Grain size-induced changes in carbon and nitrogen concentrations and characteristics of tomato harvest residue biochar

The production of biochar is one of the new methods used in the recycling of organic wastes. The use of biochar may increase carbon storage in the soil for longer periods of time. In this study, carbon and nitrogen contents of different sizes of tomato harvest residue biochar (TB) produced at different temperatures and holding times were determined. TB produced at five different temperatures (300, 400, 500, 600, 700 °C) and three different durations (20, 40, and 80 min) was divided into three different grain size classes (< 0.5, 0.5–2, and 2–4 mm). Total carbon (C) and nitrogen (N) contents of each size class were examined. Production temperature significantly affected N contents of TB (p < 0.05) while changes in the C contents due to temperature were not statistically significant (p > 0.05). On the other hand, effect of TB size on C and N contents was statistically significant (p < 0.05). As TB grain size increases, BET surface area and total pore volume increased, while average pore radius decreased. The larger size of biochar (2–4 mm) demonstrated highly porous and honey-combed structures and contained both micro- and mesopores, which create a high specific surface area. It was suggested that the properties of biochar produced under the same conditions may vary according to their size and different sizes of biochar may be preferred according to the intended use.