Decomposition and nutrient dynamics of Quercus pubescens leaf litter in a naturally enriched CO2 Mediterranean ecosystem

Introduction

The atmospheric concentration of CO2 is currentlyincreasing largely owing to anthropogenic emissions,and this is expected to continue throughout the nextcentury, with important implications for terrestrialecosystems (IPCC 1995). In particular, elevated CO2

has been reported to affect the chemical compositionof plant material, inducing a reduction in the nitrogen(N) concentration of plant tissues (Cotrufo, Ineson &Scott 1998) and an increase in lignin and other

polyphenolic compounds (Cipollini, Drake &Whigham 1993; Cotrufo, Ineson & Rowland 1994;Cotrufo & Ineson 1996). However, available data onchanges in chemical composition of plant tissues inresponse to elevated CO2 are mostly from green tis-sues and they may not lead to similar changes in thechemical composition of plant litter.

Nutrient mineralization during decomposition rep-resents an important source of nutrients for primaryproduction in terrestrial ecosystems and the rate and

FunctionalEcology 199913,343–351

© 1999 BritishEcological Society

ORIGINAL ARTICLE OA 000 EN

Decomposition and nutrient dynamics of Quercuspubescensleaf litter in a naturally enriched CO2

Mediterranean ecosystem

M. F. COTRUFO,* A. RASCHI,† M. LANINI† and P. INESON‡*Dipartimento di Scienze Ambientali, II Universitá di Napoli, via Arena 22, 81100 Caserta and †CNR-I.A.T.A.,Piazz.le Delle Cascine 18, 50144, Firenze, Italy, and ‡Institute of Terrestrial Ecology, Merlewood ResearchStation, Grange-over-Sands LA11 6JU, UK

Summary

1. The chemical composition (i.e. N, P, C, lignin and polyphenol concentrations) ofQuercus pubescensleaf litter derived from a natural CO2 spring in Tuscany (Italy) wasanalysed and compared to litter from a nearby reference site. Litter was incubated for25 months at both the natural CO2 spring and the reference site, and monitored fordecomposition rates, nutrient and lignin concentrations.2. Long-term exposure to elevated CO2 concentrations from the natural spring wasassociated with a change in the chemical composition of the Oak leaf litter, withdecreases in P and polyphenol concentrations and increases in lignin. No differences inN concentrations were observed between the enriched CO2 litter from the naturalspring and the reference litter.3. Decomposition was reduced in the CO2 spring, with the lower P concentration ofthe native litter, combined with the lack of soil fauna observed at that site, being thefactors most probably responsible for the measured decreases in mass loss. However,litter from the CO2 spring and reference litter decomposed at the reference site showedsimilar rates of decomposition.4. All litter showed similar N concentrations during decomposition, with N beingmineralized throughout the incubation period from both litter regardless of the site ofincubation. In contrast, P dynamics differed between litter, with P being immobilizedin the litter derived from the spring, and mineralized from the reference litter. Whenthe litter from the spring was incubated at the reference site, there was a trend for net Puptake from the surrounding environment. The chemical composition of decomposinglitter from the spring appeared to match that of the reference litter after 3 months ofincubation at the reference site.5. The results from the CO2 spring suggest that litter decomposition may be retardedunder elevated levels of atmospheric CO2. However, results from field surveys aroundCO2 vents should be viewed with caution because differences may relate to factorsother than the known differences in CO2 concentrations.

Key-words: CO2 spring, lignin, litter decomposition, phosphorus, nitrogen

Functional Ecology(1999)13,343–351

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dynamics of nutrient release during decompositionhas been shown to be correlated with the initial chem-ical composition of the decomposing litter (Berg1986; Stump & Binkley 1993). Decomposition ofplant material is largely mediated by fungi and bacte-ria, which usually have lower carbon-to-nitrogen(C/N) and carbon-to-phosphorus (C/P) ratios than thelitter on which they grow. These micro-organismstherefore have high requirements for these nutrients,and detritus with high concentrations of N and phos-phorus (P) will decompose faster than detritus withlow N and P concentrations, because of the associatedfaster growth of decomposer populations (Enrìquez,Duarte & Sand-Jensen 1993).

