Role of atmospheric resonance and land-atmosphere feedbacks as a precursor to the June 2021 Pacific Northwest Heat Dome event.

We demonstrate an indirect, rather than direct, role of quasi-resonant amplification of planetary waves in a summer weather extreme. We find that there was an interplay between a persistent, amplified large-scale atmospheric circulation state and soil moisture feedbacks as a precursor for the June 2021 Pacific Northwest "Heat Dome" event. An extended resonant planetary wave configuration prior to the event created an antecedent soil moisture deficit that amplified lower atmospheric warming through strong nonlinear soil moisture feedbacks, favoring this unprecedented heat event.

Weaker land-atmosphere coupling in global storm-resolving simulation.

The debate on the sign of the soil moisture-precipitation feedback remains open. On the one hand, studies using global coarse-resolution climate models have found strong positive feedback. However, such models cannot represent convection explicitly. On the other hand, studies using km-scale regional climate models and explicit convection have reported negative feedback. Yet, the large-scale circulation is prescribed in such models. This study revisits the soil moisture-precipitation feedback using global, coupled simulations conducted for 1 y with explicit convection and compares the results to coarse-resolution simulations with parameterized convection. We find significant differences in a majority of points with feedback that is weaker and dominantly negative with explicit convection. The model with explicit convection is more often in a wet regime and prefers the triggering of convection over dry soil in the presence of soil moisture heterogeneity, in contrast to the coarse-resolution model. Further analysis indicates that the feedback not only between soil moisture and evapotranspiration but also between evapotranspiration and precipitation is weaker, in better agreement with observations. Our findings suggest that coarse-resolution models may not be well suited to study aspects of climate change over land such as changes in droughts and heatwaves.

Response of the root anatomical structure of Carex moorcroftii to habitat drought in the Western Sichuan Plateau of China.

MAIN CONCLUSION: The anatomical structures of Carex moorcroftii roots showing stronger plasticity during drought had a lower coefficient of variation in cell size in the same habitats, while those showing weaker plasticity had a higher coefficient of variation. The complementary relationship between these factors comprises the adaptation mechanism of the C. moorcroftii root to drought. To explore the effects of habitat drought on root anatomy of hygrophytic plants, this study focused on roots of C. moorcroftii. Five sample plots were set up along a soil moisture gradient in the Western Sichuan Plateau to collect experimental materials. Paraffin sectioning was used to obtain root anatomy, and one-way ANOVA, correlation analysis, linear regression analysis, and RDA ranking were applied to analyze the relationship between root anatomy and soil water content. The results showed that the root transverse section area, thickness of epidermal cells, exodermis and Casparian strips, and area of aerenchyma were significantly and positively correlated with soil moisture content (P < 0.01). The diameter of the vascular cylinder and the number and total area of vessels were significantly and negatively correlated with the soil moisture content (P < 0.01). The plasticity of the anatomical structures was strong for the diameter and area of the vascular cylinder and thickness of the Casparian strip and epidermis, while it was weak for vessel diameter and area. In addition, there was an asymmetrical relationship between the functional adaptation of root anatomical structure in different soil moisture and the variation degree of root anatomical structure in the same soil moisture. Therefore, the roots of C. moorcroftii can shorten the water transport distance from the epidermis to the vascular cylinder, increase the area of the vascular cylinder and the number of vessels, and establish a complementary relationship between the functional adaptation of root anatomical structure in different habitats and the variation degree of root anatomical structure in the same habitat to adapt to habitat drought. This study provides a scientific basis for understanding the response of plateau wetland plants to habitat changes and their ecological adaptation strategies. More scientific experimental methods should be adopted to further study the mutual coordination mechanisms of different anatomical structures during root adaptation to habitat drought for hygrophytic plants.

The intricacies of vegetation responses to changing moisture conditions.

