Soil Water Content Treatments Research Articles

Plant growth and physiological responses of the invasive plant Acmella radicans to contrasting light and soil water conditions

Acmella radicans (Jacquin) R.K. Jansen is a new invasive species record for Yunnan Province, China, as of 2017 and little is known about its invasion mechanisms. To better understand its invasive strategies, we investigated the growth, physiological and soil nutrient use parameters of the invader under combined conditions of light (25%, 50%, 75%, and 100% of light availability) and soil water content (full, high, medium, and low soil water content) in the glasshouse. The results showed that light level, soil water content and their interaction had a significant effect on all plant morphological, physiological and soil nutrient parameters for A. radicans (P < 0.05). For the most part, plant height, total branch length, leafstalk length, leaf area, inflorescence number, seed number, leaf biomass, stem biomass, aboveground biomass, and total biomass of A. radicans were significantly increased with increased shading rate and soil water content and were generally greater at intermediate light levels and intermediate to high soil water levels. Under high-irradiance conditions, the chlorophyll content was greatly reduced compared to other treatments. We also observed that leafstalk length, leaf area, Pn, chlorophyll a, and chlorophyll b of A. radicans were markedly increased with increased shading rate and soil water content, and were generally highest under intermediate irradiance and soil water conditions, but many plant physiological parameters also exhibited relatively high values under waterlogging conditions. The concentrations of organic matter, available N and available K of A. radicans soils at high-irradiance and full-high soil water content treatments and medium-irradiance and high-medium soil water content treatments were often less than those of other treatments indicated that hypothetically soil absorption was increased by moderate shading and high soil water. This was the first study demonstrating that A. radicans thrives best under moderate light conditions in combination with high soil water. Furthermore, we surmised that its higher phenotypic and physiological plasticity contributes to its invasion success.

Critical Factors Affecting Water and Nitrogen Losses from Sloping Farmland during the Snowmelt Process

Water and nitrogen losses from farmland during the snowmelt process play a vital role in water and nitrogen management in cold regions. To explore the mechanisms and factors contributing to water and nitrogen loss from different sloping farmlands during the snowmelt period, field experiments were conducted under two slope treatments (8° and 15°), two soil water content (SWC) treatments, and two snow water equivalent (SWE) (5 mm and 10 mm) treatments in a seasonal freezing agricultural watershed of Northeast China. The results showed that during the snowmelt process, SWE was the most important factor affecting water and nitrogen production through the surface and total runoff of the sloping farmland, followed by the slope. The water and nitrogen yield in the high snow (HS) treatments ranged from 1.76 to 8.15 and 1.65 to 12.62 times higher than those in the low snow (LS) treatments. The generation of nitrogen was advanced compared with that of water induced by the preferential production of nitrogen. A higher slope promoted this preferential production function of nitrogen. Enhanced infiltration combined with the preferential yield of nitrogen resulted in a greatly decreased yield of water and nitrogen in the gentle slope and LS (GS_LS) treatments. These findings are valuable for accurately describing the water and nitrogen cycling in seasonally freezing sloping farmland.

Effects of Freezing–Thawing Processes on Net Nitrogen Mineralization in Salinized Farmland Soil

Nitrogen is an indispensable and limiting element for plant and microbial growth. To investigate the combined effects of salinity and freezing–thawing (FT) processes on soil inorganic nitrogen (SIN) transformation in seasonally freezing salinized farmland, laboratory incubation experiments were conducted under five soil salt content (SSC) treatments (0.08%, 0.25%, 0.35%, 0.50%, and 0.70%), four FT temperature treatments (C (5 °C), FT (−5 + 5 °C), FT (−10 + 5 °C), and FT (−15 + 5 °C)), and two soil water content (SWC) treatments (40% and 80% of maximum water holding capacity (WHC)). Ammonium (NH4+-N) and nitrate (NO3−-N) nitrogen were monitored at the first, second, fifth, and eighth incubation days. The FT processes increased relative NH4+-N content by 13%, 39%, and 77% with the decreasing of freezing temperature from −5 °C to −15 °C compared with C (5 °C) treatments, respectively. FT (−5 + 5 °C) and FT (−15 + 5 °C) treatments decreased the relative NO3--N contents by 4% and 6% compared with C (5 °C) treatments, respectively. Under FT treatments, the increment of relative NH4+-N content was higher in low-SSC treatments and lower in high-SSC treatments. The relationship between relative NO3–-N content and SSC gradually changed from a decrease in C (5 °C) to an increase in FT (−15+5 °C) treatments. SWC decreased NH4+-N content in high-SSC and low-freezing temperature treatments (SSC × freezing temperature &lt; −2.5%· °C), while NH4+-N increased in low-SSC and unfrozen treatments. The variations of SIN/Rmin (nitrogen mineralization rate) were mostly affected by NO3–-N/Rnit (net nitrification rate) and NH4+-N/Ra (net ammonification rate) in C (5 °C) and FT treatments, respectively. Overall, the results suggested that enhanced salinity inhibited the effects of freezing temperature on NH4+-N and NO3−-N formation, respectively. The increase in SWC weakened the NH4+-N formation induced by the decrease in freezing temperature, and this function increased with the increase in salinity.

