Soil N-alkane Research Articles

Climatic and environmental implications from n-alkanes in glacially eroded lake sediments in Tibetan Plateau: An example from Ximen Co

Gas chromatography-mass spectrometry was used to identify a series of n-alkanes in the sediments of a typical glacially eroded lake in the eastern Tibetan Plateau. By comparing the distribution patterns of n-alkanes in lake sediments, surface soils and cow manure, it was shown that n-C27-n-C33 alkanes in the soil ecosystem of Ximen Co are derived from vascular plant species and that the distribution pattern of n-C27-n-C33 alkanes remains unchanged during the feeding and digestion processes of herbivores. The relative percentage of C27, C29 and C31 n-alkanes decreased from the bottom to the top of the sediment core showing a trend of degradation of higher plants in the Ximen Co lake region during the formation of the 44 cm core. 210Pb dating, combined with pre-existing AMS 14C dating results showed that the depositional core reflects climatic and environmental variations since about 900 years before present. The n-alkane indexes (ACL27–33, P aq, P wax) are comparable with regional temperature variation, especially recording the Little Ice Age event (LIA). This study highlights that n-alkanes are valid proxies for paleo-climate and paleo-environment reconstruction, despite the same distribution patterns in n-alkane molecular fossils derived from a typical glacially eroded lake.

Effect of precipitation regime on δD values of soil n-alkanes from elevation gradients – Implications for the study of paleo-elevation

We measured dD values of long chain n-alkanes isolated from 30 surface soil samples along two elevation transects on the Tibetan Plateau differing in precipitation regime and water source. The East Asian Monsoon precipitation dominates the wetter regime on the eastern slope (from 1230 to 4300 m) of Gongga Shan on the eastern Tibetan Plateau. Precipitation from the Polar Westerlies dominates the drier region on the slope from 1900 to 5000 m in the West Kunlun Shan on the northwestern Tibetan Plateau. The decrease in dD value with elevation in the wetter region greatly exceeded that in drier region by, � 1.9 ± 0.1‰/100 m and � 1.4 ± 1.0‰/100 m respectively. The apparent fractionation between leaf wax and precipitation ewax–p values in the wetter region (ca. � 164‰) were more negative than those in drier region (ca. � 125‰ above 3200 m). We also measured dD values in leaves of six common living trees (values from � 287‰ to � 193‰) from Gongga Shan, ranging from about 2900–4200 m. The abundance-weighted average values of the nalkanes (dDwax) show a strong reverse correlation with sample source elevation (R 2 0.78 for soils from 2 0.85 for soils from West Kunlun Shan above 3200 m), suggesting that n-alkane dDwax faithfully records the precipitation dD and that the isotopic altitude effect of precipitation controls dDwax altitudinal gradients in the mountains. The data show a fairly strong monotonic dependency of n-alkane dD values on elevation for the eastern Plateau, but a complex relationship between n-alkane dD values and elevation for the northwestern Plateau. The dDwax values at sites below 3200 m from the Kunlun Shan area exhibit an unexpected positive correlation with elevation. The study confirms the potential for using sediment dDwax values to reconstruct paleo-elevation in wetter regions, but suggests caution in applying the approach to dry regions. Our results also show it is essential to consider the intricacy of the pattern of atmospheric circulation and water sources and their influence on the lapse rate of dD values with elevation.

Effect of plant life form on relationship between δD values of leaf wax n-alkanes and altitude along Mount Taibai, China

We present δD wax values from different forms of plants and soils, and δD sw values from soil water along the northern slope of Mount Taibai, China. The results show a highly negative linear correlation of the δD wax values for soils with altitude ( R 2 0.74) and we observed the same correlation for δD sw values of soil water with altitude ( R 2 0.68). The δD wax of living plants behaves like the soil, but does not exhibit a significant linear correlation with altitude ( R 2 0.11). The δD wax values of woody plants and grasses also show a similar trend with respect to altitude with significant and no linear correlation, respectively ( R 2 0.50 for woody plants and 0.17 for grass), which suggest that the “altitude effect” can not be well documented for the δD wax values of living plants, which may be due to differences in plant type and/or evapotranspiration controlled by the plant microclimate. The ε wax-sw values of woody plants, grasses and soil show minor fluctuations with altitude. However, the ε wax-sw and δD wax values of woody plants are roughly 51‰ and 50‰ more positive, respectively, than those of grasses, suggesting that an “altitude effect” could be documented in the δD wax of woody plants and grasses, with each responding independently to changes in precipitation along the altitude transect. Additionally, the ε wax-sw values of soil are relatively constant with altitude, suggesting that the altitudinal change in the proportions between woody plant and grass input to soils will likely change the relationship between the δD wax values of soil n-alkanes and altitude.

