Seasonal Soil Water Research Articles

BrGDGT lipids in cold regions reflect summer soil temperature and seasonal soil water chemistry

The distribution of brGDGT lipids produced by soil bacteria has been used to reconstruct temperatures in marine and terrestrial settings as far back as the Cretaceous period. However, modern calibrations of this proxy have primarily relied on air rather than in situ soil temperatures, which can differ by more than 10 °C. Furthermore, the influence of other parameters such as temperature seasonality and soil chemistry on brGDGT lipids is not fully understood. We measured brGDGT distributions, in situ soil temperatures, pH, soil water content, and electrical conductivity on soils from the Eastern Canadian Arctic and Iceland. We compiled our results with those of published soil brGDGT studies that also provide in situ soil temperatures and ancilliary measurements and generated global temperature and pH calibrations from the resulting dataset. Soil temperatures outperformed air temperatures in these calibrations, with mean summer soil temperature providing the highest-performing fit among the 10 tested soil temperature parameters. When applied to a loess/paleosol sequence from the Chinese Loess Plateau, these new calibrations produced paleotemperature and paleo-pH histories consistent with the results of previous studies, encouraging the application of our new calibrations on a broader scale. We also detected 7-methyl and IIIa’’ brGDGT isomers in our Eastern Canadian Arctic and Iceland soils, which have been shown in lakes to relate to salinity and anoxia, respectively. While neither correlated with bulk soil properties such as conductivity, soil water content, or pH, these brGDGT isomers did correlate with seasonality and winter soil temperature. We hypothesize that these compounds are generated in winter by bacteria in habitable niches of more saline, sometimes anoxic liquid water in the otherwise frozen soil matrix. Finally, we report the presence of overly branched GDGTs with m/z = 1064 and suggest that these heptamethylated tetraethers should be investigated as a potential tool for improving brGDGT calibrations. Overall, our results expand our understanding of the seasonality of brGDGT production, especially at high latitudes, and provide in situ soil temperature and pH calibrations for global use.

Will trees or grasses profit from changing rainfall regimes in savannas?

Increasing rainfall variability is widely expected under future climate change scenarios. How will savanna trees and grasses be affected by growing season dry spells and altered seasonality and how tightly coupled are tree-grass phenologies with rainfall? We measured tree and grass responses to growing season dry spells and dry season rainfall. We also tested whether the phenologies of 17 deciduous woody species and the Soil Adjusted Vegetation Index of grasses were related to rainfall between 2019 and 2023. Tree and grass growth was significantly reduced during growing season dry spells. Tree growth was strongly related to growing season soil water potentials and limited to the wet season. Grasses can rapidly recover after growing season dry spells and grass evapotranspiration was significantly related to soil water potentials in both the wet and dry seasons. Tree leaf flushing commenced before the rainfall onset date with little subsequent leaf flushing. Grasses grew when moisture became available regardless of season. Our findings suggest that increased dry spell length and frequency in the growing season may slow down tree growth in some savannas, which together with longer growing seasons may allow grasses an advantage over C3 plants that are advantaged by rising CO2 levels.

Evaluation of Canopy Temperature Based Crop Water Stress Index for Deficit Irrigation Management of Sugar Beet in Semi-Arid Climate

