Increased soil nitrogen associated with dinitrogen-fixing, terricolous lichens of the genus Peltigera in northern Minnesota Rebecca D. Knowles, John Pastor and David D. Biesboer Knowles, R. D., Pastor, J. and Biesboer, D. D. 2006. Increased soil nitrogen associated with dinitrogen-fixing, terricolous lichens of the genus Peltigera in northern Minnesota.  Oikos 114: 37  48. Dinitrogen (N 2 )-fixing lichens (cyanolichens) have long been recognized as a source of exogenous N invarious ecosystems, yet the effects of nitrogen inputs from these lichens have been little studied. Cyanolichens of the genus Peltigera Willd. are often the dominant members of the earth-dwelling (terricolous) cyanolichen community. They occur in diverse habitats around the globe, but no studies of their influence on soil N have been reported. We conducted field and laboratory investigations with the primary objective of determining whether soil N availability is increased near healthy thalli of terricolous dinitrogen-fixing lichens. We measured available soil N in situ with ion-exchange resin bags, potentially mineralizable N with laboratory incubations, total soil N, and soil temperature. Measurementswere taken along transects that extended away from thalli of Peltigera, perpendicular to and parallel to topographic contours. Studies were conducted in ten types of forest across northern Minnesota, including two contrasting sites that represent extremes of habitat for Peltigera. Soil N availability, potentially mineralizable N, and soil %N increased significantly with proximity to healthy thalli of Peltigera (P-values B /0.05). Moreover, potential N mineralization was highly correlated with soil %N (R 2  /0.765). Our results suggest a potential zone of influence that extends 1.5 m from thalli of Peltigera. The data indicate that soil temperature is not a primary factor in these associations. R. D. Knowles and D. D. Biesboer, Dept of Plant Biology, Univ. of Minnesota, St. Paul, MN 55108, USA. Present address for RDK: Leech Lake Band of Ojibwe, 6530 Highway 2 NW, Cass Lake, MN 56633, USA (rknowles@lldrm.org).  J. Pastor, Natural Resources Research Institute, Univ. of Minnesota, Duluth, MN 55811, USA. The availability of soil nitrogen (N) limits the growth of plants in northern forests (Pastor et al. 1984, Binkley and Ho ¨gberg 1997, Reich et al. 1997, Bhatti et al. 2002). Most plants in the southern boreal  northern hardwood forests of northern Minnesota, USA, obtain nitrogen from atmospheric deposition and from microbial soil processes that decompose organic matter. However, atmospheric N deposition has been moderate in this region, with deposits of inorganic N varying between 1.5 and 5.3 kg ha 1 yr 1 since 1980 (NADP 2004). In addition, a cool, wet climate and lignin-rich coniferous litter limit decomposition of soil organic matter and the subsequent availability of N. A few nutrients are provided by abundant, poikilo- hydric lichens that leak N, phosphorus (P), and carbon upon rehydration (Farrar and Smith 1976, Knops et al. 1996). The N-enriched leachate from dinitrogen (N 2 )-fixing lichens (cyanolichens) is particu- larly important because it provides exogenous, atmospheric N 2 to these very N-limited forests (Critten- den 1983, Millbank and Olsen 1986, Knowles et al. 2004). Accepted 9 December 2005 Subject Editor: Heikki Seta ¨la ¨ Copyright # OIKOS 2006 ISSN 0030-1299 OIKOS 114: 37  48, 2006 OIKOS 114:1 (2006) 37