Combined biochar and organic waste have little effect on chemical soil properties and plant growth

Biochar has received great attention as a soil conditioner since it can potentially sequester carbon (C) in soil, enhance soil physical, chemical and biological properties and improve crop productivity. This study reports the results of a pot experiment with olive (Olea europaea L.), carried out in an acidic and clay loam textured soil, and cultivated during two growing seasons under eight fertilization treatments. They included mineral fertilization equivalent to a rate of 100 kg ha^-1 of N, P₂O₅ and K₂O (NPK), biochar applied at a rate of 10 (B10), and at 20 (B20) t biochar ha^-1, biochar-NPK mixture (B10+NPK), biochar-waste mixtures with mushroom waste compost (B10+MWC), olive mill waste (B10 + OMW), and municipal solid waste (B10+MSW), the organic materials applied at a rate of 20 t ha^-1, together with a treatment without fertilization (control). Biochar in the B20 treatment increased the soil C content in comparison to the control. Biochar in the B10+NPK treatment reduced soil nitrate levels compared to NPK treatment. No other benefits to soil properties, nutrient uptake or plant growth were observed with the use of biochar or any positive synergistic effect with the mixture of biochar with the other organic amendments. MSW, OMW and MWC tended to increase soil pH in comparison to the control. Most of the studies with biochar were carried out in soils with edaphic limitations or harsh environmental conditions limiting plant growth, which may have facilitated the detection of favorable effects. Under less limited soils or stressful conditions for plants, such as the ones established in this experiment, the benefits of using biochar were poor.