Application of a N tracer to simulate and track the fate of atmospherically deposited N in the coastal forests of the Waquoit Bay Watershed, Cape Cod, Massachusetts.

We examined patterns of N retention in the coastal forests of the Waquoit Bay watershed on Cape Cod, Masschusetts using N tracer techniques. A solution of 99.6% enriched N -NO, at a concentration similar to that of background throughfall, was applied to forest plots established along a gradient of soil texture to simulate and track the fate of throughfall NO deposition. The tracer solution was applied to replicate plots during both the spring and fall to examine seasonal differences in ecosystem retention. N enrichment was subsequently measured in litter, O2 horizon, 0-15 cm mineral soil, fine roots, microbial biomass in the O2 horizon and mineral soil, and lysimeter leachate over a 6 month period following each application. The O2 horizon contained the largest fraction of N in all sites immediately following the spring application (19-45%) but was less important following the fall application (10-25%). The mineral soil N pool generally contained the largest fraction of applied N (7-28%) in all sites at the end of both 6-month sampling periods. Microbial uptake of applied N provided an initial barrier against leaching loss as well as a mechanism for its long-term incorporation into soil organic matter. Microbial processing was less important in the most coarsely textured site, perhaps as a result of lower substrate availability and smaller microbial pool sizes. The highest cumulative leaching losses of applied N were observed in the coarse sand site (40, 51%) followed by the fine sand (13, 43%) and loamy sand (4, 19%) sites for the spring and fall applications, respectively. More than 90% of all N captured in lysimeters occurred within two days following the applications, and 25-43% of all N captured in lysimeters after 2 days was in the form of dissolved organic nitrogen (DON) indicating that it had been assimilated by microbes prior to leaching.