Effects of Olive Mill Wastewater and Two Natural Extracts as Nitrification Inhibitors on Activity of Nitrifying Bacteria, Soil Nitrate Leaching Loss, and Nitrogen Metabolism of Celery (Apium graveolens L.)

Minimizing nitrification of fertilizer ammonium (NH4+) can reduce nitrate (NO3−) contamination of groundwater and increase nitrogen (N) use efficiency (NUE). Olive mill wastewater (OMW), hydroalcoholic extracts of Mentha piperita L. (Mp) and Artemisia annua L. (Aa), and synthetic nitrification inhibitor (NI) dicyandiamide (DCD) were investigated. All NIs reduced activity of nitrifying bacteria and NO3− leached and increased efficiency of N metabolism of celery (Apium graveolens L.) during 56 days in a soil mixture fertilized with ammonium sulfate [(NH4)2SO4]. Soil NO3− leaching losses of DCD, OMW, Mp, and Aa treatments were 24%, 26%, 67%, and 78% of the untreated control loss, respectively, at 35 and 56 days after planting (DAP). Decreased nitrification by NIs resulted in greater concentrations of soil NH4+ correlated with less nitrate reductase (NR) activity in roots and leaves and less soil acidification compared to the control. At 35 and 56 DAP, DCD and OMW treatments decreased NR and increased glutamine synthetase activities in leaves and roots, compared to the control. NIs increased leaf and root protein and amino acids. OMW significantly decreased leaching loss of NO3− to 10% of fertilizer N applied to the soil, compared to 38% of applied NH4+-N leached from the control. OMW proved an effective alternative to DCD to improve NUE of NH4+-fertilizers.