Geophysical Research Letters

Soil n-alkane δ13C along a mountain slope as an integrator of altitude effect on plant species δ13C

Authors

  • Kai Wei,

    1. CAS Key lab of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
    2. Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, China
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  • Guodong Jia

    1. CAS Key lab of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
    2. State Key Lab of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China
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Abstract

[1] Carbon isotopic compositions of leaf wax-derived n-alkanes (δ13Cwax) and bulk organic matter (δ13CSOM) in surface soils from ca. 1000 to 3800 m above sea level along Mount Gongga, China were investigated for their altitudinal variations. There is a breakpoint, suggesting a precipitation threshold, at 2050 m in both δ13Cwax and δ13CSOM trends. Above the threshold, δ13Cwax and δ13CSOM increase in a similar pattern of the average species-level response to altitude reported from various humid mountainous areas. However, a significant decreasing trend with altitude occurs below the threshold, within which climate changes upward from arid to humid, consistent with plant foliar δ13C response to precipitation at water-limited areas. Because δ13Cwax and δ13CSOM naturally integrate plant species and time, this work demonstrates that the altitude-induced environmental impact on plant species-level δ13C has been conveyed to soils, and that the species-level δ13C could be linearly scaled up to the community level.

1. Introduction

[2] Investigations on C3 plants at the species level have shown that plant carbon isotope composition (δ13C) increases with altitude in humid areas [Körner et al., 1988, 1991; Friend et al., 1989; Morecroft and Woodward, 1990], meaning that the ratio of carbon gained to water lost in leaf gas exchange increases with altitude [Farquhar et al., 1989; Hultine and Marshall, 2000]. This phenomenon has been attributed to decreasing temperature and atmospheric partial pressure of CO2 and O2 with altitude [e.g., Körner et al., 1991; Kelly and Woodward, 1995]. While in arid areas, drought stress may overtake other factors in determining the sign of plant δ13C with elevation, and usually leads to a decreasing trend upward due to the increase of precipitation with altitude [e.g., Van de Water et al., 2002]. However, it is unknown to what extent the altitudinal trends in species-level δ13C reflect geographical variations in ecosystem characteristics of structure and function. Because the range of reported species responses to altitudes in humid areas is large (−0.9‰ to +2.7‰ km−1 [Körner et al., 1991]), species within a community can differ in δ13C by as much as 4‰ [Tsialtas et al., 2001], species composition changes with altitude, and leaf lifespan varies substantially among species [e.g., Ewers and Schmid, 1981; Reich et al., 1992], the scaling up of species-level δ13C to higher ecological levels appears not straightforward. Therefore an integrated approach is needed to address the issue.

[3] δ13C measurement on soil organic matter (δ13CSOM) reflects spatially and temporally integrated values of plants, and hence relevant for the issue. To date, few works have been done in this way [Bird et al., 1994; Townsend-Small et al., 2005], and they did not give consistent results. For example, studies of altitudinal trends in δ13CSOM from Papua New Guinea have found patterns mirroring those of plants [Bird et al., 1994]. However, observations along an altitudinal transect in the Andes of Peru showed that δ13CSOM does not exhibit similar trends of concurrent plant δ13C that increases with elevation [Townsend-Small et al., 2005]. In addition to the uncertainties of scaling up species-level δ13C to higher ecological levels, this inconsistency could be caused by several other uncertainties concerning the translation of species-level δ13C to δ13CSOM. These uncertainties include: (1) there are large differences in δ13C values of different tissue types within individual plants [Leavitt and Long, 1986] that will ultimately be incorporated into bulk SOM. For example, woody tissues can be expected to have δ13C values 1 to 2‰ higher than the leaves of the same tree [e.g., Hedges et al., 1986]; (2) Even the surface SOM integrates local vegetation signal over a time period of years or decades, during which climate and vegetation hardly remain constant; and (3) SOM is either derived from plants growing above or from root, microbial, and/or fungal biomass growing in the soils [Ehleringer et al., 2000]. Plant degradation, root inputs, and microbial growth can lead to 13C-enriched soils, and the dominant mechanism controlling soil isotope ratios is uncertain [Lehmann et al., 2002].

