This document discusses co-limitation of nitrogen and phosphorus in northern hardwood forests. It summarizes previous research showing N and P co-limitation based on fertilization studies. Co-limitation is defined as a larger productivity response to adding N and P together versus individually. The project aims to determine if northern hardwood forests are co-limited by N and P, and if soil nutrient interactions contribute to any co-limitation. Studies are being conducted at multiple sites testing the effects of N, P, and N+P additions on tree growth and soil nutrients. Preliminary results suggest forests are mostly P-limited, but N and P interactions could induce sequential limitations over time through microbial uptake effects.

1. What is co-limitation, really?
Team Shoestring:
Joel Blum
Tim Fahey
Melany Fisk
Christy Goodale
Shinjini Goswami
Mariann Johnston
Ed Rastetter
Craig See
Matt Vadeboncoeur
Adam Wild
Ruth Yanai
And many, many more!

2. Nutrient limitation of productivity
Elser et al. (2007) Ecology Letters
• Syntheses of fertilization studies predict N and P co-limitation

3. Nutrient limitation of productivity
• Syntheses of fertilization studies predict N and P co-limitation
• Defined by the way we detect it: larger productivity response to
N+P together than to either alone
• What underlies that treatment response?
– Additive
– Synergistic
NP NPCon
NP NPCon

4. General Project Questions
1. Is productivity in the northern hardwood forest co-limited
by N and P?
2. Do soil nutrient interactions contribute to co-limitation?

5. Mid-age forest Mature forest
Young: 15-20 yrs
Mid-age: 35-40 yrs
Mature: 100+ yrs
Study Sites

6. 3 sites:
•Jeffers Brook
•HBEF
•BEF
2 stands per site:
1 mid-age
1 mature
9 stands at BEF:
3 young
3 mid-age
3 mature
Study Sites

7. Study Sites
4 (sometimes 5) plots per stand

8. Treatments
(And sometimes
Ca)
N (30 kg/ha/yr)
P (10 kg/ha/yr)
N+P
Control
• DBH (trees > 10 cm) and soil processes quantified pre- and post-treatment

9. General Project Questions
1. Is productivity in the northern hardwood forest co-limited
by N and P?
- no (varies, but mostly P-limited)
2. Do soil nutrient interactions contribute to co-limitation?

10. Interactions between soil N and P availability
Soil
solution
Organic nutrients
Microorganisms
Plants
Leaching
Resin strips measure what’s in soil solution

11. Interactions between soil N and P availability

12. • Threshold P,
above which N is
very low
• Plant and
microbial
evidence suggest
that microbial
uptake of N
suppresses
availability to
plants
Interactions between soil N and P availability

13. Interactions between soil N and P availability
Soil
solution
Organic nutrients
Microorganisms
Plants
Leaching
P
P
N
P
N
• Microbial uptake of N suppresses availability to plants
• Expect plant productivity in NP plots to begin to exceed that
in P plots
• Co-limitation?

14. Conclusions
1. Is productivity in the northern hardwood forest co-limited
by N and P?
- no
2. Do soil nutrient interactions contribute to co-limitation?
- if “co-limitation” is detected, it will be sequential
- arising from nutrient interactions, rather than
simultaneous limitations by multiple nutrients
3. New hypothesis: alleviating microbial P limitation induces
N limitation over time
1. predict that plant and microbial responses to N+P will
soon exceed those to P