Soil N-alkane Research Articles

Using δDn-alkane as a proxy for paleo-environmental reconstruction: A good choice to sample at the site dominated by woods

Some studies have demonstrated that leaf wax δDn-alkane values for a single species varied significantly with seasons. However, it is still not clear that the seasonality patterns of leaf wax δDn-alkane values in higher plants. Meanwhile, few efforts have been pursued to assess the effect of the light slopes (sunny vs. cloudy) on leaf wax δDn-alkane values. In this study, we systematically investigated plant wax δDn-alkane values and soil n-alkane δD values along different light slopes in different seasons (spring vs. autumn), as well as the relationship of n-alkane δD values between plant leaves and soil. We found that plant wax δDn-alkane values were D-enriched by ca. 20‰ in spring relative to autumn, and ca. 10‰ in the sunny slope than in the cloudy slope. Moreover, surface soil n-alkane δD values varied consistently with plant wax δDn-alkane values for different seasons and light slopes. More importantly, plant wax δDn-alkane values showed clear seasonal variations, but varied slightly with light slopes. The variations of plant wax δDn-alkane values can be recorded in soil n-alkane δDn-alkane values. In addition, we found that leaf wax δDn-alkane values in a majority of species differed significantly among woods, non-woods and grasses at a site. Therefore, we suggested a good choice to sample at the site dominated by woods when leaf wax δDn-alkane values are utilized as a proxy for the reconstruction of the paleoenvironment.

Source apportionment of polycyclic aromatic hydrocarbons and n-alkanes in the soil-sediment profile of Jianghan Oil Field, China.

Surface soil in oil exploration area always contains high contents of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. To investigate the migration possibility of PAHs and n-alkanes from surface through aquitard and aquiclude to aquifer, the distribution, together with the source apportionment using several indicators, such as composition pattern, fluoranthene/(fluoranthene+pyrene) (Flt/(Flt+Pyr)), anthracene/(anthracene+phenanthrene) (Ant/(Ant+PA)), and the carbon preference index (CPI) of n-alkanes, in a 30-m-deep soil-sediment profile were studied. Results showed that there were considerable PAHs and n-alkanes not only in surface soil but also in aquitard, aquiclude, and aquifer sediments. The PAHs and n-alkanes in surface soil strongly suggested petroleum pollution. The high molecular weight PAHs and the n-alkanes with both long and short chains could not migrate into deep sediments as their sources in surface soil and deep sediment were different. Whereas the aquitard and aquiclude had significant input of low molecular weight PAHs (LMWPAHs) from petroleum sources, the LMWPAHs in confined aquifer suggested pyrogenic sources. Therefore, LMWPAHs migrated from surface to aquitard and aquiclude, but did not cause aquifer pollution in Jianghan Oil Field. However, the high mobility of LMWPAHs from surface to aquitard and aquiclude suggested that the long-term risk of groundwater pollution from oil exploration should be concerned.

Hydrogen isotope fractionation of leaf wax n-alkanes in southern African soils

The hydrogen isotope composition of plant leaf wax (δDwax) has been found to record the isotope composition of precipitation (δDp). Hence, δDwax is increasingly used for palaeohydrological reconstruction. It is, however, also affected by secondary factors, such as vegetation type, evapotranspiration and environmental conditions, complicating its direct application as a quantitative palaeohydrological proxy. Here, we present δDwax data from soils along vegetation gradients and climatic transects in southern Africa to investigate the impact of different environmental factors on δDwax. We found that δDwax correlated significantly with annual δDp (obtained from the interpolated Online Isotopes in Precipitation Calculator data set) throughout eastern and central South Africa, where the majority of the mean annual precipitation falls during the summer. We found evidence for the effect of evapotranspiration on δDwax, while vegetation change was of minor importance. In contrast, we found that δDwax did not correlate with annual δDp in western and southwestern South Africa, where most of the annual precipitation falls during winter. Wide microclimatic variability in this topographically variable region, including distinct vegetation communities and high vegetation diversity between biomes as well as a potential influence of summer rain in some locals, likely compromised identification of a clear relationship between δDwax and δDp in this region. Our findings have implications for palaeoenvironmental investigations using δDwax in southern Africa. In the summer rain dominated eastern and central region, δDwax should serve well as a qualitative palaeohydrological recorder. In contrast, the processes influencing δDwax in the winter rain- dominated western and southwestern South Africa remain unclear and, pending further analyses, potentially constrain its use as palaeohydrological proxy in this region.