The Physiological Role of Abscisic Acid in Regulating Root System Architecture of Alfalfa in Its Adaptation to Water Deficit

Alfalfa (Medicago sativa L.) is a perennial leguminous plant, with a strong tap root system that plays an important role in alfalfa’s adaptation to drought stress. However, a better understanding of root functional traits and how these root traits are related to whole plant responses in order to improve pasture productivity under water deficit. Two greenhouse experiments were conducted: Experiment 1 used three alfalfa cultivars and four levels of soil water content treatments to investigate herbage productivity, growth point density, residual shoot weight, and root weight. Experiment 2 assessed relationships among root-sourced abscisic acid (ABA), root system architecture and plant biomass in response to water deficit. The results demonstrated that root system was used as a useful tool to improve tolerant and adaptation when alfalfa copied with lower levels of soil water content. On average, maintaining 60–65% soil water-holding capacity alfalfa had the highest herbage accumulation (6.7 g DM pot-1), growing point density (46.5 pot-1), and residual shoot biomass (1.8 g DM pot-1). At the level of water stress, Medicago sativa L. cv Zhaodong (ZD) and cv Aohan (AH) tended to exhibit a herringbone branching pattern with less root tips, root forks, altitude, and magnitude than cv Golden empress (GE). Principal component analysis and structural equation modeling revealed that root-sourced ABA positively regulated the altitude and magnitude of root system architecture, root tips and root forks, and was closely associated with plant root biomass and herbage biomass. It was concluded that these findings can contribute to developing optimum irrigation strategies and help alfalfa breeders in the development of new cultivars with improved drought tolerance based on root system architecture, plant hormone, and plant growth.

Allocation of photoassimilated carbon of radiata pine (Pinus radiata) seedlings as affected by soil water stress

ABSTRACT This study was carried out to determine the carbon (C) allocation of tree components following water stress in radiata pine (Pinus radiata) seedlings. The seedlings were exposed to various water contents (low, moderate and high soil water content) for 30 days and labelled with 14CO2 gas for six hours on the 31st day. Biomass in all seedling components (foliage, stems and roots) was significantly higher in the moderate soil water content treatment than in the low soil water content treatment, while seedling biomass did not significantly differ between the moderate soil water and high soil water content treatments. The shoot/root ratio of seedlings decreased when soil water content decreased. The C concentrations of radiata pine seedlings were not affected by the soil water content, whereas the soil water stress-induced difference in the C allocation of seedlings was attributed to differences in seedling biomass. The translocation of pulse-labelled 14C from the foliage to the roots was enhanced by low soil water content. The distribution of 14C was highest in foliage, followed by roots, stems and soil. The results indicate that soil water content was one of the primary factors influencing biomass allocation in the early growth of radiata pine seedlings.

