Hydrogen isotopic composition of plant leaf wax in response to soil moisture in an arid ecosystem of the northeast Qinghai-Tibetan Plateau, China

The hydrogen isotopic composition of plant leaf wax (delta D-wax) is used as an important tool for paleohydrologic reconstruction. However, the understanding of the relative importance of environmental and biological factors in determining delta D-wax values still remains incomplete. To identify the effects of soil moisture and plant physiology on delta D-wax values in an arid ecosystem, and to explore the implication of these values for paleoclimatic reconstruction, we measured delta D values of soil water (delta D-water) and delta D-wax values in surface soils along two distance transects extending from the lakeshore to wetland to dryland around Lake Qinghai and Lake Gahai on the northeast Qinghai-Tibetan Plateau. The results showed that the delta D-water values were negatively correlated with soil water content (SWC) (R-2=0.9166), and ranged from -67 parts per thousand to -46 parts per thousand with changes in SWC from 6.2% to 42.1% in the arid areas of the Gangcha (GCh) and Gahai (GH) transects. This indicated that evaporative D-enrichment in soil water was sensitive to soil moisture in an arid ecosystem. Although the shift from grasses to shrubs with increasing aridity occurred in the arid area of the GH transect, the delta D-wax values in surface soils from the arid areas of the two transects still showed a negative correlation with SWC (R-2=0.6835), which may be due to the controls of primary evaporative D-enrichment in the soil water and additional transpirational D-enrichment in the leaf water on the delta D-wax values. Our preliminary research suggested that delta D-wax values can potentially be applied as a paleo-humidity indicator on the northeast Qinghai-Tibetan Plateau.</p

Distributions and hydrogen isotopic compositionsof plant leaf wax from Orinus kokonorica along ageneral aridity gradient around Lake Qinghai,China

To quantitatively analyze the response of distributions and hydrogen isotopic compositions (&delta;D) of plant leaf wax to moisture, and to better understand their implications for paleoclimatic reconstruction, we measured average chain length (ACL) and &delta;D values of n-alkanes and n-fatty acids (n-FAs) from Orinus kokonorica, a typical and representative plant in Lake Qinghai area, along a distance transect extending from lakeshore to wetland to dryland in the arid ecosystem. The results showed that the ACL values of n-alkanes and n-FAs were negatively correlated with soil water content (SWC) with R2=0.593 and R2=0.924, respectively. This is as a result of plant&rsquo;s response to water loss with more abundance in long-chain n-alkyl lipids under increasing aridity by analyzing relationships between the molecular ratios of long-chain n-alkyl lipids (n-alkanes and n-FAs) from O. kokonorica and SWC. The &delta;D values of C29 n-alkane and C28 n-FA were also negatively correlated with SWC with R2=0.778 and R2=0.760, respectively, which may due to enhanced D-enrichment in leaf water by evapotranspiration (soil water evaporation and leaf water transpiration) with increasing aridity. Our results demonstrated that moisture exerts a significant control on the ACL and &delta;D values from O. kokonorica in an arid ecosystem. This preliminary study on a modern single plant (O. kokonorica) sets a foundation for comprehending these values as quantitative proxies for paleo-humidity reconstruction.</p

Occurrence of Organochlorine Pesticides in Surface Soils from College School Yards of Xi'an, China

In this study, surface soil samples from 14 representative college school yards in Xi&#39;an, the capital city of Shaanxi province, China, were collected and analyzed for 21 organochlorine pesticides (OCPs). The total concentrations were in the ranges of 0.2-67.0 ng/g. HCHs and DDTs were the most dominant compounds among the 21 OCPs, and their concentrations ranged from 0.1-8.5 to 0.1-56.3 ng/g, respectively. Source identification analysis indicated that the residues of HCHs and DDTs were mostly due to historical use of these chemicals or from other source regions. The quality of all the Xi&#39;an college school yard soils except sample XAUAT was classified as low pollution by OCPs according to the National Environmental Quality Standards for Soils of China (GB15618-1995).</p