A long-standing debate looks at whether air or soil dryness is more limiting to vegetation water use and productivity. The answer has large implications for future ecosystem functioning, as atmospheric dryness is predicted to increase globally while changes in soil moisture are predicted to be far more variable. Here, I review the complexities that contribute to this debate, including the strong coupling between atmospheric and soil dryness, and the widespread heterogeneity in vegetation hydraulic traits, acclimations, and adaptations to water stress. I discuss solutions to improve understanding and modeling of vegetation sensitivity to dryness, including how different types of observational data can be used together to gain insight into vegetation response to water stress across spatial and temporal scales.

Forest plant indicator values for moisture reflect atmospheric vapour pressure deficit rather than soil water content.

Soil moisture shapes ecological patterns and processes, but it is difficult to continuously measure soil moisture variability across the landscape. To overcome these limitations, soil moisture is often bioindicated using community-weighted means of the Ellenberg indicator values of vascular plant species. However, the ecology and distribution of plant species reflect soil water supply as well as atmospheric water demand. Therefore, we hypothesized that Ellenberg moisture values can also reflect atmospheric water demand expressed as a vapour pressure deficit (VPD). To test this hypothesis, we disentangled the relationships among soil water content, atmospheric vapour pressure deficit, and Ellenberg moisture values in the understory plant communities of temperate broadleaved forests in central Europe. Ellenberg moisture values reflected atmospheric VPD rather than soil water content consistently across local, landscape, and regional spatial scales, regardless of vegetation plot size, depth as well as method of soil moisture measurement. Using in situ microclimate measurements, we discovered that forest plant indicator values for moisture reflect an atmospheric VPD rather than soil water content. Many ecological patterns and processes correlated with Ellenberg moisture values and previously attributed to soil water supply are thus more likely driven by atmospheric water demand.

Effects of drought and moisture stress on the growth and ecophysiological traits of Schima superba seedlings.

Changes in rainfall patterns are important environmental factors affecting plant growth, especially when larger precipitation events and prolonged drought periods occur in subtropical regions. There are many studies on how drought reduces plant biomass through drought-sensitive functional traits, but how excess water affects plant growth and ecophysiology is still poorly understood. Therefore, a greenhouse experiment was conducted on Schima superba (Theaceae), a dominant tree species in subtropical forests and commonly used in forestry, in a closed chamber under control (25% soil water content (SWC) as in local forests), drought stress (D, 15% SWC) and moisture stress (W, 35% SWC). Plant growth and ecophysiological traits related to morphology, leaf gas exchange, water potential and structural traits were measured. Compared to control, S. suberba under dry conditions significantly decreased its aboveground biomass, photosynthetic rate (A), leaf water potential and nitrogen use efficiency, but increased intrinsic water use efficiency, root to shoot ratio and specific root length. S. superba under wet conditions also significantly decreased its total biomass, aboveground biomass and specific root length, while W had no effect on A and leaf water potential. Our results indicate that S. superba shows a decrease in carbon gain under drought stress, but less response under wet conditions. This emphasizes the need to consider the strength and frequency of rainfall pattern changes in future studies because rainfall may either alleviate or intensify the effects of drought stress depending on the moisture level, thus suitable water conditions is important for better management of this tree species in subtropical China.

Abscisic acid increase correlates with the soil water threshold of transpiration decline during drought.

By regulating carbon uptake and water loss by plants, stomata are not only responsible for productivity but also survival during drought. The timing of the onset of stomatal closure is crucial for preventing excessive water loss during drought, but is poorly explained by plant hydraulics alone and what triggers stomatal closure remains disputed. We investigated whether the hormone abscisic acid (ABA) was this trigger in a highly embolism-resistant tree species Umbellularia californica. We tracked leaf ABA levels, determined the leaf water potential and gravimetric soil water content (gSWC) thresholds for stomatal closure and transpiration decline during a progressive drought. We found that U. californica plants have a peaking-type ABA dynamic, where ABA levels rise early in drought and then decline under prolonged drought conditions. The early increase in ABA levels correlated with the closing of stomata and reduced transpiration. Furthermore, we found that transpiration declined before any large decreases in predawn plant water status and could best be explained by transient drops in midday water potentials triggering increased ABA levels. Our results indicate that ABA-mediated stomatal regulation may be an integral mechanism for reducing transpiration during drought before major drops in bulk soil and plant water status.

