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

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). HighlightsWe review the role of pore characteristics of hydrochars for bacterial colonizationWe identify opportunities for engineering hydrochars to provide favorable habitat conditions to PGPR240–260 °C, low pH, non-lignocellulosic feedstocks, and adding mineral amendments increase the habitable pore spaceHydrochars offer suitable pore characteristics and high labile C amounts and are promising PGPR carriers/vectors 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

水热炭化法（Hydrothermal carbonization, HTC）是将湿废弃生物质转化为水热炭的优选方法。其多孔结构可作为植物促生根际菌（plant-growth-promoting rhizobacteria, PGPR）的微环境，助力其生长、存活与活性维持。鉴于现有研究尚未系统梳理与细菌定殖相关的水热炭孔隙特征，本研究收集公开可用数据，详细阐述了孔隙特征对炭化工艺参数、原料种类及孔隙系统表征方法的依赖关系。本研究分析发现，1~20 μm区间的孔隙占比极高，这类孔隙可保护PGPR免受捕食者侵害，同时水热炭中的营养物质与易降解碳（labile C）可支撑细菌生长。除优化工艺参数与原料选择（240~260℃、低原料pH值、非木质纤维素类生物质）外，在HTC前添加矿物改良剂，可进一步制备出拥有更大比例适配细菌定殖孔隙空间的工程化水热炭。结合生物炭（biochar）领域的全面研究成果，本研究证实炭材料的内部孔隙空间决定了其作为PGPR接种载体的潜力，进而可助力植物获取营养、抵御病害与非生物胁迫。水热炭的孔隙特征与生物炭相当，且在提供PGPR可快速利用的易降解碳库方面，水热炭整体更具优势。本研究认为，HTC可作为低成本转化路径，以湿废弃生物质为原料制备PGPR载体/接种剂，可满足多种环境管理目标（废弃物资源化、土壤肥力提升、土壤修复技术）及循环生物经济发展需求（可持续农业、替代不可再生载体材料与肥料）。

## 研究亮点
1. 本综述梳理了水热炭孔隙特征在细菌定殖中的作用
2. 明确了工程化改造水热炭以构建PGPR适宜栖息环境的可行路径
3. 240~260℃、低pH值、非木质纤维素类原料及添加矿物改良剂可提升适配细菌栖息的孔隙空间占比
4. 水热炭具备适宜的孔隙特征与丰富的易降解碳含量，是极具应用前景的PGPR载体/接种剂
本综述梳理了水热炭孔隙特征在细菌定殖中的作用；明确了工程化改造水热炭以构建PGPR适宜栖息环境的可行路径；240~260℃、低pH值、非木质纤维素类原料及添加矿物改良剂可提升适配细菌栖息的孔隙空间占比；水热炭具备适宜的孔隙特征与丰富的易降解碳含量，是极具应用前景的PGPR载体/接种剂