Highlights Sugar beet irrigation scheduling was based on daily average crop water stress index between 13:00 and 16:00 hours. Three crop water stress index thresholds, 0.2, 0.35, and 0.55, were evaluated for irrigation scheduling. Season evapotranspiration decreased and soil water extraction increased as crop water stress threshold increased. There was no significant difference in root or sucrose yield between full irrigation and 0.2 crop water stress index, while seasonal irrigation depths were reduced from133 to 185 mm. Abstract. Sugar beet is an economically important crop in the semi-arid Intermountain Western U.S., with seasonal water use ranging from 500 to 900 mm. Sugar beet is a deep-rooted crop (1.5-2 m) in unrestricted soil profiles that can utilize stored soil water to reduce seasonal irrigation requirements. Effective use of stored soil water below 0.6 m requires precise irrigation scheduling and knowledge of soil water availability below 0.6 m, which is usually unknown due to the labor and expense of soil water monitoring at deeper depths and uncertainty in effective rooting depth and soil water holding capacity. Deficit irrigation (DI) management of sugar beet using a thermal-based crop water stress index (CWSI) has the potential to overcome soil water monitoring limitations and facilitate the utilization of stored soil water to reduce seasonal irrigation requirements. The objective of the research summarized in this paper was to implement and evaluate the effect of automated DI scheduling of sugar beet using three daily average CWSI thresholds (0.2, 0.35, and 0.55) on seasonal irrigation requirement, crop evapotranspiration, seasonal soil water depletion, root yield, estimated recoverable sugar (ERS) yield, and water use efficiency compared to full irrigation. There were no significant differences in root and ERS yield between full irrigation and 0.2 CWSI DI treatment, while seasonal ET was significantly decreased, seasonal soil water extraction was significantly increased, and seasonal irrigation depths were reduced from 133 to 185 mm. Root and ERS yield water production functions were curvilinear with a downward concave. Root and ERS yield water use efficiencies were constant or increased slightly for crop evapotranspiration reductions up to 85% of full irrigation evapotranspiration. The results indicate that irrigating when the average daily CWSI sugar beet exceeds 0.2 is an effective means for mild deficit irrigation scheduling to reduce seasonal irrigation requirements with no significant effect on root and ERS yield. Keywords: Crop water stress index, Evapotranspiration, Irrigation, Irrigation scheduling, Root yield, Sucrose yield, Sugar beet.

Fall/winter cover crop roots change soil hydrology to control the drought status of subsequent season summer maize in Ultisol

Climate change has increased drought frequency, necessitating strategies to reduce water stress, increase water use efficiency, and improve agricultural productivity. The impacts of different fall/winter cover crops on alleviating maize drought in the subsequent season by changing soil water content remain unclear in Ultisol. Our aim was to investigate whether different fall/winter cover crops (taproot and fibrous root) can improve soil water content to attenuate subsequent season maize drought and its underlying mechanism. This research compared four fall/winter cover crops (2 rapeseed cultivars, lucerne and vetiver) and fallow control (no cover crop in winter) to investigate their root traits, root effect on subsequent season soil water content (SWC), soil drought degree (D), maize leaf and root water potential (Ψl, Ψr), maize crop water stress index (CWSI), and maize growth components (aboveground plant and root parameters) in 2019 (a dry year), 2021 (a normal wet year), and 2022 (a severe dry year). The fibrous-rooted vetiver displayed higher average root diameter and higher average root length density, leading to increased SWC (17%, 15%, 15%), and lower D values in all three years. Additionally, it exhibited a more substantial gradient between Ψl, and Ψr and lower CWSI than taproot (rapeseed cultivars and lucerne) and fallow treatment. Furthermore, the cover crop treatments enhanced the gradient between maize Ψl and Ψr to varying degrees in different drought years. Ultimately, the improvement in SWC resulting from cover crop treatments increased maize aboveground growth (plant height, stem diameter, and leaf area) and maize root development, ultimately improving maize yield. This study introduces a new perspective on investigating the role of cover crops in alleviating seasonal drought in subsequent crops, especially by changing soil water properties in the subtropical climate.

Young temperate tree species show different fine root acclimation capacity to growing season water availability

Background and aimsChanges in water availability during the growing season are becoming more frequent due to climate change. Our study aimed to compare the fine-root acclimation capacity (plasticity) of six temperate tree species aged six years and exposed to high or low growing season soil water availability over five years.MethodsRoot samples were collected from the five upper strata of mineral soil to a total soil depth of 30 cm in monoculture plots of Acer saccharum Marsh., Betula papyrifera Marsh., Larix laricina K. Koch, Pinus strobus L., Picea glauca (Moench) Voss and Quercus rubra L. established at the International Diversity Experiment Network with Trees (IDENT) field experiment in Sault Ste. Marie, Ontario, Canada. Four replicates of each monoculture were subjected to high or low water availability treatments.ResultsAbsorptive fine root density increased by 67% for Larix laricina, and 90% for Picea glauca, under the high-water availability treatment at 0–5 cm soil depth. The two late successional, slower growing tree species, Acer saccharum and Picea glauca, showed higher plasticity in absorptive fine root biomass in the upper 5 cm of soil (PIv = 0.36 & 0.54 respectively), and lower plasticity in fine root depth over the entire 30 cm soil profile compared to the early successional, faster growing tree species Betula papyrifera and Larix laricina.ConclusionTemperate tree species show contrasting acclimation responses in absorptive fine root biomass and rooting depth to differences in water availability. Some of these responses vary with tree species successional status and seem to benefit both early and late successional tree species.

