Effects of amendment with treated and untreated olive oil mill wastewaters on soil properties, soil humic substances and wheat yield

Catalytic digestion with MnO2 was shown to be a suitable treatment for promoting organic matter humification in olive oil mill wastewater in order to increase its potential as a soil amendment and reduce environmental risk. In this study, the effects of addition of either lagooned wastewater (LW) or catalytically digested wastewater (CW) at two rates on soil and soil humic acid (HA) properties and durum wheat yield were investigated in a field experiment. Amendment with LW or CW increased soil electrical conductivity and contents of total organic C, total extractable C, humified and non-humified C forms, and available P and K. In comparison to LW-amended soils, CW-amended soils featured larger contents of total extractable C and humified C. The compositional, structural and functional characteristics of soil HA were modified by the partial incorporation of HA-like components typical of applied LW and CW. In particular, amended soil HAs featured an increase of C, H and phenolic OH group contents, C/N ratio and aliphaticity, whereas C/H ratio, N, O and COOH group contents, aromatic polycondensation and humification degree decreased. These modifications were generally more evident in HAs from LW-amended soils than in HAs from CW-amended soils. Addition of LW, and especially CW, affected positively the wheat grain yield by increasing the number of kernels per unit area, spike density, and kernel weight. In conclusion, the catalytic treatment of olive oil mill wastewater with MnO2 is able to produce an efficient soil amendment.