1. Ecological Perspectives on Critical Loads - Linkages between Biogeochemical Cycles and Ecosystem Change Differences and Similarities in N and S Cycling with an Emphasis on Forested Ecosystems in the United States by Myron J. Mitchell SUNY-ESF Syracuse, NY

2. Similarities between N and S Loading/Biogeochemistry of Forested Ecosystems

3. Organic Nitrogen NH 4 + Wet Deposition Dry Deposition Clays Leaching to Surface Waters Mineralization Immobilization Uptake Litter Inputs NH 4 fixation NO 3 - Nitrification Gaseous N Losses Dissimilatory Reduction Of NO 3 - Nitrogen Loading DON Abiotic N Retention

4. Organic Sulfur Adsorbed Sulfate Wet Deposition Dry Deposition Sulfur Minerals Weathering Leaching to Surface Waters Mineralization Immobilization Uptake Litter Inputs Adsorption Desorption Oxidation Dissimilatory Reduction Gaseous S Losses Sulfur Loading DOS SO 4 2-

5. Differences between N and S Loading/Biogeochemistry of Forested Ecosystems

6. Organic Nitrogen NH 4 + Wet Deposition Dry Deposition Clays Leaching to Surface Waters Mineralization Immobilization Uptake Litter Inputs NH 4 fixation NO 3 - Nitrification Gaseous N Losses Dissimilatory Reduction Of NO 3 - Unique or important attributes DON Abiotic N Retention

7. Organic Sulfur Adsorbed Sulfate Wet Deposition Dry Deposition Sulfur Minerals Weathering Leaching to Surface Waters Mineralization Immobilization Uptake Litter Inputs Adsorption Desorption Oxidation Dissimilatory Reduction Gaseous S Losses Unique or Important Attributes DOS SO 4 2-

8. We know that in general sulfur loadings are more closely linked to sulfate losses than nitrogen loadings to nitrate loss.

9. Johnson & Mitchell, 1998

10. Spring (n=216) WV CAT ADK VT NH ME Summer (n=354) Estimated N Deposition (kg ha -1 yr -1 ) NO 3 - (  mol/L) Aber et al. (2003) BioScience Summer nitrate = 2.5 * N Deposition – 14.4, R 2 = 0.30, P < 0.0001 Spring nitrate = 6.7 * N Deposition – 40.7, R 2 = 0.38, P < 0.0001 Threshold Response

11. What causes variation in relationships between S loadings and SO 4 losses in drainage waters? Generally relationship is better with highest S loadings. Sulfate adsorption relationships have a major influence spatial patterns. Weathering contributions can be important in some watersheds. With decreasing loading internal S sources become more important (weathering and organic S mineralization).

12. Rochelle et al. (1987) Soils with high SO 4 2- adsorption

13. Harvest followed by enhanced nitrification Low pH (from nitrification) enhances SO 4 2- adsorption (Mitchell et al., 1989)

14. Drier conditions result in higher SO 4 2- concentrations in Ontario, Canada (Eimers and Dillon, 2002) 0 Days Discharge [SO 4 2- ]

15. What causes the spatial variation associated with N loadings Land use history including harvesting and fire. Forms of N input (NH 4 versus NO 3 ). Types of vegetation affecting N mineralization and nitrification rates. More closely linked with other biotically regulated processes including soil freezing (disrupts fine root uptake) and carbon dioxide availability. Also seems to be highly sensitive to climatic effects including overall temperature effects and the role of the snow pack.

16. kg DIN ha -1 yr -1 0 1 2 3 4 5 6 7 8 9 Input Output F4 F10 F13 LR BSB SR HB9 HB6 CP HW EBB DIN INPUT-OUTPUT BUDGETS AT THE MOST INTENSIVELY MONITORED WATERSHEDS Campbell et al.,2004 Substantial variation in DIN losses even for sites with similar DIN inputs

17. Concentration of nitrate in B-horizon soil solution in mixed- species stands in the Adirondack Mountains plotted against the percentage of sugar maple in the stand (Lovett and Mitchell, 2004) More sugar maple more nitrate

18. Ca rich site has greater proportion of sugar maple resulting in higher NO 3 - generation in two adjacent watersheds in Adirondacks (Christopher et al., 2006).

19. Importance of biotic cycling of N and S Stable isotopic results

20. Catskills of New York State

21. NO 3 - Isotope Data (Burns & Kendall 2002) Microbial processing of N

23. Streams  18 O (NO 3 ) Winter snow Rainfall Snowmelt Microbial nitrate Spring snow Groundwater Loch Vale nitrate isotopes, 1995 (Campbell et al., 2002)

24. 0 2 4 6 8 10 12 18 O of sulfate, ‰  Precipitation Streamwater At Sleepers River, VT: values of  34 S and  18 O of SO 4 in streams between bedrock and precipitation (Shanley et al., 2005).

25. How do S and N responses affect estimates of critical loading? Importance of weathering as sources of SO 4. Adsorption/Desorption reactions need to be considered for SO 4, but not for NO 3. Dissimilatory reduction reactions can have substantial effects on S and N. –Substantial gaseous loss of N, but not S. –Mobilization of previously reduced S with changing hydrology (wetlands).

26. How do S and N responses affect estimates of critical loading? (continued) Biotic regulation –More important for N versus S. Tree species, soil organic matter dynamics, nutrient demand, etc. need to be considered for N. –Both N and S show substantial amounts of biological cycling before being released into drainage waters and these processes become especially important at lower N and S loadings.