Effects of Biochar Derived from Various Feedstock Sources on some Physical and Chemical Properties of Soil

Organic matter is one of the factors affecting soil fertility and quality. The presence of organic matters increases the stability of aggregate, increases water retention in soil, reduces bulk density and thus reduces soil erosion. Biochar is one of these organic matters which is a carbon-containing and porous solid produced by the chemical heating of organic matter in an oxygen-free atmosphere. It has good physicochemical characteristics for long-term storage of carbon and its preservation in the environment. Different types of raw materials such as plant residues (rice bran, soybean forage, and cotton residues), fertilizers and wood waste (sawdust) are used to produce biochar. The presence of this matter in the soil increases fertility, reduces climate change, reduces nutrient leaching from the soil, increases the soil moisture retention and increases the cation exchange capacity, resulting in increased soil quality. Given the importance of biochar and its role in improving soil quality indices, the present study was conducted with the aim of investigating the effect of different levels of biochar prepared from pistachio and walnut hulls and shells on some physical and chemical indicators of soil.

The effect of four types of biochar prepared from hull and shell of pistachio and walnut, at three levels (zero, two and four percent by weight) on two soils with sandy loam and clay loam textures and with three replications were examined in a completely randomized factorial design. To produce biochar, pistachio and walnut hulls and shells were collected from pistachio and walnut orchards in Iran-Kerman Province-Rafsanjan region. These materials were converted to biochar under pyrolysis conditions. The temperature used in this process was 400 C° and the conversion time of raw materials to biochar was four to five hours. Particle size selection was different according to different articles and in this study; therefore, a particle size smaller than 45 meshes was selected. Biochar (at the mentioned levels) was added to five kilograms of soil and poured into pots. The samples remained intact for four months. During the period, the moisture content of the samples was maintained within the field capacity. At the end of the period, the soil was sampled and the physical and chemical characteristics of the soil were examined.

The results showed that with increasing the levels of biochar, no significant effect was observed on the electrical conductivity of soils. In relation to the changes in pH, the effects of the biochar obtained from the hull of walnut and shell of pistachio were significant, so that the application of the first increased and the second decreased the soil pH in a significant manner. On the other hand, after adding biochar, the soil bulk density decreased. Furthermore, the application of the biochar obtained from the hull of pistachio and shell of walnut in clay loam soil and that of shell of pistachio in sandy loam soil had significant positive effects on aggregation. Moreover, the use of the biochar obtained from the hull of walnut increased the soil water holding capacity in sandy loam soil. Finally, in both clay loam and sandy loam soils, the biochar obtained from the hull of walnut resulted in the highest values of S index among the used biochar types.

In general, the results of this study mainly indicate the positive effects of the use of biochar on soil properties, and it seems that the amount of their impact is a function of the type of biochar and soil texture. Also, based on the results of this study, using walnut hull biochar in sandy loam soil increases the water retention capacity of the soil, and this is due to the micro porosity of biochar. This can be a good solution, especially in arid and semi-arid regions. However, it should be noted that increasing biochar level leads to increased soil salinity, so appropriate levels of this material in the soil should be used. Also, the biochar produced from the walnut hull in both clay loam and sandy loam soils yielded the highest values of S index among the biochar types used. Dexter index, as one of the indices of physical quality of soil, has a special effect on root growth capacity of plants. Finally, it is recommended that the combined effect of these biochar types (hull and shell of each biochar together) be investigated in the soil to determine whether their functional groups compete with each other or complement each other.