The effect of organic amendment on native soil humic substances, with emphasis on the use of olive-oil mill wastewaters

Low organic matter (OM) levels are often responsible for low soil fertility and crop production and high soil exposure to degradation, erosion, desertification and pollution. Currently, wastes of various origin and nature are widely used as organic amendments to increase and/or restore soil OM. The agronomically efficient and environmentally safe use of any organic amendment requires, however, an adequate control not only of the chemical quality of the humic acid (HA)- like fraction contained, which is an important indicator of OM maturity and stability, but also of the effects that these HAs may have on native soil HA. Olive-oil mill wastewaters (OWs), whose disposal represents a major and yet unresolved problem for several Mediterranean countries, are potential candidates for use as liquid organic soil amendment because of the relatively high amount of OM dissolved and suspended in a large volume of water, which is also a precious resource for agriculture in and and semiarid areas. This paper has two objectives: (1) to shortly review available data on the effects of some representative organic amendments on the chemical and spectroscopic properties of HAs isolated from amended soils in comparison to soil HA and HAs from the corresponding amendments; and (2) to present and discuss comparatively chemical (elemental and functional composition) and spectroscopic (Fourier transform infrared and fluorescence) data obtained on HAs isolated from: (a) three OW samples either freshly-collected from an olive-oil mill plant in Southern Italy, or aged or catalytically-treated with Mn (IV) oxide for 8 weeks at room temperature under continuous mechanical stirring and aeration; (b) parcel soils amended once with 60 L m(-2) of each type of OW, and sampled at a 0-20 cm depth after 15 and 120 days from OW application; and (c) the control, unamended soil.