Range Of Soil Water Potentials Research Articles

Root hydraulic phenotypes impacting water uptake in drying soils.

Soil drying is a limiting factor for crop production worldwide. Yet, it is not clear how soil drying impacts water uptake across different soils, species, and root phenotypes. Here we ask (1) what root phenotypes improve the water use from drying soils? and (2) what root hydraulic properties impact water flow across the soil–plant continuum? The main objective is to propose a hydraulic framework to investigate the interplay between soil and root hydraulic properties on water uptake. We collected highly resolved data on transpiration, leaf and soil water potential across 11 crops and 10 contrasting soil textures. In drying soils, the drop in water potential at the soil–root interface resulted in a rapid decrease in soil hydraulic conductance, especially at higher transpiration rates. The analysis reveals that water uptake was limited by soil within a wide range of soil water potential (−6 to −1000 kPa), depending on both soil textures and root hydraulic phenotypes. We propose that a root phenotype with low root hydraulic conductance, long roots and/or long and dense root hairs postpones soil limitation in drying soils. The consequence of these root phenotypes on crop water use is discussed.

Water relations and drought sensitivity of Folsomia candida eggs (Collembola: Isotomidae)

Drought tolerance of juvenile and adult life stages is relatively well understood, but very little is known about the tolerance of eggs to drought in this group of animals. The aim of the present study was therefore to investigate the water relations and drought sensitivity of eggs of the hygrophilic springtail, Folsomia candida Willem, 1902 (Isotomidae), exposed to a range of soil water potentials above and below the permanent wilting point of plants (-1.5 MPa). Under saturated conditions, eggs absorbed water during development and increased water content from 1.1 to 2.9 mg mg-1 dry weight. By increasing drought conditions, water absorption was gradually reduced and was nullified approximately at the soil water potential equivalent to the osmolality of egg fluids (630 mOsm corresponding to -1.53 MPa). Eggs had a lower permeability for water (68 ± 13 µg water cm-2 h-1 mm Hg-1) than adults (about 400 µg water cm-2 h-1 mm Hg-1), but eggs were much more sensitive to drought than adults. Eggs did not survive exposure to -1.5 MPa, whereas adults readily survive this level of drought by absorbing water vapour. In conclusion, eggs of F. candida are sensitive to drought and would perish if soil water potential in the field approaches the wilting point of plants, which is often reached during summer droughts. The persistence of this species depends on the survival of post-embryonic life stages.

Laboratory Method to Evaluate Wheat Seedling Emergence from Deep Planting Depths

Core Ideas Stand establishment is the biggest factor afffecting winter wheat yield in the low‐precipitation region. Winter wheat seed is planted as deep as 20 cm below the soil surface to reach adequate soil moisture. Determination of a cultivar's emergence ability in the field is limited to a short time window once a year. We developed a laboratory method to to accurately measure emergence in pots from deep planting depths. Laboratory and field results were strongly correlated under a wide range of soil water potentials. Planting depth effect on seedling emergence is an important concern for many crops grown around the world. Farmers in the low‐precipitation (<300 mm annual) winter wheat (Triticum aestivum L.) (WW) production region of the Inland Pacific Northwest of the United States (PNW) plant seed as deep as 20 cm below the surface of summer‐fallowed soils with deep‐furrow drills to reach adequate seed‐zone moisture. Seedlings need to emerge through 12 to 15 cm of soil cover, most often under marginal seed‐zone water potentials. Successful stand establishment is the most critical factor affecting WW grain yield potential in the region. We developed a laboratory method to accurately assess WW emergence from deep planting depths in pots. To test the methodology, we first conducted a 4‐yr field experiment to measure emergence of four WW cultivars having either standard‐height or semi‐dwarf growth habit. Depth of soil cover over the seed was 14 cm and seed‐zone water potential over the 4 yr ranged from very dry (–0.69 MPa) to wet (–0.40 MPa). Next, a factorial laboratory pot experiment was conducted using the same WW cultivars and soil seed‐zone water potentials similar to those during the 4 yr in the field. Statistical comparison between field and laboratory emergence data showed a strong correlation (r = 0.71, p < 0.01) for median time to emerge. We describe the step‐by‐step procedure for conducting a laboratory pot experiment to measure WW emergence from deep planting depths under a wide range of water potentials.

