Assessing the Fertilizing Potential of Two Tannery Bio-Wastes Through Short-Term Soil Incubation and Plant Rhizosphere Bioassay

The tannery industry is a significant global manufacturing sector, producing over 4 million tons of tannery bio-wastes (TBWs) annually, and to reintegrate TBWs into the productive cycle offers a significant alternative to landfill disposal. Despite TBWs are rich in organic matter, nitrogen (N), and carbon (C) they pose important environmental risks, such as the presence of heavy metals like chromium (Cr) that can be released into the environment. This study aimed at evaluating the fertilizing potential of two TBWs-based fertilizers through a complementary laboratory-scale approach by using a short-term soil incubation experiment and the Rhizotest (ISO16198: 2015) bioassay. Two TBWs-based fertilizers, Tannery Sludge (TS) and Integrated Leather Meal (ILM), were subjected to 42 days of soil incubation and to Rhizotest bioassay. Nitrogen release, chemical and biochemical indicators were assayed after the soil incubation. Tomato plants uptake of heavy metals and rhizosphere enzyme activities were assayed after Rhizotest. TS and ILM released 11% and 35% of their total N content respectively, with TS acting more as a slow-release fertilizer. Heavy metal contamination was negligible, except for Cr, which however remained in the soil in its trivalent form and was not absorbed by plants. Rhizotest allowed to highlight that TS and ILM stimulated broad and specific enzyme activities in the rhizosphere soil. This complementary approach enabled rapid, reproducible, and sensitive characterization of organic fertilizers produced from TBWs, which have the potential to be employed as organic fertilizers safely.