When dealing with bioavailable arsenic (As) in acid soils, it is important to use extraction methods that are suitable for such conditions. Here are two commonly used extraction methods for bioavailable As in acid soils:

1. Mehlich-3 extraction: The Mehlich-3 method is widely used for extracting various nutrients and trace elements from acidic soils. It involves the use of a mixture of strong acids (such as hydrochloric acid and sulfuric acid) at a pH of approximately 2.5. This method is effective for extracting bioavailable As in acid soils and is commonly used in agricultural and environmental research.

2. Modified Morgan extraction: The Modified Morgan method is another commonly used extraction method for assessing bioavailable As in acid soils. It utilizes a mixture of weak acids, including acetic acid and ammonium nitrate, at a pH of approximately 4.8. This method is suitable for acidic pH and has been used extensively for extracting bioavailable As in both agricultural and contaminated soils.

Regarding the specific Soil As extraction method using 0.5M NaHCO3 at pH 8.5, it is not suitable for acidic pH conditions. Sodium bicarbonate (NaHCO3) at pH 8.5 is typically used to extract As in neutral to slightly alkaline soils. The higher pH helps in solubilizing As from solid phases. However, in acidic soils, the use of NaHCO3 at pH 8.5 may not be effective for extracting bioavailable As due to the low pH of the soil.

It is recommended to choose an extraction method that is appropriate for the specific pH conditions of the soil you are working with. The Mehlich-3 or Modified Morgan extraction methods mentioned above would be more suitable for acidic pH soils.