Does long-term forest N enrichment enhance microbial P limitation? a case study

Farrah Fatemi, Ivan Fernandez, Kevin Simon, David Bryan Dail, Lindsey Rustad, Stephen Norton

Globally, N deposition is still a problem

Galloway et al. 2004 Biogeochem 70: 153-226

1860 1990 2050

Forest N enrichment may enhance P limitation

N P CaN P Ca

N enrichment

Can we detect a shift towards enhanced microbial P limitation in N-enriched soils?

Magnitude of nutrient demand relative to supply

An Ecological Observatory

The Bear Brook Watershed in Maine (BBWM)

WEST BEAR(treated)

EAST BEAR(reference)

36 kg-1 ha-1 yr-1

N as (NH4)2SO4

The BBWM: A long-term N-enrichment experiment

8 kg-1 ha-1 yr-1 N

WBSW• WB has been experimentally acidified with N and S since 1989

• EB receives ambient deposition of N and S

EBHW

EBSW

WBHW

Watershed scale studies

Q1 -Are there signs of enhanced microbial P limitation due to the whole-watershed, long-term N enrichment and acidification?

phosphatases Bioavailable P

• Phosphatase activity• Microbial biomass

Insoluble organic P

Q2- What is the microbial response to P additions?

P addition experiments: field and lab incubations

C

Control (H2O) Phosphorus

P

N - enriched

reference

• soil phosphatase activity• N mineralization• soil respiration

Watershed scale results: soil phosphatase activity

0.0

0.5

1.0

1.5

2.0

2.5

phos

phat

ase

activ

ity (

µmol

gO

M-1

hr-1

) EB (reference)

WB (N-enriched) p= 0.04

p= 0.08

Hardwoods Softwoods

0.0

1,000.0

2,000.0

3,000.0

4,000.0

5,000.0

6,000.0

micr

obial

bio

mas

s C

µg g

-1

EB (reference)WB (N-enriched)

Hardwoods Softwoods

Watershed scale results: microbial biomass

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

phos

phat

ase

activ

ity (

µmol

gO

M-1

hr-

1) EB (reference)

WB (N-enriched)

So, phosphatase activity per unit microbial biomass is the same in reference and N-enriched soils

Hardwoods Softwoods

Microbial biomass is higher in the reference watershed

p=0.13p=0.08

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

phos

phat

ase

activ

ity (µ

mol

gO

M-1

hr-

1)Results from P additions: phosphatase response

EBHW WBHW EBSW WBSW

P addition in the field decreased phosphatase activity

+P

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

∆ C

O2

efflux

(µg

C g-1

day-1

)

EB (reference)

WB (N-enriched)

Results from P additions: Laboratory incubation

Hardwoods Softwoods

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

∆ so

il m

iiner

aliza

tion

(mg

N k

g-1

d-1)

EB (reference)WB (N-enriched)

Soil respiration

N mineralization

Hardwoods Softwoods

SUMMARY

A1- No, phosphatase activity data show no significant differences in microbial P demand

N PN P

Q1- Are there signs of enhanced microbial P limitation due to long-term N enrichment?

Q2- What is the microbial response to P additions?

A2- * soil phosphatase activity decreases * N mineralization and soil respiration do not respond significantly

Conclusions

• Extent of mirobial P limitation is not greater in N-enriched soils

• Phosphatase results highlight importance of considering microbial biomass in interpreting enzyme activity as an indicator of P limitation

• The University of Maine:- Department of Plant, Soil and Environmental Sciences- Graduate Program in Ecology and Environmental Sciences- Climate Change Institute

• The National Science Foundation

Thanks to many who helped with research design, field research and laboratory efforts!

Acknowledgements