Gasifier biochar effects on nutrient availability, organic matter mineralization, and soil fauna activity in a multi-year Mediterranean trial

• Three-year gasifier biochar field trial showed no effect on microbial biomass or yield.
• Gasifier biochar decreased nitrate mineralization.
• Soil available potassium and sulfur were increased by gasifier biochar addition.
• Negative effects of gasifier biochar addition on soil fauna activity.
• No increase in nitrogen fertilizer efficiency was found.

Gasifier pine biochar is a carbon-rich material which may be useful as a soil amendment. In Europe and elsewhere there may be potential added value of char produced in industrial gasifiers, up to now considered as wastes. Pine gasification biochar was tested as a soil amendment in a multi-year Mediterranean barley crop field trial, applied at 12 and 50 t ha−1 while applying half the usual N rate at 50 kg ha−1, contrasted with a full 100 kg ha−1 N fertilizer treatment without biochar. Over the 6–30 month period following the application, biochar treatments did not have any significant effect on soil microbial biomass, respiration, or metabolic coefficient. N mineralization as NO3− was decreased by biochar at 6 and 12 months from experiment start and coincided with ammonium accumulation. Biochar increased overall soil concentrations of K+ and SO42−, attributed to a direct additive effect, agreeing with data from other sources. Biochar treatments (with half usual N fertilization) did not have any significant effects on barley crop parameters, and when biochar treatments were contrasted against full N fertilization with no biochar, the usual N dosage was clearly more beneficial to crop development. Finally, soil fauna activity was negatively impacted by gasifier biochar treatments in years two and three, indicating a risk to soil processes mediated by soil invertebrates. Though this gasifier biochar is expected to be highly stable and therefore of interest for carbon sequestration, its utilization therefore risks negative effects on some biologically-mediated soil processes at high application rates.