Steam-Exploded Pruning Waste as Peat Substitute: Physiochemical Properties, Phytotoxicity and Their Implications for Plant Cultivation.

Peat is a nonrenewable resource that we are using at alarming rates. Development of peat alternative from pruning waste (PW) could be a cost- and environment-friendly way of disposal. Steam explosion (SE) is a commonly used pretreatment of lignocellulosic biomass, but its impact on the properties of PW as a growing substrate is largely unknown. To address this issue, PW was treated using five SE temperatures (160, 175, 190, 205 and 220 °C) and three retention times (1, 3 and 5 min) and evaluated for key traits of growing substrate. Results indicate that bulk density, total porosity, EC, total carbon, and concentration of phytotoxins including phenol, flavonoid, and alkaloid significantly increased or tended to increase with increasing temperature and/or retention time. A reversed trend was observed for water-holding capacity, pH, content of hemicellulose and lignin, and germination index. Cation exchange capacity and total N showed minimal response to SE. Steam explosion had inconsistent impacts on acid soluble nutrients. Phytotoxicity was a major factor limiting the use of SE-treated PW as growing substrate. Higher pretreatment severity led to higher phytotoxicity but also facilitated subsequent phytotoxicity removal by torrefaction. Pruning waste treated by SE and torrefaction under certain conditions may be used as peat substitute for up to 40% (v/v).