Hydrogen and oxygen isotope signal transmission in rainfall, soil water, and cave drip water in Liangfeng Cave, Southwest China

δD and δ18O isotope signatures were widely used as tracers to investigate recharge processes of rainfall transfer into caves in the vadose zones of karst regions. The present research systematically monitored rainfall, soil water, and drip water at the Liangfeng Cave in Guilin City, China, from January 2020 to September 2022, as these aspects remain poorly quantified. The δD and δ18O compositions of rainfall were depleted during the rainy season and enriched during the dry season. The local meteoric water line (LMWL) was described as δD = 7.98δ18O + 11.52. The dry season is mainly characterized by resident soil water, with little mobile soil water, whereas the primary source of recharged drip water is stored bedrock water on the top of the cave. During the rainy season, resident and mobile soil water exchanged with each other, resulting in homogenous δD and δ18O compositions across different soil depths and indicating a lack of ecohydrological separation; however, δD and δ18O signature in drip water may differ from the original observed in rainfall, suggesting that the residence time affected the response time of the drip water to rain. Moreover, mixed stored older water in the overlying bedrock was the primary drip water recharge source each season; thus, drip water's isotope amplitude values were more depleted than rainfall's. Distinct flow paths also created differences in the lag time and amplitude at each drip site. During high-intensity rain, the isotopic signals were rapidly transmitted by preferential flow at different soil depths and via drip water. The δD and δ18O signals in the drip water showed significant depleted excursions several months after high-intensity rainfall. These findings indicate that ecohydrological separation did not occur under any circumstances within the study area, and care should be taken when interpreting significant depleted excursions of δD and δ18O signals in drip water during the summer monsoon or the amount of rainfall in stalagmites across seasonal or interannual scales.

Impact of biochar amendments on soil water and plant uptake dynamics under different cropping systems

AbstractApplication of biochar amendments in agricultural systems has received much attention in recent years. In this study, we assess the 5‐year impacts of biochar application on soil water and plant interactions for an irrigated fresh market tomato (Solanum lycopersicum) and a rainfed pasture (Poaceae) cropping system. In particular, we focus on three varieties of locally produced biochar from agricultural waste materials—almond shell, walnut shell, and almond pruning residues that are pyrolyzed using a mobile pyrolysis unit. We used the soil hydrological model HYDRUS‐1D to explicitly track seasonal and annual soil water fluxes through changes in water retention, drainage, evaporation, and plant water uptake under biochar application. Modeling results show that the application of biochar at 5% increased soil water availability within the top 20 cm for a rainfed system, irrespective of biochar amendment type. This is clearly indicative of higher plant water uptake and greater water use efficiency (WUE) under biochar application. In contrast, a similar biochar amendment for the irrigated system did not affect WUE, instead reducing seasonal soil evaporation loss and thereby reducing irrigation demand. In both cropping systems, year‐to‐year variability in precipitation significantly impacted the total amount of water saved under biochar application with certain amendments retaining more water than others. Given that biochar application increased water retention irrespective of cropping systems, we further used a simple approach to determine yield trade‐off, if any, between control and biochar treatments. Our economic balance clearly demonstrates that the water saved by amending soil with biochar does not offset the yield disparity if compensated with carbon credits and therefore, application of biochar should be actively considered for both its direct and indirect benefits to potential greenhouse gas mitigation (e.g., diverting orchard waste from open burning), water savings, and soil health.

