Response Of Water Use Efficiency Research Articles

Responses of ecosystem water use efficiency to meteorological drought under different biomes and drought magnitudes in northern China

Future climate change scenarios predict that warming may increase drought stress over northern China, causing ecological deterioration and food insecurity. Water use efficiency (WUE) is an important indicator to understand the response of ecosystem productivity to water availability. At present we lack a clear picture of how ecosystem WUE responds to drought magnitude across a broad range of vegetation types and environments. Here, we applied Moderate Resolution Imaging Spectrometer (MODIS) satellite images and meteorological data to obtain a regional estimation of gross primary production (GPP) and evapotranspiration (ET) during the period of 2000–2014. We then investigated the relationships between WUE (GPP/ET) and drought magnitude (based on the Standardized Precipitation Evapotranspiration Index, SPEI) for different climate zones and biomes. The mean annual WUE decreased with increasing Aridity Index (AI) in arid regions. However, the relationships between WUE and AI followed a logarithmic function across semi-arid and humid regions. In hype-arid, sub-humid and humid regions, a drying trend resulted in increased WUE. Contrasting responses of WUE to drought were found in arid and semi-arid regions. The negative effect of drought that caused decreased WUE was most significant in the transition zone between arid and semi-arid lands. Besides, the relationships between relative changes in WUE and drought magnitude followed a logarithmic function of SPEI. Severe and extreme drought resulted in WUE reductions regardless of different hydroclimatic conditions and biomes. The ecosystem resistant to drought can be evaluated by the magnitude of WUE reductions during drought, together with the WUE-SPEI correlations. Forests were most resistant to drought, followed by croplands, grasslands, and deserts. Differences in the ecosystem resistance to water stress were attributed to diversities in drought survival traits and strategies. Given that hydroclimatic conditions break down ecosystem resistance under severe and extreme drought, we highlight an urgent need to formulate adaptation strategies aimed at reducing drought risk in the eastern and central portions of northern China.

Separating the effects of climate change and human activity on water use efficiency over the Beijing-Tianjin Sand Source Region of China

Water use efficiency (WUE) is a central parameter for linking carbon and water exchange processes in terrestrial ecosystems. The Beijing-Tianjin Sand Source Region (BTSSR) in China has undergone tremendous vegetation restoration and climate change. Understanding the WUE responses to climate change and human activity and their relative contributions to the trends and inter-annual variations (IAVs) in WUE is necessary to improve water use efficiency and strengthen water resource management. The evapotranspiration (ET) dataset based on the model tree ensemble (MTE) algorithm which was a machine learning approach using flux-tower ET measurements and the GLASS GPP dataset, as well as the variance decomposition method, were used to analyze the spatiotemporal changes in water use efficiency and inherent water use efficiency (IWUE) and the impacts of climate change and human activities. The results showed that the annual WUE and IWUE exhibited significantly increase in most regions of the BTSSR. The trend of human activity played the most important role in the increases of WUE and IWUE, with relative contributions of 88.2% and 85.9%, respectively, followed by the IAV of human activity for WUE (6.1%) and the trend of climate change (8.7%) for IWUE. The contribution of IAV to climate change was relatively small. Moreover, WUE and IWUE were all positively correlated with precipitation and temperature in most regions. Our results indicated that ecological restoration projects had significantly improved water use efficiency in BTSSR and may decrease the water burden in the BTSSR.

Effect of planting density on deep soil water and maize yield on the Loess Plateau of China

Dryland farmers tend to increase maize plant density with drought and density stress tolerance hybrids to achieve higher grain yield in recent years. However, could this strategy improve yield or water use efficiency (WUE) and be sustainable without decreasing deep soil water in drought-prone environments is not clear. A 4-year of successive field study was carried out with three different drought and density stress tolerance maize hybrids and four plant density arrange from 52,500 to 97,500 plants ha−1. To quantify the responses of grain yield formation and WUE to increasing plant density under various rainfall condition and evaluate the effect on deep soil water balance. Results showed that using of drought and density stress tolerance hybrids could achieve higher grain yield and WUE with higher plant density in normal years, which was associated with an increase in kernels number per square meter. But in dry year, as fewer water was available during reproductive growth stage in higher plant density, grain yield and WUE was gradually decreased with increasing plant density, especially in density stress sensitive hybrid. Soil water balance at 0 to 200 cm depth was not broken by high plant density from the perspective of same water availability at sowing in each year, despite of the lower soil water content during maize growth stage. However, high plant density tended to consume more deep soil water which was hardly been replenished by precipitation, especially in high density tolerance hybrids. Hence, higher density that exceed 60000 plants ha−1 couple with drought and density stress tolerance hybrids is a potential way to improve maize production in dryland, but it increases the risk of deep soil desiccation.

