Early acidification during dredged sediment composting can alter phosphorus (P) availability and increase heavy metal mobility. Nano-hydroxyapatite (nHAP) was evaluated at addition rates of 0, 50, 150 and 250 mg·kg−1 dry matter (CK, N50, N150 and N250, respectively). The minimum pH was 4.6 in CK, 5.2 in N150 and 5.5 in N250. Amended composts contained more Olsen-P and larger labile and moderately labile P pools. At maturity, N250 contained 58.4 mg·kg−1 NaHCO3-Pi and 88.7 mg·kg−1 highly labile P, compared with 24.4 and 36.6 mg·kg−1 in CK. Humic-like dissolved organic matter increased during maturation. Sequential extraction indicated lower mean operational metal mobility in N150. The Cd mobility index decreased from 38.3 % to 30.9 %, and residual Cu and Cr proportions increased from 28.3 % to 42.2 % and from 69.2 % to 83.5 %, respectively. The Cu and Cr residual-fraction differences were not statistically significant after adjustment for multiple comparisons. Changes in P-cycling gene abundance indicated altered functional potential. In a 58-day field-plot trial, N150 compost increased mean leaf fresh mass of Brassica rapa L. by 156.4 % relative to CK, alongside improved nutrient availability and lower plant metal concentrations. These findings support the potential of nHAP-assisted composting to improve P availability and short-term plant performance in sediment resource recovery.
