Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue4.2000:

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Elevated atmospheric carbon dioxide concentration: eects of

increased carbon input in a

Lolium perenne

soil on

microorganisms and decomposition

J.H. van Ginkel, A. Gorissen*, D. Polci

Plant Research International, P.O. Box 16, 6700 AA Wageningen, Netherlands

Accepted 28 May 1999

Abstract

Eects of ambient and elevated atmospheric CO2 concentrations (350 and 700 ml lÿ1) on net carbon input into soil, the

production of root-derived material and the subsequent microbial transformation were investigated. Perennial ryegrass plants (L. perenne L.) were labelled in a continuously labelled14C-CO2atmosphere to follow carbon ¯ow through the plant and all soil

compartments. After 115 days, root biomass was 41% greater at elevated CO2 than at ambient CO2 and this root biomass

seemed to be the driving force for the increase of 14C-labelled carbon in all compartments examined, i.e. carbon in the soil solution, soil microbial biomass and soil residue. After incubation for 230 days at 148C, roots grown at elevated CO2

decomposed slower (14%) than roots grown at ambient CO2. Increasing the incubation temperature of the roots grown at

elevated CO2 by 28C could not compensate for this delay in decomposition. In addition, `elevated CO2' root-derived material

(14_{C-labelled soil microorganisms plus}14_{C-labelled soil residue) decomposed signi®cantly slower (29%) than `ambient CO} 2'

root-derived material. At the end of the incubation experiment, the ratio between 14C-labelled microorganisms and total 14CO2

evolved showed no dierence among root incubation and incubation of root-derived material. Thus, the substrate use eciency of microorganisms, involved with decomposition of roots and root-derived material, seems not to be aected by an increase in atmospheric CO2concentrations. Therefore, the lower decomposition rate at elevated CO2is not due to a change in the internal

1. Introduction

Atmospheric CO2 levels are increasing steadily (Keeling et al., 1995), mainly due to combustion of fossil fuels and deforestation. Increasing CO2levels are expected to have numerous direct and indirect eects on terrestrial ecosystems (Bazzaz, 1990). Among those eects, changes in above-ground primary production and litter fall are relatively easy to assess, but changes in carbon allocation below-ground are more dicult to determine. Yet, this process is thought to be of great importance both for the functioning of terrestrial

eco-systems and for carbon sequestering. Most intriguing is the impact of root growth on soil microorganisms and associated transformation processes. After all, microorganisms play an essential role in the cycling of nutrients associated with primary production (Paul and Clark, 1989; Killham, 1994) and this largely deter-mines the overall ecosystem response. To date, the re-sponse of the soil microorganisms to elevated atmospheric CO2 concentrations is poorly understood (O'Neill, 1994). Newton et al. (1995) and Ross et al. (1995) found no change in microbial biomass in soils with plants grown elevated CO2. Rillig et al. (1997) measured dierent carbon-substrate utilisation of rhi-zosphere extracts applied to Biolog microplates with fewer polymers oxidised at elevated CO2. Because only a minority of the microbial community is active, while the majority is dormant and forms a high background

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noise when measuring soil microbial biomass, re-sponses of soil microorganisms to elevated CO2 are hard to measure. Therefore, the use of carbon tracers such as 14C or 13C is a good tool to distinguish between already present native-soil organic carbon and incoming plant-derived carbon in the soil and to measure soil microorganisms actively involved in the transformation of either of the carbon sources.

To study the metabolic behaviour of soil microor-ganisms, we calculated the ratio between the 14 C-labelled soil microbial biomass and total 14CO2 evolved at the end of an incubation with roots and root-derived material originated from ambient and el-evated CO2. What consequences a temperature increase accompanying the raise of atmospheric CO2 will have on microbial behaviour and subsequently on decomposition of root material grown at elevated CO2 is not clear yet. Gorissen et al. (1995) found that slow decomposition of grass roots grown at elevated CO2 could be compensated by incubation at a 68C higher temperature. However, the projected 28C increase with atmospheric CO2 levels rising from 350 to 700 ml l

ÿ1

(IPPC, 1995) may make soil a sink for atmospheric CO2.