Relating Macropore Flow to Soil Parametric and Morphological Variables

Core Ideas A new parameter, “slope of macropore flow (s)”, was introduced The parameter s allowed the identification of soil variables controlling macropore flow Soil clay content was the key parameter controlling macropore flow The parameter s may be used as a soil physical indicator in environmental studies Preferential flow is an important soil hydraulic property as it controls water flow and solute transport in saturated and near‐saturated conditions. Preferential flow occurs in a small range of soil water potentials and soil structure predominantly controls soil water flow; it is thus challenging to model preferential flow using soil parametric variables. The aim of this study was to identify soil properties that affect macropore‐type preferential flow. Thirty undisturbed soil samples were collected from the topsoil of a paddy field and adjacent grassland, and hydraulic conductivity (K) was measured at saturation and near saturation at different tension (h) values. Soil parametric and morphological features were then recorded in the laboratory. To evaluate correlations between K(h) and different soil properties, the log‐log slope of K(h) at near saturation (s) was obtained for each sample by plotting logK versus logh. The values of s were then correlated with soil parametric and morphometric properties. Values of Ks ranged from 1.2 × 10−6 to 7.5 × 10−4 cm h−1 with a mean of 3.77 × 10−4 cm h−1 and standard deviation (SD) of 2.14 × 10−4 cm h−1. Application of 7 cm tension resulted in an approximately 80‐fold decrease in Ks. Values for s ranged from –3.64 to –0.98 with a mean of –2.32 and SD of 0.77. These results demonstrated that the soil clay content, cation exchange capacity, specific surface area, coefficient of linear extensibility, rupture resistance, and plasticity were negatively correlated with s, while BD, structure size, structure grade, pore number and pore size were positively correlated with s. A correlation analysis further revealed that soil clay content was the key variable controlling s in the studied soils. These findings suggest that s can be used as a physical soil indicator that links environmental impact, water, and solute dynamics in near‐saturated and saturated soils.

Climate change-induced water stress suppresses the regeneration of the critically endangered forest tree Nyssa yunnanensis.

Climatic change-induced water stress has been found to threaten the viability of trees, especially endangered species, through inhibiting their recruitment. Nyssa yunnanensis, a plant species with extremely small populations (PSESP), consists of only two small populations of eight mature individuals remaining in southwestern China. In order to determine the barriers to regeneration, both in situ and laboratory experiments were performed to examine the critical factors hindering seed germination and seedling establishment. The results of in situ field experiments demonstrated that soil water potentials lower than -5.40 MPa (experienced in December) had significantly inhibitory effects on seedling survival, and all seedlings perished at a soil water potential of -5.60 MPa (January). Laboratory experiments verified that N. yunnanensis seedlings could not survive at a 20% PEG 6000 concentration (-5.34 MPa) or 1/5 water-holding capacity (WHC; -5.64 MPa), and seed germination was inhibited in the field from September (-1.10 MPa) to November (-4.30 MPa). Our results suggested that soil water potentials between -5.34 and -5.64 MPa constituted the range of soil water potentials in which N. yunnanensis seedlings could not survive. In addition to water deficit, intensified autotoxicity, which is concentration-dependent, resulted in lower seed germination and seedling survival. Thus, seed establishment was probably simultaneously impacted by water deficit and aggravated autotoxicity. Meteorological records from the natural distribution areas of N. yunnanensis indicated that mean annual rainfall and relative humidity have declined by 21.7% and 6.3% respectively over past 55 years, while the temperature has increased by 6.0%. Climate change-induced drought, along with a poor resistance and adaptability to drought stress, has severely impacted the natural regeneration of N. yunnanensis. In conclusion, climate change-induced drought has been implicated as a regulating factor in the natural regeneration of N. yunnanensis through suppressing seed germination and screening out seedlings in the dry season. Based on the experimental findings, habitat restoration and microclimate improvement should both be highlighted in the conservation of this particular plant species.

