Gradients in soil solution composition between bulk soil and rhizosphere – In situ measurement with changing soil water content

Soil solution composition changes with time and distance from the root surface as a result of mass flow, diffusion, plant nutrient uptake and root exudation. A model system was designed, consisting of a root compartment separated from the bulk soil compartment by a nylon net (30 m mesh size), which enabled independent measurements of the change of soil solution composition and soil water content with increasing distance from the root surface (nylon net). K⁺ concentration in the rhizosphere soil solution decreased during the initial growth stage (12 days after planting, DAP). Thereafter K⁺ accumulated with time, due to mass flow as the dominating process. The extend of K⁺ accumulation depended on the initial fertiliser application. As K⁺ concentrations in soil solution increase, not only as a result of transport exceeding uptake, but also as a result of decreasing soil water content, it is hypothesised that K concentration in soil solution is not the only trigger for the activity of K transporters in membranes, but ABA accumulation in roots induced by decreasing soil matric potentials may add to the regulation. A strong decrease of rhizosphere pH with time is observed as a result of H⁺ efflux from the roots in order to maintain cation-anion balance. In addition the K⁺ to Ca²⁺ ratio was altered continuously during the growing period, which has an impact on Ca²⁺ uptake and thus firmness of cell walls, apoplast pH, membrane integrity and activity of membrane transporters. The value of osmotic potential in the rhizosphere soil solution increased with time indicating decreasing soil water availability. Modelling approaches based on the data obtained with the system might help to fill in the time gaps caused by the low temporal resolution of soil solution sampling method.     

Root:soil adhesion in the maize rhizosphere: the rheological approach

This study was designed to investigate the strength of attachment of plant seedling roots to the soil in which they were grown. The study also assessed the effects of differing soil textures and differing soil matric potentials upon the strength of the root:soil attachment. A device for growing roots upon a soil surface was designed, and was used to produce roots which were attached to the soil. In order to quantify root:soil adhesion, roots of maize seedlings, grown on the soil surface, were subsequently peeled off using a universal test machine, in conjunction with simultaneous time-lapse video observation. To clarify the partitioning of energy in the root:soil peeling test, separate mechanical tests on roots, and on two adherent remoulded topsoil balls were also carried out. The seedling root was characterised by a low bending stiffness. The energy stored in bending was negligible, compared to the root:soil adhesion energy. The mechanical properties of two adherent remoulded topsoil balls were a decrease of the soil:soil adhesion energy as the soil:soil plastic energy increased. These two parameters were therefore interdependent. Using a video-camera system, it was possible to separate the different processes occurring during the root:soil peeling test, in particular, the seed:soil adhesion and the root:soil soil adhesion. An interpretation of the complex and variable force:displacement curves was thus possible, enabling calculation of the root:soil interfacial rupture energy. At a given suction (10 kPa), the results of the peeling test showed a clear soil texture effect on the value of the root:soil interfacial rupture energy. In contrast, for the same silty topsoil, the effect of the soil water suction on the value of the interfacial rupture energy was very moderate. The root:soil interfacial rupture energy was controlled mainly by a product of microscopic soil specific surface area and the macroscopic contact surface area between the root and the soil. Biological and physical interactions contributing to root:soil adhesion such as root:soil interlocking mechanics were also analysed and discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrological and biogeochemical controls on the timing and magnitude of nitrous oxide flux across an agricultural landscape

