ELEVATED ATMOSPHERIC NITRATE DEPOSITION IN NORTHERN

HARDWOOD FORESTS: IMPACTS ON MICROBIAL MECHANISMS OF PLANT

LITTER DECOMPOSITION

Jared L. DeForest

Earth, Ecological, & Environmental Sciences

University of Toledo

Global rates of atmospheric nitrogen deposition

Galloway & Cowling, (2002)

50.020.010.07.55.02.51.00.50.30.1

kg N ha-1

Total Nitrogen Deposition(2002)

Values in 1012 g; From Schlesinger (1997)

Global Nitrogen Cycle

150

Human activities have doubled the amount of available nitrogen

Values in 1012 g; From Schlesinger (1997)

Global Nitrogen Cycle

150

Human activities have doubled the amount of available nitrogen

The deposition of nitrogen can be in two forms:

Nitrate (NO3-) or Ammonium (NH4

+)

Nitrate represents the majority of total nitrogen deposition in the Midwest

Nitrate is rapidly assimilated by the microbial community and through the process of cell

death, that nitrogen is released as ammonium

Ammonium can represent 75% of extractable total inorganic nitrogen in soil

HumanNitrate

Deposition

GPP

Respiration

Decomposition

Soils1500 Gt C

Land Plants560 Gt C

The doubling of available nitrogen can be a potent modifier of the carbon cycle

Atmosphere

750 Gt C

Adapted from Schlesinger (1997)

120 Gt C yr-1

60 Gt C yr-1

60 Gt C yr-1

Increases in nitrogen deposition can inhibit decomposition because high levels of soil

nitrogen can suppress the activity of enzymes that degrade plant litter

Lignin degrading enzymes are the most likely to be suppressed by increases in soil

ammonium availability

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6

0

25

50

75

100

Ligninolytic Activity

Total Extracellular

Nitrogen

Ligninolytic activity is often inhibited byammonium (NH4

+)

Culture Age (days)Adapted from Keyser et al., 1978

Lignin

oly

tic Activ

ity

Am

moniu

m (

mM

)

ExtracellularAmmonium

LigninolyticActivity

Basidiomycetes are the primarydecomposers of lignin

White-rot fungi are considered the primary decomposers of lignin because they produce an array

of enzymes that can fully degrade lignin.

Degrading lignin is a specialized function giving lignin-degrading microorganism access to lignified

carbohydrates.

A relatively small population of soil bacteria, actinomycete, and fungi have the ability to

depolymerize lignin by non-enzymatic and enzymatic means.

Evidence of White-RotDecomposition

White-rot fungi are a physiological, rather

than a taxonomic, grouping of fungi.

At least 21 genera are considered

white-rot fungi.

The decomposition of lignin is important because:

Lignin is the second most abundant

organic molecule

Lignin protectsplant tissue from decomposition

Lignin

Time

Rem

ain

ing M

ass

Lignin

Lignified Cellulose

Non Lignified Cellulose

Labile Compounds

The Decomposition of Plant Litter

Adapted from Berg (1986)

Time

Rem

ain

ing M

ass

Lignin

Non Lignified Cellulose

Labile Compounds

Phase regulated by lignindecomposition rate

Phase regulatedby nutrient

level and readilyavailable carbon

Adapted from Berg (1986)

Lignified Cellulose

Time

Mass

Loss

AmbientNitrogen

ElevatedNitrogen

Phase regulatedby nutrient

level and readilyavailable carbon

Phase regulatedby lignin

decomposition rate

Adapted from Fog (1988)

LessLignolyticEnzymeActivity

Human NitrateDeposition

More Available

NH4+

Less LigninDecay

MicrobialNitrate Assimilation

And Turnover

LessLitter

Decomposition

ReducedCarbonFlow

Hypothesis

Chronic nitrate additions can suppress the lignin-degrading

activity of soil microbial communities

PredictionsPredictions

Nitrate amended soils will have:

A microbial community composition with less fungi

Lower activity of enzymes that degrade lignin and cellulose

StudySites7

9

12(kg N ha-1 y-1)

12

PLOTS

AmbientNitrogen

Deposition

Ambient NDeposition Plus

30 kg N-NO3-

ha-1 y-1A

B

D

C

Cell membranes can be used to determine Cell membranes can be used to determine microbial community compositionmicrobial community composition

Microbialcell

Cell membraneLipid bilayer

PhospholipidPhospholipid

Phospholipid Fatty AcidsPhospholipid Fatty Acids

Common to manysoil microorganisms

Unique tofungi

The length of fatty acid tails and position of double bonds on the tails can be unique to broad

taxonomic groups

FattyAcidsTails

Cellulose

Lignin

Plant Litter Compound

-glucosidase

Peroxidase

Extracellular Enzymes

Cellobiohydrolase

Phenol oxidase

Cellulose

Lignin

Enzyme AnalysisEnzyme Analysis

% m

ol f

ract

ion

Nitrate additions had no noticeable effect Nitrate additions had no noticeable effect on microbial community compositionon microbial community composition

Nitrate additions decreased microbial Nitrate additions decreased microbial biomassbiomass

0

2

4

6

8

10

p = 0.012

Control N Amended

Tot

al P

LF

A (

nm

ol P

LF

A

g-1 C

)

Nitrate addition suppressed activity of soil Nitrate addition suppressed activity of soil lignin & cellulose degrading enzymes lignin & cellulose degrading enzymes

-40% -30% -20% -10% 0% 10%

Change in Enzyme Activity

Cellobiohydrolase

-glucosidase

Peroxidase

Phenol Oxidase

*

*

* p < 0.05

Nitrate addition suppressed activity of Nitrate addition suppressed activity of lignin degrading enzymes in litterlignin degrading enzymes in litter

-40% -30% -20% -10% 0% 10%

Change in Enzyme Activity

Cellobiohydrolase

-glucosidase

Peroxidase

Phenol Oxidase*

* p < 0.05

NitrateAdditions

MicrobialCommunityComposition

Total PLFA(Microbial Biomass)

NoApparentChange

Decrease

LignolyticActivity

Decrease

Decreases in -glucosidase activity can help explain lower microbial

biomass in nitrate amended soils.