Atmospheric Water Demand Dominates Daily Variations in Water Use Efficiency in Alpine Meadows, Northeastern Tibetan Plateau

AbstractEcosystem water use efficiency (WUE) acts as an integrated functional indicator for understanding land‐atmosphere interactions. The temporal patterns in the daily variations of WUE and their underlying drivers during different seasons in alpine meadow ecosystems, which are particularly vulnerable to changing climate, still remain poorly understood in spite of increasing efforts. In this study, we investigated the potential divergence in the response of WUE to climatic and biological drivers during different seasons at two alpine meadow ecosystems in the northeastern Tibetan Plateau using continuous eddy‐covariance measurements of carbon and water fluxes between 2013 and 2015. We found that variations in CO2 concentration exert significantly positive effects on variations in WUE in spring, but not in summer and autumn. Notably, vapor pressure deficit (VPD) overrode other factors playing a dominant role in regulating daily variations in WUE during all seasons in these alpine meadow ecosystems. Variations in VPD explained 29.5 to 52.3% of the variance in WUE between different seasons. We further showed that carbon gain and water loss processes responded divergently to different drivers; higher VPD significantly increased ecosystem evapotranspiration; whereas, variations in soil moisture and leaf area index significantly and positively affected gross primary productivity. Our findings highlighted the increasing importance of atmospheric drought in shaping land‐atmosphere interactions in alpine meadow ecosystems, particularly in a warming climate.

Observing the Whole-Canopy Short-Term Dynamic Response to Natural Step Changes in Incident Light: Characteristics of Tropical and Temperate Forests

We examine the physical and biological responses of forest canopies to step changes in light caused by passing low cumulus clouds that intermittently block the direct solar beam. Using data obtained at a tropical rainforest and at a midlatitude deciduous forest, we estimate the course of sensible heat flux, net ecosystem exchange, evapotranspiration, and water-use efficiency in response to the rapid changes in the incident radiative flux. To describe these fluxes during the interval over which the effects of stomatal time delays can be most influential, eddy fluxes are estimated over minute or shorter intervals by invoking a conditional-sampling procedure based on forming a Reynolds-average ensemble. The most important differences between the two forests’ physical responses are in the thermal balances and heat-flux time response constants. During the initial period after the light transition the only mean variables that show appreciable changes are the blackbody and air temperatures, the other scalars being little affected. We find that a distinct transient thermal internal boundary layer appears ≈ 20 m thick above the temperate deciduous forest and ≈ 45 m thick above the tropical rainforest. At each forest, the effective thickness of the inferred thermal outer-canopy ‘big leaf’ is about 1 mm. Twenty minutes after the abrupt change in incident light, ensemble eddy-flux estimates approach those found using conventional time averaging, confirming the validity of the ensemble approach. Previously unrecognized transient maxima in net ecosystem exchange and evapotranspiration are evident 5–10 min following the shadow-to-light transition, longer than the average light interval between shadows observed on partly-cloudy days in each case. Short-term variations in sensible heat flux, net ecosystem exchange, and evapotranspiration approximate an exponential adjustment, implying that first-order time-dependent single-leaf models are adequate to describe whole-canopy processes in these cases, providing an experimental method for determining whole-canopy bulk stomatal time constants. During the sunlit interval (direct and diffuse radiative fluxes combined) net ecosystem exchange is enhanced, while under cloud shadow (only diffuse radiative flux) water-use efficiency increases. This light and shadow alternation provides a mechanism describing the observed enhanced net ecosystem exchange and water-use efficiency under certain types of partly-cloudy sky. We apply empirical flux-response curves to an idealized case of radiative flux varying in a regular on–off light and shadow pattern. For this case, an analytical solution for mean net ecosystem exchange flux as a function of diffuse-radiation fraction yields results that strongly resemble previous findings based on conventional time-averaged fluxes. Our analysis and modelling indicates that a well-known correlation between diffuse radiative flux and enhanced net ecosystem exchange and water-use efficiency on cloudy days is in many cases not causal, but rather to be a consequence of time averaging over light-and-shadow intervals. By linking processes associated with photosynthesis in fluctuating light at the leaf scale to the canopy scale, our efforts facilitate the scaling-up of leaf responses to the ecosystem scale.

