Effects of N Deposition on Soil Respiration in the

Harvard Forest

By: Christian Arabia

Mentors:

Kathleen Savage

Eric Davidson

Werner Borken

The Chronic N Study SiteLocated on Prospect Hill tract at Harvard Forest

Designed and initiated by John Aber of University of New Hampshire

2 sites: red pine stand, and mixed hardwood stand.

3 plots within each site: high N addition site

(150 kg N Ha-2 yr-1)

low N addition site (50 kg N Ha-2 yr-1)

control plot (no N)

Canopy and ground level vegetation

• After 13 years of ammonium nitrate additions, the treatment plots show marked differences in canopy and ground level vegetation growth and structure.

• Generally, as the amount of N fertilization increases, canopy coverage and ground level vegetation decrease

Canopy Structure (Pine)

Clockwise from left:

High treatment

Low treatment

Control

Canopy Structure (Hardwood)

Clockwise from left:

High treatment

Low treatment

Control

Ground Level Vegetation (Pine)

Clockwise from left:

High Treatment

Low Treatment

Control

Ground Level Vegetation (Hardwood)

Clockwise from left:

High Treatment

Low Treatment

Control

• Ground level soil respiration was measured at 6 randomly placed collars within each site (36 total)

• A Licor 6252™ backpack system was used to record CO2

concentrations every 12 sec.

• A linear regression was run on concentrations to calculate fluxes of CO2

Soil Respiration Sampling

Chart 1. Average CO2 fluxes by plot at Chronic N study site (June 6 – August 6, 2001)

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H150 H50 H Control P150 P50 P Control

Site

Flu

x (

mg

C m

-2 h

r-1)

Trends in Soil Respiration

Explaining Differences

Why/How does N fertilization decrease CO2 fluxes?

2 Hypothesis:

Decreased root respiration

Decreased microbial activity

Soil Incubation Experiment

• 6 soil samples were taken from each of the plots

• 10cm of soil from “A” layer down

• Approx. 30g of soil for each replication

• Roots were removed and soil was sieved (1mm)

• Field moisture was measured gravimetrically

Why: To measure CO2 respiration that can be attributed solely to microbial decomposition

Incubating the soils• 20g of soil (at field moisture) from each sample was placed into a 1pt. Mason jar.

• The jars were opened before the incubation so that they would equilibrate with ambient (outdoor) air.

• The jars were sealed one at a time and 5ml of gas was collected at times 0, 60, 120 and 180 min.

• Samples were injected immediately into a Licor 6252™ I.R.G.A.

• A linear regression was run on the concentrations in Mason Jars over time and was used to calculate the flux rate of CO2

Taking gas samples

Running the samplesPump Soda Lime Scrubber

IRGAMixing Chamber

Valve

Sample

Trial 1 CO2 Fluxes

Chart 2. Average CO2 fluxes from incubation.

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H150 H50 HC P150 P50 PC

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Flu

x (

mg

C k

g s

oil-1

hr-1

)

Comparing Trends

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H150 H50 H Control P150 P50 P Control

Site

Flu

x (

mg

C m

-2 h

r-1)

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H150 H50 HC P150 P50 PC

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r-1)

Summary• Trends in average respiration fluxes from soil incubations are similar to those seen in field respiration sampling

• N saturation shown for both pine and hardwood

• Data suggests microbial decomposition is primary factor affected by N saturation.

Acknowledgements

I would like to thank my mentors, Kathleen Savage, Eric Davidson and Werner Borken

for their help and guidance with this project, as well as my partners, Rosa Navarro and Linda Wan. I would also like to show my appreciation to Allison Magill and John Aber for their willingness to allow me to sample from their experimental plots.