Reductions in -glucosidase activity can diminish the physiological

capacity of the microbial community to metabolize cellulose.

This reduction could reduce the energy enzymatically derived from

cellulose degradation.

Conclusions

Anthropogenic nitrate Anthropogenic nitrate deposition may diminish the deposition may diminish the physiological capacity of soil physiological capacity of soil

microbial communities to microbial communities to degrade plant litter.degrade plant litter.

Does a suppression of lignin & Does a suppression of lignin & cellulose degrading enzymes cellulose degrading enzymes

indicate a reduction in the flow indicate a reduction in the flow of carbon from these of carbon from these

compounds?compounds?

HypothesisHypothesis

Nitrate additions will inhibitthe ability of soil microorganismsto metabolize and assimilate theproducts of lignin and cellulose

degradation

13C Vanillin

MicrobialAssimilation

LigninCHO

OCH3

OHH

13C Cellobiose MicrobialAssimilation

Cellulose

13C Sequential Extractions:

Soil was incubated for 48 hours and 13C was traced into respiration, dissolved organic carbon (DOC), microbial carbon, and soil carbon.

13C PLFA Analysis:

Traced the flow of labeled 13C vanillin and cellobiose into cell membranes.

1313C PLFA AnalysisC PLFA AnalysisMicrobial

MembraneExtraction &Separation

Analysis

CHO

OCH3

OHH

13C13C

13C

13C

13C13C

13C 13C

13C13C

N additions increased the incorporation N additions increased the incorporation of vanillin into PLFAsof vanillin into PLFAs

**

*

Cellobiose

Vanillin

N additions did not alter the flow of N additions did not alter the flow of 1313C C vanillin into carbon poolsvanillin into carbon pools

0

20

40

60

80

Microbial

Respiration

DOC Microbial

Biomass

Soil Organic

Matter

% R

ecov

ery

13C

Control

N Amended

N additions did not alter the flow of N additions did not alter the flow of 1313C C cellobiose into carbon poolscellobiose into carbon pools

0

20

40

60

80

Microbial

Respiration

DOC Microbial

Biomass

Soil Organic

Matter

% R

ecov

ery

13C

Control

N Amended

0

10

20

30

40

50

60

p < 0.001

Control N Amended

N additions increased soil organic N additions increased soil organic carboncarbon

Soil O

rgan

ic C

arb

on

(g

C g

-1)

Chronicnitrate additions

UNCHANGEDVanillin or

Cellobiose intoCarbon Pools

INCREASED Soil Organic

Carbon

Excess nitrogen likely inhibits lignocellulose degradation more than vanillin or cellobiose

degradation

ConclusionsConclusions

Nitrate additions have apparently stemmed the

flow of carbon through the soil food web evident by increasing soil organic

matter formation through a reduction in lignolytic

activity.

ImplicationsImplications

Northern Hardwood Forests

AtmosphericCO2

Pools

SlowerDecomposition

HumanNitrogen

Deposition

Global ImplicationGlobal Implication

The same mechanism that decreases lignin

decomposition could be used to understand the

impact nitrogen deposition may have on broad global patterns of decomposition

Plant litter decay

LitterBiochemistry

Environmental Conditions

Global Controls of Decomposition

Actual Evapotranspirat

ion(AET)

EnvironmentalConditions

High > 1000 mm Low < 300 mm

Faster Decomposition Slower Decomposition

350-550 mm

EnvironmentalConditions

Wet Tropical Boreal

Years Required to Decompose95% of Leaf Litter

~0.5 years ~14 years

TemperateDeciduous

~4 years

LitterBiochemistry

Lignin and Nitrogen

Concentrations

Lower LigninHigher Nitrogen

Faster Decomposition Slower Decomposition

Higher LigninLower Nitrogen

HumanNitrogen

Deposition

Increase LitterNitrogen Concentrations

Decrease theDecomposition of Lignin

Increase DecompositionRates

Decrease DecompositionRates

Time

Mass

Loss

AmbientNitrogen

ElevatedNitrogen

Phase regulatedby nutrient

level and readilyavailable carbon

Phase regulatedby lignin

decomposition rate

Adapted from Fog (1988)

“Low”Lignin

“High”Lignin

Increased Decay

Nitrogen deposition impact on decomposition may depend on lignin concentrations

Decreased Decay

0

25

50

75

100

0 5 10 15 20 25 30

AET

1000 (mm)

500 (mm)

250 (mm)

Lignin Control of Decay is Greater at Higher AET A

nn

ual D

ecom

posit

ion

Rate

(%

)

Lignin Concentration (%)

Slope = -1.50

Adapted from Meentemeyer (1978)

Slope = -0.75

Slope = -0.40

As slope decreases, higher lignin

concentrations require more energy and

moisture to cause decay.

LessImpact

MoreImpact

Anthropogenic nitrogen deposition may Anthropogenic nitrogen deposition may have a larger impact on decomposition have a larger impact on decomposition

in wet-tropical environmentsin wet-tropical environments

SummarySummary

Nitrogen deposition has the potential to diminish the physiological capacity of lignin-degrading microorganisms to depolymerize lignin.

Reductions in lignocellulose-degrading enzymes and microbial biomass suggests a reduction in energy

available for microbial metabolism

Nitrogen deposition may have a greater impact on decomposition in wet tropical regions than arid or

cold regions