Decreasing Soil Water Potential Research Articles

Sap flow and growth response of Norway spruce under long-term partial rainfall exclusion at low altitude.

Under ongoing climate change, more frequent and severe drought periods accompanied by heat waves are expected in the future. Under these conditions, the tree's survival is conditioned by fast recovery of functions after drought release. Therefore, in the presented study, we evaluated the effect of long-term water reduction in soil on tree water use and growth dynamics of Norway spruce. The experiment was conducted in two young Norway spruce plots located on suboptimal sites at a low altitude of 440 m a.s.l. In the first plot (PE), 25% of precipitation throughfall was excluded since 2007, and the second one represented the control treatment with ambient conditions (PC). Tree sap flow, stem radial increment, and tree water deficit were monitored in two consecutive growing seasons: 2015-2016, with contrasting hydro-climatic conditions. Trees in both treatments showed relatively isohydric behavior reflected in a strong reduction of sap flow under the exceptional drought of 2015. Nevertheless, trees from PE treatment reduced sap flow faster than PC under decreasing soil water potential, exhibiting faster stomatal response. This led to a significantly lower sap flow of PE, compared to PC in 2015. The maximal sap flow rates were also lower for PE treatment, compared to PC. Both treatments experienced minimal radial growth during the 2015 drought and subsequent recovery of radial growth under the more the humid year of 2016. However, treatments did not differ significantly in stem radial increments within respective years. Precipitation exclusion treatment, therefore, led to water loss adjustment, but did not affect growth response to intense drought and growth recovery in the year after drought.

Arbuscular mycorrhizal fungi inoculation and phosphorus application improve growth, physiological traits, and grain yield of rice under alternate wetting and drying irrigation

Climate change and agricultural malpractices are exacerbating drought in many parts of the world causing a substantial agricultural production loss. The improvement of drought tolerance in rice is crucial for maintaining productivity and ensuring global food security. Alternate wetting and drying (AWD) irrigation along with plant-microbe interaction through arbuscular mycorrhizal fungi (AMF) is a potential approach for enhancing rice production through AMF-induced up-regulation of tolerance and resilience against drought stress. Therefore, the ameliorative role of AMF inoculation and phosphorus (P) application on growth, physiological traits, and grain yield of rice was evaluated under water stress imposed through AWD irrigation. A factorial experiment consisting of four fertilizer treatments where the P percentage varied along with the recommended dose of nitrogen (N) with or without AMF inoculation (P100 as the control, P100 + AMF, P75 + AMF, and P50 + AMF), three soil water potential levels (0, −15, and −30 kPa), and two cultivation methods (wet direct seeding and transplanting) was conducted in a polyhouse. The subscript values of 100, 75, and 50 under P represent 100%, 75%, and 50% of the recommended field application dose. Data were collected on selected growth parameters, physiological traits, levels of mycorrhizal colonization, yield and its components, and water productivity of rice. The results revealed that P100 + AMF inoculated plants had 11%, 14%, 74%, and 54% higher leaf greenness, leaf relative water content, net photosynthetic rate, and grain yield, respectively, for wet direct-seeded plants at reduced soil water potential (−30 kPa) compared with non-inoculated plants (P100). Free proline accumulation gradually enhanced with decreasing soil water potential, and it was maximized by 77% at −30 kPa compared with 0 kPa for P50 + AMF (for transplanted plants). Free proline accumulation was also higher with decreasing soil water potential in AMF-inoculated plants than non-inoculated plants regardless of cultivation methods. Leaf osmotic potential was reduced by −0.5 to −1.2 MPa at −30 kPa compared with 0 kPa under different fertilizer doses. However, AMF inoculation (P100 + AMF and P75 + AMF) improved leaf osmotic potential of plants under severe water stress (−30 kPa) maintained through AWD irrigation resulting in better osmotic adjustment than non-inoculated plants. AMF inoculation improved the response of most of the evaluated physiological traits of rice and enhanced grain yield with higher P availability (even with a 25% reduction in its recommended dose) in the rhizosphere under drought stress. Thus, it can be concluded that AMF inoculation coupled with judicious P management is a promising approach for improving physiological and biochemical traits, grain yield, and water productivity of rice under AWD irrigation regardless of cultivation methods.

