Stable carbon isotope characteristics of different plant species and surface soil in arid regions

Abstract

The stable carbon isotope composition in surface soil organic matter (δ¹³C_soil) contains integrative information on the carbon isotope composition of the standing terrestrial plants (δ¹³C_leaf). In order to obtain valuable vegetation information from the δ¹³C of terrestrial sediment, it is necessary to understand the relationship between the δ¹³C value in modern surface soil and the standing vegetation. In this paper, we studied the δ¹³C value in modern surface soil organic matter and standing vegetation in arid areas in China, Australia and the United States. The isotopic discrepancy between δ¹³C_soil and δ¹³C_leaf of the standing dominant vegetation was examined in those different arid regions. The results show that the δ¹³C_soil values were consistently enriched compared to the δ¹³C_leaf. The δ¹³C_leaf values were positively correlated with δ¹³C_soil, which suggests that the interference of microorganisms and hydrophytes on the isotopic composition of surface soil organic matter during soil organic matter formation could be ignored in arid regions. The averaged discrepancy between δ¹³C_soil and δ¹³C_leaf is about 1.71‰ in Tamarix L. in the Tarim Basin in China, 1.50‰ in Eucalytus near Orange in Australia and 1.22‰ in Artemisia in Saratoga in the United States, which are different from the results of other studies. The results indicate that the discrepancies in the δ¹³C value between surface soil organic matter and standing vegetation were highly influenced by the differences in geophysical location and the dominant species of the studied ecosystems. We suggest that caution should be taken when organic matter δ¹³C in terrestrial sediment is used to extract paleovegetation information (C₃/C₄ vegetation composition), as the δ¹³C in soil organic matter is not only determined by the ratio of C₃/C₄ species, but also profoundly affected by climate change induced variation in the δ¹³C in dominant species.

Keywords

stable carbon isotope composition / surface soil / vegetation composition

Introduction

On account of the requirements for the quantification of paleoenvironmental information, more and more attention has been paid to study the environmental and ecological significance of stable carbon isotope composition in terrestrial sediments. Since typical characteristics in sediment can be used to obtain paleoenvironmental information, the stable carbon isotope of sediment has been widely used to study paleovegetational components. The stable carbon isotope (δ¹³C) in sediment is controlled by the carbon isotope composition of standing vegetation, while the carbon isotope characteristics of standing vegetation are determined by climate. Therefore, the carbon isotope composition in sediment can be used as an important approach to obtain information on previous standing vegetation and climate in paleoenvironmental change research (Cerling, 1984; Cerling and Hay, 1986; Cerling et al., 1989; Stanley and Nancy, 1991; Gu, 1991; Sukumar et al., 1993; Han et al., 1995; Chen et al., 1996; Wang et al., 1997; Wang and Follmer, 1998; Yang et al., 1999; Hong et al., 2001; Lin et al., 2001; He et al., 2002). Many researchers have tried to explore paleoclimatic and paleoenvironmental information through the relationship between foliar δ¹³C and environmental factors (Feng et al., 2000, 2003; Wang and Han, 2001; Wang et al., 2002). In order to validate the reliability of paleoecological information acquired from the sediment carbon isotope composition, considerable effort is required to clarify the relationship between the carbon isotope composition in modern standing vegetation and that of surface soil organic matter.

The stable carbon isotope composition in surface soil is the integrative information of stable carbon isotope composition in modern standing vegetation. Isotopic fractionations during the formation of soil organic matter are likely to occur because soil microorganisms preferentially remove isotopically lighter carbon, which causes surface soil organic matter to tend to be enriched in the ¹³C (Melillo et al., 1989; Balesdent et al.,1996; Connin et al., 2001). Those fractionations potentially lead to discrepancies in carbon isotope composition between the standing vegetation and surface soil (Stout et al., 1975; Stout and Rafter, 1978). The vegetation information induced from the δ¹³C in surface soil is likely different from that in the reality. Currently, we are still not clear about whether those discrepancies are systematical or not, and the variation range of this discrepancy has been debated. In order to obtain reliable vegetation information from the δ¹³C value of surface soil, the discrepancies between the δ¹³C value of modern standing vegetation and that of surface soil organic matter should be intensively studied.