Geochemical and ecological significance of soil lipids under Rhododendron ponticum stands

The bio-geographical significance of Rhododendron ponticum spp. baeticum (Ericaceae) as a relict species is well recognized. However, out of its native habitat it is an invasive exotic considered a major threat to natural ecosystems in areas of Atlantic Western Europe. The studies on the impact of Rhododendron influence on soil organic matter composition and associated ecological implications, i.e. presence of bioactive compounds with ecological significance, are limited. This work describes the soil lipid assemblage in three sites under Rhododendron stands and adjacent sites with deciduous oak (Quercus canariensis), both in their native habitats in Southern Spain (Sierra de Luna, Cádiz). The results are discussed in terms of organic matter dynamics and the presence of molecules that may be associated with Rhododendron invasive success. The soils are acid Xerochrepts formed on siliceous sands. Composite soil samples were taken at two depths (0–10 cm and 10–20 cm) and soxhlet extracted with a dichloromethane-methanol mixture (3:1). Soil lipid assemblage was studied by GC/MS after fractionation and appropriate derivatization of extracts. The qualitative chemical composition of soil extractable lipids under Rhododendron is reported here for the first time. Our results show that soil n-alkane and fatty acid distributions are compatible with an input from plant epicuticular waxes, as well as with the occurrence of selective preservation of long-chain fatty acids with depth. The pattern of short-chain n-alkanes found in surface samples indicates an anthropogenic contamination threat from nearby industrialized areas of “Campo de Gibraltar”. The presence of branched iso and anteiso C15 and C17, β-hydroxy fatty acids and the sterol brassicasterol points to high microbial soil activity. Finally, the pentacyclic triterpenes taraxerone and taraxerol were detected in soils with Rhododendron but not with Quercus. These are known bio-active plant compounds and could be related with the effectiveness of Rhododendron as an invasive exotic species.

Assessment of soil <i>n</i>-alkane δ<i>D</i> and branched tetraether membrane lipid distributions as tools for paleoelevation reconstruction

Abstract. δ18O values of pedogenic minerals forming from soil water are commonly used to reconstruct paleoelevation. To circumvent some of the disadvantages of this method, soil n-alkane δD values were recently proposed as a new tool to reconstruct elevation changes, after showing that soil n-alkane δD values track the altitude effect on precipitation δD variations (r2=0.73 along Mt. Gongga, China). To verify the suitability of soil n-alkane δD values as a paleoelevation proxy we measured the δD of soil n-alkanes along Mt. Kilimanjaro (Tanzania). At midslope, soil n-alkane δD values are possibly influenced by the present precipitation belt, causing D-depletion in precipitation, and hence in the soil n-alkanes. Consequently, soil n-alkane δD values do not linearly relate with altitude (r2=0.03), suggesting that, in this case, they can not serve as an unambiguous proxy to infer past elevation changes. In contrast, it was recently shown that the MBT/CBT temperature proxy, which is based on the distribution of branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids, is linearly related with MAT, and thus altitude (r2=0.77), at Mt. Kilimanjaro. This suggests that this proxy may be more suitable for paleoelevation reconstruction for this region. However, application of the MBT/CBT proxy on the altitude gradient along Mt. Gongga showed that, although the MBT/CBT-derived temperature lapse rate (−5.9°C/1000 m) resembles the measured temperature lapse rate (−6.0°C/1000 m), there is a relatively large degree of scatter (r2=0.55). Our results thus show that both proxies can be subject to relatively large uncertainties in their assessment of past elevation changes, but that a combination of the soil n-alkane δD and MBT/CBT proxies can likely result in a more reliable assessment of paleoelevation.

A compound-specific n-alkane δ13C and δD approach for assessing source and delivery processes of terrestrial organic matter within a forested watershed in northern Japan

Abstract We measured molecular distributions and compound-specific hydrogen (δD) and stable carbon isotopic ratios (δ13C) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. δ13C values of odd carbon numbered C23–C33 n-alkanes ranged from −37.2‰ to −31.5‰, while δD values of these alkanes showed a large degree of variability that ranged from −244‰ to −180‰. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane δD values are much higher in forest soils than wetland peat. The higher δD values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A δ13C vs. δD diagram of n-alkanes among forest, wetland and lake samples showed that C25–C31 n-alkanes deposited in lake sediments are mainly derived from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific δD analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment.