The C/N ratio of plant litter has frequently been neg-atively correlated with litter decomposition rates(Floate 1970; Edmonds 1980; Taylor, Parkinson &Parsons 1989). However, Fog (1988) demonstrated thatsuch correlations cannot be generalized, there being astrong distinction between different resource types.Initial litter N content is an important rate-regulatingfactor in the first stages of decomposition (Cotrufo,Ineson & Roberts 1995), whilst, in later stages, ligninconcentration becomes a better predictor of decomposi-tion rates (Berg 1984). Many studies have shown sig-nificant negative correlations between litter mass lossand lignin concentrations (Blair 1988; Tian, Kang &Brussaard 1992; van Vuuren, Berendse & De Visser1993), but lignin-to-N (lignin/N) ratio appears to be theparameter which best predicts litter mass-loss rates(Melillo, Aber & Muratore 1982; Camiré, Côté &Brulotte 1991; Stump & Binkley 1993).

The few studies which have investigated the effectsof elevated CO2 on decomposition of hardwood leaf lit-ter in the field have demonstrated that when elevatedCO2 results in lower litter quality (sensu litter chemicalcomposition), this leads to a decrease in litter decompo-sition rates (Cotrufo & Ineson 1996; Cotrufo, Briones& Ineson 1998). In contrast, when elevated CO2 has notaffected litter quality, changes in litter decompositionrates have not been found, with litter raised at high CO2

decomposing at a similar rate to that of control litter(O’Neil & Norby 1996; Raiesi Gahrooee 1998).Moreover, there are also studies on crops in which ele-vated CO2 appeared to decrease litter quality, but with-out subsequent reductions in litter decomposition rates(Torbert, Prior & Rogers 1995; Henning et al. 1996).However, most of the decomposition studies to datehave been carried out at ambient CO2 levels and nutri-ent or lignin dynamics during decomposition haverarely been followed, with the exception of the study ofO’Neil & Norby (1996), who monitored N dynamicsduring decomposition of Liriodendron tulipifera leaflitter grown at ambient and elevated CO2.

Studies of the effects of elevated atmospheric CO2

concentration on trees have mainly been performed asshort-term exposures of small plants to elevated CO2

regimes in chambers (see Ceulemans & Mousseau1994) with results from these studies being extrapolated

to mature trees. Natural CO2 springs, in addition tobeing unique environments of intrinsic ecologicalinterest, offer the opportunity for studying long-termresponses of entire communities and, in particular,mature trees to enriched CO2 (Miglietta, Raschi & vanGardingen 1997).

Central Italy is a volcanic area with several naturalCO2 springs which have been identified as potentiallyuseful for studies into the effects of elevated CO2

(Miglietta et al. 1993). Of these mineral vents,Bossoleto has been one of the most widely exploitedfor CO2 enrichment studies (Körner & Miglietta1994; Johnson, Michelozzi & Tognetti 1997; vanGardingen et al. 1997), and this location was chosenfor a study of the decomposition of tree leaf litter gen-erated under a natural CO2 enriched atmosphere.

The aim of the study was to determine if tree leaflitter generated in a natural CO2 spring had a differentchemical composition to tree leaf litter, of the samespecies, derived from a comparable and nearby smallwoodland exposed to ambient levels of CO2 (refer-ence site), and to monitor the subsequent decomposi-tion of these litters. Additionally, the study examinedthe effect of the incubation in the natural CO2 springon decomposition of a standard litter. Quercuspubescens (Mill.) leaf litter derived from theBossoleto spring was incubated both inside theBossoleto spring and at the nearby reference site.Rates of mass loss and of N, P and lignin dynamicsduring decomposition were investigated.

