Resources, Conservation & Recycling 197 (2023) 107113 Available online 10 July 2023 0921-3449/© 2023 Published by Elsevier B.V. Perspective Hydrochar: A promising activator for legacy phosphorus in soil Lei Luo a, b, * , Zhengang Liu a, b , Yong-Guan Zhu a, b a Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China b University of Chinese Academy of Sciences, Beijing 100049, China A R T I C L E INFO Keywords: Legacy phosphorus Use effciency Biomass waste recycling Priming effect Phosphorus (P), a critical nutrient for crop growth, is frequently over-applied in soil to sustain high crop yields. The excessive application of P in cropland, together with the strong binding of available P to soil components and the inputs of stable organic P from biomass waste application, has resulted in the low use effciency of applied P and the build-up of P in soil (known as legacy P). The global legacy P in soil has accumulated to 212 kg P ha 1 in 2019 (Zou et al., 2022), which serves as a great potential P source for not only future crop production but also leaching to adjacent water bodies. On the other hand, the majority of commercial P fertilizers currently used in global food production orig- inate from nonrenewable phosphate rock mines. P shortage induced by unevenly distributed P rock resources has become a serious obstacle to be addressed for achieving the Sustainable Development Goal target of food security in many countries. Recycling P from P-containing biomass wastes such as animal manure and sewage sludge provides plausible sustainable solutions to stave off the crisis. However, recovering P from sewage sludge, albeit technologically feasible, has not been well actu- alized because of economic and policy reasons. The rapid P release (due to high mobile P species) in animal manure, even after composting treatments, usually outpaces plant utilization rate, triggering P leaching in short terms and causing water pollution problems. Converting these P-rich biomass wastes into pyrochar via (dry) pyrolysis process at high temperatures (usually ≥ 300 ◦ C), which transforms mobile P into rela- tively stable species, can act as slow-release P sources in soil (Sun et al., 2022). Nevertheless, seven-year feld trials indicate that low-P bio- mass-derived pyrochar contributes minutely to the release of legacy P in soil and facilitates the detainment of available P instead, owing to its abundant porosity and cations (Yuan et al., 2022). Innovative tech- niques and/or products need to be developed to substantially enhance the availability of legacy P and/or effectively recycle P in biomass wastes to tackle the looming P crisis. Legacy P is not a specifc species but a complex mixture with multiple species, with organic P accounting for ~50% of total legacy P in soil. The annual accumulated phytate, a recalcitrant species of organic P with low availability, in soil is almost equivalent to 65% of the P fertilizers applied, which mainly originates from the application of crop straw and monogastric animal manure. Enhancing the availability of legacy P while avoiding its build-up becomes essential in increasing its use eff- ciency in soil. Amendments such as oxalic acid and synthetic humic acids have been verifed to be effective in solubilizing otherwise insol- uble P in soil (Yang et al., 2019). Labile organic carbon (OC) like glucose has also been applied to increase the availability of legacy P in soil via microbial regulation. However, these strategies are currently only viable at laboratory scales. Nevertheless, these results suggest that labile OC and the associated microbial activities control the speciation and availability of P in the environment. We therefore argue that converting biomass wastes into hydrochar through hydrothermal carbonization (or wet pyrolysis) at relatively low temperatures (usually < 350 ◦ C) will not only effectively recycle P resources in these biowastes but also signif- cantly enhance the availability thus the use effciency of legacy P in soil. During hydrothermal process, organic P in biowastes tends to be transformed into inorganic phosphate, and thus unnecessary accumu- lation of organic P can be avoided. The derived hydrothermal products include value-added solid hydrochar and humic acid-rich liquid byproduct. Compared to the highly carbonized pyrochar, hydrochar and its liquid byproduct contain high levels of labile OC which can solubilize fxed P via competitive complexation with cations and ligand exchange reactions (Fig. 1). More importantly, they can stimulate the P- * Corresponding author. E-mail address: leiluo@rcees.ac.cn (L. Luo). Contents lists available at ScienceDirect Resources, Conservation & Recycling journal homepage: www.elsevier.com/locate/resconrec https://doi.org/10.1016/j.resconrec.2023.107113 Received 11 May 2023; Received in revised form 7 July 2023; Accepted 7 July 2023