The impact on carbon dioxide (CO2) and nitrous oxide (N2O) emissions when applying hydrothermally carbonized (HTC) char to soil was investigated in a laboratory experiment with two HTC chars made from hemp (Cannabis sativa L.) dust and incubated for 131 d. Two fractions of hemp dust were collected during fiber processing (from fractionation and suction) and were carbonized at 230 °C for 6 h in water. Non-treated and water-washed HTC chars were used in incubation experiments, doubling the carbon concentration of the soil. As a result of adding HTC char to soil, CO2 emissions increased significantly in all cases compared to the control treatment. Washing the HTC chars easily removed dissolvable carbon (C) compounds, which significantly decreased CO2 emissions. Nitrous oxide emissions, following the incorporation of HTC char, did not differ from those of the control sample; however, washed HTC char treatments tended to emit less N2O than the corresponding unwashed samples. Hydrothermally carbonized char obtained from the suction of dust may play a greater role as a soil conditioner than HTC char from dust by fractionation because dust from suction accumulates to a larger degree during hemp fiber processing.