Threshold Of Soil Water Content Research Articles

Unexpectedly, triple super phosphate fertilizer induces maize drought resilience

Phosphorus fertilizer is commonly applied to soils in crop production as diammonium phosphate (DAP). To decrease ammonium addition to the environment, triple super phosphate (TSP) is being considered as a DAP replacement. This study was undertaken to compare the response of maize plants to soil fertilization with TSP vs. DAP under well-watered conditions, under soil-drying conditions, and root hydraulic conductance. It was found for well-watered conditions in a controlled environment that there was no difference in plant growth between DAP to TSP treatments. In soil dry-down experiments, however, the initiation of the decrease in transpiration rate was unexpectedly quite different between DAP and TSP treatments. The soil water content threshold for initiation of decrease in transpiration rate with DAP treatment (average fraction transpirable soil water for two experiments = 0.285) was consistent with common observations, but the threshold associated with TSP treatment occurred at an unusually high soil water content (average fraction transpirable soil water for two experiments = 0.545). The higher threshold with TSP resulted in an extended period of soil water use, i.e. soil water conservation. Soil water conservation resulting from the TSP treatment was associated with a 27% greater shoot mass accumulation following a 4-wk re-watering period after the water-deficit treatment than measured with the DAP treatment. The high threshold for decrease in transpiration resulting from TSP was consistent with measured lower root hydraulic conductance as compared to DAP treatment. The unexpected discovery of TSP-induced initiation of transpiration rate decrease at high soil water content is consistent with greater crop drought resilience.

Contrasting water-use strategies revealed by species-specific transpiration dynamics in the Caatinga dry forest.

In forest ecosystems, transpiration (T) patterns are important for quantifying water and carbon fluxes and are major factors in predicting ecosystem change. Seasonal changes in rainfall and soil water content can alter the sensitivity of sap flux density to daily variations in vapor pressure deficit (VPD). This sensitivity is species-specific and is thought to be related to hydraulic strategies. The aim of this work is to better understand how the sap flux density of species with low versus high wood density differ in their sensitivity to VPD and soil water content and how potentially opposing water-use strategies influence T dynamics, and ultimately, correlations to evapotranspiration (ET). We use hysteresis area analysis to quantify the sensitivity of species-specific sap flux density to changes in the VPD, breakpoint-based models to determine the soil water content threshold instigating a T response and multiscalar wavelet coherency to correlate T to ET. We found that low wood density Commiphora leptophloeos (Mart.) Gillett had a more dynamic T pattern, a greater sensitivity to VPD at high soil water content, required a higher soil water content threshold for this sensitivity to be apparent, and had a significant coherency correlation with ET at daily to monthly timescales. This behavior is consistent with a drought avoidance strategy. High wood density Cenostigma pyramidale (Tul.) E. Gagnon & G. P. Lewis, conversely, had a more stable T pattern, responded to VPD across a range of soil water content, tolerated a lower soil water content threshold to T, and had a significant coherency correlation with ET at weekly timescales. This behavior is consistent with a drought-tolerant strategy. We build on previous research to show that these species have contrasting water-use strategies that should be considered in large-scale modeling efforts.

Effect of soil water content threshold on kiwifruit quality at different growth stages with drip irrigation in the humid area of Southern China