In this study, grass plants (L. perenne) were grown and homogeneously 14C-labelled at two dierent 14 C-CO2concentrations (350 and 700ml lÿ1). Our objectives were to determine the eects of elevated atmospheric CO2 concentrations on: (i) possible changes in beha-viour of the soil microbial biomass due to altered below-ground 14C-carbon input and (ii) the in¯uence of such changes on decomposition of 14C-roots, incu-bated at two dierent temperatures.

2. Materials and methods

2.1. Plant and soil

We used L. perenne L. cv. `Barlet' and fresh soil from the top layer (20 cm) of an arable loamy sand soil. Soil properties were: particle size distribution 3% < 2 mm, 12% 2±50 mm and 85% > 50 mm; organic carbon 1.7%; total nitrogen 0.10%; pH 6.4.

Grass seeds were germinated on moist paper for two days in the dark and transferred to 50 ml pots contain-ing 70 g of soil. Half of the seedlcontain-ings were grown for 30 days in a greenhouse at 350 ml lÿ1 CO2 and the other half at 700ml lÿ1CO2. Subsequently, eight plants were transferred to 1150 ml polythene soil containers (one plant per container) ®lled with 1745 g of mois-tened soil with a bulk density of 1.3 g cmÿ3. (dry weight basis). These containers were transferred to the ESPAS (Experimental Soil Plant Atmosphere System; Gorissen et al., 1996) growth chambers and the four plants were grown in a continuously labelled14CO2

at-mosphere (speci®c activity 0.70 kBq mgÿ1 C) with either CO2 concentrations of 350 or 700 ml lÿ1. The plants were grown under the following conditions: light 16h dayÿ1; PAR (Photosynthetically Active Radi-ation) 400 mmol mÿ2 sÿ1; temperature 18/148C (day/ night); relative humidity 65/70% (day/night) and wind speed 0.1 m sÿ1. All environmental variables were checked with a third independent meter to ensure iden-tical conditions in the growth chambers. During the experiment, soil water was kept at about 14% w/w (60±70% of ®eld capacity) by adjusting to a predeter-mined weight adjusted for the growth of the plants. The mineral N content (KCl extractable NH4+ and NO3ÿ) of the soil at the start of the experiment was 10.5 mg N gÿ1dry soil. In addition, at the start of the experiment all soil containers received phosphorus and potassium at a concentration of 29 and 78 mg gÿ1, re-spectively. The shoots were cut after 49, 66, 83 and 115 days.

On day 115, the plants were removed from the growth chambers and the shoots and any stubble were cut o at ground level. The roots and soil were separ-ated by gently shaking the soil-root core and the remaining roots were removed from the soil by hand-picking. Subsequently, the root material was rinsed with tap water to remove adhering soil particles. Shoots, roots and soil were dried (708C) and ground (1 mm) and dry weight, total carbon and 14C-carbon were determined. Roots were also analysed for total nitrogen.

Plant weight at the time of transfer to ESPAS and the start of 14C-labelling (day 32) was less than 1% of plant weight at the end of the experiment (data not shown) and was therefore assumed negligible. Before and after analysis, the soils of the harvested plants were kept at 48C to minimise microbial activity, pend-ing the incubation experiment.

2.2. Incubation experiment

Soil from the pots of the harvested plants was incu-bated: (1) 60 g of root-free 14C-labelled soil and, (2) as (1) but mixed with the dried and ground 14C-labelled roots in the same root/soil ratio as was present at har-vest. This resulted in four treatments: root-free 14 C-labelled soil (includes 14C-labelled soil microbial bio-mass plus 14C-labelled microbial products) from plants grown at 350 and 700 ml lÿ1 CO2 (later referred to as 350S and 700S) and 14C-labelled soil amended with 14

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tem-perature (700R-HT). During incubation, soil water was about 14% w/w (60±70% of ®eld capacity) and as the containers were hermetically closed, no water was lost. After 32 and 128 days, the containers were ¯ushed with CO2-free air to prevent lack of oxygen. NaOH solutions were refreshed and analysed after 1, 2, 4, 8, 16, 32, 64, 128 and 230 days.

