Dry Soil Surface Research Articles

Surface soil moisture and stubble management practice effects on the progress of infection of wheat by Fusarium pseudograminearum

The influence of surface soil moisture and stubble management practices on the progress of infection of wheat by Fusarium pseudograminearum, the cause of crown rot, was assessed in a field trial at Moree in northern New South Wales during the growing seasons of 1994, 1995 and 1996 by analysis of infection progress curves. During the dry season of 1994, wheat was sown into dry surface soil. Increases in incidence of infection followed rainfall events that raised the water content of the surface soil above the equivalent of a water potential of –1.5 MPa. The rate of increase in incidence of infection was more uniform in the 1995 and 1996 seasons, which had more regular rainfall. The area under the infection progress curve (AUIPC) was consistently greater when stubble was retained on the surface compared with incorporation with a disc plough, and this difference was significant in 2 out of 3 years. Comparison of AUIPCs indicated greater epidemiological differences between stubble management treatments than did comparisons of incidence of infection at single points during the season.

Recovery of citrus surface roots following prolonged exposure to dry soil

The effects of prolonged exposure to dry surface soil on the capacity of roots to take up water and phosphorus were examined in mycorrhizal sour orange (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In the first experiment, upper portions of the pots were either irrigated every 2–3 d, droughted for 14 d, droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrigated and fertilized every 2–3 d. Phosphorus uptake capacity was estimated in excised roots using 32P in aerated 50, 750, and 1500 μM P solutions. Exposure to dry soil had no appreciable effect on P uptake capacity. In the second experiment, the ability of intact roots to acquire water and P in the 8 d following rewatering after roots were exposed to localized drought for 14 and 43 d was examined. Roots were observed non-destructively using small transparent tubes (2 cm diameter) and a rigid borescope. Soil water depletion was monitored using time-domain reflectrometry. Phosphorus (32P) was added at various depths in the soil in the upper compartment and uptake was assessed by non-destructively counting beta particle emissions from leaves using a scintillation probe. Similar to the first experiment, localized drought had no effect on P uptake and soil water depletion in citrus roots compared to continuously irrigated plants. Water and P uptake in the first few days apparently occurred from existing roots because of delayed production of new roots in the droughted treatment. Thus, citrus roots exposed to extended periods of dry soil appar ently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an overall rooting strategy where essentially no surface roots are shed following prolonged exposure to dry soil.

Recovery of citrus surface roots following prolonged exposure to dry soil

ently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an The effects of prolonged exposure to dry surface soil overall rooting strategy where essentially no surface on the capacity of roots to take up water and phos- roots are shed following prolonged exposure to dry phorus were examined in mycorrhizal sour orange soil. (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In Key words: Drought, split-root, 32P, citrus, uptake kinetics, the first experiment, upper portions of the pots were phosphorus uptake, water uptake, sour orange, Citrus either irrigated every 2‐3 d, droughted for 14 d, aurantium. droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrig- Introduction ated and fertilized every 2‐3 d. Phosphorus uptake capacity was estimated in excised roots using 32 Pi n The soil surface layers, with their accumulation of roots, aerated 50, 750, and 1500 mM P solutions. Exposure organic matter and nutrients, commonly undergo large to dry soil had no appreciable effect on P uptake fluctuations in temperature, water content, and nutrient capacity. In the second experiment, the ability of intact availability. These fluctuations promote pulses of nutrient roots to acquire water and P in the 8 d following release by increasing microbial biomass turnover (Reid, rewatering after roots were exposed to localized 1974; Stark, 1994; Cui and Caldwell, 1997). Moisture drought for 14 and 43 d was examined. Roots were and temperature fluctuations may cause opportunities for observed non-destructively using small transparent nutrient uptake by roots in the litter, organic and surface tubes (2 cm diameter) and a rigid borescope. Soil water mineral layers to be brief and unpredictable (Grime, depletion was monitored using time-domain reflectro- 1994). There are at least two basic strategies for exploiting metry. Phosphorus (32P) was added at various depths resources in soil surface layers that periodically dry and in the soil in the upper compartment and uptake was which may develop high temperatures. One strategy is to assessed by non-destructively counting beta particle grow rapidly relatively thin inexpensive roots that have emissions from leaves using a scintillation probe. high absorptive capacity when the soil is wet, and then Similar to the first experiment, localized drought had rapidly shed these roots when the soil becomes dry or no effect on P uptake and soil water depletion in citrus otherwise unfavourable. This kind of root strategy is roots compared to continuously irrigated plants. Water commonly found in hot desert climates with desert succuand P uptake in the first few days apparently occurred lents (Huang and Nobel, 1992) and in certain desert from existing roots because of delayed production of half-shrubs like Cryptantha (personal observation). In new roots in the droughted treatment. Thus, citrus addition, many annual crops seem to exhibit this strategy ( Eissenstat and Yanai, 1997). roots exposed to extended periods of dry soil appar