The role of biochar porosity and surface functionality in augmenting hydrologic properties of a sandy soil

This paper reports studies to elucidate the potential relationships between porosity and surface functionality of biochar and soil water retention characteristics. The biochars studied were produced from pine wood (PW), hybrid poplar wood (HP), and pine bark (PB) at temperatures of 350°C and 600°C. The resulting materials were then oxidized under air at 250°C to generate oxygenated functional groups on the surface. All biochar were thoroughly characterized (surface and bulk properties) and their hydrological properties measured in blends with Quincy sand. We prepared 39 microcosms for this study to examine the effect of biochar functionalities and porosity on the hydro-physical properties of Quincy sand. Each biochar was thoroughly mixed with the soil at 20gkg−1. The field capacity, wilting point, and total available soil moisture of the bio-char/Quincy sand mixtures were measured for both dry and wet ranges. The soil water potentials and soil water contents were fitted using the model of van Genuchten. Our results indicated that the amount of oxygenated functional groups on the surface of biochars clearly differentiated the biochars in terms of hydrophilicity, with the oxidized biochars being superior, followed by the low-temperature biochars, while the high temperature biochars possessed lowest hydrophilicity. As a result, oxidized biochars exhibited better wettability compared to unoxidized biochars, regardless their feedstock source. Significant correlation occurred between the total acidic functional groups on biochar surface and water contents at different matric potentials. Over a wide range of soil water potentials, oxidized biochar-soil mixtures held more water than the unoxidized biochar-soil mixtures except in the region between −0.1 and −5kPa of ψ, which is near saturation. Soil water contents at different matric potentials were significantly inter-correlated (P<0.01) and correlated with bulk densities of biochar-amended soil samples.

Effect of biochar on the properties of loamy sand Spodosol soil samples with different fertility levels: A laboratory experiment

The effect of biochar on the properties of a loamy sand Spodosol with different fertility level in the ploughed horizon has been studied in a 60-day laboratory experiment. It has been found that the application of biochar resulted in a significant increase of the soil water content in the range of soil-water potential from −5 to −50 kPa, a significant decrease of the total shrinkage of the soil with high level of fertility after three wetting-drying cycles, and the accumulation of a significantly higher content of nitrates in the soil. At a water content of 21%, less favorable conditions for the denitrification development were formed in the soil with high level of fertility than in the medium-fertile soil, which resulted in lower cumulative N2O emission from the former. The addition of biochar resulted in no significant changes in the cumulative emission of CO2 from the studied soil but significantly reduced the emission of N2O from the soil at the simultaneous addition of biochar and clover residues.

Addition of a Two-Dimensional Water Balance Model to the DSSAT-CSM

Abstract. There is increasing pressure for more efficient production of agricultural commodities. To this end, cropping systems models are useful tools for identifying efficient production practices and understanding complex relationships between management, system drivers, production, and environmental consequences. In this research, the Decision Support System for Agrotechnology Transfer Cropping Systems Model (DSSAT-CSM) was amended to allow for water-limited simulation under cultural practices common for intensive vegetable production, including drip irrigation under plastic-mulched raised beds. A two-dimensional water balance model was developed with an emphasis on creating a practical model with flexible input requirements and reasonable computation time. Model outputs were evaluated against lab and field measurements to assess the model’s ability to simulate soil water dynamics under drip irrigation regimens. Evaluation of a parameter estimation method designed to utilize the standard DSSAT soil inputs indicated that the method provided unbiased characterization of soil water retention curves (SWRC), with an overall root mean square error (RMSE) of 0.0458 cm3 cm-3. However, SWRCs were more accurately characterized when estimated from soil water measurements from a wider range of soil water potentials, with an overall RMSE of 0.0277 cm3 cm-3. Thus, detailed measurement of soil hydraulic properties was recommended to improve parameter estimation and simulation of soil water dynamics. Evaluation of soil water simulations against field-measured values demonstrated the model’s ability to adequately simulate soil water flow patterns under drip irrigation for a fairly uniform soil under typical vegetable production management practices, with overall RMSE values of 0.0257 to 0.0378 cm3 cm-3 and 0.0245 to 0.0254 cm3 cm-3 using the simplified and the more detailed parameter estimation methods, respectively.