Breeding phenology of a terrestrial‐breeding frog is associated with soil water potential: Implications for conservation in a changing climate

AbstractAmphibian breeding is often linked to environmental cues. Given accelerating global climate change and habitat alteration, it is important to understand how environmental changes may affect male calling activity, the necessary precursor to mating. Here, we investigate the calling phenology and activity of Geocrinia alba, a critically endangered terrestrial‐breeding frog with a highly restricted distribution in southwestern Australia – a region undergoing significant warming and drying. We investigated whether calling periods differed across eight sites during a single breeding season, and the environmental conditions associated with calling activity at the start, peak and end of the calling period. We found consistent and prolonged calling activity over several months of the austral spring, but the length of the calling period varied two‐fold across sites, ranging from 3 to 6 months. Initiation of calling by G. alba was relatively similar across sites and was associated with warmer temperatures and higher soil water potential. However, the end of the calling period varied from October to January and was significantly associated with decreasing soil water potential. Calling only occurred when soils were close to saturation point, between −8 to −11 kPa, and therefore, breeding opportunities were likely also constrained by the length of time that soils were close to saturation. Changes in the regional climate, such as declining winter rainfall, could result in shorter breeding periods and consequently reduced breeding opportunities. More broadly, our study highlights the sensitivity of amphibians, particularly terrestrial‐breeding species, to changes in soil water potential and temperature, and the importance of maintaining suitable hydrological conditions during the breeding period.

“Responsiveness of mature oak trees (Quercus robur L.) to soil water dynamics and meteorological constraints in urban environments”

Urban trees contribute to the regulation of microclimates of cities through transpirational cooling. The main environmental factors determining transpiration are evaporative demand and soil water availability. However, growth conditions in urban areas are often unfavorable and may promote drought stress. The responses of trees to these conditions may vary between species and age classes, and in addition, can be modified by their environment. For a better understanding of water use patterns of urban tree species, these responses were investigated as a function of different meteorological and soil conditions of their urban environments. To identify physiological responses of mature oaks (Quercus robur L.) to contrasting site conditions, sap flow measurements were carried out in two consecutive growing seasons (2013 and 2014) in urban and suburban areas of Hamburg, Germany. Even considering the varying and sometimes challenging local soil and meteorological conditions, the oaks showed predominantly uniform response patterns. At all study sites, sap flow dynamics were characterized by saturation responses to increasing vapor pressure deficits during both daytime and nighttime conditions, reflecting stomatal responsiveness and hence, a stomatal down-regulation of water loss at demanding atmospheric conditions. Accordingly, the water use of oak trees was characterized by an isohydric strategy, which can help to avoid the risk of cavitation. Moreover, the maximum values of nighttime sap flow exceeded 20% of the daily total at all sites, indicating high rates of nocturnal transpiration and thus a relevant cooling effect during the night. Actual sap flow followed potential sap flow at all sites despite decreasing soil water potentials in the upper soil layers. This suggests actual sap flow during both growing seasons was only limited by atmospheric water demand. Accordingly, the studied mature oak trees maintained high rates of transpiration even in times of reduced soil water availability and thus could provide cooling under challenging site conditions. In contrast to suburban areas, however, oaks in the urban site showed reduced absolute sap flow rates, indicating transpiration sensitivity to different situations of water demand and supply. Due to its water use strategy and its ability to cope with moderate drought, Q. robur can be considered a suitable urban tree species in view of current and future demanding urban growth conditions.

