemical results for five manure-based biochars pyrolyzed at 350 and 700°C: swine separated-solids; paved-feedlot manure; dairy manure; poultry litter; and turkey litter. Elemental and FTIR analyses of these alkaline biochars demonstrated variations and similarities in physicochemical characteristics. The FTIR spectra were similar for (1) turkey and poultry and (2) feedlot and dairy, but were distinct for swine biochars. Dairy biochars contained the greatest volatile matter, C, and energy content and lowest ash, N, and S contents. Swine biochars had the greatest P, N, and S contents alongside the lowest pH and EC values. Poultry litter biochars exhibited the greatest EC values. With the greatest ash contents, turkey litter biochars had the greatest biochar mass recoveries, whereas feedlot biochars demonstrated the lowest.

Qualitative analysis of volatile organic compounds on biochar

Qualitative identification of sorbed volatile organic compounds (VOCs) on biochar was conducted by headspace thermal desorption coupled to capillary gas chromatographic-mass spectrometry. VOCs may have a mechanistic role influencing plant and microbial responses to biochar amendments, since VOCs can directly inhibit/stimulate microbial and plant processes. Over 70 biochars encompassing a variety of parent feedstocks and manufacturing processes were evaluated and were observed to possess diverse sorbed VOC composition. There were over 140 individual chemical compounds thermally desorbed from some biochars, with hydrothermal carbonization (HTC) and fast pyrolysis biochars typically possessing the greatest number of sorbed volatiles. In contrast, gasification, thermal or chemical processed biochars, soil kiln mound, and open pit biochars possessed low to non-detectable levels of VOCs. Slow pyrolysis biochars were highly variable in terms of their sorbed VOC content. There were no clear feedstock dependencies to the sorbed VOC composition, suggesting a stronger linkage with biochar production conditions coupled to post-production handling and processing. Lower pyrolytic temperatures (⩽350°C) produced biochars with sorbed VOCs consisting of short carbon chain aldehydes, furans and ketones; elevated temperature biochars (>350°C) typically were dominated by sorbed aromatic compounds and longer carbon chain hydrocarbons. The presence of oxygen during pyrolysis also reduced sorbed VOCs. These compositional results suggest that sorbed VOCs are highly variable and that their chemical dissimilarity could play a role in the wide variety of plant and soil microbial responses to biochar soil amendment noted in the literature. This variability in VOC composition may argue for VOC characterization before land application to predict possible agroecosystem effects.

Biochars impact on soil moisture storage in an Ultisol and two Aridisols

Abstract Biochar additions to soils can improve soil-water storage capability; however, there is sparse information identifying feedstocks and pyrolysis conditions that maximize this improvement. Nine biochars were pyrolyzed from five feedstocks at two temperatures, and their physical and chemical properties were characterized. Biochars were mixed at 2% wt wt−1 into a Norfolk loamy sand (Fine-loamy, kaolinitic, thermic Typic Kandiudult), a Declo silt loam (Coarse-loamy, mixed, superactive, mesic xeric Haplocalcid), or a Warden silt loam (Coarse-silty, mixed, superactive, mesic xeric Haplocambid). Untreated soils served as controls. Soils were laboratory incubated in pots for 127 days and were leached about every 30 days with deionized water. Soil bulk densities were measured before each leaching event. For 6 days thereafter, pot-holding capacities (PHC) for water were determined gravimetrically and were used as a surrogate for soil-moisture contents. Water tension curves were also measured on the biochar-treated and untreated Norfolk soil. Biochar surface area, surface tension, ash, C, and Si contents, in general, increased when produced under higher pyrolytic temperatures (≥500°C). Both switchgrass biochars caused the most significant water PHC improvements in the Norfolk, Declo, and Warden soils compared with the controls. Norfolk soil-water tension results at 5 and 60 kPa corroborated that biochar from switchgrass caused the most significant moisture storage improvements. Significant correlation occurred between the PHC for water with soil bulk densities. In general, biochar amendments enhanced the moisture storage capacity of Ultisols and Aridisols, but the effect varied with feedstock selection and pyrolysis temperature.