The multidimensionality of plant drought stress: The relative importance of edaphic and atmospheric drought.

Drought threatens plant growth and related ecosystem services. The emergence of plant drought stress under edaphic drought is well studied, whilst the importance of atmospheric drought only recently gained momentum. Yet, little is known about the interaction and relative contribution of edaphic and atmospheric drought on the emergence of plant drought stress. We conducted a gradient experiment, fully crossing gravimetric water content (GWC: maximum water holding capacity-permanent wilting point) and vapour pressure deficit (VPD: 1-2.25 kPa) using five wheat varieties from three species (Triticum monococcum, T. durum & T. aestivum). We quantified the occurrence of plant drought stress on molecular (abscisic acid), cellular (stomatal conductance), organ (leaf water potential) and stand level (evapotranspiration). Plant drought stress increased with decreasing GWC across all organizational levels. This effect was magnified nonlinearly by VPD after passing a critical threshold of soil water availability. At around 20%GWC (soil matric potential 0.012 MPa), plants lost their ability to regulate leaf water potential via stomata regulation, followed by the emergence of hydraulic dysfunction. The emergence of plant drought stress is characterized by changing relative contributions of soil versus atmosphere and their non-linear interaction. This highly non-linear response is likely to abruptly alter plant-related ecosystem services in a drying world.

Trajectories and tipping points of piñon-juniper woodlands after fire and thinning.

Piñon-juniper (PJ) woodlands are a dominant community type across the Intermountain West, comprising over a million acres and experiencing critical effects from increasing wildfire. Large PJ mortality and regeneration failure after catastrophic wildfire have elevated concerns about the long-term viability of PJ woodlands. Thinning is increasingly used to safeguard forests from fire and in an attempt to increase climate resilience. We have only a limited understanding of how fire and thinning will affect the structure and function of PJ ecosystems. Here, we examined vegetation structure, microclimate conditions, and PJ regeneration dynamics following ~20 years post-fire and thinning treatments. We found that burned areas had undergone a state shift that did not show signs of returning to their previous state. This shift was characterized by (1) distinct plant community composition dominated by grasses; (2) a lack of PJ recruitment; (3) a decrease in the sizes of interspaces in between plants; (4) lower abundance of late successional biological soil crusts; (5) lower mean and minimum daily soil moisture values; (6) lower minimum daily vapor pressure deficit; and (7) higher photosynthetically active radiation. Thinning created distinct plant communities and served as an intermediate between intact and burned communities. More intensive thinning decreased PJ recruitment and late successional biocrust cover. Our results indicate that fire has the potential to create drier and more stressful microsite conditions, and that, in the absence of active management following fire, there may be shifts to persistent ecological states dominated by grasses. Additionally, more intensive thinning had a larger impact on community structure and recruitment than less intensive thinning, suggesting that careful consideration of goals could help avoid unintended consequences. While our results indicate the vulnerability of PJ ecosystems to fire, they also highlight management actions that could be adapted to create conditions that promote PJ re-establishment.

Drought thresholds that impact vegetation reveal the divergent responses of vegetation growth to drought across China.