• Effects of soil water content threshold (SWCT) on kiwifruit quality depended on SWCT timing and degree. • SWCT of 55% field capacity (FC) increased fruit volume and fruit weight at bud burst to leafing stage. • SWCT of 65% FC elevated total soluble solids, soluble sugar and solid-acid ratio at fruit growth stage. • SWCT of 60% FC improved firmness, total soluble solids, soluble sugar, vitamin C and solid-acid ratio at fruit maturation stage. Fruit quality as affected by soil water content threshold levels (SWCT) at different growth stages remains largely elusive. Therefore, a drip irrigation experiment was conducted to investigate the effect of SWCT on kiwifruit quality at bud burst to leafing stage (stage I), flowering to fruit set stage (stage II), fruit growth stage (stage III) and fruit maturation stage (stage IV) during 2018 and 2019 in the humid area of southern China, respectively. The results showed that SWCT at stage I mainly affected fruit weight (Fw). I-55% treatment significantly elevated Fw by 11.6%-8.0% ( P < 0.05). SWCT at stage II had little effect on fruit quality. SWCT at stage III influenced fruit volume (Fv) and chemical quality. Specifically, Fv was increased by 8.0%-2.2% under III-75% treatment, total soluble solids (TSS), soluble sugar (SS) and solids-acid ratio (SOAR) were increased by 9.4%-10.0% ( P < 0.05), 2.4%-5.8% and 7.9%-10.6% ( P < 0.05) under III-65% treatment, respectively. SWCT at stage IV impacted fruit firmness (Fn) and chemical quality. IV-60% treatment increased Fn, TSS, SS, VC and SOAR by 5.6%-10.5%, 12.0%-12.6% ( P < 0.05), 6.6%-9.1% ( P < 0.05), 2.1%-14.0% ( P < 0.05), and 8.1%-8.3% ( P < 0.05), respectively, which was beneficial to the improvement of nutritional value and taste, and also the storage and transportation of kiwifruit. Collectively, SWCT of 55% FC (field capacity) at stage I or 65% FC at stage III or 60% FC at stage IV was an effective measure to improve the kiwifruit quality.

Prediction of Soil Water Thresholds for Trees in the Semi-Arid Region on the Loess Plateau

It is important to obtain the soil water content threshold (θTHR) for agricultural water management. However, the measurement of θTHR is time consuming and needs specialized and expensive equipment. The accuracy of the empirical estimates is often low. Therefore, the development of a simple, rapid, and accurate prediction method for θTHR is the focus of the present study. The value of θTHR is regarded as the soil water content at the capillary break capacity (θCB). A formula based on field capacity (θFC) and soil bulk density (Db) is proposed to calculate θCB, expressed as θCB=θFC−0.21*1−Db/2.65. Six soils from six published studies on the response of tree physiological processes to water deficit were used to calculate θCB using this formula. The calculated θCB values were compared with the measured θTHR. The results showed that the calculated θCB values were nearly equal to the measured θTHR. A highly significant (adj R2 = 0.9826, p &lt; 0.001) linear relationship with a slope of 0.9506 and a y intercept of 0.0072 was found between the calculated θCB and measured θTHR. The formula proposed in this study provides a novel approach for estimating the θTHR of trees in the semi-arid regions on the Loess Plateau.

Experimental Study of Site-Specific Soil Water Content and Rainfall Inducing Shallow Landslides: Case of Gakenke District, Rwanda

Shallow landslides are among the natural threats causing death and damage. They are mostly triggered by rainfall in mountainous areas where precipitation used to be abundant. The amount of rainfall inducing this natural threat differs from one site to another based on the geographical characteristics of that area. In addition to the rainfall depth, the determination of soil water content in a specific zone has a major contribution to the landslide prediction and early warning systems. Rwanda being a country with hilly terrains, some areas are susceptible to both rainfall and soil water content inducing landslides. But an analytical study of the physical threshold determination of both rainfall and soil water content inducing landslides is lacking. Therefore, this experimental study is conducted to determine the rainfall and soil water content threshold that can be fed in to the landslide early warning system (LEWS) for alert messages using the Internet of Things (IoT) technology. Various experiments have been conducted for the real-time monitoring of slope failure using the toolset composed of a rain gauge, soil moisture sensors, and a rainfall simulating tool. The results obtained show that the threshold for landslide occurrence does not solely correlate with the total rainfall amount (or intensity) or soil moisture, but also influenced by internal (geological, morphological) and environmental factors. Among the sampled sites, the sites covered by forest indicated no sign of slope failure, whereas sites with crops could slip. The experiments revealed that for a specific site, the minimum duration to induce slope failure was 8 hours, 41 minutes with the rainfall intensity of 8 mm/hour, and the soil moisture was above 90% for deeper sensors. These values are used as thresholds for LEWS for that specific site to improve predictions.

