Soil Respiration Responds to Nutrient Additionin Northern Hardwood Forests

Tim Fahey, Cornell University

Components of Soil Respiration

• Heterotrophic respiration by microbial decomposers

• Root-associated respiration (supplied by belowground C allocation)

- respiration of fine roots

- respiration of mycorrhizal fungi

- respiration of other rhizosphere microbes

Seasonal Pattern of Soil Respiration in Hubbard Brook Sites

Pre-treatment Pattern of Soil Respiration

Both soil respiration and estimated belowground carbon allocation declined significantly with increasing soil nutrient availability across the MELNHE sites

-3 0 3 6 9

400

500

600

700

800

Oe

0 2 4 6 8

Bel

ow

gro

und

C a

llocat

ion (

gC

m-2

yr-1

)

400

500

600

700

800

Oa

net nitrification (ug g-1

)

0.2 0.3 0.4 0.5 0.6 0.7 0.8

400

500

600

700

800

0-10 cm

2000 3000 4000 5000 6000 7000

Oe

400 800 1200 1600 2000

Oa

exchangable Ca (ug g-1

)

0 100 200 300 400 500

0-10 cm

R2=0.96

R2=0.80

R2=0.90

R2=0.73

R2=0.94

Pre-treatment observations for a sub-set of the sites

Hypotheses

1. Addition of a tree growth-limiting nutrient will reduce belowground carbon allocation resulting in lower root-associated respiration

1a. Colimitation would be indicated by strong response of soil respiration to addition of N + P

2. Reduction of soil respiration will be greatest in most infertilesites

3. Nitrogen addition might suppress activity of microbial decomposers thereby complicating interpretation of respiration response

Response ratio of soil respiration to nutrient additions

We express the treatment effect on soil respiration as the ratio:

% response ratio= ((fertilized – control)/control) * 100

Thus a negative response ratio indicates a reduction of soil respiration in the treated plots

N + P Plots

Note: no clear evidence of a decline of heterotrophic respiration in response to nutrient addition (next talk)

Conclusions

• Response of soil respiration to nutrient addition varies linearly with pre-treatment site fertility

• Belowground carbon allocation in infertile sites decreases significantly in response to nutrient additions (resulting in tree aboveground growth increase?)

• Some indication of possible co-limitation by N and P on infertile sites

Acknowledgements

Kikang, Hongzhang, Melany, Ruth and a cast of thousands

What limits microbial respiration?

Oie

CO2

incorportation

into Oa

Litter and root inputs

Questions:

• Do N or P limit microbial respiration in forest floor?

• Is this limitation secondary to that of C?

• Does forest age or site affect respiratory responses?

3 sites:

• Jeffers Brook

• HBEF

• BEF

Approach:

• Lab incubations

• Treatments:

Control

C (litter)

nutrient (N or P)

C + nutrient

• N suppressed

respiration

• N suppressed

respiration

• With added C, P

increased

respiration

N effect

microbial biomass

accumulation: 256

(101)

DON accumulation:

440 (969)

Litter effect

inorganic N reduced

by 82 (23) ug N/g

Where the added N (mg/g soil) went:

Ni:

137 (27)DON:

115 (14)

MBN:

797 (95)

Ni:

56 (7)DON:

114 (25)

MBN:

877 (77)

Ni:

840 (87)DON:

554 (99)

MBN:

1082 (93)

control

+ litter

+N

cellulases

Hypothesized C, N, P interactions: Low N

Microbial biomass synthesis

CO2

Respiration

cellulases

Hypothesized C, N, P interactions: High N

Microbial biomass synthesis

Respiration

CO2

Why would P limitation be

induced by added C?

Hypothesized C, N, P interactions:

N may also limit enzyme production, C availability.

We predict that adding N and P together should increase microbial

respiration