Analysis of two biochars and one hydrochar from different feedstock: focus set on environmental, nutritional and horticultural considerations

• Biochar is more recalcitrant to decomposition than hydrochar.
• Alkaline cation-rich biochars function as traps for CO2.
• BCH-OMW is a good source of P for fertilising.
• BCH-FW immobilize nitrogen in the short term.
• HYD-FW does not immobilize but mineralize N and supply nutrients in the short term.

Two biochars, one from forest waste (BCH-FW) and the other from olive mill waste (BCH-OMW), and one hydrochar from forest waste (HYD-FW) were analyzed from the horticultural and environmental point of view. Several incubation assays were performed to foresee the material evolution when used as soilless growth media. HYD-FW was slightly alkaline and rich in nutrients and showed high microbial respiration Moreover, it did not caused N immobilization but N mineralization which contradicts previous reports. Biochars were highly alkaline. BCH-FW was poor in nutrients whilst BCH-OMW was saline and rich in soluble cations. Both materials had low respiratory activity, similar to that of coir fibre (CF). Whilst BCH-FW and CF immobilize N, BCH-OMW did not. Both biochars captured CO2 from the atmosphere, particularly BCH-OMW, which concomitantly showed a decrease in soluble cations. We conclude that pure hydrochar does not immobilize N and that alkaline biochars act as CO2 traps.

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