Short-term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO2 concentrations

global0091

Global Change Biology (1997) 3, 29–35

Short-term decomposition of litter produced by plantsgrown in ambient and elevated atmospheric CO2concentrations

A N D R E W S . B A L L * and B E R T G . D R A K ESmithsonian Environmental Research Centre, Edgewater, Maryland, USA

Abstract

The effects of elevated atmospheric CO2 (ambient 1 340 µmol mol–1) on above-groundlitter decomposition were investigated over a 6-week period using a field-based mesocosmsystem. Soil respiratory activity in mesocosms incubated in ambient and elevatedatmospheric CO2 concentrations were not significantly different (t-test, P . 0.05)indicating that there were no direct effects of elevated atmospheric CO2 on litterdecomposition.

A study of the indirect effects of CO2 on soil respiration showed that soil mesocosmsto which naturally senescent plant litter had been added (0.5% w/w) from the C3 sedgeScirpus olneyi grown in elevated atmospheric CO2 was reduced by an average of 17%throughout the study when compared to soil mesocosms to which litter from Scirpusolneyi grown in ambient conditions had been added. In contrast, similar experimentsusing senescent material from the C4 grass Spartina patens showed no difference in soilrespiration rates between mesocosms to which litter from plants grown in elevated orambient CO2 conditions had been added.

Analysis of the C:N ratio and lignin content of the senescent material showed that,while the C:N ratio and lignin content of the Spartina patens litter did not vary withatmospheric CO2 conditions, the C:N ratio (but not the lignin content) of the litter fromScirpus olneyi was significantly greater (t-test; P , 0.05) when derived from plants grownunder elevated CO2 (105:1 compared to 86:1 for litter derived from Scirpus olneyi grownunder ambient conditions). The results suggest that the increased C:N ratio of the litterfrom the C3 plant Scirpus olneyi grown under elevated CO2 led to the lower rates ofbiodegradation observed as reduced soil respiration in the mesocosms. Further long-term experiments are now required to determine the effects of elevated CO2 on Cpartitioning in terrestrial ecosystems.

Keywords: carbon dioxide enrichment, climate change, plant litter, decomposition, soil respiration,tidal marsh ecosystem

Received 21 December 1995; revision received 24 April 1996; accepted 19 May 1996

IntroductionThe atmospheric concentration of CO2 has been steadily a number of biological processes. Changes in atmospheric

CO2 concentrations may have significant effects on plantrising due to increased emissions of CO2 from the use ofgrowth (Anderson 1991). The responses of both C3 andfossil fuels, industry and changes in land use (BazzazC4 plants to elevated atmospheric CO2 concentrations1990; Vitousek 1992). The concentration of CO2 in theinclude an increased water-use efficiency, while only C3atmosphere is regulated by a number of factors includingspecies show an instantaneous increase in net photo-the physical chemistry of the oceans and the interaction ofsynthesis (Curtis & Whigham 1989). The overall effect ofthese responses is an increased production of plant

Correspondence and Current Address: Dr Andrew S. Ball,biomass (Eamus & Jarvis 1989; Long & Drake 1992).Department of Biological and Chemical Sciences, John Tabor

The effects of elevated atmospheric CO2 concentrationsLaboratories, University of Essex, Wivenhoe Park, Colchesteron soil C cycling has yet to be fully elucidated despiteCO4 3SQ, UK, fax 144-(0)1206–873416, e-mail andrew@

essex.ac.uk the fact that the amount of carbon and nitrogen present

© 1997 Blackwell Science Ltd. 29

30 A . S . B A L L & B . G . D R A K E

in soils may be 2–3 times that found in plant tissues Drake 1991). In addition the use of an open top chamberin which the atmospheric CO2 concentration could be(Bouwman 1990). It is thought generally that there would

be no direct effects of increased atmospheric CO2 on soil controlled enabled us to set up mesocosms in fieldchambers exposed to both ambient and elevated CO2.processes due to the relatively high CO2 concentrations

present in soils (Van de Geijn & van Veen 1993). Although This provided us with the opportunity to determinewhether there were any direct effects of elevated atmo-work by Koizumi, Nakadai and co-workers has shown

that microbial respiratory activity was reduced by µ 50% spheric CO2 on soil respiration.when the CO2 concentration of ventilated air wasincreased from 0 to 1000 µmol mol–1 (Koizumi et al. 1991;

