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Short Communication

Elevated atmospheric [CO

2

] from a natural soda spring aects

the initial mineralization rates of naturally senesced C3 and C4

leaf litter

A. Sowerby

a

, A.S. Ball

a,

*, T.R.G. Gray

a

, P.C.D. Newton

b

, H. Clark

b

a

Department of Biological Sciences, John Tabor Laboratories, Essex University, Wivenhoe Park, Colchester, Essex, CO4 3SQ, UK

b

AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand

Abstract

Naturally senesced leaves from the grasses Holcus lanatus (Yorkshire fog; C3 photosynthetic pathway) and Pennisetum clandestirum (Kikuyu; C4 photosynthetic pathway), grown in elevated atmospheric [CO2] surrounding a natural soda spring,

were decomposed within respiratory chambers and the initial mineralization (0±30 days) of the litter measured. Signi®cantly greater rates of initial mineralization of the C3 litters previously grown in elevated atmospheric [CO2] were detected compared to

the C3 litters grown in ambient [CO2]. For example, within the ®rst 15 days the rate of CO2production from the litters grown

in the elevated atmospheric [CO2] was two to ®ve times greater than that seen from the litters grown in the ambient

concentrations of CO2.

The eect of elevated atmospheric [CO2] on the mineralization of the C4 litter was less pronounced, however rates of

mineralization were still signi®cantly greater from litters grown originally in elevated [CO2]. A signi®cant decline in the C:N

ratios and nitrogen content of the leaves of both the C3 and C4 grasses was observed, although analysing the relationship between the carbon and nitrogen content, or the C:N ratio, with the initial mineralization rate indicated that these litter quality parameters were not good indicators of the rate of mineralization of the litter from these grasses.72000 Elsevier Science Ltd. All rights reserved.

Keywords:C mineralization; CO2soda spring; Elevated CO2; C3/C4 photosynthetic pathway; Senesced litter

Previous work on the eects of elevated atmospheric CO2 concentrations on the rates of litter decompo-sition has been contradictory. For example Kemp et al. (1994) found no eect of elevated [CO2] on de-composition processes. Others have observed decreased rates of litter decomposition resulting from exposure of plants to elevated [CO2] (Cotrufo and Ineson, 1996; Ball and Drake, 1997). A few authors have even reported positive eects of elevated [CO2] on the de-composition of litter from certain species or at certain times through the decomposition process (Couteaux et

al., 1991; Taylor and Ball, 1994). This variability may re¯ect dierences in plant species responses to CO2, which are manifested through dierences in the chemi-cal composition of the litter and through subsequent rates of decomposition. Nitrogen content or the C:N ratio are frequently cited as key factors regulating the rate of litter decomposition (Mellilo et al., 1982; Tay-lor et al., 1989; Cotrufo et al., 1994). A plant species photosynthetic type (i.e. C3/C4 photosynthetic path-way) is also thought to aect the response to elevated [CO2], with C4 species responding to a lesser extent than C3 species, as the rate of photosynthesis within their tissues is generally independent and not limited by the external concentration of CO2 in the atmos-phere (Ball and Drake, 1997; Cotrufo et al., 1998).

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* Corresponding author. Tel.: 873332; fax: +44-1206-873416.

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Our objectives with this work were (1) to assess whether elevated [CO2] aected the basic litter quality of senesced litter from an area with naturally elevated levels of atmospheric [CO2], (2) to see if the subsequent initial mineralization of these litters was aected, and (3) to compare the responses, in terms of litter quality and subsequent decomposition, of a C3 and C4 grass species when grown in elevated [CO2].

Hakanoa soda spring, Northland, New Zealand (located at 35₈ 40_'S and 174₈ 16_'E), is a cold CO2 spring, where levels of atmospheric [CO2] are naturally raised through emission of CO2 from cold springs (Newton et al. 1996). Atmospheric levels of CO2 are enriched through both drift from the vents and from evolution of CO2 from the soil (Newton et al., 1996). The soil at the site is gley (Fluvaquent); further site details can be found in Newton et al., (1996). Two grasses, Yorkshire fog (Holcus lanatus; C3 photosyn-thetic pathway) and Kikuyu (Pennisetum clandestirum; C4 photosynthetic pathway), were selected from the vegetation growing at the site. Atmospheric CO2 con-centration was measured on ®ve occasions over the period of 1995±8 at the Hakanoa site (approx. 20 cm above ground level) with a portable photosynthesis system (LCA-3 ADC, Hoddesdon, Herts, UK). In ad-dition, the N content of plant leaves taken along the CO2 gradient at the spring was compared with the N content of leaves taken from a CO2 response surface experiment in a controlled environment room. There was no dierence between the slopes of the lines relat-ing CO2 concentration to plant N content, indicating that the CO2concentrations we measured at the spring represented biologically meaningful estimates of the at-mospheric CO2 concentration experienced by the plants (Ross et al., 2000). Five areas were selected according to their average atmospheric CO2 concen-trations, one site was selected at a distance from the vent so as to be unaected by the spring and therefore had ambient levels of atmospheric [CO2] (38 Pa, stan-dard deviation of readings 6 Pa). Yorkshire fog (H. lanatus) was sampled at two further sites surrounding the CO2 spring, elevated [CO2] site 1 (mean CO2 con-centration 67 Pa CO2, standard deviation 14 Pa) and elevated [CO2] site 2 (mean CO2concentration 112 Pa CO2, standard deviation 82 Pa). Two sites were also selected for Kikuyu (P. clandestirum), elevated [CO2] site 1 (mean CO2 concentration 57 Pa, standard devi-ation 18 Pa) and elevated [CO2] site 2 (mean CO2 con-centration 111 Pa, standard deviation 38 Pa).