[4] Despite those uncertainties, several results still show the possibility of conveying to soils of the altitude effect on plant species-level δ13C. For example, the altitude effects on δ13C of individual plant species have been found scaled up linearly to community and landscape level by grazer hair δ13C analysis in the northern European Alps [Männel et al., 2007]. Surface litter and SOM δ13C show consistent relationship with δ13C of CO2 respired by the ecosystem representing a spatially integrated estimate of isotope signals [Ponton et al., 2006]. Therefore more investigations on community- and landscape-level δ13C possibly recorded in soils from different climate areas are of significance. Besides, as SOM is heterogeneous in origin and composition, and prone to diagenetic alteration, we propose that organic compounds specifically derived from higher plants and well preserved in soils, e.g. the odd carbon-numbered long-chain n-alkanes from plant leaf wax, could be more suitable for such investigations. In addition, whether or not soil organic δ13C could reflect community-wide isotopic signature is fundamental to isotopic paleo-ecological studies, in which paleo-vegetation records collectively documented in paleosols and/or sediments are generally extracted for isotopic analysis. Therefore, δ13C data of higher plant-derived n-alkanes, as well as of SOM, extracted from soil samples at various altitudes along Mount (Mt.) Gongga, China are presented here in order to assess whether soil δ13C can be an integrator of plant species δ13C and exhibits the similar pattern of altitudinal trend reported for plant δ13C.

2. Area, Sampling, Experiments, and Statistical Analysis

[5] Mt. Gongga (29°20′∼30°20′N, 101°30′∼102°15′E) is the highest peak (7556 m above sea level (asl)) in the eastern part of the Tibetan Plateau. The eastern slopes reach down into the deep Dadu River valley (1100 m asl) with a horizontal distance of less than 30 km [Thomas, 1997, 1999]. Regional climate shows typical monsoon pattern of temperature, precipitation, and evaporation. The hottest months occur well within the rainy season from May to September while evaporation peaks in the sunny pre-monsoon months [Thomas, 1997, 1999]. Mean annual precipitation increases from 644 mm at 1321 m asl to 1938 mm at 3000 m asl, while evaporation is reverse, 1916 mm at 1321 m asl and 327 mm at 3000 m asl [Thomas, 1999]. Mean annual temperature declines upward from 15.4°C at 1320 m to 3.9°C at 3000 m asl. The prominent changes in climate, vegetation, and soils over the slope transect create vertical geoecological zonations [Thomas, 1999; Zhong et al., 1999]. Over a vertical range of 4200 m from the low-altitude subtropical arid river valley to the high-altitude snowline, an intact, continuous vertical vegetation spectrum from the subtropical zone to the frigid zone can be observed (Figure 1): arid shrub and grass (Z1; 1000∼1600 m); evergreen broadleaved forest (Z2; 1600∼2000 m); evergreen-deciduous mixed broadleaved forest (Z3; 2000∼2400 m); deciduous broadleaved-coniferous mixed forest (Z4; 2400∼2800 m); dark coniferous forest (Z5; 2800∼3600 m); shrub and grass (Z6; 3600∼4200 m). Mt. Gongga thereby is an area with very high biodiversity: about 2500 plant species belonging to 869 genera and 185 families have been identified [Thomas, 1999].

Figure 1.

Altitudinal trends in δ13CSOM and δ13Cwax along Mt. Gongga. There is a statistically significant breakpoint at 2.05 km asl in each isotopic trend. The breakpoint, suggesting a precipitation threshold for vegetation response, is determined by piecewise regression analysis using the Joinpoint Regression Program (Version 3.3; Statistical Research and Applications Branch, National Cancer Institute; http://srab.cancer.gov/joinpoint/). Z1∼Z6 are vertical vegetation zones described in the text. Main plant genera for these zones are as follows: Z1, Dioscorea, Setaria, and Bidens; Z2, Quercus, Cyclobalanopsis, and Rubus; Z3, Acer, Lithocarpus, and Rhododendron; Z4, Acer, Pinus, and Picea; Z5, Abies, Picea, and Betula; Z6, Rhododendron, Carex, and Primula. There are three meteorological stations along the slope as shown by diamonds.