Materials and methodsNakadai et al. 1993), because of the concentration of CO2

in most soils being measured in terms of 103 µmolStudy site

mol–1 it is still likely that the main effects of elevatedatmospheric CO2 concentrations on soil processes would The study site for growth of plant material was located

on Kirkpatrick Marsh, Chesapeake Bay, Maryland, at abe indirect (Van Veen et al. 1991).From our laboratory-based studies on the effects of latitude of 36° 539N and longitude of 76° 339W. Vegetation

in this study site occurred as mono-specific stands of aelevated atmospheric CO2 concentrations on soilrespiratory activity we concluded that for sorghum, a C4 C3 sedge, Scirpus olneyi and a C4 grass, Spartina patens

(see Drake 1992 for a fuller description).plant, one effect of growth in elevated atmospheric CO2

concentrations was an increase in plant litter biodegrad-ability, as assessed by the increase in soil respiratory

Chamber design and operationactivity (Taylor & Ball 1994). Our previous study hadshown that wheat, a C3 plant, exhibited an opposite Study plots used for the growth of the two plant species

used in this study, Spartina patens and Scirpus olneyi,effect, i.e. that elevated-CO2 grown plant material wasmore difficult to degrade (Ball 1991). We suggested were located in a blocked design with two plots having

chambers. One of the two paired chambers receivedthat these two annual crops may have shown oppositeresponses according to their photosynthetic type. normal ambient air and the other received enough addi-

tional CO2 to increase that concentration to the presentAlthough the reasons for the observed differences werenot elucidated it was suggested that the increase in C:N normal ambient CO2 level plus 340 µmol mol–1. Addi-

tional details of the chamber design and operation canratio of plant material from wheat may have reduced itsbiodegradability. The C:N ratio of plant material has been be found in Leadley & Drake (1992). The chambers used

for the mesocosm study were located adjacent to the tidalshown to be important in determining decompositionrates (Berg 1984). The C:N ratio of plant material from marsh study site. These large open chambers were 4 m

in diameter and 3.4 m in height. In the elevated chamberssorghum grown under elevated atmospheric CO2 remainsunchanged (Taylor & Ball 1994). If this response is (N 5 3) the levels of CO2 was the ambient level plus

340 µmol mol–1.repeated in perennial vegetations of natural ecosystems,this will have important implications for future carbonand nitrogen cycling.

Mesocosm design and measurement of respiratoryIn this paper we further examine the effects of elevated

activityatmospheric CO2 on above-ground litter decompositionusing plant material from perennial C3 and C4 plants Mesocosms (N 5 36), consisting of 10 cm-diameter plastic

tubing cut into 20 cm-lengths, with one end covered withgrown in a natural ecosystem in elevated and ambientatmospheric CO2 concentrations. We selected a wetland wire mesh (1 mm-mesh size), were randomly placed

(arranged using random number tables) in a plastic trayecosystem situated on Chesapeake Bay. This study site,located on the Kirkpatrick Marsh consisted of a brackish (6 mesocosms per tray, number of trays 5 6). The plastic

trays were pre-filled to a depth of 5 cm with sterile sand.marsh in which the vegetation was organized into mono-specific stands of a C3 sedge, Scirpus olneyi and a C4 The mesocosms on each tray were filled to within 1 cm

of the top with soil (approximate mass of soil 5 1.000 kg).grass, Spartina patens. This site was selected for a numberof reasons. First, the site had been exposed to elevated Wire mesh (1 mm mesh size) was then placed over the

mesocosms. One tray was placed in each of 6 largeCO2 since 1987. Secondly, a complete investigation intothe effects of exposure of the native plant to elevated chambers. Three of the chambers were exposed to ambient

air while three others were exposed to air containingCO2 had been undertaken (Drake 1992). In this ecosystem,exposure of plants to elevated CO2 led to an increased elevated concentrations of CO2. Following stabilization

of respiratory activities in soils, plant material (0.05 kg,photosynthetic rate, a reduction in plant respiration andincreased carbon accumulation (Ziska et al. 1990; Arp & derived from either Scirpus olneyi or Spartina patens)

© 1997 Blackwell Science Ltd., Global Change Biology, 3, 29–35

E F F E C T S O F E L E VA T E D C O 2 O N P L A N T L I T T E R D E C O M P O S I T I O N 31

Table 1 Mean soil respiratory activity (µmol CO2 m–2 s–1) in soils incubated in ambientand elevated atmospheric CO2 concentrations and mesocosms to which plant materialgrown under elevated or ambient CO2 had been added. The results presented are themeans of 30 measurements.

