Methods: Ecosystem-scale ¹⁵N tracer in burned and unburned black spruce forests

Site description: In 1999, we set up an ecosystem scale ¹⁵N tracer addition to determine the fate of N inputs to unburned forest, and the fate of N following fire. We added the tracer to replicate burned and unburned plots on Helmer’s Ridge, a flat, slightly north-facing ridgeline within the Caribou-Poker Creek Research Watershed (CPCRW). Purchase of the tracer was funded by the Bonanza Creek Longterm Ecological Research Program (BNZ-LTER). Samples and data have been and will continue to be archived in LTER facilities

Treatment plots burned in a medium severity stand-initiating fire on July 9, 1999 as part of the NSF-funded Frostfire experiment (see http://lter.alaska.edu/. This area was previously forested with a ca. 100 year old closed-canopy black spruce stand (C. Fastie, unpublished data) on moderately well drained gravelly silt loam with no permafrost. Similar intact stands still exist ~200m from our study area on the unburned side of the fire line. Extensive pre- and post fire data on vegetation, soils, carbon and N pools, energy balance, climate, and hydrology exist for this site. Although the Frostfire funding period has ended, several LTER researchers will continue to monitor post-fire conditions, including hydrologic fluxes, permafrost, soil respiration, and vegetation recovery. Post-fire fluxes of N in groundwater and stream water are currently being measured lower on the hill slope. Our proposed research will compliment these measurements by assessing controls over N retention and loss from the soil profile.

¹⁵N tracer application: Addition of a ¹⁵N label to ecosystem-scale plots is a powerful tool for tracing the path of N as it enters and cycles through ecosystem pools. This method combines a mass-balance approach with tracer techniques commonly used in small scale studies (e.g., Jackson et al. 1988, Schimel 1989, and Hobbie and Chapin 1998), and has been used to identify sinks for N in both disturbed (Matson et al. 1987) and undisturbed ecosystems (Nadelhoffer and Fry 1994, Nadelhoffer et al. 1995, Tietema et al. 1998, Nadelhoffer et al. 1999). This is made possible by the relatively small and constant range of ¹⁵N values found in natural materials and the fact that fractionation of N during movement among pools is generally small relative to level of enrichment (Nadelhoffer and Fry 1994). The method is especially useful for tracing the movement of tracer N into large pools, such as soil organic matter N, where changes in total N are difficult to detect.

The goal of the ¹⁵N tracer experiment was to shift the isotopic composition of the pool of N that was mineralized during or soon after fire in order to trace the flux of this N into ecosystem compartments, and determine its overall retention in the ecosystem. As a control for the effects of fire, we similarly added ¹⁵N to unburned plots at each of the sites to examine partitioning and retention in unburned forest. We will monitor these unburned plots, but will not discuss them further in this proposal. In the future, we will collaborate with other researchers to model ecosystem N fluxes as part of an LTER cross-site comparison of ¹⁵N studies.

In August of 1999, we established four pairs of 12 x 12 m plots in burned forest at Helmer’s Ridge. The ¹⁵N addition was randomly assigned to one member of the pair; in the other, we will monitor natural abundance of ¹⁵N over the course of the experiment. The latter plots will offer valuable information about how the natural abundance changes with time since fire. Plots were located within permanent plots that had been established as part of the Frostfire Experiment before the fire, and were selected to have similar stem density, soil depth, and vegetation composition. Thus for these plots, we have access to measurements of pre-fire ecosystem pools and fluxes and vegetation composition, and measurements of forest floor consumption during fire.

This method requires an accurate determination of the natural abundance ¹⁵N signature of each ecosystem pool prior to tracer addition. Prior to addition of the ¹⁵N tracer, we volumetrically sampled three 2.5 cm diameter soil cores down to parent material in each plot, including unburned plots and both sets of paired plots in the burned area. >Burned cores were separated into a char layer, an organic layer, and mineral soil, which was further separated into 5 cm increments for the top 15 cm, and into 10 cm increments thereafter. Cores were processed according to standard soils methods (SSJA 1996), and roots and soil components were archived for later analyses. From these cores, we will determine soil C and N pools, and the ¹⁵N signature of these pools. We sampled whole plants for ¹⁵N signature from areas outside the plots because plants were extremely small and we wanted to avoid altering natural colonization within the plots.

After quantifying the initial ¹⁵N of vegetation and soils, we sprayed a solution of 99% ¹⁵N-(NH₄)₂SO₄ onto the charred forest floor of treatment plots at 0.05g ¹⁵N/m². This areal amount was chosen to raise the signature of the NH₄⁺ pool to a high enough enrichment such that we could trace its fate through all ecosystem pools. For example, if the relatively small NH₄⁺ pool was completely transferred to the mineral soil, the largest pool of N in this ecosystem, then the enrichment of the mineral soil would be raised to a level 25 times greater than natural abundance levels (based on data from Van Cleve et al. 1983). Given low levels of N inputs to this system, this level of application should be visible in the ecosystem for the next 50-plus years. All soil and plant samples will be analyzed for total C and N, and ¹⁵N content on a Finnegan Delta Plus mass spectrometer coupled to a C and N analyzer at the University of Florida.

Plants and soils from labeled plots were harvested at peak biomass in summer 2000, 2001, 2002, 2003, and will be harvested again in 2005. In the unburned treatment, we used a combination of inventory and allometric techniques combined with destructive harvests to measure biomass pools, and element and isotope concentrations. We used destructive harvests in the burned treatment.

Literature cited:

Hobbie, S.E. and F.S. Chapin, III. 1998. The response of tundra plant biomass, aboveground production, nitrogen and CO₂ flux to experimental warming. Ecology.

Jackson, L.E., R.B. Strauss, M.K. Firestone and J.W. Bartolome. 1988. Plant and soil nitrogen dynamics in California annual grassland. Plant and Soil 110:9-14.

Matson, P.A., P.M. Vitousek and J. Ewel. 1987. Nitrogen transformations following tropical forest felling and burning on a volcanic soil. Ecology 68:491-502.

Nadelhoffer, K.J., M.R. Downs, B. Fry, J.D. Aber, A.H. Magill and J.M. Melillo. 1995. The fate of 15N labelled nitrate additions to a northern hardwood forest in eastern Maine. Oecologia 103:292-301.

Nadelhoffer, K.J., M. Downs and B. Fry. 1999. Sinks for N additions to an oak forest and a red pine plantation at the Harvard Forest, Massachuttsetts, USA. Ecological Applications 9:72-86.

Nadelhoffer, K.J. and B. Fry. 1994. Nitrogen isotope studies in forest ecosystems. Stable isotopes in ecology and environmental science. K. Lajtha and R. H. Michener. Oxford, Blackwell Scientific Publications: 22-44.

Schimel, J. 1989. Spatial and temporal effect on plant-microbial competition for inorganic nitrogen in a California annual grasslands. Soil Biology and Biochemistry 21:1059-1066.

Tietema, A., B.A. Emmett, P. Gundersen, O.J. Kjonaas and C.J. Koopmans. 1998. The fate of 15N labelled nitrogen decomposition in coniferous forests. Forest Ecology and Management 101:19-27.

Van Cleve, K., L. Oliver, R. Schlentner, L.A. Viereck and C.T. Dyrness. 1983. Productivity and nutrient cycling in taiga forest ecosystems. Canadian Journal of Forest Research 13:747-766.