Frontiers in root ecology: recent advances and future challenges

Background and aims: Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France. Methods : Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year. Results : Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter. Conclusions: We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined

Linking above- and belowground phenology of hybrid walnut growing along a climatic gradient in temperate agroforestry systems

Plant phenology is a sensitive indicator of plant response to climate change. Observations of phenological events belowground for most ecosystems are difficult to obtain and very little is known about the relationship between tree shoot and root phenology. We examined the influence of environmental factors on fine root production and mortality in relation with shoot phenology in hybrid walnut trees (Juglans sp.) growing in three different climates (oceanic, continental and Mediterranean) along a latitudinal gradient in France. Eight rhizotrons were installed at each site for 21 months to monitor tree root dynamics. Root elongation rate (RER), root initiation quantity (RIQ) and root mortality quantity (RMQ) were recorded frequently using a scanner and time-lapse camera. Leaf phenology and stem radial growth were also measured. Fine roots were classified by topological order and 0–1 mm, 1–2 mm and 2–5 mm diameter classes and fine root longevity and risk of mortality were calculated during different periods over the year. Root growth was not synchronous with leaf phenology in any climate or either year, but was synchronous with stem growth during the late growing season. A distinct bimodal pattern of root growth was observed during the aerial growing season. Mean RER was driven by soil temperature measured in the month preceding root growth in the oceanic climate site only. However, mean RER was significantly correlated with mean soil water potential measured in the month preceding root growth at both Mediterranean (positive relationship) and oceanic (negative relationship) sites. Mean RIQ was significantly higher at both continental and Mediterranean sites compared to the oceanic site. Soil temperature was a driver of mean RIQ during the late growing season at continental and Mediterranean sites only. Mean RMQ increased significantly with decreasing soil water potential during the late aerial growing season at the continental site only. Mean root longevity at the continental site was significantly greater than for roots at the oceanic and Mediterranean sites. Roots in the 0–1 mm and 1–2 mm diameter classes lived for significantly shorter periods compared to those in the 2–5 mm diameter class. First order roots (i.e. the primary or parents roots) lived longer than lateral branch roots at the Mediterranean site only and first order roots in the 0–1 mm diameter class had 44.5% less risk of mortality than that of lateral roots for the same class of diameter. We conclude that factors driving root RER were not the same between climates. Soil temperature was the best predictor of root initiation at continental and Mediterranean sites only, but drivers of root mortality remained largely undetermined.

A model exploring whether the coupled effects of plant water supply and demand affect the interpretation of water potentials and irrigation management

Water potential is a useful predictive tool in irrigation scheduling as it, or a component, is associated with physiological responses to water deficit. Increasing atmospheric demand for water increases transpiration and decreases water potential for the same stomatal conductance. However, based on supply by the soil-plant-atmosphere-continuum, decreasing soil water potential should decrease stomatal conductance and thus transpiration but also decrease water potential. Such contradictory behavior of supply and demand responses, may limit the value of water potential as an indicator of plant water status. This work studied the relationship between plant water potential and transpiration affected by supply (soil moisture) and atmospheric evaporative demand, and has implications for interpretation of water potentials and irrigation management. Results were that plant water potential has a narrow range of sensitivity to variation in supply and demand in hydrated soils, but greatly varying sensitivity in dry soils, limiting interpretation under dry conditions. Loss of soil conductance in dry, coarse soil types affects the trajectory of plant water potential response to supply and demand. Sapflow measurements on almonds indicated that variation in reference evapotranspiration and/or soil moisture deficit led to similar variation in stem water potentials to that predicted by the model. The model indicates hypotheses that with further testing may have important repercussions on the measurement of plant water use and irrigation scheduling.

The response of intra-annual stem circumference increase of young European beech provenances to 2012-2014 weather variability