Mesocosms containing soil to which Soil respiration (µmol CO2 m–2 s–1) in mesocosmsthe following plant material had incubated under:been added: ambient CO2 elevated CO2

1

soil only (control) 1.6 1.7Scirpus olneyi elevated CO2 grown 3.12 3.22

Scirpus olneyi ambient CO2 grown 3.92 3.72

Spartina patens elevated CO2 grown 3.5 3.2Spartina patens ambient CO2 grown 3.2 3.1

1 No significant difference (analysis of variance; P . 0.05) was observed between similarmesocosms when incubated under elevated or ambient CO2 chambers. 2 Significantdifferences (t-test; P , 0.05) between mesocosms containing plant material grown inelevated and ambient CO2 were detected in soil respiration measurements taken on thesame day.

grown in either ambient or elevated atmospheric CO2 cosm studies and contained (dry weight) 6.5% C and0.5% N and 50% moisture (w/w).was added to one mesocosm in each tray. The mesocosms

remaining were used as controls. Measurements of soilrespiratory activity were recorded regularly from July

Experimental detailsto September 1994 using a portable environmental gasmonitor (EGM-1, PP Systems, UK) connected to a soil The carbon:nitrogen ratio of soil and leaf material usedrespiration chamber (SRC-1, PP Systems, UK). Measure- in the study was taken using an automated Perkin-Elmerments of soil respiratory activity were taken three times CHNS/O analyser (Series ii, 2400). Soil samples (20 mg)each sample day; each single measurement, taking and plant samples (5 mg) were ground to small particlesµ 5 min, was repeated three times. The soil respiration and placed in foil capsules for analysis by combustionchamber (diameter 10 cm) was placed on the top of and spectral characterization.the mesocosm and the respiratory rate determined as The polymeric components of plant material used indescribed by the manufacturer (Anon. 1990). Briefly, the this study were analysed by sequential extraction ofinfra-red gas analyser sampled the air sealed inside the triplicate samples of Scirpus olneyi and Spartina patensrespiration chamber every 8 s over a 90-s period and, if grown in either ambient or elevated atmospheric CO2a linear increase in CO2 was detected, the the soil concentrations, followed by gravimetric analysis as previ-respiration rate was logged (g CO2 m–2 h–1). To allow ously described (Harper & Lynch 1985).easier comparison to other published data soil respiratoryactivity has been expressed in terms of µmol CO2 m–2 s–1.

Statistical tests

Differences between mean values were determined usingPlant material and soil samples the analysis of variance. Where appropriate, tests of two

samples were compared with the t-test. For analysis ofFreshly fallen plant material from both Scirpus olneyi andthe C:N ratios of the plant litter, data were transformedSpartina patens was gathered from chambers exposed toto a fraction prior to statistical treatment.both elevated and ambient CO2. This plant material was

used throughout this study after being cut into uniformsize (µ 2 cm in length). The plant material was placed Resultsin the mesocosms on the day they were collected (20July 1994). Soil respiratory activity in mesocosms

The soil used in this study, a sediment soil was takenfrom undisturbed soil (non-chamber site, 0–15 cm depth) Soil respiration measurements were taken over the period

July to September 1994. The mesocosms were left for onefrom the edge of Kirkpatrick Marsh. The soil was passedthrough a 2 mm-mesh in order to remove plant material week to stabilise prior to the addition of plant material.

At the time of addition the soil respiration rate in theand provide a relatively homogeneous soil for the meso-


32 A . S . B A L L & B . G . D R A K E

Fig. 1 Soil respiration values (µmol CO2 m–2 s–1) taken throughout July September 1994 in soil mesocosms to which plant litter fromScirpus olneyi grown in elevated and ambient atmospheric CO2 had been added. The results from a control mesocosm are also given.All mesocosms were placed in chambers exposed to elevated concentrations of CO2. 1 indicates a significant difference (P , 0.05)between soil respiration rates in mesocosms to which plant material from Scirpus olneyi exposed to elevated and ambient atmosphericCO2 concentrations had been added. . . .. .. . . , control soil (no litter added); u, soil containing ambient-CO2 grown S. olneyi; j, soil containingelevated-CO2 grown S. olneyi.