Water salinity and productivity recorded by ostracod assemblages and their carbon isotopes since the early Holocene at Lake Qinghai on the northeastern Qinghai-Tibet Plateau, China

Lake Qinghai, the largest saline lake in China, is a closed-basin lake with a salinity of 16 g/L and is situated in the sensitive semi-arid zone between the Asian monsoon-controlled area and the westerly jet stream-influenced area. With the support of the International Continental Drilling Program (ICDP), Lake Qinghai was drilled in 2005 using the ICDP GLAD800 drilling system. Two ostracod species, i.e., Limnocythere inopinata and Eucypris mareotica, were found only in the upper 5.15 m of the drilled core 1F, covering 12 ka, where the depths of 5.15 to 3.15 m (12.0-7.4 ka) are dominated by E. mareotica and the depths of 3.15 to 0 m (7.4 ka-present) are dominated by L inopinata. The presence of the ostracod Eucypris mareotica was related to the high salinity of the water ( exceeding 30 g/L), and the dominant species of ostracod (Limnocythere inopinata) was related to the brackish water (3-16 g/L) in the Lake Qinghai area (Li et al., 2010). The changes in the carbon isotope in the modem ostracod shells were related to the water salinity and productivity on the lake bottom in Lake Qinghai (Li et al., 2012). On this basis, this study evaluated the changes in lake salinity and productivity and their relation to climatic change using changes in the relative abundances of ostracod fossil species assemblages and the stable carbon isotope in ostracod shells from core 1F and other published data (e.g., total organic carbon content, delta C-13(TOC) and ice core delta O-18). The single occurrence of ostracod Eucypris mareotica and the highest average delta C-13 values (-02 parts per thousand) in their shells indicated that the lake salinity and productivity were very high, a phenomenon that was related to the high temperature and low lake level (caused by intense evaporation even though the precipitation increased sharply) in the early Holocene. The deceased abundance of ostracod E. mareotica and the lowest average delta C-13 values (-2.0 parts per thousand.) in ostracod shells showed that the lake salinity and productivity decreased because of the increased lake level and decreased temperatures in the middle Holocene. Finally, the lake level decreased, but the water salinity and the lake productivity gradually increased because of the high E/I ratio related to the increased temperature in the late Holocene.</p

An improved method of ion exchange for nitrogen isotope analysis of water nitrate

Nitrate nitrogen and oxygen isotopes have been widely used to trace the nitrogen biogeochemical cycle by identifying NO(3)(-) sources. An improved method of anion exchange was developed to measure delta(15)N-NO(3)(-) in fresh water by continuous-flow elemental analyzer/isotope ratio mass spectrometry (EA-IRMS). We used a custom-built exchange resin column, a peristaltic pump and the oven-drying method in our experiments. Consequently, the amount of Ag(2)O used as a neutralizer was reduced, time was saved, and operation became simpler than before. Meanwhile, analytical precision remained identical to previous studies. KNO(3) solutions were prepared at 0.2, 5 and 25 mg-N L(-1) from KNO(3) standard salt (delta(15)N = +6.27 parts per thousand), and the average delta(15)N values of the solutions after having been absorbed on and subsequently stripped from anion columns were +6.62 +/- 0.22 parts per thousand (n = 6), +6.38 +/- 0.09 parts per thousand (n = 6), and +6.26 +/- 0.07 parts per thousand (n = 6), respectively. In addition, the &quot;natural&quot; water sample delta(15)N-NO(3)(-) showed consistency in comparison to standards, and the mean standard deviation by the different approaches was 0.08 parts per thousand. Accordingly, by these improvements the anion exchange resin technique is demonstrated to be more suitable for measuring delta(15)N in NO(3)(-) than original techniques. (C) 2010 Elsevier B.V. All rights reserved.</p