A Post-domestication Mutation, Dt2, Triggers Systemic Modification of Divergent and Convergent Pathways Modulating Multiple Agronomic Traits in Soybean

The semi-determinate stem growth habit in leguminous crops, similar to the “green revolution” semi-dwarf trait in cereals, is a key plant architecture trait that affects several other traits determining grain yield. In soybean semi-determinacy is modulated by a post-domestication gain-of-function mutation in the gene, Dt2, which encodes an MADS-box transcription factor. However, its role in systemic modification of stem growth and other traits is unknown. In this study, we show that Dt2 functions not only as a direct repressor of Dt1, which prevents terminal flowering, but also as a direct activator of putative floral integrator/identity genes including GmSOC1, GmAP1, and GmFUL, which likely promote flowering. We also demonstrate that Dt2 functions as a direct repressor of the putative drought-responsive transcription factor gene GmDREB1D, and as a direct activator of GmSPCH and GmGRP7, which are potentially associated with asymmetric division of young epidermal cells and stomatal opening, respectively, and may affect the plant's water-use efficiency (WUE). Intriguingly, Dt2 was found to be a direct activator or repressor of the precursors of eight microRNAs targeting genes potentially associated with meristem maintenance, flowering time, stomatal density, WUE, and/or stress responses. This study thus reveals the molecular basis of pleiotropy associated with plant productivity, adaptability, and environmental resilience.

Projecting climate change impacts on rainfed wheat yield, water demand, and water use efficiency in northeast Iran

The frequency and severity of high temperature and drought extremes are expected to increase under future climate change (CC) and considerably affect the agricultural sector particularly in water-limited ecosystems. This study was conducted to assess future CC impacts on rainfed wheat yield, water requirement (CWR), water use efficiency (WUE), precipitation use efficiency (PUE), reference crop evapotranspiration (ET0), and agricultural rainfall index (ARI) in northeast of Iran. The outputs of five global climate models (GCMs) under RCP-4.5 and RCP-8.5 during three time periods (i.e., the 2025s, 2055s, and 2085s) downscaled by MarkSimGCM model were applied. CWR was estimated using the CROPWAT 8.0 model. Further, the CSM-CERES-Wheat model was employed to simulate rainfed wheat yield, WUE, PUE, and ET0 responses to CC. The results showed that the mean monthly ET0 and CWR would likely increase under both emission pathways over the studied sites. The mean monthly ARI is also anticipated to decline in the future indicating a drier climatic condition over northeastern Iran by 2100. Furthermore, CC is highly likely to decrease rainfed grain yield, WUE, and PUE during the current century. The largest changes in ET0, ARI, CWR, yield, WUE, and PUE were projected in the late twenty-first century (the 2085s) under RCP-8.5. The CC-induced wheat yield loss will likely endanger food security in the country. Yield reduction can be partially offset by adopting appropriate adaptation measures.