mesocosms was found to vary from 1.8 to 1.9 µmol m–2 Scirpus olneyi exposed to ambient CO2 had been addedincreased to a maximum level, µ 4 times the levelss–1. Table 1 shows the mean respiration rates for the soils

from each of the treatments measured throughout the detected in the control mesocosm (Fig. 1). While the soilrespiration rates in the mesocosms to which plant litterexperiment. No significant difference (analysis of vari-

ance; P . 0.05) could be detected between mesocosms from Scirpus olneyi grown in elevated CO2 had beenadded also increased during the period, on 7 out ofincubated in either ambient or elevated atmospheric CO2

concentrations prior to the introduction of plant material the 11 days in which sampling occurred, a significantdifference (t-test; P , 0.05) in respiratory rates occurred(Table 1). The addition of plant material (either Scirpus

olneyi or Spartina patens) led to an approximate doubling between mesocosms to which litter from plants grownin ambient and elevated CO2 had been added. Theof soil respiration rates over the length of the experiment.

For plant material derived from Spartina patens, soil increase in soil respiration was less with elevated-CO2

grown plant litter than with ambient-CO2 grown plantrespiration was unaffected by the original CO2 concentra-tion in which the plant was grown (Table 1). For meso- (Fig. 1.). Similar results were observed in mesocosms

exposed to ambient air (data not shown), indicating thatcosms to which plant material from Scirpus olneyi hadbeen added, the respiration rates were highest in meso- the concentration of CO2 in the air during degradation

of plant material had no effect on soil respiration.cosms to which plant material grown in ambient CO2

had been added. Soil respiratory activity in mesocosmsto which litter from Scirpus olneyi grown in elevated

Analysis of plant litteratmospheric CO2 concentrations had been added was,on average, µ 17% lower than soil respiration in meso- Analysis of the carbon and nitrogen present in the plant

litter used in this study, grown in elevated and ambientcosms to which litter from Scirpus olneyi grown in ambientCO2 had been added. atmospheric CO2 concentrations, indicated that the C:N

ratio of Spartina patens remained unchanged (no signific-Variations in recorded respiratory rates were observedthroughout the experiment for all mesocosms. However, ant difference; t-test, P . 0.86) during growth in elevated

atmospheric CO2 concentrations (C:N ratio 77:1 and 81:1the only significant differences in daily respiration ratesbetween litter-containing mesocosms occurred with for elevated and ambient CO2 litter, respectively). There

was, however, a significant increase in the C:N ratio (t-Scirpus olneyi. These variations in respiratory rates areshown in Fig. 1. The data shown are from the set test; P , 0.05) in plant litter from Scirpus olneyi grown in

elevated atmospheric CO2 concentrations (105:1 com-of mesocosms incubated in elevated atmospheric CO2

concentrations. While the soil respiratory activity of the pared to a C:N ratio of 86:1 for plant litter from Scirpusolneyi grown in ambient atmospheric CO2 concentration.control mesocosms was not found to vary throughout

the study period, the mesocosms containing soil amended Gravimetric analysis of the main polymeric componentsof plant litter revealed no significant differences in thewith plant material were found to approximately double

their soil respiration rate after 24 h. After 30 days, the polymeric composition of plant material grown in elev-ated and ambient atmospheric CO2 concentrations (Tablerespiration of the mesocosm to which plant litter from



Table 2 Elemental and gravimetric analysis of plant litter from Spartina patens and Scirpus olneyi grown under elevated and ambientCO2 concentrations. Carbon and nitrogen content (%) and percentage lignin, cellulose, hemicellulose and hot water soluble content oflitter. The results are the means of three replicates with standard deviations within 5% of the values shown.

Percent present in plant litter (%):Plant litter derived from: C:N ratio Carbon Nitrogen Lignin Cellulose Hemicellulose Hot water solubles

Scirpus olneyi-ambient CO2 86:11 38.01 0.44 20.5 42.2 27.3 10.0Scirpus olneyi-elevated CO2 105:11 42.11 0.40 20.1 44.4 24.8 10.7Spartina patens-ambient CO2 81:1 41.1 0.51 14.2 48.3 26.4 11.1Spartina patens-elevated CO2 77:1 40.2 0.53 14.4 49.1 25.3 11.2

1Significant differences (t-test; P , 0.05) between plant material grown in elevated and ambient CO2 were detected in soil respiration.