Identifying droughts and accurately evaluating drought impacts on vegetation growth are crucial to understanding the terrestrial carbon balance across China. However, few studies have identified the critical drought thresholds that impact China's vegetation growth, leading to large uncertainty in assessing the ecological consequences of droughts. In this study, we utilize gridded surface soil moisture data and satellite-observed normalized difference vegetation index (NDVI) to assess vegetation response to droughts in China during 2001-2018. Based on the nonlinear relationship between changing drought stress and the coincident anomalies of NDVI during the growing season, we derive the spatial patterns of satellite-based drought thresholds (T SM ) that impact vegetation growth in China via a framework for detecting drought thresholds combining the methods of feature extraction, coincidence analysis, and piecewise linear regression. The T SM values represent percentile-based drought threshold levels, with smaller T SM values corresponding to more negative anomalies of soil moisture. On average, T SM is at the 8.7th percentile and detectable in 64.4% of China's vegetated lands, with lower values in North China and Jianghan Plain and higher values in the Inner Mongolia Plateau. Furthermore, T SM for forests is commonly lower than that for grasslands. We also find that agricultural irrigation modifies the drought thresholds for croplands in the Sichuan Basin. For future projections, Earth System Models predict that more regions in China will face an increasing risk for ecological drought, and the Hexi Corridor-Hetao Plain and Shandong Peninsula will become hotspots of ecological drought. This study has important implications for accurately evaluating the impacts of drought on vegetation growth in China and provides a scientific reference for the effective ecomanagement of China's terrestrial ecosystems.

Does pollination interact with the abiotic environment to affect plant reproduction?

BACKGROUND AND AIMS: Abiotic and biotic components of the environment both limit plant reproduction, but how they interact with one another in combination is less understood. Understanding these interactions is especially relevant because abiotic and biotic environmental components respond differently to various global change drivers. Here we aim to understand whether the effects of pollination (biotic component) on plant reproduction depend on soil moisture (abiotic component), two factors known to affect plant reproduction and that are changing with global change. METHODS: We conducted pollen supplementation experiments for two plant species, Delphinium nuttallianum and Hydrophyllum fendleri, in subalpine meadows in the Western USA across four years that varied in soil moisture. In a separate one-year field experiment, we factorially crossed water addition with pollen supplementation. We measured proportion fruit set, seeds per fruit, and seeds per plant, in addition to stomatal conductance, to determine whether plant physiology responded to watering. KEY RESULTS: In the four-year study, only H. fendleri reproduction was pollen limited, and this occurred independently of soil moisture. Experimental water addition significantly increased soil moisture and stomatal conductance for both species. The effect of pollen addition on reproduction depended on the watering treatment only for H. fendleri fruit production. Reproduction in D. nuttallianum was not significantly affected by pollen addition or water addition, but it did respond to interannual variation in soil moisture. CONCLUSIONS: Although we find some evidence for the effect of a biotic interaction depending on abiotic conditions, it was only for one aspect of reproduction in one species, and it was in an unexpected direction. Our work highlights interactions between the abiotic and biotic components of the environment as an area of further research for improving our understanding of how plant reproduction responds to global change.

Nitrogen addition alleviates the adverse effects of drought on plant productivity in a temperate steppe.

Drought and nitrogen enrichment could profoundly affect the productivity of semiarid ecosystems. However, how ecosystem productivity will respond to different drought scenarios, especially with a concurrent increase in nitrogen availability, is still poorly understood. Using data from a 4-year field experiment conducted in a semiarid temperate steppe, we explored the responses of aboveground net primary productivity (ANPP) to different drought scenarios and nitrogen addition, and the underlying mechanisms linking soil properties, plant species richness, functional diversity (community-weighted means of plant traits, functional dispersion) and phylogenetic diversity (net relatedness index) to ANPP. Our results showed that completely excluding precipitation in June (1-month intense drought) and reducing half the precipitation amount from June to August (season-long chronic drought) both significantly reduced ANPP, with the latter having a more negative impact on ANPP. However, reducing half of the precipitation frequency from June to August (precipitation redistribution) had no significant effect on ANPP. Nitrogen addition increased ANPP irrespective of drought scenarios. ANPP was primarily determined by soil moisture and nitrogen availability by regulating the community-weighted means of plant height, rather than other aspects of plant diversity. Our findings suggest that precipitation amount is more important than precipitation redistribution in influencing the productivity of temperate steppe, and nitrogen supply could alleviate the adverse impacts of drought on grassland productivity. Our study advances the mechanistic understanding of how the temperate grassland responds to drought stress, and implies that management strategies to protect tall species in the community would be beneficial for maintaining the productivity and carbon sequestration of grassland ecosystems under climate drought.