Data-driven irrigation scheduling increases the crop water use efficiency of Cabernet Sauvignon grapevines

In the context of water management in agriculture, irrigation scheduling is critically important as it optimises water application to crops and can also target specific production goals. However, there is no consensus on the ideal irrigation scheduling strategy regarding crop water use efficiency (WUEc). In a premium Cabernet Sauvignon vineyard in Coonawarra, South Australia, over three growing seasons, irrigation scheduling strategies based on experience or historical knowledge (‘GROW’ treatment) were compared to data-driven strategies including crop evapotranspiration, and plant and soil water status thresholds to evaluate their effects on leaf- and vine-level WUEs. A final treatment, GROW + , that doubled the GROW level of irrigation was also evaluated in the third season. The WUE metrics were determined at the leaf, vine, and fruit scales as intrinsic WUE (WUEi), crop WUE (WUEc), and carbon isotope ratio (δ13C), respectively. Furthermore, the irrigation strategies were evaluated in the background of two contrasting soil types: Terra Rossa (light clay, well-drained) and Rendzina (heavier clay, poorly drained). Seasonal soil and vine water status, leaf gas exchange, and light interception were measured, and yield components and pruning weights were obtained following harvest. The amount of seasonal irrigation water based on the data-driven strategies was up to 65% lower across both soil types compared with the GROW or GROW + approaches. WUEi and δ13C were largely similar between treatments. However, for vines grown on Terra Rossa soil, little to no yield penalty was observed when data-driven irrigation scheduling was applied, in addition to increased WUEc values of up to 41%. It can be concluded that irrigation scheduling decisions based on data were superior to the conventional irrigation scheduling method on account of reducing irrigation water volume and increasing WUE, particularly in Terra Rossa soils.

Negative pressure irrigation as a potential technique for increasing vegetable yields and decreasing nitrous oxide emissions

Irrigation is a common practice used to promote vegetable yields through high agricultural inputs, which potentially increases nitrous oxide (N2O) emissions. However, it is still unclear how an effective irrigation strategy could increase yield by regulating soil water conditions, ultimately mitigating N2O emissions. In this study, negative pressure irrigation (NPI) and furrow irrigation (FI) were applied to control two different soil conditions (i.e., the stable soil water content and the dry-wet cycles) in a vegetable greenhouse growing amaranth (Amaranthus hybridus L.) and lettuce (Lactuca sativa L.). The key objective was to assess the effect of irrigation on vegetable yields and N2O emissions and further reveal their controlling factors. Compared to FI, NPI increased the seasonal stability of soil moisture, NH4+-N, and NO3−-N by 11.4%, 26.4%, and 21.2%, respectively. NPI also increased vegetable yields by 16.6%−20.3%, whereas the average area-scaled and yield-scaled N2O emissions under NPI were 15.1% and 29.2% lower than FI, respectively. Notably, the N2O emissions could be affected by soil moisture, temperature, NH4+-N, NO3−-N, and soil microbial biomass carbon and nitrogen, but the relative importance of these factors was different under the two irrigation systems due to different soil water conditions. The random forest (RF) model showed that soil moisture was the most crucial factor driving N2O flux under FI, whereas soil NO3−-N and NH4+-N were the key governing factors under NPI. Overall, this study highlights the importance of increasing seasonal soil water stability for improving the sustainability of irrigation and NPI was a promising irrigation strategy to promote vegetable yields while mitigating N2O emissions.

Seasonal variation in water uptake patterns of two greening species and their responses to rainfall events in a subtropical megacity of China

With rapid urbanization and climate change, water consumption and land-use pattern has dramatically changed, resulting in altered eco-hydrological processes and high ecological water requirements in megacities. However, the water uptake strategies may differ in urban and natural environment, and which remains largely unknown. Therefore, this study investigated the water use patterns of two greening plants species (Ficus concinna and Ligustrum vicaryi) and their responses to rainfall events in a megacity of subtropical China using the stable isotope methods. The results indicated that the two greening plants species showed different water use strategies. F. concinna mainly absorbed water from the shallower soil layer (0–20 cm, 56.29 %) in the wet season and deeper soil water (30–50 cm, 41.13 %) in the dry season, whereas L. vicaryi mainly relied on the shallower soil water (0–20 cm, 48.28 %) throughout the whole year. L. vicaryi absorbed water from the shallower soil layer (0–20 cm) before rainfall events and changed into deeper soil water (30–50 cm) after rainfall events in both dry and wet season; on the contrary, F. concinna did not show these dynamics throughout the year. These results suggested that the water use pattern of F. concinna showed more ecological plasticity, facilitating the adaptation of the plant to seasonal drought and other environment fluctuations in subtropical China urban areas.