Abstract: The increasing frequency and severity of extreme weather events, especially droughts, arising from on-going climate changes negatively affect productivity and stability of forest ecosystems. Understanding species responses and suitable ecotypes that are able of adapting to new environmental conditions is increasingly important. The objective of this study was to quantify the relationships between the inter-annual stem circumference increase (SCI) of five European beech (Fagus sylvatica L.) provenances and weather variability during 2012-2014 in a provenance trial located in central Slovakia. SCIs were extracted on daily and weekly scale from hourly data of circumference changes. To detect the main environmental factors influencing SCI seasonal dynamics, intra-seasonal moving correlation functions were calculated. All five provenances responded synchronously to weather conditions, with high correlations among them during the growing season on both daily and weekly scale. The photoperiod exhibited a synchronizing effect on the seasonal peak of SCI as a sign of tree adaptation to long-term seasonal variations in climate. Temperature was the most significant factor influencing SCI dynamics at the beginning of the season. During the summer months, a precipitation deficit, heat waves and the consequently decreased soil water potential significantly affected the SCI of young beech trees, despite the fact that the provenance plot was situated in an area of optimum beech growth. Not only the severity and duration were important but also the timing of drought within a season. Within all seasons, the lowest SCI values were recorded for the provenance from the lowest altitude and the most oceanic climate (northern Germany). A comparison of daily and weekly SCI with first derivatives of growth functions indicated that SCIs were closely related to theoretical incremental processes, especially on a weekly scale. In young beech trees, SCI seemed to represent an appropriate proxy for studying intra-seasonal incremental processes. A newly designed SASB (self adjusting sharp beginning) function fit these processes better than the Gompertz function.

Effects of Soil Water Potential and Nitrogen Fertilization on Characteristics of Photosynthesis and Chlorophyll Fluorescence Induction in Schisandra chinensis Baillon

Management of soil water and fertilization is known to primarily affect physiological properties and yield in plant. The effect of soil water potential and nitrogen application on characteristics of photosynthesis and chlorophyll fluorescence in Schisandra chinensis Baillon was investigated on a sandy loam soil. Net photosyntheis rate and transpiration rate increased as a photon flux density and was highest at -50kPa of soil water potential. Light compensation point (<TEX>$1.5{\mu}molm^{-1}s^{-1}$</TEX>) and dark respiration (<TEX>$0.13{\mu}molCO_2m^{-1}s^{-1}$</TEX>) was lowest at -50 kPa but maximum photosynthesis rate (<TEX>$13.10{\mu}molCO_2m^{-1}s^{-1}$</TEX>) and net apparent quantum yield (<TEX>$0.083{\mu}molCO_</TEX><TEX>2m^{-1}s^{-1}$</TEX>) was highest at -50 kPa. As results of chlorophyll fluorescence by OJIP analysis, maximum quantum yield (Fv/Fm) of photosystem II (PSII) and PIabs was higher in treatments of -50 kPa and -60 kPa respectively, which reflects the relative reduction state of PSII. But the relative activities per reaction center such as ABS/RC and DIo/RC were low with decreasing soil water potential. Net photosyntheis rate and transpiration rate were highest at treatment of soil testing 1.0 times (<TEX>$92kgha^{-1}$</TEX>). Application of nitrogen resulted in high Fv/Fm, <TEX>$PI_{abs}$</TEX> and low ABS/RC, DIo/RC. This result implies that -50 kPa of soil water potential and nitrogen fertilizer may improve the efficiency of photosynthesis through controlling a photosystem in Schisandra chinensis Baillon.

Root-soil air gap and resistance to water flow at the soil-root interface of Robinia pseudoacacia.

During periods of water deficit, growing roots may shrink, retaining only partial contact with the soil. In this study, known mathematical models were used to calculate the root-soil air gap and water flow resistance at the soil-root interface, respectively, of Robinia pseudoacacia L. under different water conditions. Using a digital camera, the root-soil air gap of R. pseudoacacia was investigated in a root growth chamber; this root-soil air gap and the model-inferred water flow resistance at the soil-root interface were compared with predictions based on a separate outdoor experiment. The results indicated progressively greater root shrinkage and loss of root-soil contact with decreasing soil water potential. The average widths of the root-soil air gap for R. pseudoacacia in open fields and in the root growth chamber were 0.24 and 0.39 mm, respectively. The resistance to water flow at the soil-root interface in both environments increased with decreasing soil water potential. Stepwise regression analysis demonstrated that soil water potential and soil temperature were the best predictors of variation in the root-soil air gap. A combination of soil water potential, soil temperature, root-air water potential difference and soil-root water potential difference best predicted the resistance to water flow at the soil-root interface.