Materials and methods

SITE DESCRIPTION

Bossoleto (Rapolano Terme, Siena, Italy) is a naturalCO2 spring within a circular depression 20 m deepand 80 m broad, formed after the collapse of an under-ground cavern several hundred years ago. At the bot-tom of the depression CO2 concentration variesthroughout the day and according to wind speed. Aclear CO2 concentration gradient was detectable fromthe bottom of the depression, and on its flanks (I.Bettarini, unpublished data). During the night, understable atmospheric conditions, CO2 accumulates andconcentrations build up to a maximum of around 75%in early morning. During the day, as direct sunlight isincreasingly incident on the area, turbulence removesthe gas, leading to concentrations of about 0·1% in themid-afternoon (van Gardingen et al. 1995). The ventCO2 is totally depleted of 14C (van Gardingen et al.1995) and 14C abundance in plant tissues has beenused to measure the effective CO2 concentration atwhich the plants have been living. Analyses performedon Phragmites australis growing at the same height asQ. pubescenson the depression flanks showed an aver-age CO2 concentration of 734 p.p.m. (van Gardingenet al. 1995). The emitted gas, in which the CO2 con-centration is above 99%, is slightly contaminated with

344M. F. Cotrufoet al.

© 1999 BritishEcological Society,Functional Ecology,13, 343–351

hydrogen sulphide (H2S). The concentration of H2Sin the proximity of the main emission point at thebottom of the depression ranges from c. 5 p.p.b. dur-ing daylight hours, increasing during the night andreaching highest concentrations in the early morning(22 p.p.b.). Some SO2 pollution has also beenrecorded at the bottom of the depression but not on theflanks (H. Rennenberg, personal communication).

More than 70 plant species, from 39 families, con-stitute the natural flora of the Bossoleto spring, withhemi-cryptophytes dominating whilst half of the basinis forested, mainly with Oaks (Quercus ilex and Q.pubescens). The Bossoleto CO2 spring is protectedalong the entire external perimeter by ancient walls,which have prevented human disturbance to the nativevegetation, and excluded herbivores (Körner &Miglietta 1994).

The climate at the site is typical Mediterranean,with air temperatures during the period of study rang-ing from – 9 °C in winter to 35 °C in summer.

LEAF LITTER COLLECTION

Senescent leaves were collected as they fell from Q.pubescenstrees in November 1993, at two locations:Bossoleto and Poggio Santa Cecilia, a small decidu-ous woodland nearby, which was used as a referencesite. One of the main difficulties in using naturalsprings for CO2 enrichment studies is the identifica-tion of a suitable reference site. Poggio Santa Ceciliahad been used previously as reference site for otherstudies performed at Bossoleto, having a comparableclimate and vegetation structure (Chaves et al. 1995;van Gardingen et al. 1997; Michelozzi et al. 1997).

Three Oak trees of comparable age (between 25 and30 years) and structure were sampled at each site froman area of c.50 m2. Leaves were collected by trappingfalling leaves from the entire tree canopy in nylon netsplaced around the perimeter of the sampled trees. ForBossoleto, CO2 concentrations at the level whereleaves were collected were measured by a portableinfrared gas analyser (IRGA, PPSystem EGM-1) andranged between 500 and 800 p.p.m. After collection,leaf litter was air-dried and stored until used for thedecomposition study and chemical analyses.

DECOMPOSITION STUDY

Decomposition of Oak leaf litter derived from theBossoleto spring and from Poggio Santa Cecilia wasstudied in the field using the litterbag technique(Bocock & Gilbert 1957). Bags (12 cm× 18 cm) weremade of a PVC coated fibreglass net (Simpkin Machin& Co. Ltd, Sheffield, UK), with a mesh size of 2 mm.A air-dried litter (2·0 g), with petioles removed, wasplaced in each bag, and identified by a Dymotapelabel. At the time of bag preparation, litter water con-tent was determined for each of the two litter types onthree litter subsamples by drying in an oven (80 °C).

A two-letter nomenclature is used to describe treat-ments, with the letters ‘A’ and ‘E’ representing ambi-ent (reference site) and elevated CO2 (Bossoleto site),respectively. The first letter of the nomenclature givesthe source of the litter, whilst the second letter identi-fies the site where the litter was decomposed; hence,for example, ‘EA’ denotes litter grown at theBossoleto site and decomposed in the reference site.

