Biological activity of soil organic matter mobilized by root exudates S. Nardi a, * , E. Sessi a , D. Pizzeghello a , A. Sturaro b , R. Rella b , G. Parvoli b a Dipartimento di Biotecnologie Agrarie, Universit  a di Padova, via Romea 16, Legnaro, Padova I 35020, Italy b Servizio di Prevenzione e Protezione del Consiglio Nazionale delle Ricerche, Corso Stati Uniti 4, Padova I 35127, Italy Received 8 December 2000; accepted 7 May 2001 Abstract Inordertostudythebiologicalactivityofsoilorganicmattermobilizedbyagrarian(Zea mays :cultivarsMytosand Samantha)andforest(Picea abies Karst. and Pinus sylvestris L.)rootexudates,twodifferentsoils,anEutricCambisol (EC) and a Rendzic Leptosol (RL), were considered. Soil organic matter extracts were obtained by treating the soils with water (control) or plant root exudates. The extracts were characterized by hormone-like activities and gas chro- matographic/mass spectrometric (GC/MS) measurements. Their effects on the nitrogen metabolism in maize seedlings were evaluated. The nitrogen organification in the maize seedlings has been greatly stimulated by all the organic acid extracts from the agrarian soil, while the extracts from the forest soil had no influence upon the metabolism; this in- dicated a probable link between the plant and the environment. The different biological activities of the extracts are discussed. Ó 2002 Published by Elsevier Science Ltd. Keywords: Organic acids; Pinus sylvestris; Picea abies; Auxin- and gibberellin-like activities; Gas chromatographic/mass spectrometric measurements; Dissolved organic matter 1. Introduction Low-molecular-weight organic acids play an impor- tant role in soil genesis and plant physiology (Jones, 1998).Theyareproducedandreleasedbysoilorganisms and in great quantities by roots and germinating seeds (Shen et al., 1996). Organic acid root exudated have been implicated in many soil processes including the mobilization and uptake of nutrients by plants and microorgasnisms, detoxification of metals by plants, microbial proliferation in the rhizosphere and the dis- solution of soil minerals leading to pedogenesis (Mars- chner, 1995). Recently organic acid root exudates, in plant–root– microbial interactions, have attracted much interest not so much in terms of carbon source but as ‘signals’ for recognition or as precursors of phytohormone produc- tion (Marschner, 1995). Previous studies have demon- strated that organic acids (fumaric and succinic acids) occurring in the root exudates of three maize cultivars seedlings shift the humic matter from high to low mo- lecular weight in size exclusion chromatograms (Nardi etal.,1997).Thissuggestsamicellarbehaviourofhumic substances in solution and the importance of hydro- phobic bonds that held the humic molecules together (Piccoloetal.,1996a,b,2000).Exudateswerealsousedto separatesoilorganicfractionsfromthesoilitself(Nardi et al., 1997, 2000a). The fractions mobilized by organic acidsareevidencedbytheirauxin-and/orgibberellin-like behaviour. The scope of this paper is to investigate if organic acid extracts are able to stimulate the N meta- bolisminmaizeseedlings.Forthispurposethebiological Chemosphere 46 (2002) 1075–1081 www.elsevier.com/locate/chemosphere * Corresponding author. Tel.: +39-049-8272905; fax: +39- 049-8272929. E-mail address: serenella.nardi@unipd.it (S. Nardi). 0045-6535/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII:S0045-6535(01)00160-6