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Ecosystem water use efficiency (EWUE) is a popular issue in the comprehensive study of climate change, ecology, and hydrology. Currently, views on the response of EWUE to temperature, precipitation, and drought remain controversial. Based on ecosystem net primary productivity (NPP) and evapotranspiration (ET) datasets, both of which were retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) using the Carnegie Ames Stanford approach (CASA) and surface energy balance algorithms for land (SEBAL) models, respectively, this study comprehensively examined the relationship between EWUE and temperature, precipitation, and drought in the Tianshan Mountains of Central Asia. The results showed that EWUE had an obvious temporal change trend in the Tianshan Mountains. The EWUEs of all vegetation types presented an increasing trend in spring and a decreasing trend in autumn. These results led to a phase shift in the annual cycle of EWUE over the years. Compared with 2000 to 2003, from 2012 to 2016, the annual EWUE cycle had advanced by 32 days. Precipitation generally had a negative effect on EWUE, while temperature had an obvious positive effect on EWUE. The EWUE responses to drought for the different vegetation types showed a variety of change trends. With the increase in drought stress, EWUE not only showed a simple upward or downward trend but also showed an upward trend followed by a downward trend or a downward trend followed by an upward trend. EWUE is more sensitive to changing environments than NPP or ET and is more suitable for analyzing ecosystem responses to global change.

Ecosystem water use efficiency (EWUE) is a popular issue in the comprehensive study of climate change, ecology, and hydrology. Currently, views on the response of EWUE to temperature, precipitation, and drought remain controversial. Based on ecosystem net primary productivity (NPP) and evapotranspiration (ET) datasets, both of which were retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS) using the Carnegie Ames Stanford approach (CASA) and surface energy balance algorithms for land (SEBAL) models, respectively, this study comprehensively examined the relationship between EWUE and temperature, precipitation, and drought in the Tianshan Mountains of Central Asia. The results showed that EWUE had an obvious temporal change trend in the Tianshan Mountains. The EWUEs of all vegetation types presented an increasing trend in spring and a decreasing trend in autumn. These results led to a phase shift in the annual cycle of EWUE over the years. Compared with 2000 to 2003, from 2012 to 2016, the annual EWUE cycle had advanced by 32 days. Precipitation generally had a negative effect on EWUE, while temperature had an obvious positive effect on EWUE. The EWUE responses to drought for the different vegetation types showed a variety of change trends. With the increase in drought stress, EWUE not only showed a simple upward or downward trend but also showed an upward trend followed by a downward trend or a downward trend followed by an upward trend. EWUE is more sensitive to changing environments than NPP or ET and is more suitable for analyzing ecosystem responses to global change.

Abstract Induction time and agglomeration of methane hydrate formation in dispersed systems play an important role in exploitation of natural gas hydrate, prevention of gas hydrate plug, and application of hydrate-based technologies. In this work, an autoclave with particle video microscope (PVM) probe was used to detect induction time of methane hydrate formation as a function of the water cut, dosage of sorbitan monolaurate (Span 20), and subcooling. Forty-one experiments and thirty-six experiments of induction time have been conducted for methane hydrate formation at constant pressure and at nonconstant pressure, respectively. The results showed subcooling was the major factor that affects induction time during methane hydrate formation process. Subcooling of 4 K can be seen as an inflection point because the average methane hydrate formation time was less than 200 min when the subcooling was greater than 4 K, while methane hydrate formation time exhibited more stochastic when the subcooling was less than 4 K. The results also suggested that there exists a transformation range of subcooling (TRS) during methane hydrate formation process. The agglomerated mechanism of methane gas hydrate may be changed when the subcooling is greater than TRS, and subcooling of 4 K is included in the TRS.

