The Impact of Water Potential and Temperature on Native Species' Capability for Seed Germination in the Loess Plateau Region, China.

Global warming is increasing the frequency and intensity of heat waves and droughts. One important phase in the life cycle of plants is seed germination. To date, the association of the temperature and water potential thresholds of germination with seed traits has not been explored in much detail. Therefore, we set up different temperature gradients (5-35 °C), water potential gradients (-1.2-0 MPa), and temperature × water potential combinations for nine native plants in the Loess Plateau region to clarify the temperature and water combinations suitable for their germination. Meanwhile, we elucidated the temperature and water potential thresholds of the plants and their correlations with the mean seed mass and flatness index by using the thermal time and hydrotime models. According to our findings, the germination rate was positively correlated with the germination percentage and water potential, with the former rising and the latter decreasing as the temperature increased. Using the thermal time and hydrotime models, the seed germination thresholds could be predicted accurately, and the germination thresholds of the studied species varied with an increase in germination percentage. Moreover, temperature altered the impact of water potential on the germination rate. Overall, the base water potential for germination, but not the temperature threshold, was negatively correlated with mean seed mass and was lower for rounder seeds than for longer seeds. This study contributes to improving our understanding of the seed germination characteristics of typical plants and has important implications for the management and vegetation restoration of degraded grasslands.

Contribution of microbial necromass to soil organic carbon formation during litter decomposition under incubation conditions.

We conducted a 512-day incubation experiment to study the dynamics of microbial necromass and soil carbon fraction in the 'litter-soil' transformation interface soil layer (TIS) during litter decomposition, using a perennial C3 herb, Stipa bungeana, in the loess hills. The results showed that soil microbial necromass was dominated by fungi in the early and middle stages, and by bacteria in the late stage. The contribution of fungal necromass C to mineral-associated organic C (MAOC) was significantly higher (38.7%-75.8%) than that of bacteria (9.2%-22.5%) and 2-3 times more than the contribution rate of bacterial necromass. Soil organic C (SOC) content was decreasing during litter decomposition. The input of plant C resources stimulated microbial utilization of soil C fractions. The continuous decrease in particulate organic C during the early and late stages of decomposition was directly responsible for the decrease in SOC content. In contrast, the fluctuating changes in microbial necromass C and MAOC played an indirect role in the reduction of SOC. The increase in soil microbial necromass C caused by a single exogenous addition of litter did not directly contribute to SOC accumulation.

Determination of plutonium in cryoconite on glacier surfaces in the northeast Tibetan Plateau: Implications for source identification and accumulation.

Plutonium isotopes have drawn public attention because of their high radiotoxicity and risk for internal radiation. Cryoconite, dark sediments on the surface of glaciers, is rich in anthropogenic radionuclides. Therefore, glaciers are regarded as not only a temporary sink for radioactive pollutants in the past decades, but also a secondary source during their melting. However, study on activity concentration and source of Pu isotopes in cryoconite materials in Chinese glaciers have not been studies thus far. The present study determined the 239+240Pu activity concentration and 240Pu/239Pu atom ratio in cryoconite and other environmental samples collected in August-one ice cap, northeast Tibetan Plateau. The results showed that the 239+240Pu activity concentration in cryoconite is 2-3 orders of magnitude higher than the background value, suggesting the exceptional capacity of cryoconite to accumulate Pu isotopes. The high 239+240Pu level in cryoconite in the study area was significantly correlated with organic matter and slope, indicating their dominant influence. The average values of the 240Pu/239Pu atom ratio for proglacial sediments (0.175) and grassland soils (0.180) suggested that the global fallout is the dominant source of Pu isotopes pollution. In contrast, the measured 240Pu/239Pu atom ratios in the cryoconite were distinctly lower at 0.064-0.199, with an average of 0.157, indicating that the close-in fallout Pu isotopes from Chinese nuclear test sites is another source. In addition, although the relatively lower activity concentrations of 239+240Pu in proglacial sediments implies that most Pu isotopes are retained in this glacier instead of being redistributed along with cryoconite by meltwater, the potential health and ecotoxicological risks to the proglacial environment and downstream areas cannot be ignored. These results are important for understanding the fate of Pu isotopes in the cryosphere and can be applied as baseline data for radioactivity evaluation in the future.