Endophytes Alleviate Drought-Derived Oxidative Damage in Achnatherum inebrians Plants Through Increasing Antioxidants and Regulating Host Stress Responses.

Endophytes generally increase antioxidant contents of plants subjected to environmental stresses. However, the mechanisms by which endophytes alter the accumulation of antioxidants in plant tissues are not entirely clear. We hypothesized that, in stress situations, endophytes would simultaneously reduce oxidative damage and increase antioxidant contents of plants and that the accumulation of antioxidants would be a consequence of the endophyte ability to regulate the expression of plant antioxidant genes. We investigated the effects of the fungal endophyte Epichloë gansuensis (C.J. Li & Nan) on oxidative damage, antioxidant contents, and expression of representative genes associated with antioxidant pathways in Achnatherum inebrians (Hance) Keng plants subjected to low (15%) and high (60%) soil moisture conditions. Gene expression levels were measured using RNA-seq. As expected, the endophyte reduced the oxidative damage by 17.55% and increased the antioxidant contents by 53.14% (on average) in plants subjected to low soil moisture. In line with the accumulation of antioxidants in plant tissues, the endophyte increased the expression of most plant genes associated with the biosynthesis of antioxidants (e.g., MIOX, crtB, gpx) while it reduced the expression of plant genes related to the metabolization of antioxidants (e.g., GST, PRODH, ALDH). Our findings suggest that endophyte ability of increasing antioxidant contents in plants may reduce the oxidative damage caused by stresses and that the fungal regulation of plant antioxidants would partly explain the accumulation of these compounds in plant tissues.

Warming positively promoted community appearance restoration of the degraded alpine meadow although accompanied by topsoil drying.

On-going climate warming is threatening the ecological function of grassland ecosystems. However, whether warming has positive effects on community microhabitats and appearance, especially in degraded grasslands, remains elusive. To address this issue, we conducted a 2-year field experiment on the severely degraded alpine meadow and undegraded alpine meadow with no warming and warming treatments. Community coverage and height in degraded meadow significantly increased under warming, while these changes were not significant in undegraded meadow. Two-year warming increased the community height of degraded meadow and undegraded meadow by 56.55% and 10.99%, respectively. Warming also increased community coverage by 41.88% in degraded meadow and decreased community coverage by 3.01% in undegraded meadow. Moreover, the response of topsoil temperature to warming was stronger in degraded meadow (6.89%) than in undegraded meadow (- 0.26%), while the negative response of topsoil moisture to warming was weaker in degraded meadow (- 13.95%) than in undegraded meadow (- 20.00%). The SEMs further demonstrated that warming had positive effects on topsoil temperature and community height, while had negative effects on topsoil moisture both in degraded and undegraded meadows. Our results confirm that warming-induced soil drying is an important pathway affecting the community appearance in alpine meadows. These findings highlight that warming has positive effects on community height and coverage and is particularly effective in improving community coverage appearance in severely degraded alpine meadow with topsoil drying.

Digestate induces significantly higher N2O emission compared to urea under different soil properties and moisture.

Digestate is considered as an option for recycling resources and a part of the substitution for chemical fertilizers to reduce environmental impacts. However, its application may lead to significant nitrous oxide (N2O) emissions because of its high concentration of ammonium and degradable carbon. The research objectives are to evaluate how N2O emissions respond to digestate as compared to urea application and whether this depends on soil properties and moisture. Either digestate or urea (100 mg N kg-1) was applied with and without a nitrification inhibitor of 3,4-dimethylpyrazole phosphate (DMPP) to three soil types (fluvo-aquic soil, black soil, and latosol) under three different soil moisture conditions (45, 65, and 85% water-filled pore space (WFPS)) through microcosm incubations. Results showed that digestate- and urea-induced N2O emissions increased exponentially with soil moisture in the three studied soils, and the magnitude of the increase was much greater in the alkaline fluvo-aquic soil, coinciding with high net nitrification rate and transient nitrite accumulation. Compared with urea-amended soils, digestate led to significantly higher peaks in N2O and carbon dioxide (CO2) emissions, which might be due to stimulated rapid oxygen consumption and mineralized N supply. Digestate-induced N2O emissions were all more than one time higher than those induced by urea at the three moisture levels in the three studied soils, except at 85% WFPS in the fluvo-aquic soil. DMPP was more effective at mitigating N2O emissions (inhibitory efficacy: 73%-99%) in wetter digestate-fertilized soils. Overall, our study shows the contrasting effect of digestate to urea on N2O emissions under different soil properties and moisture levels. This is of particular value for determining the optimum of applying digestate under varying soil moisture conditions to minimize stimulated N2O emissions in specific soil properties.

