Shenggong Li

A meta-analysis of water vapor deuterium-excess in the midlatitude atmospheric surface layer

Deuterium-excess (d) in water is a combination of the oxygen (delta O-18) and hydrogen (delta D) isotope ratios, and its variability is thought to indicate the location and environmental conditions of the marine moisture source. In this study, we analyze d of water vapor (d(v)) from six sites, all between 37 and 44 degrees N to examine patterns in the atmospheric surface layer and identify the main drivers of variability. Two sites are in urban settings (New Haven, CT, USA and Beijing, China), two sites are in agricultural settings (Rosemount, MN, USA and Luancheng, China), and two sites are in natural ecosystems, a forest (Borden Forest, Ontario, Canada) and a grassland (Duolun, China). We found a robust diurnal cycle in d(v) at all sites with maximum values during mid-day. Isotopic land surface model simulations suggest that plant transpiration is one mechanism underlying the diurnal pattern. An isotopic large-eddy simulation model shows that entrainment of the free atmosphere into the boundary layer can also produce high d(v) values in mid-day. Daily mid-day means of d(v) were negatively correlated with local mid-day relative humidity and positively correlated with planetary boundary layer height at the North American sites, but not the Chinese sites. The mechanism for these differences is still undetermined. These results demonstrate that within the diurnal time scale, d(v) of the surface air at continental locations can be significantly altered by local processes, and is therefore not a conserved tracer of humidity from the marine moisture source region as has previously been assumed.

Dew water isotopic ratios and their relationships to ecosystem water pools and fluxes in a cropland and a grassland in China

Dew formation has the potential to modulate the spatial and temporal variations of isotopic contents of atmospheric water vapor, oxygen and carbon dioxide. The goal of this paper is to improve our understanding of the isotopic interactions between dew water and ecosystem water pools and fluxes through two field experiments in a wheat/maize cropland and in a short steppe grassland in China. Measurements were made during 94 dew events of the D and 18O compositions of dew, atmospheric vapor, leaf, xylem and soil water, and the whole ecosystem water flux. Our results demonstrate that the equilibrium fractionation played a dominant role over the kinetic fractionation in controlling the dew water isotopic compositions. A significant correlation between the isotopic compositions of leaf water and dew water suggests a large role of top-down exchange with atmospheric vapor controlling the leaf water turnover at night. According to the isotopic labeling, dew water consisted of a downward flux of water vapor from above the canopy (98%) and upward fluxes originated from soil evaporation and transpiration of the leaves in the lower canopy (2%).

Response of gross ecosystem productivity, light use efficiency, and water use efficiency of Mongolian steppe to seasonal variations in soil moisture

8 [1] The examination of vegetation productivity and use of light and water resources is 9 important for understanding the carbon and water cycles in semiarid and arid 10 environments. We made continuous measurements of carbon dioxide and water vapor 11 fluxes over an arid steppe ecosystem in Mongolia by using the eddy covariance (EC) 12 technique. These measurements allow an examination of EC-estimated gross ecosystem 13 productivity (GEP), light use efficiency (LUE), and water use efficiency (WUE) of the 14 steppe. Daily variations of GEP, LUE, and WUE were associated with daily variations of 15 incident photosynthetically active radiation (PAR), ambient temperature (Ta), and vapor 16 pressure deficit (VPD). The magnitudes of these variations were also dependent on canopy 17 development. On the daily basis, GEP linearly correlated with evapotranspiration rate and 18 PAR. LUE correlated positively with leaf area index, Ta, and soil moisture availability but 19 negatively with the surface reflectivity for short-wave solar radiation. Throughout the 20 growing season, both GEP and LUE responded strongly to precipitation-fed soil moisture 21 in the top 20 cm of the soil. An examination of the responses of LUE and WUE to PAR 22 under different soil moisture conditions shows that when soil water availability exceeded 23 VPD, the steppe was most efficient in light use, whereas it was less efficient in water use. 24 The multivariate analysis of variance also suggests that soil moisture availability, 25 especially water status in the upper 20-cm soil layer with dense distribution of grass roots, 26 is the most significant factor that governs GEP, WUE, and LUE. This study provides a 27 preliminary assessment of the use of available water and light by the Mongolian arid 28 steppe ecosystems under seasonally varying soil moisture conditions. A better 29 understanding of these functional responses is required to predict how climate change may 30 affect arid steppe ecosystems.