Naturally senesced litter from the C3 grass, York-shire fog (H. lanatus) and the C4 grass Kikuyu (P. clandestirum) was collected from each of the sites during March 1996, and dried. Seived soil (2 mm mesh; 100 g) from Wivenhoe Park, Colchester, Essex (sandy silt loam, 9% soil moisture w/w), was placed in respiratory chambers and connected to an infra red

gas analyser (IRGA; ADC 225). For 30 s every hour, air from each of the chambers was measured for CO2 concentration. The soil was left to equilibrate until levels of soil respiration were consistent (four days), then background soil respiration was measured for a further 24 h. Litter from Hakanoa was then chopped into approximately 1 cm sections and mixed in to the soil in chambers. Soil moisture levels were maintained at 9% w/w, throughout the duration of the exper-iment. Each hour CO2 production within the soil res-piratory chambers was calculated, and values were meaned for each 24 h period over 30 days. Elemental analysis (carbon and nitrogen concentrations and the C:N ratio (% of dry mass)) of the litter was measured using an automated CHNS\O analyser.

The most important aspect of the results obtained from the mineralization of the dierent litters was the comparative dierence between the respiration rates of the soils inoculated with the same species litter, grown under dierent concentrations of CO2. This variation of soil respiratory activity can be seen clearly when the data is transformed to give cumulative CO2production from the soil (Fig. 1). However, for both the C3 and C4 litter, the amount of CO2produced did not corre-late with the atmospheric CO2 concentration in which the plant was originally grown. In each case, the great-est amount of CO2 was produced from the litter grown at the elevated [CO2] site 1. The litter grown in elevated [CO2] site 2 also produced greater mineraliz-ation rates over that of the ambient litter (Fig. 1). Analying the dierences between the mineralization of each of the litters showed signi®cant dierences between all sites for both litter types (Fig. 1). The eect of the increasing levels of CO2on the mineraliz-ation rates of the C4 litter was less pronounced (Fig. 1), however, still statistically signi®cant.

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1998), yet a dierence was still seen in the nitrogen concentration and C:N ratio of the senesced leaves from the C4 species grown near the CO2 spring, com-pared to those originally grown in ambient [CO2]. As the litter is derived from a C4 plant, a down regulation of the rubisco content (as seen in many C3 species) cannot be responsible for the decrease in tissue nitro-gen content. Perhaps the long-term input of litter with a lower nitrogen content has altered the nitrogen avail-ability in the soil at the CO2spring, resulting in a

gen-eral (or increased) limitation of nitrogen in the plant community. Further experimental work would be needed to con®rm this hypothesis.

Much of the research to date has used arti®cial methods of raising levels of atmospheric [CO2], such as greenhouses. These have often only been in operation over a relatively short time scale, and often involve a sudden step increase in atmospheric [CO2] to the com-munity. This may represent an acclimation to the CO2, rather than indicating how the community would

Fig. 1. Cumulative mean daily CO2production from soil inoculated with litter from (A) Yorkshire fog (C3 photosynthetic type), (B) Kikuyu (C4

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behave once equilibrated to the elevated [CO2] (van de Geijn and van Veen, 1993). Curtis et al. (1994) esti-mated that up to 10 years of exposure to elevated [CO2] may be needed before any eects on community structure or biochemical properties can be seen in soil. Hakanoa soda spring has been emitting CO2 for dec-ades and so represents a community fully equilibrated to elevated levels of atmospheric CO2. O'Neill and Norby (1996) stated that increases in C:N found in some research were a result of limitations of the exper-imental procedures, i.e. restricted pot size, and that increased C:N ratios were not realised in the ®eld. In our work, where no experimental procedures could

have aected the chemical properties of the litter, the increase in C:N ratio under elevated [CO2] was still seen, potentially with even more consequence, as the increase was still seen even though the entire commu-nity has fully acclimated to the levels of elevated [CO2].

Our results show that the CO2 emitted from the spring at Hakanoa is aecting both the basic litter quality and initial degradability of both C3 and C4 grass species. This potentially could aect the sub-sequent decomposition rates of the litter. This is despite the long time period that the community has been exposed to elevated atmospheric [CO2], and

Fig. 2. Elemental analysis of the litters grown at Hakanoa soda spring.a±b_{show dierences between values at the 5% signi®cance level (Unpaired}

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suggests that any dierences seen in quality and sub-sequent decomposition of these litters may be perma-nent and not in fact a short term acclimation response to the elevated [CO2].

Acknowledgements

The award of a NERC studentship to Alwyn Sowerby is gratefully acknowledged. Many thanks also to Dr. Debbie Lloyd for help with the statistics.

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