Phosphorus-nitrogen feedbacks maintain balanced availability in hardwood forest

soils Tera Ratliff

11-July-2012

Time (millions of years)

Tota

l Soi

l Nut

rient

s Walker and Syers, 1976

US Forest Service

Identifying the processes underlying limitations to productivity help us understand forest ecosystem response to change

Resource Optimization

High C:N:P

High N:PP-limiting

Investment in phosphatse

N:P

Low N:PN-limiting

Investment in decomposition

N:P

Low C:N:PC-limiting

Investment in C-mineralizing

enzymes

C:N:P

Resource optimization in response to N and P fertilization

• When N is high, organisms allocate to P

• Do N and P equilibrate over the long-term?

Marklein and Houlton, 2011

Response Ratio = Mean outcome of treatment: mean outcome of control

• Pretreatment (2008 to 2009) soil collection MELNHE plots

• Cores divided into Oe and Oa

• Resin strips buried in-situ (2010)

C9 -1-A

PO4-

• N and P availability covary in forests of varying ages in the northern hardwoods

• N availability appears to promote P availability via phosphatase production.

Conclusions• Tight coupling of N and P could contribute to

colimitation at the ecosystem level• Resource optimization could work to prevent P-

limitation in the short-term by increasing investment when N is high

• Mechanisms of P redistribution from slowly available pools are of interest for understanding long-term effects of anthropogenic changes in N and P availability

Questions?

AcknowledgmentsMany people have helped in carrying out this project, so many thanks to Kevan Minick, Mark Dempsey, Brittany Coyne, Stephanie Bailey, Carrie Rose Levine, members of the Fisk Lab, and collaborators at HBEF for assistance in the field and laboratory.