Modeling the Emergence of Winter Wheat in Response to Soil Temperature, Water Potential, and Planting Depth

Abstract. Seedling emergence is a critical stage in wheat (Triticum aestivum L.) establishment, particularly in dryland agriculture. Soil temperature, water potential, and planting depth are important factors influencing emergence. These factors can have considerable spatio-temporal variation, making it difficult to predict the timing and percentage of wheat seedling emergence. Our objectives were to: (1) measure the effect of soil temperature, water potential, and planting depth on seedling emergence of winter wheat; and (2) develop a population-based hydrothermal model to estimate time and percentage of winter wheat emergence planted at a given soil depth. The experiment was a 3 (wheat cultivars) A— 3 (planting depths) A— 5 (temperatures) A— 5 (water potentials) factorial in a randomized complete block design (RCBD) with three replicates. Decreasing soil water potential from 0 to -1 MPa delayed emergence of Moro, Buchanan, and Rod by 13, 14, and 18 days after planting and prolonged the time to reach maximum emergence by 19, 21, and 27 days after planting, respectively. Decreasing soil temperature from 25°C to 5°C delayed the average starting emergence of the three wheat cultivars by 19 days and delayed their average maximum emergence by 25 days after planting. Increasing soil planting depth from 5 to 10 cm delayed starting emergence time for Moro, Buchanan, and Rod by 6, 4, and 7 days, while maximum emergence was delayed by 12, 16, and 9 days after planting, respectively. Increasing the planting depth from 5 to 15 cm hindered the three cultivars from reaching 50% emergence. A hydrothermal time emergence model was developed that estimated observed emergence with 154 out of 207 data points having a root mean square error (RMSE) of ≤15%, whereas 19% of the simulated results did not exceed 19% RMSE and only 7% of the simulated data had RMSE greater than 20%. The simulations of the emergence model for maximum percent seedling emergence (E max ) and number of days after planting to reach maximum emergence (DAP max ) were compared with field data for three years and two cultivars (Rod and Moro). The emergence model gave reasonably good simulations of E max and DAP max for both winter wheat cultivars and in the three years of sowing dates.

Simplified modeling of catchment-scale evapotranspiration via boundary condition switching

Evapotranspiration is one of the most difficult terms of the water balance to estimate and model, because of its complex links with atmospheric, hydrological, and ecological processes. In this paper we show how a switching procedure for the boundary conditions at the soil surface achieves a good description of evapotranspiration in a catchment-scale process-based hydrological model. The switching algorithm relies on one parameter, a threshold soil water pressure head (ψmin), to distinguish between atmosphere-controlled and soil-limited evapotranspiration. We successfully applied the model to a small water-limited catchment in southwestern Victoria, Australia, mainly used as pasture and where an extensive hydrological data set is available. Our simulation results show that the model is capable of reproducing satisfactorily the hydrological regime of the catchment without the need for a detailed multiparameter calibration. Specifically, the observed daily flow hydrographs, typical of ephemeral streams, are reproduced satisfactorily for both the calibration and validation phases. Water table levels proved to be more difficult to match, even though the overall groundwater dynamics are well captured by the model. The comparison between measured and observed evapotranspiration rates demonstrates the capability of ψmin to describe the conversion of potential evaporative demand into actual evapotranspiration. The effect of ψmin on the components of the catchment water balance and the model numerical performance were investigated for two different soil types through a sensitivity analysis. The modeled reduction of evapotranspiration with decreasing soil water potential is shown to be analogous to the commonly adopted Feddes formulation of water stress. The results show that the boundary condition-switching algorithm, with a proper choice of ψmin and soil retention curves, can represent a simple and effective way to account for the impacts exerted on the catchment hydrological response by shallow rooted vegetation.

