Response Of Water Use Efficiency Research Articles

Response of Ecosystem Carbon–Water Fluxes to Extreme Drought in West Asia

Global warming has resulted in increases in the intensity, frequency, and duration of drought in most land areas at the regional and global scales. Nevertheless, comprehensive understanding of how water use efficiency (WUE), gross primary production (GPP), and actual evapotranspiration (AET)-induced water losses respond to exceptional drought and whether the responses are influenced by drought severity (DS) is still limited. Herein, we assess the fluctuation in the standardized precipitation evapotranspiration index (SPEI) over the Middle East from 1982 to 2017 to detect the drought events and further examine standardized anomalies of GPP, WUE, and AET responses to multiyear exceptional droughts, which are separated into five groups designed to characterize the severity of extreme drought. The intensification of the five drought events (based on its DS) increased the WUE, decreased the GPP and AET from D5 to D1, where both the positive and negative variance among the DS group was statistically significant. The results showed that the positive values of standardized WUE with the corresponding values of the negative GPP and AET were dominant (44.3% of the study area), where the AET values decreased more than the GPP, and the WUE fluctuation in this region is mostly controlled by physical processes, i.e., evaporation. Drought’s consequences on ecosystem carbon-water interactions ranged significantly among eco-system types due to the unique hydrothermal conditions of each biome. Our study indicates that forthcoming droughts, along with heightened climate variability, pose increased risks to semi-arid and sub-humid ecosystems, potentially leading to biome restructuring, starting with low-productivity, water-sensitive grasslands. Our assessment of WUE enhances understanding of water-carbon cycle linkages and aids in projecting ecosystem responses to climate change.

Simulating short-term light responses of photosynthesis and water use efficiency in sweet sorghum under varying temperature and CO2 conditions.

Climate change, characterized by rising atmospheric CO2 levels and temperatures, poses significant challenges to global crop production. Sweet sorghum, a prominent C4 cereal extensively grown in arid areas, emerges as a promising candidate for sustainable bioenergy production. This study investigated the responses of photosynthesis and leaf-scale water use efficiency (WUE) to varying light intensity (I) in sweet sorghum under different temperature and CO2 conditions. Comparative analyses were conducted between the A n-I, g s-I, T r-I, WUEi-I, and WUEinst-I models proposed by Ye etal. and the widely utilized the non-rectangular hyperbolic (NRH) model for fitting light response curves. The Ye's models effectively replicated the light response curves of sweet sorghum, accurately capturing the diminishing intrinsic WUE (WUEi) and instantaneous WUE (WUEinst) trends with increasing I. The fitted maximum values of A n, g s, T r, WUEi, and WUEinst and their saturation light intensities closely matched observations, unlike the NRH model. Despite the NRH model demonstrating high R 2 values for A n-I, g s-I, and T r-I modelling, it returned the maximum values significantly deviating from observed values and failed to generate saturation light intensities. It also inadequately represented WUE responses to I, overestimating WUE. Across different leaf temperatures, A n, g s, and T r of sweet sorghum displayed comparable light response patterns. Elevated temperatures increased maximum A n, g s, and T r but consistently declined maximum WUEi and WUEinst. However, WUEinst declined more sharply due to the disproportionate transpiration increase over carbon assimilation. Critically, sweet sorghum A n saturated at current atmospheric CO2 levels, with no significant gains under 550 μmol mol-1. Instead, stomatal closure enhanced WUE under elevated CO2 by coordinated g s and T r reductions rather than improved carbon assimilation. Nonetheless, this response diminished under simultaneously high temperature, suggesting intricate interplay between CO2 and temperature in modulating plant responses. These findings provide valuable insights into photosynthetic dynamics of sweet sorghum, aiding predictions of yield and optimization of cultivation practices. Moreover, our methodology serves as a valuable reference for evaluating leaf photosynthesis and WUE dynamics in diverse plant species.

