Oecologia (2003) 137:252–257 DOI 10.1007/s00442-003-1334-0 ECOSYSTEMS ECOLOGY Jörgen Persson · Peter Högberg · Alf Ekblad · Mona N. Högberg · Anders Nordgren · Torgny Näsholm Nitrogen acquisition from inorganic and organic sources by boreal forest plants in the field Received: 22 October 2002 / Accepted: 6 June 2003 / Published online: 15 July 2003  Springer-Verlag 2003 Abstract A wide range of recent studies have indicated that organic nitrogen may be of great importance to plant nitrogen (N) nutrition. Most of these studies have, however, been conducted in laboratory settings, excluding important factors for actual plant uptake, such as com- petition, mycorrhizal associations and soil interactions. In order to accurately evaluate the importance of different N compounds to plant N nutrition, field studies are crucial. In this study, we investigated short- as well as long-term plant nitrogen uptake by Deschampsia flexuosa, Picea abies and Vaccinium myrtillus from 15 NO 3  , 15 NH 4 + and (U- 13 C, 15 N) arginine, glycine or peptides. Root N uptake was analysed after 6 h and 64 days following injections. Our results show that all three species, irrespective of their type of associated mycorrhiza (arbuscular, ecto- or ericoid, respectively) rapidly acquired similar amounts of N from the entire range of added N sources. After 64 days, P. abies and V. myrtillus had acquired similar amounts of N from all N sources, while for D. flexuosa, the uptake from all N sources except ammonium was significantly lower than that from nitrate. Furthermore, soil analyses indicate that glycine was rapidly decarboxy- lated after injections, while other organic compounds exhibited slower turnover. In all, these results suggest that a wide range of N compounds may be of importance for the N nutrition of these boreal forest plants, and that the type of mycorrhiza may be of great importance for N scavenging, but less important to the N uptake capacity of plants. Keywords Nutrition · Organic nitrogen · Ammonium · Nitrate · Soil respiration Introduction Nitrogen (N) is a fundamental element regulating plant growth and development (Vitousek et al. 1997). Recent research does, however, give disparate pictures of key elements of the N cycle, particularly concerning the role and importance of organic N for plant N nutrition. Several studies have shown amino acid uptake by a wide range of different plants (Kielland 1994; Raab et al. 1999; Schmidt and Stewart 1999; Persson and Näsholm 2001a; Henry and Jefferies 2002) and plant uptake of small peptides has also been observed (Sopanen et al. 1985; Steiner et al. 1994). Moreover, some studies have shown that plants in symbiosis with ecto- or ericoid mycorrhiza have the ability to grow on complex N sources, such as protein (Abuzinadah and Read 1986; Finlay et al. 1992; Turnbull et al. 1995) and chitin (Kerley and Read 1995, 1997). In addition, a large number of transporters of organic N compounds have been identified in several plant and mycorrhizal species (Chalot et al. 1996; Ortiz-Lopez et al. 2000; Stacey et al. 2002). In all, these studies suggest that organic N compounds may function as N sources for plant growth, and since the bulk of soil N in many ecosystems is present in organic form (Schulten and Schnitzer 1997; Murphy et al. 2000; Jones and Kielland 2002) the potential use of these compounds for plant N nutrition, bypassing mineraliza- tion, may be of great importance and could have profound effects on our understanding of N cycling in these ecosystems. In contrast, however, several studies have implied that plant uptake of organic N sources is small and may be of only limited significance to plant N nutrition (Hodge et al. 2000b; Owen and Jones 2001). J. Persson ( ) ) · T. Näsholm Department of Forest Genetics and Plant Physiology, Umeå Plant Science Center, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden e-mail: jorgen.persson@genfys.slu.se Tel.: +46-90-7866302 Fax: +46-90-7865901 P. Högberg · M. N. Högberg · A. Nordgren Department of Forest Ecology, Section of Soil Science, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden A. Ekblad Department of Natural Sciences, Örebro University, 70182 Örebro, Sweden