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 Statesby
Myron J. Mitchell
SUNY-ESF
Syracuse, NY
Similarities between N and S Loading/Biogeochemistry
of Forested Ecosystems
OrganicNitrogen
NH4+
WetDeposition
DryDeposition
Clays
Leaching to Surface Waters
MineralizationImmobilizationUptake
LitterInputs
NH4 fixation
NO3-
Nitrification
Gaseous
N Losses
Dissimilatory Reduction
Of NO3-
Nitrogen Loading
DON
Abiotic N Retention
OrganicSulfur
Adsorbed Sulfate
WetDeposition
DryDeposition
Sulfur MineralsWeathering
Leaching to Surface Waters
Mineralization ImmobilizationUptake
LitterInputs
AdsorptionDesorption
Oxidation Dissimilatory
Reduction
Gaseous
S Losses
Sulfur Loading
DOS
SO 42-
SO42-
Differences between N and S Loading/Biogeochemistry
of Forested Ecosystems
OrganicNitrogen
NH4+
WetDeposition
DryDeposition
Clays
Leaching to Surface Waters
MineralizationImmobilization
Uptake
LitterInputs
NH4 fixation
NO3-
Nitrification
Gaseous
N Losses
Dissimilatory Reduction
Of NO3-
Unique or important attributes
DON
Abiotic N Retention
OrganicSulfur
Adsorbed Sulfate
WetDeposition
DryDeposition
Sulfur MineralsWeathering
Leaching to Surface Waters
Mineralization ImmobilizationUptake
LitterInputs
AdsorptionDesorption
Oxidation Dissimilatory
Reduction
Gaseous
S Losses
Unique or ImportantAttributes
DOS
SO 42-
SO42-
We know that in general sulfur loadings are more closely
linked to sulfate losses than nitrogen loadings to nitrate
loss.
Johnson &Mitchell, 1998
Spring (n=216)
4 6 8 10 12
WVCATADKVTNHME
Summer (n=354)
0
10
20
30
40
50
60
4 6 8 10 12
Estimated N Deposition (kg ha-1 yr-1)
NO
3- (
mo
l/L
)Aber et al. (2003) BioScience
Summer nitrate = 2.5 * N Deposition – 14.4, R2 = 0.30, P < 0.0001Spring nitrate = 6.7 * N Deposition – 40.7, R2 = 0.38, P < 0.0001
Threshold Response
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).
Rochelle et al. (1987)
Soils with high SO4
2- adsorption
Harvest followed by enhanced nitrification
Low pH(fromnitrification)enhancesSO4
2-
adsorption(Mitchell et al., 1989)
Drier conditions result in higher SO42- concentrations
in Ontario, Canada (Eimers and Dillon, 2002)
0 Days Discharge
[SO42-]
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.
kg D
IN h
a-1 y
r-1
0
1
2
3
4
5
6
7
8
9
Input Output
F4
F10
F13
LR
BSB
SRHB9
HB6CPHW
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
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
Stream
Low Ca2+
Low NO3-
NO3-
Ca Rich Site (S14) Ca Poorer Site (S15)
Stream
High Ca2+
High NO3-
LitterHigh Ca
Litter Low C:N
Ca2+
Ca-rich parent material Ca-poorer parent material
Litter High C:N
LitterLow Ca
NO3-
Ca2+
UptakeUptake
Ca rich site has greater proportion of sugar maple resulting in higher NO3
- generation in two adjacent watersheds in Adirondacks (Christopher et al., 2006).
Importance of biotic cycling of N and S
Stable isotopic results
Catskills of New York State
NO3- Isotope Data
(Burns & Kendall 2002)
0
20
40
60
80
-8 -6 -4 -2 0 2 4 615N - NO3
- (o/oo)
18 O
- N
O3- (
o / oo)
Stream
O-Horizon
B-Horizon
C-Horizon
Precipitation
Microbial processing of N
Streams
18O
(N
O3)
Winter snow
RainfallSnowmelt
-20
20
60
100
15N (NO3)-4 -2 0 2 4 6
Microbial nitrate
Spring snow
Groundwater
Loch Vale nitrate isotopes, 1995
(Campbell et al., 2002)
0.0
0.4
0.8
1.2
0
50
100
150
Dis
char
ge, m
m h
r-1
2000
20 30 9 19 29March April
Sulfate, µeq L
-1
5
6
7
834
S, ‰
Precipitation
Bedrock
Streamwater
0
2
4
6
8
1012
18O
of s
ulfa
te, ‰
Precipitation
Streamwater
At Sleepers River,VT: values of 34S and 18O of SO4 in streams between bedrock and precipitation (Shanley et al., 2005) .
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).
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.