Extreme drought triggers parallel shifts in wood anatomical and physiological traits in upper treeline of the Mediterranean Andes

BackgroundTreeline ecotones of Mediterranean ecoregions have been affected by the increasing intensity and severity of droughts. Even though the effect of droughts on forest dynamics has been widely documented, knowledge is relatively scarce of how extreme climate episodes affect the hydraulic structure and, therefore, the physiology of woody plants. The Mediterranean Andes have experienced an uninterrupted period of drought since 2010, including an extremely dry year in 2019 with approximately 80% rainfall deficit. Here, we investigated shifts in wood anatomical and physiological traits of Kageneckia angustifolia, an endemic treeline species, in response to this drought period.MethodsWe evaluated the xylem plasticity of three K. angustifolia populations across their natural distribution (31–35° SL) based on anatomical (vessel structure and distribution) and physiological (intrinsic water-use efficiency) variables in the tree rings. We focused on the period 2000–2020 that corresponds to before the megadrought (2000–2007), (ii) megadrought (2008–2018) and (iii) hyperdrought (2019–2020). The variables were annualized and analyzed by linear mixed-effects models.ResultsOur results provide insights to the anatomical and physiological mechanisms underlying the resilience of treeline forests to persistent droughts in central Chile. We found that the extreme drought in 2019–2020 triggered shifts in vessel size and frequency that increased hydraulic safety. These significant shifts in vessel traits occurred in parallel with a decrease in pit aperture area and an increase in water-use efficiency, further increasing the resilience of K. angustifolia to extreme drought stress.ConclusionsOur results revealed coordinated shifts in vessel size and frequency and water-use efficiency in response to the megadrought, thereby reducing vulnerability to hydraulic failure. The apparent resilience of K. angustifolia to extreme droughts suggests that this adaptation to drought stress may increase its ability to tolerate novel climatic conditions of treeline environments of the Mediterranean Andes, although it is not clear whether these adaptations will be sufficient to persist in scenarios that predict intensification of climate stress. Finally, our results provide empirical evidence that integrating wood anatomical and physiological traits facilitates the understanding of resilience mechanisms that treeline forests develop in the face of increasing drought stress.

Genotypic Differences in Morphological, Physiological and Agronomic Traits in Wheat (Triticum aestivum L.) in Response to Drought.

Drought is one of the main environmental factors affecting crop growth, and breeding drought-tolerant cultivars is one of the most economic and effective ways of increasing yields and ensuring sustainable agricultural production under drought stress. To facilitate the breeding of drought-tolerant wheat, this study was conducted to evaluate genotypic differences in the drought tolerance of 334 wheat genotypes collected from China and Australia with the aim of screening for drought-tolerant and -sensitive genotypes and to elucidate the corresponding physiological mechanisms. A hydroponic-air experiment (roots exposed to air for 7 h/d and continued for 6 d) showed significant genotypic differences in shoot and root dry weights among the genotypes. The relative shoot and root dry weights, expressed as the percentage of the control, showed a normal distribution, with variation ranges of 20.2-79.7% and 32.8-135.2%, respectively. The coefficients of variation were in the range of 18.2-22.7%, and the diversity index was between 5.71 and 5.73, indicating a rich genetic diversity among the wheat genotypes for drought tolerance. Using phenotypic differences in relative dry weights in responses to drought stress, 20 of each of the most drought-tolerant and drought-sensitive genotypes were selected; these were further evaluated in pot experiments (watering withheld until the soil moisture content reached four percent). The results showed that the trends in drought tolerance were consistent with the hydroponic-air experiment, with genotypes W147 and W235 being the most drought-tolerant and W201 and W282 the most sensitive. Significant genotypic differences in water use efficiency in response to drought were observed in the pot experiment, with the drought-tolerant genotypes being markedly higher and the two sensitive genotypes being no different from the control. A marked increase in bound water content in the drought stress plants was observed in the two drought-tolerant genotypes, while a decrease occurred in the free water. The reductions in photochemical efficiencies of PSII, transpiration rates, net photosynthesis rates, chlorophyll contents and stomatal conduction in the drought-sensitive genotypes W201 and W282 under drought stress were higher than the two tolerant genotypes. This study provides a theoretical guide and germplasm for the further genetic improvement of drought tolerance in wheat.