By increasing infiltration and reducing evaporation, mulching can improve the soil water environment and apple yield of orchards in semiarid areas

Infiltration and evaporation are two important components of soil water balance in semiarid areas. For local orchards using drip irrigation, rainfall and irrigation events have a profound impact on soil infiltration and evaporation. As an effective water conservation technique, soil surface mulching is widely used in semiarid areas. The aim of this study was to investigate the effect of mulching (e.g., horticultural fabric mulching and corn straw mulching) on soil infiltration and evaporation after irrigation and rainfall. Through field experiments, we investigated soil infiltration and evaporation during three wet-dry cycles and for a rainy month and studied the effects of mulching on soil physical properties and apple yields. The results showed that both horticultural fabric mulching (FM) and straw mulching (SM) improved soil physical properties by reducing soil bulk density and increasing porosity and that SM had a better effect. Mulching increased infiltration after irrigation and heavy rain, while SM reduced infiltration when light rain occurred. Regardless of whether the soil was in the energy-limited stage or falling-rate stage, mulching can effectively reduce soil evaporation. We determined that the threshold of soil water content (SWC) between the energy-limited stage and falling-rate stage was 22.09–22.75%. When SWC is lower than the threshold, there is a positive significant correlation between evaporation and SWC, while no significant correlation was found when it is above the threshold. Moreover, mulching effectively increased apple yields and irrigation water use efficiency (IWUE) for three consecutive years. Therefore, both FM and SM have potential for application in orchards in semiarid areas to improve soil and moisture conditions in the root zone.

Land‐Atmosphere Interactions Exacerbated the Drought and Heatwave Over Northern Europe During Summer 2018

AbstractThe 2018 drought and heatwave over northern Europe were exceptional, with unprecedented forest fires in Sweden, searing heat in Germany and water restrictions in England. Monthly, daily, and hourly data from ERA5, verified with in situ soil water content and surface flux measurements, are examined to investigate the subseasonal‐to‐seasonal progression of the event and the diurnal evolution of tropospheric profiles over Britain to quantify the anomalous land surface contribution to heat and drought. Data suggest the region entered an unprecedented condition of becoming a “hot spot” for land‐atmosphere coupling, which exacerbated the heatwave across much of northern Europe. Land‐atmosphere feedbacks were prompted by unusually low soil water over wide areas, which generated moisture limitations on surface latent heat fluxes, suppressing cloud formation, increasing surface net radiation, and driving temperatures higher during several multiweek episodes of extreme heat. We find consistent evidence in field data and reanalysis of a threshold of soil water content at most locations, below which surface fluxes and daily maximum temperatures become hypersensitive to declining soil water. Similar recent heatwaves over various parts of Europe in 2003, 2010, and 2019, combined with dire climate change projections, suggest such events could be on the increase. Land‐atmosphere feedbacks may play an increasingly important role in exacerbating extremes, but could also contribute to their predictability on subseasonal time scales.

Analysis of floodplain forest sensitivity to drought.

Floodplain forests are very complex, productive ecosystems, capable of storing huge amounts of soil carbon. With the increasing occurrence of extreme events, they are today among the most threatened ecosystems. Our study's main goal was to assess the productivity of a floodplain forest located at Lanžhot in the Czech Republic from two perspectives: carbon uptake (using an eddy covariance method) and stem radius variations (using dendrometers). We aimed to determine which conditions allow for high ecosystem production and what role drought plays in reducing such production potential. Additionally, we were interested to determine the relative soil water content threshold indicating the onset and duration of this event. We hypothesized that summer drought in 2018 had the most significant negative effects on the overall annual carbon and water budgets. In contrast with our original hypothesis, we found that an exceptionally warm spring in 2018 caused a positive gross primary production (GPP) and evapotranspiration (ET) anomaly that consequently led in 2018 to the highest seasonal total GPP and ET from all of the investigated years (2015–2018). The results showed ring-porous species to be the most drought resistant. Relative soil water content threshold of approximately 0.45 was determined as indicating the onset of drought stress.This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’.