On 20 December 1993 the litterbags were placedin the field. Sixty bags, half of which contained thelitter derived from the Bossoleto spring, and theother half containing litter from Poggio SantaCecilia, were incubated in the reference site (EA andAA, respectively). Thirty litterbags, all containinglitter derived from the spring, were incubated atBossoleto (EE), underneath the sampled Oak trees.At both sites, bags were placed using random co-ordinates in the litter layer, and fastened to theground using metal pegs.

During the decomposition experiment (25 months)six samplings were made, with five replicate bagsbeing collected on each occasion. Once collected, bagswere immediately returned to the laboratory, where lit-ter was carefully cleaned of soil and dried in an oven(80 °C) for determination of mass remaining. Afterweighing, dried litter was milled and stored separatelyfor each litterbag until used for chemical analyses.

CHEMICAL ANALYSES

Chemical composition was determined on the initialmaterial, using three milled subsamples for each of thetwo litter types, and for each litter-bag retrieved fromthe field at the first, third and last sampling. Analyseswere performed at a EU-licensed chemical laboratory(Neotron S.r.l., Modena, Italy) using conventionalmethods (Allen 1989): C was determined by theSpringeer–Klee method, N using the Kjeldahl method,P was determined spectrophotometrically usingammonium molybdate, after acid digestion. Ligninwas determined gravimetrically, after acid digestion(72% H2SO4) and corrected for ash content followingthe Van Soest method. Polyphenols were extractedwith acid methanol and determined spectrophotometri-cally using the Folin–Ciocolteau reagent.

DATA ANALYSIS

Differences in initial chemical composition betweenthe Bossoleto and the reference litters were analysedusing Student’s t-test. One-way ANOVA was used totest for significant differences in mass remaining andchemical composition between EE, EA and AA litterat each sampling, and within each litter type acrosssamplings. Significant differences are reported atP < 0·05. The a posterioriStudent–Newman–Keulstest was applied to identify different litter types.

Decomposition rate constants were calculated afterOlson’s (1963) modelk = ln (X0/Xt)/t – t0, where X0 is

345Decompositionin a natural CO2

spring


the initial (time t0) litter dry mass and Xt is the litterdry mass at time t. Additionally, turnover time (1/K)was calculated for each litter type. Total amounts of N,P and lignin in the decomposing litter were calculatedby multiplying measured concentrations (%) with litterdry mass, for each litterbag, and sampling occasion.

Results

LEAF LITTER QUALITY

The chemical composition of the Oak leaf litterderived from the CO2 spring and the reference site arereported in Table 1. The two litter types showed simi-lar N concentrations, which were within the normalrange for Oak (Rodin & Bazilevich 1967), andshowed no signs of N deficiency. Both litters had thesame C concentrations, but the Bossoleto leaf litterhad significantly lower P concentrations than the ref-erence litter (P < 0·001). Litter derived from thespring also had higher concentrations of lignin, whichresulted in significantly higher values of lignin/N ratiothan the reference litter. In contrast, the concentration

of total polyphenols was significantly higher in thereference litter than in the Bossoleto litter.

LITTER DECOMPOSITION

After 25 months of field exposure, litter still retainedaround 50% of its original mass, with the litterderived from Bossoleto and incubated inside thespring (EE) showing the highest value of massremaining (55%), when compared to both the littersincubated in the reference site (EA and AA), 51% and47%, respectively (Fig. 1).

The EE litter had lower decomposition rates thanthe EA litter throughout the entire experimentalperiod except between July 1995 and February 1996,with this difference being statistically significant onmost occasions (Fig. 1), showing that litter decayrates were slower when litter was decomposed insidethe CO2 spring. No significant difference in massloss rates was observed between the EA and the AAlitters, and only during the second year of field incu-bation did the litter derived from the CO2 spring



Table 1. Initial chemical composition of Oak leaf litter. Values are means, n = 3, with SE in parentheses. Significance levelsafter Student’s t are given

Sample N (%) P (%) C (%) Lignin (%) Polyphenols (%) C/N Lignin/N

Bossoleto (1·42 (0·068 (47·0 (26·77 (1·50 (33·17 (18·9(0·07) (0·008) (0·47) (1·47) (0·043) (2·69) (1·8)

Reference site (1·38 (0·169 (47·5 (19·43 (7·13 (34·48 (14·11(0·07) (0·013) (0·37) (0·49) (0·162) (1·77) (0·95)NS P< 0·02 NS P< 0·001 NS P< 0·001 P< 0·0001

Fig. 1. Field decomposition rates for Oak leaf litter derived and incubated at the Bossoleto spring (EE), derived fromBossoleto and incubated at the reference site (EA), derived and incubated at the reference site (AA). Vertical bars indicate SE;n = 5. Significance levels after ANOVA are given: ****P< 0·0001; **P< 0·01.

appear to show a reduced, although not statisticallysignificant, rate of decay (Fig. 1).

