Biogeochemistry 42: 325–343, 1998. c 1998 Kluwer Academic Publishers. Printed in the Netherlands. Transformation and retention of nitrogen in a coastal forest ecosystem B. SEELY 12 , K. LAJTHA 13 & G.D. SALVUCCI 4 1 Department of Biology, Boston University, Boston, MA 02215, USA; 2 Department of Forest Sciences, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4 (Current address); 3 Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331-2902, USA (Current address); 4 Departments of Earth Sciences and Geography, Boston University, Boston, MA 02215, USA Key words: atmospheric deposition, dissolved organic nitrogen, eutrophication, forest ecosystem, lysimeter, nitrogen leaching Abstract. Transformations and fluxes of N were examined in three forested sites located along a gradient of soil texture in the coastal forests of the Waquoit Bay watershed on Cape Cod. Total N leaching losses to ground water were 0.5 kg ha 1 yr 1 in the loamy sand site and 1.5 kg ha 1 yr 1 in the fine sand site. Leaching loss to groundwater was not measured in the coarse sand site due to the prohibitive depth of the water table but total N leaching loss to 1m depth in the mineral soil was 3.9 kg ha 1 yr 1 . DON accounted for most of the leaching losses below the rooting zone (77–89%) and through the soil profile to ground water (60%– 80%). Differences in DON retention capacity of the mineral soil in the sites along the soil texture gradient were most likely related to changes in mineral soil particle surface area and percolation rates associated with soil texture. Forests of the watershed functioned as a sink for inorganic N deposited on the surface of the watershed in wet and dry deposition but a source of dissolved organic N to ground water and adjoining coastal ecosystems. Introduction Dramatic increases in atmospheric N deposition over the past few decades have fueled concerns about the degradation of forest health and water quality in many regions throughout the world. Accordingly, research has focused on understanding how forest ecosystems process this N input. Most of this research, which has been conducted in upland forest ecosystems, suggests that retention of atmospherically deposited N is largely dependent upon the successional stage of the forest as well as soil C:N ratios. Young aggrading stands that are actively sequestering N in biomass and soil tend to exhibit small leaching losses of N while more mature stands have N outputs that are approximately equal to inputs (Vitousek & Reiners 1975; Van Miegroet et al. 1992; Emmet et al. 1993). Less attention has been directed towards understanding the dynamics of atmospherically deposited N in coastal forest ecosystems of the eastern US.