Journal of Conference Abstracts

Linking the Carbon and Nitrogen Cycles in Forested Catchments: Isotopic Studies of the Sources and Transformations of Dissolved Organic Matter,
Nitrate, and Ammonium

S. L. Schiff¹ (sschiff@ sciborg.uwaterloo.ca), R. O. Aravena¹ (roaraven@ sciborg.uwaterloo.ca), R. J. Elgood¹ (rjelgood@ sciborg.uwaterloo.ca), S. Lamontagne¹ (slamonta@ cgrpc.watstart.uwaterloo.ca), P. McCrindle¹ (pmmccrin@ cgrpc.watstart.uwaterloo.caa), S. E. Trumbore² (setrumbo@ uci.edu), K. J. Devito³ (kdevito@ eratos.erin.utoronto.ca),
D. S. Jeffries⁴ (Dean.Jeffries@CCIW.ca), R. G. Semkin⁴,
P. J. Dillon⁵ (dillonpe@ epo.gov.on.ca) & K. A. Peebles¹

¹ Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1 Canada.

² Department of Earth System Science, University of California-Irvine, Irvine, California 92717 U.S.A.

³ Department of Geography, University of Toronto, Erindale College, Mississauga, Ontario L5L 1C6 Canada.

⁴ National Water Research Institute, Burlington, Ontario L7R 4A6 Canada.

⁵ Dorset Research Centre, Ontario Ministry of Environment and Energy, Dorset, Ontario P0A 1E0 Canada.

Linkages between the carbon and nitrogen cycles in forested catchments have been examined at 3 sites in Ontario that span a range of N deposition: The Experimental Lakes Area (ELA), Turkey Lakes Watershed (TLW) and Harp Lake. Dissolved organic matter (DOM) is the major form of C and N exported from upland catchments at ELA but the relative importance of DON decreases in importance at the other sites as N deposition and soil depth increases. In aquatic systems, DOM is an important component in the food web and the acid-base balance of softwaters, has a major role in the transport and toxicity of trace metals and other contaminants, affects light penetration and protects aquatic organism from the effects of UV radiation.

At Harp Lake, DOC exported in streams from upland forested catchments can be divided into two end members based on radiocarbon dating. DOC with ¹⁴C activities substantially less than modern is exported when water table levels are low and is similar in ¹⁴C activity to groundwaters located in or below the B soil horizon. DOC with modern ¹⁴C activity is derived from shallow riparian soil horizons. During the fall when the basin moisture conditions are replenished following late summer drought and the decrease in evapotranspiration following leaf-off, DOC in the stream can be observed to increase in D ¹⁴C from low negative values to positive near modern values. These two DOM end members differ in ¹⁵N, ¹³C, C/N ratio and chemical characteristics such as charge equivalents per mg DOC and ability to absorb UV-B radiation. Values of ¹⁵N and C/N observed in stream and groundwater DOC are compared to soil organic matter and DOC leached from specific soil horizons. Peatlands and upland forests with poor soil development export recently fixed carbon with high C/N ratio and low ¹⁵N.

Preliminary analyses show that the ¹⁵N in DOM at Harp Lake and ELA is generally not enriched compared to precipitation ¹⁵N-NH₄⁺ and ¹⁵N-NO₃^- and ¹⁵N in growing leaves. Enriched ¹⁵N values in soil horizons below the litter layer demonstrate the influence of forest floor recycling.

Atmospheric nitrate differs substantially in ¹⁸O signature from soil-derived nitrate. ¹⁵N in combination with ¹⁸O is a useful indicator of denitrification. Both ¹⁸O and ¹⁵N of NO₃^- and ¹⁵N of DOM of samples collected from precipitation, first order streams, lake outflow, piezometers and forest floor lysimeters are used to examine the fate of atmospheric NO₃^- in forested catchments at TLW and Harp Lake.