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

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

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

Profile distribution of soil inorganic carbon along a chronosequence of grassland restoration on a 22-year scale in the Chinese Loess Plateau

Understanding the distribution of soil inorganic carbon (SIC) is meaningful and necessary for assessing soil carbon sequestration. However, changes in SIC following vegetation restoration are still not well studied in the Chinese Loess Plateau (CLP), which has undergone extensive changes from farmland to grassland. Here, we chose three study sites, farmland (FL), grassland restored for 12 years (RG-1) and grassland restored for 22 years (RG-2). The changes of SIC storage were determined under the conversion from a farmland ecosystem to extensively used grassland during the past twenty years. Results showed that the SIC content decreased with vegetation restoration, which averaged 18.5, 13.2 and 11.8 g kg(-1) in FL RG-1 and RG-2, respectively. The SIC content was significantly lower in the top 80 cm in RG-1 and RG-2 than in FL, but showed little variance at depths of 80-200 cm among the study sites. It illustrated that the portion of carbonate was most likely released to the atmosphere as CO2 temporarily due to soil acidification and the increase of soil moisture in restored grassland. The lower delta C-13 value of SIC in RG-1 and RG-2 than in FL was most likely due to the soil carbonate exchanging with more soil respired CO2, with increasing vegetation coverage in restored grassland. Our results indicate that the SIC storage deceases in the restored grassland in situ probably due to the decrease of soil pH and the increase in soil water content The SIC plays an important role when assessing the capacity of carbon sequestration with vegetation restoration in the semi-arid CLP.</p

Carbon isotopic composition of isoprenoid tetraether in surface sediments of Lake Qinghai and surrounding soils

Isoprenoid GDGTs (iGDGTs), along with their constituent biphytanyl moieties, are biomarkers for archaea. In order to obtain more information on identifying the carbon source and potential carbon assimilation pathway of archaea in surface sediments of Lake Qinghai and the surrounding soils, the stable carbon isotopic composition of iGDGT-derived biphytanes as well as its relationship with delta C-13 values of total organic carbon (TOC) and dissolved inorganic carbon (DIC) was investigated. The delta C-13 values of iGDGT-derived biphytanes ranged from -42.7% to -20.3%. For example, the values for acyclic biphytanes (BP-0), the most abundant, varied from -25.0% to -22.1% in offshore sediments, -30.5% to -25.4% in surrounding soils and -42.7% to -32.0% in nearshore sediments. The wider variation than that of delta C-13(TOC) (-26.9% to -25.1%) in offshore sediments and that in surrounding soils indicated that delta C-13 values of BP-0 may be a sensitive indicator for examining the depositional environments between terrestrial and lacustrine systems. With average delta C-13 values of -23.6%, crenarchaeol-derived biphytanes (BP-cren) in offshore sediments were enriched in C-13 (ca. 3.6%) relative to TOC. In addition, the carbon isotopic fractionation between BP-cren and DIC was -21.3%, consistent with that reported for marine Thaumarchaeota. The enrichment in C-13 relative to TOC and the similar carbon isotopic fractionation indicated that Thaumarchaeota in offshore sediments of Qinghai Lake are autotrophic. With average delta C-13 values of -29.2%, BP-cren in surrounding soils was depleted by 0.9% to 12.8% in C-13 relative to DIC. The quite large scatter in carbon isotopic fractionation between BP-cren and DIC indicated that the biomass of soil Thaumarchaeota originated from inorganic and organic carbon sources, suggesting a mixotrophic lifestyle.</p

Branched and isoprenoid tetraether (BIT) index traces water content along two marsh-soil transects surrounding Lake Qinghai: Implications for paleo-humidity variation

The BIT index represents the relative abundances of branched glycerol dialkyl glycerol tetraethers (bGDGTs) and the isoprenoid GDGT, crenarchaeol. While bGDGTs are produced mainly by soil (anaerobic) bacteria, crenarchaeol is known to be a biomarker for aerobic ammonia oxidation by chemolithoauto-trophic Thaumarchaeota, particularly in the open ocean or lakes. Thus, the index in marine and lacustrine settings has been widely used as a proxy for soil input. Here, we have investigated GDGT distribution along two transects extending from the lake shore marsh to upland soils on the NE Qinghai-Tibetan plateau. The results show that soil water content (SWC) correlates positively with concentration of bGDGTs and negatively with concentration of crenarchaeol; consequently, there is a significant positive correlation between SWC and the BIT index. Our study highlights a new potential application of BIT as a humidity proxy in loess/soil and peat deposits.</p

Distribution of glycerol dialkyl glycerol tetraether lipids along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau, China