Responses of water use efficiency to phenology in typical subtropical forest ecosystems—A case study in Zhejiang Province

Ecosystem-scale water-use efficiency (WUE) is an important indicator for understanding the intimately coupled relationship between carbon and water cycles in ecosystems. Previous studies have suggested that both abiotic and biotic factors have significant effects on WUE in forest ecosystems. However, responses of WUE to phenology in the context of climate change remain poorly understood. In this study, we analyzed the sensitivity and response patterns of seasonal WUE to phenology in Zhejiang Province where typical subtropical forest ecosystems are located, and discussed potential causes of the changes of the sensitivity and response patterns along different climate gradient during 2000–2014. The results of interannual partial correlation analysis showed widespread negative correlations between WUE and the start of growing season (SOS) in spring. This is because the increase in gross primary product (GPP) is larger than that of evapotranspiration (ET), resulting from an advanced SOS. The positive correlation between WUE and SOS was widely observed in summer mainly because of water stress and plant ecological strategy. The autumn WUE enhanced with the delay in the end of growing season (EOS) mainly because of the increase in GPP meanwhile the decrease or steadiness in ET, resulting from a delayed EOS. In space, the sensitivity of spring WUE to SOS significantly decreased along the radiation gradient, which might be related to strong soil evaporation in high radiation area; the sensitivity of WUE to SOS in summer showed a positive correlation with precipitation and a negative correlation with temperature, respectively, which might be attributed to the compensation of GPP to the delayed SOS and water stress caused by high temperature. The sensitivity of WUE to EOS increased significantly along the radiation and precipitation gradients in autumn, which may be because the increase of radiation and precipitation provides more water and energy for photosynthesis.

Gas exchange, biomass allocation and water-use efficiency in response to elevated CO2 and drought in andiroba (Carapa surinamensis, Meliaceae)

Abstract: Prolonged droughts are predicted for some parts of the Amazon; however, it is still unclear how Amazonian trees will respond to water stress under the ongoing increase in CO2 concentration. The aim of this study was to assess the effect of elevated CO2 (eCO2) and drought on photosynthetic rates, water-use efficiency, and biomass allocation in andiroba (Carapa surinamensis). The plants were grown in pots at ambient (400 ppm CO2) and eCO2 (700 ppm) at two water regimes, soil at 50% field capacity, FC (drought) and soil at 100% FC for 163 days. We measured light saturated photosynthesis on a mass basis (Asat-mass), stomatal conductance to CO2 on a mass basis (gsCO2-mass), whole-plant water-use efficiency (WUEP), biomass accumulation, specific leaf area (SLA) and total leaf area. At eCO2, Asat-mass increased 28% in well-watered plants and 93% under drought, whereas gsCO2-mass declined 39% in well-watered plants at eCO2, with no effect of drought on gsCO2-mass at eCO2. The total biomass gain improved 73% at eCO2 and over CO2 levels it was reduced (54%) by drought. WUEP improved (188%) at eCO2 in well-watered plants and 262% under drought. SLA declined 23% at eCO2, but the effect of drought on SLA was null. On the contrary, total leaf area was greatly reduced (67%) by drought, but it was not affected by eCO2. The large increase in total biomass and the substantial improvement in WUEP under eCO2, and the sharp decline in leaf area under water stress widen our knowledge on the physiology of this important species for the forest management of large areas in the Amazon region.

Water-Use Efficiency: Advances and Challenges in a Changing Climate.

Water use efficiency (WUE) is defined as the amount of carbon assimilated as biomass or grain produced per unit of water used by the crop. One of the primary questions being asked is how plants will respond to a changing climate with changes in temperature, precipitation, and carbon dioxide (CO2) that affect their WUE At the leaf level, increasing CO2 increases WUE until the leaf is exposed to temperatures exceeded the optimum for growth (i.e., heat stress) and then WUE begins to decline. Leaves subjected to water deficits (i.e., drought stress) show varying responses in WUE. The response of WUE at the leaf level is directly related to the physiological processes controlling the gradients of CO2 and H2O, e.g., leaf:air vapor pressure deficits, between the leaf and air surrounding the leaf. There a variety of methods available to screen genetic material for enhanced WUE under scenarios of climate change. When we extend from the leaf to the canopy, then the dynamics of crop water use and biomass accumulation have to consider soil water evaporation rate, transpiration from the leaves, and the growth pattern of the crop. Enhancing WUE at the canopy level can be achieved by adopting practices that reduce the soil water evaporation component and divert more water into transpiration which can be through crop residue management, mulching, row spacing, and irrigation. Climate change will affect plant growth, but we have opportunities to enhance WUE through crop selection and cultural practices to offset the impact of a changing climate.