Response of WUE of maize at ear stage to the coupling effect of CO2 and temperature

In the face of global warming, the photosynthesis and transpiration of plants will change greatly, which will ultimately affect the water use efficiency (WUE) of plants. In order to study the coupling effects of CO2 and temperature on WUE of maize at ear stage, ‘Zhengdan 958’ was taken as the research object, and 5 temperatures (20 °C, 25 °C, 30 °C, 35 °C and 40 °C) and 11 CO2 concentration (400, 300, 200, 150, 100, 50, 400, 400, 600, 800 and 1000 μmol mol−1) were set to measure the parameters such as net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci) of single leaves. The response of WUE (Pn/Tr) to CO2 and temperature was evaluated by a CO2 response model. The results show that at the same temperature, Pn and WUE increased with CO2 level, while Tr decreased as CO2 level increases; at the same CO2 concentration, Pn and Tr were both positively correlated with temperature, while WUE decreased with the increase of temperature. The maximum value of WUE was obtained when the CO2 concentration was 1000 μmol mol−1 and the temperature was 20.0 °C. The results suggest that global warming will not improve WUE of maize, which will bring more severe challenges to water-saving agriculture and food security.

Responses of water use efficiency of Chinese fir to mixed planting and meteorological factors

Chinese fir in China are generally inefficient plantations with single species, unreasonable stand density, and low productivity. The introduction of broadleaved species is usually adopted as a strategy to improve Chinese fir plantations. Taking the pure forests and mixed forests of the Guanshan Forest Farm in Jiangxi Province as example, we quantified the intrinsic water-use efficiency (iWUE) of trees based on the stable isotope carbon method, as well as its response to meteorological factors, and investigated the improvement of stand quality after introducing Phoebe zhennan into Chinese fir plantation. The results showed that the basal area increment was 0.23 cm2 in pure forest, being higher than that of 0.19 cm2 in mixed forest. The δ13C and iWUE of pure forest were -27.4‰ and 52.9%, respectively, being lower than those of -26.7‰ and 62.8% in the mixed forest. Tree δ13C in pure forest was more sensitive to changes in mean annual precipitation and mean annual relative humidity, while that in mixed forest was not significantly correlated with meteorological factors. Pure forest iWUE was positively correlated with mean annual temperature, mean annual atmospheric CO2 concentration, and mean annual maximum temperature, and negatively correlated with mean annual precipitation and mean annual relative humidity, while mixed forest iWUE was positively correlated with mean annual atmospheric CO2 concentration only. Our results indicated that pure forests was more sensitive to climate than mixed forests.

Drought resistance, quality characteristics and water‐yield relationships of some wheat (Triticum aestivum L.) lines and varieties

AbstractThe aim of this study was to determine the drought tolerance of four different genotypes, including two wheat varieties (Ceyhan 99 and Sagittario) and two wheat lines (Zdeb101 and Zdeb102), and create irrigation schedules. This study was performed with treatments applied at three irrigation levels (S100: treatment where the entire water need of the plant was met, S50: 50% of the water provided in the S100 treatment, S0: Rainfed), with four genotypes and four replications in a split‐plot design in Kahramanmaras East Mediterranean Transitional Zone Agricultural Research Institute in 2019 and 2020. The Ceyhan 99 and Sagittario varieties, which are widely cultivated in the region and the Zeb101 and Zdeb102 lines that are candidates for varieties were used in the study. In the study, 217.1 and 213.77 mm of water was given to S100, 108.54 and 106.88 mm of water was given to S50, and no irrigation was provided to S0 in 2019 and 2020, respectively. The effects of different irrigation levels on yield, water use efficiency (WUE), irrigation water use efficiency (IWUE), protein content and wet gluten content were investigated. At the end of this study, seasonal evapotranspiration (ET), irrigation water and yield decreased from S100 to S0. WUE, IWUE and yield response factor (Ky) values showed different trends in both years. There was no significant difference in yield between the varieties. However, in the stress susceptibility index (SSI) analyses of the genotypes, it was found that the most drought‐tolerant genotypes were Zdeb102 (0.50) and Ceyhan 99 (0.54). The highest protein content and wet gluten content were found as 11.38% and 22.24% in the Sagittario variety, respectively. It is recommended that Zdeb102 and Ceyhan 99 be cultivated in regions such as Kahramanmaras which have semi‐arid climate characteristics in the Mediterranean.