In general, the variations in the δ¹³C value in sediment (such as loess) has been explained as changes in the relative abundance of the C₃ and C₄ species. One possible situation is that C₃ and C₄ species composition has not changed apparently, but the plant δ¹³C values have changed due to variations in the climatic and environmental conditions (Hatté et al., 1998, 1999, 2001; Rao et al., 2005, 2006). Therefore, investigations of the δ¹³C value of surface soil organic matter and that of similar vegetation grown under the different climatic conditions will help explain this situation reasonably.

In order to examine the relationship between the δ¹³C value of surface soil organic matter and that of modern standing vegetation in different climatic regions, we studied the carbon isotope composition of leaf and surface soil in the ecosystems dominated by Tamarix in China, Eucalytus in Australia and Artemisia in the United States. The objectives of this study were to obtain the relationship and the discrepancies between the δ¹³C value of modern standing vegetation and that of surface soil in arid ecosystems dominated by different species, and to validate the reliability of using the organic matter δ¹³C in sediment to obtain paleoenvironmental, paleoecological and standing vegetation information.

Materials and methods

Study sites

Three arid regions in China, Australia and the United States were selected for this study. The C₃ plants Tamarix, Eucalytus and Artemisia are the dominant species in ecosystems in the Tarim Basin in China (77°23′—88°45′ E and 36°45′—41°33′ N), Orange in Australia (148°12′—149°22′ E and 32°32′—33°26′ S) and Saratoga in the United States (106°15′—106°49′ W and 40°47′—41°24′ N), respectively. The contribution of the C₄ species to the total biomass in those three regions is very small compared to the C₃ species and can be ignored.

Materials

Surface soil and plant foliar samples were collected from the Tarim Basin in China in October 2005, Orange in Australia in August 2007 and Saratoga in the United States in August 2008. Plant foliar and corresponding surface soil samples were collected simultaneously at the locations with little anthropogenic and livestock influence. 56, 5 and 6 sampling sites were chosen in Tarim Basin in China, Orange in Australia and Saratoga in the United States, respectively. Soil at 2—3 cm below ground were collected and each soil sample was a pool of 4—5 individual soil samples.

Stable carbon isotope analysis

The plant foliar samples were rinsed and oven-dried at 70°C to constant weight and ground finely. The soil samples were sieved through a 120 mesh screen to remove roots and gravels. Excess amounts of 10% hydrochloric acid (HCl) was added to the samples for 24 hours at room temperature to remove all carbonates and other acid soluble organic material. The residues were washed with distilled water until neutral. The samples were oven-dried at 70°C for 24 h and ground in an agate mortar for homogenization.

The samples were then combusted at 960°C in excess of O₂. CO₂ derived from the combustion of the samples was collected. The isotope ratios of the collected CO₂ were measured with Finigan Delta Plus mass spectrometer in the Stable Isotope Laboratory, Key Laboratory of Western China’s Environmental System (Ministry of Education), Lanzhou University, China.

The carbon isotope ratios (δ¹³C) are expressed in per mil deviation relative to the PDB (Pee Dee Belemnite) standard:

(1){\delta }^{\mathrm{13}}C=(R_{\mathrm{sample}}/R_{\mathrm{standard}}-\mathrm{1})\times \mathrm{1000},

where δ¹³C is the carbon isotope ratio of the sample in parts per mil (‰), R_sample and R_standard are the ¹³C/¹²C ratios of the sample and standard, respectively (Craig, 1957). The precision of the isotopic measurement was 0.2‰.

Results and discussion

The surface soil organic matter was enriched in ¹³C compared to the corresponding standing vegetation, and the magnitude of the enrichment was highly site-dependent (Fig. 1, Table 1). The largest isotopic difference between soil organic matter and standing vegetation was 6.62‰, and the lowest was 0.04‰, with an average of 1.71‰ positive in Tarim Basin, China (Fig. 1). The δ¹³C_leaf values in Tamarix ranged from -29.81‰ to -20.88‰, with an average of -25.32‰. The variability in δ¹³C_soil values was less than that of the δ¹³C_leaf in Tamarix, with the δ¹³C values fluctuating between -26.37‰ to -20.27‰, with an average of -23.60‰. The magnitude of variation in δ¹³C_leaf and δ¹³C_soil in Eucalytus and Artemisia was less than that of Tamarix. The δ¹³C_leaf values in Eucalytus and Artemisia ranged from -28.65‰ to -26.37‰ and -26.40‰ to -24.50‰, and δ¹³C_soil ranged from -27.09‰ to -25.22‰ and -25.10‰ to -23.20‰, respectively (Table 1). The observed lesser variation in δ¹³C_leaf and δ¹³C_soil in Eucalytus and Artemisia are likely due to the relatively small sample size. A large variation in δ¹³C_leaf and δ¹³C_soil in Eucalytus and Artemisia would be obtained if a larger area was sampled.

