N Deposition Effects on Vegetation and Soils in Alpine Ecosystems

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1 N Deposition Effects on Vegetation and Soils in Alpine Ecosystems William D. Bowman Mountain Research Station, Institute of Arctic and Alpine Research, University of Colorado, Boulder thanks to Cory Cleveland, Jill Baron, Lubos Halada, Courtney Meier, Anthony Darrouzet-Nardi

2 Two phases of N deposition: 1) eutrophication- production in many temperate terrestrial ecosystems is limited by the supply of N (often associated with increases in nitrophilic plants) 2) acidification- excess NH 4+ & NO 3- lead to leaching of base cations & mobilization of soluble Al Linked, due to the N sink of biomass reactive N plant & soil uptake leaching losses, acidification, increased Al mobility

3 Aber N Saturation Model for Forest Ecosystems (from Aber et al. 1998) how are eutrophication and acidification (leaching) linked temporally? how much of a sink is alpine vegetation?

4 N Deposition in the alpine of the S. Rockies 25 NADP-NTN trends in inorganic N concentration Niwot Ridge, CO μeq/l P < 0.01 NO 3 - NH P < Year estimated total annual deposition 6 kg N/ha

5 Estimates of N critical loads in the alpine: Amount: source: (kg ha -1 yr -1 ) 4-10 Bowman et al. (2006) basis: vegetation change 4 * Williams & Tonnessen (2000) 1.5 Baron (2006) 3-4 Baron et al. (1994) Bobbink et al. (2002) surface water chemistry hindcasting analysis/ diatoms CENTURY model (N leaching) vegetation change (Europe) *wet only

6 Experimental N deposition in an alpine dry meadow, Niwot Ridge Vegetation and soil resonses to present treatments of 20, 40, & 60 Kg N /ha/ yr modest increase in aboveground biomass and N uptake (due entirely to changes in species abundances) how much buffering of soil effects? 200 N content P< total aboveground biomass (g m -2 ) biomass N content (g m -2 ) N added (kg ha -1 yr -1 ) 0

7 Soil responses to simulated N deposition Niwot Ridge dry meadow * increasing soil acidity * trend towards loss of base cations T otal cation charge Soil ph H2O 6.0 P < r 2 = 0.75 Total charge (mmol /100 g) N input (Kg ha -1 yr -1 ) N input (Kg ha -1 yr -1 )

8 Soil responses to simulated N deposition Extractable cations * trend towards increasing Al [Ca 2+ ] mg/g Calcium (dominant ) [Al 3+ ] mg/g Aluminum P = N input (Kg ha -1 yr -1 ) similar trends with increased loss of other nutrient base cations (K, Mg) N addition (Kg ha -1 yr -1 )

9 Results indicate eutrophication effects, but insufficient buffering to prevent effects of N deposition on soil ph, loss of nutrient cations, and increased Al Ongoing research addressing N critical loads, vegetation and soil responses to simulated N deposition in Rocky Mountain and Glacier National Parks

10 (from Aber et al. 1998) where are we on the N saturation trajectory? are we taking a short cut? what happens later in the later stages?

11 Western Tatra Mountains, Slovakia Western Tatras High Tatras

12 NH 4 + NO 3 - Current estimated total N deposition 12 Kg N/ha/yr) historic rates ca. 20 Kg N/ha/yr, 25 Kg S/ha/yr from Kopácek 2001

13 Biomass responses to simulated N deposition * decreased production aboveground biomass (g m -2 ) aboveground biomass P< N inputs (kg ha-1 yr-1)

14 Soil responses to simulated N deposition, Mount Salatin * increased acidity * loss of already low base cations Soil ph N input (Kg ha -1 yr -1 ) extractable cation (mg g -1 ) Nutrient/ base cations P< P< N input (Kg ha -1 yr -1 ) K Ca Mg

15 Soil responses to simulated N deposition, Mount Salatin * loss of Al, increase in Fe * transition from base cation to Al to Fe buffering system extractable Fe 3+, Al 3+ (mg / g) Fe 3+ ; r 2 =0.39, P = Al 3+ ; r 2 =0.21, P = 0.05 (dominant) N addition (Kg Ha -1 yr -1 )

16 Summary Alpine dry meadow on Niwot Ridge shows simultaneous eutrophication and leaching (acidification) effects at higher rates of N deposition Biological buffering in alpine insufficient to prevent adverse environmental impacts with increased rates of N deposition At extreme levels of soil acidity in the Western Tatras, N deposition forces system towards Fe dominated buffering of soil Alpine systems with acidic parent material are at high risk to detrimental impacts of N deposition