Effects of soil surface electrochemical properties on soil detachment regulated by soil types and plants.

Soil surface electrochemical properties play an important role in soil detachment. However, the effects of soil surface electrochemical properties on soil detachment have received little attention. This study selected three typical soil types (sandy loam, silt loam and silt clay loam) and two typical herbaceous plants (Stipa bungeana Trin. and Artemisia gmelinii Web. ex Stechm., with an abandoned cropland) to quantify the effects of varied soil surface electrochemical properties caused by soil types and plants on soil detachment on the Loess Plateau, China. A hydraulic flume test was conducted under five flow discharges (0.1, 0.2, 0.3, 0.4 and 0.6 L s-1) on a 15° slope. The results showed that large differences in soil surface electrochemical properties occurred among different soil types and plant species. The exchangeable sodium percentage (ESP), cation exchange capacity (CEC), specific surface area (S), surface charge density (σ0), surface potential (φ0) and surface electric field strength (E0) ranged from 1.02% to 2.26%, from 7.26 to 24.34 cmolc kg-1, from 8.32 to 147.38 m2 g-1, from 0.09 to 2.02C m-2, from -81.33 to -149.92 mV and from 1.23 × 108 to 28.46 × 108 V m-1, respectively. The soil detachment capacity (Dc) was significantly correlated with ESP (P < 0.01), E0 (P < 0.01) and |φ0| (the absolute value) (P < 0.05). Moreover, Dc exponentially increased with |φ0| (R2 = 0.99) and linearly increased with ESP (R2 = 0.85) and E0 (R2 = 0.95). Therefore, soil surface electrochemical properties greatly varied with soil types and plants and had a significant effect on soil detachment.

Effect of vegetation patchiness on the subsurface water distribution in abandoned farmland of the Loess Plateau, China.

Patchiness of grassland results in important effects on ecohydrological processes in arid and semiarid areas; however, the influences on subsurface water flow and soil water distribution remain poorly understood, particularly during vegetation succession on slopes. This study examined these effects by comparing the water flow behaviors and preferential infiltration between vegetation patches (VP) and interspace patches (IP) in three sites at different states of vegetation succession (grass, subshrub and shrub) in abandoned farmland of the Loess Plateau, China. Dye tracer infiltration showed that patchiness of vegetation increased spatial variations of soil water and preferential infiltration by increasing the densities of fine root length and fine root volume in the soil profile. Moreover, the more abundant and intricate roots following a lateral direction beneath VP likely contributed to lateral flow and infiltration variability. However, differences between VP and IP were not significant because considerable living fine roots and decayed roots of IP also provided preferential flow pathways. Our finding indicated that IP could compete with VP for access to soil water resources, which potentially increased hillslope-scale infiltration and reduced surface runoff and erosion risk. Under the different states of vegetation succession, subshrub patches showed significantly greater preferential infiltration volume (28.53 mm) and contribution of preferential infiltration to total infiltration (60.58%) than grass and shrub patches. Vegetation patch size made positive effects on improving preferential flow and water movement. Greater preferential flow in subshrub patches played a positive role in soil water storage and replenishment. Therefore, natural restoration of a slope area with small heterogeneity in preferential flow can be successfully applied in the Loess Plateau, particularly during the subshrub succession state.

Spatial and temporal variations of the greenhouse gas emissions in coastal saline wetlands in southeastern China.

Coastal wetlands are crucial to global climate change due to their roles in modulating atmospheric concentrations of greenhouse gases (GHGs) (CO2, CH4, N2O). Under a warming climate, we investigated spatial and temporal variations of GHGs emissions over the coastal wetlands in southeastern China during 2012-2014. Five dominant land cover types in coastal wetlands have been considered, including the bare mud flat (BF), the Spartina alterniflora flats (SAF), the Suaeda glauca flats (SGF), the Phragmites australis flat (PAF), and the Scripus triqueter flat (STF). The results showed that the annual average CO2 fluxes were 305.8, 588.8, 370.2, and 136.5 mg m-2 h-1 from spring to winter. CH4 fluxes presented to be a sink in spring (- 0.02 mg m-2 h-1), and functioned as a source in the following seasons. Correlation analysis indicated that the surface air temperature and the cumulative precipitation could be two main factors that influenced the seasonal and inter-annual variations of GHGs emissions. In addition, we provided a regional budget of GHGs emissions that suggested the variations of GHGs emissions under a warming climate.