When litter decay rates were calculated afterOlson’s (1963) model, the AA litter showed the high-est decomposition rate constant (k) across the entireincubation period, with a turnover time of 1·4 years,which was faster than the litter from the spring (EAand EE, 1·6 and 1·8 years, respectively; Table 2).

NUTRIENT AND LIGNIN DYNAMICS

Nitrogen concentrations in the three litter typesshowed similar values throughout the incubationperiod, and only on one occasion did the decomposingEE litter have a significantly (P < 0·05) lower N con-centration than the AA litter (Fig. 2a). The three littertypes appeared to have similar N dynamics duringdecomposition, with litter N concentrations mostlyincreasing with time (Fig. 2a). The observed increasein litter N concentration did not correspond to net Nimmobilization. In fact, total N content in all littertypes significantly decreased during decomposition,resulting in a net N mineralization from all the decom-posing litter (Fig. 2b).

Phosphorus concentrations in the EE litter were sig-nificantly lower than in the EA and AA litters for mostof the incubation period, but showed similar values atthe last sampling (Fig. 3a). Phosphorus dynamics dur-ing decomposition differed significantly between thelitter treatments and, in the first 3 months of incuba-tion, the AA litter which showed an initial P concentra-tion more than two times higher than that of the CO2

spring litter released almost 40% of the initial P,whereas the EE and EA litters increased their initial Pconcentration by 10% and 36%, respectively (Fig. 3a).Thereafter, P concentrations in the EE and EA litterscontinued to increase, but at a lower rate and, at the endof the experiment, P concentrations had increased by54% and 76%, respectively (Fig. 3a). However, whentotal P contents are calculated, a significant P mineral-ization was observed for the AA litter, whereas nostatistically significant changes were reported for Pcontent in the EA and EE litters (Fig. 3b).

Only at the start of the experiment did lignin con-centrations significantly differ between the litter types(Table 1, Fig. 4a). During the first 3 months of decom-position, the AA litter significantly increased in ligninconcentration by 40%, thereafter lignin concentra-tions remain unchanged in all litter types (Fig. 4a). Atthe start of the experiment the Bossoleto litter had asignificantly higher lignin content than the referencelitter (Fig. 4b), but this difference disappeared whenthe Bossoleto litter was incubated at the reference siteand, by the end of the experiment, only the EE litterstatistically differed in lignin content from the AA lit-ter (Fig. 4b). The amount of lignin in the EE and EAlitters significantly diminished throughout the incuba-tion time, whereas in the AA litter lignin started to bedegraded after the third month (Fig. 4b).

Discussion

In the present study, the long-term exposure to naturallyenriched CO2 atmosphere was associated with differ-ences in the concentration of some chemical compo-nents of Q. pubescensleaf litter (e.g. decreased P andpolyphenols and enhanced lignin concentrations).No differences were observed for N concentration.


spring

Table 2. Decomposition rate constants, calculated afterOlson’s (1963) model k = ln (X0 – Xt)/t – t0, and turnovertime (1/k) for Oak leaf litter derived and incubated at theBossoleto spring (EE), derived from Bossoleto and incu-bated at the reference site (EA), derived and incubated at thereference site (AA)

Sample k (year–1) Turnover time (1/k) (year–1)

EE 0·57 1·76EA 0·63 1·59AA 0·72 1·39

Fig. 2. Dynamics of (a) N concentration and (b) total N content during decomposi-tion of Oak leaf litter derived and incubated at the Bossoleto spring (EE), derivedfrom Bossoleto and incubated at the reference site (EA), derived and incubated atthe reference site (AA). Vertical bars indicate SE; n = 5. Significance levels afterANOVA are given: *P< 0·05.

