Pore characteristics of hydrochars and their role as a vector for soil bacteria: A critical review of engineering options

Abstract

Hydrothermal carbonization (HTC) is the method of choice to convert wet waste biomass to hydrochars. Their porous structure can serve as a microenvironment to plant-growth-promoting rhizobacteria (PGPR), supporting their growth, survival, and activities. As published work lacks the systematic compilation of pore characteristics of hydrochars related to bacterial colonization, we collect available data and elaborate on their dependence on the carbonization process conditions, feedstocks, and methodology of pore system characterization. Our analysis indicates a high abundance of pores sized between 1 and 20 μm relevant for the protection of PGPR from predators, and of nutrients and labile C in hydrochars supporting bacterial growth. In addition to the selection of optimized process parameters and feedstocks (240–260 °C, low feedstock pH, non-lignocellulosic biomass), adding mineral amendments prior to HTC offers opportunities for engineering hydrochars with an even larger share of pore space suited for bacterial colonization. Using the comprehensive literature on biochars, we demonstrate that the interior pore space in chars determines the potential to serve as an inoculum carrier to PGPR, thereby enhancing nutrient acquisition and protecting plants from diseases and abiotic stresses. The pore characteristics of hydrochars are comparable to biochars, and hydrochars are generally superior in providing a labile C reservoir that PGPR can readily access. We argue that HTC provides a cost-effective conversion route to produce PGPR vectors/carriers from wet (waste) biomass serving various environmental management objectives (waste recycling, soil fertility, soil remediation technologies) and circular bioeconomy (sustainable agriculture, substituting non-renewable carrier materials and fertilizers).

Highlights

• We review the role of pore characteristics of hydrochars for bacterial colonization

• We identify opportunities for engineering hydrochars to provide favorable habitat conditions to PGPR

• 240–260 °C, low pH, non-lignocellulosic feedstocks, and adding mineral amendments increase the habitable pore space

• Hydrochars offer suitable pore characteristics and high labile C amounts and are promising PGPR carriers/vectors

Graphical abstract

Keywords:

• Hydrothermal carbonization
• porosity
• inoculant carriers
• microbial habitat
• pore analysis

HANDLING EDITORS:

• Frederic Coulon and Lena Ma

Disclosure statement

The authors declare that there is no conflict of interest.

Additional information

Funding

This work was funded by FFG—Forschungsförderungsgesellschaft, Wien, Austria through the Research Study Austria FERTI-MINE (From waste to fertilizer—phosphorus and carbon waste mining as nutrient recycling strategy for the future) and by AGRO INNOVATION INTERNATIONAL, Saint Malo, France through the project “Pyrolysis/HTC based nutrient recycling technology: Tailoring P, Si and micronutrient availabilities for diversified markets (PSiMi)” and the Roullier Award project “BioAW.”