Effects of saline irrigation on CO2 emission and chemical properties of aeolian sandy soil under litter addition

Calligonum mongolicum is one of the dominant species in the Taklimakan Desert highway shelterbelt, the litter of which plays an important role in carbon cycling. After litter addition, we carried out a laboratory incubation experiment to investigate the dynamics of soil CO2 emission, soil organic carbon (SOC), soluble organic carbon (DOC), pH, and electrical conductivity (EC) under the condition of salt water (SW) and fresh water (FW) with field water holding capacity of 25%, 50%, 75%, and 100%. The results showed that saline water irrigation had an inhibitory effect on soil CO2 emission. Under the four soil water content treatments, the cumulative CO2 emission of freshwater irrigation increased by 1.9%-29.1% compared with that of saline irrigation. Cumulative soil CO2 emissions increased with increasing soil water content. With litter addition, SOC decreased rapidly in the early stage, then gradually increased, and finally tended to be stable. The DOC contents of each treatment following the incubation increased by 41.3%-92.4% compared with that before the incubation. At the end of incubation, soil pH of each treatment increased by 0.20-0.35. The EC increased with the increases of soil water content. Under the four water content conditions and compared with the situation before the incubation, the EC values irrigated with SW increased by 0.11-0.79 mS·cm-1, while those with FW increased or decreased at the end of incubation. Cumulative soil CO2 emission was positively correlated with SOC, DOC, and pH, but not with soil water content. Both saline irrigation and lower water content could inhibit CO2 emission of aeolian sandy soil under litter addition, while EC was significantly affected by the quality of irrigation water and soil water content.

SPORE PROPAGATION OF INDIGENOUS ENDOMYCHORIZA FROM SEVERAL ROOTING AREAS OF SNAKE FRUIT ON DIFFERENT SOIL WATER CONTENT

Cultivation of organic snake fruit in Bali which is done on dry land with the irrigation depends on rainfall and the fertilization generally only uses uncertain amount of fallen leaves, it reduces the productivity, quality, and continuity of fruit production over time. In rhizosphere of snake fruit there are various types of indigenous endomycorrhiza that potentially can be developed as biofertilizer, but as a biofertilizer, the low number of spores population for inoculum becomes a limiting factor in using indigenous endomycorrhiza. The effort that can be done are to propagate the spores by giving water stress treatment. This study aimed to obtain the best rhizosphere location of snake fruit as the source of spores and the level of soil water content to multiply the spores. The research was conducted in the green House, Farm Station, Agriculture Faculty, Udayana University at Jalan Pulau Moyo, Denpasar, from October 2017 to January 2018. The spore propagation used nested experiment with Randomized Block Design patterns. The treatments were rhizosphere location as the source of indigenous endomycorrhizas spore consists of three levels (Bebandem District Karangasem Regency, Payangan District Gianyar Regency, and Pupuan District Tabanan Regency) and soil water content consists of three levels (100%, 70%, and 40% field capacity). The results of the research showed that the source of indigenous endomychorrizae from snake fruit rhizosphere in Payangan District gave the highest number of spores found and the highest percentage of the spore increase after propagation. While in the soil water content treatments, the highest number of spores found and the percentage of the spore increase after propagation were obtained at soil water content of 40% field capacity. The percentage of root infections from different sources of indigenous endomychorrhizae and different levels of soil water content is same i.e 100%.

Impact of Soil Water Content on Hybrid Bermudagrass Athletic Fields

Soil water content (SWC) influences the consistency and performance of athletic field surfaces. Two studies were conducted at the University of Tennessee Center for Athletic Field Safety, Knoxville, TN, to determine how SWC affects wear tolerance of hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt‐Davy, ‘Tifway’] on root zones composed of either silt loam (cohesive) or sand meeting US Golf Association specifications (noncohesive). Soil water content treatments for cohesive root zones averaged low (0.06–0.13 m3 m−3), medium (0.14–0.21 m3 m−3), medium‐high (0.22–0.29 m3 m−3), and high (0.30–0.37 m3 m−3); comparatively, SWC on noncohesive averaged low (0.05–0.11 m3 m−3), medium (0.12–0.19 m3 m−3), and high (0.20–0.27 m3 m−3). Differences in the amount of ranges between root zones were due to plant available water of the soil texture. Plots were subjected to 50 traffic events for 5 wk each fall over a 2‐yr period. Green turfgrass cover was reduced four times faster at high SWC than the low and medium SWC treatments on cohesive soil. All SWC treatments on noncohesive soil lost green turfgrass cover at a predictable rate. Surface hardness increased as SWC decreased for both root zones. Turfgrass shear strength decreased with traffic for all treatments on cohesive soils. Soil water content of noncohesive soils did not compound the effect of traffic on turfgrass shear strength. The optimal mean SWC ranges to maximize hybrid bermudagrass wear tolerance on cohesive soils were low to medium, and low to medium on noncohesive soils.