Anticipated increases in precipitation intensity due to climate change may affect hydrological controls on soil N₂O fluxes, resulting in a feedback between climate change and soil greenhouse gas emissions. We evaluated soil hydrologic controls on N₂O emissions during experimental water table fluctuations in large, intact soil columns amended with 100 kg ha^?1 KNO₃‐N. Soil columns were collected from three landscape positions that vary in hydrological and biogeochemical properties (N= 12 columns). We flooded columns from bottom to surface to simulate water table fluctuations that are typical for this site, and expected to increase given future climate change scenarios. After the soil was saturated to the surface, we allowed the columns to drain freely while monitoring volumetric soil water content, matric potential and N₂O emissions over 96 h. Across all landscape positions and replicate soil columns, there was a positive linear relationship between total soil N and the log of cumulative N₂O emissions (r²= 0.47; P= 0.013). Within individual soil columns, N₂O flux was a Gaussian function of water‐filled pore space (WFPS) during drainage (mean r²= 0.90). However, instantaneous maximum N₂O flux rates did not occur at a consistent WFPS, ranging from 63% to 98% WFPS across landscape positions and replicate soil columns. In contrast, instantaneous maximum N₂O flux rates occurred within a narrow range (?1.88 to ?4.48 kPa) of soil matric potential that approximated field capacity. The relatively consistent relationship between maximum N₂O flux rates and matric potential indicates that water filled pore size is an important factor affecting soil N₂O fluxes. These data demonstrate that matric potential is the strongest predictor of the timing of N₂O fluxes across soils that differ in texture, structure and bulk density.     

Does Soil Strength Play a Role in Wheat Yield Losses Caused by Soil Drying?

Shoot growth in wheat is sensitive to high soil strength, but as high strength and drying tend to occur together it has proved difficult to separate the effects of water stress and mechanical impedance. The results of two field experiments in 2003 and 2004, where soil strength was manipulated by compaction and irrigation, demonstrated that the yield of wheat (Triticum aestivum L.) was sensitive to physical stress in the root zone. We obtained linear relationships between yield and soil strength and between yield and accumulated soil moisture data (accumulation analogous to thermal time), with similar slopes for both seasons. We were unable to detect root-sourced signals of xylem-sap ABA concentration, despite changes in stomatal conductance. When mechanical impedance and matric potential were varied independently in controlled environments, the growth of wheat was sensitive to mechanical impedance, but not to small changes in matric potential. While the response of stomatal conductance to soil drying in the field could be interpreted as evidence of hydraulic signalling, we suggest that the role of high soil strength, in limiting growth rates on moderately dry soil, requires further research.     

Strain Variation in Tolerance of Water Stress by Idriella (Microdochium) bolleyi,a Biocontrol Agent of Cereal Root and Stem Base Pathogens

Three randomly chosen isolates of Idriella bolleyi differed markedly in tolerance of water stress down to—5 MPa in vitro. The differences were seen with respect to osmotic potential of media adjusted with KCl or matric potential of media adjusted with polyethylene glycol 8000. They were consistent when assessed by linear extension of colonies, spore germination, biomass production in liquid culture and sporulation in liquid culture. In comparative tests, one strain of I. bolleyi showed similar stress tolerance to that oF Fusarium culmorum, noted as a pathogen of cereals in dry conditions, whereas another strain of I. bolleyi showed low tolerance equivalent to that of the take‐all fungus Gaeumannomyces graminis and a Phialophora sp. from maize. The extreme variation in water stress tolerance of I. bolleyi might be used to select strains for biocontrol of different cereal root‐ and foot‐rot pathogens or strains might be combined in seed‐applied inocula for consistency of biocontrol in different site or seasonal conditions.     

Water stress effects on toluene biodegradation by Pseudomonas putida

We quantified the effects of matric and solute waterpotential on toluene biodegradation by Pseudomonasputida mt-2, a bacterial strain originally isolated fromsoil. Across the matric potential range of 0 to – 1.5 MPa,growth rates were maximal for P. putida at – 0.25MPa and further reductions in the matric potentialresulted in concomitant reductions in growth rates.Growth rates were constant over the solute potential range0 to – 1.0 MPa and lower at – 1.5 MPa. First ordertoluene depletion rate coefficients were highest at0.0 MPa as compared to other matric water potentialsdown to – 1.5 MPa. Solute potentials down to – 1.5 MPadid not affect first order toluene depletion ratecoefficients. Total yield (protein) and carbon utilizationefficiency were not affected by water potential, indicatingthat water potentials common to temperate soils were notsufficiently stressful to change cellular energyrequirements. We conclude that for P. putida: (1)slightly negative matric potentials facilitate faster growthrates on toluene but more negative water potentials resultin slower growth, (2) toluene utilization rate per cell massis highest without matric water stress and is unaffected bysolute potential, (3) growth efficiency did not differ acrossthe range of matric water potentials 0.0 to – 1.5 MPa.     

