The historical Japanese city of Kyoto boasts a great many old Buddhist temples and Shinto shrines, many of which are surrounded by sizable forests that have long been preserved as sacred forests. However, acidic deposition has been fallen on the forests in Kyoto for many years. For this study, we conducted soil surveys and investigated the extent of decline of the trees in two Shinto shrines as historic monuments of ancient Kyoto. Our study revealed clear decline in two key tree species (Cryptomeria japonica (Japanese cedar) and Chamaecyparis obtusa (Japanese cypress)) in both shrines, with some trees showing signs of mortality. The soil was acidic, with an average pH of 4.35. Nutrient salt content too was only about one tenth the national average, with exchangeable Ca (0.52 cequiv./kg) and Mg (0.23 cequiv./kg) for 0-20 cm surface soil. The (Ca+Mg+K)/Al molar ratios were also very low, with 80% of all soil samples having a ratio of 10 or below. Such soil conditions are thought to hamper the sound growth of both Japanese cedar and Japanese cypress, and soil acidification is one of the most likely causes of the decline of temple and shrine forests in Kyoto.

The bioavailability of cadmium (Cd) in agricultural soils is a significant health concern due to the potential risk of human exposure via foods grown in Cd-contaminated fields. Biochar has been known to have a highly porous structure and high pH, as well as containing various functional groups; as such, it can immobilize heavy metals. Although it has found that biochar amendment in Cd-contaminated agricultural soils could be effective in reducing Cd bioavailability in previous studies, differences in plant Cd accumulation from Cd-contaminated soils amended with biochars produced from various types of biomass have not been fully discussed yet; we aimed to address this shortcoming in the present work. The soil investigated was an acid soil (pH 5.1) and had an elevated concentration of Cd (total Cd: 3.3 mg kg-DW⁻¹). Six kinds of biochar were produced, i.e., from woodchips (Japanese cedar [CE] and Japanese cypress [CY]), moso bamboo (MB), rice husk (RH), poultry manure (PM), and wastewater sludge (WS), at a pyrolysis temperature of 600 °C. Biochars were incorporated into the Cd-contaminated soil at 3% (w/w) and pot experiments using Brassica rapa var. perviridis were conducted for 28 days in a growth chamber. The Cd concentrations in the above-ground portion of the plants were significantly decreased as a result of the incorporation of all biochars compared to the unamended soil, with reduction ratios following the order PM (78%) > > WS (31%) ≈ RH (29%) ≈ MB (28%) ≈ CY (26%) > CE (19%). Among all biochar-amended soils, soil pH and shoot biomass were highest for those amended with PM-derived biochar. These results suggest that in Cd-contaminated soils, PM-derived biochar may offer significant potential in reducing plant Cd accumulation due to the immobilization of soil Cd and an effect of dilution resulting from enhanced plant shoot biomass.