Soil n-alkane δD and glycerol dialkyl glycerol tetraether (GDGT) distributions along an altitudinal transect from southwest China: Evaluating organic molecular proxies for paleoclimate and paleoelevation

Organic molecular proxies provide a record of the hydrogen isotopic composition of ambient precipitation and paleoenvironmental temperature. In terrestrial settings, two classes of organic biomarkers are commonly utilized to reconstruct these past environmental conditions: straight-chained normal alkanes (n-alkanes) and branched glycerol dialkyl glycerol tetraethers (brGDGTs). We measured the δDn-alkane and the ratio of cyclisation and methylation indexes of branched tetraethers (CBT/MBT) from surface soils along a ∼1000km transect along the southeast margin of the Tibetan Plateau to assess how these proxies record changes in climate parameters that are related to elevation, despite variations in ecosystem. Abundance-weighted δDnC27-31 data record the δD of precipitation across the orogen and indicate minimal change in apparent fractionation (εn-alkane/water=−133‰) with elevation. n-Alkane δD mirrors the pattern of Rayleigh distillation of an air mass undergoing rainout as moisture moves from the Bay of Bengal into the SE margin of the Tibetan plateau. Soil tetraether temperatures provide a reliable measure of modern temperature with an MBT/CBT calculated temperature lapse rate (5.8°C/km) close to the moist adiabat. These data reflect “ground-level” conditions that are independent of the shape of the orogen or the source of moisture. In combination with a Rayleigh distillation model that incorporates temperature and precipitation isotope data, paired tetraether temperature and leaf wax isotope data produce a reliable record of changes in environmental conditions associated with modern elevation change across the largest orogen. However, some tetraether calibrations may impart systematic temperature bias, most notably in cold or dry climates. These calibration uncertainties can result in underestimates of paleoelevations by up to 1.5–2km. Paired isotopic and temperature paleoelevation reconstructions can minimize errors associated with an individual proxy method, however we recommend that application of tetraether temperature paleoelevation reconstruction take potential calibration biases into account in paleoelevation estimates.

Elevation-dependent changes in n-alkane δD and soil GDGTs across the South Central Andes

Surface uplift of large plateaus may significantly influence regional climate and more specifically precipitation patterns and temperature, sometimes complicating paleoaltimetry interpretations. Thus, understanding the topographic evolution of tectonically active mountain belts benefits from continued development of reliable proxies to reduce uncertainties in paleoaltimetry reconstructions. Lipid biomarker-based proxies provide a novel approach to stable isotope paleoaltimetry and complement authigenic or pedogenic mineral proxy materials, in particular outside semi-arid climate zones where soil carbonates are not abundant but (soil) organic matter has a high preservation potential. Here we present δD values of soil-derived n-alkanes and mean annual air temperature (MAT) estimates based on branched glycerol dialkyl glycerol tetraether (brGDGT) distributions to assess their potential for paleoelevation reconstructions in the southern central Andes. We analyzed soil samples across two environmental and hydrological gradients that include a hillslope (26–28°S) and a valley (22–24°S) transect on the windward flanks of Central Andean Eastern Cordillera in NW Argentina. Our results show that present-day n-alkane δD values and brGDGT-based MAT estimates are both linearly related with elevation and in good agreement with present-day climate conditions. Soil n-alkanes show a δD lapse rate (Δ(δD)) of −1.64‰/100 m (R2=0.91, p<0.01) at the hillslope transect, within the range of δD lapse rates from precipitation and surface waters in other tropical regions in the Andes like the Eastern Cordillera in Colombia and Bolivia and the Equatorial and Peruvian Andes. BrGDGT-derived soil temperatures are similar to monitored winter temperatures in the region and show a lapse rate of ΔT=−0.51°C/100 m (R2=0.91, p<0.01), comparable with lapse rates from in situ soil temperature measurements, satellite-derived land-surface temperatures at this transect, and weather stations from the Eastern Cordillera at similar latitude. As a result of an increasing leeward sampling position along the valley transect lapse rates are biased towards lower values and display higher scatter (Δ(δD)=−0.95‰/100 m, R2=0.76, p<0.01 and ΔT=−0.19°C/100 m, R2=0.48, p<0.05). Despite this higher complexity, they are in line with lapse rates from stream-water samples and in situ soil temperature measurements along the same transect. Our results demonstrate that both soil n-alkane δD values and MAT reconstructions based on brGDGTs distributions from the hillslope transect (Δ(δD)=−1.64‰/100 m, R2=0.91, p<0.01 and ΔT=−0.51°C/100 m, R2=0.91, p<0.01) track the direct effects of orography on precipitation and temperature and hence the combined effects of local and regional hydrology as well as elevation.