GRACE Follow-On revealed Bangladesh was flooded early in the 2020 monsoon season due to premature soil saturation.

The overall size and timing of monsoon floods in Bangladesh are challenging to measure. The inundated area is extensive in low-lying Bangladesh, and observations of water storage are key to understanding floods. Laser-ranging instruments on Gravity Recovery and Climate Experiment (GRACE) Follow-On spacecraft detected the peak water storage anomaly of 75 gigatons across Bangladesh in late July 2020. This is in addition to, and three times larger than, the maximum storage anomaly in soil layers during the same period. A flood propagation model suggested that the water mass, as shown in satellite observations, is largely influenced by slow floodplain and groundwater flow processes. Independent global positioning system measurements confirmed the timing and total volume of the flood water estimates. According to land surface models, the soils were saturated a month earlier than the timing of the peak floodplain storage observed by GRACE Follow-On. The cyclone Amphan replenished soils with rainfall just before the monsoon rains started, and consequently, excessive runoff was produced and led to the early onset of the 2020 flooding. This study demonstrated how antecedent soil moisture conditions can influence the magnitude and duration of flooding. Continuous monitoring of storage change from GRACE Follow-On gravity measurements provides important information complementary to river gauges and well levels for enhancing hydrologic flood forecasting models and assisting surface water management.

Early warm-season mesoscale convective systems dominate soil moisture-precipitation feedback for summer rainfall in central United States.

Land-atmosphere interactions play an important role in summer rainfall in the central United States, where mesoscale convective systems (MCSs) contribute to 30 to 70% of warm-season precipitation. Previous studies of soil moisture-precipitation feedbacks focused on the total precipitation, confounding the distinct roles of rainfall from different convective storm types. Here, we investigate the soil moisture-precipitation feedbacks associated with MCS and non-MCS rainfall and their surface hydrological footprints using a unique combination of these rainfall events in observations and land surface simulations with numerical tracers to quantify soil moisture sourced from MCS and non-MCS rainfall. We find that early warm-season (April to June) MCS rainfall, which is characterized by higher intensity and larger area per storm, produces coherent mesoscale spatial heterogeneity in soil moisture that is important for initiating summer (July) afternoon rainfall dominated by non-MCS events. On the other hand, soil moisture sourced from both early warm-season MCS and non-MCS rainfall contributes to lower-level atmospheric moistening favorable for upscale growth of MCSs at night. However, soil moisture sourced from MCS rainfall contributes to July MCS rainfall with a longer lead time because with higher intensity, MCS rainfall percolates into deeper soil that has a longer memory. Therefore, early warm-season MCS rainfall dominates soil moisture-precipitation feedback. This motivates future studies to examine the contribution of early warm-season MCS rainfall and associated soil moisture anomalies to predictability of summer rainfall in the major agricultural region of the central United States and other continental regions frequented by MCSs.

Moisture contents regulate peat water-leachable concentrations of methylmercury, inorganic mercury, and dissolved organic matter from boreal peat soils.