Quantifying the variation in water use efficiency across climates and biomes on the Qinghai-Tibetan Plateau

Water use efficiency (WUE) is an important indicator in linking the carbon and water cycles of ecosystems. Previous studies performed in alpine and cold regions have contributed greatly to understanding the WUE response of single ecosystem or specific species in small-scale regions to climatic factors; however, studies focusing on differences in the WUE response among ecosystems or species are insufficient, which has limited the understanding of the climatic adaptation of WUE in alpine and cold regions. Here, based on the 134 leaf δ13C records measured for the 46 dominant species in two main ecosystems on the Qinghai-Tibetan Plateau, variation in the response of WUE among biomes was analyzed by grouped linear regression, redundancy analysis (RDA), and structural equation models (SEMs). The results showed that (1) photosynthetically active radiation (PAR0), moisture index (MI) and vapor pressure deficit (VPD) were significantly correlated with the variation in WUE; (2) compared with the WUE of alpine grasslands, alpine desert grasslands was more sensitive to climatic factors, which presenting a steeper slope that varied with modified growing degree days (MGDD0), PAR0, MI, VPD and CO2; (3) composition of genera distributions explained 32.22% of the variation in WUE, while the WUE of Stipa was more sensitive compared to other species to most climatic factors; (4) climate (composition of genera) and their joint effects explained 58.98% (63.53%) of the variation in WUE of alpine desert grasslands (alpine grasslands), while altitude indirectly controlled the WUE variation in alpine grasslands and MI directly controlled the WUE variation in alpine desert grasslands. This study quantified the driving effects of climates and biomes on the variation in WUE and contributed to the understanding of vegetation adaptation in alpine and cold regions.

Observed divergence in the trends of temperature controls on Chinese ecosystem water use efficiency

How have plants addressed the trade-off between carbon gain and water loss in this warming world? Ecosystem water-use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is a key indicator of the carbon–water relationship. WUE is expected to change due to climate change, yet the extent to which GPP or ET affects WUE changes remains unclear. Moreover, the potential time-varying variations in the WUE responses to recent warming have been overlooked. In this study, we assessed the relative contributions of GPP and ET to WUE using variance decomposition and investigated trends in temperature controls on WUE using moving windows and partial correlation analysis. Our results include: (1) the national multi-year average WUE in China increased significantly at a rate of 0.0174 gC·kg-1H2O·a-1, with 86.88% of the study area exhibiting increased trends. Forest ecosystems, except for ENF, had relatively higher WUE, and the highest value was observed in the deciduous broad-leaf forest (3.77 gC·kg-1H2O), while grassland ecosystems had the lowest WUE of only 1.05 gC·kg-1H2O. (2) GPP, rather than ET, primary drove WUE variations in most areas, except in Northeast China and Southwest China. GPP always contributed more to WUE than ET in each land cover type. In forests, ET contributed more to WUE than in other land cover types. (3) WUE exhibited significantly positive correlations with temperature and radiation, with a positive correlation with temperature in 63.73% (14.83% significant in level of p < 0.05 and the same as below) of the study area and positive correlation with radiation in 74.12% (20.71% significant) of the study area. The control of precipitation on the WUE was complex, with a positive correlation with precipitation in 54.29% of the study area (9.38% significant). (4) The correlation coefficients with temperature exhibited an increasing trend in 52.26% of the study areas and a decreasing trend in 47.74% of the study areas, with a notable divergence in the spatial distribution of the trend of the temperature controls on WUE. Warming is projected to enhance ecosystem functioning in northern regions but may have adverse effects in the south. These findings shed light on the dynamic response of ecosystem functioning to ongoing warming, and would improve our understanding of the terrestrial carbon–water cycle.