The occurrence of short chain n-alkanes with an even over odd predominance in higher plants and soils

In this study we provide data which support the interpretation that tissues of higher plants can constitute a significant source of the short chain n-alkanes with an even/odd predominance (EOP) found in soil organic matter. Gas chromatographic analyses of vegetation (C 3 trees, C 4 grasses) and associated soil samples (woodland and grassland) from a study site in central Queensland, Australia, revealed that (1) woody vegetation (leaves) and grasses (leaves, roots) contain short chain n-alkanes (C 14−C 20) with pronounced EOP and (2) such homologues dominate the n-alkane assemblages in its woodland and grassland soils. The presence of short chain n-alkanes with an EOP in some of the vegetation suggests that these may represent a significant source of such alkanes in the woodland and grassland soils. Previous studies have shown that combustion induced thermal breakdown of long chain n-alkanes may produce short chain homologues with an EOP. A history of repeated bushfires at the study site may have contributed to the presence of these n-alkanes in its soils. The co-occurrence of polyaromatic hydrocarbons derived from fire (e.g. retene) indicates that heat related generation of short chain n-alkanes indeed may have played an additional part in the formation of the observed soil n-alkane patterns. These two potential origins were further investigated by compound specific carbon and hydrogen isotope analyses of the plant and soil n-alkane assemblages. These data show that the δ 13C and δD signatures of the short chain n-alkanes in the soil resemble those of the plants. Our study therefore provides strong evidence that EOP among short chain n-alkanes can represent a primary (i.e. non-diagenetic) signature, which originates directly from biological sources. In the case of this Queensland soil, the leaves and roots of higher plants are likely to be the principal sources, together with a smaller secondary contribution from the combustion of associated long chain n-alkanes during bushfires.

Short-chain n-alkanes (C 16–20) in ancient soil are useful molecular markers for prehistoric biomass burning

The incorporation of plant biomass into soil usually leads to long-chain n-alkanes with a relative predominance of odd carbon numbered homologues. Contrastingly, an increase in short-chain even carbon numbered n-alkanes was found in charred biomass with progressing temperatures. We applied lipid analysis to buried ancient topsoils that contained charred organic matter and to corresponding control soils, which were characterized by a lighter color and lower contents of charred materials. Commonly, the proportion of lipid extracts was found to be lower in the ancient soil than in the control samples, which indicated an enhanced degradation of organic matter, e.g., by thermal degradation. All samples displayed a particular pattern of short-chain and even carbon numbered n-alkanes (maximum at n-C 16 or n-C 18). The ratios CPI (carbon preference index) and ACL (average chain length) for the investigated soil samples matched the ratios found for maize and rye straw charred at 400 °C or 500 °C, respectively. These molecular ratios indicate the presence of charred biomass. The predominance of short-chain and even carbon numbered n-alkanes was a result of thermal degradation of biomass. The degradation products were preserved in ancient soils and could be applied as molecular markers in archaeological or palaeoenvironmental research.

Thermal degradation of rye and maize straw: Lipid pattern changes as a function of temperature

Future climatic conditions may coincide with an increased potential for wildfires in grassland and forest ecosystems, whereby charred biomass would be incorporated into soils. Molecular changes in biomass upon charring have been frequently analysed with a focus on black carbon. Aliphatic and aromatic hydrocarbons, known to be liberated during incomplete combustion of biomass have been preferentially analysed in soot particles, whereas determinations of these compounds in charred biomass residues are scarce. We discuss the influence of increasing charring temperature on the aliphatic and aromatic hydrocarbon composition of crop grass combustion residues. Straw from rye, representing C3 grasses and maize, representing C4 grasses, was charred in the presence of limited oxygen at 300, 400 and 500°C. Typical n-alkane distribution patterns with a strong predominance of long chain odd-numbered n-alkanes maximising at C31 were observed in raw straw. Upon combustion at 300°C aliphatic hydrocarbons in char were dominated by sterenes, whereas at 400°C sterenes disappeared and medium chain length n-alkanes, maximising around n-C20, with a balanced odd/even distribution were present. At a charring temperature of 500°C n-alkane chain length shifted to short chain homologues, maximising at C18 with a pronounced predominance of even homologues. Even numbered, short chain n-alkanes in soils may thus serve as a marker for residues of charred biomass. Aromatic hydrocarbons indicate an onset of aromatization of biomass already at 300°C, followed by severe aromatization upon incomplete combustion at 400–500°C. The diagnostic composition of aliphatic and aromatic hydrocarbons from charred biomass affords potential for identifying residues from burned vegetation in recent and fossil soils and sediments.