Monthly changes in chain length distributions and stable carbon isotope composition of leaf n-alkanes during growth of the bamboo Dendrocalamus ronganensis and the grass Setaria viridis

The links between growth condition change, molecular distributions and δ13C values of individual n-alkanes from leaves of the C3 bamboo Dendrocalamus ronganensis Q.H. Dai and D.Y. Huang and the C4 grass Setaria viridis (L.) Beauv. were studied to improve interpretation of the paleoenvironmental signatures of plant wax n-alkanes in sediments and soils. The molecular composition and isotopic composition of the dominant n-alkanes in fresh leaves sampled every month over the growing seasons of the plants were determined and compared against climate conditions. The average n-alkane chain length from both species was lower in the early stages of the growing season and became larger in the summer months with higher temperature and lower precipitation. The δ13C values of C27 and C29 from the leaves of D. ronganensis and of C29 and C33 from the leaves of S. viridis correlated with monthly mean temperature and precipitation amount. The isotope differences between the two dominant long chain n-alkanes in both plant species also showed positive correlation with mean monthly temperatures. The δ13C values for individual n-alkanes and especially the isotope difference between two individual n-alkanes may be good proxies for reconstructing paleoenvironmental conditions from plant wax components in geological archives.

The fate of plant wax lipids in a model forest ecosystem under elevated CO2 concentration and increased nitrogen deposition

Atmospheric CO2 concentration and nitrogen (N) deposition have been altered by anthropogenic activity and they affect global biogeochemical cycles. It is still not clear how these environmental changes influence the storage and cycling of organic matter (OM) in soils, although this plays a key role in the biogeochemistry of terrestrial ecosystems. Here, we used n-alkanes as biomarkers for plant-derived OM in specific soil fractions. We investigated the effect of elevated CO2 concentration and increased N deposition on the molecular and isotopic (δ13C) composition of n-alkanes in above-ground and below-ground tree biomass (beech and spruce) and soil density fractions.The n-alkane distribution patterns of spruce needles and root biomass were well reflected in those of particulate soil OM. Long chain (C27–34) n-alkanes were preserved in soil by association with soil minerals, while mid-chain (C20–27) n-alkanes from plant biomass and particulate soil OM were ultimately degraded due to lack of physical protection. Renewal of n-alkanes was lower in roots and spruce needles than in beech leaves. Similar low renewal of n-alkanes in soil reflected the low input of n-alkanes from plant biomass to the soil n-alkane pool. n-Alkane biosynthesis in beech leaves was modified under high N deposition and similar effects were observed for n-alkanes in mineral soil fractions and bulk soil. Here, different biosynthesis of n-alkanes in beech leaves under high N deposition consequently led to a modified input of n-alkanes from plant biomass to soil, with a relatively low proportion of new (experimentally-derived) n-alkanes bound to soil minerals.

Evaluation of microbial population and functional genes during the bioremediation of petroleum-contaminated soil as an effective monitoring approach

This study investigated the abundance and diversity of soil n-alkane and polycyclic aromatic hydrocarbon (PAH)-degrading bacterial communities. It also investigated the quantity of the functional genes, the occurrence of horizontal gene transfer (HGT) in the identified bacterial communities and the effect that such HGT can have on biostimulation process. Illumina sequencing was used to detect the microbial diversity of petroleum-polluted soil prior to the biostimulation process, and quantitative real-time PCR was used to determine changes in the bacterial community and functional genes (alkB, phnAc and nah) expressions throughout the biostimulation of petroleum-contaminated soil. The illumine results revealed that γ-proteobacteria, Chloroflexi, Firmicutes, and δ-proteobacteria were the most dominant bacterial phyla in the contaminated site, and that most of the strains were Gram-negative. The results of the gene expression results revealed that gram-negative bacteria and alkB are critical to successful bioremediation. Failure to maintain the stability of hydrocarbon-degrading bacteria and functional gene will reduce the extend to which alkanes and PAHs are degraded. According to the results of the study, the application of a C:N:P ratio of was 100:15:1 in the biodegradation experiment resulted in the highest rate at which petroleum hydrocarbons were biodegraded. The diversity of pollutant-degrading bacteria and the effective transfer of degrading genes among resident microorganisms are essential factors for the successful biostimulation of petroleum hydrocarbons. As such, screening these factors throughout the biostimulation process represents an effective monitoring approach by which the success of the biostimulation can be assessed.