Mandarin irrigation scheduling by means of frequency domain reflectometry soil moisture monitoring

The accurate estimation of plant water needs is the first step for achieving high crop water productivity. The main objective of the work was to develop an irrigation scheduling procedure for mandarin orchards under Mediterranean conditions based on replacing the amount of consumed water using reference values of soil moisture according to different phenological periods. The proposed methodology includes a definition part where the threshold values were established relating the trees’ stem water potential and the volumetric soil water content measured with Frequency Domain Reflectometry probes. A second part includes the steps for standardizing measurements from capacitance probes by using the LEACHM hydrological simulation model to take into account the sensor-to-sensor variations. Finally, an extrapolation procedure based on soil water retention curves was used for adapting critical soil water content thresholds to different soil conditions. Field evaluations were made in a citrus orchard located in eastern Spain during two seasons. Standardize critical soil water contents were: i) 24 % vol. for post-harvest, bloom - fruit set and phase III of fruit growth; ii) 27 % vol. for phase I of fruit growth, and iii) 29 % vol. for phase II of fruit growth with average daily air vapour pressure deficit values ranging between 0.2 - 0.4; 0.9–1.1 and 1.1–1.3 kPa, respectively. When implemented in the orchard, the sensor-based strategy resulted in water saving of 26 % respect to a control treatment, irrigated using the standard FAO-56 approach, without significant differences in yield and increasing the crop water productivity by 33 %. In conclusion, we suggest that the determination and use of the critical soil water content is a useful tool for scheduling irrigation. The proposed standardization and extrapolation methodology allows the irrigation strategy to be applied to other mandarin orchards under similar climatic conditions.

Soil Respiration Dynamics in Bromus erectus-Dominated Grasslands under Different Management Intensities

Reduction of soil greenhouse gas emissions is crucial to control increases in atmospheric CO2 concentrations. Permanent grasslands are of considerable importance in climate change mitigation strategies as they cover about 13% of the global agricultural area. However, uncertainties remain for the effects of management practices on soil respiration, especially over the short term. This study investigated the influence of different mowing intensities on soil respiration over the short term for Bromus erectus-dominated grasslands in the central Apennines. From 2016 to 2018, soil respiration, temperature, and moisture were measured under three different management systems: customary management, intensive use, and abandonment. Both soil water content and temperature changed over time, however mowing did not affect soil water content while occasionally altered soil temperature. The intensive use promoted higher seasonal mean soil respiration compared to the abandonment only during the 2016 growing season. Soil temperature was the main driver of soil respiration above a soil water content threshold that varied little among treatments (18.23–22.71%). Below the thresholds, soil moisture was the main driver of soil respiration. These data suggest that different mowing regimes have little influence on soil respiration over the short term in Bromus erectus-dominated grasslands. Thus, more intensive use would not have significative impacts on soil respiration, at least over the short term. Future studies need to clarify the role of root mycorrhizal and microbial respiration in the light of climate change, considering the seasonal redistribution of the rainfall.

Predicting Stormwater Retention Capacity of Green Roofs: An Experimental Study of the Roles of Climate, Substrate Soil Moisture, and Drainage Layer Properties

Due to the ever-increasing degree of urbanization, blue and green infrastructures are becoming important tools for achieving stormwater management sustainability in urban areas. Concerning green roofs, although scientists have investigated their behaviors under different climates and building practices, their hydrological performance is still a thought-provoking field of research. An event scale analysis based on thirty-five rainfall–runoff events recorded at a new set of experimental green roofs located in Southern Italy has been performed with the aim of identifying the relative roles of climate, substrate moisture conditions, and building practices on retention properties. The retention coefficient showed a wide range of variability, which could not be captured by neither simple nor multiple linear regression analysis, relating the latter to rainfall characteristics and substrate soil water content. Significant improvements in the prediction of the retention coefficient were obtained by a preliminary identification of groups of rainfall–runoff events, based on substrate soil water content thresholds. Within each group, a primary role is played by rainfall. At the experimental site, building practices, particularly those concerning the drainage layer properties, appeared to affect the retention properties only for specific event types.