Threshold response of ecosystem water use efficiency to soil water in an alpine meadow

Ecosystem water use efficiency (WUE) is a coupled index of carbon (gross ecosystem productivity, GEP) and water fluxes (transpiration, Tr or evapotranspiration, ET), reflecting how ecosystem uses water efficiently to increase its carbon uptake. Though ecosystem WUE is generally considered to decrease with increasing precipitation levels, it remains elusive whether and how it nonlinearly responds to extreme water changes. Here, we performed a 5-year precipitation halving experiment in an alpine meadow, combined with extremely interannual precipitation fluctuations, to create a large range of soil water variations. Our results showed that WUETr and WUEET consistently showed a quadratic pattern in response to soil water. Such quadratic patterns were steadily held at different stages of growing seasons, with minor changes in the optimal water thresholds (25.0–28.4 %). Below the water threshold, more soil water stimulated GEP but reduced Tr and ET by lowering soil temperature, resulting in a positive response of ecosystem WUE to soil water. Above the threshold, soil water stimulated GEP less than Tr (ET), leading to a negative response of ecosystem WUE to soil water. However, biological processes, including plant cover and belowground biomass as well as vertical root biomass distribution, had less effect on ecosystem WUE. Overall, this work is among the first to reveal the nonlinearity and optimal water thresholds of ecosystem WUE across a broad range of soil water, suggesting that future extreme precipitation events will more frequently surpass the water threshold and differently change the coupling relationships of carbon and water fluxes in alpine grasslands.

Drought-induced resource use efficiency responses in an alpine meadow ecosystem of northern Tibet

Drought is a threat universally faced by global terrestrial ecosystems under global climate change. The habitat of alpine ecosystems is harsh, and they necessitate to use the limited resources efficiently to survive. However, drought occurred frequently during the short growing season, which greatly affected their resource use efficiency (RUE). In this study, based on long-term flux and microclimate observations in a typical alpine meadow ecosystem in northern Tibet, the effects of drought on RUE, including water use efficiency (WUE), light use efficiency (LUE), and carbon use efficiency (CUE), were studied. The results indicate that droughts could change the driving paths of abiotic and biotic factors on carbon and water fluxes and hence affect RUE. When vegetation exhibits robust physiological activity, droughts could enhance the control effects of biotic factors. The droughts that occurred at different periods of the growing season showed divergent effects on RUE. In the early-growing season, the strong photosynthetic capacity of the nascent leaves facilitated enhancements in WUE and LUE in response to drought conditions. However, the CUE did not increase significantly due to the heightened consumption of organic matter by respiration. The mid-growing season droughts led to a sharp decrease in gross primary productivity (GPP), consequently leading to a decline in WUE, LUE and CUE. During the late-growing season, droughts together with the decomposition of chlorophyll reduced the photosynthetic capacity and led to a significant decrease in LUE. The diminished physiological activity of the senescent leaves led to insensitive responses of WUE and CUE to droughts. These findings indicate that RUE might primarily be regulated by factors affecting GPP rather than others. This study would be beneficial for assessing the responses and adaptation characteristics of alpine meadow ecosystems to future climate change and could help accurately predict their possible evolution trends.