Soil n-alkane δD vs. altitude gradients along Mount Gongga, China

The altitude effect on the isotopic composition of precipitation and its application to paleoelevation reconstruction using authigenic or pedogenic minerals have been intensively studied. However, there are still no studies on variations in biomarker δD along altitude transects to investigate its potential as a paleoelevation indicator, although it has been observed that δD of higher plant lipid may record changes in precipitation δD (δD p). Here, we present δD values of higher plant-derived C 27, C 29, and C 31 n-alkanes from surface soil along the eastern slope of Mount Gongga, China with great changes in physical variables and vegetation over a range from 1000 to 4000 m above sea level. The weighted-mean δD values of these n-alkanes (δD wax) show significant linear correlations with predicted δD p values ( R 2 = 0.76) with an apparent isotopic enrichment ( ε wax–p) of −137 ± 9‰, indicating that soil δD wax values track overall δD p variation along the entire altitudinal transect. Leaf δD wax is also highly correlated with mountain altitude by a significant quadratic relationship ( R 2 = 0.80). Evapotranspiration is found declining with altitude, potentially lowering δD wax values at higher elevations. However, this evapotranspiration effect is believed to be largely compensated by the opposing effect of vegetation changes, resulting in less varied ε wax–p values over the slope transect. This study therefore confirms the potential of using leaf δD wax to infer paleoelevations, and more generally, to infer the δD of precipitation.

Plant and soil lipid modifications under elevated atmospheric CO 2 conditions: I. Lipid distribution patterns

Grassland soils are regarded as one potential sink for atmospheric CO 2 via photosynthetic fixation in plant biomass and subsequent transformation into soil organic matter upon degradation. In the future, an enrichment in atmospheric CO 2 concentration is expected, leading to modified photosynthetic activity in plant biomass. Free air CO 2 enrichment (FACE) experiments provide an opportunity for investigating under field conditions plant behaviour expected under future atmospheric composition. Lipid components are important constituents of plant surfaces, whereby their position at the plant/atmosphere interface leads to a high susceptibility towards environmental change. The main focus of this study was an investigation of the modification in lipid distribution patterns within plant biomass and the translocation of these lipids towards, and fixation within, soil organic matter as a result of enhanced CO 2 concentration. We demonstrate which lipids are mainly influenced under modified CO 2 concentrations and show how this affects the lipid composition of plant biomass and soil. Carboxylic acid, alcohol and aliphatic hydrocarbon distribution patterns of plant biomass and soils are discussed. While short chain acids reveal a uniform depletion in unsaturated C 18 acids in plants and soils under enhanced CO 2 concentration, the alcohol fraction shows diverse trends for Lolium perenne and Trifolium repens plants and soil. Long chain alcohols increase in abundance for L. perenne and decrease for T. repens samples. The n-alkanes in soil, as degradation products of plant-derived acids and alcohols, exhibit minor compositional variation. Decreasing amounts of plant-derived acids vs. increasing concentrations of alcohols are noted for T. repens samples. The study demonstrates the response on the molecular level of selected plants under enhanced atmospheric CO 2 concentration. Lipid compositional variation is modified by photosynthetic activity and adapted biosynthesis under future atmospheric conditions may be expected.

The use of plant hydrocarbon signatures in characterizing soil organic matter

Abstract Resistant compounds associated with vegetation have potential for understanding and uniquely describing soil. Although much of the forensic identification of soils has focused on the mineral component, this study illustrates how the origin of the organic component can be a useful tool in soil identification. The epicuticular wax of most plants containts mixtures of hydrocarbons (mainly n -alkanes) and plant species differences are persistent. Evidence from three separate studies is compiled to show the validity of this approach. In the first example, on upland grassland vegetation, the n -alkane pattern of the soil at one site reflected that of the overlying grass, whereas at another site, it reflected that of the previous vegetation, heather. In the second study, n -alkane analysis data indicated the presence of heather in a buried horizon, matching independent evidence from pollen identification. The third study was one covering the whole of Scotland, using an unbiased grid-sampling strategy. Results show that the patterns in the soil n -alkane profiles reflected the overlying vegetation. Where this was not the case, the profiles matched previously grown vegetation. Such biomarker information, derived from plant wax signatures, coupled with soil spatial information, has potential in the unique identification of soils.