2.3. Analyses

Total carbon and 14C-carbon content of shoots, roots and soil were determined using a modi®ed wet combustion method (Dalal, 1979). Dried and ground plant material (30 mg) and soil (1 g) were digested in 5 ml of a 10% (w/v) solution of K2Cr2O7, dissolved in a mixture of concentrated H2SO4 and H3PO4 (3:2 v/v), at 1608C for 2 h. The CO2 evolved was trapped in 10 ml of a 0.5 N NaOH solution. Total CO2(12CO2and 14

CO2) adsorbed in the NaOH solution was deter-mined by titrating the remaining NaOH with HCl after precipitation of HCO3ÿ/CO32ÿ species by excess BaCl2. Liquid scintillation counting was used to deter-mine 14C-carbon: 0.5 ml of the 0.5 N NaOH solution was mixed with 3 ml of Ultima Gold (Packard) and counted on a liquid scintillation counter (Tri-Carb 2100TR; Packard). Total nitrogen in the roots was determined by acid digestion and determination of nitrogen with Nessler's reagent by continuous ¯ow-analysis (Van Ginkel and Sinnaeve, 1980). The 14 C-labelled soil microbial biomass (14C-smb) was deter-mined using the fumigation±centrifugation technique: soil solutions were obtained by centrifugation of non-fumigated and chloroform-non-fumigated (20 h) soils and in an aliquot of the soil solution14C was subsequently determined (Van Ginkel et al., 1994). The proportion-ality factor Kcc relating the ¯ush size obtained by cen-trifugation with the microbial biomass of the soil was determined by in situ labelling of the microbial bio-mass with D(U-14C)glucose (Van Veen et al., 1985), assuming that after 3 days the 14C-label is totally in-corporated into the microbial biomass. The propor-tionality factor so obtained for the FC-method (Kcc) was 0.152.

2.4. Calculations

Total carbon (mg C) recovered in the dierent plant-soil compartments was calculated by dividing the 14

C-recovered (kBq) in all compartments by the mean speci®c activity (kBq mgÿ1 C) of the ESPAS atmos-phere. The percentage respired 14C-labelled material from the soil was calculated as 14CO2respired (mg C) divided by the initial 14C-labelled soil content (mg C) times 100. The respiration of the native-soil organic carbon was calculated as the total respiration deter-mined by acid digestion (12C-CO2plus

14

C-CO2) minus

the 14C-CO2 respiration determined by liquid scintil-lation counting (see Analyses).

2.5. Statistics

Four plants were exposed to 350 ml lÿ1 CO2 and four plants to 700 ml lÿ1 CO2. The data of the incu-bation experiment were analysed based on a comple-tely randomised design with ®ve treatments in four replicates. All pots were incubated in duplicate and the

Fig. 1. (A) Relation between the amount of 14C-labelled carbon in the roots (mg C) and the amount of 14_{C-labelled carbon in the soil} solution (mg C gÿ1 _{soil) after 115 days of}

L. perennegrowth at 350 and 700ml Lÿ1_CO2_{(four replicates). (B) The same relation between} the amount of14_{C-labelled carbon in the soil solution (}

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analysis was performed based on the averaged values. The signi®cance of dierences and LSD test were assessed by ANOVA (Genstat 5; release 3.1). Treat-ment eects are considered signi®cant when p values were lower than 0.05, unless stated otherwise.

