Tim Fahey discusses fine root biomass responses to nutrient additions. Adding both nitrogen and phosphorus causes a large increase in fine root biomass in young tree stands, likely because new root growth is colimited by both nutrients. While increased soil fertility may generally decrease belowground carbon allocation and fine root biomass, nitrogen-phosphorus colimitation could stimulate increased fine root biomass if only one nutrient is added. The presentation acknowledges contributions from the shoestring research team including Alexis Heinz, Shiyi, and Geoff Wilson.

1. Nitrogen, phosphorus and fine roots
Tim Fahey
Cornell University

2. Audience Poll: How will fine root biomass respond to
nutrient additions?
• Increase
• Decrease
• No change

3. Background: Fine root biomass
Depends upon both root production and root turnover rate
(or root lifespan)
Higher root biomass could represent either or both higher
production or lower turnover rate (longer lifespan)

4. Functional considerations
• Lower soil resource supply should require more roots
• Evidence: high root:shoot ratio in dry or infertile sites

5. Simplest hypothesis
Increasing soil fertility should reduce belowground carbon
allocation and fine root biomass

6. Complication of nitrogen-phosphorus colimitation
If N and P are colimiting, then adding only one (N or P) might
create greater demand for the other, stimulating increased
fine root biomass

7. Complication of nitrogen-phosphorus colimitation
If N and P are colimiting, then adding only one (N or P) might
create greater demand for the other, stimulating increased
fine root biomass
However, Shinjini’s results…

8. Fine root biomass in young (grad student) stands

9. Fine root biomass in mid-age (struggling asst. prof.) stands

10. Fine root biomass in old (emeritus) stands

11. Fine root biomass in young (grad student) stands

12. Why does adding both N and P cause a big increase in
fine root biomass in the young stands?
Hypothesis: Growth of new fine roots might be nutrient
colimited – i.e. new roots require balanced mineral nutrition

13. Why does adding both N and P cause a big increase in
fine root biomass in the young stands?
Hypothesis: Growth of new fine roots might be nutrient
colimited – i.e. new roots require balanced mineral nutrition
Tentative plan: factorial experiment using ingrowth cores to
determine whether root production is colimited

14. Acknowledgments
The shoestring team
Especially Alexis Heinz, Shiyi, Geoff Wilson