Abstract Induction time and agglomeration of methane hydrate formation in dispersed systems play an important role in exploitation of natural gas hydrate, prevention of gas hydrate plug, and application of hydrate-based technologies. In this work, an autoclave with particle video microscope (PVM) probe was used to detect induction time of methane hydrate formation as a function of the water cut, dosage of sorbitan monolaurate (Span 20), and subcooling. Forty-one experiments and thirty-six experiments of induction time have been conducted for methane hydrate formation at constant pressure and at nonconstant pressure, respectively. The results showed subcooling was the major factor that affects induction time during methane hydrate formation process. Subcooling of 4 K can be seen as an inflection point because the average methane hydrate formation time was less than 200 min when the subcooling was greater than 4 K, while methane hydrate formation time exhibited more stochastic when the subcooling was less than 4 K. The results also suggested that there exists a transformation range of subcooling (TRS) during methane hydrate formation process. The agglomerated mechanism of methane gas hydrate may be changed when the subcooling is greater than TRS, and subcooling of 4 K is included in the TRS.

Understanding the responses of vegetation characteristics and soil properties to grazing disturbance is useful for grassland ecosystem restoration and management in semiarid areas. Here, we examined the effects of long-term grazing on vegetation characteristics, soil properties, and their relationships across four grassland types (meadow, Stipa steppe, scattered tree grassland, and sandy grassland) in the Horqin grassland, northern China. Our results showed that grazing greatly decreased vegetation cover, aboveground plant biomass, and root biomass in all four grassland types. Plant cover and aboveground biomass of perennials were decreased by grazing in all four grasslands, whereas grazing increased the cover and biomass of shrubs in Stipa steppe and of annuals in scattered tree grassland. Grazing decreased soil carbon and nitrogen content in Stipa steppe and scattered tree grassland, whereas soil bulk density showed the opposite trend. Long-term grazing significantly decreased soil pH and electrical conductivity (EC) in annual-dominated sandy grassland. Soil moisture in fenced and grazed grasslands decreased in the following order of meadow, Stipa steppe, scattered tree grassland, and sandy grassland. Correlation analyses showed that aboveground plant biomass was significantly positively associated with the soil carbon and nitrogen content in grazed and fenced grasslands. Species richness was significantly positively correlated with soil bulk density, moisture, EC, and pH in fenced grasslands, but no relationship was detected in grazed grasslands. These results suggest that the soil carbon and nitrogen content significantly maintains ecosystem function in both fenced and grazed grasslands. However, grazing may eliminate the association of species richness with soil properties in semiarid grasslands.

Understanding the responses of vegetation characteristics and soil properties to grazing disturbance is useful for grassland ecosystem restoration and management in semiarid areas. Here, we examined the effects of long-term grazing on vegetation characteristics, soil properties, and their relationships across four grassland types (meadow, Stipa steppe, scattered tree grassland, and sandy grassland) in the Horqin grassland, northern China. Our results showed that grazing greatly decreased vegetation cover, aboveground plant biomass, and root biomass in all four grassland types. Plant cover and aboveground biomass of perennials were decreased by grazing in all four grasslands, whereas grazing increased the cover and biomass of shrubs in Stipa steppe and of annuals in scattered tree grassland. Grazing decreased soil carbon and nitrogen content in Stipa steppe and scattered tree grassland, whereas soil bulk density showed the opposite trend. Long-term grazing significantly decreased soil pH and electrical conductivity (EC) in annual-dominated sandy grassland. Soil moisture in fenced and grazed grasslands decreased in the following order of meadow, Stipa steppe, scattered tree grassland, and sandy grassland. Correlation analyses showed that aboveground plant biomass was significantly positively associated with the soil carbon and nitrogen content in grazed and fenced grasslands. Species richness was significantly positively correlated with soil bulk density, moisture, EC, and pH in fenced grasslands, but no relationship was detected in grazed grasslands. These results suggest that the soil carbon and nitrogen content significantly maintains ecosystem function in both fenced and grazed grasslands. However, grazing may eliminate the association of species richness with soil properties in semiarid grasslands.

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