The Mechanism of Changes in Hydraulic Properties of Populus euphratica in Response to Drought Stress

Stable hydraulic conductivity in forest trees maintains the survival of trees which contribute to productivity in forest ecosystems. Drought conditions break down this relationship, but the mechanisms are poorly known. To increase the understanding of the mechanism of hydraulic characteristics during drought, we determined hydraulic parameters in Populus euphratica Oliv. (P. euphratica) in a time-series of drought using a high-pressure flow meter. We found that P. euphratica could enhance hydraulic transport in severe drought stress under a threshold of soil water content. Drought-induced loss of hydraulic conductance could seriously impair water transport capacity. The soil water content of about 4.5% in the rhizosphere could lead to canopy mortality yet maintain live roots. Hydraulic conductance could be changed under drought stress as a consequence of changes in the anatomical structure and physiology. Furthermore, there was also a trade-off between hydraulic efficiency and safety. The consideration of hydraulic efficiency was first within the range of hydraulic safety limit. Once the hydraulic safety limit was reached, safety would be taken as the first consideration and hydraulic efficiency would be reduced. Research on the mechanism of hydraulic properties in riparian plants in arid areas provides a scientific basis for riparian forest restoration.

Terrestrial and Remote Indexes to Assess Moderate Deficit Irrigation in Early-Maturing Nectarine Trees

Monitoring plant water status is relevant for the sustainable management ofirrigation under water deficit conditions. Two treatments were applied to an early-maturingnectarine orchard: control (well irrigated) and precise deficit irrigation (PDI, based on soilwater content thresholds). Moderate water deficits generated by PDI were assessed bycomparing terrestrial: stem water potential (Ψstem) and gas exchange parameters, with remote:canopy temperature, normalized difference vegetation (NDVI), and soil adjusted vegetationindex (SAVI), plant water status indicators. The Ψstem was the only indicator that showedsignificant differences between treatments. NDVI and SAVI at the postharvest period wereappropriate indexes for estimating winter pruning, although they did not serve well as plantstress indicator. Vapor pressure deficit along with Ψstem values were able to predict remotesensing data. Ψstem and canopy to air temperature difference values registered the highestsignal intensity and NDVI the highest sensitivity for detecting water deficit situations. Theresults suggest that care should be taken when using instantaneous remote indicators toevaluate moderate water deficits in deciduous fruit trees; more severe/longer water stressconditions are probably needed. The proposed PDI strategy promoted water saving whilemaintaining yield, and could be considered a promising tool for semi-arid agrosystems.

Comparison of the Roles of Optimizing Root Distribution and the Water Uptake Function in Simulating Water and Heat Fluxes within a Maize Agroecosystem

Roots are an important water transport pathway between soil and plant. Root water uptake (RWU) plays a key role in water and heat exchange between plants and the atmosphere. Inaccurate RWU schemes in land surface models are one crucial reason for decreased model performance. Despite some types of RWU functions being adopted in land surface models, none have been certified as suitable for maize farmland ecosystems. Based on 2007–2009 data observed at the maize agroecosystem field station in Jinzhou, China, the RWU function and root distribution (RD) in the Common Land Model (CoLM) were optimized and the effects of the optimizations on model performance were compared. Results showed that RD parameters calculated with root length density were more practical relative to root biomass in reflecting soil water availability, and they improved the simulation accuracy for water and heat fluxes. The modified RWU function also played a significant role in optimizing the simulation of water and heat fluxes. Similarly, the respective and integrated roles of two optimization schemes in improving CoLM performance were significant during continuous non-precipitation days, especially during the key water requirement period of maize. Notably, the improvements were restrained within a threshold of soil water content, and the optimizations were inoperative outside this threshold. Thus, the optimized RWU function and the revised RD introduced into the CoLM model are applicable for simulation of water and heat fluxes for maize farmland ecosystems in arid areas.