Effects of decreasing water potential on gross ammonification and nitrification in an acid coniferous forest soil

Abstract Changes in the soil water regime, predicted as a consequence of global climate change, might influence the N cycle in temperate forest soils. We investigated the effect of decreasing soil water potentials on gross ammonification and nitrification in different soil horizons of a Norway spruce forest and tested the hypotheses that i) gross rates are more sensitive to desiccation in the Oa and EA horizon as compared to the uppermost Oi/Oe horizon and ii) that gross nitrification is more sensitive than gross ammonification. Soil samples were adjusted by air drying to water potentials from about field capacity to around −1.0 MPa, a range that is often observed under field conditions at our site. Gross rates were measured using the 15N pool dilution technique. To ensure that the addition of solute label to dry soils and the local rewetting does not affect the results by re-mineralization or preferential consumption of 15N, we compared different extraction and incubation times. T0 times ranging from 10 to 300 min and incubation times of 48 h and 72 h did not influence the rates of gross ammonification and nitrification. Even small changes of water potential decreased gross ammonification and nitrification in the O horizon. In the EA horizon, gross nitrification was below detection limit and the response of the generally low rates of gross ammonification to decreasing water potentials was minor. In the Oi/Oe horizon gross ammonification and nitrification decreased from 37.5 to 18.3 mg N kg−1 soil d−1 and from 15.4 to 5.6 mg N kg−1 soil d−1 when the water potential decreased from field capacity to −0.8 MPa. In the Oa horizon gross ammonification decreased from 7.4 to 4.0 mg N kg−1 soil d−1 when the water potential reached −0.6 MPa. At such water potential nitrification almost ceased, while in the Oi/Oe horizon nitrification continued at a rather high level. Hence, only in the Oa horizon nitrification was more sensitive to desiccation than ammonification. Extended drought periods that might result from climate change will cause a reduction in gross N turnover rates in forest soils even at moderate levels of soil desiccation.

Biophysical and morphological leaf adaptations to drought and salinity in salt marsh grasses

Leaf energy budgets were constructed for 13 species of estuarine C 4 grasses (Poaceae) to elucidate the biophysical effects of drought and salinity on the interception and dissipation of solar energy. Spartina alterniflora, S. anglica, S. argentinensis, S. bakeri, S. cynosuroides, S. densiflora, S. foliosa, S. foliosa × S. alterniflora hybrids, S. gracilis, S. patens, S. pectinata, S. spartinae, and Distichlis spicata plants were grown under controlled soil water potential gradients in a greenhouse. Species were grouped into four major ecological functional types, based on elevational zonation ranges: low marsh species, middle marsh species, high marsh species, and freshwater species. Different functional types are adapted to different environmental conditions, and responded differently to reduced water potentials. Latent heat flux decreased similarly across species in response to decreasing water potential. Latent heat loss was found to decrease by as much as 65% under decreasing water potential, leading to an increase in leaf temperature of up to 4 °C. Consequently, radiative and sensible heat losses increased under decreasing water potential. Sensible heat flux increased as much as 336% under decreasing water potential. Latent heat loss appeared to be an important mode of temperature regulation in all species, and sensible heat loss appeared to be more important in high marsh species compared to low marsh species. High marsh species are characterized by narrower leaves than middle and low marsh species, leading to a smaller boundary layer, and providing higher conductance to sensible heat loss. This may be an adaptation for high marsh species to regulate leaf temperature without access to large amounts of water for transpirational cooling. Stomatal conductance decreased with decreasing water potential across species: leaf conductances to water vapor and CO 2 decreased as much as 69% under decreasing water potential. Additionally, oxidative stress appeared to increase in these plants during times of drought or salinity stress. Ascorbate peroxidase activities increased with decreasing soil water potential, indicating increased cellular reactive oxygen species. High marsh species had higher ascorbate peroxidase activities compared to low marsh species, indicating higher tolerance to drought- or salinity-induced stresses. It was concluded that different species of marsh grasses are adapted for growth in different zones of salt marshes. Adaptations include biophysical, biochemical, and morphological traits that optimize heat exchange with the environment.