Dynamics of Structural Dry Matter, Water Soluble Carbohydrates and Leaf Senescence Mediate the Response of Winter Wheat Yield to Soil Cover and Water Availability

Plastic film mulching often increases the yield of winter wheat in the Loess Plateau of China, but the physiological mechanisms are unclear, especially in response to the interaction between mulch and water supply. In this study, we investigated the interactive effects of initial soil water (dry, moderate, and wet), soil cover (plastic mulch, bare soil), and seasonal conditions on the dynamics of dry matter partitioning, water-soluble carbohydrates (WSC), and flag leaf senescence, and their relations with yield and its components. Plastic mulch increased dry matter accumulation at anthesis and maturity relative to bare soil, with no interaction with season or initial soil water. Allocation of dry matter to leaf, stem, and spike did not change with soil cover. Compared with bare soil, mulch increased WSC accumulation by 14% at anthesis and its translocation by 16%. Soil cover did not influence the senescence of flag leaf after anthesis as indicated by similar dynamics of the C:N ratio. Grain yield was higher under plastic mulch than bare soil in two out of three seasons, and was associated with a higher translocation amount of WSC and post-anthesis dry matter that linked grain weight, grain number, and harvest index.

Elevation Gradients Limit the Antiphase Trend in Vegetation and Its Climate Response in Arid Central Asia

Vegetation in arid central Asia (ACA) has been experiencing significant changes due to substantial warming and humidification since the 1980s. These changes are inhomogeneous due to the ecological vulnerability and topographic complexity of ACA. However, the heterogeneity of vegetation changes has received limited attention in the literature, which has focused more on the region’s overall general features. Thus, this paper analyzes the regional heterogeneity of vegetation changes during the growing season in ACA and further explores their underlying drivers. The results reveal an antiphase trend of vegetation, with an increase in eastern ACA and a decrease in western ACA. This antiphase pattern is primarily constrained by the divergent hydrothermal and climatic contexts of different elevation gradients. At elevations higher than 300 m (in the eastern ACA), increased growing season precipitation dominates vegetation greening. Conversely, vegetation at elevations lower than 300 m (in western ACA) is influenced by growing season soil water, which is driven by winter precipitation (pre-growing season precipitation). Additionally, the temperature could indirectly impact vegetation trends by altering precipitation, soil water, glaciers, snow cover, and runoff. Our findings have implications for restoring the ecosystem and sustainable development in ACA.

Characterization of dominant factors on evapotranspiration with seasonal soil water changes in two adjacent forests in the semiarid Loess Plateau

Evapotranspiration is an essential process of ecosystem water consumption and is controlled by meteorological factors and soil water conditions. In this study, evapotranspiration based on soil water budget was investigated with respect to major influencing factors during the growing seasons of 2012–2015 for two adjacent forest communities in the semiarid Loess Plateau region: a natural secondary forest dominated by oak (Quercus liaotungensis) and a plantation of exotic black locust (Robinia pseudoacacia). Based on the combination of precipitation, soil water storage, and evapotranspiration dynamics, four distinct periods were differentiated to explore the evapotranspiration response to the main meteorological factors. The correlation matrixes showed that, in most investigated days, evapotranspiration was limited by soil water storage, while an increase in soil water storage shifted dominance from water-related to energy-related factors. The stepwise regression of ≥5-day dry events in each period had a better fitness when paired with the relationship between the summations of evapotranspiration and dominant factors (R2 > 0.7, p < 0.01 for all) than the stepwise results for daily averages. A receiver operating characteristic (ROC) curve was applied to validate the division and determine thresholds. All divisions were thoroughly validated, and the soil water storage thresholds of the black locust (120.2 mm and 150.7 mm) were higher than those of the oak (105.8 mm and 137.9 mm). Our results suggest that the oak had an adaptive water consumption strategy, while the black locust had a higher sensitivity to soil water insufficiency. The method for determining the threshold of soil water storage to separate the dominant factors of evapotranspiration was validated. The approach may be applied to other semiarid forest communities with distinct evapotranspiration response patterns.

