Site-specific nutrient mass balances and critical loads for forests in Canada Shaun Watmough*, Julian Aherne, Rock Ouimet, Paul Arp, Ian Demerchant Critical loads allow soils to acidify to the critical chemical criterion used in the SMB model. Large parts of Canada currently receive acid deposition in excess of the critical load. Are soils acidifying? Do site specific critical loads compare with regional assessments? The way ahead (Parks, Forest Health and Dynamic Modeling) *ERS Program, Trent University, Peterborough, ON. K9J 7B8

1. Are Soils Acidifying? Base Cation (and S and N) mass balances conducted for 21 sites in Canada, eastern United States and Europe.

Site Mass Balances: Contributing Authors: Shaun A. Watmough, Julian Aherne, Christine Alewell, Paul Arp, Scott Bailey, Tom Clair, Peter Dillon, Louis Duchesne, Catherine Eimers, Ivan Fernandez, Neil Foster, Thorjorn Larssen, Eric Miller, Myron Mitchell, Stephen Page. Funded by NERC

Mass Balances Mass Balance = Input – Output Inputs = deposition + (mineral weathering) Outputs = stream export

Weathering Soil calcium pool Deposition Soil leaching losses Net forest uptake

The Sites:

Sulphate Deposition Annual sulphate deposition (1990s) was between 7.3 and 28.4 kg/ha (excluding WF). No notable differences between Canada, US and Europe.

Nitrogen Deposition Annual N deposition (1990s) was between 2.8 kg/ha and 13.7 kg/ha (excluding WF). Nitrogen deposition was generally higher at the European sites where a greater proportion of the annual N deposition was as NH 4.

Calcium Deposition Annual Ca deposition (1990s) was between 0.8 kg/ha and 4.2 kg/ha. Calcium deposition was highest at the eastern Canadian and central European sites.

Sulphate export exceeds input at 18/21 sites:

The majority of N is retained: Between 31 and 100% of inorganic N input in deposition was retained across the study sites; median retention of 94%

Estimates of base cation weathering: Site nameCaMgKNa Rawson NW (ELA) a Rawson NE (ELA) a Rawson E (ELA) a Turkey Lakes a Plastic (MUS-HAL) a Chub (MUS-HAL) a Harp (MUS-HAL) a Lac Laflamme a Lac Clair a Lac Tirasse a Bear Brook a Moose Pit a Cone Pond Arbutus a Hubbard Brook Whiteface Langtjern Birkenes a Estimated using the PROFILE model (version 4.0; Warfvinge and Sverdrup 1992).

Mass balances including weathering: Calcium Magnesium Annual Ca and Mg losses represent appreciable proportions of the current exchangeable soil Ca and Mg pools, although losses at some of the sites likely occur from weathering reactions beneath the rooting zone. There is also considerable uncertainty associated with mineral weathering estimates. Ca and Mg export exceeded inputs at 14/18 and 10/18 sites respectively:

Mass balances including weathering: Sodium Potassium Inclusion of mineral weathering balances K and Na budgets (i.e. deposition + weathering ≥ export) at 16/18 and 10/18 sites, respectively:

Conclusions Mass Balance Studies indicated that base cation losses are occurring at the majority of sites. Sulphate export generally exceeds sulphate deposition (is it all dry deposition?) Losses are occurring despite the fact that % of the N is currently retained (not the assumption of the SMB model)

How do site specific critical load estimates compare?

< – – –1000 > 1000 Critical loads (eq ha –1 yr –1 )

What are the critical loads for Parks? An example with the Kawartha Highlands Park in Ontario (The First Steps).

< – – –1000 > 1000 Critical loads (eq ha –1 yr –1 ) Forest health plot Survey lake Pencil lake (3337) Bottle lake (833) Cavendish lake (McGinnis) (1265) Litte horse shoe lake (5675) Loon call lake (1162)

> 400 (EXC) 100–400 (EXC) 0–100 (EXC) -100–0 (no EXC) < -100 (no EXC) Exceedance (eq ha –1 yr –1 ) Forest health plot

The Next Steps More Detailed Site Specific Data (MOE Plots, MNR Plots, CWS, Surface Water Chemistry). Ground-Truth Maps. Target Sampling. Link to Forest (Ecosystem) Health. Dynamic Approach (combining surface water/soil data). Include Disturbance in Dynamic Scenarios.

Median lake ANC (µmol c L –1 ) Sulphate deposition scale factor Median soil base saturation (%) Deposition Soil base saturation continues to decline, despite large reductions Lake ANC (acid neutralizing capacity) increases in response to reduced deposition, but will start to slowly decline again as soil base saturation decreases

Forest health monitoring plots exceedance > 100 exceedance 0–100 no exceedance 0–100 no exceedance > 100 Exeedance (eq ha –1 yr –1 )

sites were mis-classified (yellow dots on the previous slide)– soil data indicated high weathering potential Regression excluding 5 sites Relationship between exceedance of the critical load and forest health

Many factors may be responsible for poor crown condition

Forests are not at steady state and disturbances such as harvesting or fire must be considered.

The Ca dynamics of Fire Effect of forest fire on stream Ca and SO 4 concentration at Rawson lake (NW basin; S. Page, unpublished data)

Conclusions General agreement with regional maps and site specific data. Maps may be used to highlight potentially sensitive areas (target sampling). Mass balance estimates at 18 sites indicate that soils are acidifying at the majority of sites, despite the fact that N is currently retained. Dynamic models have much potential, but require more data and the issue of uncertainty with respect to N and disturbance remains. Link to forest health?