2). The lignin content remained unchanged in both the was therefore subject to natural changes in environmentalconditions. Fluctuations in the soil moisture potentialC3 and the C4 plant litter (20% and 14% for Scirpus olneyi

and Spartina patens, respectively). and the soil temperature will have occurred during thestudy. These environmental changes are probably partlyreflected in the variations in soil respiration observed

Discussionduring the study (Fig. 1). However, it is interesting tonote that the variations in soil respiratory activity of

Soil respiratory activity in mesocosmsthe control soil were much lower than for mesocosmscontaining soil to which plant material had been addedThe addition of plant material to the soil mesocosms led

to an approximate doubling of respiration when averaged (Fig. 1). One possible explanation for this is that thecontrol soils are depleted in readily utilizable carbon andthroughout the experiment (Table 1). Soil respiratory

activity has often been used as an indicator of biodegrad- nitrogen and therefore respiratory activity is substratelimited even when other environmental parametersability (Sharabi & Bartha 1993). Any observed differences

in respiratory rates between soils is due to the biodegrada- (moisture potential; temperature) are favourable.Soil respiratory rates in mesocosms to which planttion of the fresh litter. This mesocosm study necessarily

examined the effects of elevated CO2 on the initial stages litter from the C4 grass Spartina patens grown in elevatedor ambient elevated atmospheric CO2 concentrations hadof biodegradation. It is likely that the soil respiration rate

would decrease once this initial activity has utilized all been added were not significantly different (Table 1).However, mesocosms to which plant litter from the C3the labile components of the fresh litter. In order to study

the later stages of degradation it would be necessary to sedge Scirpus olneyi grown in elevated atmospheric CO2

concentrations had been added showed a lower soilextend the study period to at least 2–3 years, and to addunnaturally large quantities of plant litter to soil or to respiration rate than that obtained for mesocosms to

which Scirpus olneyi litter from plants grown in ambientuse radiolabelled plant litter (Taylor & Ball 1994).A comparison of soil respiration rates in replicate atmospheric CO2 concentrations had been added (Table

1; Fig. 1). These results are similar to those previouslychambers incubated in elevated and ambient atmosphericCO2 concentrations failed to detect any differences in observed for the annual C3 plant wheat (Ball 1991 and

Ball, unpublished data) and the C4 plant sorghum (Taylorrespiration rates, indicating that elevated atmosphericCO2 concentrations did not directly affect soil respiration & Ball 1994). However, the present results were obtained

using a field-based mesocosm system while the previousunder the experimental conditions described (Table 1).The concentration of CO2 in most soils is greater than data was obtained in laboratory conditions of constant

temperature and moisture conditions. These currentthe CO2 concentrations of even elevated-CO2 air (VanVeen et al. 1991; Van de Geijn & van Veen 1993) and results may therefore be of more relevance to changing

environmental conditions. However, it must be remem-therefore it was thought that, at the concentration ofCO2 used in current studies involving elevated CO2 bered that these studies were carried out in the absence

of animal communities which are also involved in the(µ 700 µmol mol–1), soil respiration would not be affected.The results from this study support this hypothesis, decomposition of plant material in natural ecosystems

(Couteaux et al. 1991). Nevertheless, the results suggestalthough studies have shown that microbial respirationis reduced with increasing atmospheric CO2 concentration that changes in atmospheric CO2 will affect soil respira-

tion indirectly through modifications in plant compositionfrom 0 to 1000 µmol mol–1 (Koizumi et al. 1991; Nakadaiet al. 1993). of C3 plants in natural ecosystems. Other studies compar-

ing the rate of decomposition of senescent material fromThe mesocosm study was carried out in field sites and


34 A . S . B A L L & B . G . D R A K E

plants exposed to ambient and elevated CO2 in open-top long-term experiments in which constant fractions ofchambers failed to detect significant differences in the plant growth in elevated and ambient atmospheric CO2rate of degradation of litter from plants grown under concentrations are added to mesocosms.ambient, ambient 1 150 µmol mol–1 and ambient1 300 µmol mol–1 CO2 concentrations (O’Neill & Norby1996). This two year litter bag study, using material from Acknowledgementsyellow poplar (Liriodendron tulipifera) grown in open top

We thank R. Matamala, M. Gonzalez Meyler and G. Peresta forchambers, did however, show an increased nitrogenhelpful comments and suggestions. This study was funded incontent in litter grown in ambient atmospheric CO2 part by grants from the Natural Environment Research Council,

concentrations. the North Atlantic Treaty Organization, the Smithsonian Institu-tion and the US. Department of Energy

Analysis of plant litter

The C:N ratio of plant material is an important determin- Referencesant of decomposition rates (Berg 1984). Analysis of theplant litter used in this study shows that, for the C4 plant Anderson JM (1991) The effects of climate change onSpartina patens, no significant differences were observed decomposition processes in grassland and coniferous forests.

