Application of a ¹⁵N tracer to simulate and track the fate of atmospherically deposited N in the coastal forests of the Waquoit Bay Watershed, Cape Cod, Massachusetts

Abstract

We examined patterns of N retention in the coastal forests of the Waquoit Bay watershed on Cape Cod, Masschusetts using ¹⁵N tracer techniques. A solution of 99.6% enriched ¹⁵N -NO₃ ⁻, 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 NO₃ ⁻ deposition. The tracer solution was applied to replicate plots during both the spring and fall to examine seasonal differences in ecosystem retention. ¹⁵N 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 ¹⁵N 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 ¹⁵N (7–28%) in all sites at the end of both 6-month sampling periods. Microbial uptake of applied ¹⁵N 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 ¹⁵N 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 ¹⁵N captured in lysimeters occurred within two days following the applications, and 25–43% of all ¹⁵N 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.