Crop Yield and Water Use Efficiency as Affected by Different Soil-Based Management Methods for Variable-Rate Irrigation in a Semi-Humid Climate

Abstract. To improve the management of variable-rate irrigation (VRI) systems in semi-humid climates, three different soil-based irrigation management methods were evaluated on their potential for reducing irrigation water use and maximizing crop yield and water use efficiency (WUE) during the 2016 and 2017 growing seasons of summer maize in the North China Plain. The three irrigation management methods evaluated were soil water balance modeling (SWB), measured soil water content (SWC), and a combination of SWB and the rain forecast for the next three days (RF). The experiments were implemented on four management zones delineated by available soil water holding capacity of a center-pivot VRI system. A similar irrigation trigger point (70% of field capacity) was used for the three irrigation management methods in the four management zones. In the two seasons, the total water application in the SWC treatments varied in a larger range among the management zones, and the irrigation water applied was 22% and 21% less than in the SWB and RF treatments, respectively. Similar yields were obtained among the irrigation management methods in both seasons. The maximum WUE was always observed with the SWC treatments for the four management zones in the 2017 season. The WUE with the SWC treatments was 36% and 23% higher than with the SWB and RF treatments, respectively. Considering the amount of irrigation water applied, yield, and WUE, our results demonstrated that the SWC method was more suitable for VRI management than the SWB and RF methods in this semi-humid climate. Keywords: Center pivot, Soil water balance, Soil water content, Rain forecast, Summer maize, Yield.

Effects of Soil Water Deficit on Insecticidal Protein Expression in Boll Shells of Transgenic Bt Cotton and the Mechanism.

This study was conducted to investigate the effects of soil water deficit on insecticidal protein expression in boll shells of cotton transgenic for a Bt gene. In 2014, Bt cotton cultivars Sikang 1 (a conventional cultivar) and Sikang 3 (a hybrid cultivar) were planted in pots and five soil water content treatments were imposed at peak boll stage: 15% (G1), 35% (G2), 40% (G3), 60% (G4), and 75% field capacity (CK), respectively. Four treatments (G2, G3, G4, and CK) were repeated in 2015 in the field. Results showed that the insecticidal protein content of boll shells decreased with increasing water deficit. Compared with CK, boll shell insecticidal protein content decreased significantly when soil water content was below 60% of maximum water holding capacity for Sikang 1 and Sikang 3. However, increased Bt gene expression was observed when boll shell insecticidal protein content was significantly reduced. Activity assays of key enzymes in nitrogen metabolism showed that boll shell protease and peptidase increased but nitrogen reductase and glutamic-pyruvic transaminase (GPT) decreased. Insecticidal protein content exhibited significant positive correlation with nitrogen reductase and GPT activities; and significant negative correlation with protease and peptidase activities. These findings suggest that the decrease of insecticidal protein content associated with increasing water deficit was a net result of decreased synthesis and increased decomposition.

Influence of irrigation during the growth stage on yield and quality in mango (Mangifera indica L).

Although being one of the few drought-tolerant plants, mango trees are irrigated to ensure optimum and consistent productivity in China. In order to better understand the effects of soil water content on mango yield and fruit quality at fruit growth stage, irrigation experiments were investigated and the object was to determine the soil water content criteria at which growth and quality of mango would be optimal based on soil water measured by RHD-JS water-saving irrigation system through micro-sprinkling irrigation. Five soil water content treatments (relative to the percentage of field water capacity) for irrigation (T1:79%-82%, T2:75%-78%, T3:71%-74%, T4: 65%-70%, T5:63%-66%) were compared in 2013. Amount of applied irrigation water for different treatments varied from 2.93m3 to 1.08 m3. The results showed that mango fruit production and quality at fruit growth stage were significantly affected under different irrigation water amounts. Variation in soil water content not only had effects on fruit size, but also on fruit yield. The highest fruit yield and irrigation water use efficiency were obtained from the T4 treatment. Irrigation water amount also affected fruit quality parameters like fruit total soluble solids, soluble sugar, starch, titratable acid and vitamin C content. Comprehensive evaluation of the effect of indexs of correlation on irrigation treatment by subordinate function showed that when the soil moisture content were controlled at about 65–70% of the field water moisture capacity, water demand in the growth and development of mango could be ensured, and maximum production efficiency of irrigation and the best quality of fruit could be achieved. In conclusion, treatment T4 was the optimum irrigation schedule for growing mango, thus achieving efficient production of mango in consideration of the compromise among mango yield, fruit quality and water use efficiency.

