Using topsoil for rabbit digging boxes can be safe as long as certain considerations are taken into account. Here are a few things to keep in mind:

1. Quality and Source: Ensure that the topsoil you use is free from contaminants, pesticides, herbicides, and other harmful substances. Use organic or chemical-free topsoil to minimize the risk of exposing your rabbits to potentially harmful substances.

2. Sterilization: It is recommended to sterilize the topsoil before using it in the rabbit digging boxes. This can be done by baking the soil in the oven at a low temperature (around 250°F or 120°C) for about 30 minutes. This helps eliminate any potential pathogens or parasites that may be present.

3. Allergies and Sensitivities: Some rabbits may be sensitive or allergic to certain types of soil. Monitor your rabbits closely after introducing the topsoil to ensure they don't develop any adverse reactions or allergies. If you notice any signs of discomfort or allergies, discontinue the use of that particular soil and consult a veterinarian.

4. Soil Composition: Consider the composition of the topsoil. Rabbits may dig and potentially ingest small amounts of soil, so it's important to use soil that is not excessively clayey, sandy, or high in organic matter. A balanced soil composition can help reduce the risk of digestive issues or impaction if the rabbits accidentally ingest small amounts of soil.

5. Regular Cleaning and Maintenance: Keep the digging boxes clean and regularly remove any soiled or contaminated soil. Replace the topsoil periodically to maintain cleanliness and prevent the buildup of waste or bacteria.

Remember to provide alternative safe and enriching activities for your rabbits, such as providing appropriate chew toys, tunnels, and other mentally stimulating objects, to prevent excessive digging or destructive behavior.