Körner & Miglietta (1994) also found significantchanges in tissue quality among the 40 species studiedaround the Bossoleto spring. These authors found aconsistent and significant increase in total non-struc-tural carbohydrates (TNC) for all the species exam-ined, and Q. pubescensleaves inside the vent areacontained twice as much TNC as outside. Such anincrease in TNC was mainly attributed to starch accu-mulation. Leaf N concentration generally decreasedunder elevated CO2 but for Q. pubescens, the differ-ence in leaf N concentration between trees grown inand outside the spring area disappeared when calcu-lated on a TNC-free dry matter basis (Körner &Miglietta 1994). In a previous survey, Ineson &Cotrufo (1997) found a 19% lower N concentration in

Q. pubescensleaf litter from the Bossoleto springwhen compared to a reference litter, but this reduc-tion was not statistically significant. None of theseprevious studies examined P contents for plant mate-rial grown in the Bossoleto basin. According toConroy (1992) the critical foliar concentration of P ishigher at elevated CO2, resulting in increased plantdemand for P under a high CO2 regime. The lower Pvalues measured at the Bossoleto could be the resultof a dilution effect owing to higher carbohydrate con-tent in leaf litter (sensu Körner & Miglietta 1994), orof a lower P availability in the soil, when compared tothe reference site.

The higher lignin concentration in the Oak leaf lit-ter derived from the Bossoleto spring confirmed fre-quent observations that tree leaf litter generated underelevated CO2 has higher lignin concentration thancontrol litter grown at ambient CO2 levels (Cotrufoet al. 1994; Cotrufo & Ineson 1996). In contrast, theBossoleto litter showed lower concentrations of totalpolyphenols than the reference litter. A recent compi-lation of the few data available on the concentration ofcarbon-based secondary compounds in plant materialraised at high CO2 (Peñuelas, Estiarte & Llusià 1997)showed an overall average increase of 14% in greentissues at 700 p.p.m. CO2, but the response wasdependent on the nutritional status of plants. At low Navailability plants raised at elevated CO2 decreasedconcentrations of phenols in their green tissues(Peñuelas et al. 1997). Furthermore the work ofJulkunen-Tiitto, Tahvanainen & Silvola (1993) onSalix myrsinifolia highlighted that generalizationscannot readily be made, as plant responses to ele-vated CO2 differ between individual phenolic com-pounds. Unfortunately, in the present work wemeasured the bulked concentration across all pheno-lic hydroxyl groups and may have missed more sub-tle differences. Future work on the effects ofelevated CO2 on the concentration of phenols inplant tissues should distinguish between phenolics ofdifferent ecological significance.

There is also a very limited amount of publishedwork on the effect of elevated CO2 on litter decompo-sition performed in the field and, in all these studies,decomposition has been carried out under naturalatmospheric CO2 concentrations (Kemp et al. 1994;Boerner & Rebbeck 1995; Cotrufo & Ineson 1996;O’Neill & Norby 1996). The emerging result, in gen-eral, has been that hardwood leaf litter that exhibitedchanges in N or lignin concentrations when grownunder elevated CO2, showed reduced field decompo-sition rates (Boerner & Rebbeck 1995; Cotrufo &Ineson 1996). No such changes were observed ineither decomposition rates or N dynamics duringdecomposition when decomposing leaf litter had notshown significantly reduced initial N concentration asa response to CO2 treatment (O’Neill & Norby 1996).In the present study, the Bossoleto litter had a similarinitial N concentration to that of the reference litter,


Fig. 3. Dynamics of (a) P concentration and (b) total P content during decompositionof Oak leaf litter derived and incubated at the Bossoleto spring (EE), derived fromBossoleto and incubated at the reference site (EA), derived and incubated at the refer-ence site (AA). Vertical bars indicate SE, n = 5. Significance levels after ANOVA aregiven: *P< 0·05.

which may explain why the two litters showed similardecomposition rates, as well as similar N dynamicsduring decomposition, when incubated in the refer-ence site under ambient levels of atmospheric CO2.Neither the significant initial differences in lignin or Pconcentrations appeared to have an effect on litterdecomposition rates, and when both litters whereincubated in the reference site these chemical differ-ences disappeared after the first three months of fieldexposure. These results suggest that nutrients andlignin dynamics in decomposing litter may be con-trolled by site of incubation more than by the initialchemical composition of the litter.

