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.