We observed a strongly positive correlation (δ¹³C_soil=-9.03+0.58×δ¹³C_leaf, R²=0.43, P<0.001) between the δ¹³C_leaf and δ¹³C_soil, which implies that the carbon isotope composition of surface soil organic matter in the arid area is determined by the isotope composition of its surrounding vegetation. The isotopic discrepancy between δ¹³C_leaf and δ¹³C_soil might be influenced by the δ¹³C_leaf values, with the discrepancy decreasing with the enrichment of δ¹³C_leaf.

Surface soil organic matter is formed by organic molecules derived from plants, bacteria and aquatic organisms through a series of complex physical and chemical processes. Cayet and Lichtfouse (2001) reported that the variation in the δ¹³C values of soil total organic carbon were consistent with that of the molecular components in the arid area due to limited bacteria and few aquatic organisms. Our study indicates that the δ¹³C_leaf is significantly positively correlated with that of corresponding δ¹³C_soil (Fig. 2). The δ¹³C values of surface soil organic matter are consistent with that of the corresponding standing vegetation and thus the interfering factors of microorganisms and hydrophytes can be ignored in arid regions. It is feasible to estimate the variation of the δ¹³C of surface soil organic matter by measuring the δ¹³C of modern standing plants in the arid areas.

Soil organic carbon isotope is a proxy that can be used to indicate the vegetation shift history because δ¹³C soil is controlled by the carbon isotope composition of vegetation (Cerling et al., 1989; Wang and Follmer, 1998; Hatté et al., 1998, 2001; Schwartz et al., 1986). However, potential isotopic differences between soil organic matter and vegetation should be taken into consideration when using soil organic matter δ¹³C in sediment to reconstruct vegetation change. The isotopic fractionations during the conversion from plant litter to soil organic matter could lead to an isotopic discrepancy between δ¹³C_soil and δ¹³C_leaf. For example, Wang (2001) observed that the δ¹³C values of surface soil organic matter were 2.2‰ enriched compared to that of the corresponding standing vegetation on a precipitation gradient located on the northern grassland in China. Liu et al. (2002) assumed that the organic carbon isotope difference between soil organic matter and the corresponding standing vegetation was 1‰ in his study on the paleosols in the Chinese Loess Plateau. Lee et al. (2005) found that the δ¹³C values of standing vegetation were consistent with that of surface soil organic matter in the area from Baoji, China to Hatgal City, Mongolia. They observed that the δ¹³C values of standing vegetation were on average only 0.5‰ depleted compared to that of surface soil organic matter. Our study showed that the δ¹³C values of modern standing vegetation were 1.7‰, 1.50‰ and 1.22‰ more negative than that of the corresponding surface soil organic matter in China, Australia and the United States, respectively. The inconsistent results in isotopic discrepancy between the δ¹³C_soil and δ¹³C_leaf suggested that the degree of isotopic enrichment in δ¹³C_soil relative to that of δ¹³C_leaf likely depends on the vegetation composition and climatic conditions.

We observed that isotopic discrepancies between surface soil organic matter and C₃ standing vegetation differ among various arid regions. The relationship between the δ¹³C_soil and δ¹³C_leaf in C₄ species dominated ecosystems are largely unknown. Therefore, we argue that the proportion of C₃ and C₄ plants calculated from the δ¹³C values of surface soil organic matter would not necessarily reflect the actual vegetation composition. In order to reveal the actual paleovegetation C₃/C₄ composition through analysis of δ¹³C of paleosols, one should watch for the possible effects of isotopic discrepancies between the δ¹³C_soil and δ¹³C_leaf in C₃ and C₄ species, respectively.

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