Ecological Applications, 1, 326–347.in the C:N ratio of litter from plants grown in ambientAnon (1990) Operators Manual Version 1.8. PP Systems, Hitchin,or elevated atmospheric CO2 concentrations (Table 2).

UK.However, for the C3 sedge, Scirpus olneyi, a significantArp WJ, Drake BG (1991) Increased photosynthetic capacity ofincrease (t-test of transformed data, P , 0.05) in the C:N

Scirpus olneyi after 4 years exposure to elevated CO2. Plant,ratio of litter from plants grown in elevated atmosphericCell and Environment, 14, 1003–1006.CO2 concentrations was obtained. The results of the C:N

Ball AS (1991) Degradation by Streptomyces viridosporus T7 A ofanalysis show a reduction of 10% in the N concentration

plant material grown under elevated CO2 conditions. FEMSof plant litter from Scirpus olneyi grown in elevated Microbiology Letters, 84, 139–142.atmospheric CO2. Plants grown in elevated CO2 com- Bazzaz FA (1990) The response of natural ecosystems to the risingmonly show changes in nitrogen content (Norby et al. global CO2 levels. Annual Review of Ecology and Systematics, 21,1986). The N content of plant litter is an important rate- 167–196.regulating factor in the preliminary stages of decomposi- Berg B (1984) Decomposition of root litter and some factors

regulating the process: long- term root litter decompositiontion (Berg 1984). Lignin concentrations of plant litter havein a Scots pine forest. Soil Biology and Biochemistry, 16, 609–617.also been shown to be an important determinant in the

Bouwman AF (1990) Exchange of greenhouse gases betweendecomposition of plant litter (Melillo et al. 1982) andterrestrial ecosystems and the atmosphere. In: Soils and theincreased lignin concentrations have been reported inGreenhouse Effect (ed. Bouwman AF), pp. 61–192. John Wileyplants exposed to elevated CO2 (Cipollini et al. 1993).and Sons, Chichester, UK.Analysis of the plant litter used in this study showed

Cipollini ML, Drake BG, Whigham D (1993) Effects of elevatedthat lignin concentrations remained unaffected by the

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complexities. Oikos, 61, 54–64.patens, no significant reduction in the initial rates ofCurtis PS, Whigham DF (1989) Nitrogen and carbon dynamicsbiodegradation could be detected. In addition, no signi-

in C3 and C4 estuarine marsh plants grown under elevatedficant difference in the composition or C:N content ofCO2 in situ. Oecologia, 78, 297–301.plant litter was observed. In contrast, for the C3 sedge,

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Sharabi NE, Bartha R (1993) The use of soil respiration as aLeadley PW, Drake BG (1992) Open top chambers for exposingmeasurement of decomposition. Applied and Environmentalplant canopies to elevated CO2 concentration and forMicrobiology, 9, 1201–1205.measuring net gas exchange. Vegetatio, 104/105, 3–15.

Taylor J, Ball AS (1994) The effect of plant material grown underLong SP, Drake BG (1992) Photosynthetic CO2 assimilation andelevated CO2 on soil respiratory activity. Plant and Soil, 162,rising atmospheric CO2 concentrations. In: Crop315–318.Photosynthesis: Spatial and Temporal Determinants (eds Baker

Van de Geijn SC, van Veen JA (1993) Implications of increasedNR and Thomas H), pp. 69–107. Elsevier, Amsterdam.carbon dioxide levels for carbon input and turnover in soils.

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Norby RJ, Pastor J, Mellilo JM (1986) Carbon–nitrogen Ziska LH, Drake BG, Chamberlain S (1990) Long-terminteractions in CO2-enriched white oak: physiological and photosynthetic response in single leaves of a C3 and C4 saltlong term perspectives. Tree Physiology, 2, 233–241. marsh species grown at elevated atmospheric CO2 in situ.

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Short-term decomposition of litter produced by plants grown in ambient and elevated atmospheric CO2 concentrations