Biochar for Environmental Restoration: Multi-level Investigations into Structure and Performance
Biochar is a product that has been used since at least 1000 BCE for the enhancement of soil fertility. These original applications were in the Amazon jungle, where soils are naturally sub-par for most forms of conventional or industrial agriculture. Adapting this technology to other climates, soils, and farming uses requires a broad base of knowledge. The current state of biochar science seeks to understand how specific environmental behaviors arise from the physical and chemical structure of biochars, and in turn how variation in pyrolysis and feedstock can be manipulated to construct useful biochars. This body of work seeks to contribute to that growing body of scientific knowledge by at multiple levels, from the bench top to the greenhouse. Specifically, two studies focus on enhancing the utility of certain methods of biochar characterization. First, the thermal stability of biochars in air and an inert atmosphere (N2) were compared. Using spectroscopic and chemical characterization, the structural causes of differing thermal stability were examined. Not all biochars with high stability in N2 were highly stable in air. A single pathway of pyrolytic conversion was observed, regardless of how feedstocks responded to pyrolysis (i.e. the ease of pyrolytic conversion). All high N2 stability biochars were highly pyrolyzed with high aromaticity, greater order in aromatic structures, and low H and low O content. However, only aromatic order and O content were significantly correlated with thermal stability in air. The second study examined the use of Raman spectroscopy to characterize the aromatic structures of condensed carbons, including biochars. This study examined the use of principle component analysis (PCA) as a method of spectral decomposition, contrasting it with a peak fitting model and measuring the absolute intensities of certain spectral features. Additionally, this study used several other measures of aromatic condensation to study the difference between aromatic condensation (i.e. how many aromatic rings are in an aromatic cluster) and aromatic order (not only how many rings but also the number of specific defects). PCA was found to provide a useful method of decomposition and provided a simple index to rank aromatic order. It was also observed that aromatic order and aromatic condensation are not synonymous, especially when biochar are created at high temperatures (700 °C). At lower temperatures of creation, aromatic condensation and aromatic order are closely proportional, at higher temperatures the relationship between the two becomes less predictable (i.e. some highly condensed biochars are not as ordered as one might predict from low temperature trends). In a third study, the ability of two biochars to sorb nutrients in a simulated contact with liquid fertilizer (in both an acidic and unaltered environment) and the likelihood of those sorbed nutrients to be released during simulated rainwater leaching were examined. While the biochar with greater CEC and greater ion-exchanging moieties (observed through FTIR) did sorb more nutrients, CEC was a poor predictor and overestimated the quantity of nutrients sorbed. Both biochars sequestered (i.e. sorbed but did not leach) enough nutrients to pose a possible threat to crop yield but were unlikely to cause total crop failure. Additionally, the amount of nutrients sorbed and sequestered would likely decrease in most agronomic applications, as the nutrient concentrations used were in excess of normal agronomic uses. Low pH did increase the amount of inorganic nutrients release of biochars but did not increase sorption or sequestration. Finally, a greenhouse study contrasted the ability of a biochar and mulch (or a mix of the two) to support rehabilitation of a denuded minesoil in both fertilized and unfertilized systems. The unfertilized system developed greater aggregate stability despite lower root mass and above-ground growth of wheat grass. Aggregate stability was closely associated with microbial activity in the unfertilized soils; in both metrics biochar enhanced the effect of mulch in the mix treatment but was not as effective as a greater quantity of mulch without biochar. In fertilized soils, microbial activity was not connected to development of aggregate stability, and aggregate development was greatest in the biochar with the least growth above and below ground growth due to boron toxicity.
Johnston, Purdue University.
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