Root turnover and relocation in the soil profile in response to seasonal soil water variation in a natural stand of Utah juniper (Juniperus osteosperma)

Juniper species are noted for long-lived foliage, low and persistent gas exchange activity and drought tolerance. Because leaves and roots of the same species are thought to be similar in structure and life history, we hypothesized that Juniperus osteosperma (Torr.) Little (Utah juniper) fine roots would reflect the persistent aboveground foliage characteristic of this species. We monitored fine roots, less than 1 mm in diameter, by minirhizotron imaging to a depth of 150 cm over two growing seasons from April 2002 to December 2003. We measured fine root numbers, lengths and diameters, and noted the time of birth and death of root segments. We correlated our root data with soil water potential measured by thermocouple psychrometry and ecosystem evapotranspiration measured by ecosystem eddy flux. Median fine root lifespan, determined by the Kaplan-Meier product-limit method, was about one year, much less than foliage lifespan estimates of more than five years. Yet, roots of juniper live much longer than those of other Great Basin species. The median survivorship of shallow and deep roots was 144 and 448 days, respectively. Production of new roots was observed during periods of favorable soil water potential and there was a seasonal progression of increased new roots and root length during the warm season toward lower soil depths with root loss in the upper soil layers. This was also reflected in water extraction which progressed to greater soil depths later in the warm season. Aboveground, rates of ecosystem evapotranspiration decreased with decreasing soil water potentials in a similar manner in both 2002 and 2003, reflecting the relocation of roots to available water at depth. Juniper exhibited a flexible root depth distribution throughout the 20 months of this study, indicating the potential to respond to shifting soil water resources despite long fine root lifespans.

Assessment of C:N Ratios and Water Potential for Nitrogen Optimization in Diesel Bioremediation

ABSTRACT Sandy clay loam soil contaminated with 5000, 10,000 or 20,000 mg/kg of diesel fuel no. 2 was amended with 0 (ambient nitrogen only), 250, 500, or 1000 mg/kg nitrogen (NH4Cl) to evaluate the role of C:N ratios and soil water potential on diesel biodegradation efficacy. The soil was incubated at 25°C for 41 days and microbial O2 consumption measured respirometrically. Highest microbial respiration was observed in the 250 mg N/kg soil treatments regardless of diesel concentration. Higher levels of nitrogen fertilization decreased soil water potential and resulted in an extended lag phase and reduced respiration. Application of 1000 mg/kg nitrogen reduced maximum respiration by 20% to 52% depending on contaminant levels. Optimal C:N ratios among those tested were 17:1, 34:1, and 68:1 for the three diesel concentrations, respectively, and were dependent on contaminant concentration. Nitrogen fertilization on the basis of soil pore water nitrogen (mg N/kg soil H2O) is independent of hydrocarbon concentration but takes into account soil moisture content. This method accounts for both the nutritional and osmotic aspects of nitrogen fertilization. In the soil studied the best nitrogen augmentation corresponded to a soil pore water nitrogen level of 1950 mg N/kg H2O at all diesel concentrations.

Bioremediation of hydrocarbon-contaminated polar soils

Bioremediation is increasingly viewed as an appropriate remediation technology for hydrocarbon-contaminated polar soils. As for all soils, the successful application of bioremediation depends on appropriate biodegradative microbes and environmental conditions in situ. Laboratory studies have confirmed that hydrocarbon-degrading bacteria typically assigned to the genera Rhodococcus, Sphingomonas or Pseudomonas are present in contaminated polar soils. However, as indicated by the persistence of spilled hydrocarbons, environmental conditions in situ are suboptimal for biodegradation in polar soils. Therefore, it is likely that ex situ bioremediation will be the method of choice for ameliorating and controlling the factors limiting microbial activity, i.e. low and fluctuating soil temperatures, low levels of nutrients, and possible alkalinity and low moisture. Care must be taken when adding nutrients to the coarse-textured, low-moisture soils prevalent in continental Antarctica and the high Arctic because excess levels can inhibit hydrocarbon biodegradation by decreasing soil water potentials. Bioremediation experiments conducted on site in the Arctic indicate that land farming and biopiles may be useful approaches for bioremediation of polar soils.