Isotopic indicators of carbon and nitrogen cycles in river catchments during soil erosion in the arid Loess Plateau of China

Water and soil erosion are serious issues in the Loess Plateau in China. The carbon and nitrogen cycles along river catchments during the soil erosion process have historically been poorly understood. In this study, we investigated the concentrations and isotopic signatures of carbon and nitrogen in river water, river-suspended materials and soil. Our results revealed that, in river catchments, from the water sources to their confluence with larger rivers, (1) the isotopic signatures of dissolved inorganic carbon (DIC) varied from -9.2 to -6.7%. in May and from -12.9 to -9.3%. in August. In addition, delta C-13(DIC) values were more negative in August than in May. These differences may be related to changes in the source and soil erosion. (2) The NO3- nitrogen isotopic signature of the water ranged from 5.1 to 10.3 parts per thousand in May and from 3.5 to 6.4 parts per thousand in August. The delta N-15(org) of the suspended material also became more positive; similar trends can help us to understand the N cycle during soil erosion. Our investigation suggests that soil CO2/carbonates and soil NO3- are dissolved into the Loess Plateau rivers during rainfall and soil erosion. The concentration and the isotopic signatures of carbon and nitrogen in rivers can be used for the quantitative evaluation of carbon and nitrogen transfer from the soil to the Loess Plateau rivers.</p

Ion-Exchange Method for Nitrogen Isotope Analysis of Nitrate in Soil

The N isotopic composition of nitrate in soil extraction samples can deliver critical information regarding the soil N cycle for biogeochemical processes. Here, we present a preparatory ion-exchange resin method for the N isotope analysis of NO3- in saturated CaSO4 soil extracts. This method showed that the NO3- in saturated CaSO4 soil extracts could be effectively separated from SO42- and other anions to avoid interference from the large amounts of coexisting anions in soil extracts. The nitrate recovery rate ranged from 95.54 to 105.04%. The resulting average natural abundance of N-15 (delta N-15) values of the KNO3 solutions and the KNO3-CaSO4 solutions were +6.04 +/- 0.03 parts per thousand (n = 6) and +6.03 +/- 0.12 parts per thousand (n = 6), respectively. The average delta N-15 value of the KNO3 salt was +6.07 +/- 0.06 parts per thousand (n = 12). Therefore, we concluded that N isotopic fractionation did not occur throughout the process. In addition, the delta N-15-NO3- values of natural soils extracted using saturated CaSO4 were reproducible between separate analytical runs. The standard deviations of the delta N-15-NO3- for the three different soils were +/- 0.17 parts per thousand (n = 4), +/- 0.04 parts per thousand (n = 3), and +/- 0.34 parts per thousand (n = 3). Our ion-exchange method was effective at separating NO3- from other anions in the soil extracts and was suitable for measuring the delta N-15-NO3 of saturated CaSO4 soil extracts.</p

Carbon chain length distribution in n-alkyl lipids: A process for evaluating source inputs to Lake Qinghai

Lake sediments generally contain a mixture of terrestrial and aquatic source inputs, and determining the major inputs is important for understanding geological records in paleoenvironment and paleoclimate research. In this study we describe the distribution of n-alkanes and n-fatty acids (FAs) in representative modern plants from around Lake Qinghai. We found a significant difference in the average length of n-FA carbon chains (ACL Fa(16-32)) in terrestrial (23.3) and aquatic plants (18.6). The results reveal that ACL Fa(16-32) may essentially serve as a proxy for evaluating the major source inputs to lake sediments. Assessment of surface sediments from the lake showed that the FAs originated from a mixture of inputs, with the aquatic source input predominant at most sites. Additionally, the delta D values of sediment mid-chain n-acids (C-22) showed a relationship with the ACL Fa(16-32) proxy: an increased Fa ACL corresponded to more negative hydrogen isotope ratio values. We suggest that different sources should be considered and ACL Fa(16-32) could be a potential calibration proxy before using delta D values to extract reliable isotopic information from lake water. More attention should be paid to source inputs and their relationship to other geochemical proxies in future studies of lake sediments.</p