Effects of Maize Residue Quality and Soil Water Content on Soil Labile Organic Carbon Fractions and Microbial Properties

Investigating the effects of residue chemical composition on soil labile organic carbon (LOC) will improve our understanding of soil carbon sequestration. The effects of maize residue chemical composition and soil water content on soil LOC fractions and microbial properties were investigated in a laboratory incubation experiment. Maize shoot and root residues were incorporated into soil at 40% and 70% field capacity. The soils were incubated at 20 °C for 150 d and destructive sampling was conducted after 15, 75, and 150 d. Respiration, dissolved organic carbon (DOC), hot-water extractable organic carbon (HEOC), and microbial biomass carbon (MBC) were recorded, along with cellulase and β-glucosidase activities and community-level physiological profiles. The results showed that the cumulative respiration was lower in root-amended soils than in shoot-amended soils, indicating that root amendment may be beneficial to C retention in soil. No significant differences in the contents of DOC, HEOC and MBC, enzyme activities, and microbial functional diversity were observed between shoot- and root-amended soils. The high soil water content treatment significantly increased the cumulative respiration, DOC and HEOC contents, and enzyme activities compared to the low soil water content treatment. However, the soil water content treatments had little influence on the MBC content and microbial functional diversity. There were significantly positive correlations between LOC fractions and soil microbial properties. These results indicated that the chemical composition of maize residues had little influence on the DOC, HEOC, and MBC contents, enzyme activities, and microbial functional diversity, while soil water content could significantly influence DOC and HEOC contents and enzyme activities.

Effect of mulching with maize straw on water infiltration and soil loss at different initial soil moistures in a rainfall simulation

Mulching and soil water content (SWC) have a significant impact on soil erosion, and this study investigated the effect of straw mulching on water infiltration and soil loss under different initial SWC treatments in a rainfall simulation experiment conducted in northern China. Increasing initial SWC can decrease soil infiltration and increase soil loss. During an 80mm rainfall event (80 mm$h -1 for 60 min), 8% ,1 2% and 16% initial SWC treatments decreased cumulative infiltration by 8.7%, 42.5% and 58.1%, and increased total sediment yield by 44, 146 and 315 g, respectively, compared to 4% initial SWC. However, in all the straw mulching treatments, there was no significant difference in stable infiltration rate between the different initial SWC treat- ments. For all initial SWC treatments, straw mulching of 30% or more significantly enhanced water infiltration by over 31% and reduced soil loss by over 49%, compared to the unmulched treatment. Taking into consideration the performance of no-till planters, a maize straw mulching rate of 30% to 60% (1400-3100 kg$hm -2 ) is recom- mended for the conservation of water and soil in northern China.

Soil organic matter and texture affect responses to dry/wet cycles: Changes in soil organic matter fractions and relationships with C and N mineralisation

The extent to which SOM content and texture affect C and N dynamics during dry/wet cycles is poorly understood. A laboratory incubation study was conducted to quantify short-term changes in SOM (C & N) fractions and their relationship to C and N mineralisation in response to dry/wet cycles along a SOM gradient in two soil types of differing texture. The experiment consisted of three phases: pre-incubation, treatment and recovery. Three soil water content (SWC) treatments were established: continuously wet (WW; field capacity (FC)), moderately dry (MD; 120% of SWC at wilting point (WP)) and very dry (VD; 80% of SWC at WP). Each of the two ‘dry’ treatments were either maintained continuously dry (MD & VD) or subjected to three sequential 20 d-long dry/wet cycles (MDW & VDW) during the experiments treatment phase. All soils were returned to FC at the start of the recovery phase and analyses were carried out at the end of each phase. Over all, the results of this study showed that SOC content and texture are important factors affecting the size of several commonly measured fractions of soil organic matter, but the stability and resilience of these fractions depended on the duration and amplitude of dry/wet cycles. Whereas most of the measured C and N fractions (cold water extractable C [CWEC] and hot water extractable C [HWEC], microbial biomass C [MBC] and N [MBN], inorganic N) were affected by both the duration (e.g. MDW vs. MD) and amplitude (e.g. MDW vs. VDW) of dry/wet cycles, the response differed between fractions and the effects tended to be much stronger in silt loam than in clay loam soils. The duration and amplitude of dry/wet cycles also suppressed the rate of both C and N mineralisation in both soils compared to continuously wet conditions. There was also strong evidence that the C mineralised from both soils during the recovery phase (i.e. following rewetting of dry soils) compensated for the reduction in C mineralised during the treatment phase. For N mineralisation, the amplitude of dry/wet cycles was at least as important as the duration of the cycles in affecting N mineralisation during the recovery phase. Much higher rates of N mineralisation were observed in soils that had previously been exposed to very dry conditions, particularly in the silt loam soil. There was some evidence that HWEC was the primary source of the C made available during the rewetting of dry soil and that it contributed to the increased availability of CWEC and supported an increase in MBC and C mineralisation in both soils during the recovery phase. In contrast, there was no evidence that differences in availability of C and N fractions affected the rate of N mineralisation following the return of dry soils to continuously wet conditions. Further research is need to resolve the primary factors that regulate N mineralisation response to the recovery from dry soil conditions.

