Critical assumptions to test include:• Is leaf litter chemistry affected by elevated [CO2]?

• Does a change in litter chemistry in elevated [CO2] alter decomposition?

Ecosystem models are used to explore the consequences for N cycling, primary productivity, and C storage of CO2 effects on litter chemistry and decomposition.

A consensus is needed from experimental data on how to best represent these effects in models.

The GCTE Synthesis of Litter Chemistry and Decomposition in Elevated CO2

R. J. Norby1, M. F. Cotrufo2, P. Ineson3, E. G. O’Neill1, and J. G. Canadell4

1Oak Ridge National Laboratory, USA; 2II Universitá di Napoli, Italy; 3University of York, UK; 4GCTE Project Office, Australia

The Litter Quality Hypothesis

Plant tissue grown in elevated concentrations of atmospheric CO2 is usually found to have lower concentrations of N than comparable tissue grown in ambient CO2.

If plant detritus (e.g., leaf litter) also has lower [N] in CO2-enriched

ecosystems, then the community of decomposing organisms will have food of lower quality, and the transfer of organically-bound N to mineral N pools available to support plant growth could slow.

Ecosystem models show that the accelerated growth of plants in high CO2 atmospheres would be self-limited by this litter-quality feedback.

Methods of Meta-Analysis

Litter chemistry data (N and lignin concentrations) were assembled from observations of naturally senseced leaves of plants exposed to elevated [CO2] (600-700 ppm) in the field or in field chambers.

Decomposition data base (mass loss and CO2 release) also included other plant tissues and experiments using different CO2 enrichment regimes.

Categorical variables were defined to describe source material characteristics, exposure protocol, and measurement protocol.

The effect size for meta-analysis was lnRR where RR is the response ratio (mean in elevated [CO2] divided by mean in ambient [CO2]). A mixed model was used.

GCTE Synthesis – Approach & Products

GCTE Focus 1 (Ecosystem Physiology) identified litter quality to be a critical response that could influence ecosystem response to rising CO2.

A workshop “Litter Quality and Decomposition under Elevated CO2” was held in Capri, Italy, in September, 1998, with the primary objective of reaching consensus on the experimental results.

Workshop products included a meeting report [Nature (1998) 396: 17-18] and a volume of research papers [Plant & Soil (2000) 224, No. 1].

A meta-analysis of all relevant published and pre-publication data on CO2 effects on litter chemistry and decomposition was published in Oecologia (2001) 127: 153-165.

The workshop developed a set of recommendations for standardizing measures of litter quality and decomposition in ongoing and future CO2 enrichment experiments.

0 50 100 150 200 250 300

ligninambient (mg g-1

)

0

50

100

150

200

250

300

ligni

nel

evat

ed (m

g g

-1)

b

0.9 1.0 1.1 1.2 1.3 1.4

Response ratio

OTC

solardome

no chamber

[N] < 10 mg g-1

[N] > 10 mg g-1

all

woody

herbaceous

pot

in ground

46

31

15

9

37

25

21

15

7

24

0 5 10 15 20 25 30 35

Nambient (mg g-1

)

0

5

10

15

20

25

30

35

Nel

evat

ed (m

g g

-1)

a

0.6 0.7 0.8 0.9 1.0 1.1

Response ratio

OTC

solardome

no chamber

[N] < 10 mg g-1

[N] > 10 mg g-1

all

32

42

19

6

49

74

Litter Nitrogen

0 20 40 60 80 100

mass loss ambient (%)

0

20

40

60

80

100

mas

s lo

ssel

evat

ed (%

)

a

0.7 0.9 1.1 1.3 1.5

Response ratio

101

48

53

20

3

23

20

10

21

11

11

48OTC

solardome

no chamber

greenhouse

root

all

leaf

shoot

woody

herbaceous

growth chamber

wood

Litter Lignin Decomposition

Conclusions

The hypothesis that CO2-induced changes in leaf litter chemistry result in impacts on decomposition is not supported.

N concentration of naturally senesced leaves of plants grown in elevated [CO2] was 7.1% lower than that of ambient-grown plants. This result was:

• usually not significant in individual experiments• much less than that often observed in green leaves• less in leaves with incomplete N resorption

The small, but consistent decline in litter [N], coupled with a 6.5% increase in lignin, would be predicted to result in slower decomposition in CO2-enriched litter. However, there was no consistent effect of CO2 treatment on mass loss or CO2 release.

CO2 effects on litter chemistry or decomposition were smallest under experimental conditions more similar to field conditions.

Any changes in decomposition rate resulting from exposure of plants to elevated [CO2] are small compared to other potential impacts of [CO2] on C and N cycling.

-20 0 20 40 60 80 100

REambient (%)

-20

0

20

40

60

80

100

RE

elev

ated

(%)

N Resorption

CO2 Release

growthchamber

SD OTC CO2spring

all 0.0

0.5

1.0

1.5

2.0

Res

po

nse

rat

io

b

Symbolsopen – herbaceousclosed – woodycircles – OTCssquares – open-fieldtriangles - solardomes

Median of 67 observations: 9.8 mg/g in ambient 9.2 mg/g in elevated CO2

Symbolsopen – herbaceousclosed – woodycircles – OTCssquares – open-fieldtriangles - solardomes

Mean of 20 observations: 49% in ambient 48% in elevated CO2

Note: no significant effect of [CO2] if 95% confidence interval includes a response ratio of 1. The number of observations is indicated. “no chamber” includes FACE, CO2 springs, and SACC.

Symbolsopen – herbaceousclosed – woodyShapes represent different species

Median of 46 observations: 138 mg/g in ambient 149 mg/g in elevated CO2

Symbolsopen – herbaceousclosed – woodycircles – OTCssquares – open-fielduptriangles – solardomesdowntriangles – growth

chambers

Symbolsopen – herbaceousclosed – woody

RE = (green [N] –litter [N])/green [N]

Note: the data set did not support meta-analysis

Acknowledgements -- This synthesis was made possible with the financial support of the U.S. Department of Energy and the contributions of published and unpublished data from many researchers in the GCTE CO2 network.