[6] Surface soil samples (0–5 cm) in A horizon for this study were collected after removing the litter layer along the slope transect of Mt. Gongga from ca. 1000 to 3800 m asl over 1-week period in late May, 2004. Samples at each locality were the mixture of three sub-samples randomly taken within a radius of ∼10 m, using a small metal scoop. Samples sealed in bags on site were frozen immediately after being carried to the laboratory. The altitude of each locality was determined using a handheld GPS unit with an error of ±10 m.

[7] Approximate 1 g freeze-dried soil sample was pulverized to 60 mesh, and then decarbonated with 1 N HCl. An aliquot of treated sample was measured for δ13CSOM using a Finnigan Delta Plus XL mass spectrometer interfaced with a CE Flash 1112 elemental analyzer. All samples were measured twice and the final results were reported as average in per mil units (‰) relative to Vienna Peedee belemnite (VPDB) standard. The standard deviation (σ) for every sample measurement was ≤0.5‰.

[8] Freeze-dried soil samples were ultrasonically extracted three times with dichloromethane. The hydrocarbon fraction was isolated from the total extract using silica gel column chromatography (∼2 g silica) by eluting with hexane (10 ml), and then purified for n-alkanes using urea adduction. Purified n-alkanes were then identified by comparison of retention times defined by gas-chromatography (GC) analysis of a mixed n-alkane standards.

[9] δ13C of n-alkanes was analyzed by gas chromatography-isotope ratio mass spectrometry (GC-IRMS), using a HP 6890 GC connected to a Delta Plus XL mass spectrometer via a GC-C III interface. Prior to δ13C analyses, the CO2 reference gas was calibrated relative to VPDB. Instrument performance was routinely checked using an n-alkane standard mixture with known δ13C values provided by Indiana University. For isotopic standardization, CO2 reference gas was automatically introduced into the mass spectrometer in a series of pulses at the beginning and the end of each analysis. Every sample was analyzed at least twice, and the average value, with σ ≤ 0.5‰, was reported here.

[10] Because we found that our δ13C results did not monotonically change with altitude, we performed piecewise linear regression to model thresholds where δ13C trend changed significantly using a statistical software, Joinpoint Regression Program (Version 3.3; Statistical Research and Applications Branch, National Cancer Institute). The program starts with the minimum number of breakpoint (e.g., 0 breakpoint, which is a straight line) and tests whether more breakpoints are statistically significant and must be added to the model.

3. Results

[11] We got data of δ13CSOM and δ13Cn-alkanes values for 39 samples from Mt. Gongga spanning from 1180 to 3819 m asl. δ13CSOM exhibits a range of values from −28.2‰ to −22.9‰, with an average of −25.7‰. High molecular-weight n-alkanes with carbon numbers ranging from 24 to 32 display a high odd-over-even predominance (OEP) with OEP index from 5.5 to 13.4, suggesting the plant leaf wax origin of the odd carbon-numbered (i.e., C25, C27, C29, and C31) n-alkanes [Eglinton and Hamilton, 1967]. The δ13C values of these odd carbon-numbered n-alkanes are in comparable ranges (−32.8 ± 1.2‰ for δ13C25; −33.0 ± 1.0‰ for δ13C27; −33.8 ± 1.2‰ for δ13C29; and −34.1 ± 1.8‰ for δ13C31) and show significant correlations (e.g., r2 = 0.57 between δ13C25 and δ13C27, r2 = 0.69 between δ13C27 and δ13C29, r2 = 0.49 between δ13C29 and δ13C31; n = 39, p < 0.001), demonstrating their similar origins. Here we use the mean δ13C values weighted by the respective concentrations of nC25/27/29/31 alkanes (δ13Cwax) in our discussion because we think it is more suitable to represent the mean isotope signal of various local plant leaf input into soils. The obtained δ13Cwax varies between −36.3‰ and −31.0‰, with an average of −33.6‰.

