Soil chemical properties and organic matter composition of a subtropical Cambisol after charcoal fine residues incorporation

Brazil is the world's greatest charcoal producer with an annual production of 10 million tons. However, about 15 % is lost as charcoal fines, which can be used as soil fertilizer and source of soil stable carbon (C). In this study we, investigated the impact of charcoal fines application on soil chemical properties and organic matter (SOM) stability and composition. Four doses (0, control, 10, 20 and 40 Mg ha(-1)) of charcoal fines were incorporated to the soil at 10 cm. Soil samples were collected at four soil depths (0-5, 5-10, 10-20, and 20-30 cm) 20 months after charcoal incorporation. Soil chemical properties were determined, and SOM composition was evaluated by infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM-EDX) and carbon stable isotopic analysis. Soil chemical properties were affected mainly in the upper depth (0-5 cm) and only at the highest dose (40 Mg ha(-1)), which also increased the total C content in 37.7 % compared to the control. Probably, the observed effects solely on the surface are due the concentration of charcoal on the upper depth, despite its incorporation. The dose of 40 Mg ha(-1) also increased the total C content at 10-20 cm depth compared to the control. Such result was assigned to the leaching of some finest charcoal particles. The SEM-EDX and SOM delta C-13 signature confirmed the presence of charcoal at both 0-5 and 10-20 cm soil depths. FTIR and TGA indicated an increase in SOM aromaticity and thermostability until 20 cm depth after charcoal incorporation. No effects were observed at the deepest soil depth (20-30 cm), suggesting that charcoal was not leached to this depth within 20 months. The application of charcoal fines ameliorated the soil chemical properties. In general, the charcoal increased the SOM aromaticity and thermostability and changed the delta C-13 signature at 0-5 and 10-20 cm. Charcoal leaching from the 0-10 to the 10-20 cm depth was observed, but further downward transport was not evidenced by our data. Charcoal fine residues show a potential to be used as an effective soil fertilizer, as well as a stable C source into the soil.