Different altitude effect of leaf wax n-alkane δD values in surface soils along two vapor transport pathways, southeastern Tibetan Plateau

Here we present 76 hydrogen isotope values for leaf wax-derived n-alkanes (δDwax, being the abundance-weighted average δD values of C29 and C31n-alkanes) in surface soils along two vapor transport pathway transects running across the southeastern Tibetan Plateau (TP). The first is the South Himalayan (SH) Transect (with an altitude range of 1660–5050m∗ above sea level, and representing the Southern Himalayan air mass), and the second a combination of the Zayu–Bomi (ZB; altitude 1468–4800m) and Bayi-Lhasa (BL; altitude 3050–4970m) transects, together representing the Brahmaputra–Yarlung Zangbo river valley air mass. δDwax values and their relation to altitude did exhibit some variability. Nonetheless, altitude was the predominant factor in determining soil δDwax values, with a lapse rate of −2.2‰/100m (R2=0.78; n=76) based on overall soil δDwax values, despite changes in precipitation amount, relative humidity (RH) and vegetation type. Furthermore, our εwax/rw values (the isotopic fractionation between the δDwax and δD values of river water (δDrw)) remained approximately constant at −104.5‰ (ranging between −99‰ and −110‰), confirming that the relation between altitude and isotopic hydrology (river water and precipitation) controls the n-alkane δDwax altitudinal gradients for all of the sample localities in the Southern Himalaya (SH) and the southern TP. This endorses the efficacy of using δDwax to reconstruct paleoelevation. Along the SH Transect, soil δDwax values varied from −151‰ to −264‰, with an altitudinal lapse rate of −2.4‰/100m (R2=0.84, n=32). This was due to the significant change in altitude over short distances. Along the ZB Transect, soil δDwax values varied from −174‰ to −263‰, with a lower lapse rate of −1.9‰/100m (R2=0.68, n=29). The altitudinal lapse rate was −3.3‰/100m above 3000m (R2=0.63, n=19), and −1.35‰/100m below 3000m (R2=0.64, n=10); in the lower sub-transect, the latitudinal effect significantly counteracted the altitude effect. Along the BL Transect, soil δDwax values varied from −205‰ to −244‰, with an altitudinal lapse rate of −1.4‰/100m (R2=0.74, n=15). The lower lapse rate for soil δDwax reflects a combination of local continental and altitudinal effects. The differences we observed in soil δDwax altitudinal lapse rates along the two vapor transport pathways imply that they are affected by subtle local environmental conditions, moisture sources, latitude and topography. This should be considered fully during any reconstruction of paleoelevation.

Cuticular waxes in alpine meadow plants: climate effect inferred from latitude gradient in Qinghai-Tibetan Plateau.

Alpine meadow ecosystems are susceptible to climate changes. Still, climate impact on cuticular wax in alpine meadow plants is poorly understood. Assessing the variations of cuticular wax in alpine meadow plants across different latitudes might be useful for predicting how they may respond to climate change. We studied nine alpine meadows in a climate gradient in the east side of Qinghai-Tibetan Plateau, with mean annual temperature ranging from -7.7 to 3.2°C. In total, 42 plant species were analyzed for cuticular wax, averaged 16 plant species in each meadow. Only four plant species could be observed in all sampling meadows, including Kobresia humilis,Potentilla nivea,Anaphalis lacteal, and Leontopodium nanum. The amounts of wax compositions and total cuticular wax in the four plant species varied among sampling meadows, but no significant correlation could be observed between them and temperature, precipitation, and aridity index based on plant species level. To analyze the variations of cuticular wax on community level, we averaged the amounts of n-alkanes, aliphatic acids, primary alcohols, and total cuticular wax across all investigated plant species in each sampling site. The mean annual temperature, mean temperature in July, and aridity index were significantly correlated with the averaged amounts of wax compositions and total cuticular wax. The average chain length of n-alkanes in both plant and soil linearly increased with increased temperature, whereas reduced with increased aridity index. No significant correlation could be observed between mean annual precipitation and mean precipitation from June to August and the cuticular wax amounts and average chain length. Our results suggest that the survival of some alpine plants in specific environments might be depended on their abilities in adjusting wax deposition on plant leaves, and the alpine meadow plants as a whole respond to climate change, benefiting the stability of alpine meadow ecosystem.

