Ecological Perspectives on Critical Loads - Linkages between Biogeochemical Cycles and Ecosystem Change

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/Biogeochemist
ry of Forested Ecosystems
3
Wet Deposition
Dry Deposition
Nitrogen Loading
Litter Inputs
Gaseous N Losses
Uptake
Organic Nitrogen
Immobilization
Dissimilatory Reduction Of NO3-
Mineralization
NH4
NH4 fixation
Clays
Nitrification
Abiotic N Retention
NO3-
Leaching to Surface Waters
DON
4
Wet Deposition
Dry Deposition
Sulfur Loading
Litter Inputs
Uptake
Gaseous S Losses
Organic Sulfur
Mineralization
Immobilization
Oxidation Dissimilatory Reduction
Adsorbed Sulfate
Adsorption
Desorption
SO42-
SO42-
Sulfur Minerals
Leaching to Surface Waters
Weathering
DOS
5
Differences between N and S Loading/Biogeochemistr
y of Forested Ecosystems
6
Unique or important attributes
Wet Deposition
Dry Deposition
Litter Inputs
Gaseous N Losses
Uptake
Organic Nitrogen
Immobilization
Dissimilatory Reduction Of NO3-
Mineralization
NH4
NH4 fixation
Clays
Nitrification
Abiotic N Retention
NO3-
Leaching to Surface Waters
DON
7
Wet Deposition
Dry Deposition
Unique or Important Attributes
Litter Inputs
Uptake
Gaseous S Losses
Organic Sulfur
Mineralization
Immobilization
Oxidation Dissimilatory Reduction
Adsorbed Sulfate
Adsorption
Desorption
SO42-
SO42-
Sulfur Minerals
Leaching to Surface Waters
Weathering
DOS
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
Aber et al. (2003) BioScience
Threshold Response
Spring (n216)
Summer (n354)
NO3- (mmol/L)
Estimated N Deposition (kg ha-1 yr-1)
Summer nitrate 2.5 N Deposition 14.4, R2
0.30, P lt 0.0001 Spring nitrate 6.7 N
Deposition 40.7, R2 0.38, P lt 0.0001
11
What causes variation in relationships between S
loadings and SO4 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
Soils with high SO42- adsorption
Rochelle et al. (1987)
13
Low pH (from nitrification) enhances SO42- adsorpt
ion (Mitchell et al., 1989)
Harvest followed by enhanced nitrification
14
SO42-
0 Days Discharge
Drier conditions result in higher SO42-
concentrations in Ontario, Canada (Eimers and
Dillon, 2002)
15
What causes the spatial variation associated with
N loadings

• Land use history including harvesting and fire.
• Forms of N input (NH4 versus NO3).
• 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
Campbell et al.,2004
DIN INPUT-OUTPUT BUDGETS AT THE MOST INTENSIVELY
MONITORED WATERSHEDS
SR
HB9
9
HB6
EBB
8
BSB
CP
HW
7
LR
kg DIN ha-1 yr-1
6
F10
5
F4
F13
4
3
2
Input
1
Output
0
Substantial variation in DIN losses even for
sites with similar DIN inputs
17
More sugar maple more nitrate
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)
18
Ca rich site has greater proportion of sugar
maple resulting in higher NO3- 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
NO3- Isotope Data (Burns Kendall 2002)
Microbial processing of N
22
(No Transcript)
23
Loch Vale nitrate isotopes, 1995
Winter snow
Rainfall
Snowmelt
d18O (NO3)
Spring snow
Streams
Groundwater
Microbial nitrate
(Campbell et al., 2002)
24
At Sleepers River, VT values of ?34S and ?18O of
SO4 in streams between bedrock and precipitation
(Shanley et al., 2005) .
Precipitation
Streamwater
25
How do S and N responses affect estimates of
critical loading?

• Importance of weathering as sources of SO4.
• Adsorption/Desorption reactions need to be
  considered for SO4, but not for NO3.
• 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.