Nutrient amendments for contaminated peri-glacial soils: use of cod bone meal as a controlled release nutrient source

The lack of available nutrients, particularly nitrogen, often limits the rate of microbial petroleum hydrocarbon degradation in contaminated cold region soils. Microbial activity in many peri-glacial soils responds to addition of nitrogen, although excess levels can inhibit biodegradation by decreasing soil water potentials. Aqueous soluble inorganic fertilizer quickly partitions into soil water, increasing the salt concentration, and imposing an osmotic potential. Strategies that can be used to avoid microbial inhibition include the use of controlled release fertilizers. We studied the use of an organic fertilizer, cod bone meal, as a nutrient source for bioremediation. Nitrogen mineralization from cod bone meal was greater at 20 °C (first-order reaction rate constant k=0.0206 d −1) than at 10 °C ( k=0.0154 d −1) and greater at pH 6.5 and 7.5 ( k=0.0208 and 0.0189 d −1, respectively) than at pH 5.5 ( k=0.0143 d −1). Net O 2 consumption from diesel fuel degradation in a contaminated soil was greatly increased by addition of nitrogen and phosphorus in the form of diammonium phosphate (DAP) or cod bone meal relative to unfertilized soil. Cod bone meal fertilized soils had greater net O 2 consumption than DAP fertilized soils. However, residual soil hydrocarbon analyses indicated no difference in petroleum loss between the two nutrient sources.

Sensitivity of life history parameters in the earthworm Aporrectodea caliginosa to small changes in soil water potential

The effects of decreasing soil water potential on cocoon production, cocoon development and growth of juveniles were investigated in the earthworm Aporrectodea caliginosa. The earthworms were subjected to water potentials between −2 and ca. −300 kPa, using two soil types with various wetness. The duration of the drought exposure was 14 days, whereafter the earthworms were returned to ideal moisture conditions. Cocoon production was negatively affected when the water potential was lower than −12 kPa, and below −40 kPa cocoon production was completely arrested. The negative effect on cocoon production persisted for at least 2 weeks after the worms were moved back to soil with optimal moisture conditions. For those worms that were exposed to the most severe drought level, −330 kPa, cocoon production was still significantly impaired two months after drought exposure. Juvenile growth was sensitive to decreasing water potential. Even at −6 kPa the growth of juveniles was significantly lower than in controls and at ca. −20 kPa there was no growth at all. Cocoon development was less sensitive to water potential than growth and cocoon production. Thus, development seemed not to be affected by a 14 day exposure at water potentials above ca. −700 kPa, whereas at higher levels there was an increasing effect. When cocoons were exposed to ca. −3 MPa the development was immediately arrested.

Aluminum diffusion in Oxisols as influenced by soil water matric potential, ph, lime, gypsum, potassium chloride, and calcium phosphate

Plant root exposure to soil aluminum (Al) depends on the soil solution Al concentration and transport to the root by diffusion. Changes in Al diffusive flux for two Oxisols was measured under laboratory conditions as a function of pH, water matric potential, and applications of gypsum, potassium chloride, and calcium phosphate. Double‐faced cation exchange resin sheets served as sinks for Al transported during 10‐day incubations through chambers containing 314 cm3 of soil. Across a range of soil pH values from 4.5 to 5.5, maximum diffusive flux of Al occurred at pH values of 4.7–4.8 in both soils and corresponded to increases of 2.2–3.0% relative to the unlimed treatment. Between pH values of 4.7–4.8 and 5.4, diffusive flux of Al decreased by 38 and 46% in the two Oxisols. Diffusive flux of Al decreased by 16–20% for the two Oxisols as soil water potentials decreased from ‐10 to ‐200 kPa. Magnitude of the reductions in diffusive flux of Al with decreasing soil water potential were less than those reported for diffusive flux of phosphorus (P) in prior investigations. Diffusive flux of Al increased by as much as 4‐fold with additions of CaSO4 and KCl, which increased the soil solution Al concentration. Additions of 400 mg P dm‐3 of soil had no effect on Al diffusion in either Oxisol.