Do n-alkane biomarkers in soils/sediments reflect the δ2H isotopic composition of precipitation? A case study from Mt. Kilimanjaro and implications for paleoaltimetry and paleoclimate research

During the last decade compound-specific deuterium (2H) analysis of plant leaf wax-derived n-alkanes has become a promising and popular tool in paleoclimate research. This is based on the widely accepted assumption that n-alkanes in soils and sediments generally reflect δ2H of precipitation (δ2Hprec). Recently, several authors suggested that δ2H of n-alkanes (δ2Hn-alkanes) can also be used as a proxy in paleoaltimetry studies. Here, we present results from a δ2H transect study (∼1500 to 4000 m above sea level [a.s.l.]) carried out on precipitation and soil samples taken from the humid southern slopes of Mt. Kilimanjaro. Contrary to earlier suggestions, a distinct altitude effect in δ2Hprec is present above ∼2000 m a.s.l., that is, δ2Hprec values become more negative with increasing altitude. The compound-specific δ2H values of nC27 and nC29 do not confirm this altitudinal trend, but rather become more positive both in the O-layers (organic layers) and the Ah-horizons (mineral topsoils). Although our δ2Hn-alkane results are in agreement with previously published results from the southern slopes of Mt. Kilimanjaro [Peterse F, van der Meer M, Schouten S, Jia G, Ossebaar J, Blokker J, Sinninghe Damsté J. Assessment of soil n-alkane δD and branched tetraether membrane lipid distributions as tools for paleoelevation reconstruction. Biogeosciences. 2009;6:2799–2807], a re-interpretation is required given that the δ2Hn-alkane results do not reflect the δ2Hprec results. The theoretical framework for this re-interpretation is based on the evaporative isotopic enrichment of leaf water associated with the transpiration process. Modelling results show that relative humidity, decreasing considerably along the southern slopes of Mt. Kilimanjaro (from 78 % in ∼2000 m a.s.l. to 51 % in 4000 m a.s.l.), strongly controls δ2Hleaf water. The modelled 2H leaf water enrichment along the altitudinal transect matches well the measured 2H leaf water enrichment as assessed by using the δ2Hprec and δ2Hn-alkane results and biosynthetic fractionation during n-alkane biosynthesis in leaves. Given that our results clearly demonstrate that n-alkanes in soils do not simply reflect δ2Hprec but rather δ2Hleaf water, we conclude that care has to be taken not to over-interpret δ2Hn-alkane records from soils and sediments when reconstructing δ2H of paleoprecipitation. Both in paleoaltimetry and in paleoclimate studies changes in relative humidity and consequently in δ2Hn-alkane values can completely mask altitudinally or climatically controlled changes in δ2Hprec.

Assessment of soil organic contamination in a typical petrochemical industry park in China.

The concentrations of total petroleum hydrocarbons (TPH), n-alkanes (n-C8 through n-C40), and 16 polycyclic aromatic hydrocarbons (PAHs) in soils were determined to assess the level of organic contamination in soils from the Da-gang Petrochemical Industry Park with several big state-run enterprises, a recent rapid flourishing park in China. The results showed that the concentration of TPH in soil was high, up to 20 ng/g-12.8478%; in particular, the content in most sites ranged from 1 to 2%. Thus, it is clear that soil environment in the Da-gang Petrochemical Industry Park has been seriously polluted by TPH according to the Nemerow pollution index method. Furthermore, the average concentration of Σ(n-C>16 through n-C34) in 30 sampling sites was above the maximum limit set for F3 under all the conditions in the Canada-wide standards for petroleum hydrocarbons (PHC CWS) with 43.33-93.33% soil samples exceeding F3 standards, and n-alkanes possessing higher concentrations were proved much abundant alkanes in this study. Besides, the predominance of even n-alkanes and lower carbon preference index (CPI) demonstrated that n-alkanes in surface soils were mainly caused by anthropogenic inputs, while the concentration of Σ16-PAHs was in the range of 1652.5-8217.3 ng/g and the BaA/(BaA + Chr) and Flu/(Flu + Pyr) ratios indicated that pyrogenic PAHs may be the dominant PAHs in most soils with the contribution of petrogenic hydrocarbons in some sites.

