The accumulation of heavy metals in rice (Oryza sativa L.) compromises food safety and endangers public health. Previous studies have postulated that ecological co-cultivation systems can potentially improve soil quality and reduce crop absorption of heavy metals. Herein, three treatment groups, rice mono-culture (CG), low-density rice–frog co-culture (LRF), and high-density rice–frog co-culture (HRF), were employed to evaluate the effects of rice–frog co-culture on the physicochemical properties of soils in reclaimed rice fields and heavy metal accumulation in rice. Notably, the rice–frog co-culture markedly increased levels of soil organic matter (SOM), dissolved organic carbon (DOC), cation exchange capacity (CEC), pH, and redox potential (Eh) (p < 0.05), particularly under high-density conditions, compared to the mono-culture system. These changes significantly reduced the bioavailable fractions of Cd, As, and Hg in the soil and substantially diminished their uptake in the roots, stems, leaves, and grains of rice. Conversely, the co-cultivation systems increased the bioavailable content and plant uptake of Pb, particularly under high-density conditions. These findings highlight the feasibility of the rice–frog co-cropping systems in improving soil conditions and reducing the accumulation of specific toxic metals within rice, thereby enhancing the safety of rice grown in reclaimed fields. However, increased Pb accumulation warrants further investigation.

The Chinese giant salamander (CGS, Andrias davidianus) is an endangered aquatic amphibian in China. Using environmental DNA (eDNA), this study aimed to monitor the spatial distribution of this species in Zhejiang Province, providing a scientific basis for species assessment, evaluation of reintroduction effectiveness, and non-invasive monitoring. Based on historical records of species presence, 18 representative water localities were selected for sampling. Among these localities, eDNA signals of the CGS were detected at nine. After excluding sampling sites affected by historical CGS releases and surrounding breeding farms, Jinzi Peak and Huyuan were inferred as candidate potentially natural stream habitats within the province. Additionally, eDNA monitoring was conducted at two release sites, Gutian Mountain and Banshanyuan, for two consecutive years. Results revealed lower detection rates in both areas in the second year of monitoring, potentially attributed to low survival rates, dispersal and disappearance, or sampling variability. A principal component analysis (PCA) of water quality factors (including parameters such as NH₃-N, TN, TP, COD) revealed distinct geographic clustering of CGS-positive and CGS-negative sampling sites that corresponded to their separation in topographic space, suggesting that the measured physicochemical conditions may be associated with the CGS eDNA detection probability. This study demonstrates that eDNA is a feasible and efficient method for surveying CGS in Zhejiang Province. Further, it provides an efficient non-invasive monitoring approach for endangered amphibians, with broad application prospects.