Phosphorous Retention and Release by Sludge‐Derived Hydrochar for Potential Use as a Soil Amendment

To solve both the problems of P deficiency in arable soil and excessive waste sludge disposal, we evaluated hydrothermal carbonization of sludge with the aim of recycling sludge P resources for soil amendment. In contrast with pyrochars obtained through pyrolysis, hydrochars generated from hydrothermal carbonization often feature variable properties and therefore require detailed characterization. In this study, sludge‐derived hydrochars were evaluated to determine their P content and fractionation, release and availability of P, and P adsorption and desorption behavior. We also assessed changes in P availability after soil was amended with the hydrochars. Our results showed that the chars were rich in total P (up to 25,175 mg kg −1 ), but most of the free fractions were transformed to bound fractions, thereby reducing the available P concentrations. However, available P content was >417 mg kg −1 , which was far higher than soil demand. The hydrochars shifted from releasing to adsorbing P adsorbent when the environmental P concentration increased above 20 mg L −1 . The hydrochars showed a high phosphate adsorption capacity (up to 23,815 mg kg −1 ) and the adsorbed P could be readily released. The addition of 1% P‐laden hydrochar significantly enhanced the soil available P content by 8.93 mg kg −1 . These findings have important implications for further development of hydrochar‐based P carriers as a slow‐releasing fertilizer. Core Ideas Charring transformed P from readily available fractions to moderately available ones. Sludge‐derived hydrochars showed the best performance in long‐term P release. Sludge‐derived hydrochars had better P retention capacity than their feedstock. Applying the sludge‐derived hydrochar improved the available P content of soil.