A simulation of winter wheat crop responses to irrigation management using CERES-Wheat model in the North China Plain

To improve efficiency in the use of water resources in water-limited environments such as the North China Plain (NCP), where winter wheat is a major and groundwater-consuming crop, the application of water-saving irrigation strategies must be considered as a method for the sustainable development of water resources. The initial objective of this study was to evaluate and validate the ability of the CERES-Wheat model simulation to predict the winter wheat grain yield, biomass yield and water use efficiency (WUE) responses to different irrigation management methods in the NCP. The results from evaluation and validation analyses were compared to observed data from 8 field experiments, and the results indicated that the model can accurately predict these parameters. The modified CERES-Wheat model was then used to simulate the development and growth of winter wheat under different irrigation treatments ranging from rainfed to four irrigation applications (full irrigation) using historical weather data from crop seasons over 33 years (1981–2014). The data were classified into three types according to seasonal precipitation: <100 mm, 100–140 mm, and >140 mm. Our results showed that the grain and biomass yield, harvest index (HI) and WUE responses to irrigation management were influenced by precipitation among years, whereby yield increased with higher precipitation. Scenario simulation analysis also showed that two irrigation applications of 75 mm each at the jointing stage and anthesis stage (T3) resulted in the highest grain yield and WUE among the irrigation treatments. Meanwhile, productivity in this treatment remained stable through different precipitation levels among years. One irrigation at the jointing stage (T1) improved grain yield compared to the rainfed treatment and resulted in yield values near those of T3, especially when precipitation was higher. These results indicate that T3 is the most suitable irrigation strategy under variable precipitation regimes for stable yield of winter wheat with maximum water savings in the NCP. The application of one irrigation at the jointing stage may also serve as an alternative irrigation strategy for further reducing irrigation for sustainable water resources management in this area.

Response of water use efficiency and carbon emission to no-tillage and winter wheat genotypes in the North China Plain

No-tillage management practices reduce net CO2 losses from farmland and keep soil from degrading, but also decrease winter wheat grain yield and water use efficiency (WUE) in the North China Plain (NCP). Suitable management practices, namely, the choice of genotypes, could enhance crop yield and WUE; however, how the WUE and CO2 exchange responds to no-tillage practices and winter wheat genotypes remains unclear. In the 2015–2016 and 2016–2017 winter wheat growing seasons in the NCP, a field experiment was carried out, and tested two tillage methods (no-tillage with mulching and conventional tillage) and two winter wheat genotypes (‘Tainong 18’ and ‘Jimai 22’). The goal of the study was to identify the relationship between winter wheat grain yield, water consumption, and carbon emissions in no-tillage practices. The results showed that, compared to conventional tillage, no-tillage significantly reduced the net CO2-C cumulative emissions and water consumption; however, the grain yield was significantly reduced by 6.8% and 12.0% in the first and second growing seasons, respectively. Compared with Jimai 22, Tainong 18 had a compensatory effect on the yield reduction caused by no-tillage. As a result, the yield carbon utilization efficiency (R) and WUE were the highest in no-tillage with Tainong 18 (NT18), and the carbon emission per unit water consumption was the lowest in NT18. The results support the idea that a combination of no-tillage with genotype can improve the regulation of soil carbon emissions and water consumption of winter wheat, thus, providing theoretical support for sustainable crop production and soil development in the NCP.

Production components, grain yield and water use efficiency of irrigated cowpea

Cowpea cultivars, which are indicated for irrigated cropping, are recommended without proper assessment of their productive potential and water use efficiency in response to water regimes in the soil from different production environments. The aim of this study was to evaluate the production components, the yield performance and the efficiency of water use of cowpea cultivars in response to the application of irrigation depths under the soil and climate conditions of Bom Jesus, PI. The experiment was performed from June to August 2011 in a soil classified as sandy-textured Fluvic Neosol. The study was conducted in a randomized blocks design, with four replicates. The treatments were arranged in subdivided plots (irrigation depths in plots and cultivars in subplots). The cultivars BRS Aracê (semi-prostrate) and BRS Tumucumaque (semi-erect) were submitted to five irrigation depths (108.2, 214.7, 287.9, 426.1 and 527.7 mm). The number of pods per plant and the weight of one hundred grains are the main components related to grain yield. The cultivar BRS Tumucumaque presented a better yield performance (1697.2 kg ha-1) than the cultivar BRS Aracê (1233.8 kg ha-1), with the application of the irrigation depths of 527.7 and 522.4 mm, respectively. The highest water use efficiency (0.420 kg m-3) is obtained with the application of the irrigation depth of 108.2 mm in both cultivars.