3. Results

3.1. Elevated CO and14C-allocation after 115 days plant growth

At the end of the growth period, elevated CO2 had signi®cantly increased the accumulated carbon content of the shoots by 27% and of roots by 41% (Table 1). Root mass showed a positive correlationr0:75,p

0:03with the amount of soluble

14

C-carbon in the soil solution of the nonfumigated soil (Fig. 1A). Sub-sequently, the amount of soluble 14C-labelled carbon in the soil solution showed a positive correlation r0:79, p0:02 with the amount of

14

C-labelled soil microbial biomass (Fig. 1B). The 14C-labelled soil microbial biomass was 46% greater at elevated CO2 than that at ambient CO2 (Table 1) and showed a strong positive correlation r0:95, p < 0.001) with

the amount of 14C-labelled soil residue (Fig. 1C). Expressing the 14C-labelled soil residue per unit 14 C-labelled soil microbial biomass showed that at elevated CO2 this ratio was signi®cantly higher (100%, p < 0.001). The amount of 14C-labelled soil residue was about three times greater at elevated CO2 than at ambient CO2 (Table 1). Increasing the atmospheric CO2concentration from 350 to 700 ml lÿ1caused total 14

C-labelled carbon in the soil (14C-labelled roots,14 C-labelled soil microbial biomass and 14C-labelled soil residue) to increase by 53% (Table 1).

3.2. Incubation of roots and root-derived material

The mean decomposition of the `elevated CO2' roots after 230 days (700R) was 36%, which was less (p< 0.01) than the 42% of the `ambient CO2' roots (350R, Table 2). Raising the incubation temperature by 28C increased the decomposition of the `700' roots

(700R-HT) to 39%, which was not signi®cantly dierent from the 700R treatment, but still signi®cantly lower than the decomposition in the 350R treatment. The de-composition of the root-derived material (consisting of 14

C-labelled soil microbial biomass plus 14C-labelled microbial products) in the `elevated' soil (700S) and in the `ambient' soil (350S) was 27 and 38% (p< 0.001), respectively.

The14C-labelled soil microbial biomass (14C-smb) in the 700R soil was 28% larger (p< 0.01) than that in the 350R soil (Table 2). The 14C-smb in the 700R-HT soil was 20.5mg C gÿ1dry soil, which was not dierent from that in the 350R soil but signi®cantly lower (41%; p< 0.01) than that in the 700R soil. The 14 C-smb of 350S was 32% lower than that of 700S, but this dierence was not signi®cant.

No dierence was observed between the decompo-sition of native-soil organic matter (SOM) of the 350R and 700R treatment (290mg C gÿ1dry soil). The SOM decomposition of the 700R-HT treatment was 422 mg C gÿ1, which was signi®cantly higher than the SOM decomposition at the lower temperature. There was no dierence in SOM decomposition between 350S and 700S treatment (248 mg C gÿ1), but it was signi®cantly lower (p< 0.01) than the SOM decomposition of the 350R and the 700R.

A ratio between 14C-labelled soil microbial biomass (mg C) and total 14CO2evolved (mg C) at the end of the incubation experiment was calculated. There was no dierence in this ratio between the 350R and 700R treatment or between the 350S and 700S treatment (Fig. 1). This ratio was signi®cantly lower in the 700R-HT treatment than in the 700R treatment.

4. Discussion

4.1.14C-allocation after 115 days of plant growth

The 27% increase in shoot yield at elevated CO2 agrees well with the observations reviewed by Kimball (1983). The total plant-derived below-ground carbon input was 53% higher at elevated CO2than at ambient CO2 and agrees with studies by Newton et al. (1994;

Table 1

Total amounts of14C-labelled C recovered in shoots, roots, soluble C in the soil solution, soil microbial biomass, soil residue and total below-ground ofL. perennegrown at 350 and 700ml lÿ1for 115 days. Each value represents four replicates

CO2a_treatment _{Shoot (mg C)} _{Root (mg C)} _{Soluble (}

mg C gÿ1₎ _smbb₍

mg C gÿ1₎ _{Residue (}

mg C gÿ1₎ _Total14_{C-soil content (}

mg C gÿ1₎

350 264 182 0.126 14.8 10.9 147

700 335 257 0.163 21.6 31.4 225

LSD0.05 36 59 0.057 4.6 8.6 50

a

350=atmosphere 350ml lÿ1CO2; 700=atmosphere 700ml lÿ1CO2. b 14

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1995), i.e. +64% for pasture turves, by Cotrufo and Gorissen (1997), i.e. +26% for L. perenne, Agrostis capillaris and Festuca ovina and by Van Ginkel et al. (1996; 1997), i.e. +45 and +57%, respectively, forL. perenne. It emphasises the relatively large impact of increased CO2 concentrations in the atmosphere on below-ground processes.