Soil moisture and soil-litter mixing effects on surface litter decomposition: A controlled environment assessment

Recent studies suggest the long-standing discrepancy between measured and modeled leaf litter decomposition in drylands is, in part, the result of a unique combination of abiotic drivers that include high soil surface temperature and radiant energy levels and soil-litter mixing. Temperature and radiant energy effects on litter decomposition have been widely documented. However, under field conditions in drylands where soil-litter mixing occurs and accelerates decomposition, the mechanisms involved with soil-litter mixing effects are ambiguous. Potential mechanisms may include some combination of enhanced microbial colonization of litter, physical abrasion of litter surfaces, and buffering of litter and its associated decomposers from high temperatures and low moisture conditions. Here, we tested how soil-litter mixing and soil moisture interact to influence rates of litter decomposition in a controlled environment. Foliar litter of two plant species (a grass [Eragrostis lehmanniana] and a shrub [Prosopis velutina]) was incubated for 32 weeks in a factorial combination of soil-litter mixing (none, light, and complete) and soil water content (2, 4, 12% water-filled porosity) treatments. Phospholipid fatty acids (PLFAs) were quantified one week into the experiment to evaluate initial microbial colonization. A complementary incubation experiment with simulated rainfall pulses tested the buffering effects of soil-litter mixing on decomposition.Under the laboratory conditions of our experiments, the influence of soil-litter mixing was minimal and primarily confined to changes in PLFAs during the initial stages of decomposition in the constant soil moisture experiment and the oscillating soil moisture conditions of the rainfall pulse experiment. Soil-litter mixing effects on CO2 production, total phospholipid concentrations, and bacterial to total PLFA ratios were observed within the first week, but responses were fairly weak and varied with litter type and soil moisture treatment. Across the entire 32-week incubation experiment, soil moisture had a significant positive effect on mass loss, but soil-litter mixing did not. The lack of strong soil-litter mixing effects on decomposition under the moderate and relatively constant environmental conditions of this study is in contrast to results from field studies and suggests the importance of soil-litter mixing may be magnified when the fluctuations and extremes in temperature, radiant energy and moisture regimes common dryland field settings are in play.

Effect of water management on cadmium and arsenic accumulation by rice (Oryza sativa L.) with different metal accumulation capacities