The impact on area‐averaged heat fluxes from using remotely sensed data at different resolutions: A case study with Washita '92 data

Landscape‐scale fluxes are derived using an energy balance model with remotely sensed input data at different pixel resolutions. The model explicitly evaluates energy flux contributions from the soil and vegetation using remotely sensed near‐surface soil moisture and normalized difference vegetation index (NDVI) to define key model variables. The remotely sensed data used in the model were collected as part of the Washita '92 Experiment conducted in the Little Washita watershed, a subhumid catchment in southwest Oklahoma. Near‐surface soil moisture maps covering approximately an 800 km2 area at 0.2 km resolution were generated using aircraft‐based passive microwave observations from the L band electronically scanned thinned array radiometer (ESTAR). A spatially distributed NDVI map was derived from a SPOT satellite image. Daily values of the midday Bowen ratio (BO, ratio of sensible to latent heat flux) computed by the model indicated that areas with low vegetation cover or bare soil and senescent vegetation were drying out significantly (i.e., dramatic increases in BO) while other areas with higher vegetation cover showed smaller increases in BO in response to a drying soil surface. After a few days of drying, BO values computed by the model ranged from ∼0.1 to values ≳2 over the image. The resulting spatially distributed maps of BO suggested a heterogeneous surface at the field or patch scale. A satellite‐based L band ESTAR would have a pixel resolution on the order of 101 km or larger (i.e., landscape scale). Since fluxes are nonlinearly related to model input variables/parameters, defining model inputs at resolutions significantly larger than the patch scale may cause significant errors in flux calculations. Potential errors in flux calculations were investigated by evaluating differences in area‐averaged flux estimates for the ∼18 × 45 km study area using the model with the remotely sensed near‐surface soil moisture and NDVI data at the following pixel resolutions: (1) 0.2 km (original pixel resolution), (2) 9 km, and (3) the whole image, which has an effective pixel resolution on the order of 28 km. Differences in the area‐ averaged sensible and latent fluxes computed by the model at the three resolutions were within 10%. Such minor discrepancies in the area‐average fluxes suggests that the heterogeneity in soil moisture and vegetation type and cover was not of the type to affect regional flux predictions using significantly coarser resolution information. There was a consistent decrease in area‐averaged latent heat flux estimates at the coarser resolution caused by biases in area‐averaged values of the other three energy flux components. The consistent decrease in area‐average latent heat flux caused the area‐averaged midday BO to increase with decreasing pixel resolution resulting in ≈15% increase from 200 m to using the full image. These results, however, depended on the method used in aggregating leaf area index to the coarser resolutions, indicating that techniques for scaling up key model parameters have a significant effect on area‐average flux predictions.

Mapping surface energy flux partitioning at large scales with optical and microwave remote sensing data from Washita '92

A model evaluating the energy balance of the soil/substrate and vegetation (i.e., two‐source) was applied to remotely sensed near‐surface soil moisture maps generated from passive microwave data collected during the Washita '92 experiment. Model parameters were derived from a soil texture and a land‐use/land cover database along with a normalized difference vegetation index map created from a SPOT satellite image. The Bowen ratio (BO, ratio of sensible to latent heat flux) was used for investigating the temporal and spatial variability in model output. Comparisons between predicted and observed heat fluxes were made with values summed over the daytime period. Daily maps of midday BO indicated areas with low vegetation cover or bare soil and senescent vegetation were drying out significantly (i.e., dramatic increases in BO), while other areas with higher vegetation cover showed smaller increases in BO in response to a drying soil surface. This result agrees with the profile soil moisture and surface flux observations indicating adequate moisture was available to the vegetation for meeting atmospheric demand. The predicted daytime fluxes agreed to within 1 mm of the observations with ≈25% difference between modeled and observed daytime evapotranspiration. Differences between modeled and measured surface temperatures averaged ≈2 K. The discrepancies between model output and observations are similar to the uncertainty in these measurements, indicating that the model provided reliable daytime energy flux maps for the Washita '92 study area using remotely sensed near‐surface soil moisture.