Switchgrass Biochar Affects Two Aridisols

The use of biochar has received growing attention because of its ability to improve the physicochemical properties of highly weathered Ultisols and Oxisols, yet very little research has focused on its effects in Aridisols. We investigated the effect of low or high temperature (250 or 500°C) pyrolyzed switchgrass () biochar on two Aridisols. In a pot study, biochar was added at 2% w/w to a Declo loam (Xeric Haplocalcids) or to a Warden very fine sandy loam (Xeric Haplocambids) and incubated at 15% moisture content (by weight) for 127 d; a control (no biochar) was also included. Soils were leached with 1.2 to 1.3 pore volumes of deionized HO on Days 34, 62, 92, and 127, and cumulative leachate Ca, K, Mg, Na, P, Cu, Fe, Mn, Ni, Zn, NO-N, NO-N, and NH-N concentrations were quantified. On termination of the incubation, soils were destructively sampled for extractable Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Zn, NO-N, and NH-N, total C, inorganic C, organic C, and pH. Compared with 250°C, the 500°C pyrolysis temperature resulted in greater biochar surface area, elevated pH, higher ash content, and minimal total surface charge. For both soils, leachate Ca and Mg decreased with the 250°C switchgrass biochar, likely due to binding by biochars functional group sites. Both biochars caused an increase in leachate K, whereas the 500°C biochar increased leachate P. Both biochars reduced leachate NO-N concentrations compared with the control; however, the 250°C biochar reduced NO-N concentrations to the greatest extent. Easily degradable C, associated with the 250°C biochars structural make-up, likely stimulated microbial growth, which caused NO-N immobilization. Soil-extractable K, P, and NO-N followed a pattern similar to the leachate observations. Total soil C content increases were linked to an increase in organic C from the biochars. Cumulative results suggest that the use of switchgrass biochar prepared at 250°C could improve environmental quality in calcareous soil systems by reducing nutrient leaching potential.

Macroscopic and molecular investigations of copper sorption by a steam-activated biochar.

Excessive Cu concentrations in water systems can negatively affect biological systems. Because Cu can form strong associations with organic functional groups, we examined the ability of biochar (an O-C-enriched organic bioenergy by-product) to sorb Cu from solution. In a batch experiment, KOH steam-activated pecan shell biochar was shaken for 24 h in pH 6, 7, 8, or 9 buffered solutions containing various Cu concentrations to identify the effect of pH on biochar Cu sorption. Afterward, all biochar solids from the 24-h shaking period were air-dried and analyzed using X-ray absorption fine structure (XAFS) spectroscopy to determine solid-phase Cu speciation. In a separate batch experiment, biochar was shaken for 30 d in pH 6 buffered solution containing increasing Cu concentrations; the Cu sorption maximum was calculated based on the exponential rise to a maximum equation. Biochar sorbed increasing amounts of Cu as the solution pH decreased from 9 to 6. The XAFS spectroscopy revealed that Cu was predominantly sorbed onto a biochar organic phase at pH 6 in a molecular structure similar to Cu adsorbed on model humic acid (Cu-humic acid [HA]). The XAFS spectra at pH 7, 8, and 9 suggested that Cu was associated with the biochar as three phases: (i) a complex adsorbed on organic ligands similar to Cu-HA, (ii) carbonate phases similar to azurite (Cu(CO)(OH)), and (iii) a Cu oxide phase like tenorite (CuO). The exponential rise equation fit to the incubated samples predicted a Cu sorption maximum of 42,300 mg Cu kg. The results showed that KOH steam-activated pecan shell biochar could be used as a material for sorbing excess Cu from water systems, potentially reducing the negative effects of Cu in the environment.

Carbon mineralization in two ultisols amended with different sources and particle sizes of pyrolyzed biochar.

Biochar produced during pyrolysis has the potential to enhance soil fertility and reduce greenhouse gas emissions. The influence of biochar properties (e.g., particle size) on both short- and long-term carbon (C) mineralization of biochar remains unclear. There is minimal information on the potential effects of biochar particle sizes on their breakdowns by soil microorganism, so it is unknown if the particle size of biochar influences C mineralization rate and/or stability in soils. In order to evaluate the effect of different sources (BS) and particle sizes (BF) of biochar on C loss and/or stability in soils, an incubation study on C mineralization of different biochar sources and particle sizes was established using two soils (ST): Norfolk soil (fine loamy, kaolinitic, thermic, typic Kandiudults) and Coxville soil (fine loamy kaolinitic, thermic, Paleaquults). In separate incubation vessels, these soils were amended with one of two manure-based biochars (poultry litters, PL; swine solids, SS) or one of two lignocellulosic-based biochars (switchgrass, SG; pine chips, PC) which were processed into two particle sizes (dust, <0.42 mm; pellet, >2 mm). The amount of CO2 evolved varied significantly between soils (p≤0.0001); particle sizes (p≤0.0001) and the interactions of biochar source (p≤0.001) and forms of biochars (p≤0.0001) with soil types. Averaged across soils and sources of biochar, CO2-C evolved from dust-sized biochar (281 mg kg(-1)) was significantly higher than pellet-sized biochar (226 mg kg(-1)). Coxville soils with SS biochar produced the greatest average CO2-C of 428 mg kg(-1) and Norfolk soils with PC had the lowest CO2-C production (93 mg kg(-1)). Measured rates of carbon mineralization also varied with soils and sources of biochar (Norfolk: PL>SS>SG≥PC; Coxville: PC>SG>SS>PL). The average net CO2-C evolved from the Coxville soils (385 mg kg(-1)) was about threefold more than the CO2-C evolved from the Norfolk soils (123 mg kg(-1)). Our results suggest different particle sizes and sources of biochar as well as soil type influence biochar stability.