Nitrogen use efficiency and yield of rice under different nitrogen and water stress conditions at grain-filling stage

以杂交籼稻"冈优527"和常规粳稻"农垦57"为材料, 设置硫酸铵(铵硝配比100∶0)、硝酸铵(铵硝配比50∶50)、硝酸钠(铵硝配比0∶100)3种形态氮肥及结实期4种水分胁迫处理[土壤水势(ψsoil)分别为0 kPa、 25 kPa、 50 kPa、 75 kPa, 持续处理14 d], 研究其对水稻氮素吸收利用及产量的影响。结果表明: 结实期土壤水势在 25 kPa时, 铵硝比50∶50处理较铵硝比100∶0处理的水稻籽粒产量增加显著, 铵态氮比例≥50%时, 适当增加硝态氮比例可缓解土壤水分严重不足对产量形成的不利影响。当土壤水势在0~ 25 kPa范围内适当增加硝态氮肥比例, 有利于促进稻株氮素累积, 尽管与纯铵态氮处理间未达到显著水平, 但与纯硝态氮处理间差异均达到显著水平。土壤水势≤ 50 kPa时, 增加硝态氮产量优势减弱, 相反增加铵态氮肥的比例更有利于产量形成。增加铵态氮有利于分蘖盛期前稻株对氮的吸收, 但在保证一定铵态氮比例下, 适当增加硝态氮有利于加快中、后期对氮素的吸收速度和氮素累积量, 为结实期氮素向籽粒转运及提高氮素利用效率提供保证。适度水分胁迫能促进结实期水稻对氮素的吸收, 促进结实期干物质累积, 提高各器官中营养物质向籽粒运转, 进而有利于收获指数的提高。杂交籼稻"冈优527"和常规粳稻"农垦57"对不同形态氮肥与结实期水分胁迫下氮素利用及产量的响应趋势基本一致。

Resiliences to water deficit in a phenotyping platform and in the field: How related are they in maize?

Resilience to soil water deficit is a priority of many maize breeding programmes. Its genetic analysis requires estimators that characterise each genotype in a reproducible way. We have tested and compared three methods for that, namely (i) the ability of hybrids to maintain leaf growth in a range of soil water potentials in a phenotyping platform, (ii) a direct estimator of resilience of seed number to water deficit in a network of field experiments and (iii) classical methods involving tolerance indices and variance analysis. A drought index was obtained by averaging soil water potential, measured with tensiometers, for the phenological phase during which seed number is determined (evaluated for each individual hybrid in each site). It closely correlated with seed number in each of the 19 hybrids that were analysed over 14 environmental situations in France, Hungary and Chile. The slope of the regression line between drought index and seed number, established for each hybrid, was taken as an estimate of the resilience to soil water deficit of that hybrid. Resilience estimated in this way varied 2-fold in the set of studied hybrids, and correlated with resilience of leaf growth to soil water deficit in short-term experiments in the phenotyping platform. In contrast, the resilience estimated via the interaction genotype×watering treatment resulted non-significant because of large differences in drought indices between genotypes in a given watering treatment. We therefore propose that direct estimation of resilience to water deficit is feasible in the field with a minimum amount of environmental measurements. This provides estimations that are consistent with those measured in a phenotyping platform. The combination of both estimates provides insight into the mechanisms associated with resilience of each hybrid, potentially contributing to accelerate the genetic gain.