Mechanisms of evaporation from soil with a dry surface

Evaporation from bare soil proceeds in three stages as the soil surface dries. The formation of a dry surface layer (DSL), where water moves only in the vapour phase, is the most remarkable and symbolic feature of the last stage, and the following phenomena, observed recently, seem to be closely related to it: (a) humidity inversion across the soil surface. Water vapour density increases sharply from just under the dry surface to just above it in the daytime on fine days; (b) humidity inversion in the surface boundary layer. The mixing ratio of water vapour increases with height, at least, in the 1 to 20 m layer in desert areas on fine days; (c) counter-gradient flow of water vapour. Evaporation occurs from soil with a dry surface under the humidity inversion conditions, so that water vapour is transported against its density gradient near the surface in the daytime on fine days. A hypothesis is proposed to explain these phenomena, such that hot, dry soil surfaces produce dry air, or the humidity inversion and make the water vapour moving upwards in the DSL go through the surface against its density gradient by thermal diffusion. The hot, dry air which is made and trapped in the uppermost soil pores can stay there under moist boundary air layers with lower temperatures. When instability occurs, however, the trapped hot, dry air bursts out like jets and the humidity inversion across the surface is transported upwards by convection, by which means humidity inversions are made and kept in the surface boundary layer. 1998 John Wiley & Sons, Ltd.

An improved methodology to measure evaporation from bare soil based on comparison of surface temperature with a dry soil surface

Accurate estimation of the resistances to water vapor movement is a major difficulty in evaluating evaporation from soil. By including the temperature of a dry soil surface (the temperature of the surface of a dry soil column buried in the field), a method for estimating evaporation from soil is proposed. The necessary input variables for the suggested method are temperature, net radiation, and soil heat flux. There are three advantages of the proposed method over the conventional methods. First, soil surface resistance and aerodynamic resistance are not required. Second, the variables included are fewer. Third, measurement and analysis of the parameters involved are relatively easy. Sensitivity analysis shows that the suggested method is sensitive to temperatures. Test experiments were conducted in a sandy field, where a weighing lysimeter was installed. Evaporation from soil, together with the variables specified above, were measured. For temperatures measured by thermocouples, experimental results showed that the mean absolute error (MAE) for the daily evaporation over 22 days was 0.17 mm day1. The regression between calculated and measured evaporation was highly significant ( r 2 = 0.89). Moreover, the intercept and slope of the regression equation were not significantly different from zero and unity, respectively, at the 0.05 probability level. Furthermore, by using the temperatures measured by infrared thermometers, the MAE between measured evaporation and estimated evaporation was 0.15 mm day −1. The regression between them was highly significant ( r 2 = 0.94) In addition, the intercept and slope of the regression equation were not significantly different from zero and unity, respectively, at the 0.05 probability level. These results show that evaporation calculated using the proposed method is in good agreement with lysimeter measured values. By comparing with the temperature difference method, it was shown that the suggested method estimated soil evaporation more accurately than the temperature difference method. Therefore, it is concluded that the proposed method is not only a simple way for application, but also an accurate way to estimate soil evaporation.

Photodegradation of mecoprop and dichlorprop on dry, moist and amended soil surfaces exposed to sunlight

The effects of environmental conditions on the photodecomposition of two phenoxy-alkanoic acid herbicides were investigated on different soil surfaces conditions under sunlight exposition. A technique has been developed to study this process on moist nonsterile soil surfaces. A slow rate of disappeareance occurs in the three soils exposed to sunlight in absence of water. Photolysis of these herbicides is controlled by soil texture and its adsorption capacity. The organic matter does not have a sensitizing effect on the photodecomposition of these herbicides on dry soil surfaces. On nonsterile moist soil surfaces exposed to sunlight, the photolytic process prevails in the two first days of exposure and the transformation kinetics fits the Hoerl function better than the first-order exponential equation.

