APPLICATION OF A N-15 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 Waqu oit Bay watershed on Cape Cod, Masschusetts using N-15 tracer techniqu es. A solution of 99.6% enriched N-15-NO3-, at a concentration similar to that of background throughfall, was applied to forest plots establ ished along a gradient of soil texture to simulate and track the fate of throughfall NO3- deposition. The tracer solution was applied to rep licate plots during both the spring and fall to examine seasonal diffe rences in ecosystem retention. N-15 enrichment was subsequently measur ed in litter, O2 horizon, 0-15 cm mineral soil, fine roots, microbial biomass in the O2 horizon and mineral soil, and lysimeter leachate ove r a 6 month period following each application. The O2 horizon containe d the largest fraction of N-15 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-15 (7-28%) in all sites at the end of bo th 6-month sampling periods. Microbial uptake of applied N-15 provided an initial barrier against leaching loss as well as a mechanism for i ts long-term incorporation into soil organic matter. Microbial process ing was less important in the most coarsely textured site, perhaps as a result of lower substrate availability and smaller microbial pool si zes. The highest cumulative leaching losses of applied N-15 were obser ved in the coarse sand site (40, 51%) followed by the fine sand (13, 4 3%) and loamy sand (4, 19%) sites for the spring and fall applications , respectively. More than 90% of all N-15 captured in lysimeters occur red within two days following the applications, and 25-43% of all N-15 captured in lysimeters after 2 days was in the form of dissolved orga nic nitrogen (DON) indicating that it had been assimilated by microbes prior to leaching.