Variation in Feedstock Wood Chemistry Strongly Influences Biochar Liming Potential

Abstract

Chars intended for use as soil amendment (“biochars”) vary greatly in their chemical and physical properties. In the present study, 19 Canadian temperate wood feedstocks were charred across a range of pyrolysis temperatures from 300–700 °C. The resulting 95 biochars were tested for their physio-chemical properties and liming capacity. Data indicated increasing base cation concentrations including Ca, Mg, and K (elements that characteristically form liming compounds, i.e., carbonates) as pyrolysis temperature increased. Acidic surface functional groups were analyzed with modified Boehm titration: Carboxylic and lactonic functional group concentrations decreased and phenolic group concentration increased with pyrolysis temperature. Functional group composition also varied greatly with feedstock: In particular, conifer-derived biochars produced at pyrolysis temperatures <500 °C showed much higher carboxylic and lactonic functional group concentrations than did angiosperm-derived biochars. Liming capacity was assessed using soil incubation experiments and was positively related to biochar pH. Both acidic surface functional group concentration and nutrient element concentration influenced biochar pH: we developed a non-linear functional relationship that predicts biochar pH from the ratio of carboxylic to phenolic moieties, and concentrations of Ca and K. Biochar’s liming components that are inherited from feedstock and predictably modified by pyrolysis temperature provide a basis for optimizing the production of biochar with desired pH and liming characteristics.

Variability of Physical and Chemical Properties of TLUD Stove Derived Biochars

“…Pinewood biochars (WB) were much more limey than bamboo biochars (BB), but their EC was less than half as large, in accordance with lower ash contents. Biochars generally have a substantial liming capacity, to an extent due to mineral content present in the feedstock [4]. Significant differences (p < 0.05) in pH were found between replicates showing higher values of pH in biochars with a higher degree of thermal alteration, in agreement with previous works (e.g., [15,21]).…”

“…According to Keiluweit et al [53] decomposition of cellulose occurs rapidly at low temperatures (between 230 and 400 • C), while lignins decompose in a wider range of temperatures (160-900 • C). The differences in the patterns of stove temperatures between bamboo and wood, observed in Figure 1, may obey to differences in the biomass composition [4], due to the temperatures at which decomposition reactions take place, such as oxidation and dehydration. As previously discussed, higher contents of metallic cations have been related to lower pyrolysis temperatures [36][37][38].…”

“…These contributions of biochar vary with its quality. Biochar quality for soil amendment relies primarily on the volume and size of its pores and on the functional groups on the surface [4]. These aspects are driven by key factors; known as the feedstock type [5,6], the biochar particle size [7] and the pyrolysis conditions, for instance, the charring temperature, the heating rate and the residence or holding time [8].…”

“…Probably, the major contributions that added biochar can make to the agronomic value of soils are the entailing improvements in the soil's cation exchange capacity (CEC), the liming effect [6], and its water retention capacity [13]. CEC is a measure of the negative charges in a material, which allows it to retain positive exchangeable cations such as K + , Ca 2+ and NH 4 + , and is therefore a desirable quality in a soil. Specific studies for application of biochars in soil remediation [14] have shown that pyrolysis conditions affect the chemical functional groups formed at the surface, determining their CEC.…”

Role of biochar in promoting circular economy in the agriculture sector. Part 1: A review of the biochar roles in soil N, P and K cycles

“…In addition, increased concentration of K in biochar, together with Mg and Ca, functions as a liming agent to neutralise acid soils [109]. Meta-analysis of the literature showed that biochar application led to a reduction in the acidity of the soil in multiple studies [110].…”