Experimental Analyses of the Evaporation Dynamics in Bare Soils under Natural Conditions

In spite of recent progresses in evaporation estimate through advanced models and laboratory experiments, the drying process of bare soils through its successive stages remains difficult to predict. A study which addresses evaporation modeling in natural bare soils is presented. It relies upon hydro-meteorological measurements performed in a plot with a bare silt loam soil maintained in natural conditions. The following steps are involved: estimate of daily actual evaporation, Ea, through the hydrological balance, scaling with pan evaporation measurements, Epan, and analysis of the relation of Ea/Epan with the soil moisture vertical profile. The results enable us (1) to check the occurrence of the first stage of the evaporation process, characterized by not limited-soil water supply and high evaporative rates, and (2) to identify the transition from the first to the second stage, with decreasing soil water-limited Ea values. The last point requires the introduction of a soil water content threshold at 5 cm depth, that is associated with the soil field capacity. The adopted procedure provides insights on the soil water dynamics at depths differently involved through the successive stages of the evaporative process. Finally, indications on the use of pan evaporation measurements in evaporative rate estimate at least during the first stage of the process are also given.

Determination of soil erodibility using fluid energy method and measurement of the eroded mass

The resistance of soil to erosional processes, especially in hillslopes is generally well recognized and documented in many researches, however, the full implications of tillage (soil disturbance) and slope degree on erodibility of a soil are not considered and well investigated. Thus, in this paper we particularly examined the impact of tillage (soil disturbance) and surface slope on the erodibility of four types of soils (Masa, Oxisols, Andosols, Sand soil), and determined their thresholds of soil water content (SWC) and erosivity that initiate soil erosion. The soil samples were obtained in disturbed form, where the natural conditions of the soils such as structure, density and shear strength were changed. Hence, systematic compaction (soil pressing) treatment was carried out for a couple of months with the concept that the treated soil will be a surrogate of the soil at field condition (untilled-fallow soil) and the disturbed soil represents the soil in its tilled condition. We used dripper type of rainfall simulator to generate runoff and the suspended sediment concentration (SSC) in the runoff was measured using optical backscatter sensor, and the infiltration rate was measured with Mini disk portable tension infiltrometer. We used optical disdrometer under simulated rainfall to measure raindrop size, splash kinetic energy and erosivity of rain events; thereby, we applied fluid energy method to determine erodibility of different soils. The results revealed that both in disturbed and treated conditions, erodibility of Oxisols was the highest followed by Andosols, Masa and Sand soil. After the treatment, erodibility and threshold of SWC significantly decreased in most of the soils while their threshold of erosivity increased; and this implies that the treated soils had better resistance to erosion than disturbed soils. Slope had also notable impact on erodibility (thah/MJmmha) of all soils, where the measured soil erodibility varied from 0.08 to 0.21 in the gentlest slope (5.2°) and from 0.21 to 0.49 for the steepest slope (35.3°). Particularly, the erodibility of Oxisols and Andosols showed a remarkable increase in the steeper slope. Furthermore, the finding showed that erodibility, thresholds of SWC (%) and erosivity were different for different soils considered in the study. Masa soil had the lowest threshold of SWC and erosivity to initiate erosion followed by Sand soil, Oxisols and Andosols. In general, the treated soils had lower erodibility than the disturbed soil. The erodibility of all soils increased with slope, however, Oxisols and Andosols showed a significant increase towards higher slope degree. Thus, tilling steep slope areas of the latter two soils might cause significant erosion and soil loss due to their substantial increase in erodibility towards higher slope.