Water management affects the bioavailability of cadmium (Cd) and arsenic (As) in the soil and hence their accumulation in rice grains and grain yields. However, Cd and As show opposite responses to soil water content, but information, particularly on irrigation, is missing on a field scale. The purpose of the present study was therefore to find a water management regime that can lower accumulation of both Cd and As in grain without yield loss. Two rice (Oryza sativa L.) cultivars, A16 and A159, with different grain Cd accumulation capacities were employed in field plot experiments with four water management regimes comprising aerobic, intermittent, conventional practice and flooded. The dynamics of Cd and As bioavailability in the soil and Cd and As concentrations in roots, straw and grains were determined at the early tillering, full tillering, panicle initiation, filling and maturity stages of crop growth. The lower water content regimes (aerobic and intermittent) mostly led to higher soil HCl-extractable Cd than the higher soil water content regimes (conventional and flooded). HCl-extractable As in contrast was favoured by the higher soil water content treatments. Conventional and flooded irrigation accordingly gave higher plant As concentrations but lower Cd compared to aerobic and intermittent irrigation. Cd concentrations in roots and straw of both varieties increased with growth stage, especially in aerobic and intermittent regimes, while As concentrations in plants showed little change or a slight decrease. As the water irrigation volume increased from aerobic to flooded, brown rice Cd decreased from 1.15 to 0.02 mg kg(-1) in cultivar A16 and from 1.60 to 0.05 mg kg(-1) in cultivar A159, whereas brown rice As increased. Aerobic and flooded treatments produced approximately 10-20 % lower grain yields than intermittent and conventional treatments. Cultivars with low Cd accumulation capacity show higher brown rice grain As than those with high Cd uptake capacity. Of the four water management regimes, the conventional irrigation method (flooding maintained until full tillering followed by intermittent irrigation) ensured high yield with low Cd and As in the brown rice and so remains the recommended irrigation regime.

Relative effect of soil moisture on emissions and distribution of 1,3-dichloro-propene and chloropicrin in soil columns

Emissions of soil fumigants are regulated to protect air quality in California. Irrigation prior to fumigation can reduce fumigant emissions at relatively low costs; however, the optimum range of soil water content that reduces emissions without reducing efficacy is not clearly defined. The objective of this study was to determine the effects of soil water content [at 30, 45, 60, 75, 90 and 100% field capacity (FC)] on the emission and distribution of fumigants 1,3-dichloropropene (1,3-D) and chloropicrin (CP) in columns packed with a sandy loam soil. After injecting equal amounts of cis-1,3-D, trans-1,3-D, and CP, fumigant emissions and distribution in soil were monitored for 14 days. Emissions of all three compounds showed similar response to soil water content except that CP emissions were lower than both isomers of 1,3-D. The emission peak flux was highest and occurred earliest in the driest soil while it was reduced and delayed as soil water content increased. After the peak, emission flux decreased rapidly in the driest soil but more slowly in higher water content treatments. Initially, higher soil water content resulted in substantially lower cumulative emissions among the treatments, but as time progressed, the differences in cumulative emissions decreased or even disappeared. These trends were likely due to the effect of the closed-bottom short soil columns which allowed fumigants to only move upward and contribute to emission. Higher fumigant concentrations in the soil–gas phase were observed in high soil water content treatments, due to less emission loss and more fumigant retained in the soil.

Effects of decreasing soil water content on seminal lateral roots of young maize plants

AbstractSoil micropores that contain water at or below field capacity cannot be invaded by seminal or first‐order lateral roots of maize plants because their root diameters are larger than 10 μm. Hence, at soil‐water levels below field capacity plant roots must establish a new pore system by displacement of soil particles in order to access soil water. We investigated how decreasing soil water content (SWC) influences growth and morphology of the root system of young maize plants. Plants were grown in rhizotrons 40 cm wide, 50 cm high, and approximately 0.7 cm thick. Five SWC treatments were established by addition of increasing amounts of water to soil and thorough mixing before filling the rhizotrons. No water was added to treatments 1–4 throughout the experiment. Treatment 5 was watered frequently throughout the experiment to serve as a control. Seminal‐root length and SWC in soil layers 0–10, 10–20, 20–30, 30–40, and 40–50 cm were measured at intervals of 2–3 d on scanner images by image analysis. At 15 d after planting, for treatments 1–4 shoot dry weight and total root length were directly related to the amount of water added to the soil, and for treatments 4 and 5, total root length and shoot dry weights were similar. Length of seminal roots visible at the transparent surface of the rhizotron for all treatments was highest in the uppermost soil layer and decreased with distance from the soil surface. For all layers, seminal‐root elongation rate was at maximum above a SWC of 0.17 cm3 cm–3, corresponding to a matric potential of –30 kPa. With decreasing SWC, elongation rate decreased, and 20% of maximum seminal root elongation rate was observed below SWC of 0.05 cm3 cm–3. After destructive harvest for treatment 1–4, number of (root‐) tips per unit length of seminal root was found uninfluenced over the range of initial SWC from 0.10 to 0.26 cm3 cm–3. However, initial SWC close to the permanent wilting point strongly increased number of tips. Average root length of first‐order lateral (FOL) roots increased as initial SWC increased, and the highest length was found for the frequently watered treatment 5. The results of the study suggest that the ability to produce new FOL roots across a wide range of SWC may give maize an adaptive advantage, because FOL root growth can rapidly adapt to changing soil moisture conditions.