Plant Wax n-Alkanes Trapped in Soil Humin by Noncovalent Bonds

Soil organic matter is a very complex mixture of compounds derived from the decay of numerous living organisms. Earlier reports suggest that soil organic molecules may occur unaltered in a form, e.g. by encapsulation in the humic substances matrix or by binding involving weak forces such as H bonds and Van der Waals forces. This issue is relevant to the release of pesticides and other toxic compounds in waters during several years, which suggests a particular mechanim of storage of organic compounds in soil organic matter. However, so far, the mechanim of weak binding is poorly known due to possible analytical bias in the isolation of weakly-bound molecules, and to the lack of analytical approaches that can distinguish a compound from its weakly-bound counterpart. Here, we analysed the 13 C isotope composition of plant-derived soil n-alkanes in a soil sample from an experimental field cropped 23 years with maize to label soil organic carbon. Indeed, cultivating maize, a C 4 plant with 13 C-enriched carbon, on a soil previously cropped with C 3 plants, introduces 13 C-enriched carbon in the soil. Thus maize-derived soil carbon can be distinguished from previously-cropped plant soil carbon by 13 C analysis. We used a recently developped analytical technique, gas chromatography - isotope ratio monitoring mass spectrometry (GC-IRMS), that allows to measure the 13 C isotope ratio of individual substances occurring in complex mixtures. We analysed three pools of C 27 -C 33 n-alkanes in the same soil sample: 1) n-alkanes that were extracted with CHCl 3 -MeOH, 2) n-alkanes that were extracted from humin, the macromolecular part of soil organic matter, and 3) n-alkanes that were released by pyrolysis of the pre-extracted humin. Our results show that free n-alkanes are 13 C-enriched by +5.8-7.0‰ versus their bound n-alkanes counterparts. This finding have several implications. First, this clear isotope difference of the the same substance occurring either in free or bound form proves that there is no analytical bias such as a lack of exhautive extraction. Second, the higher 13 C content of free n-alkanes evidences their higher turnover versus bound n-alkanes. Here a first-order kinetic law yields an age difference of 7 years between free and bound n-alkanes. Third, since n-alkanes are apolar compounds our results demonstrate the existence of a mechanism of weak binding involving either encapsulation or weak forces. This mechanism could explain why chemicals such as pesticides are well preserved in soil then released in waters several years after the end of their use in cropping systems. It is also relevant to the storage of organic-N compounds as possible plant nutrient precursors.

Molecular, 13C, and 14C evidence for the allochthonous and ancient origin of C 16C 18n-alkanes in modern soils

The heterogeneous isotopic composition of C 3 and C 4 plants can be used to to follow the fate of plant carbon into soil organic molecules. Thus, after 23 years of maize cropping (C 4) on a soil which was previously under C 3 vegetation, C 25C 33 soil n-alkanes are 13C-enriched up to 9ℵ. relatively to the initial C 3 soil, reflecting the input of 13C-enriched n-alkanes from maize waxes. In sharp contrast, C 16C 18 soil n-alkanes do not show any significant 13C/ 12C variation over the same time interval. This absence of isotopic variation, along with consideration of their relative concentration, absolute concentration, and biodegradability, demonstrate that these substances must represent a regular input from an external source. Evidence of a large contribution of an ancient source, amounting to more than 65% of the alkane fraction, is given by a 14C-age of 8510 yrs bp. Moreover, short-chain n-alkanes from soils, diesel fuel, diesel automobile exhaust, and petroleum products exhibit similar distributions and δ 13C values. These findings suggests that C 16C 18 soil n-alkanes represent a non-point source pollution of ancient hydrocarbons either carried by aerosols or entering the soil via continuous hydrocarbon seepage from the deep sedimentary rocks of the Paris basin.