[12] Piecewise regression analysis gave a unique statistically significant breakpoint (p < 0.01) at 2050 m asl for each of the δ13CSOM and δ13Cwax trends. From 2050 m upward, δ13CSOM increases with a slope of 1.3 ± 0.5‰ km−1 (r2 = 0.41, p = 0.02), and δ13Cwax with 1.2 ± 0.6‰ (r2 = 0.30, p = 0.04). However, below the breakpoint, δ13CSOM decreases with a slope of −3.2 ± 1.6‰ km−1 (r2 = 0.25, p = 0.06), and δ13Cwax with −2.6 ± 1.6‰ (r2 = 0.26, p = 0.05) (Figure 1).

4. Discussion

[13] It has been established that C3 and C4 plants show distinct δ13C values: from −23‰ to −34‰ and from −10‰ to −16‰ in C3 and C4 bulk tissues, respectively [Schidlowski, 1987; Collister et al., 1994]. Leaf wax lipids during biosynthesis become more depleted in 13C than the total biomass so that their δ13C values vary between −31‰ and −39‰ in C3 plants and between −18‰ and −25‰ in C4 plants [Rieley et al., 1991, 1993; Collister et al., 1994]. At present there is no reported investigation on the C3 vs. C4 plants distribution along the eastern slope of Mt. Gongga. However, we consider that C4 plant contributions to total biomass, and hence their inputs to SOM, could be minimal, because both δ13CSOM and δ13Cwax values in our results lie in the typical range for C3 plants. The difference between δ13CSOM and δ13Cwax is rather constant, 7.9 ± 1.4‰, similar to the results from modern higher plant studies. Therefore, our isotopic results suggest that bulk surface SOM is predominantly higher plant origin, which is consistent with the observations that the potential effects of changing microbial activity and soil C turnover rates with elevation on δ13CSOM, although more significant at depth, could be neglected for surface soils [e.g., Powers and Schlesinger, 2002].

[14] Several studies have shown that there is a threshold value of annual precipitation for plant δ13C response to environment. Above the threshold, additional precipitation has little impact on leaf δ13C [Leffler and Evans, 1999; Leffler and Enquist, 2002; Song et al., 2008]. The global pattern of plant species-level δ13C increase with altitude is established from humid areas avoiding drought stress, and hence above this precipitation threshold, reflecting the response of gas exchange physiology to altitude, i.e., reducing stomatal conductance and decreasing the ratio of internal to external CO2 concentrations (ci/ca) due to the decreasing temperature and atmospheric partial pressure of CO2 and O2 with altitude [e.g., Körner et al., 1991; Kelly and Woodward, 1995]. However, below the precipitation threshold, a negative correlation occurs between total annual precipitation and plant δ13C, suggesting that precipitation is an important factor influencing plant physiology [Leffler and Evans, 1999; Leffler and Enquist, 2002; Van de Water et al., 2002; Song et al., 2008], i.e., a reduction in stomatal conductance and leaf ci/ca as soil moisture declined. Both scenarios likely occur along the eastern slope of Mt. Gongga at the community and landscape levels as suggested by the variation patterns of δ13CSOM and δ13Cwax in our results showing a reversal of δ13C-altitude slope at the breakpoint at 2050 m asl (Figure 1). We therefore speculate that the precipitation of ∼1300 mm at the site (interpolated from precipitation data between 1600 and 3000 m asl as shown in Figure 1) is the likely threshold for the landscape-scale δ13C response on this mountain slope, as discussed below.