Phosphorus transformations in a ferralsol through ingestion by Pantoscolex corethrurus, a geophagous earthworm

This paper analyses the transformations that result in increased inorganic phosphorus (Pi) content in a ferralsol after transit through the gut of the earthworm Pontoscolex corethrurus. Total, inorganic and organic P contents were measured in bulk samples of dry soil, incubated and non-ingested soil and surface casts, and in chemical extracts on samples previously separated into three particle-size fractions (> 20, 2–20 and < 2 μm). The total P content was the same in control, non-ingested soil and casts but remoistening and transit through the earthworm gut decreased organic P and increased inorganic P contents. Clay content in surface casts (19.7 %) was significantly higher than in the control soil (9.1 %). Phosphate content significantly increased in the NaHCO 3 extract from 3.17 in non-ingested soil to 5.04 ug Pi·g −1 soil in casts; the same trend was observed in NaOH extract with 27.4 in noningested soil and 45.8 μg Pi·g −1 soil in casts. Phosphate in casts was mostly concentrated in the finest fractions (0–2 μm) with 83 % of extracted phosphate, opposed to 61 % in non-ingested soil. These changes resulted from a combination of selective ingestion of small particles and partial mineralisation of organic P.

Citrus root responses to localized drying soil: A new approach to studying mycorrhizal effects on the roots of mature trees

Because fine roots tend to be concentrated at the soil surface, exposure to dry surface soil can have a large influence on patterns of root growth, death and respiration. We studied the effects of arbuscular mycorrhizas (AM) formation on specific root length (SRL), respiration and mortality of fine roots of bearing red grapefruit (Citrus paradisi Macf.) trees on Volkamer lemon (C. volkameriana Tan. & Pasq.) rootstock exposed to drying soil. For each tree, the fine roots were removed from two woody lateral roots, the roots were surface sterilized and then each woody root was placed in a separate pair of vertically divided and independently irrigated soil compartments. The two split-pot systems were filled with sterilized soil and one was inoculated with arbuscular mycorrhizal fungi (Glomus etunicatum/G. intraradices). New fine lateral roots that emerged from the woody laterals were permitted to grow inside the pots over a 10-month period. Irrigation was then removed from the top compartment for a 15-week period. At the end of the study, roots inoculated with AM fungi exhibited about 20% incidence of AM formation, whereas the uninoculated roots were completely void of AM fungi. Arbuscular mycorrhizal roots exhibited lower SRL, lower root/soil respiration and about 10% lower fine root mortality than nonmycorrhizal roots after 15 weeks of exposure to dry surface soil. This study demonstrates the feasibility of examining mycorrhizal effects on the fine roots of adult trees in the field using simple inexpensive methods.

THE EFFICIENCY OF THE STRATEGY OF MUCILAGINOUS SEEDS OF SOME COMMON ANNUALS OF THE NEGEV ADHERING TO THE SOIL CRUST TO DELAY COLLECTION BY ANTS

Groups of dry seeds of four annual plant species which occur in the Negev highlands were placed on a natural, dry or wet loess soil crust surface near Sede Boker on the Zin plateau during the autumn before the first rains, and on the first day with rain (1.15 mm). Ant nests ofMessor rugosuswere 8 to 14m from the experimental plot. The length of time it took these ants to collect the free or adhered seeds was observed. When the mucilaginous ombrohydrochoric seeds ofAnastatatica hierochuntica, Plantago coronopus, andCarrichtera annuaadhere to wet soil that remains moist, most of the seeds may have time to germinate in proper conditions before they are collected by ants. However, all but 5% of theReboudia pinnataseeds were collected within 2 h. The adhered seeds that had been moistened by wet soil crust and then dried, were collected by ants, in most cases, faster than when seeds and soil remained moist. Within 2 h none of the dry and free seeds situated on the dry soil surface remained. The first free seeds were collected after 7 min. Findings are discussed together with the mechanisms and strategies involved in seed dispersal by rain and germination of these plant species.

Spatially Distributed Sensible Heat Flux over a Semiarid Watershed. Part II: Use of a Variable Resistance Approach with Radiometric Surface Temperatures

Radiometric surface temperature images from aircraft observations over the Walnut Gulch Experimental Watershed, a semiarid rangeland watershed, were used with ground-based meteorological data at a reference site for extrapolating estimates of surface sensible heat flux across the basin. Two approaches were used. One method assumed that the resistance to heat transport and other meteorological data at a reference site were constant over the watershed. This resulted in a simple scheme (constant resistance approach) for computing spatially distributed sensible heat flux since the variation in sensible heat flux was directly proportional to surface temperature differences from the reference site. The second approach (the variable resistance approach) used spatially distributed estimates of the surface roughness for momentum and heat, as well as air temperature and wind speed. The sensible heat flux values derived by both techniques were compared to measurements made at several other locations in the watershed for three different days. The environmental conditions for these days ranged from uniformly dry surface soil moisture to variably wet conditions caused by several high intensity and spatially variable rainfall events. Comparisons between these two schemes with observations indicated that the more detailed method of accounting for changes in surface roughness over the basin gave significantly better agreement than the simpler scheme. The average percentage of difference with measured values was 30% for the constant resistance approach compared to approximately 20% for the variable resistance method.