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

We present delta D(wax) values from different forms of plants and soils, and delta D(sw), values from soil water along the northern slope of Mount Taibai, China. The results show a highly negative linear correlation of the delta D(wax) values for soils with altitude (R(2) 0.74) and we observed the same correlation for delta D(sw) values of soil water with altitude (R(2) 0.68). The delta D(wax) of living plants behaves like the soil, but does not exhibit a significant linear correlation with altitude (R(2) 0.11). The delta D(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 &quot;altitude effect&quot; can not be well documented for the delta D(wax) values of living plants, which may be due to differences in plant type and/or evapotranspiration controlled by the plant microclimate. The epsilon(wax-sw) values of woody plants, grasses and soil show minor fluctuations with altitude. However, the epsilon(wax-sw) and delta D(wax) values of woody plants are roughly 51 parts per thousand and 50 parts per thousand more positive, respectively, than those of grasses, suggesting that an &quot;altitude effect&quot; could be documented in the delta D(wax) of woody plants and grasses, with each responding independently to changes in precipitation along the altitude transect. Additionally, the epsilon(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 delta D(wax) values of soil n-alkanes and altitude.</p

Variations in the concentration and isotopic composition of nitrate nitrogen in wet deposition and their relation with meteorological conditions in Xi'an city, Northwest China

The characterization of N isotopes in NO(3) is an effective method to determine NO(3) sources and NO(X) transformation mechanisms. Xi&#39;an city is located in a semi-arid to arid region and is representative of most cities in the north and NW of China. Few studies have been carried out on N isotopes in wet deposition of this region. In this study, 29 wet deposition samples were collected from Xi&#39;an city in northwestern China between March 2008 and May 2009 and analyzed for their volume-weighted NO(3)(-) and NH(4)(+) concentrations and N isotopic compositions. Volume-weighted mean concentrations were 3.9 mu mol L(-1) for NO(3)(-) and 13.5 mu mol L(-1) for NH(4)(+). The wet deposition samples showed pronounced seasonal variation in the N isotopic composition of NO(3)(-) with delta(15)N-NO(3)(-) ranging from -2.0 parts per thousand to +9.2 parts per thousand over the course of a year, with a mean value of +2.6 +/- 2.6 parts per thousand. Higher delta(15)N-NO(3)(-) values were observed in the cool season (November-May, +3.8 parts per thousand) than in the warm season (June-October, +1.0 parts per thousand). In autumn and winter in particular, the mean delta(15)N-NO(3)(-) values were clearly different (+0.9 parts per thousand and +6.7 parts per thousand, respectively). Various factors are shown to lead to variation in the delta(15)N-NO(3)(-) values in wet deposition. One effect is changing ratios of N derived from agricultural/soil sources (low delta(15)N-NO(3)(-), dominant in the warm season) and fossil fuel-derived NO(X) (high delta(15)N-NO(3)(-), relatively more important in the cool season). However, in the cool season delta(15)N-NO(3)(-) unexpectedly shows a strong negative correlation with SO(2) concentration, which should correlate with fossil fuel combustion inputs. This may be explained by interactions of N-species with other atmospheric pollutants. The results also suggest that temperature may be a significant factor affecting delta(15)N-NO(3)(-). The NO(X) reacts in different ways depending on temperature, leading to a correlation of delta(15)N-NO(3)(-) with temperature. Moreover, the effects of dilution cannot be neglected here, as there is clear seasonal variation in precipitation. When combined with meteorological parameters, the volume-weighted concentrations of NO(3)(-) and NH(4)(-) and air pollutant precursor data, the delta(15)N of NO(3)(-) in wet deposition can be used as a reliable tool for the precise identification of NO(X) transformation mechanisms in the environment and the fate of NO(X) emissions.</p