Yes. First, decide the weight of 1 hectare grassland. As the soil is rich in organic carbon, it may have a bulk density of 1.35 g/cm3. The height of top soil is assumed to be 15cm.
Weight of I hectare grassland soil (15 cm height) = 10 000 m2 X 0.15 m X 1.35 Mg/m3 = 2025 Mg = 2 025 000 kg
Soil organic carbon = 2 025 000 kg X (2.96/100) = 59.94 Mg/ha (or tonnes/ha)