Altitudinal shift in stable hydrogen isotopes and microbial tetraether distribution in soils from the Southern Alps, NZ: Implications for paleoclimatology and paleoaltimetry

The Southern Alps are an ideal locality for studying patterns of isotopic fractionation associated with orographic precipitation. We have evaluated whether altitudinal change is reflected in the stable hydrogen isotopic composition (δ2H) of stream water, plant stem water and leaf wax lipids (n-alkanes) from living plants and soils, as well as in soil temperature. Samples were collected along an altitudinal transect from the windward side of the Southern Alps to Lake Hawea in the rain shadow. The results indicate that δ2H values of stem water overlap with stream water, demonstrating a gradual decrease with elevation that complied with modeled Rayleigh distillation, reflecting an isotopic lapse rate of −18.0 (±1.1, 1σ)‰/km. Leaf and soil n-alkanes shared similar δ2H values and were 2H depleted relative to stem/stream waters. The values for soil n-alkanes indicated an isotopic lapse rate of −21.8 (±2.0, 1σ)‰/km, consistent with precipitation data and long term observations. MBT/CBT derived soil temperature values based on the relative distribution of microbial tetraether lipids were similar to midsummer temperature observations, displaying an elevational decrease rate of −5.6 (±1.5, 1σ)°C/km, consistent with regional and global observations.The results indicate that sedimentary lipid δ2H and microbial tetraether temperature estimates captured altitudinal trends in the isotopic composition of precipitation and mean temperature and further support their application in the reconstruction of past climate and surface uplift histories. However, notable differences in isotopic composition and temperature estimates between in situ soils and those with downslope transport of material emphasize the importance of facies analysis when interpreting past systems.

Abundance and diversity of soil petroleum hydrocarbon-degrading microbial communities in oil exploring areas.

Alkanes and polycyclic aromatic hydrocarbons (PAHs) are the commonly detected petroleum hydrocarbon contaminants in soils in oil exploring areas. Hydrocarbon-degrading genes are useful biomarks for estimation of the bioremediation potential of contaminated sites. However, the links between environmental factors and the distribution of alkane and PAH metabolic genes still remain largely unclear. The present study investigated the abundances and diversities of soil n-alkane and PAH-degrading bacterial communities targeting both alkB and nah genes in two oil exploring areas at different geographic regions. A large variation in the abundances and diversities of alkB and nah genes occurred in the studied soil samples. Various environmental variables regulated the spatial distribution of soil alkane and PAH metabolic genes, dependent on geographic location. The soil alkane-degrading bacterial communities in oil exploring areas mainly consisted of Pedobacter, Mycobacterium, and unknown alkB-harboring microorganisms. Moreover, the novel PAH-degraders predominated in nah gene clone libraries from soils of the two oil exploring areas. This work could provide some new insights towards the distribution of hydrocarbon-degrading microorganisms and their biodegradation potential in soil ecosystems.

Downstream alteration of the composition and biodegradability of particulate organic carbon in a mountainous, mixed land-use watershed

Despite increasing recognition of storm-induced organic carbon (C) export as a significant loss from the terrestrial C balance, little is known about the biodegradation and chemical transformation of particulate organic carbon (POC) in mountainous river systems. We combined analyses of C isotopes, solution-state 1H NMR, and lipid biomarkers with biodegradable dissolved organic C (BDOC) measurements to investigate downstream changes of POC composition and biodegradability at a mountainous, mixed land-use watershed in South Korea. Water and suspended sediment (SS) samples were collected in a forested headwater stream, a downstream agricultural stream, and two downstream rivers during peak flow periods of four storm events, using either sequential grab sampling from the headwater stream to the most downstream river within a few hours around the peak flow or sediment samplers deployed during the whole storm event. DOC concentrations exhibited relatively small changes across sites, whereas POC concentrations were highest in the agricultural stream, and tapered along downstream reaches. The δ13C and δ15N of SS in the agricultural stream were distinct from up- and downstream signatures and similar to those for erosion source soils and lake bottom sediment, although increases in radiocarbon age indicated continuous compositional changes toward the lake. 1H NMR spectra of SS and deposited sediment exhibited downstream decreases in carbohydrates and lignin but enrichment of organic structures related to microbial proteins and plant wax. The downstream sediments had more microbial n-alkanes and lipid markers indicating anthropogenic origin such as coprostanol compared to the forest soil n-alkanes dominated by plant wax. While the BDOC concentrations of filtered waters differed little between sites, the BDOC concentrations and protein- to humic-like fluorescence ratios of DOC leached from SS during a 13-day incubation were higher in downstream rivers, pointing to contribution of labile POC components to the enhanced biodegradation. Overall, inputs of microbial and anthropogenic origin, in interplay with deposition and mineralization, appear to substantially alter POC composition and biodegradability during downstream transport, raising a question on the conventional view of mountainous river systems as a passive conduit of storm pulses of POC.

