Bot. Bull. Acad. Sin. (1997) 38: 45_48

Kao M. transmorrisonensis in a mountain grassland

Contribution of Miscanthus transmorrisonensis to soil organic carbon in a mountain grassland: Estimated from stable carbon isotope ratio

Wen-Yuan Kao1

Institute of Botany, Academia Sinica, Nankang, Taipei, Taiwan, Republic of China

(Received March 18, 1996; Accepted Novmeber 27, 1996)

Abstract. To estimate the contribution of Miscanthus transmorrisonensis to soil organic matter in a high mountain grassland at Tartarchia Anpu (altitude of 2,600 m) in Taiwan, I analyzed the stable carbon isotope ratios (d13C) of dominant plant species and soil organic matter. The d13C analysis reveals that among the five dominant plant species M. transmorrisonensis (d13C of -12.37) is the only species that uses C4 photosynthetic pathway. In contrast, the d13C values of other plants species ranged from -29.21 to -23.3 (average of -26.25), typical values for terrestrial plants with C3 photosynthetic pathway. The average d 13C value of soil samples collected from ten locations over the grassland was -18.56 0.36 again indicating that the vegetation contains a mixture of C3 and C4 type plants. Using two-end member linear mixing models, I then estimated that about 55% of soil organic carbon in this grassland came from M. transmorrisonensis.

Keywords: C3 and C4 photosynthetic pathways; Grassland; Miscanthus transmorrisonensis; Soil organic carbon; Stable carbon isotopes.

Introduction

The concentration of greenhouse gasses, mainly carbon dioxide, methane, and nitrous oxide in the atmosphere has been increasing since the Industrial Revolution. As a consequence, the earth warms up and climates change (Houghton et al., 1990). It has been suggested that C4 plants evolved as an adaptation to low CO2 concentration in Late Tertiary (Ehleringer et al., 1991). The CO2 concentrating mechanism of C4 pathway confers a higher productivity, better water use efficiency, and greater temperature tolerance as compared with C3 plants, however, the advantages may be diminished at elevated CO2. Thus, concern about the potential threat posed to plants containing the C4 photosynthetic pathway by increasing atmospheric carbon dioxide has been raised (Henderson et al., 1994). To evaluate the potential effect of changes in climate on the distribution of C4 plants, it is hence essential to study the current distribution of C4 plants in the various ecosystems.

Stable carbon isotopes have been used in agricultural and ecological research for decades (Tieszen and Boutton, 1989). The two stable carbon isotopes are 12C and 13C, which comprise 98.89 and 1.11%, respectively, of all carbon in nature (Ehleringer and Rundel, 1989). Currently, the carbon isotope ratio (d13C) of atmospheric CO2 is about -7.5 (Keeling et al., 1984). Plants contain less 13C than the atmosphere because the physical and chemical pro

cesses involved in CO2 uptake discriminate against the isotope (Berry, 1989). During photosynthetic uptake of CO2, the carboxylation enzyme ribulose bisphosphate carboxylase discriminates against 13C more than phosphoenol pyruvate carboxylase (O'Leary, 1981; Roeske and O'Leary, 1984). As a consequence, C3 plants have less 13C than C4 plants. For example, d13C values of C3 plants vary from -23 to -34 , whereas C4 plants have values of approximately -10 to -15 (O'Leary, 1988). Therefore, 13C analysis has become an accepted method for determining the pathway of CO2 fixation of terrestrial plants.

In natural ecosystems, soil organic carbon is derived predominantly from the residues of native vegetation, hence the stable carbon isotope ratio of the vegetation has a direct impact on the d13C of the soil organic matter (d13Csom). It has been shown that the natural isotopic abundance of soil organic matter corresponds closely to that of the vegetation cover from which it originated (Nadelhoffer and Fry, 1988). Thus, the characteristic d13C of the vegetation serves as a marker to indicate the origin of soil organic matter (Balesdent et al., 1987; Rao et al., 1994; Kessel et al., 1994). For example, mass-balance calculations based on the d13C values of a geological sample and assumed end-member d13C values for C3 and C4 plants have been used to estimate the relative proportion of C3- and C4- derived carbon in a sample of organic carbon (Bird et al., 1994).

The objective of this study was to analyze the carbon isotopes ratio of soil organic carbon to estimate the relative contribution of C3 and C4 plants to soil organic carbon in a mountain grassland.

1Fax: 886-2-782-7954.


