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Measurement of carbon dynamics of soybean ( Glycine max L.) ecosystems outside Corn Belt of the United States (U.S.) is lacking. This study examines the seasonal variability of net ecosystem CO 2 exchange (NEE) and its components (gross primary production, GPP and ecosystem respiration, ER), and relevant controlling environmental factors between rainfed (El Reno, Oklahoma) and irrigated (Stoneville, Mississippi) soybean fields in the southern U.S. during the 2016 growing season. Grain yield was about 1.6 t ha − 1 for rainfed soybean and 4.9 t ha − 1 for irrigated soybean. The magnitudes of diurnal NEE (~ 2-weeks average) reached seasonal peak values of − 23.18 and − 34.78 μmol m − 2 s − 1 in rainfed and irrigated soybean, respectively, approximately two months after planting (i.e., during peak growth). Similar thresholds of air temperature (T a , slightly over 30 °C) and vapor pressure deficit (VPD, ~ 2.5 kPa) for NEE were observed at both sites. Daily (7-day average) NEE, GPP, and ER reached seasonal peak values of − 4.55, 13.54, and 9.95 g C m − 2 d − 1 in rainfed soybean and − 7.48, 18.13, and 14.93 g C m − 2 d − 1 in irrigated soybean, respectively. The growing season (DOY 132–243) NEE, GPP, and ER totals were − 54, 783, and 729 g C m − 2 , respectively, in rainfed soybean. Similarly, cumulative NEE, GPP, and ER totals for DOY 163–256 (flux measurement was initiated on DOY 163, missing first 45 days after planting) were − 291, 1239, and 948 g C m − 2 , respectively, in irrigated soybean. Rainfed soybean was a net carbon sink for only two months, while irrigated soybean appeared to be a net carbon sink for about three months. However, grain yield and the magnitudes and seasonal sums of CO 2 fluxes for irrigated soybean in this study were comparable to those for soybean in the U.S. Corn Belt, but they were lower for rainfed soybean. [ABSTRACT FROM AUTHOR]

Drought, a natural hydrometeorological phenomenon, has been more frequent and more widespread due to climate change. Water availability strongly regulates the coupling (or trade-off) between carbon uptake via photosynthesis and water loss through transpiration, known as water-use efficiency (WUE). Understanding the effects of drought on WUE across different vegetation types and along the wet to dry gradient is paramount to achieving better understanding of ecosystem functioning in response to climate change. We explored the physiological and environmental control on ecosystem WUE in response to drought using observations for 44 eddy covariance flux sites in the Northern Hemisphere. We quantified the response of WUE to drought and the relative contributions of gross primary production (GPP) and evapotranspiration (ET) to the variations of WUE. We also examined the control of physiological and environmental factors on monthly WUE under different moisture conditions. Cropland had a peak WUE value under moderate drought conditions, while grassland, deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), and evergreen needleleaf forest (ENF) had peak WUE under slight drought conditions. WUE was mainly driven by GPP for cropland, grassland, DBF, and ENF but was mainly driven by ET for EBF. Vapor pressure deficit (VPD) and canopy conductance (G c) were the most important factors regulating WUE. Moreover, WUE had negative responses to air temperature, precipitation, and VPD but had a positive response to G c and ecosystem respiration. Our findings highlight the different effects of biotic and abiotic factors on WUE among different vegetation types and the important roles of VPD and G c in controlling ecosystem WUE in response to drought. Unlabelled Image • Effects of biotic & abiotic factors on WUE varied among different vegetation types • VPD and canopy conductance are most important drivers of WUE in response to drought • WUE is more sensitive to ET for evergreen broadleaf forest and to GPP for others [ABSTRACT FROM AUTHOR]

Ecosystem water use efficiency (WUE), defined as the ratio between gross primary productivity (GPP) and evapotranspiration (ET), is an indicator of the tradeoff between carbon assimilation and water loss that is controlled by climate and ecosystem structure. However, how GPP and ET impact WUE remains poorly understood. In this study, we provide a global analysis of WUE trends from 1982 to 2011 using multi-model ensemble mean WUE values derived from seven process-based carbon cycle models and investigate the relative effects of leaf area index (LAI), soil moisture (SM), and vapor pressure deficit (VPD) on GPP and ET. Increasing WUE trend was derived for all models, with an average rate of 0.0057 ± 0.0018 g C·kg−1 H 2 O·yr−1 (p = 0.00), with a spatially increasing WUE across ~84% of the global land area, and increasing trends which are statistically significant over ~72% (p < 0.05). Spatially, GPP primarily dominated WUE variability in humid regions, i.e., boreal Eurasia, eastern America, and the tropics, whereas ET dominated WUE variability in dryland regions, i.e., northeast China, the Middle East, southern South America, and South Australia. Soil moisture is likely the most influential factor on GPP and ET variations, with ~63% and ~61% of the global land area dominated by SM, and therefore WUE, for GPP and ET respectively from 1982 to 2011. Our findings enrich the understanding of WUE trends and provide direct evidence for SM-induced variability in WUE. Unlabelled Image • The results show significant increasing trends in water use efficiency for all models from 1982-2011 • Gross primary productivity dominates the water use efficiency variability in the boreal Eurasia region and tropical region • Evapotranspiration dominates the water use efficiency variability in northeast China, the Middle East, and Australia • Soil moisture controls the changes in gross primary productivity and evapotranspiration and thereof water use efficiency [ABSTRACT FROM AUTHOR]

Droughts and heatwaves have been and will continue to bring large risks to terrestrial ecosystems. However, the understanding of how plants respond to drought and heatwave over broad spatial scales is still limited. In this paper, we use the 2009/2010 drought in Yunnan and the 2013 heatwave over southern China as case studies to investigate the potential of using satellite-observed solar-induced chlorophyll fluorescence (SIF) to monitor vegetation responses to drought and heatwave over broad spatial scales. The 2009/2010 drought onset follows a strong soil moisture deficit due to the yearlong below-normal precipitation in Yunnan from the autumn of 2009 to the spring of 2010. In the summer of 2013, southern China experienced the strongest heatwave due to the sudden temperature increase and rainfall deficit. The results show that SIF can reasonably capture the spatial and temporal dynamics of drought and heatwave development, as indicated by the large reduction in fluorescence yield (SIF yield). Moreover, SIF yield demonstrates a significant reduction and earlier response than traditional vegetation indices (enhanced vegetation index, EVI) during the early stages of drought and heatwave events. For both study areas, the spatial and temporal correlation analysis demonstrates that the SIF yield anomalies are more sensitive to a high vapor pressure deficit (VPD) than low soil moisture. This study implies that satellite observations of SIF have great potential for accurate and timely monitoring of drought and heatwave developments. Unlabelled Image • Dynamics of 2009/2010 drought and the 2013 heatwave in China are investigated. • SIF yield shows earlier responses to drought and heatwave than traditional VIs. • SIF yield is more sensitive to high VPD than low soil moisture. [ABSTRACT FROM AUTHOR]