Heterogeneous Turnover of Molecular Organic Substances from Crop Soils as Revealed by 13 C Labeling at Natural Abundance with Zea mays

Understanding the dynamics of soil organic matter is of major importance for agronomical and environmental studies. In particular, soil organic matter holds soil physical stability and thus prevents erosion; soil organic matter stores pollutants; and soil organic carbon impacts climate change as either a source or sink of atmospheric CO2. However, knowledge on the dynamics of soil carbon is still incomplete, particularly at the molecular level. Here, using a recently developped analytical technique that allows measuring the 13C isotope composition of individual substances within a complex organic mixture I show that plant-wax derived soil n alkanes have a higher turnover than bulk soil organic matter. Soil samples were collected in a field experiment where maize was cropped 23 years. Since maize, a C4 plant, is 13C-enriched versus previous vegetation made of C3 plants, cropping maize introduces 13C-enriched carbon in the soil over time. This labelling at natural abundance thus allows to distinguish maize-derived carbon from initial C3 plant-derived carbon. The 13C composition of bulk soil carbon was measured by isotope ratio monitoring mass spectrometry (IRMS), whereas that of individual soil n-alkanes was measured by gas chromatography-IRMS. The results show that after 23 years of maize cropping the increase of 13C composition is faster for the C31 soil n-alkane (+7.6‰) than for bulk soil carbon (+5.6‰). This finding evidences the faster turnover of soil n-alkanes, which is rather unexpected because n-alkanes are chemically stable compounds. Using a first order kinetic model, it is estimated that the complete C turnover will last 192 years for bulk soil carbon and 148 years for the plant-derived soil n-alkane.

13C and 14C evidence of pollution of a soil by fossil fuel and reconstruction of the composition of the pollutant

n-Alkanes from soils in which C 3 plants have been cultivated were analysed for their distributions and individual δ 13C ratios. For long-chain n-alkanes from unpolluted soils, both the strong odd-carbon number predominance, with a carbon preference index (CPI) of 9.2, and the light isotope composition, ∼ −35‰, are typical of modern C 3-plant waxes. In contrast, long-chain n-alkanes from another soil sample show a weak odd-carbon number predominance, with a CPI of 1.2, indicating a large anthropogenic contribution from ancient and thermally mature organic matter. This derivation from ancient organic matter is strongly supported by (1) the δ 13C ratios of those alkanes, ∼ −30‰, which does not match reported values for C 3 plant n-alkanes and which falls within the range of values reported for fossil-fuel n-alkanes, and (2) the 14C analysis of the alkane fraction which gives an age of 8770 yrs BP. Moreover, assuming that each soil n-alkane is a mixture of C 3-plant n-alkane and fossil-fuel n-alkane, the isotopic composition provides a means of reconstructing the composition of the fossil-fuel pollution. This method, using both isotopic and quantitative analyses of individual substances, represents a powerful means to resolve multiple sources in environmental and geochemical investigations.

Isotope and molecular evidence for direct input of maize leaf wax n-alkanes into crop soils

The contribution of plant carbon to crop soils can be followed by isotope labelling at natural abundance, such as growing a C 4 plant on a soil which was previously under C 3 vegetation. For this purpose, carbon isotope compositions and relative abundances of n-alkanes of maize leaf waxes and maize crop soils were compared. Isotope values of soil n-alkanes increased with time of maize cultivation as the result of maize carbon integration into soil organic matter. With increasing time of cultivation, the increase in isotopic difference between n-heptacosane (C 27) and n-nonacosane (C 29) is explained, at least partly, by a direct input of maize leaf n-alkanes. The amount of maize-derived carbon within each n-alkane has been calculated by isotopic means.

Diploptene: an indicator of terrigenous organic carbon in Washington coastal sediments.

The pentacyclic triterpene 17 beta(H),21 beta(H)-hop-22(29)-ene (diploptene) occurs in sediments throughout the Columbia River drainage basin and off the southern coast of Washington state in concentrations comparable to long-chain plantwax n-alkanes. The same relationship is evident for diploptene and long-chain n-alkanes in soils from the Willamette Valley. Microorganisms indigenous to soils and soil erosion are indicated as the biological source and physical process, respectively, for diploptene in coastal sediments. Similarity between the stable carbon isotopic composition (delta 13CPDB) of diploptene isolated from soil in the Willamette Valley (-31.2 +/- 0.3%) and from sediments deposited throughout the Washington coastal environment (-31.2 +/- 0.5%) supports this argument. Values of delta for diploptene in river sediments are variable and 8-17% lighter, indicating that an additional biological source such as methane-oxidizing bacteria makes a significant contribution to the diploptene record in river sediments. Selective biodegradation resulting from a difference in the physicochemical association within eroded particles can explain the absence of the more-13C-depleted form of diploptene in Washington coastal sediments, but this mechanism remains unproven.