Nitrogen, phosphorus and fine roots
Tim Fahey
Cornell University

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

Simplest hypothesis
Increasing soil fertility should cause a reduction in
belowground carbon allocation and fine root biomass and
production

Complication of nitrogen-phosphorus co-limitation
If N and P are co-limiting, then adding only one (N or P) might
create greater demand for the other, stimulating increased
fine root biomass and production

Response of fine root biomass in three young northern hardwood
stands to long-term additions of N and P

Why does adding both N and P cause a big increase in
fine root biomass in young northern hardwood stands?
Hypothesis: Growth of new fine roots might be nutrient co-
limited – i.e. growing new roots requires balanced mineral
nutrition

Why does adding both N and P cause a big increase in
fine root biomass in young northern hardwood stands?
Hypothesis: Growth of new fine roots might be nutrient co-
limited – i.e. new roots require balanced mineral nutrition
Approach: factorial experiment using ingrowth cores to
evaluate whether root production is co-limited by N and P

Microsite NP addition in control plots did not cause increased local root production

Fine root growth increased in response to N plus P addition
at the plot level in two young stands

Increased proportional allocation to roots vs. leaves:
Ratio of fine root production to fine litter production

Conclusion and Speculation
• In young northern hardwood stands, fine root biomass and growth
are co-limited by N and P, but this limitation is expressed at the stand
level not at the soil microsite level.
• This co-limitation is not observed in older stands.
• Perhaps in young stands, where fine root systems have not fully
exploited soil volume, competition among trees for co-limiting N and
P induces greater allocation to root growth

Acknowledgments
The shoestring team, especially Shiyi Li and Geoff Wilson.
The MELNHE Project is funded by USDA NIFA (2019-67019-
29464) and NSF (DEB-1637685) . For more information, please
visit www.esf.edu/melnhe

Evidence of N-P co-limitation in a young northern hardwood forest (C2)

Microsite NP addition in control plots did not cause increased local root production