Pedosphere 21(3): 309–318, 2011 ISSN 1002-0160/CN 32-1315/P c  2011 Soil Science Society of China Published by Elsevier B.V. and Science Press Dissolved Organic Nitrogen in Mediterranean Ecosystems *1 M. DELGADO-BAQUERIZO ∗2 , F. COVELO and A. GALLARDO Department of Physical, Chemical and Natural Systems, University Pablo de Olavide, Ctra. de Utrera km. 1, Seville 41013 (Spain) (Received December 17, 2010; revised March 15, 2011) ABSTRACT Dissolved organic nitrogen (DON) in soils has recently gained increasing interest because it may be both a direct N source for plants and the dominant available N form in nutrient-poor soils, however, its prevalence in Mediterranean ecosystems remains unclear. The aims of this study were to i) estimate soil DON in a wide set of Mediterranean ecosystems and compare this levels with those for other ecosystems; ii) describe temporal changes in DON and dissolved inorganic nitrogen (DIN) forms (NH + 4 and NO − 3 ), and characterize spatial heterogeneity within plant communities; and iii) study the relative proportion of soil DON and DIN forms as a test of Schimel and Bennett’s hypothesis that the prevalence of different N forms follows a gradient of nutrient availability. The study was carried out in eleven plant communities chosen to represent a wide spectrum of Mediterranean vegetation types, ranging from early to late successional status. DON concentrations in the studied Mediterranean plant communities (0–18.2 mg N kg −1 ) were consistently lower than those found in the literature for other ecosystems. We found high temporal and spatial variability in soil DON for all plant communities. As predicted by the Schimel and Bennett model for nutrient-poor ecosystems, DON dominance over ammonium and nitrate was observed for most plant communities in winter and spring soil samples. However, mineral-N dominated over DON in summer and autumn. Thus, soil water content may have an important effect on DON versus mineral N dominance in Mediterranean ecosystems. Key Words: dissolved inorganic nitrogen, nitrogen cycle, soil ammonium, soil N availability, soil nitrate Citation: Delgado-Baquerizo, M., Covelo, F. and Gallardo, A. 2011. Dissolved organic nitrogen in Mediterranean ecosys- tems. Pedosphere. 21(3): 309–318. INTRODUCTION Nitrogen is one of the nutrients most frequently limiting primary production in terrestrial ecosystems (Vitousek and Howarth, 1991; LeBauer and Treseder, 2008). The role of dissolved organic nitrogen (DON) in the N cycle was scarcely studied until the last decades, as most research centred on inorganic N (NH + 4 and NO − 3 ) as the only source of N for plants in the classic N cycle. However, recent studies show that DON can play an important role in the N cycle as a potential source both for microorganisms and plants (Kalbitz et al., 2000; Neff et al., 2003; Schimel and Bennett, 2004; Jones et al., 2005; Bardgett, 2005). A growing num- ber of studies have shown that inorganic N availability in alpine and boreal forests explain less than 50% of annual plant growth; thus, plants may take up low molecular weight organic N molecules as well (Chapin III et al., 1988; Schimel and Chapin III, 1996; Kaye and Hart, 1997). Recently, plants have been observed to directly take up organic N forms without micro- bial mineralization, which may alter competitive rela- tionships between plant and soil microorganisms, es- pecially in nutrient-poor ecosystems (Kaye and Hart, 1997;N¨asholm et al., 1998; Owen and Jones, 2001; Harrison et al., 2007). Direct uptake of DON has been observed in boreal ecosystems where DON was more than 80% of the soil-soluble N forms (Kranabetter et al., 2007). Furthermore, DON uptake has also been ob- served in agricultural systems, where DON is less than ∗1 Supported by the Ministerio Espa˜ nol de Ciencia e Innovaci´on of the Spanish government (Nos.REN2003-08620-C02-01 and CGL2006-13665-C02-01). ∗2 Corresponding author. E-mail: mdelbaq@upo.es.