Comparison of Laboratory- and Field-Aged Biochars for Agronomic Benefits.

In order to assess the long term impacts of biochar on soil properties and agronomic systems, it is necessary to investigate soil and plant responses to weathered or “aged” biochar rather than fresh biochar. Fresh biochars, however, are often used in studies because aged biochars are generally unavailable. Therefore, a need exists to develop a method for the rapid aging of biochars in the laboratory. The objectives of this study were to compare the physico-chemical properties of fresh, laboratory aged, and field aged (≥3 years) biochars, and to assess the appropriateness of a laboratory aging procedure that integrates acidification, oxidation, and incubations. Twenty two biochars (11 freshly made, 6 lab-aged, and 5 field-aged) produced by both fast and slow pyrolysis techniques from four different biomass feedstocks (hardwood, corn stover, soybean and switchgrass) were studied here. The laboratory aging procedure included a one month incubation at 40°C in 1M HCl with weekly additions of 30% H₂O₂, Ca-saturation, and then a one month incubation at 40°C in a dissolved organic carbon solution (prepared by leaching compost). In general, both laboratory and field aging caused increases in ash-free volatile matter (% w/w), cation- and anion exchange capacities, specific surface area as measured by EGME adsorption and modifications in oxygen-containing surface functional groups. Relative to fresh biochars, however, the pH and ash content decreased to a greater extent for laboratory aged than field aged biochars, suggesting that the laboratory based acidification treatments were excessive or that the field aged biochars adsorbed clay and other inorganic components from the soil. We conclude that the proposed rapid laboratory aging procedure causes similar physico-chemical changes in biochars as field aging although the extent of these changes differs.