Effect of Waterlogged and Aerobic Incubation on Enzyme Activities in Paddy Soil

ABSTRACT An incubation experiment with soil water content treatments of 0.15 (W1), 0.20 (W2), and 0.40 (W3) g g−1 soil was carried out for two months to investigate the activities of important enzymes involved in C, N, P, and S cycling in a paddy soil from the Taihu Lake region, China, under waterlogged and aerobic conditions. Compared with air-dried soil, waterlogging resulted in a significant decrease (P ≤ 0.05) of fluorescein diacetate (FDA) and β-D-glucosidase activities, and this effect was enhanced with increasing waterlogging time. Waterlogging also significantly inhibited (P ≤ 0.05) arylsulfatase as well as alkaline and acid phosphatase activities, but did not decrease the activities with the increase in waterlogging time. Short-term waterlogging did not affect urease activity, but prolonged waterlogging decreased it markedly. In contrast, the aerobic incubation (W1 and W2 treatments) significantly increased (P ≤ 0.05) FDA, alkaline phosphatase, and β-D-glucosidase activities. With aerobic treatments the activities of FDA and alkaline phosphatase increased with incubation time, whereas β-D-glucosidase activity decreased. A significant difference (P ≤ 0.05) was usually observed between the W1 and W2 treatments for the activities of FDA as well as alkaline and acid phosphatase; however, β-D-glucosidase and urease were usually not significant (P ≤ 0.05). No activity differences were observed between waterlogging and aerobic incubation for arylsulfatase and urease.

Elevated atmospheric CO 2 alleviates drought stress in wheat

Rising levels of atmospheric CO 2 affect transpiration and water absorption processes that influence total leaf water potential ( Ψ W) of wheat ( Triticum aestivum L.). The objective of this case study was to determine the effect of a rise in atmospheric CO 2 concentration on predawn, midday, and sunset Ψ W as soil matric potential ( Ψ M) ranged from −0.03 to −1.50 MPa. ‘Yecora Rojo’ was sown on 15 December 1992 (130 plants m −2) and 7–8 December 1993 (180 plants m −2) in an open field at Maricopa, AZ (33.1°N, 112.0°W), in air enriched for 24 h per day to a atmospheric CO 2 concentration of ∼200 μmol mol −1 (550 μmol mol −1) above ambient (370 μmol mol −1) using a free-air CO 2 enrichment (FACE) apparatus (main-plot) in four replicates. A sub-surface drip-tape irrigation system provided two soil–water content treatments: 50% (dry) and 100% (wet) replacement of potential evapotranspiration (split-plot). Treatments were control-dry (CD), FACE-dry (FD), control-wet (CW) and FACE-wet (FW). A pressure chamber was used to measure Ψ W on 3–5 upper-canopy fully-expanded sunlit leaves at predawn, midday, and sunset between tillering and hard dough during 1993 and 1994. The Ψ W for the dry plots was regressed on Ψ M from field capacity (−0.03 MPa) to the permanent wilting point (−1.50 MPa). Slopes (standard error) were derived for CD ( Ψ W[CD] (MPa) Ψ M[CD] (MPa) −1) compared with FD ( Ψ W[FD] (MPa) Ψ M[FD] (MPa) −1) at predawn (5.1 (1.2) compared with 1.1 (0.2)), midday (4.3 (1.8) compared with 0.7 (0.3)), and sunset (5.9 (1.5) compared with 1.0 (0.9)). Regardless of time of day, more negative Ψ W for a given Ψ M occurred in control compared with FACE ( P<0.01). As Ψ M ranged from −0.03 to −1.50 MPa, a ∼200 μmol mol −1 rise in atmospheric CO 2 concentration resulted in a curvilinearly increase in drought stress alleviation from 0 to 77% at predawn, 0 to 67% at midday, and 0 to 79% at sunset. Hence, as the CO 2 concentration of the atmosphere rises, both drought avoidance and tolerance mechanisms will be enhanced resulting in improved water relations in wheat.