Boreal peatlands are "hotspots" of net methylmercury (MeHg) production and may become drier in the future due to climate change. This study investigates a critical gap by analyzing the nuanced relationship between soil moisture content and the release of MeHg, inorganic mercury (IHg), sulfate (SO42-), and dissolved organic matter (DOM) in a laboratory incubation of boreal peat soils. Dried peat soils exhibited heightened releases of IHg, MeHg, and SO42- during re-wetting events. Both dried and saturated peat soils released more DOM than moist peat soils during re-wetting events, and DOM released from dried soils had higher bioaccessibility than that from the saturated soils (p<0.05). There was an equilibrium of IHg concentrations between peat soils and pore waters, but long-term severe drought may disrupt this equilibrium and then release more IHg to pore waters during re-wetting events. Contrary to expectations, positive relationships between IHg concentrations and SUVA254 did not exist in all treatments. MeHg and SO42- were depleted quickly because there was no external input of Hg and SO42- to this static system. More bioaccessible DOM than aromatic DOM was released from peat soils with different soil moisture contents after 32 weeks during the re-wetting event (p<0.05). These results imply that re-wetting of peat soils after droughts can increase the release of MeHg from peat soils and may also increase net MeHg production due to the release of SO42- and bioaccessible DOM from peat soils, reshaping our understanding of soil moisture's role in mercury dynamics. This novel insight into soil moisture and MeHg dynamics carries significant implications for mitigating mercury contamination in aquatic ecosystems.

Calibration of Low-Cost Moisture Sensors in a Biochar-Amended Sandy Loam Soil with Different Salinity Levels.

With the increasing focus on irrigation management, it is crucial to consider cost-effective alternatives for soil water monitoring, such as multi-point monitoring with low-cost soil moisture sensors. This study assesses the accuracy and functionality of low-cost sensors in a sandy loam (SL) soil amended with biochar at rates of 15.6 and 31.2 tons/ha by calibrating the sensors in the presence of two nitrogen (N) and potassium (K) commercial fertilizers at three salinity levels (non/slightly/moderately) and six soil water contents. Sensors were calibrated across nine SL-soil combinations with biochar and N and K fertilizers, counting for 21 treatments. The best fit for soil water content calibration was obtained using polynomial equations, demonstrating reliability with R2 values greater than 0.98 for each case. After a second calibration, low-cost soil moisture sensors provide acceptable results concerning previous calibration, especially for non- and slightly saline treatments and at soil moisture levels lower than 0.17 cm3cm-3. The results showed that at low frequencies, biochar and salinity increase the capacitance detected by the sensors, with calibration curves deviating up to 30% from the control sandy loam soil. Due to changes in the physical and chemical properties of soil resulting from biochar amendments and the conductive properties influenced by fertilization practices, it is required to conduct specific and continuous calibrations of soil water content sensor, leading to better agricultural management decisions.

Integrated UAV and Satellite Multi-Spectral for Agricultural Drought Monitoring of Winter Wheat in the Seedling Stage.

Agricultural droughts are a threat to local economies, as they disrupt crops. The monitoring of agricultural droughts is of practical significance for mitigating loss. Even though satellite data have been extensively used in agricultural studies, realizing wide-range, high-resolution, and high-precision agricultural drought monitoring is still difficult. This study combined the high spatial resolution of unmanned aerial vehicle (UAV) remote sensing with the wide-range monitoring capability of Landsat-8 and employed the local average method for upscaling to match the remote sensing images of the UAVs with satellite images. Based on the measured ground data, this study employed two machine learning algorithms, namely, random forest (RF) and eXtreme Gradient Boosting (XGBoost1.5.1), to establish the inversion models for the relative soil moisture. The results showed that the XGBoost model achieved a higher accuracy for different soil depths. For a soil depth of 0-20 cm, the XGBoost model achieved the optimal result (R2 = 0.6863; root mean square error (RMSE) = 3.882%). Compared with the corresponding model for soil depth before the upscaling correction, the UAV correction can significantly improve the inversion accuracy of the relative soil moisture according to satellite remote sensing. To conclude, a map of the agricultural drought grade of winter wheat in the Huaibei Plain in China was drawn up.