Landscape-scale patterns in N biogeochemistry of high-elevation watersheds Chuck Rhoades1, Eugene Kelly2, Mary Stromberger2 & Robert L. Sanford Jr3 1U.S. Forest Service, Rocky Mountain Research Station, Ft. Collins, Collins, Colorado, [email protected] of Crop and Soil Sciences, Colorado State University; 3Department of Biology, University of Denver 2Department Alpine Biogeochemistry Influences Watershed N Export Redistribution 150 Prevailing Wind b 200 Snowpack Depth (cm) 30 0 Leeward East Nitrate Windward Ammonium 0.25 Initial Pools (mg N /L) 0 .5 0 0.20 e t ne pin eli res Al Tre Fo 0.05 Nov Jan Mar May July Nov Jan Mar May July 25 Treeline Net N Turnover (mg / L / 28 d) Net Nitrification 0.5 S ne pi Al lo pe T n eli re eA e in lp T n eli re eF es or 40 50 60 70 The strong relationship between streamwater inorganic N & alpine cover in Fraser watersheds (d) demonstrates the importance of alpine biogeochemistry on watershed N export. 15 It is unclear to what extent landscapelandscapescale variability in snow accumulation & soil development contribute to this pattern. Windward / Erosional 10 5.0 5 4.5 4.0 5 ge st ne orest pe lpi ore Rid Slo eF eA eF ine lin eelin ine pin e l p Alp l e a r A b T Tr Su Subalpine When incubated at constant moisture & temperature alpine topography had little effect on nitrification. Substrate supports comparable nitrifier activity on both alpine aspects. Similar vegetation occurs on both aspects, so it appears that differences originate from factors controlling nitrate production or leaching. e 15 Alpine As expected, resinresinavailable nitrate is higher in the alpine. Environmental factors may be more critical than organic matter quality in regulating alpine microbial processes. Leeward / Depositional t b Su i alp ne re Fo st Treeline differences in N dynamics relate to the : - Short alpine growing season & low plant N demand - WeaklyWeakly-developed alpine soils & limited residence time of snowmelt within groundwater flow paths - Tundra v.s. v.s. forest differences in litter & soil organic matter quality B Ochric Alpine C Alpine Alpine Tundra Alpine Tundra Subalpine Forest This project samples transects on 3 ridges in central Colorado where treeline occurs from 3340 - 3630 m. Subalpine Physical processes, such as cryoturbation, consume H+ & may buffer against increased acidification. In contrast, on leeward & subalpine slopes, greater organic acids from litter combined with increased snowpack & soil moisture to favor chemical weathering & lower pH. Organic A E Albic Leeward / Depositional Subalpine Forest Alpine Soil acidity & nitrate display similar patterns, possibly indicating a shift in relative importance of physical vs. chemical weathering across the alpine ridge. Soil Horizons C% N% Windward Windward Leeward Leeward 6.0 5.5 10 Windward / Erosional 1.0 e 30 20 Subalpine Subalpine Soil nitrate & nitrification decline abruptly below treeline Net Mineralization dg Ri C:N 20 Treeline 2.0 20 P e r c e n t A lp in e Leeward Within the alpine landscape, windblown areas differ from depositional areas Treeline 0.10 10 30 0.15 ne pi Al Subalpine y = 0.06 + 0.006x + 0.0002x2 0 -10 0 0.0 Adj. = 0.77 p < 0.001 0 .0 0 Windward Windward Leeward Leeward 0 More interestingly, windward & leeward alpine soils differ by about 100%. r2 0 .2 5 -10 st e ine re lin Fo ee Alp Tr Treeline Patterns 1.5 S t r e a m w a te r N itr a te (m g /L ) 0 .7 5 0 Total Mineral Soil C and N East Wind Direction 400 0 North Windward West 1 .5 0 1 .0 0 10 South d 1 .7 5 1 .2 5 Alpine Tundra Windswept Tundra Subalpine Forest Alpine Tundra Windswept Tundra Depositional Slope Depositional Subalpine Slope Forest 10 West c Soil Temperature Soil Temperature (oC) 20 50 600 Alpine 20 100 6.5 0 30 Soil pH Net Nitrification 25 Windward Leeward 200 Our preliminary work on alpine ecosystems at the Fraser Experimental Forest in central Colorado indicates that nitrogen nitrogen dynamics & the soil chemical environment differ between windswept & leeward alpine slopes & across the alpinealpine-subalpine ecotone. a 7.0 30 Resin Nitrate 800 μg / bag In contrast, increased snow accumulation on leeward slopes (b) dampens subnivean temperature fluctuations (c) and maintains adequate conditions to sustain wintertime soil microbial activity. activity. Soil Processes 1000 (μg /g/28d) Alpine Landscape Patterns Removal of snow from windward slopes exposes these landscapes to colder wintertime soil temperatures & higher frequency of freeze freeze-thaw events (a). Hypothetical model (e) of highhigh-elevation soil development on windward & leeward aspects. Cryoturbation (dashed arrows) occurs more frequently on windswept slopes resulting in poorlypoorly-defined horizons. On leeward sites with persistent snowpack, snowpack, combined chemical & physical weathering result in more distinct soil horizons. Pedogenic Links This onon-going study will characterize the distinct biogeochemical & pedological conditions within alpine landscapes & across the treeline ecotone in order to explain patterns of watershed N export & improve predictions regarding the impact of climate change on watershed processes.