Partitioning of evapotranspiration through oxygen isotopic measurements of water pools and fluxes in a temperate grassland

Stable isotopic measurements of water provide a promising tool for partitioning of ecosystem evapotranspiration (ET). This approach, however, is still facing some challenges due to the uncertainties in estimating the isotopic compositions of ET and its components. In this study, a tunable diode laser analyzer was deployed for in situ measurements of the oxygen isotopic compositions of water vapor. Using these measurements together with samples of water in plant and soil pools, we partitioned ET via estimating the oxygen isotopic compositions of ET ((ET)) and that of its two components, i.e., plant transpiration ((T)) and soil water evaporation ((E)). A new (T) model was developed in this study, which illustrated consistent estimations with the traditional model. Most of the variables and parameters in the new model can be measured directly with high accuracy, making its potential to be used at other sites high. Our results indicate that the ratio of plant transpiration to evapotranspiration (T/ET) illustrates a U shape diurnal pattern. Mean T/ET at 0630-1830 during the sampling days was 83%. Soil depth of 15 cm is a reasonable depth for soil water sampling for estimating (E) at this site. We also investigated the uncertainties in estimating these three terms and their effects on partitioning. Overall, in terms of partitioning, the uncertainties are relatively small from (T) and (E) but quite large from (ET). Quantifying and improving the precision of (ET) should be a priority in future endeavors of ET partitioning via the stable isotopic approach. Key Points A new model is developed to estimate (T) Fifteen centimeter is a reasonable depth for soil water sampling for estimating (E) The (ET) is the most critical member for partitioning

Intercomparison of Four Commercial Analyzers for Water Vapor Isotope Measurement

The d 18 O and dD of atmospheric water vapor are important tracers in hydrological and ecological studies. Isotope ratio infrared spectroscopy (IRIS) provides an in situ technology for measuring d 18 O and d Di n ambient conditions. An intercomparison experiment was carried out with four commercial IRIS analyzers to characterize their performance and transferability of calibration methods. Over a 15-day atmospheric measurement,duringwhichthewatervaporconcentrationrangedfrom14to27 mol mol 21 andtheisotopicratios spanned about 90& and 13& for dD and d 18 O, respectively, these analyzers tracked the natural variability in ambient conditions very well and achieved an average difference between one another within 2& for dD and within 0.1& for d 18 O after calibration at appropriate frequencies. Two of the calibration methods (discrete liquid water injection and continuous dripping) agreed with each other within the tolerance thresholds of 2& for dD and 0.1& for d 18 O. The Rayleigh distillation technique appeared to be acceptable as a calibration standard for dD but not for d 18 O. The dD measurements were less prone to concentration dependence errors than the d 18 O measurements. The concentration dependence underscores the importance of using a calibration procedure at multiple mixing ratios to bracket the range of natural variability.

Seasonal variation in oxygen isotope composition of waters for a montane larch forest in Mongolia

Measurements of water oxygen isotopic composition were conducted in the 2003 growing season for a montane larch (Larix sibirica Ledeb.) forest in northern Mongolia, a transitional area from the south Siberian taiga to the Asian steppe. Oxygen isotopic composition of foliar water and its daily variability were found to be sensitive to atmospheric evaporative demand. During most of the growing season, water sources used by larch trees were from the upper 30-cm surface layer of the soil when precipitation input was large, and were from the deeper layer when the water supply at the upper soil layer was limited. The Keeling plot method suggested that the forest returned soil water to the atmosphere predominantly by means of canopy transpiration during the peak growth period (in August).

Temporal variation of δ 13 C of larch leaves from a montane boreal forest in Mongolia