Responses of ecosystem water use efficiency to spring snow and summer water addition with or without nitrogen addition in a temperate steppe.

Water use efficiency (WUE) is an important indicator of ecosystem functioning but how ecosystem WUE responds to climate change including precipitation and nitrogen (N) deposition increases is still unknown. To investigate such responses, an experiment with a randomized block design with water (spring snowfall or summer water addition) and nitrogen addition was conducted in a temperate steppe of northern China. We investigated net ecosystem CO2 production (NEP), gross ecosystem production (GEP) and evapotranspiration (ET) to calculate ecosystem WUE (WUEnep = NEP/ET or WUEgep = GEP/ET) under spring snow and summer water addition with or without N addition from 2011 to 2013. The results showed that spring snow addition only had significant effect on ecosystem WUE in 2013 and summer water addition showed positive effect on ecosystem WUE in 2011 and 2013, as their effects on NEP and GEP is stronger than ET. N addition increased ecosystem WUE in 2012 and 2013 both in spring snow addition and summer water addition for its increasing effects on NEP and GEP but no effect on ET. Summer water addition had less but N addition had greater increasing effects on ecosystem WUE as natural precipitation increase indicating that natural precipitation regulates ecosystem WUE responses to water and N addition. Moreover, WUE was tightly related with atmospheric vapor-pressure deficit (VPD), photosynthetic active radiation (PAR), precipitation and soil moisture indicating the regulation of climate drivers on ecosystem WUE. In addition, it also was affected by aboveground net primary production (ANPP). The study suggests that ecosystem WUE responses to water and N addition is determined by the change in carbon process rather than that in water process, which are regulated by climate change in the temperate steppe of northern China.

Component crop physiology and water use efficiency in response to intercropping

Abstract Interspecies specific interactions are generally regarded as drivers of plant productivity in multispecies agroecosystems. Complementary use of resource in diverse communities can enhance community productivity through optimal use of plant-available resources and positive interactions such as facilitation can ameliorate high abiotic stress conditions. We studied the effects on physiological response, leaf traits and water use efficiency of a multifunctional species intercropping system consisting of peanut (Arachis hypogaea L.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], okra [Abelmoschus esculentus (L.) Moench], cowpea [Vigna unguiculata (L.) Walp.], and pepper (Capsicum annuum L.) planted alone or in various intercropping combinations in a low fertilizer input system in the peak of summer heat in Texas. Differences in gas exchange measurements were detected only in watermelon in year 2 of the study when okra was the dominant crop. This same year watermelon specific leaf area (SLA) was significantly higher when okra was present in a treatment and particularly in the three and four species combinations, Wpwo and Wpwoc, 27.5 and 31.0 m2 kg−1, respectively, as compared to watermelon grown in monoculture, strip intercropped with peanut (Spw) and within row intercropped with peanut (Wpw), 20.4, 20.1, and 19.8 m2 kg−1, respectively. This corresponds with an increase watermelon leaf N concentration and a decrease in leaf C:N ratio in Wpwo and Wpwoc treatments. No differences in d13C composition, a measure of water use efficiency over the leaf lifespan, were detected across cropping system for each species. Water use efficiency based on per plant production (WUEyield) indicated an increase in water use efficiency in dominant crops such as watermelon in 2011 and okra in 2012, but a reduction in WUEyield subordinate crops such as cowpea and pepper both years of the study. Peanut grown in monoculture and strip intercropped with watermelon had significantly lower leaf water potential values in 2012, −2.2 and −2.1 MPa, respectively, as compared to intercropping systems increasing in level of integration (Wpw = − 1.1, Wpwo = − 0.6, Wpwoc = − 1.3, Wall = −1.1 MPa), indicating peanut benefited from alterations to microclimate and facilitative interactions with companion crops in some intercropping systems through a reduction in plant water stress. The results from this study suggest there may be a benefit to a multifunctional intercropping system in the form of increased food production per unit of water input in dominant crops and reduced water stress for some component species. This is important to producers; showing a method to increase overall crop production without increasing water inputs.