[15] The significant decreasing trends with altitude in both δ13CSOM and δ13Cwax below the breakpoint in our results have also been reported in a study of δ13C of nC29 alkane extracted from lake and reservoir sediments along an altitude gradient from 500 m to 2500 m asl on the same mountain range several hundred km north to our study sites [Wang et al., 2005]. This pattern of δ13CSOM and δ13Cwax responses to altitude has been observed for C3 plants on species-level δ13C investigations in many arid areas as well, and is attributed to the physiological effect of restricted water availability at lower elevations [Ehleringer and Cooper, 1988; Lajtha and Getz, 1993; Van de Water et al., 2002]. The low altitudes of the eastern flank of Mt. Gongga are characterized by climate shift from arid to humid, e.g., the annual precipitation at 1320 m asl is 644 mm, and at 1640 m asl 993 mm (Figure 1). Responding to this climate shift, vegetation changes from the valley arid shrub and grass (below ∼1600 m) to the evergreen broad-leaved forest (1600–2000 m). We therefore suppose that water stress controls the δ13C pattern of plants below the breakpoint suggestive of a precipitation threshold, i.e., the less water availability the heavier δ13C of plants, which in turn is translated into community level δ13C, and recorded in SOM and soil n-alkanes. There is also a possibility of more contributions of C4 plants at lower arid altitudes that likewise leads to increasing δ13C with decreasing altitude, as has been found in many low altitude mountain areas [e.g., Tieszen et al., 1979; Bird et al., 1994; Cabido et al., 1997]. Unfortunately, in our study area, no investigations on C4 plants distribution along the eastern slope of Mt. Gongga have been reported although local climate conditions meet the demand of C4 plants. Therefore, future work on the occurrence of C4 plants in this area is needed. Nevertheless, it appears that C4 plant contribution is not significant even at the lowest altitude from our isotopic results as stated above.

[16] Both the δ13CSOM and δ13Cwax values above the suggested precipitation threshold in this study exhibit remarkable increasing trends along the altitudinal gradient (Figure 1). The slopes of their regression lines, 1.3‰ km−1 for δ13CSOM vs. altitude and 1.2‰ km−1 for δ13Cwax vs. altitude, respectively, are quite similar to that of the average species-level response to altitude, i.e., 1.1‰ km−1 or 1.2‰ km−1, established from investigations on hundreds of C3 plant species from all major mountain ranges of the globe [Körner et al., 1988, 1991; Männel et al., 2007]. Recently, investigations on δ13C of 174 leaf samples belonging to 89 plant species growing from 2800 to 4400 m asl in eastern Tibetan Plateau around our study area give an average rate of change of 1.1‰ km−1 [Li et al., 2007]. When compared to these measurements of vegetation, δ13CSOM and δ13Cwax offer the advantage of an easily obtained value that naturally integrates over plant species, space, and time. Therefore, the similarity of δ13CSOM and δ13Cwax trends above the precipitation threshold in our results with the reported average species-level response to altitude in humid areas implies that the altitude effect on species-level δ13C scales up linearly to community and landscape levels. Similar observations and conclusions have also been reported by Männel et al. [2007], who analyzed grazer hair δ13C in the northern European Alps.

[17] Overall, the altitudinal variations of δ13CSOM and δ13Cwax in our results show similar patterns to those reported for plants from various mountain areas, suggesting that the impact of altitude-induced environmental change on plant species-level δ13C has been successfully conveyed to δ13CSOM and δ13Cwax, and that the linear scaling of δ13C responses from average species level to community and landscape levels is feasible. This would imply that δ13C responds to altitude-induced environmental changes in the same way at species and community levels. This implication justifies the application of soil organic δ13C record for paleoecological reconstructions. However, although surface δ13CSOM correlates with δ13Cwax in our results, at depth profiles of δ13CSOM would reflect environmental factors or microbial input during degradation [Ehleringer et al., 2000; Powers and Schlesinger, 2002], which would impair the original isotopic signals of vegetation. For comparison, leaf wax n-alkanes from higher plants are usually isotopically well preserved in soils and sediments [e.g., Huang et al., 1997; Mazeas et al., 2002]. Consequently, although our results do not show significant differences between surface δ13CSOM and δ13Cwax patterns, we suggest δ13Cwax for further studies, especially for paleoecological reconstructions, as have been done in numerous works [Eglinton and Eglinton, 2008].

Acknowledgments

[18] We would like to thank Mingzhong Ren, Tongyang Li, and Wei Li for collaboration in collecting soil samples. We also thank Huashan Chen for technical support in the SKLOG, Guangzhou Institute of Geochemistry. Two anonymous reviewers are appreciated for their helpful comments. This is contribution IS-1074 from GIGCAS.

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