15N tracer recoveries in red pine and oak forests 16 years after labeling as constraints on estimates of N deposition effects on forest C storage

Background/Question/Methods

We analyzed forest floor, soil, and biomass samples collected from the Harvard Forest Chronic N Addition Study plots in 2008 for ¹⁵N, after 20 years of N additions (0 and 50 kg N ha^-1 yr^-1) and 16 years following 2 years of ¹⁵N labeling, in order to track long-term fates of tracer additions and assess N deposition effects on forest C balances.  Fertilizer was applied at 50 kg NH₄NO -N ha^-1 yr^-1 to 30m x 30m plots in a red pine and an oak forest from 1988 through the present.  ¹⁵N tracers were applied in 1991 and 1992 as either ¹⁵NH₄ or ¹⁵NO₃ to halves of control (background ~8 kg N ha^-1 yr^-1) and fertilized (background + fertilizer = 58 kg N ha^-1 yr^-1) plots.  We traced decadal scale movements of N deposition into ecosystem pools under background and elevated N additions and estimated the effects of chronic N additions on forest ecosystem C storage using ¹⁵N values of plant tissue, forest floor and soil samples collected in 1999 and 2008, N pool sizes, and mass balance equations.

Results/Conclusions

Tree biomass was a weak but persistent sink for N additions, with ¹⁵N tracer recoveries in tree biomass (wood+bark+roots+foliage) after 16 years ranging from 8.1 to 14.9% of total tracer additions across forest types, N addition rates, and tracer forms (¹⁵NH₄ and¹⁵NO₃).  Tracer recoveries in fine roots decreased from 1992 through 2008 in both oak and pine forests under all treatments, whereas recoveries in foliage decreased in oak but either remained constant or increased slightly between 1999 and 2008 in red pine.  Woody tissues continued to accumulate ¹⁵N tracer:  the recoveries in pine forest wood+bark increased between 1999 and 2008 from 1.4 to 3.0% under ambient deposition and from 1.6 to 3.0% with fertilization.  Percent recoveries in oak wood+bark increased from 4.2 to 6.0% under ambient deposition and from 6.0 to 6.6% with fertilization.  Also, ¹⁵N was mobilized from older wood to younger wood and bark across all treatments, even as the total recovery in wood increased through time.  Overall, tracer recoveries in soils and forest floors were much higher than in tree biomass, ranging from 49 to 101% of additions across forest types and N addition rates.  Stoichiometric analyses of these recoveries suggest that N additions are contributing to soil C accumulation to a greater extent than to biomass accumulation in these forests.