Organic fertilizers obtained from metal-free and chromium tanned leather: Evaluation of nitrogen release and the effects on the soil microbiome

Tannery industry generates over 4 million tonnes of waste annually, with 600,000 tonnes containing 2.5-6 % chromium (Cr). Tannery wastes can be employed as fertilizers due to their N and organic matter content. However, their use raises concerns about Cr and trace element contamination. Metal-free tanning processes (wet white, WW) offer an environmentally friendly alternative to Cr-based tanning (wet blue, WB), however 90 % of leather production still relies on WB tanning. The fertilizing potential of WW tanning residues has been little investigated, while this knowledge will be essential for a rational reuse of these materials in sustainable agriculture. This study evaluates fertilizers produced from hydrolysed leather residues (WW and WB) by assessing: i) N release dynamics; ii) effects on key soil fertility indicators; iii) microbial community physiological profiling; iv) Cr and trace element bioavailability to plants. Physicochemical characterization was first performed, afterwards, a soil incubation experiment was conducted. Both fertilizers released N fitting a first-order kinetics model, releasing 40-45 % of their total N after 42 days. DTPA-extractable Cr increased but remained low (31 mu g kg-1) in WB-treated soils. Biochemical indicators were positively affected by WW and WB fertilizers, and microbial community profiles were influenced by their composition but not by the presence of Cr in WB. Overall, WW and WB fertilizers effectively supplied N and enhanced microbial activity, without adversely impacting soil fertility indicators. Rhizotest suggested that Cr was not bioavailable for tomato plants in both fertilizers, even though total Cr reached about 60 mu g kg-1 in WB-fertilized soils.