Pedosphere 24(4): 469–475, 2014 ISSN 1002-0160/CN 32-1315/P c  2014 Soil Science Society of China Published by Elsevier B.V. and Science Press Soil Microbial Biomass After Three-Year Consecutive Composted Tannery Sludge Amendment ∗1 M. D. M. SILVA 1 , M. BARAJAS-ACEVES 2 , A. S. F. ARA ´ UJO 1, * 2 , F. F. ARA ´ UJO 3 and W. J. MELO 4 1 Soil Quality Lab., Agricultural Science Center, Federal University of Piau´ ı, Teresina, PI 64049-550 (Brazil) 2 CINVESTAV-IPN, Departamento de Biotecnolog´ ıa y Bioingenier´ ıa, Mexico D.F., PO Box 14-740 (Mexico) 3 West S˜ao Paulo University, UNOESTE, Presidente Prudente, SP 19067-175 (Brazil) 4 Department of Technology, S˜ ao Paulo State University, Jaboticabal, SP 14801-902 (Brazil) (Received August 27, 2013; revised May 27, 2014) ABSTRACT Brazilian industry produces huge amounts of tannery sludge as residues, which is often disposed by landfilling or land application. However, consecutive amendment of such composted industrial wastes may cause shifts in soil microbial biomass (SMB) and enzyme activity. This study aimed to evaluate SMB and enzyme activity after 3-year consecutive composted tannery sludge (CTS) amendment in tropical sandy soils. Different amounts of CTS (0.0, 2.5, 5.0, 10.0, and 20.0 t ha -1 ) were applied to a sandy soil. The C and N contents of SMB, basal and substrate-induced respiration, respiratory quotient (qCO 2 ), and enzyme activities were determined in the soil samples collected after CTS amendment for 60 d at the third year. After 3 years, significant changes were found in soil microbial properties in response to different CTS amounts applied. The organic matter and Cr contents significantly increased with increasing CTS amounts. SMB and soil respiration peaked following amendment with 10.0 and 20.0 t ha -1 of CTS, respectively, while qCO 2 was not significantly affected by CTS amendment. However, soil enzyme activity decreased significantly with increasing CTS amounts. Consecutive CTS amendment for 3 years showed inconsistent and contrasting effects on SMB and enzyme activities. The decrease in soil enzyme activities was proportional to a substantial increase in soil Cr concentration, with the latter exceeding the permitted concentrations by more than twofold. Thus, our results suggest that a maximum CTS quantity of 5.0 t ha -1 can be applied annually to tropical sandy soil, without causing potential risks to SMB and enzyme activity. Key Words: Cr concentration, enzyme activity, industrial wastes, soil microbial properties, tropical soil Citation: Silva, M. D. M., Barajas-Aceves, M., Ara´ ujo, A. S. F., Ara´ ujo, F. F. and Melo, W. J. 2014. Soil microbial biomass after three-year consecutive composted tannery sludge amendment. Pedosphere. 24(4): 469–475. INTRODUCTION The tannery industry occupies an important place in the Brazilian economy with assets of 21 billion dol- lars per year (Silva et al., 2010). However, this indu- stry releases 1 million tons of tannery sludge annu- ally, of which 3% is solid waste (Santos et al., 2011), and this waste presents chemical elements produced after the tanning process. Landfilling is the main dis- posal method for tannery sludge in Brazil. Alterna- tively, application to soil has been suggested as a sui- table method (Singh and Agrawal, 2008). As tannery sludge contains trace elements, mainly Cr, its direct application to soil may cause environmental contami- nation (Singh and Agrawal, 2008; Silva et al., 2010) and may change the functioning of soil. Recently, al- ternative methods for tannery sludge recycling have been evaluated, such as composting process (Rihani et al., 2010; Santos et al., 2011; Gon¸ calves et al., 2014). Composting has long been recognized as one of the most cost-effective and environmental-friendly al- ternatives for organic waste recycling (Santos et al., 2011), and has been used to process sludge from diffe- rent origins, such as sewage, tannery, and textile sludge (Bernal et al., 1998; Ara´ ujo et al., 2007; Gon¸ calves et al., 2014). During composting, plant nutrients present in the wastes are converted by microbial action into soluble forms that are available to plants (Ndegwa and Thompson, 2001). In addition, as reported by Ara´ ujo et al. (2009) for textile sludge, composting can also de- crease or eliminate the toxicity of tannery sludge. The knowledge of the effects of composted tannery sludge (CTS) on soil microbial properties, such as mi- crobial biomass and enzyme activities, is important to *1 Supported by the Conselho Nacional de Desenvolvimento Cient´ ıfico e Tecnol´ogico, Brazil (No.302999/2011-6) and the Funda¸c˜ao de Amparo a Pesquisa do Estado do Piau´ ı, Brazil (No. 032/2011). *2 Corresponding author. E-mail: asfaruaj@yahoo.com.br.