Environmental control on eastern broadleaf forest species’ leaf wax distributions and D/H ratios

Local climate and environment broadly affect the deuterium/hydrogen (D/H) ratios of plant materials, however the degree to which an individual plant’s leaf waxes D/H ratios are affected by these parameters remains in question. Understanding these issues is particularly important in order to reconstruct past floral transitions and changes in the paleohydrologic cycle. For this study, we sampled five co-occurring tree species, Acer rubrum, Platanus occidentalis, Juniperus virginiana, Pinus taeda, and Pinus strobus and soils at forty sites along the East Coast of the US, from Florida to Maine. Hydrogen isotopic compositions of leaf wax n-alkanes, stem and surface waters were analyzed and compared against high-resolution temperature, precipitation, relative humidity, and vapor pressure deficit data to determine environmental controls on isotopic composition.Our results demonstrate that each tree species produce a unique distribution of n-alkanes with distinct chain length pattern. Average n-alkane chain lengths recovered from soils, A. rubrum, and J. virginiana leaves show significant correlations with mean annual temperature. δD values of A. rubrum leaf n-alkanes were strongly correlated to modeled mean annual precipitation δD values and other climate parameters related to latitude (i.e. temperature, relative humidity, vapor pressure deficit), while the δD values of J. virginiana n-alkanes were not. Differences in correspondence may reflect the timing of leaf wax synthesis between the two species. Further, soil n-alkane D/H compositions were strongly correlated to modeled mean annual precipitation δD values, while the apparent hydrogen isotopic fractionation was not. These findings indicate that the isotope ratio of n-alkanes from soils in Eastern North American forests and similar ecosystems likely represents a time-averaged value that smooth out the environmental influence any one plant experiences.

Latitudinal variations of CPI values of long-chain n-alkanes in surface soils: Evidence for CPI as a proxy of aridity

Carbon preference index (CPI) of long-chain n-alkanes preserved in surface soil increases gradually from southeastern China to the north margin of Loess Plateau. Along this latitudinal transect, the CPI value correlates to relative humidity, precipitation, and temperature with a negative linear relationship, respectively, whereas the correlation of CPI to temperature is relatively weak. In the Wuyi, Shennongjia, and Tianshan Mountains, CPI values do not change systemically with altitude increasing (or temperature decreasing). However, mean value of CPI for the individual mountain increases in turn from the humid mountain to the arid. These results jointly suggest that aridity (or humidity) is a dominate climate factor in altering soil CPI value. High CPI values of geological records therefore indicate the arid paleoclimate. Though long-chain n-alkanes in soil are derived mainly from leaf wax of terrestrial vascular plants, the regular latitudinal variations of soil CPI might not be caused by the change of vegetation. We speculate that increased long-chain n-alkanes from microbes and/or enhanced biodegradation in the humid climate lead to the decrease of soil CPI.

Effects of humic acid on phytodegradation of petroleum hydrocarbons in soil simultaneously contaminated with heavy metals

