Monitoring carbon, nitrogen, and particulate matter exchange in a northern hardwood forest subject to high N deposition

We describe the establishment of a long-term monitoring site to study the linked biosphere-atmosphere exchange of carbon, nitrogen, water, and energy in a northern hardwood forest. The site is located in the remote Haliburton Forest and Wildlife Reserve in central Ontario (45.28°N, 78.55°W). Land morphology is relatively heterogeneous with an undulating topography on the granitic Canadian Shield. The average annual precipitation for the area is 1050 mm and the mean annual temperature is 5°C(Environment Canada). The site is an uneven-aged managed forest dominated by sugar maple (Acer saccharum Marsh), with American Beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga canadensis L.) and yellow birch (Betula alleghaniensis Britt.). The average leaf area index is ~6, while the canopy height and crown base height are estimated as 22 m and 8 m. Based on Raupach (1992) and Hamman and Finnigan (2007), the displacement height has been estimated to be 17.6 m and z₀ to be 1.1 m. Regional estimates of nitrogen deposition are among the highest in North America, and recent observations suggest that Haliburton Forest has transitioned to phosphorus limitation (Gradowski and Thomas, 2006, 2008).

Eddy covariance flux instruments were mounted at the top of a 30.5 meter walk-up scaffolding tower, which reached approximately 8 m above the average forest canopy. Half hourly fluxes of momentum, sensible heat, latent heat, and CO₂ were measured using an open path system consisting of a CSAT3 sonic anemometer (Campbell Scientific), a LI-7500 infrared CO₂ and H₂O gas analyzer (LI-COR), an HMP45C temperature and humidity probe, and a fine wire thermocouple. Here we report a preliminary analysis of the forest carbon flux during the first 8 months of monitoring, from August 15, 2009 - March 3, 2010.

During the late summer (Aug 15 - Sep 11), the average temperature was 15°C (high of 24°C, low of 7°C). The forest was a carbon sink (-1.6 ± 5.9 g m^-2 d^-1 (average and 95% confidence interval)), with a clear midday peak in carbon uptake, and high levels of respiration during the night. Throughout the early fall (Sep 19 - Oct 16), the average temperature dropped to 4°C (high of 21°C, low of -13°C). The forest was a significant source of carbon during this time (4.8 ± 3.9 g m^-2 d^-1 ), as daytime carbon uptake decreased while nighttime respiration maintained its summer levels. By late fall (Oct 24 - Nov 20), the average temperature was -4°C (high of 10°C, low of -15°C) and there were no peaks in daytime carbon uptake or nighttime respiration. The forest remained a small source of carbon with an average daily net carbon flux of 2.8 ± 1.2 g m^-2 d^-1 . Finally, during the winter (Jan 8 - Mar 3), the average temperature was -18°C (high of -4°C, low of -31°C). During this time, the forest was approximately carbon neutral, with an average daily carbon flux around -0.9 ± 0.7 g m^-2 d^-1 .

The contribution of wet deposition to reactive nitrogen deposition was evaluated between July and November of 2009. Ion chromatography was used to analyze samples for major ions, including nitrate and ammonium, collected during nine precipitation events. The protocol included both open and throughfall collectors, allowing a separation of external and in-canopy sources of nitrogen. Throughfall samples collected in the summer were significantly enriched in nitrate, while those collected during leaf senescence had no additional nitrate, but were depleted in ammonium. Additional leaf wash experiments at multiple heights in the canopy suggest that the enrichment of nitrate in throughfall has contributions both from dry deposition wash-off and canopy leaching, the latter of which may be indicative of nitrogen saturation.

The characterization of nitrogen deposition will be augmented in 2010 by the deployment of a suite of instruments designed to make measurements required for the direct quantification of dry deposition of trace gas and particulate reactive nitrogen. Reactive nitrogen oxide compounds (NO, NO₂, and NO_y) will be measured using a custom-built, fast time response (~ 1 Hz) two channel chemiluminescent analyzer (Air Quality Design, Inc.). Selective conversion of NO₂ to NO is achieved by a blue light LED converter, and NO_y to NO by a heated molybdenum converter (350 °C). The converters are positioned in an inlet box mounted to the top of the tower to minimize sampling losses of sticky compounds, such as HNO3, and connected to the rest of the system via a 40m umbilical. In-lab and field tests have shown 1 Hz detection limits of between 10-20 ppt for NO_x and NO_y. Ammonia will be measured using a quantum cascade tunable infrared laser differential absorption spectrometer (QC-TILDAS, Aerodyne Research Inc.) operating at 10 Hz, also with a custom-designed sampling inlet mounted to the top of the tower. Instrument and inlet characterization demonstrate a 1 Hz detection limit of 600 ppt, and the ability to restrict the majority (> 90%) of the bi-exponential time response to less than 1 second when using a heated inlet with a hydrophobic coating and critical orifice (Ellis et al, 2010). The inlets of these chemical sensors will be located close the sonic anemometer to allow us to calculate eddy covariance fluxes of NO, NO₂, NO_y, and NH₃.

To estimate the influence of particulate species on nitrogen deposition, a low-pressure micro-orifice uniform deposition impactor (MOUDI, MSP) will be used to collect size-resolved (11 stages with cut points between 0.056 and 18 μm) integrated particle samples to be analysed by ion chromatography for water-soluble compounds. Additionally, high time resolution continuous particle sizing systems (Differential Mobility Particle Sizer DMPS and Aerodynamic Particle Sizer APS, TSI Inc., USA) will be used for inter-comparison and validation purposes. Such detailed measurements will provide us with average size-resolved mass distributions for each component (e.g. NH₄⁺ and NO₃^-) that allow us to deduce an average deposition flux to the forest. To do so, we rely on a detailed mechanistic model that accounts for the properties of forest morphology and turbulence as well as particle characteristics (Petroff et al., 2009).