Chlorophyll Response to Water Stress and the Potential of Using Crop Water Stress Index in Sugar Beet Farming.

Field experiments were conducted in 2019 and 2021 growing seasons to evaluate the chlorophyll readings and crop water stress index (CWSI) response to full and deficit irrigation for drip-irrigated sugar beet (Beta vulgaris L.) under sub-humid climate of Bursa, Turkey. In addition, the changes of soil water content under different irrigation treatments and statistical relationships between chlorophyll and CWSI values and ETc, root yield and sugar yield were investigated. Experiments were carried out in a completely randomized blocks design with three replications. Irrigations were scheduled based on the replenishment of 100 (S1), 66 (S2), 33 (S3), and 0% (S4) of soil water depletion within the soil profile of 0–90 cm using 7 day irrigation intervals. Lower and upper baselines obtained by measurements based on the canopy temperature from the treatments full irrigated and non-irrigated were used to calculate CWSI. The variations in CWSI values were consistent with the variations of seasonal soil water contents induced by the different irrigation practices. CWSI values generally varied between 0 and 1 throughout the experimental periods. In 2019, seasonal mean chlorophyll readings varied between 203.3 and 249.1, and mean CWSI values varied between 0.12 and 0.85. In 2021, seasonal mean chlorophyll readings varied between 232.7 and 259.3 and mean CWSI values between 0.19 and 0.89. Unlike chlorophyll values, CWSI decreased with increased irrigation water amount. In both years, statistically significant relationships were determined between chlorophyll readings and CWSI and ETc, root yield and sugar yield. The greatest root yield was achieved with a seasonal mean CWSI value of 0.12. An exponential equation determined as “Root Yield = 10.804e−1,55CWSI” between seasonal average CWSI values and root yield can be used for estimation of root yield in sugar beet farming. The mean CWSI values determined by infrared thermometer technique can be used in determination of crop water stress and irrigation scheduling of sugar beet cultivation under sub-humid climatic conditions.

Development of an irrigation regime for winter wheat to save water resources by avoiding irrigation at anthesis stage

AbstractLatest published information is limited on agronomic responses of winter wheat to irrigation quantity and the necessity of irrigation at the anthesis stage. This study was conducted to (1) evaluate winter wheat yield, water use, assimilate redistribution and economic benefit with respect to water input and (2) quantify relationship between water input and yield to develop a standard for withholding irrigation at anthesis. A 4‐year long field experiment was conducted to evaluate winter wheat water use, yield formation pathway and farmers' income under three irrigation regimes: rainfed, irrigation at sowing and jointing (SJ‐W) and irrigation at sowing, jointing and anthesis (SJA‐W). The yield formation pathway was correlated with the water‐induced variation in assimilate redistribution and accumulation. Throughout the experimental period, wheat yield was 19–38% lower in rainfed than that under other irrigation treatments. Moreover, SJ‐W treatment substantially increased biomass accumulation at anthesis, accelerated assimilate redistribution in vegetative organs and eventually resulted in a similar wheat yield to that of SJA‐W. Simultaneously, the SJ‐W treatment had lower irrigation water, reduced additional irrigation cost, suppressed yield loss and obtained a similar farmer's net income to the SJA‐W treatment. Water‐induced variations in yield were determined by irrigation, rainfall and soil water storage. SJ‐W plots receiving 204–331 mm water input (rainfall + irrigation) before anthesis and holding 549–587 mm soil water during anthesis stage achieved higher irrigation water use efficiency and yield relative to the rainfed and SJA‐W plots. In contrast, water input under rainfed plots exceeded 200 mm before anthesis, limiting yield substantially even when seasonal soil water consumption exceeded 160 mm. Developing a standard for withholding irrigation at the anthesis stage should incorporate 204–331 mm of water input (rainfall + irrigation) before anthesis and 549–587 mm soil water storage at anthesis, which could achieve a high wheat yield and save water resources.