Irrigation and Nitrogen Regimes Promote the Use of Soil Water and Nitrate Nitrogen from Deep Soil Layers by Regulating Root Growth in Wheat.

Unreasonably high irrigation levels and excessive nitrogen (N) supplementation are common occurrences in the North China Plain that affect winter wheat production. Therefore, a 6-yr-long stationary field experiment was conducted to investigate the effects of irrigation and N regimes on root development and their relationship with soil water and N use in different soil layers. Compared to the non-irrigated treatment (W0), a single irrigation at jointing (W1) significantly increased yield by 3.6–45.6%. With increases in water (W2, a second irrigation at flowering), grain yield was significantly improved by 14.1–45.3% compared to the W1 treatments during the drier growing seasons (2010–2011, 2012–2013, and 2015–2016). However, under sufficient pre-sowing soil moisture conditions, grain yield was not increased, and water use efficiency (WUE) decreased significantly in the W2 treatments during normal precipitation seasons (2011–2012, 2013–2014, and 2014–2015). Irrigating the soil twice inhibited root growth into the deeper soil depth profiles and thus weakened the utilization of soil water and NO3-N from the deep soil layers. N applications increased yield by 19.1–64.5%, with a corresponding increase in WUE of 66.9–83.9% compared to the no-N treatment (N0). However, there was no further increase in grain yield and the WUE response when N rates exceeded 240 and 180 kg N ha−1, respectively. A N application rate of 240 kg ha−1 facilitated root growth in the deep soil layers, which was conducive to utilization of soil water and NO3-N and also in reducing the residual NO3-N. Correlation analysis indicated that the grain yield was significantly positively correlated with soil water storage (SWS) and nitrate nitrogen accumulation (SNA) prior to sowing. Therefore, N rates of 180–240 kg ha−1 with two irrigations can reduce the risk of yield loss that occurs due to reduced precipitation during the wheat growing seasons, while under better soil moisture conditions, a single irrigation at jointing was effective and more economical.

Transpiration Dominates Ecosystem Water‐Use Efficiency in Response to Warming in an Alpine Meadow

AbstractAs a key linkage of C and water cycles, water‐use efficiency (WUE) quantifies how much water an ecosystem uses for carbon gain. Although ecosystem C and water fluxes have been intensively studied, yet it remains unclear how ecosystem WUE responds to climate warming and which processes dominate the response of WUE. To answer these questions, we examined canopy WUE (WUEc), ecosystem WUE (WUEe) and their components including gross ecosystem productivity, ecosystem evapotranspiration (ET), soil evaporation (E), and plant canopy transpiration (T), in response to warming in an alpine meadow by using a manipulative warming experiment in 2015 and 2016. As expected, low‐ and high‐level warming treatments increased soil temperature (Tsoil) at 10 cm on average by 1.65 and 2.77°C, but decreased soil moisture (Msoil) by 2.52 and 7.6 vol %, respectively, across the two years. Low‐ and high‐level warming increased WUEe by 7.7 and 9.3% over the two years, but rarely changed WUEc in either year. T/ET ratio determined the differential responses of WUEc and WUEe. Larger T/ET led to less difference between WUEc and WUEe. By partitioning WUEc and WUEe into different carbon and water fluxes, we found that T rather than gross ecosystem productivity or E dominated the responses of WUEc and WUEe to warming. This study provides empirical insights into how ecosystem WUE responds to warming and illustrates the importance of plant transpiration in regulating ecosystem WUE under future climate change.

Effects of nutrient supply on carbon and water economies of C4 grasses.