Botanical Bulletin of Academia Sinica, Vol. 38, 1997

Study Site

Tartarchia Anpu grassland (altitude of 2,600 m) is located in Yushan National Park (2329' N, 12048' E), Nantou county, in central Taiwan. It is classified as a mountain temperate grassland. The mean monthly air temperature ranges from 6C in December to 14C in July (Climatological data annual report, Central Weather Bureau, ROC). The mean annual precipitation is approximately 4,000 mm. A visual examination of the vegetation indicated that Miscanthus transmorrisonensis, Baeothryon subcapitatum (Thwaites) T. Koyama, Pinus taiwanensis Hayata, Yushania nittakayamensis, and Lycopodium cernuum Linn. are the five most dominant plant species of the vegetation composition.

Materials and Methods

Leaf materials were collected from five individuals of each of the five dominant species. Individuals of each species were sampled at least 1 m away from each other. Leaves were dried at 70C for at least 48 h and then ground to a fine powder using a mortar and pestle. Ten soil sampling points were chosen over the grassland, the distance between each sampling points was at least 1 m. To minimize the effect of progressive modification of isotopic composition of soil organic matter by microbial degradation (Nadelhoffer and Fry, 1988), at each sample point the top 5 cm of soil of a 0.02 m plot was collected. Soil samples were air-dried, after removal of coarse litter and roots, and sieved through a 0.5 mm mesh (Der Shuenn, Taiwan). The pH value of the soil was determined using a pH meter (Suntex TS-2, Taiwan) by mixing soil with distilled water in a proportion of 1 to 10. Total carbon contents were determined with an elemental analyzer (NA1500, Fisons, Italy).

Inorganic carbon was removed from soil samples prior to isotopic analysis by pre treatment with 1 M HCl at room temperature overnight, rinsed with distilled water, then oven dried for 12 h.

Two to 3 mg of dried plant material or 10 to 15 mg of soil sample were sealed with 1 g of copper oxide wire (Merck) and 1 piece of silver foil (2 mm 10 mm) under vacuum in a 6 mm quartz tube and heated to 850C for 4 h (Ehleringer and Osmond, 1989). The tubes were then allowed to cool slowly for 10 h. After combustion, the sealed tube contained CO2, H2O, and N2. The tube was then cracked under vacuum, and the gasses were separated by passing them through an ethanol-dry ice trap to remove H2O and a liquid nitrogen trap to collect CO2 (Ehleringer and Osmond, 1989). The carbon isotope ratio of the purified CO2 was determined on an isotope ratio mass spectrometer (SIRA10, VG Instruments, Oxford, UK). The carbon isotope ratio of the organic matter was expressed as

d13C () = [(Rsample/RPDB)- 1] 1000, where R = 13C/12C.

Thus, d13C is the difference in carbon isotope ratios between a sample (Rsample) and the PDB standard (RPDB), in

thousandths () of the isotope ratio in the standard, and PDB refers to the belemnite carbonate standard of the Peedee Formation, South Carolina, USA, which is accepted as the international standard. Measurements of organic standards were reproducible to 0.05.

The following equation was then used to calculate the proportion of the soil carbon from C4 plants:

d13Csom = d13CC4 f + d13CC3 (1-f) (eqn. 1)

where

d13Csom = carbon isotope ratio of the soil organic matter,

d13CC3 = carbon isotope ratio of C3 plants,

d13CC4 = carbon isotope ratio of C4 plants,

f = proportion of carbon from C4 plants, and

1-f = proportion of carbon from C3 plants.

Results and Discussion

d13C Values of Vegetation and Soil Organic Matter

Results of the d13C values of the five dominant species in the Tartarchia Anpu grassland are presented in Table 1. According to these values, the five dominant species can be divided into two groups. The d13C value of M. transmorrisonensis is within the range of typical C4 plants (d13C of -14 to -10 ) indicating that they use the C4 photosynthetic pathway. In contrast, L. cernuum, P. taiwanensis, Y. nittakayamensis, and B. subcapitatum have d13C values ranging from -29.21 to -23.33 , which are typical of plant species with the C3 photosynthetic pathway, indicating that these species are C3 plants.

The average pH value and total carbon content of ten soil samples were 4.5 and 8.0 0.7 (mean S.E. ), respectively. The d13C values of these samples range from -19.02 to -18.11 (Table 2). The d13C values of the soil samples were between that of C3 plants and C4 plants, implying that soil organic carbon of this grassland came from a mixture of C3 and C4 vegetation.