The distribution of glycerol dialkyl glycerol tetraethers (GDGTs) can reflect continental environmental changes. Recently, the distribution of branched GDGTs (bGDGTs) has been proposed as a novel tool for paleoelevation reconstructions. Here we report the variation in TEX86 (tetraether index of 86 carbon atoms) of isoprenoidal GDGTs (iGDGTs) and MBT (methylation of branched tetraether index) of bGDGTs along an altitudinal transect on Mt. Xiangpi, NE Qinghai-Tibetan Plateau. Both TEX86 and MBT values of surface soils showed significant linear decreases with altitude (TEX86: R-2 = 0.65; n = 50; MBT: R-2 = 0.69; n = 24). We suggest that the apparent relationships between the two indices and altitude may be related to temperature. Our preliminary investigation suggests that the TEX86 index can potentially be applied as a paleoelevation indicator in addition to the MBT index on the Qinghai-Tibetan Plateau.</p

Variation of the winter monsoon in South China Sea over the past 183 years: Evidence from oxygen isotopes in coral

Oxygen isotope (&delta;18O) data in winter months in living coral Porites lutea collected from the Xisha Islands, South China Sea is significantly correlated to the measured winter monsoon velocity (WMV) with a correlation coefficient of 0.63. Based on 40-year (1961&ndash;2000) instrumental data, a transfer function between the WMV and the winter &delta;18O is established: WMV = 2.819 &delta;18O + 19.615 (N = 40, p &lt; 0.0001). Based on the calculated WMV from &delta;18O over the past 183 years, the sequence of winter monsoon velocity (WMV) from year 1818 to 2000 in the South China Sea is re-constructed. The sequence can be divided into three stages: the first stage shows a decreasing trend of 0.009 m/s&middot;yr from 1818 to 1954, the second stage indicates an increasing trend of 0.011 m/s&middot;yr from 1955 to 1976, and the last stage shows a decreasing trend of 0.026 m/s&middot;yr from 1977 to 2000. The maximum reduction in winter monsoon velocity over the last 24 years (1977&ndash;2000) is approximately 20% of the average over the last 183 years (1918&ndash;2000). The variation of wind velocity shows two complete cycles over the past 183 years. In particular, the strongest and weakest winter monsoon velocities in the last two centuries occurred in the 1830s and 1940s, respectively. The variation in winter monsoon velocity in the 20th century is closely linked to the surface temperature of the South China Sea, as well as air temperature over continental China. In addition, the winter monsoon was weak during two warm periods, i.e. the 1940s and 1980s. Statistical analysis of the winter monsoon velocity anomaly as well as the El Ni&ntilde;o (warm event) and La Ni&ntilde;a (cold event) phenomena indicates that 70% of El Ni&ntilde;o events correspond to the weakness of the winter monsoon.</p

Precipitation changes recorded in the sedimentary total organiccarbon isotopes from Lake Poyang in the Middle and Lower YangtzeRiver, southern China over the last 1600 years

Lake Poyang, the largest freshwater lake of China, is well known for its ecological and hydrological importance as a dynamic wetland and lake system. The climatic change and its effect on ecological and hydrological system of the lake has aroused considerable interest in recent years. So, a sediment core from Lake Poyang was analyzed for d13C total organic matter (d13Corg) to study the hydrological changes in the Lake Poyang catchment. By comparing with modern meteorological data (precipitation and temperature) and hydrological data (volume of runoff), the result showed that the d13Corg values in the sediment are negatively correlated with the total runoff of the 5 main rivers supplying water to Lake Poyang and precipitation in this area over the past 50 years. Therefore, the d13Corg values of the Lake Poyang sediment can serve as reliable proxies for reconstruction of river discharge and regional precipitation, with negative d13Corg values representing increased precipitation in the Lake Poyang area. On this basis, the river discharge and regional precipitation variation in the Lake Poyang catchment over the past 1600 years were discussed. The d13Corg values suggest that the precipitation was highest in the period 340e880 AD, and then decreased with fluctuations to the minimum at approximately 1110 AD. During the period of Medieval Warm Period (1110e1350 AD, MWP), the precipitation was generally in the high level except low amount during period of 1140 AD to 1220AD. The precipitation was in low amount during the Little Ice Age (1350e1620 AD, LIA), except for a sudden increase at approximately 1550e1560 AD. The precipitation continuously increased with little fluctuation from 1610 AD until now. In general, changes in regional precipitation and river discharge were dominated by variations in the Asian Summer Monsoon (ASM) precipitation in the Lake Poyang area, consistent with other records from the ASM-controlled areas of China.</p