Chemical, physical and morphological properties of biochars produced from agricultural residues: Implications for their use as soil amendment.

Biochar is a pyrogenous organic material resulting from the pyrolysis of organic residues, which is attracting the interest from researchers and farmers for its potential to sequester carbon and its use as soil ameliorant. Pyrolysis conditions and feedstock determine the properties of the biochars produced. In order to understand the relationship between these variables we analysed in detail the physical, chemical and surface characteristics of biochars produced from three contrasting agronomic residues abundantly generated in South Spain, such as rice husk (RH), olive pit (OP) and pruning remains of olive trees (mainly composed of olive branches and leaves; OB), using a temperature range from 350 to 600 °C and residence times from 0.5 to 4 h. High pyrolysis temperature (600 °C) and time resulted in the greatest pH and C content in the biochars. In general, elemental composition and ash content were dependent on the type of organic waste used as feedstock. 13C Nuclear Magnetic Resonance Spectroscopy and thermal (TG-DSC) analyses showed that temperatures ≥500 °C are needed to achieve a high degree of aromatization of the chars. Micro-computed tomography and field emission scanning electron microscopy revealed that the structure of RH was preserved during the pyrolysis process, favouring a greater porosity for these biochars. These data are very useful for the production of stable biochars obtained from residual biomass, maximising the value of residual biomass resources. These biochars show physical and chemical properties, such as adequate pH, high water retention capacity or high porosity, of interest for their use as soil amendments.