The use of humic acid (HA) to enhance the efficiency of phytodegradation of petroleum hydrocarbons in soil contaminated with diesel fuel was evaluated in this study. A sample of soil was artificially contaminated with commercially available diesel fuel to an initial total petroleum hydrocarbons (TPH) concentration of 2300 mg/kg and four heavy metals with concentrations of 400 mg/kg for Pb, 200 mg/kg for Cu, 12 mg/kg for Cd, and 160 mg/kg for Ni. Three plant species, Brassica campestris, Festuca arundinacea, and Helianthus annuus, were selected for the phytodegradation experiment. Percentage degradation of TPH in the soil in a control pot supplemented with HA increased to 45% from 30% without HA. The addition of HA resulted in an increases in the removal of TPH from the soil in pots planted with B. campestris, F. arundinacea, and H. annuus, enhancing percentage degradation to 86%, 64%, and 85% from 45%, 54%, and 66%, respectively. The effect of HA was also observed in the degradation of n-alkanes within 30 days. The rates of removal of n-alkanes in soil planted with B. campestris and H. annuus were high for n-alkanes in the range of C11–C28. A dynamic increase in dehydrogenase activity was observed during the last 15 days of a 30-day experimental period in all the pots amended with HA. The enhanced biodegradation performance for TPHs observed might be due to an increase in microbial activities and bioavailable TPH in soils caused by combined effects of plants and HA. The results suggested that HA could act as an enhancing agent for phytodegradation of petroleum hydrocarbons in soil contaminated with diesel fuel and heavy metals.

Compound-specific hydrogen isotopes of long-chain n-alkanes extracted from topsoil under a grassland ecosystem in northern China

To investigate characteristics of H isotope variation in long-chain n-alkanes (δDn-alkanes) from higher plants in surface soils under a single ecosystem, 12 samples from a basalt regolith were randomly collected from Damaping in Wanquan County of Zhangjiakou in North China. Molecular distribution and C (δ13Cn-alkanes) and H isotopes of long-chain n-alkanes, as well as C isotopes of TOC (δ13CTOC), were analyzed. Both δ13CTOC and δ13Cn-alkanes values from four representative dominant long-chain n-alkanes (n-C27, n-C29, n-C31, n-C33) derived from terrestrial higher plants show minor variations among the 12 samples, indicating the major contributor is from local grasses with a uniform C3 photosynthetic pathway. In contrast, variations in δDn-alkanes values of the four long-chain n-alkanes are relatively large, with the more abundant homologs generally showing more negative δDn-alkanes values and less variation. However, variation of <30‰ among weighted averaged δDn-alkanes values of the four long-chain n-alkanes is not only less than that among δDn-alkanes values for different modern terrestrial C3 grasses from the specific locations, but also less than the literature values of δDn-alkanes of long-chain n-alkanes for single species over different seasons. Thus, because the sources of long-chain n-alkanes in surface soils and sediments are similarly from multiple individual plants, our results are significant in confirming that paleoclimatic, paleoenvironmental and paleohydrological information can be interpreted more accurately from δDn-alkanes values of long-chain n-alkanes from sediments, particularly terrestrial sediments with organic matter derived from in-situ plants.

Plant tolerance to diesel minimizes its impact on soil microbial characteristics during rhizoremediation of diesel-contaminated soils

Soil contamination due to petroleum-derived products is an important environmental problem. We assessed the impacts of diesel oil on plants ( Trifolium repens and Lolium perenne) and soil microbial community characteristics within the context of the rhizoremediation of contaminated soils. For this purpose, a diesel fuel spill on a grassland soil was simulated under pot conditions at a dose of 12,000 mg diesel kg − 1 DW soil. Thirty days after diesel addition, T. repens (white clover) and L. perenne (perennial ryegrass) were sown in the pots and grown under greenhouse conditions (temperature 25/18 °C day/night, relative humidity 60/80% day/night and a photosynthetic photon flux density of 400 μmol photon m − 2 s − 1 ) for 5 months. A parallel set of unplanted pots was also included. Concentrations of n-alkanes in soil were determined as an indicator of diesel degradation. Seedling germination, plant growth, maximal photochemical efficiency of photosystem II (F v/F m), pigment composition and lipophylic antioxidant content were determined to assess the impacts of diesel on the studied plants. Soil microbial community characteristics, such as enzyme and community-level physiological profiles, were also determined and used to calculate the soil quality index (SQI). The presence of plants had a stimulatory effect on soil microbial activity. L. perenne was far more tolerant to diesel contamination than T. repens. Diesel contamination affected soil microbial characteristics, although its impact was less pronounced in the rhizosphere of L. perenne. Rhizoremediation with T. repens and L. perenne resulted in a similar reduction of total n-alkanes concentration. However, values of the soil microbial parameters and the SQI showed that the more tolerant species ( L. perenne) was able to better maintain its rhizosphere characteristics when growing in diesel-contaminated soil, suggesting a better soil health. We concluded that plant tolerance is of crucial importance for the recovery of soil health during rhizoremediation of contaminated soils.