Biochar elemental composition and factors influencing nutrient retention

Biochar is the carbonaceous solid byproduct of the thermochemical conversion of a carbon-bearing organic material, commonly high in cellulose, hemicelluloses, or lignin content, for the purposes of carbon sequestration and storage. More specifically, the thermal conversion process known as pyrolysis occurs when carbon-containing substances are introduced to elevated temperatures in the absence of oxygen at varying residence times, yielding biochar. Several pyrolysis techniques employed to produce biochar differ in the temperature of reaction and residence time in the reactor. Different reactor residence times are described as slow (hours to days), fast (seconds to minutes), and flash (seconds). Fast or flash pyrolysis typically occurs around 500oC with residence times less than 500 milliseconds to 1 second and produces relatively greater gas yields with a concomitant decrease in biochar yield (~ 12%). Slow pyrolysis temperatures have ranged from 350 to 750oC but with residence times ranging from minutes to days. Slow pyrolysis yields a greater quantity of biochar (between 25 to 35%). Pyrolysis temperature and type may be varied to maximize the desired biochar end-product. In general, increasing pyrolysis temperature tends to increase biochar total carbon, potassium, and magnesium content, pH, and surface area, and decrease cation exchange capacity. Slow pyrolysis, in general, tends to produce biochars with greater nitrogen, sulfur, available phosphorus, calcium, magnesium, surface area, and cation exchange capacity as compared to fast pyrolysis. In addition to altering temperature and time, the importance of feedstock source needs to be recognized when utilizing biochar in situations such as a soil conditioner. Over the last 10 years biochar research and use has expanded exponentially and so have the feedstocks utilized. Biochars have now been created from corn, wheat, barley and rice straw, switchgrass, peanut, pecan, and hazelnut shells, sugarcane bagasse, coconut coir, food waste, hardwood and softwood species, poultry and turkey litter, swine, dairy, and cattle manure, and biosolids to name a few. Feedstock source influences end-product characteristics, and in general most plant-based biochars containing elevated carbon content and lesser quantities of necessary plant nutrients as compared to manure-based biochars. It has been demonstrated that the mineral content of the feedstock has a significant effect on product distribution, with higher amounts of chloride salts reducing the amount of the solid biochar product. In addition, chloride and other inorganic salts also impact the chemical composition of the liquid, gas, and char pyrolysis products, potentially producing products with higher economic values. Existing studies indicate that even the trace amounts of minerals present in the various biomass sources and feedstock mixtures do have an impact on the chemical compositions of the products. Furthermore, both temperature and residence time, along with feedstock source or mixtures of sources, affect end-product characteristics.

Predicting the impact of biochar additions on soil hydraulic properties

Different physical and chemical properties of biochar, which is made out of a variety of biomass materials, can impact water movement through amended soil. The objective of this research was to develop a decision support tool predicting the impact of biochar additions on soil saturated hydraulic conductivity (Ksat). Four different kinds of biochar were added to four different textured soils (coarse sand, fine sand, loam, and clay texture) to assess these effects at the rates of 0%, 1%, 2%, and 5% (w/w). The Ksat of the biochar amended soils were significantly influenced by the rate and type of biochar, as well as the original particle size of soil. The Ksat decreased when biochar was added to coarse and fine sands. Biochar with larger particles sizes (60%; >1 mm) decreased Ksat to a larger degree than the smaller particle size biochar (60%; <1 mm) in the two sandy textured soils. Increasing tortuosity in the biochar amended sandy soil could explain this behavior. On the other hand, for the clay loam 1% and 2% biochar additions universally increased the Ksat with higher biochar amounts providing no further alterations. The developed model utilizes soil texture pedotransfer functions for predicting agricultural soil Ksat as a function of soil texture. The model accurately predicted the direction of the Ksat influence, even though the exact magnitude still requires further refinement. This represents the first step to a unified theory behind the impact of biochar additions on soil saturated conductivity.