Divergent impacts of VPD and SWC on ecosystem carbon-water coupling under different dryness conditions

Ecosystem water use efficiency (WUE) is an indicator of carbon-water interactions and is defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). However, it is currently unclear how WUE responds to atmospheric and soil drought events in terrestrial ecosystems with different dryness conditions. Additionally, the contributions of GPP and ET to the WUE response remain poorly understood. Based on measurements from 26 flux tower sites distributed worldwide, the binning method and random forest model were employed to separate the sensitivities of daily ecosystem WUE, GPP, and ET to vapor pressure deficit (VPD) and soil water content (SWC) under different dryness conditions (dryness index = potential evapotranspiration/precipitation, DI). Results showed that the sensitivity of WUE to VPD was negative at humid sites (DI < 1), while the sensitivity of WUE to SWC was positive at arid sites (DI > 2). Furthermore, the contribution of GPP to VPD-induced WUE variability was 63 % at humid sites, and the contribution of ET to SWC-induced WUE variability was 68 % when SWC was less than the 60th percentile at arid sites. Consequently, one increasing VPD-induced decrease in GPP was generally linked to a decrease in WUE at humid sites, and one drying soil moisture-caused decrease in ET was linked to a WUE increase under low SWC conditions at arid sites. Finally, VPD had a stronger effect on WUE than SWC when VPD was less than the 90th percentile or SWC was greater than the 50th percentile. Our findings underscore the importance of considering ecosystem dryness when investigating the impacts of VPD and SWC on ecosystem carbon-water coupling.

The non-uniform time-lag and cumulative responses of terrestrial ecosystem water use efficiency to climate change in Lake Victoria Basin, East Africa

Revealing the response mechanisms of carbon–water cycle to maximum and minimum temperatures and daytime and nighttime warming is important for managing fragile ecosystems exposed to severe climate warming and extreme weathers. Taking water use efficiency (WUE) characterizing the coupled carbon–water cycle as the research object, and maximum and minimum temperatures as the indicators of temperature change, this study explored the response of ecosystem WUE to temperature change in Lake Victoria Basin during distinct rainy and dry seasons. The results revealed the time-lag and cumulative multiple effects of antecedent maximum and minimum temperatures on current ecosystem WUE, which marked differences between the rainy and dry seasons. The monthly average ecosystem WUE in the rainy seasons (0.99 g C kg−1 H2O) were higher than those in the dry seasons (0.83 g C kg−1 H2O) during 2001–2018. WUE were more sensitive to temperature change in the dry seasons, with significant negative correlation with minimum temperature and positive correlation with cumulative maximum temperature. In rainy seasons, WUE showed more significant time-lag and cumulative effects, with response to maximum temperature for lagging approximate 3 months, to minimum temperature for lagging 0–2 months, to cumulative maximum temperature for lagging 1 month, and to cumulative minimum temperature lagging 3–4 months, respectively. The results add new evidence for differential non-instantaneous responses of WUE to maximum and minimum temperatures under dry and wet conditions.

Response of water-use efficiency to phenology in the natural forest and grassland of the Loess Plateau in China

Response of water-use efficiency to phenology in the natural forest and grassland of the Loess Plateau in China

The effects of flash drought on the terrestrial ecosystem in Korea

Typified by rapid intensification in a short time, flash droughts (FDs) have attracted increasing attention because of their effects on water resources and ecosystems. Water use efficiency (WUE) is a key indicator for understanding the interaction between the carbon dynamic and water cycle. However, the effects of FDs on terrestrial ecosystems in Korea remain mostly unknown. Therefore, we identified FD events in Korea using the soil moisture (SM)-based rate of intensification (RI) and the evapotranspiration (ET)-based Standardized Evaporative Stress Ratio (SESR) with flux tower data. Continuous satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) were used to examine spatiotemporal patterns and trends in FD-related factors, Potential ET (PET), Actual ET (AET), Gross Primary Productivity (GPP), Evaporative Stress Ratio (ESR), and WUE from 2001 to 2020. The frequency, duration, and intensity of FDs in Korea were investigated and responses in WUE to FDs in different landcover types were examined. The SESR and RI had a similar performance in capturing FD events in flux tower site data. At the regional scale, a lower ESR was mainly observed in the western and southern parts of the study area, indicating progressively drier climate conditions. The mean frequency percentage and number of Korean FD events in the past 20 years were 61.3% and 16, respectively. In addition, more FD events occurred in the northern and eastern regions of the study area, where the main landcover type is forest, and northeastern Korea suffered from long FD events (over 30 days). FDs caused a positive response of WUE in most of the study area. However, the change in WUE after FD events varied by FD severity and landcover type. The changes in WUE after moderate FD events were largely driven by the change in GPP, and the highest sensitivity of WUE to FD intensity was found in croplands. These findings highlight the characteristics and effects of FDs on WUE across landcover types, which will be helpful in managing potential environmental risks in Korea.

