Decadal-scale fates of ¹⁵N tracers added to oak and pine stands under ambient and elevated N inputs at the Harvard Forest (USA)

We used isotope mass balances to track the fates of ecosystem-scale ¹⁵N tracer additions to control non-fertilized and nitrogen (N)-fertilized plots in oak-dominated hardwood and red pine-dominated pine over nine growing seasons. The plots used in our tracer study are the same as of those used in the Chronic Nitrogen Amendment study (see Magill et al., this volume) at the Harvard Forest in central Massachusetts, USA. Nitrogen tracers were added monthly during growing seasons (April-October) of fertilization years 4 and 5 (1991 and 1992) as either ¹⁵NH₄ or ¹⁵NO₃ to control and to N-fertilized plots (low N treatment, receiving additions of 5 g N m^-2 per year as NH ₄NO₃). Seven years after the end of ¹⁵N additions (1999), tracer recoveries were highest in organic soil, ranging from 34 to 83% of additions across ¹⁵N ion form, N loading rate and forest type. Mineral soil (0-20 cm depth) was the second largest sink, with tracer recoveries ranging from 15 to 34% of additions. Tree biomass (foliage, woody tissue, plus fine roots) accumulated 7-16% of tracer additions. Patterns of ¹⁵N recoveries after 7 years differed from recovery patterns at the end of the 2-year labeling period. At the end of labeling in 1992, recoveries in tree biomass were higher under chronic N fertilization than under ambient atmospheric N loading (control plots), higher with ¹⁵NO₃ than ¹⁵NH₄ additions, and higher in hardwoods than in pines. Seven years after the end of labeling, recoveries in trees were still higher with ¹⁵NO₃ than with ¹⁵NH₄ labeling, and trees in control plots continued to accumulate ¹⁵N tracers. In contrast, tracer recoveries in trees on the chronically fertilized plots were lower after 7 years than at the end of the labeling period. This was due mainly to transfers of ¹⁵N tracer from leaf and root tissue to other ecosystem pools during the years since labeling. Wood produced during the 2-year labeling period and the 7-year post-labeling interval continued to accumulate ¹⁵N tracers in control and chronically fertilized plots in both forest types. However, recoveries were low in woody tissues (wood formed from 1990 through 1999 plus bark), ranging from 1.1 to 6.1% of ¹⁵N additions across forest types and treatments. Our comparisons of ¹⁵N tracer recoveries at the end of a 2-year labeling period and at 7 years following the end of labeling suggest that soils are the dominant sinks for N deposition on temperate forests and that N does not contribute substantially to enhanced carbon uptake into tree biomass.