Observed strong atmospheric water constraints on forest photosynthesis using eddy covariance and satellite-based data across the Northern Hemisphere

Soil water deficit and high atmospheric dryness (vapor pressure deficit, VPD) are major environmental limitations on carbon uptake of terrestrial ecosystems. However, it is still unclear how climate seasonality influences seasonal soil water supply and atmospheric water demand, and consequently limits plant photosynthesis. Here, we analyzed the impacts of the seasonal radiation-rainfall coupling on soil moisture limitations versus atmospheric dryness limitations on plant photosynthesis across the Northern Hemisphere north of 15°N, using the eddy covariance data of 83 forest sites and multiple satellite-based data. Our results show that forest photosynthesis is strongly reduced by low soil water availability that is accompanied by a high atmospheric dryness during warm seasons for sites and regions where there is a strong negative covariation between radiation and rainfall availability, which we denote as asynchronous climate. However, under climates with positive covariation between radiation and rainfall availability, i.e. synchronous climate, forest photosynthesis experiences only a small soil water stress, but tends to be limited by high atmospheric dryness during warm seasons. Both the site and regional analyses imply that atmospheric dryness exhibits stronger constraints on forest photosynthesis in synchronous climate over a larger area than in asynchronous climate across the Northern Hemisphere.

Dry Season Transpiration and Soil Water Dynamics in the Central Amazon.

With current observations and future projections of more intense and frequent droughts in the tropics, understanding the impact that extensive dry periods may have on tree and ecosystem-level transpiration and concurrent carbon uptake has become increasingly important. Here, we investigate paired soil and tree water extraction dynamics in an old-growth upland forest in central Amazonia during the 2018 dry season. Tree water use was assessed via radial patterns of sap flow in eight dominant canopy trees, each a different species with a range in diameter, height, and wood density. Paired multi-sensor soil moisture probes used to quantify volumetric water content dynamics and soil water extraction within the upper 100 cm were installed adjacent to six of those trees. To link depth-specific water extraction patterns to root distribution, fine root biomass was assessed through the soil profile to 235 cm. To scale tree water use to the plot level (stand transpiration), basal area was measured for all trees within a 5 m radius around each soil moisture probe. The sensitivity of tree transpiration to reduced precipitation varied by tree, with some increasing and some decreasing in water use during the dry period. Tree-level water use scaled with sapwood area, from 11 to 190 L per day. Stand level water use, based on multiple plots encompassing sap flow and adjacent trees, varied from ∼1.7 to 3.3 mm per day, increasing linearly with plot basal area. Soil water extraction was dependent on root biomass, which was dense at the surface (i.e., 45% in the upper 5 cm) and declined dramatically with depth. As the dry season progressed and the upper soil dried, soil water extraction shifted to deeper levels and model projections suggest that much of the water used during the month-long dry-down could be extracted from the upper 2–3 m. Results indicate variation in rates of soil water extraction across the research area and, temporally, through the soil profile. These results provide key information on whole-tree contributions to transpiration by canopy trees as water availability changes. In addition, information on simultaneous stand level dynamics of soil water extraction that can inform mechanistic models that project tropical forest response to drought.

Plastic film mulching affects field water balance components, grain yield, and water productivity of rainfed maize in the Loess Plateau, China: A synthetic analysis of multi-site observations

