Impact of heavy metal pollution on microbial processes involved in soil N transformations Anna Carfora a , Simona Castaldi a* , Rosaria D’Ascoli a , Flora Angela Rutigliano a a Dipartimento di Scienze Ambientali, Seconda Università di Napoli, via Vivaldi 43, 81100 Caserta, Italia Abstract Heavy metals can significantly affect soil microbial biomass and functions, thus altering the role of soil microflora, which is mainly involved in organic matter degradation and recycling of soil nutrients. Microbial processes involved in N transformations are particularly important as their rates influence the amount and the form of mineral N present in the soil, which might be immobilized by organisms or lost from the system. The present work investigates the impact of pollution by Cr3+ and Cu2+ on soil processes involved in N transformations in set aside soils close to the Solofrana river (Campania). Pollution derived from agricultural practices (Cu2+) and from sequential (1980-1998) soil flooding (Cr3+ in water and sediments from tannering activity). Areas with different degree of pollution were chosen and composite soil samples were taken from the top 10 cm. N mineralization, nitrification and denitrification were measured immediately after, as well as total and extractable metals and the chemico-physical parameters (organic C, total N, mineral N, pH, water content), which might influence the investigated biological activity as well as the bioavailability of metals for the soil microbiota. Data showed that the investigated processes had different sensitivity to the two metals, which, however, presented different percentage of available fraction. The most responding activity to pollution was denitrification, suggesting its potential role as soil biomarker. © 2004 SItE. All rights reserved Keywords: biomonitoring, nitrification, mineralization, denitrification, pollution 1. Introduction Due to their functions and ubiquitous presence, soil microrganisms play a fundamental role in biogeochemical cycles of nutrients, moreover they are actively involved in forming the structure of soil. Particularly important are microbial processes involved in N transformations, the conversion of organic nitrogen compounds to ammonium (NH 4 + ) (nitrogen mineralization); the oxidation of NH 4 + to nitrite (NO 2 - ) and then to NO 3 - (nitrification); the reduction of NO 3 - under anaerobic conditions (denitrification). Rates of this processes influence the amount and the form of mineral N present in the soil, which might be immobilized by organisms or lost from the system. Heavy metal contamination of soil has been demonstrated to affect significantly soil microbial biomass and functions (Bååth 1989). Among published data, few studies on the impact of heavy metals on N-mineralization and nitrification are available (Babich and Stotzky 1985; Ross and Kaye 1994; Munn et al. 1997; Sauvè et al. 1999; Smolders et al. 2001), and even fewer assessments have been made on denitrification (Sakadevan et al. 1999; Holtan-Hartwig et al. 2002). The objective of this study was to explore the effect of heavy metals on the microbial processes involved in N transformations in soil in order to assess the relationship between metal loading and N- mineralization, nitrification and denitrification, and to