Impact of long-term N fertilisation on C stocks

in trees and soils in northern Europe

Riitta Hyvönen, Tryggve Persson, Stefan Andersson, Bengt Olsson,

Göran Ågren & Sune Linder

Biogeochemistry (Online)

Objectives

• To determine the efficiency of added N on C storage in trees and soils in 15 long-term experiments in Sweden and Finland

• To use data on N-use efficiency to draw conclusions about the role of 100 years of N deposition on C stocks

Methods

• Long-term (14-30 yrs) experiments with repeated (3-30 times) addition of N and/or NPK in doses of 30-200 kg N ha-1 occasion-1.

• Tree biomass: Stem diam, no. of trees ha-1 and Marklund (1988) expansion factors, C=50% of dry wt

• Soil sampling: Organic layer + 0-10 cm min. soil on, at least, one occasion

Norrliden (P. sylvestris )

020406080

100120140160

1970 1980 1990 2000

Tre

e b

iom

as

s C

(M

g h

a -1

)

N1N2N2PK, N3N1PKN3PK, 0PK

Stråsan (P. abies )

020406080

100120140160

1965 1975 1985 1995 2005

Tre

e b

iom

ass

C (

Mg

ha

-1)

N3PKN2PK

N1PKN2N1N3

PK0

Tree biomass C (above and below ground) in control plots (0) and after annual additions of ammonium nitrate (N), potassium and phosphorus (PK) or NPK at different doses (N1=33, N2=67, N3=109 kg N ha-1 yr-

1) at two sites.

Norrliden (P. sylvestris)

0

5

10

15

20

25

30

0 500 1000 1500 2000 2500

Cum amount of N added (kg N ha-1)

Cum

. inc

reas

e in

C p

ool i

nex

cess

of N

-free

plo

ts (M

g ha

-1)

N1

N2

N3

Stråsan (P. abies )

0

20

40

60

80

100

0 1000 2000 3000 4000Cum amount of N added (kg N ha-1)

Cu

m. i

ncr

ease

in C

po

ol i

n

exce

ss o

f N

-fre

e p

lots

(M

g h

a-1

)

N1

N2

N3

N1PK

N2PK

N3PK

Cum. increase in tree biomass C in excess of control plots as a function of the cumulative amount of N added. N1=33, N2=67, N3=109 kg N ha-1 yr-1.

Conclusion: Low application doses of N more efficient to sequester C than high N doses. Example: 35, 17, 9 kg (C) kg-1 (N) for N1, N2 and N3, respectively, at Norrliden).

N fertilisation only

-10

0

10

20

30

40

50

60

20 25 30 35 40 45 50 55C/N ratio in humus layer (control plots)

N-u

se

eff

. (k

g C

se

q. k

g-1

N)

PAFinland

PASweden

PSSweden

NPK fertilisation

-10

0

10

20

30

40

50

60

20 25 30 35 40 45 50 55C/N ratio in humus layer (control plots)

N-u

se e

ff. (

kg C

seq

. kg

-1 N

)N-use efficiency for C sequestration in tree biomass as a function of C/N ratios in humus layers of control plots after addition of N (left) or NPK (right) fertiliser at P. abies (PA) and Pinus sylvestris (PS) sites.

Conclusion: Soil N status (C/N) seems to regulate N-use efficiency in trees, but low C/N ratio often reflects low availability of P and K as seen by NPK addition.

N-use eff. in soil (N)

0

5

10

15

20

25

20 25 30 35 40 45 50 55

C/N ratio in humus layer (control plots)

N-u

se

eff

. (k

g C

se

q. k

g -1

N) P. abies

P. sylvestris

N-use eff. in soil (NPK)

0

5

10

15

20

25

20 25 30 35 40 45 50 55

C/N ratio in humus layer (control plots)

N-u

se

eff

. (k

g C

se

q. k

g -

1 N) P. abies

P. sylvestris

N-use efficiency for C sequestration in soil (org. layer + 0-10 cm min. soil) as a function of C/N ratios in humus layers of control plots after addition of N or NPK fertilisers

Relationship between N-use efficiencies in trees and soils for P. abies and P. sylvestris sites with both tree and soil studies. Figures from both N and NPK fertilisation. Conclusion: (1) Higher SOC seq. in P abies than in P sylvestris soils. (2) High SOC seq. also when trees don´t sequester extra C indicates reduced decomposition.

0

5

10

15

20

25

-10 0 10 20 30 40 50 60

N-use eff., kg (C) kg-1 (N), trees

N-u

se e

ff.,

kg (

C)

kg -1

(N

), s

oil

P. abies

P. sylvestris

C pool (kg m-2)

0

2

4

6

8

10

0 N1

LFH0-10 cm

Resp rate (mg C g-1 C d-1)

0,0

0,2

0,4

0,6

0,8

1,0

0 N1

Field Rh (g C m-2 y-1)

0

100

200

300

400

500

0 N1

LFH0-10 cm

Mean (SE, n=4) C pools, C mineralisation rates (at 15°C) and calculated field Rh to a depth of 10 cm in the mineral soil at Stråsan in control (0) and N-fertilised (N1) treatments in 2004. N1 had received, on average, 33 kg N ha-1 y-1 during 39 yrs.

Cumulative N deposition during the 20th century

0

200

400

600

800

1000

1200

1400

S Sweden N Sweden

Estim. increase in C pools (kg C

m-2) due to N deposition

0

1

2

3

4

5

S Sweden N Sweden

Trees

Soil

Estimates of cumulative N deposition in S and N Sweden during 1900-2000 and resulting tree and soil C pools as calculated from the N-use efficiencies

0.00

2.00

4.00

6.00

8.00

10.00

12.00

South Central North

Kg

C m

-2

Forest floor

Total 50 cm

Mean SOC content (±95% c.i.) for forest floor and total soil at dry and mesic forest sites in southern (55-60 oN, n= 445), central (60 – 64 oN, n=448) and northern Sweden (64-68 oN, n=329). After Mats Olsson, pers. comm.

Conclusions

• N-use efficiencies for trees (P abies) varied (0-50 kg (C) kg-1 (N)) depending on C/N ratio and PK availability.

• N-use efficiencies for SOC was generally lower than for trees and averaged 13 kg (C) kg-1 (N)) for P abies and 7 for P sylvestris.

• 10 kg N ha-1 yr-1 higher N deposition in S than in N Sweden for a whole century should have resulted in 2.0±1.0 kg m-2 more tree C and 1.3±0.5 kg m-2 more SOC.