Soil water potential does not affect leaf morphology or cuticular characters important for palaeo-environmental reconstructions in southern beech, Nothofagus cunninghamii (Nothofagaceae)

Leaf form is closely related to local prevailing abiotic conditions and thus the morphology of fossil and sub-fossil leaves is often used to reconstruct both historical and palaeo-environmental conditions. However, palaeo-environmental reconstruction is difficult because leaf form is controlled potentially by many interacting environmental factors such as temperature, CO2 concentration, light and water availability. We used a glasshouse trial to investigate the impact of water availability on the leaf and cuticle morphology of a species important for palaeo-environmental reconstruction, the southern beech, Nothofagus cunninghamii. We found that reducing soil water potential to –0.2 or –0.5 MPa had no impact on leaf form or cuticular characters, despite reducing leaf carbon assimilation and severely restricting plant growth. Although plant accession affected many leaf characters, there were few significant impacts of altitude of origin and no substantial interactions between altitude of origin and soil water potential. Thus, both cuticular and gross leaf morphology seem to be stable across a range of soil water potentials in this species, meaning that palaeo-environmental signals from this species are unlikely to be affected by changes in water availability.

Rice leaf growth and water potential are resilient to evaporative demand and soil water deficit once the effects of root system are neutralized

Rice is known to be sensitive to soil water deficit and evaporative demand, with a greatest sensitivity of lowland-adapted genotypes. We have analysed the responses of plant water relations and of leaf elongation rate (LER) to soil water status and evaporative demand in seven rice genotypes belonging to different species, subspecies, either upland- or lowland-adapted. In the considered range of soil water potential (0 to -0.6 MPa), stomatal conductance was controlled in such a way that the daytime leaf water potential was similar in well-watered, droughted or flooded conditions (isohydric behaviour). A low sensitivity of LER to evaporative demand was observed in the same three conditions, with small differences between genotypes and lower sensitivity than in maize. The sensitivity of LER to soil water deficit was similar to that of maize. A tendency towards lower sensitivities was observed in upland than lowland genotypes but with smaller differences than expected. We conclude that leaf water status and leaf elongation of rice are not particularly sensitive to water deficit. The main origin of drought sensitivity in rice may be its poor root system, whose effect was alleviated in the study presented here by growing plants in pots whose soil was entirely colonized by roots of all genotypes.

The water retention of a granite rock fragments in High Tatras stony soils

The water retention capacity of coarse rock fragments is usually considered negligible. But the presence of rock fragments in a soil can play an important role in both water holding capacity and in hydraulic conductivity as well. This paper presents results of maximum water holding capacity measured in coarse rock fragments in the soil classified as cobbly sandy loam sampled at High Tatra mountains. It is shown, that those coarse rock (granite) fragments have the maximum retention capacity up to 0.16 volumetric water content. Retention curves of the four particular granite fragments have shown water capacity available for plants expressed in units of volumetric water content of 0.005 to 0.072 in the soil water potential range (0, –0.3 MPa). Available water capacity of stone fragments can contribute to the available water capacity of soil fine earth considerably and help to plants to survive during dry spells.

The cooperative relation between non‐hydraulic root signals and osmotic adjustment under water stress improves grain formation for spring wheat varieties

Non-hydraulic root signals (nHRS) and osmotic adjustment (OA) are two important adaptive responses of plants to water stress. There is little understanding of their relationships during water stress. The threshold range of soil water potential to occurrence of nHRS, the capacity for OA, grain yield and water use efficiency (WUE) were examined in three spring wheat (Triticum aestivum L.) varieties (two bred after 1975 and one bred before 1900) under water stress conditions. The threshold range of nHRS was significantly correlated with the maintenance rate of grain yield (MRGY) (r = 0.99, P < 0.05) under moderate drought (-0.49 to -0.55 MPa) but not under severe drought (-0.70 to -0.76 MPa). There were similar correlations between OA and the MRGY. However, regulation of nHRS precedes OA during gradual water stress. The threshold range of nHRS and OA was positively correlated (r = 0.93, P < 0.05), suggesting a mechanism for adapting to drought. WUE was higher for modern than for old varieties and was correlated with the root efficiency (full biomass weight including root per root weight, r = 0.78, P < 0.05) and the root water uptake efficiency (water consumption per root weight, r = 0.72, P < 0.05). However, there was a significant negative correlation between WUE and root weight (r = -0.84, P < 0.01). The cooperative relationship between the threshold range of nHRS and OA under water stress was beneficial for improving grain formation for spring wheat varieties.