Trace Elements in Dominant Species of the Fenghe River, China: Their Relations to Environmental Factors.

The distribution of trace elements (TEs) in water, sediment, riparian soil and dominant plants was investigated in the Fenghe River, Northwestern China. The Fenghe River ecosystem was polluted with Cd, Cr, Hg and Pb. There was a high pollution risk in the midstream and downstream regions and the risk level for Cd was much higher than that of the other elements. The average values of bioconcentration coefficient for Cd and Zn were 2.21 and 1.75, respectively, indicating a large accumulation of Cd and Zn in the studied species. With broad ecological amplitudes, L. Levl. et Vant. Trin., and L. had the greatest TE concentrations in aboveground and belowground biomass of the studied species and were potential biomonitors or phytoremediators for the study area. Multivariate techniques including cluster analysis, correlation analysis, principal component analysis, and canonical correspondence analysis were used to analyze the relations between TE concentrations in plants and various environmental factors. The soil element concentration is the main factor determining the accumulation of TEs in plants. The co-release behavior of common pollutants and TEs drove the accumulation of Hg, Cd, and As in the studied plants. Significant enrichment of some elements in the Fenghe River has led to a decline in the biodiversity of plants.

[Responses of Onobrychis viciaefolia Scop and soil nitrogen contents to elevated atmospheric CO2 concentration].

The effects of elevated CO2 on terrestrial ecosystem processes are highly related to the N cycling in the ecosystem. In two closed chambers with different CO2 concentration (ambient CO2 355 - 370 micromol x mol(-1), and elevated CO2 700 micromol x mol(-1)), pot experiments were conducted to study the responses of Onobrychis viciaefolia Scop and soil nitrogen contents to elevated CO2. The results showed that compared with ambient CO2, elevated CO2 increased the plant dry biomass by 25.1% (P < 0.01), but decreased the plant N concentration by 25.3% (P < 0.001), resulting in little change of total plant N. After three months experiment, soil total N, NO3 (-) -N and NH4 (+) -N under both ambient and elevated CO2 conditions decreased, while soil microbial biomass N increased significantly. Soil NH4 (+) -N concentration had little difference under ambient and elevated CO2 conditions, but under elevated CO2 condition, soil NO3 (-) -N concentration decreased significantly, while microbial biomass N increased significantly. By the end of the experiment, the total N content of whole soil-plant ecosystem had a slight increase, especially under elevated CO2 condition, which was probably due to the higher N-fixation capability of O. viciaefolia Scop under CO2 enrichment.

[Dynamic changes of soil ecological factors in Ziwuling secondary forest area under human disturbance].

As a widespread natural phenomenon, disturbance is considered as a discrete event occurred in natural ecosystems at various spatial and temporal scales. The occurrence of disturbance directly affects the structure, function and dynamics of ecosystems. Forest logging and forestland assart, the common human disturbances in forest area, have caused the dynamic changes of forest soil ecological factors in a relatively consistent environment. A study on the dynamics of soil bulk density, soil organic matter, soil microbes and other soil ecological factors under different human disturbance (logging and assart, logging but without assart, control) were conducted in the Ziwuling secondary forest area. The results indicated that human disturbance had a deep impact on the soil ecological factors, with soil physical and chemical properties become bad, soil organic matter decreased from 2.2% to 0.8%, and soil stable aggregates dropped more than 30%. The quantity of soil microbes decreased sharply with enhanced human disturbance. Soil organic matter and soil microbes decreased more than 50% and 90%, respectively, and soil bulk density increased from 0.9 to 1.21 g x cm(-3) with increasing soil depth. Ditch edge level also affected the dynamics of soil factors under the same disturbance, with a better soil ecological condition at low-than at high ditch edge level.