Biochar is a pyrolysis-based technology that is being investigated as a means to stabilize and store plant-residue C in soils. While most studies suggest that biochar products are indeed stable in soils, some report fairly rapid degradation rates. Our objective was to determine the mineralization rate of an agronomic biochar in a full-scale field experiment under Norwegian conditions, and to verify that this biochar was safe for soil application. The biochar was produced from a miscanthus C4 feedstock between 500-700C with a PYREG pyrolyser, and applied in October 2010 to Norwegian C3 soil at rate of 8 and 25 t C ha^-1. A no-biochar control and non-pyrolyzed miscanthus control were also included. The contrasted ¹³C signature between the C4 miscanthus products and the C3 soil was used to determine mineralization rates. Here we will report on the first year of data on estimating mineralization rates of the miscanthus biochar vs. that of the non-pyrolized residues. In addition, we also conducted chemical analyses and ecotoxicological tests with earthworms, which indicate that the miscanthus biochar is a safe product for application to agricultural soil.