Soil Micropore Development and Contributions to Soluble Carbon Transport within Macroaggregates

Soil biophysical transport mechanisms promoting biogeochemical sorption of soluble organic carbon (SOC) compounds within macroaggregates control the retention and release of most soil nutrients, C‐ and N‐based polysaccharides, and contaminants. Ecosystems containing continuous supplies of soluble root exudates and particulate organic matter (POM) provide a constant supply of mobile SOC compounds to surfaces and internal pore networks of soil aggregates. Intra‐aggregate pores, especially the ultrafine pores, appear to be developed, interconnected, and blocked or disconnected by repeated drying and wetting (DW) cycling with direct but unknown contributions to movement and retention of SOC compounds. There is evidence that the severity (e.g., range of soil water potential) and frequency of severe DW cycles control intra‐aggregate micro‐ and nanopore formation and function. Heterogeneously distributed microsites within aggregates contain microbial communities that readily mineralize available C and N compounds, producing mobile SOC that can be tightly sorbed to additional mineral surfaces made available within micro‐ and nanosized fissures during repeated DW cycling. Mechanical removal of concentric soil layers of aggregates, synchrotron imaging and computer microtomographic (CMT) image processing software of three‐dimensional pore networks and connectivities, coupled with synchrotron X‐ray small angle scattering to measure pore sizes. Natural isotopes of 13C and 15N to quantify C and N sorption and CO2 respiration provide new and integrated approaches for quantifying spatially heterogeneous changes of pore diameters, connectivities, and organo‐ion‐mineral sorption within intra‐aggregate pore networks. Net C and N alterations at surfaces and within aggregates appear to modify both the microbial activities and bacterial community structures, producing integrated feedback and feed‐forward processes between the soil biological and physical components of soil aggregates.

Quantifying the Effect of Soil Moisture on the Aerobic Microbial Mineralization of Selected Pesticides in Different Soils

A standardized quantitative approach was developed to reliably elucidate the effect of increasing soil moisture on pesticide mineralization. The mineralization of three aerobically degradable and chemically different 14C-labeled pesticides (isoproturon, benazolin-ethyl, and glyphosate) was studied under controlled conditions in the laboratory at an identical soil density of 1.3 g cm(-3). The agricultural soils used are characterized by (i) large variations in soil texture (sand content 4-88%) and organic matter content (0.97-2.70% org. C), (ii) fairly diverse soil-water retention curves, and (iii) differing pH values. We quantified the effect of soil moisture on mineralization of pesticides and found that (i) at soil water potential < or = -20 MPa minimal pesticide mineralization occurred; (ii) a linear correlation (P < 0.0001) exists between increasing soil moisture (within a soil water potential range of -20 and -0.015 MPa), and increased relative pesticide mineralization; (iii) optimum pesticide mineralization was obtained at a soil water potential of -0.015 MPa, and (iv) when soil moisture approximated water holding capacity, pesticide mineralization was considerably reduced. As both selected pesticides and soils varied to a large degree, we propose that the correlation observed in this study may be also valid in the case of aerobic degradation of other native and artificial organic compounds in soils.

Carbon isotope discrimination differences within and between contrasting populations of Encelia farinosa raised under common-environment conditions.