Leaf surface wetness in sorghum and resistance to shoot fly, Atherigona soccata: role of soil and plant water potentials

In experiments with potted plants, the relationships between soil matric potential, plant water potential and production of water droplets (leaf surface wetness) on the folded central whorl leaf of seedlings of sorghum genotypes that are either resistant or susceptible to shoot fly (Atherigona soccata) damage were investigated. Differences in soil matric potentials in the pots affected the plant water status, which in turn had profound effects on the production of water droplets on the central whorl leaf of the sorghum genotype susceptible to shoot fly. There was no consistent variation in the relationship between plant water potential and soil matric potential of resistant and susceptible sorghum genotypes. However, there was very little or practically no water droplets on the central whorl leaf of the resistant genotypes, indicating that the production of water droplets is not solely the result of internal water status of the plant. It is suggested that leaf surface wetness is genetically controlled and that an understanding of the mechanism by which water is transferred to the leaf surface will enhance breeding for resistance to shoot fly.     

Topographic position affects the water regime in a semideciduous tropical forest in Panamá

The effects of topographic position on water regime in a semideciduous tropical forest on Barro Colorado Island in Panamá were assessed by measuring soil matric potential using the filter paper technique and by using measured soil water release characteristics to convert a long-term (20 years) gravimetric water content data-set to matric potential. These were also compared against predictions from a simple water balance model. Soil matric potentials on slope sites were significantly higher than on plateau sites throughout the measurement interval and slopes experienced a shorter duration of drought during the annual dry-season. Measured values of matric potential agreed with those predicted from converting the gravimetric measurements using water release characteristics. Annual duration of drought predicted by the simple water balance model agreed with values determined from the converted long term water content data-set and was able to predict the annual duration of drought on plateau sites. On slope sites, the water balance systematically and significantly overestimated the duration of drought obtained from the water content data-set, suggesting that slope sites were supplied with water from upslope. Predictions of annual drought duration from sites with higher annual rainfall than Barro Colorado Island (BCI), suggest that while plateau sites on BCI experience a water regime consistent with annual rainfall, slopes experience a water regime more similar to that of forests with much higher rainfall. We conclude that such large variations in water regime over small spatial scales may play a role in maintaining high species richness through providing opportunities for niche specialisation and by buffering slopes against possible climate change.     

Germination and seedling growth of carrots under salinity and moisture stress

Poor crop stand is a common problem in saline areas. Germination and seedling emergence may be depressed as a result of impeded aeration, saline or dry conditions. In this study, we examined the effects of salinity and moisture stress and their interactions on seed germination and seedling growth of carrots. Variable soil matric and osmotic potentials were either obtained by equilibrating soil salinized to different degrees on a 0.5 MPa ceramic plate soil moisture extractor or by adding different amounts of salt solutions to the same mass of air-dried soil, based on a previously determined soil moisture release curve, and allowing to equilibrate for 1 week. Germination decreased significantly in the investigated silty soil (Aquic Ustifluvent) at soil moisture potentials higher than −0.01 MPa, whereas osmotic potentials as low as −0.5 MPa did not influence germination. Matric potentials of −0.3 and −0.4 MPa, respectively, resulted in a strong decrease (35–95%) of germination and delayed germination by 2 to 5 days in the silty soil to which different amounts (18 and 36%, respectively) and sizes (0.8–1.2 mm and 1.5–2.2 mm, respectively) of sand particles had been added. No effect of sand and grain diameter was detected. Germination was not affected by comparable osmotic potentials. Seedling growth showed a much higher sensitivity than germination to decreasing matric potentials, but was not affected by osmotic potentials ranging from −0.05 to −0.5 MPa. Optimum shoot growth occurred at matric potentials between −0.025 and −0.1 MPa. Shoot and root growth decreased markedly at matric potentials higher than −0.01 MPa. Fresh weight of shoots decreased gradually at matric potentials lower than −0.2 MPa. Root growth was significantly increased at matric potentials of −0.1 to −0.3 MPa, whereas comparable osmotic potentials did not have equivalent effects. It is concluded that germination and seedling growth are differently affected by comparable matric and osmotic stresses and that water stress exerts a more negative effect than salt stress.     