Plastic film mulching (PM) has been widely practiced for rainfed maize (Zea mays L.) in the Loess Plateau, China. However, controversy remains regarding its impacts on field water balance components. There is also scant quantitative information on water-limited potential grain yield (GYw) and water productivity (WPw). A meta-analysis and boundary line analysis were conducted using 1421 paired observations extracted from 83 peer-reviewed publications to (1) assess PM-induced changes in soil water storage at planting (SWSP), evapotranspiration (ET), and soil water storage at harvest (SWSH), and (2) quantify GYw and WPw in PM fields. PM significantly increased SWSP by 4.0% on average due to an 83.2% average increase in precipitation storage during the fallow season. Overall, ET showed a significant positive response to PM (+2.9%), resulting in 56.4% more soil water depletion than no-mulching (NM) fields over the growing season. However, SWSH significantly increased by an average 1.8% after applying PM, indicating that increased storage of fallow season precipitation replenished increased depletion of growing season soil water. Averaged across all samples, PM had significant positive effects on GY (+56.1%) and WP (+47.5%), which were attributed to higher transpiration, ratio of transpiration to ET, and harvest index from the perspective of effective crop water use. Based on GY and WP boundary lines, GYw and WPw under NM were estimated to be 10,728 kg ha-1 and 26.8 kg ha-1 mm-1 on average, respectively, and the respective averages under PM were 13,702 kg ha-1 and 34.8 kg ha-1 mm-1. The GY and WP gaps (i.e., differences between actual and potential values) averaged around 3300 kg ha-1 and 7.0 kg ha-1 mm-1, respectively, in NM and PM fields. In the GY versus ET and WP versus ET scatter plots, the vertical distances between the measured data points on a specific field and the corresponding boundary line indicated field-specific gaps for GY and WP. This study increases our understanding of the effect sizes of PM on field water balance components and the benchmarks of GY and WP for rainfed maize in the Chinese Loess Plateau and other regions with similar conditions worldwide.

Intra-annual variation in microclimatic conditions in relation to vegetation type and structure in two tropical dry forests undergoing secondary succession

Microclimate acts as a strong filter on species performance in restored and regenerating forests, particularly in seasonally dry tropical forests (SDTF). Yet few studies have measured microclimate patterns across succession in SDTF. Furthermore, although dynamic vegetation models simulate microclimate, evaluation of these simulated variables with field observations has been relatively uncommon. Here, we investigated the seasonal patterns of soil temperature and soil water in naturally regenerated and planted successional vegetation in SDTF in Costa Rica and Puerto Rico, using complementary approaches of intensive field observations and simulation modeling with the Ecosystem Demography model. We found that plots representing later successional stages were wetter on average, but only during the dry season. During the wet season, mean soil water did not differ across vegetation types, but open, early successional vegetation experienced more frequent extreme wet and dry conditions than older forest and plantations. Soil temperature tended to decline with forest structure, and later successional vegetation also experienced less extreme daily temperature fluctuations. Basal area and leaf area index were the best predictors of differences in soil water and temperature across plots. Model simulations were consistent with observations of wet season soil temperature and soil water, but the model failed to reproduce dry season soil moisture dynamics, suggesting that further work is needed to reduce model biases in microclimate variables. Collectively, our results imply that common assumptions about how microclimates influence successional processes in SDTF should be revisited.

Response of maize hybrids under limited irrigation capacities: Crop water use

AbstractMaize (Zea mays L.) production in the semiarid Texas High Plains (THP) often suffers exceptional yield reductions when irrigation cannot meet seasonal and peak water requirements. Drought‐tolerant hybrids may reduce yield losses resulting from water stress by altering seasonal soil water use patterns to permit greater uptake during yield‐sensitive growth stages. We examined the effect of irrigation capacity (8.47 and 4.23 mm d−1) and planting rates (74,000 and 95,000 seeds ha−1) during three growing seasons (2016–2018) on water use of three maize hybrids, two of which were considered drought tolerant. Changes in stored soil water were examined for the entire profile and for segregated depth increments (0–0.4, 0.4–0.8, and 0.8–1.4 m). Seasonal crop evapotranspiration averaged 754 and 592 mm across years for the high and low irrigation capacities, respectively. Irrigation level significantly affected seasonal change in stored soil water in 1 yr (2018) with greater depletion at harvest under the low irrigation capacity. Soil water use at 0.8–1.4 m comprised &lt;4% of seasonal water use. Hybrids did not differ with respect to growing season crop water use. Stored soil water throughout the growing season within the examined depth increments were similar among hybrids and plant densities, with significant differences occurring infrequently and being inconsistent across years. Drought‐tolerant traits may have a diminished effect on transpiration in this study because of the dense soil‐limiting root proliferation or the high atmospheric demand at the study location.