Designing relevant biochars to revitalize soil quality: Current status and advances

Abstract

Chemical and physical properties of biochars can be designed to improve specific soil quality issues. In order to make appropriate biochar selections, evaluations are required of different feedstocks, pyrolysis conditions, and gross biochar particle sizes. We conducted laboratory soil incubations using a Norfolk loamy sand (fine loamy, kaolinitic, thermic Typic Kandiudult) mixed with biochars made from two different feedstocks processed into two different particle sizes. Switchgrass (Panicum virgatum L.) and loblolly pine (Pinus taeda) were individually made into (1) hammer-milled (HM) chips (<6 mm) and (2) >6-mm-diameter pellets. These prepared feedstocks were pyrolyzed into biochar for 2 h at 350°C. For each treatment (n = 4), a 1% (w w-1) biochar loamy sand mixture was incubated in triplicate pots at 10% moisture content (w w-1) for 78 days. Untreated Norfolk soil (no biochar) served as a control comparison. On incubation days 32 and 67, each pot was leached with 1.2 pore volumes of deionized water and the leachate collected. Thereafter, we gravimetrically determined each pot's daily % soil moisture content (w w-1). On days 1, 5, and 11 after both leaching events, HM switchgrass biochar had the most significant moisture storage improvement (≈6%) with respect to the control. The HM pine chip biochar showed mixed results with a longer time period required to demonstrate improvement (day 11). On the other hand, pelletized biochars caused nonsignificant or minor improvements with soil moisture content. Our results indicate that, in addition to feedstock choice, biochars processed into different particle sizes impact their ability to improve soil moisture contents.