RETENTION AND LEACHING LOSSES OF ATMOSPHERICALLY-DERIVED NITROGEN IN THE AGGRADING COASTAL WATERSHED OF WAQUOIT BAY, MA

Extensive areas of the eastern United States are being exposed to elevated levels of nitrogen in precipitation, with levels of inorganic N in wet deposition ranging from 5 to over 20 times preindustrial, background levels. This increase in N loading to the terrestrial system, coupled with changes in land use in coastal regions in particular, has dramatically increased the level of nutrient loading from watersheds to the point that coastal waters are today among the most intensely fertilized ecosystems on earth. Studies in upland, aggrading forests have generally found that precipitation N inputs are efficiently sequestered in forest biomass and soil organic matter. However, acidic soils, sandy, porous parent substrates, and chronic inputs of salt spray common to coastal watersheds may all reduce the potential for N sequestration by the terrestrial community. We assessed the role of coastal forests in the long-term storage and retention of atmospherically-derived N in the watersheds of Waquoit Bay, MA, an increasingly eutrophic estuary on Cape God, by measuring precipitation inputs, storage, and lysimeter outputs below the rooting zone in a chronosequence of sites released from agriculture at different times. Calculated annual retention efficiencies were relatively low for an N-limited, aggrading forest (40-62%), and leaching losses did not vary with site age from young pine stands to mature beech forests. Nearly all nitrogen input was retained during summer months except in months with very high rainfall events. Nitrogen was released during the dormant-season in proportion to water flux through the forest floor. The composition of lysimeter output was 76% DON, 11% NO3-, and 13% NH4+. Total water flux and infiltration appear to be more important determinants of N retention in this sandy, coastal site than in more upland forest ecosystems; sandy systems may inherently have a low N retention efficiency.