This paper reports the temporal variation (2002–2004) in foliar δ13C values, which are indicative of long-term integrated photosynthetic and water use characteristics, of Siberian larch (Larix sibirica Ledeb.) trees in a montane forest at Mongonmorit, NE Mongolia. At the stand, the δ13C value for understory shaded leaves was more negative by 2‰ on average than that for sunlit leaves sampled concurrently from open and sun-exposed environments in a forest gap. The δ13C value of both sunlit and shaded leaves showed pronounced intra- but relatively small inter-seasonal variations. The δ13C value was more positive for juvenile than mature leaves. We conjecture that juvenile leaves may derive carbon reserves in woody tissues (e.g., stems). Regardless of leaf habitats, the δ13C value was also affected by insect herbivores occurred in mid summer of 2003, being more negative in newly emerging leaves from the twigs after defoliation than in non-defoliated mature leaves. This pattern seems to contrast with that for the juvenile leaves in the early growing season. We surmise that the newly emerging leaves used stored organic carbon that was depleted due to fractionation during remobilization and translocation for leaf regrowth. There was also intra- and inter-seasonal variation in the foliar N concentrations and C:N ratios. A good positive (negative) correlation between the foliar δ13C values and N concentrations (C:N ratios) was also observed for both sunlit and shaded leaves, suggesting that the relationship between water and nitrogen use is a crucial factor affecting the plant carbon–water relationship in this mid latitude forest with a cold semiarid climate. Our isotopic data demonstrate that the larches in NE Mongolia exhibits relatively higher water use efficiency with a distinct within-season variability.

A comparison of methane emission measurements using eddy covariance and manual and automated chamber-based techniques in Tibetan Plateau alpine wetland

Comparing of different CH4 flux measurement techniques allows for the independent evaluation of the performance and reliability of those techniques. We compared three approaches, the traditional discrete Manual Static Chamber (MSC), Continuous Automated Chamber (CAC) and Eddy Covariance (EC) methods of measuring the CH4 fluxes in an alpine wetland. We found a good agreement among the three methods in the seasonal CH4 flux patterns, but the diurnal patterns from both the CAC and EC methods differed. While the diurnal CH4 flux variation from the CAC method was positively correlated with the soil temperature, the diurnal variation from the EC method was closely correlated with the solar radiation and net CO2 fluxes during the daytime but was correlated with the soil temperature at nighttime. The MSC method showed 25.3% and 7.6% greater CH4 fluxes than the CAC and EC methods when measured between 09:00 h and 12:00 h, respectively.

Sheep gain and species diversity: in sandy grassland, Inner Mongolia

Abstract A grazing experiment was conducted from 1992 to 1996 at a sandy grassland in the Horqin sandy land, located in the northeastern part of China. The grassland had been grazed by sheep for many years before the experiment at an intensity of 4.5 sheep ha−1. The experiment consisted of 4 grazing treatments: no grazing (0 sheep ha−1), light grazing (2 sheep ha−1), moderate grazing (4 sheep ha−1) and overgrazing (6 sheep ha−1). Plant species diversity, plant biomass, soil properties, and sheep liveweight under various grazing treatments were examined. Overgrazing resulted in considerable decreases in both species diversity and plant biomass. As a result, sheep liveweight gain decreased significantly in the last 3 years of the experiment in the overgrazing treatment. No grazing and light grazing treatments had higher species diversity as well as higher biomass production than moderate grazing and overgrazing treatments. The results indicate that light sheep grazing is sufficient for the recovery of overgrazed grassland in this region and for the maintenance of plant species diversity. The proper grazing intensity should be 2–3 sheep or sheep equivalents per hectare for the sandy grassland in Inner Mongolia.

Carbon uptake by karsts in the Houzhai Basin, Southwest China

Using an estimated bicarbonate concentration ([HCO(3)(-)]) in water and discharge rates of surface water and underground water from the Houzhai Basin, southwest China, from 1986 to 2007, we estimate that the mean carbon uptake rate was 20.7 g C m(-2) yr(-1). The surface water and underground water contribute about equally to the total carbon uptake from 1986 to 2007. About 97% of the interannual variation of annual carbon uptake can be explained by the discharge rate. Within a year, the net carbon uptake rate by karst during the wet season (May-October) was found to be about 2.4 times that during the dry season (November-April). If the seasonal variations of discharge rate and bicarbonate concentrations are not accounted for, estimates of annual net carbon uptake by karst can be biased by >25%, but that bias becomes very small (<5%) when averaged from 1986 to 2007 for the Houzhai Basin. We also found that one of the empirical models as used in global modeling overestimated the net carbon uptake by karst at Houzhai Basin by 29%. Carbon uptake from chemical weathering of all karsts in China is estimated to be about 12 Tg C yr(-1) at present (1 Tg = 10(12) g), or about 57% of the rate of net carbon accumulated in the forest biomass from 1981 to 1998 in China; we therefore recommend the inclusion of carbon uptake from chemical weathering in the regional carbon budget of China.