Field, plant, to leaf: A meta-analysis on crop water use efficiency response to potassium fertilization

Applying potassium (K) fertilizer to fields is considered to be an important agricultural production practice to improve crop water use efficiency (WUE). However, the magnitude of crop WUE response to K fertilization has only been reported for individual regions, crops and scales due to lack of extensive and complete global datasets. Here we quantified the responses of crop WUE to K fertilization at the three scales of field-population (WUEfield), whole-plant (WUEplant) and single-leaf (WUEleaf) through synthesizing 797 paired observations from 81 field-based studies worldwide. Meta-analysis results showed that K fertilization increased WUEfield, WUEplant and WUEleaf of crops by 19.0%, 12.0% and 29.9%, respectively, with the key factors influencing this being crop type, mean annual precipitation and soil pH, respectively. It should be noted that the magnitude of WUEfield, WUEplant and WUEleaf response to K fertilization was significantly positively correlated with mean annual precipitation, while significantly negatively correlated with soil pH, which implying that K fertilization is an effective way to improve crop WUE in the region with humid climate or acidic soil. Furthermore, the conclusive findings of this meta-analysis showed that the relative importance of environmental factors in the magnitude of the WUEfield, WUEplant and WUEleaf response to K fertilization was 49.8%, 68.8% and 82.0%, respectively, which indicated that climatic characteristics and soil properties were drivers of crop WUE response to K fertilization rather than management practices. Overall, our meta-analysis provides new insight into the crop WUE response to K fertilization from field, plant, to leaf was driven by environmental factors and highlights the great potential of improved field K fertilization strategies in developing sustainable agriculture under the condition of climate change.

Identifying the Responses of Vegetation Gross Primary Productivity and Water Use Efficiency to Climate Change under Different Aridity Gradients across China

Despite the fact that gross primary productivity (GPP) and water use efficiency (WUE) have been widely used as indicators to evaluate the water-carbon cycle, uncertainties exist in the patterns of GPP and WUE responses to climate variability along different aridity gradients. In this study, the aridity index was used to divide China into four arid-humid zones. The spatiotemporal variability of multiple vegetation types GPP and WUE in response to climate change under different arid-humid zones were investigated based on remote sensing data. The results indicated that the increasing trend of WUE in the four arid-humid zones of China was less pronounced than GPP from 2001 to 2021. The GPP value decreased gradually from the humid to the arid zone, and the WUE value in the arid zone was slightly higher than in the semi-arid zone. The GPP of all vegetation types in China showed a tendency to increase, while shrubland and wetland WUE tended to decrease. The major vegetation types (e.g., forest, cropland and grassland) in each aridity gradient contributed to the changes in local GPP and WUE. However, in individual arid-humid zones, wetland and shrubland also exhibited high GPP and WUE values that were not inferior to forest and cropland. Temperature and precipitation were the main climatic factors responsible for the increase in vegetation GPP in different aridity gradients, with a higher positive correlation for temperature than precipitation. WUE showed a distinct positive and negative correlation with the thermal factors (temperature and net radiation) and the moisture factors (precipitation and relative humidity); this pattern was more pronounced in the humid and semi-humid zones. Net radiation and precipitation may be the main climatic factors causing a slight upward trend in WUE across the arid-humid zones, while the decrease in shrubland and wetland WUE may be related to relative humidity and precipitation.

