This document summarizes research on the effects of nutrient addition on soil respiration in northern hardwood forests. It finds that: 1) Soil respiration and belowground carbon allocation declined with increasing soil nutrient availability across study sites. 2) The addition of nitrogen and phosphorus nutrients reduced soil respiration the most in the most infertile sites, likely by reducing belowground carbon allocation and root-associated respiration. 3) There were some indications that sites may have been co-limited by both nitrogen and phosphorus, as nutrient additions of both together had stronger effects on respiration in infertile sites than additions of either nutrient alone.

1. Soil Respiration Responds to Nutrient Addition
in Northern Hardwood Forests
Tim Fahey, Cornell University

3. 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

4. Seasonal Pattern of Soil Respiration in Hubbard Brook Sites

5. 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

6. -3 0 3 6 9
400
500
600
700
800
Oe
0 2 4 6 8
BelowgroundCallocation(gCm
-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

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

8. 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

10. N + P Plots

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

12. 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

13. Acknowledgements
Kikang, Hongzhang, Melany, Ruth and a cast of thousands

14. What limits microbial respiration?
Oie
CO2
incorportation
into Oa
Litter and root inputs

15. 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

16. Approach:
• Lab incubations
• Treatments:
Control
C (litter)
nutrient (N or P)
C + nutrient

17. • N suppressed
respiration

18. • N suppressed
respiration
• With added C, P
increased
respiration

19. 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

20. cellulases
Hypothesized C, N, P interactions: Low N
Microbial biomass synthesis
CO2
Respiration

21. cellulases
Hypothesized C, N, P interactions: High N
Microbial biomass synthesis
Respiration
CO2

22. 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