Effects of Protaetia brevitarsis frass on rhizosphere properties and chromium-arsenic accumulation in Scutellaria baicalensis grown in contaminated soil

This study evaluated the potential of Protaetia brevitarsis frass as an organic amendment for the safe cultivation of Scutellaria baicalensis in Cr–As-contaminated soil. A greenhouse pot experiment was conducted with five frass application gradients (CK, LD, MD, HD, and VHD) to assess rhizosphere properties, plant growth, and metal accumulation risk. Frass application improved soil water status, nutrient supply, and microbial biomass, but VHD markedly increased electrical conductivity, indicating a potential salinity risk. Plant growth showed a dose-dependent tradeoff: HD favored root development and leaf physiological activity, whereas VHD promoted shoot growth but reduced root allocation. The frass was rich in alkali-hydrolyzable nitrogen and available phosphorus, suggesting that soil N and P increases resulted from both direct nutrient input and rhizosphere transformation. Frass treatments generally reduced Cr and As accumulation in S. baicalensis, although responses varied between metals and application rates. Redundancy analysis identified alkali-hydrolyzable nitrogen as the major explanatory factor (82.6%). Because metal fractions were not determined, BCF was interpreted as an uptake-risk indicator rather than direct evidence of soil passivation. HD is recommended under the present pot conditions.