Contrasting response of water use efficiency to soil moisture availability: From leaf to ecosystem in an arid oasis

Water availability profoundly and simultaneously affects the water cycle and disturbs terrestrial carbon cycle. However, there is currently a lack of knowledge about how carbon–water coupling occurs in response to soil moisture availability, and the underlying mechanisms. Here, using two kinds of observational datasets (i.e., leaf gas exchange and ecosystem eddy-covariance), we evaluated a normalized water-use efficiency (WUE) measure (g1, the “stomatal conductance slope”), and explored its dynamic behavior in response to soil moisture availability. It was found that patterns of seasonal variations in WUE were not consistent between the two datasets. A contrasting response of G1 (the capital letter denotes g1 at the ecosystem level) to volumetric water content (VWC) during different stages of the growing season was confirmed, which was attributed to the different sensitivities of biological processes (i.e., photosynthesis and transpiration) to soil moisture availability. WUE variability during the main growing seasons was primarily controlled by transpiration, whereas WUE variability during the early and late growing seasons was mostly controlled by photosynthesis. Our findings suggest that phenology plays a significant role in regulating carbon–water coupling processes, and then exerts biotic control. The insights gained from this study will facilitate an in-depth understanding of the responses of plant biological processes to global change.

Increasing precipitation during first half of growing season enhances ecosystem water use efficiency in a semiarid grassland.

Precipitation amount and seasonality can profoundly impact ecosystem carbon (C) and water fluxes. Water use efficiency (WUE), which measures the amount of C assimilation relative to the amount of water loss, is an important metric linking ecosystem C and water cycles. However, how increasing precipitation at different points in the growing season affects ecosystem WUE remains unclear. A manipulative experiment simulating increasing first half (FP+) and/or second half (SP+) of growing-season precipitation was conducted for 4 years (2015-2018) in a temperate steppe in the Mongolian Plateau. Gross ecosystem productivity (GEP) and evapotranspiration (ET) were measured to figure out ecosystem WUE (WUE = GEP/ET). Across the four years, FP+ showed no considerable impact on ecosystem WUE or its two components, GEP and ET, whereas SP+ stimulated GEP but showed little impact on ET, causing a positive response of WUE to FP+. The increased WUE was mainly due to higher soil water content that maintained high aboveground plant growth and community cover while ET was stable during the second half of growing season. These results illustrate that second half of growing-season precipitation is more important in regulating ecosystem productivity in semiarid grasslands and highlight how precipitation seasonality affects ecosystem productivity in the temperate steppe ecosystem.

Assessing the response of water use efficiency to climate variability and land-use changes in the mountainous area of North China

Water use efficiency (WUE) is a measure used for evaluating the trade-off between carbon sequestration and water consumption of terrestrial ecosystems. Much attention has been directed on how and to what extent WUE varies with climate variability and land-use changes. In this study, the Carnegie-Ames-Stanford Approach model and remote sensing data are utilized to analyze the response of WUE to climate variability and land-use changes in the mountainous area of North China from 2000 to 2020. The results show that the annual WUE follows an increasing trend at a rate of 0.012 gC/mm·m2·yr with an average value of 1.027 gC/mm·m2. Spatially, significant heterogeneity was detected in WUE with gradients decreasing from the southeast to the northwest, while significant increases were found in Beijing, Tianjin, and some regions of Hebei. The annual average WUE of different vegetation types decreased following the order of mixed forest > needleleaf forest > broadleaf forest > grassland > shrubland > cropland. Among various meteorologic and vegetation factors, WUE was mainly influenced by temperature, precipitation, and LAI. The cumulative effect of these three factors explained 79.43 % of the variations of WUE. Finally, when precipitation was 490–510 mm, or temperature was 13 ℃, or LAI was 1.27 m2m−2, WUE reached the threshold value, i.e., vegetation had the highest degree of water use. These results have implications for the management of water resources and regional ecosystem optimization in regions with limited water resources.