However, it remains extremely dicult to assess whether roots release also more carbon per unit of root weight into the soil under elevated CO2 since carbon ¯ows through the dierent below-ground com-partments can hardly be quanti®ed (Cardon, 1996). We applied continuous 14C-labelling during the entire plant growth period and found a positive correlation between carbon ¯ows through the subsequent below-ground compartments (roots ₄carbon in the soil sol-ution4soil microbial biomass 4soil residue; Fig. 1).

This seems to support the conclusion that the root bio-mass is the driving parameter for all subsequent below-ground processes in our plant-soil system.

The positive correlation between roots and soil mi-crobial biomass is in contrast with the results of Kam-pichler et al. (1998) who found no eect of a plant community (Cardamine hirsuta,Poa annua,Senico vul-garisandSpergula arvensis) grown in an Ecotron at el-evated CO2 on soil microbial biomass. In their ®rst experiment, root biomass at elevated CO2 was 31% lower than that at ambient CO2, whereas in their sec-ond experiment root biomass at elevated CO2 was 69% higher. We consider it unlikely that 69% more root biomass would not stimulate the size or activity of the microbial biomass in the soil. In our experiment, 41% more root biomass signi®cantly increased the size of soil microbial biomass. Other experiments (Cotrufo and Gorissen, 1997; Van Ginkel and Gorissen, 1998) also showed a positive correlation between the weight of the root biomass of L. perenne, A. capillaris and F.

ovina and the size the soil microbial biomass. Kam-pichler et al. (1998) suggested that a detectable re-sponse of soil microorganisms cannot be found in experiments on ecosystem level as compared with ex-periments at a single species level. However, we feel that techniques used by Kampichler et al. (1998) are not sensitive enough to accurately measure small changes in the size of the soil microbial biomass and their suggestion seems rather premature.

4.2. Microbial behaviour in soil

Ladd et al. (1995) pointed out that faster turnover rates would result in less biomass-14C accumulated per unit 14CO2 evolved. Comparing this ratio in the 350R soil with that in the 700R soil (Fig. 2) revealed no dierence, indicating that the turnover of plant-derived

Table 2

Initial14C-labelled carbon content,14C-labelled soluble carbon in the soil solution,14C-labelled soil microbial biomass, respired native-soil or-ganic matter (som) and respired14C-carbon after incubation (four replicates) ofL. perenneroots and/or root derived material at 148C and 168C (HT) for 230 days

CO2/temperature treatment Initial14C-soil content (mg C gÿ1)

Soluble carbon (mg C gÿ1)

smba (mg C gÿ1)

Respired som (mg C gÿ1)

Respired14Cb (%)

350Rc 147 0.148 22.0 294 42.2

700R 225 0.181 28.9 286 36.2

700R-HT 225 0.146 20.5 422 38.6

350S 26 0.051 7.6 245 38.3

700S 53 0.066 10.0 251 27.1

LSD0.05 0.048 5.0 25 3.7

a 14_{C-labelled soil microbial biomass.}

b_{Expressed as a percentage of the initial total}14_{C-labelled soil carbon content.}

c_{350R=soil from atmosphere 350}_m_{l l}ÿ1_CO2_with14_{C-labelled roots,}14_{C-labelled soil microbial biomass and}14_{C-labelled microbial products;} 700R=same as 350R but from a 700 ml lÿ1

CO2 atmosphere; 700R-HT=same as 700R but with a 28C higher incubation temperature; 350S=soil from atmosphere 350ml lÿ1

CO2with14C-labelled soil microbial biomass and14C-labelled microbial products; 700S=same as 350S but from a 700ml lÿ1CO2atmosphere.