Several authors (Ball 1997; Ineson & Cotrufo 1997;van De Geijn & van Veen 1993) have suggested that

elevated atmospheric CO2 is unlikely to exert anydirect effect on soil processes, because it is unlikely toinduce any direct increase in the CO2 concentration ofthe soil air. However, there is growing evidence thatelevated atmospheric CO2 will affect soil properties(i.e. water and C content), as well as the biomass andactivity of soil microbial communities, via effects onplants (Rice et al. 1994; Hungate et al. 1996; Inesonet al. 1996; Klironomos, Rillig & Allen 1996; Cotrufo& Gorissen 1997). Therefore to understand the effectsof elevated CO2 on decomposition processes better,the litter under investigation should, ideally, be incu-bated in the field in the presence of plants and underenriched atmospheric CO2 concentrations.

In the Bossoleto site, litter decomposition wasslower than at the reference site, particularly duringthe first stage of litter decomposition when the con-centration of nutrients is thought to be the drivingforce for decay rates (Staaf & Berg 1982). In a reviewpaper, Enrìquez et al. (1993) showed that, for hard-wood leaf litter, a significant positive correlationexists between litter P concentration and decomposi-tion rate. Indeed, P deficiency could have played animportant role in the slower rate of decomposition ofthe litter from the spring at the Bossoleto site whencompared to that at the reference site. In fact, at thereference site the P content in the Bossoleto litterslightly increased, although not statistically signifi-cantly, probably owing to incorporation of P from thesurrounding native litter, richer in P. At the Bossoleto,there was no evidence for any increase in P content inthe native litter, although the P concentrationincreased owing to immobilization of the initial P.Furthermore a very limited population of soil faunawas observed in the soil of the Bossoleto spring (C. R.Bridges, personal communication), and this lack ofsoil fauna may have contributed to the decreaseddecomposition rates observed in the present work.Three ways are envisaged through which elevatedCO2 can affect soil fauna, and those are via changes:(1) in soil physico–chemical properties; (2) in com-munity and activity of micro-organisms; (3) in litterpalatabily. Future work needs to test this hypothesis ifa better understanding of the effects of elevated CO2

on decomposition processes in terrestrial ecosystemshas to be achieved.

It is difficult to say whether the lower litter decom-position rate observed in the Bossoleto, when com-pared to the reference site, is solely a result of theincreased atmospheric CO2, and whether the presentdata can be used to predict future litter turnover ratesunder elevated CO2. Natural CO2 vents are uniquesites of intrinsic scientific interest. However, severaldifficulties are associated with performing experi-ments in the CO2 springs, with the lack of an identi-cal control site and high variability in nativevegetation being major drawbacks. Although thefuture of ecosystem CO2 enrichment studies clearlylies in Free Air CO2 Enrichment (FACE) systems


spring

Fig. 4. Dynamics of (a) lignin concentration and (b) total lignin content duringdecomposition of Oak leaf litter derived and incubated at the Bossoleto spring (EE),derived from Bossoleto and incubated at the reference site (EA), derived and incu-bated at the reference site (AA). Vertical bars indicate SE, n = 5. Significance levelsafter ANOVA are given: ***P< 0·001; *P< 0·05.

(Hendrey, Lewin & Nagy 1993), which offer the pos-sibility of exposing entire plant communities to ele-vated CO2, without modifying natural environmentalconditions, the natural CO2 vents offer a uniqueopportunity to investigate long-term changes andmature ecosystems.

Acknowledgement

The work was funded by the Consiglio Nazionaledelle Ricerche.

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Received 19 May 1998; revised 20 October 1998; accepted9 November 1998


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Documents

Decomposition and nutrient dynamics of Quercus pubescens leaf litter in a naturally enriched CO2 Mediterranean ecosystem