Variations in internal water distribution and leaf anatomical structure in maize under persistently reduced soil water content and growth recovery after re-watering

Episodes of regional drought are increasing and are frequently associated with increased duration and intensity. However, relatively little is known about effects of long-lasting drought on leaf microscopic structure and physiological metabolism of plants. In this study, we investigated internal water re-distribution and leaf anatomical structure of maize (Zea mays L.) grown under persistently reduced soil water content. Meanwhile, the threshold of soil water content at which maize cannot recover growth vitality after re-watering was determined. Our results showed that during persistent reductions in the field water capacity from 75 to 25 %, plant growth declined, while the water content in maize decreased following the order from the lower to upper leaves and their leaf sheathes to the stem and roots. At 20 % of field water capacity, the volume of bulliform cells declined, accompanied by an inward shrinkage of cell walls. Under field water capacity below 20 %, the number of chloroplasts in bundle-sheath cells decreased, chloroplasts in mesophyll cells deformed from oval to round, concomitant of a near to zero net photosynthetic rate. It was demonstrated that the growth vitality of maize plants could be recovered by re-watering even if field water capacity reduced to 15 %, but not to 10 %.

Non-hydraulic root-sourced signaling and grain yield stability under shade of maize during progressive soil drying in soybean

Production practice shows shaded soybean in wheat-maize-soybean; relay strip intercropping system has better tolerance to drought as compared with sole cropping soybean. Our researches are to understand the reasons through non-hydraulic root signals (nHRS), yield and the relationships among them. Pot experiments were conducted with two soybean cultivars, under shade of maize (LI) and normal irradiance (HI). nHRS were traced during manipulative progressive soil drying period at branching stage under good soil conditions (HW), water stress treatment (LW), in 2010. Well-watered (WW), light drought (LD), moderate drought (MD) and severe drought (SD) were applied in 2009. In response to soil drying, nHRS appeared earlier in Gongxuan No.1 (GX) than Gongqiudou 05-8 (GQ) under two irradiance treatments, but it disappeared earlier for GX than GQ under normal irradiance. GX exhibited a wider average soil water content threshold range (TR) of nHRS under low irradiance. Drought stress significantly decreased the shoot dry mass, root mass and grain yield (P<0.05), excluding LD and nHRS treatments. Water use efficiency was significantly higher (P<0.05) in GX than GQ under drought conditions in both irradiances. In a conclusion, soybean cultivar GX has wider TR in low irradiance condition, light drought had no significant effect on yield, nHRS maybe beneficial to maintain soybean yield under drought condition. Key words: Soybean, drought, shade, non-hydraulic root-sourced signal, water use efficiency, yield stability.

Temperature dependence of soil CO 2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem

Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration ( R s), soil temperature ( T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of R s. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO 2 efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO 2 efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m −2 s −1 in the forest, abandoned field and olive grove, respectively. R s was largely controlled by soil temperature above a soil water content threshold value of 10% at 0–15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds R s was controlled by soil moisture. Exponential and linear models adequately described R s responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of R s in the forest and olive grove. However, in the abandoned field, a single variable – either soil temperature (growing season) or rewetting index (dry season) – was sufficient to explain between 51 and 63% of the soil CO 2 efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO 2 efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.

Scheduling drip irrigation for ginseng (Panax quinquefolius L.) grown under straw and bark mulch

Responses of ginseng to drip irrigation regimes and organic mulches were determined in two experiments beginning in 1998 and 1999. Treatments were four soil water content thresholds for irrigation: 0, 0.08, 0.12 and 0.16 m3 m-3 (θ0, θ8, θ12, and θ16, respectively); combined with either straw or bark mulch. In its first growing season, ginseng did not require irrigation in 1999 or 2000. In subsequent years, irrigation was generally more frequent when applied at higher moisture threshold levels, but precipitation affected irrigation frequency under all treatments. Water use increased with crop age to 3yr, to about 65% of the requirement of unshaded horticultural crops. Seed yield from 3-yr-old plants under straw mulch in 2001 was greater in response to θ12 and θ16 than to θ8 or θ0. Root yield response to irrigation threshold was linear for 2- and 3-yr- old plants under straw mulch in 2001, and quadratic for 3-yr-old plants under bark mulch in 2002. The optimal threshold to initiate drip irrigation was approximately 65% of field capacity. Key words: Root, soil water, water use