Tannery land plaster (TLP) is a byproduct of lime hydrolysis of leather shavings. Its use in agriculture (organic C？≈？17%, N？≈？6%？dm) could represent an alternative to landfill or incineration, but the high Cr(III) content (≈5%？dm) makes it necessary to evaluate the effect on soil biochemical properties. TLP was therefore added at the rates of 220 and 440？kg of N ha？1 to 2 agricultural soils and incubated for 56 days under controlled conditions. Extractable NH4？+-N and NO3？？-N, CO2-C evolution, microbial biomass-N, protease activity, and extractable Cr were monitored. The organic N was readily mineralized (>50% in the first week) and a significant increase in microbial activity was measured, regardless of soil type and addition rate. Extractable Cr(III) quickly decreased during the incubation. The absence of a negative impact on soil biochemical properties seems to support the use of TLP in agriculture, although further investigations in long-term field experiments are suggested. 1. Introduction The production of biosolids from municipal solid waste, sewage sludge, and waste of agroindustrial origin is continually increasing [1]. Their potential use in agriculture as an alternative method to landfill or incineration has become an increasingly attractive option, due to current trends in European waste policy [2, 3]. The agricultural use of these byproducts could help in maintaining soil organic matter (OM) content and promoting the recycling of plant nutrients, thus reducing the use of chemical fertilizers [1] and increasing agricultural production sustainability [4]. However, in order to recycle biosolids in soil it is necessary to exclude any hazardous effects for humans, animals, plants, and soil microbial populations. Appropriate control of chemical and physical characteristics of the biosolids, nutrients, and heavy metal dynamics in soils is needed in order to guarantee the agronomical value of the products and environmental safety. Tannery land plaster (TLP) is a by-product of lime hydrolysis of leather shavings, a residue of the leather production cycle classified as treated industrial sewage sludge [5]. After the alkaline hydrolysis of leather shavings, sulphuric acid is added to neutralize the suspension, and a calcium sulphate precipitate is then separated by filtration, obtaining the TLP. TLP is currently disposed of in landfill or, at best, used to correct soil acidity. However, due to the significant amount of N (≈6%？dm) its soil application as a source of organic N may represent a more suitable recycling strategy. TLP originates from Cr
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