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14

C-carbon in soil was not aected by elevated CO2. Substrate use eciency seems unaltered and soil micro-organisms seem to transform root material originating from both elevated and ambient CO2 metabolically in the same way. It remains unclear why relatively less root material in the 700R soil had been decomposed as compared with roots in the 350R soil. In absolute amounts, more carbon was transformed by microor-ganisms in the 700R soil, but signi®cantly more carbon remained in this soil than in the 350R soil due to a sig-ni®cantly higher carbon input at elevated CO2. The lowest ratio between biomass-14C and 14CO2 evolved was found in the incubation of the `700 roots' at a higher temperature, indicating that this 14C-material had the highest turnover rate. The relative amount of decomposed 14C-material in the 700R-HT soil showed no signi®cant dierence with the 700R soil; the higher turnover rate can therefore only have been caused by faster turnover of the soil microbial biomass itself as can be derived from the decrease in soil microbial bio-mass. No dierence was found in the ratio between 14

C-labelled microorganisms and total 14CO2 evolved in the 350S soil and in the 700S soil, again indicating that turnover of microorganisms and their metabolites occurs in a similar way. Microbial transformation pro-cesses involved in the turnover of roots and root-de-rived material in soil ecosystems do not seem to be aected by an increase in atmospheric CO2 concen-trations. Therefore, a lower decomposition rate at elev-ated CO2 is not due to a change in the internal metabolism of microorganisms.

One could argue that available mineral nitrogen could cause the reduced carbon mineralisation at elev-ated CO2. Soil mineral nitrogen concentration at the start of the incubation experiment was 2.4 and 3.2 mg gÿ1dry soil at ambient and elevated CO2, respectively. The amount of nitrogen mineralised in the soil during the incubation experiment can be calculated from the respiration of the native-soil organic carbon (Table 2). Assuming a substrate assimilation eciency of the soil microbial biomass for recalcitrant soil organic carbon of about 20% (Van Veen et al., 1984) for both treat-ments, 80% of this material has been evolved as 12

CO2. Division of the total amount of carbon minera-lised from soil organic matter (carbon incorporated in the microbial biomass plus CO2 evolved) by the C-to-N ratio of the soil (17.0) results in an estimated nitro-gen mineralisation of 20 mg gÿ1dry soil. This amount makes it unlikely that mineral nitrogen was a limiting factor during the incubation experiment.

4.3. Elevated CO2and decomposition of roots and root residues

Although the C-to-N ratio of the grass roots (62) was not aected, the decomposition of grass roots

grown at elevated CO2 (700R) was signi®cantly lower (14%) than that of roots grown at ambient CO2 (350R) after incubation for 230 days. The lower de-composition is in agreement with other studies on grass roots (Gorissen et al., 1995; Van Ginkel et al., 1996; Van Ginkel and Gorissen, 1998). Norby and Cotrufo (1998) argued that litter decomposition is not aected by elevated CO2, but they mainly refer to stu-dies with above-ground plant materials. However, the importance of below-ground decomposition processes on the carbon cycle is considerable since 40±60% of all assimilated photosynthates is transported below-ground in grasses (Van Ginkel and Gorissen, 1998). Although both increased carbon input and a change in decomposability play a role in soil carbon storage in grasslands (Van Ginkel et al., 1999), their relative con-tributions are still unclear. Several studies have shown a decreased decomposition of roots grown under elev-ated CO2 without a consistent correlation with the C-to-N ratio (Cotrufo and Ineson, 1995; Van Ginkel et al., 1996; Van Ginkel and Gorissen, 1998). Most likely, the C-to-N ratio is not the appropriate indicator for `CO2-induced' changes in `quality' and subsequent changes in decomposition rates. Norby and Cotrufo (1998) derived their conclusions from studies with lit-terbags. Unfortunately, using litterbags to measure the overall eect of elevated CO2 on decomposition pro-cesses, the amount and fate of (labile) carbon leaching into the soil matrix are neglected. Litterbags contain-ing 14C-labelled plant material could provide a power-ful tool to examine this artefact.