Estimate of Soil Carbon Derived from M. transmorrisonensis

The primary influence of d13C of soil organic matter is the relative contribution of C3 versus C4 plants to the total net primary productivity of the community under study. The distinct difference in d13C value of M. transmorrisonensis from other dominant plant species

Table 1. The d13C values (mean S. E., n=5) of the dominant species in the Tartarchia Anpu grassland.

Species d13CPDB ()

Miscanthus transmorrisonensis -12.37 0.14

Baeothryon subcapitatum (Thwaites)

T. Koyama -23.33 0.26

Pinus taiwanensis Hayata -26.03 0.22

Yushania nittakayamensis -26.41 0.31

Lycopodium cernuum Linn. -29.21 0.13


Kao M. transmorrisonensis in a mountain grassland

Table 2. d13C values (), total carbon content and pH of the soil organic matter and the calculated contribution of C4 plants (according to eqn.1 and the values of Table 1) in the Tartarchia Anpu grassland.

Location pH C (%) d13CPDB () % of C4

1 4.5 7.7 -18.23 57.8

2 4.3 12.2 -18.11 58.6

3 4.2 7.1 -18.07 58.9

4 4.4 9.3 -18.58 55.3

5 4.4 8.2 -19.02 52.1

6 4.5 9.4 -18.98 52.4

7 4.5 5.5 -18.78 53.8

8 4.6 6.2 -18.39 56.6

9 4.6 5.6 -18.94 52.7

10 4.5 9.1 -18.45 56.2

mean S.E. 4.5 0.0 8.0 0.7 -18.56 0.36 55.4 0.8

(Table 1) serves as a marker to trace the contribution of this species to soil organic carbon. Thus using the mass-balance equation (eqn. 1) and assuming two end-members of -12.37 (the average d13C value of M. transmorrisonensis) and -26.26 (the average d13C value of dominant C3 plants), it was estimated that M. transmorrisonensis contributes about 55% of the total soil organic carbon in this mountain grassland (Table 2).

In a steady-state system, d13C of the soil organic matter should be nearly identical to that of the plant community from which the organic matter was derived, unless the isotopic composition of plant tissue is altered during decomposition (Boutton, 1996). Direct measurements of five dominant plant species in a woodland indicate that d13C of plant tissue remains relatively constant during the early stages of decomposition (Boutton, 1996). Wedin et al. (1995) found small (0.6 to 1.5 ) shifts in the d13C of litter from four grass species. In addition, some indirect evidence also suggests that fractionation during decomposition is small (from < 1 to 2 ) especially in terrestrial ecosystems (Nadelhoffer and Fry, 1988; Boutton, 1996). The average d13C value of M. transmorrisonensis is about 14 higher than that of C3 plants. Therefore, even if there is discrimination during decomposition, the isotopic discrimination would be very small compared to the large d13C difference in C3 and C4 plants. Though fractionation during decomposition was not measured in this study, the estimate of the relative contribution of M. transmorrisonensis versus other C3 plants to soil organic carbon based on the analysis of carbon isotopic composition is still valid.

Chou et al. (1991) had previously studied the vegetation succession in the same grassland, finding that the coverage percentage of M. transmorrisonensis after a surface clearing varied in three measurements, from 3% to 22%. In addition, seasonal variation in contribution to primary production by C3 and C4 plants has been found in a mixed prairie (Ode et al., 1980). These results indicate that an estimate of the relative contribution of C3 versus C4 plants to primary production based on a one-time measurement of the vegetation coverage could result in an erroneous conclusion. In contrast, analysis of the d13C of the soil organic matter pool reflects the long-term integrated d13C of

the plant communities that contributed to the soil organic matter at a given site.

In conclusion, analysis of the stable carbon isotope ratio of soil organic carbon provides quantitative evidence of the relative contribution of C3 versus C4 plants to a community's net primary productivity.

Acknowledgments. The author is grateful to the following: The Yu-shan National Park Bureau for allowing the author access to the study site; Dr. Chung-Ho Wang for providing the mass spectrometer; Ms. Yu-Shiun Chiou and Mr. Kuo-Wei Chang for their assistance; and Dr. Chang-Huang Chou for the inspiration for this study. Drs. George R. Waller and H. H. Cheng, who provided constructive suggestions in improving the manuscript. This research was supported by the Academia Sinica, Taiwan, ROC.

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