Journal of Conference Abstracts

Changes in N Dynamics in a Small Boreal Forest Catchment Following Two Years of Nitrate Addition:
Clues from the ¹⁵N and ¹⁸O Isotopes of Labeled Nitrate

Sébastien Lamontagne (slamonta@cgrpc.uwaterloo.ca),
Sherry L. Schiff (sschiff@sciborg.uwaterloo.ca), Richard Elgood (rjelgood@sciborg.uwaterloo.ca) & Ramon Aravena (roaraven@sciborg.uwaterloo.ca)

Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada.

On the Precambrian shield of central Canada, the potential of upland boreal forest to buffer N-based acidification may be limited because of low forest productivity, cold climate and potential co-limitation of plant growth by base cations. We tested the potential of a small catchment to retain N by adding 40 kg N ha^-1yr^-1 as NaNO₃ in 1995 and 1996. The treated catchment (U3 - 0.40 ha) and two references catchments (U1 - 0.55 ha and U2 - 0.46 ha) are located at the Experimental Lakes Area in northwestern Ontario (Allan et al., 1993; Allan & Roulet, 1994). The N treatment was applied as 38 applications on the snowpack and during rain events using a back-pack sprayer. The added NO₃^- was labeled with ¹⁵N = 300 ‰ to help track the storage of retained N and ¹⁸O = 30 ‰ to trace the origin of exported NO₃^- (Durka et al., 1994).

We have measured the mass-balance for N and other elements, tree growth, soil N-mineralisation (using buried bag incubations), and the size of the biomass and soil N pools. Two pre-treatment years were available for mass-balances and one for N-mineralisation. Under pristine conditions, the catchments efficiently retain mineral N inputs, but export 1-2 kg N ha^-1 yr^-1 as dissolved organic nitrogen (DON).

After two years of NO₃^- addition, N export from U3 was 16 kg N ha^-1 yr^-1 (80 % as NO₃^- and 18 % as DON). Although most of the increased export was due to NO₃^-, NH₄⁺ and DON exports were also two-fold higher than reference catchments. The average pH of U3 runoff increased from 4.3 to 4.6 due to the consumption of H⁺ during NO₃^- reduction. With the exception of the snowmelt period, denitrification rates were low (<1 kg N ha^-1 yr^-1) indicating that most retained NO₃^- was reduced to NH₄⁺ and organic N. Nitrate was consumed during buried bag incubations and NH₄⁺ production rates doubled. The increased export of NH₄⁺ is probably related to the increased production rate (from N-mineralisation or dissimilatory NO₃^- reduction) and ion-exchange with the added Na⁺. The increased DON export may result from a higher production rate, or the deprotonation and subsequent export of weak organic acids. Preliminary isotopic data suggest most of the exported NO₃^- is treatment N that has not cycled within U3 (i.e. was not reduced and subsequently nitrified). Picea mariana and Pinus banksiana growth in U3 was not increased by N addition compared to reference catchments. However, N concentration in tree foliage, mosses, and lichens in U3 increased by 5 to 30 %. After one year of addition, plants and lichens were labeled at ¹⁵N ranging from 30 to 100 ‰ from a background of -6 to -1‰ . Work is under way to determine the proportion of retained N stored in biomass and soil.

Upland boreal forest catchments are leaky for N. Under pristine conditions, 20-40 % of N inputs are exported as DON. When elevated N inputs occur, the potential to retain N appears limited by rapid flushing rate, limited potential for denitrification, and low biological activity for half the year.

References

Allan, C.J., N.T. Roulet & A.R. Hill. Biogeochemistry 22: 37-79 (1993).

Allan, C.J. & N.T. Roulet. Hydrol. Processes 8: 369-388 (1994).

Durka, W. , E.D. Schulze, G. Gebauer & S. Voerkellus. Nature 372: 765-767 (1994).