Previous studies of the desert shrub Encelia farinosa have shown variation of morpholological and physiological integration that appears to match environmental differences among populations. Such findings led us to ask if there is a genetic basis for such differentiation that may be related to physiological control of intercellular CO(2) concentrations as indicated by carbon isotope discrimination (Delta) values, and if genetic variance for Delta is detectable within populations. Under common environment conditions, Delta values were compared between two populations of E. farinosa from desert regions with contrasting rainfall patterns: Superior, Ariz., a region with high annual rainfall and droughts of short duration, and Oatman, Ariz. a region with lower annual rainfall and longer drought periods. Superior plants had consistently greater mean Delta values than Oatman plants across a broad range of soil water potentials, indicating that there is a genetic basis for Delta variation between these populations. At the intrapopulation level only Oatman plants showed detectable genetic variance of Delta based on: (1) consistent individual-rank values for Delta among soil-drought stages, and (2) evidence of heritable genetic variance for Delta during one drought stage. No genetic variance in Delta was evident for the Superior population. It is hypothesized that the high spatio-temporal heterogeneity of water availability at Oatman may facilitate the maintenance of genetic variance for carbon isotope discrimination within this population. Both the inter- and intra-population level findings suggest that selection associated with rainfall and drought has resulted in genetic divergence of the physiological factors involved in Delta determination for these populations. There appears to be strong differences of water-use and carbon-gain strategies among populations, and broader functional breadth among plants in the habitat of greatest environmental heterogeneity.

Changes in the photosynthetic characteristics of Plantago major plants caused by soil drought stress

Changes in net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO2 concentrations (Ci), transpiration rate (E) and water use efficiency (WUE) were measured in Plantago major L. plants grown under sufficient soil water supply or under soil water stress conditions. The plants had high PN in a wide range of soil water potential and temperature regimes. Soil water had little effect on PN under ambient CO2 concentrations, which was explained by a high carboxylation rate, but increased the dark respiration rate. Carboxylation activity at low Ci depended on RuBP regeneration, whereas at high Ci it depended on the phosphate regeneration rate. The gs and E values were low in plants under stress as compared to the controls that resulted in an increase of WUE. The results obtained show that Plantago major plants have different ways of adaptation to soil water deficit conditions.

Soil moisture affects disease severity and colonisation of wheat roots by Rhizoctonia solani AG-8

Experiments were conducted to study how the soil moisture affects rhizoctonia root rot of wheat, colonisation of root by Rhizoctonia solani AG-8, spread of fungus in the soil and microbial activity of soil. Four soil moisture levels ranging from 15 to 75% water-holding capacity of a sandy soil at saturation, which equated to a soil water potential range of −10 to −3 kPa, were tested. It was found that the disease severity decreased up to 69%, as the soil became wetter. The soil moisture status at and above 20% WHC however, did not significantly affect the spread of the fungus in soil. Colonisation of roots by young unpigmented hyphae of the fungus diminished with increasing soil moisture, from 80% of root length colonised under relatively dry conditions (15% WHC) to 20–25% under wet conditions (75% WHC). Soil microbial activity increased with increasing soil moisture and may be associated with suppression of pathogen activity through competition.

Routine measurement of the soil water potential gradient near saturation using a pair of tube tensiometers

We describe a new tensiometer for routine measurements of the soil water potential near saturation. The device is called the tube tensiometer because it is a long, open-topped, vertical tube (&amp;gt;1 m long) that is filled with porous material. The tube tensiometer has advantages over other known tensiometers as it does not require maintenance when the sensing tip dries beyond its air entry pressure and it is capable of being completely buried beyond the cultivation zone so that it does not foul tillage and harvesting equipment. The disadvantage of the tube tensiometer is that it only operates in the range of soil water potentials from –L to 0 cm of water, where L is the length of the tube tensiometer. The output from the tube tensiometer was compared with mercury tensiometers in a 120-day controlled field drainage and evaporation experiment. The regression between instruments was high (r2 = 0.99) and the accuracy of the tube tensiometer was &amp;lt;0.5 cm of water. The soil water potential gradient near saturation can be measured by installing a pair of vertically separated tube tensiometers. If the soil hydraulic conductivity is known then the soil water flux near saturation can be estimated using Darcy's Law. When the installation depth is below the active rooting zone of a crop then deep drainage can be estimated. This application of the tube tensiometer was demonstrated by measuring the vertical soil water potential gradient at a depth of 1 m beneath a wheat field near Harden, NSW, in response to winter rainfall. The major limitation to the use of Darcy's Law for the routine monitoring of deep drainage remains the estimation of the soil hydraulic conductivity. Ongoing work is focussing on the use of tube tensiometers to provide simultaneous measurements of both hydraulic gradient and hydraulic conductivity.