Raising the incubation temperature by 28C, the pre-dicted increase in temperature when atmospheric CO2 rises from 350 to 700 ml lÿ1 (IPPC, 1995), increased the decomposition of the 14C-labelled `700' roots from 36.2 to 38.6%. Still, this is lower than the decompo-sition of roots in the 350R soil (42.2%) indicating that the retarded decomposition of 700R could not fully be compensated by a 28C higher temperature. In a short-term experiment, we observed that a slower decompo-sition of grass roots grown at elevated CO2 (ÿ24% compared with ambient CO2) even counteracted the increased decomposition at a 68C higher temperature (Gorissen et al., 1995).

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con-serving eect on soil organic matter decomposition. Poorter et al. (1997) pointed out that leaves of 27 C3 -species, grown at elevated CO2, contained about 50% more nonstructural carbohydrates, while Wong (1990) found double the amount of nonstructural carbo-hydrates in `elevated CO2' roots of cotton plants. It is conceivable that this energy-rich plant-derived carbon may have a conserving eect on native-soil organic carbon decomposition, since it has been hypothesised that microorganisms may prefer the more energy-rich carbon to native-soil organic carbon (Lekkerkerk et al., 1990; Van Veen et al., 1993). This preferential sub-strate use (and thus less native-soil organic carbon res-piration) can incorrectly be considered as retarded respiration of plant material grown at elevated CO2. On the other hand, energy-rich plant-derived carbon incubated in a rather nutrient-poor soil may have a priming eect on soil organic carbon decomposition to satisfy the nutrient demand of microorganisms, result-ing in extra CO2 release. Summarising, carbon bal-ances in such studies may be inconclusive with respect to decomposition of plant litter. Therefore, it seems inevitable to work with homogeneously 14C-labeled plant material for studies concerning biodegradability in soils; this also allows comparative monitoring of native-soil organic carbon decomposition.

5. Conclusions

The present experiment supports other studies show-ing that decomposition of grass roots might be retarded when grown under elevated CO2(Gorissen et al., 1995; Jongen et al., 1995; Van Ginkel et al., 1996; Van Ginkel and Gorissen, 1998). The total response of our root/soil system of L. perenne towards elevated CO2 (more carbon input, no altered microbial meta-bolic behaviour and retarded decomposition of roots and root-derived material) is most interesting in the debate whether soils may act as a sink for atmospheric CO2. Whatever the reason for altered below-ground processes, these processes will always be driven by plant carbon inputs and mediated by soil isms. Until now, the question whether soil microorgan-ism behaviour is aected by elevated CO2 has been underexposed. This is why we have tried to elucidate their role in carbon decomposition in a changing cli-mate. After all, microorganisms are the most import-ant precursors in any altered soil organic carbon build-up.

Tans et al. (1990) suggested that ecosystems (e.g. temperate grasslands) in the Northern Hemisphere could possibly function as a sink for missing carbon in the global carbon budget. Models describing the re-sponse of the soil carbon content of ecosystems to el-evated CO2 (Parton et al., 1995; Thornley and

Cannell, 1997) often suer from a lack of experimental data of elevated CO2-induced below-ground changes like increased carbon input (Newton et al., 1994; New-ton et al., 1995; Van Ginkel et al., 1997; Cotrufo and Gorissen, 1997; Schapendonk et al., 1997) and retarded decomposition of roots (Gorissen et al., 1995; Van Ginkel et al., 1996). Thornley and Cannell (1997) argued that experiments should try to lessen uncer-tainty about processes within models rather than try to predict ecosystem responses directly. This point of view seems realistic and a good guideline for future ex-perimental research.

Acknowledgements

The authors thank J.A. van Veen for critically read-ing the manuscript and useful suggestions.

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