Drought adaptation of field grown wheat in relation to soil physical conditions

In order to investigate the effects of soil texture on possible non-hydraulic signals under field conditions, spring wheat plants (Triticum aestivum L. cv. Cadensa) grown in sand and loam soils and with a well developed root system were exposed to slow soil drying in the late vegetative stage of growth. Soil water potential and content were measured daily at different depths and plant responses were measured in flag leaves. When the average soil water potential in the top soil layers (0–25 cm depth in sand and 0–45 cm depth in loam) dropped to –60 or –70 kPa and the lower soil layers were still at field capacity, morning xylem [ABA] (0.03–0.04 vs. 0.06–0.08 mmol m-3) and midday leaf ABA concentration increased (250–300 vs. 400–450 ng/g DW) and leaf conductance decreased relatively to well-watered (control) plants (0.75–0.88 vs. 0.64–0.70 mol m-2 s-1). These responses took place before any decrease in leaf water potential occurred as compared with control plants, indicating that they were triggered by root-borne signals due to reduced root water status in the top soil layers. At this stage the soil water content was as low as 6% by volume, the fraction of roots in ‘wet’ soil was 0.12 and relative available soil water was 45% in sand and still high 20%, 0.48 and 70%, respectively, in loam of the whole soil profile indicating that roots were responding to soil water availability and not soil water content at a certain evaporative demand. In addition, similar responses occurred at high and low evaporative demands (3.4–5.2 vs. 0.6–4.0 mm/day of potential evapotranspiration). This revised version was published online in July 2006 with corrections to the Cover Date.     

Observations on the maintenance and measurement of soil water in simple pot experiments and its effects on seed-borne Fusarium culmorum seedling blight of winter wheat

A simple method for maintaining and measuring soil water and the relationship between soil water and seed-borne Fusarium culmorum seedling blight of wheat was investigated under controlled environmental conditions to develop reproducible assay conditions for epidemiological studies and the testing of fungicide seed treatments. For reproducibility, soil matric potential (ψ_m) was used to define soil water, and a range of watering regimes were tested. Treatments were watered to either a maximum ψ_m or to maintain a mean ψ_m in order to establish which parameter best described the effect that soil water had on the incidence of disease symptoms. The severity of disease symptoms was closely related to the mean soil water status and not the maximum ψ_m value. Watering intervals, ranging from three times a day to once every three days, did not affect this relationship. The percentage of seedlings showing symptoms after emergence (i.e. localised or extensive necrotic lesions) was inversely related to the mean ψ_m. With increasing soil dryness (mean ψ_m from -0.14 MPa to -0.17 MPa) the percentage of seedlings with post-emergence symptoms decreased from 50% to 20%. However, as the mean ψ_m changed from -0.14 MPa to -0.17 MPa the percentage of seedlings dying before emergence increased from 25% to 55% in direct proportion to ψ_m. Overall the incidence of infection as indicated by the total number of seedlings showing symptoms either before or after emergence remained relatively constant, and was not significantly related to mean ψ_m.     

Lab-Scale Biodegradation Study of BTEX under the Unsaturated Condition and Its Effect on Soil Matric Potential

Benzene, toluene, ethylbenzene, and xylene are collectively known as BTEX which contributes to volatile environmental contaminants. This present study investigates the microbial degradation of BTEX in batch and continuous soil column experiments and its effects on soil matric potential. Batch degradation experiments were performed with different initial concentrations of BTEX using the BTEX tolerant culture isolated from petroleum-contaminated soil. In batch study, the degradation pattern for single substrate showed that xylene was degraded much faster than other compounds followed by ethylbenzene, toluene, and benzene with the highest μ_max = 0.140 h^?1 during initial substrate concentration of 100 mg L^?1. Continuous degradation experiments were performed in a soil column with an inlet concentration of BTEX of about 2000 mg L^?1 under unsaturated flow in anaerobic condition. BTEX degradation pattern was studied with time and the matric potential of the soil at different parts along the length of the column were determined at the end of the experiment. In continuous degradation study, BTEX compounds were degraded with different degradation pattern and an increase in soil matric potential was observed with an increase in depth from top to bottom in the column with applied suction head. It was found that column biodegradation contributed to 69.5% of BTEX reduction and the bacterial growth increased the soil matric potential of about 34% on an average along the column height. Therefore, this study proves that it is significant to consider soil matric potential in modeling fate and transport of BTEX in unsaturated soils.     