C3 plants can increase nutrient uptake by increasing transpiration, which promotes the flow of water with dissolved nutrients towards the roots. However, it is not clear if this mechanism of nutrient acquisition, termed 'mass flow', also operates in C4 plants. This is an important question, as differences in mass flow capacity may affect competitive interactions between C3 and C4 species. To test if mass flow can be induced in C4 species, we conducted an experiment in a semiarid seasonal savanna in South Africa. We grew six C4 grasses in nutrient-poor sand and supplied no nutrients, nutrients to the roots or nutrients spatially separated from the roots. We measured the rates of photosynthesis and transpiration, water-use efficiency (WUE), nitrogen gain and biomass. For all species biomass, N gain, photosynthesis and transpiration were lowest in the treatment without any nutrient additions. Responses to different nutrient positioning varied among species from no effect on N gain to a 50% reduction when nutrients were spatially separated. The ability to access spatially separated nutrients showed a nonsignificant positive relationship with both the response of transpiration and the response of WUE to spatial nutrient separation. This indicates that nutrient acquisition is not regulated by decreasing WUE in C4 grasses. Overall, our study suggests that under elevated CO2, when evaporative demand is lower, C4 species may be at a competitive disadvantage to C3 species when it comes to nutrient acquisition.

Water-use efficiency in response to simulated increasing precipitation in a temperate desert ecosystem of Xinjiang, China

Water-use efficiency (WUE) is a key plant functional trait that plays a central role in the global cycles of water and carbon. Although increasing precipitation may cause vegetation changes, few studies have explored the linkage between alteration in vegetation and WUE. Here, we analyzed the responses of leaf WUE, ecosystem carbon and water exchanges, ecosystem WUE, and plant community composition changes under normal conditions and also under extra 15% or 30% increases in annual precipitation in a temperate desert ecosystem of Xinjiang, China. We found that leaf WUE and ecosystem WUE showed inconsistent responses to increasing precipitation. Leaf WUE consistently decreased as precipitation increased. By contrast, the responses of the ecosystem WUE to increasing precipitation are different in different precipitation regimes: increasing by 33.9% in the wet year (i.e., the normal precipitation years) and decreasing by 4.1% in the dry year when the precipitation was about 30% less than that in the wet year. We systematically assessed the herbaceous community dynamics, community composition, and vegetation coverage to explain the responses of ecosystem WUE, and found that the between-year discrepancy in ecosystem WUE was consistent with the extent to which plant biomass was stimulated by the increase in precipitation. Although there was no change in the relative significance of ephemerals in the plant community, its greater overall plant biomass drove an increased ecosystem WUE under the conditions of increasing precipitation in 2011. However, the slight increase in plant biomass exerted no significant effect on ecosystem WUE in 2012. Our findings suggest that an alteration in the dominant species in this plant community can induce a shift in the carbon- and water-based economics of desert ecosystems.

Leaf, canopy and agronomic water‐use efficiency of field‐grown sugar beet in response to potassium fertilization

AbstractPotassium (K) fertilization is important to maintain adequate concentrations of plant available K in agricultural soils to achieve best yields and improve crop stress tolerance and water‐use efficiency (WUE). Water‐use efficiency (WUE) can be expressed on various spatiotemporal scales, and it is known that responses of WUE to external stress are not uniform across scales. Multiscale evaluations of the impact of varying K fertilization on the WUE of C3 crops under field conditions are missing so far. In the present field study, we evaluated effects of K fertilization on WUE of sugar beet (Beta vulgaris L.) on short‐termed leaf‐ (WUELeaf) and canopy‐scales (WUECanopy) and as the agronomic ratio of white sugar yield (WSY) to in‐season water use (i.e. WUEWSY). In K‐fertilized plots, WUEWSY was enhanced by 15.9%. This effect is attributed to increased beet yield and WSY, as no differences in total in‐season water use between fertilized and unfertilized plots were observed. Potassium (K) fertilization significantly enhanced the leaf area index, resulting in a more efficient depletion of soil moisture by roots in K‐fertilized plots. As a consequence, WUELeaf was increased due to stomatal adjustment. Potassium (K) improved WUECanopy only by tendency. It is concluded that K fertilization improves the WUE of field‐grown sugar beet across scales, but processes that regulate WUE are highly scale dependent.