尿素施用量对小麦根际土壤微生物数量及土壤酶活性的影响

在大田高产条件下,研究了不同尿素施用量下两种不同穗型小麦品种“兰考矮早8”和“豫麦49—198”根际微生物数量和土壤酶活性的变化。结果表明：微生物数量随小麦生育时期的进行呈规律性变化,其中微生物总量在拔节期和抽穗期时数量较多。尿素施用量对小麦根际微生物数量和酶活性均产生一定的影响,并且处理间差异达到显著水平。两个小麦品种根际微生物总量、细菌、放线菌、真菌数量随着尿素施用量的增加呈先上升后下降的趋势,但两个品种根际微生物数量最高时的尿素量略有差异。同一生育时期,随着尿素施用量的提高,土壤蛋白酶、过氧化氢酶呈先增加后降低的变化趋势,以他（391kg／hm^2）或T3（586kg／hm^2）处理的酶活性较高,T4（782kg／hm^2）处理的酶活性略有降低;脲酶活性则呈上升趋势,以T4（782kg／hm^2）处理的脲酶活性最大。表明适宜尿素施用量有益于小麦根际微生物数量和酶活性的提高,过高则微生物数量和酶活性下降。     

Effect of water potential on the development of an haploid strain of Sporisorium reilianum f.sp. zeae

Sporisorium reilianum f.sp. zeae, the causal agent of head smut, infects the roots of the maize plantlets. Little information is available concerning the development of the fungus in soil, although this saprophytic phase is an important part of the life cycle. This paper reports that water potential also affects hyphal induction, and this effect on the fungus may influence disease transmission. In response to a decrease in water potential from 0 to –1.52 MPa in presence of variable molecular weight polyethylene glycols, haploid hyphae develop from the haploid yeast. Hyphal extension is fastest at low water potentials (–1.2 MPa) controlled with high molecular weight polyethylene glycols, PEG-3350 and PEG-8000. Formation of parasitic dikaryotic hyphae following fusion between haploid hyphae was possible at low water potential (–1.2 MPa) and was not inhibited by water stress. These results are consistent with the hypothesis that the effects of low soil water potential on yeast–hyphal transition and hyphal growth facilitate the convergence of compatible haploid strains, and that this may increase disease severity.     

Matric potential evaluations and measurements for gelled substrates

A new method for evaluating the matric potential of gelled media has been developed. The method allows the derivation of the matric potential as a limit of a series of measurements of water potential values from gelled media prepared without added components, from agar powders progressively cleaned of mineral impurities. Three commercial agar brands were tested, and for these the matric potential was found to contribute only between 1 and 2% of their total water potential. Thermodynamic features relating matric and osmotic potentials are described. New hypotheses for understanding the water flux mechanism from gel to tissue cultured explants are discussed. Movement of water along polymeric chains is postulated to be a facilitated step in comparison with bulk movement.     

Calibration of Watermark soil moisture sensors for soil matric potential and temperature

Rapid, accurate, and automated measurement of soil matric potential is desirable. Evidence suggested that the Watermark resistance block might be an appropriate and inexpensive tool, so we conducted an evaluation of its relevant characteristics. A number of these blocks were calibrated under laboratory conditions to determine their individual and aggregate responses to soil matric potential, soil type, and temperature. We found that the temperature response could be expressed as a single equation, valid for all tested blocks, but comparison against matric potential revealed that each block had a characteristic response. Furthermore, block responses were different in two soils and, for a given soil, not necessarily reproducible. Given these limitations, these sensors are probably useful only as relative indicators of soil water status.