EFFECT OF WATER CHEMISTRY AND SOIL AMENDMENTS ON A SILT LOAM SOIL—PART 2: SOIL EROSION

This research examined the effect of rainfall and runoff water composition, as well as the effect of differentsoil surface amendments on infiltration, runoff, and soil loss from a silt loam soil which was highly susceptible toaggregate breakdown and sealing. This article describes the soil erosion results, while infiltration and runoff findings arepresented in a companion article. The purposes of this study were to test if soil erosion would be reduced by the presenceof electrolytes in the erosive water, if use of an anionic polyacrylamide (PAM) as a surface spray or in rain water wouldreduce soil loss, and what the combined effects of high electrolyte concentrations and PAM would be. The impacts of asoil surface application of 20 kg/ha of an anionic PAM as well as 5 t/ha of a gypsiferous power-plant byproduct on soilloss were determined. Water type applied as rainfall or inflow was deionized, tap, or tap plus 10 ppm of PAM. The soilsurface applications of the PAM were effective in controlling rill erosion, even at very high water inflow levels. Rill plotsediment discharge rates from deionized rainfall water applications to an initially dry soil surface were significantlygreater than those resulting from rainfall having greater electrolyte concentrations. However, when inflow water wasadded to actively eroding rills, electrolyte concentration had no significant effect on sediment concentration or sedimentdischarge rate. The presence of PAM in rain water and inflow water acted to enhance soil loss when sufficient runoff waspresent to transport sediment from the rills.

Responses of the Fine Roots of Citrus to Dry Surface Soil

Despite the frequent occurrence of hot, dry surface soil, little is known about the fate of roots in this soil layer during and following periods of surface drought. Phosphorus uptake kinetics were examined in surface roots of citrus seedlings exposed to different periods of drought. Exposure to dry soil for up to 43 days had no effect on phosphate uptake of excised roots measured at solution concentrations of 50, 750, and 1500 μm phosphate. Effects of surface drought on fine root behavior of seedlings and bearing trees were examined in the field using a split-pot arrangement. At the end of 5 weeks of drought, only about 1% of the roots in the top pot of either the seedling or tree had died. By 8 to 9 weeks of drought, about 26% of the fine laterals of the adult citrus had died, but only 6% had died in the seedling. After 15 weeks of drought, 33% of the mature tree fine roots had died but only 8% had died in the seedling. Root-soil respiration per gram root at this time was at least twice as high in the seedling as in the mature tree. Fine roots of citrus are remarkably tolerant to dry surface soils of about 5 weeks duration, both in terms of root survival and resumption of physiological activity after rewetting.

Morphological, physiological, and molecular characterization of actinomycetes isolated from dry soil, rocks, and monument surfaces.

In an extended study on the biodiversity of rock-dwelling bacteria, the colony and cell morphology, physiology, protein patterns, and 16S rDNA sequences of 17 bacterial strains isolated from different surfaces of rocks, stones, and monuments and from various geographical locations were characterized. All except one strain, which was found to be a Bacillus, were members of the order Actinomycetales. The majority of the strains either were closely related to Geodermatophilus obscurus, which was also analyzed in this study, or formed a closely related sister taxon. All of these strains were isolated from the surface of marble in Namibia and Greece and from limestone from the Negev desert, Israel. One strain, G10, of Namibia origin was equidistantly related to Geodermatophilus obscurus, Frankia alni, Sporichthya polymorpha, and Acidothermus cellulolyticus. Three strains from rock varnish in the Mojave desert, California, were found to be highly related to Arthrobacter (formerly Micrococcus) agilis. All clusters could be confirmed from results of studies on morphological and physiological properties and from banding patterns of whole cell proteins. Based on the results of tests, four additional strains were assigned to the lineage defined by strain G10.

Respirable‐Dust Production from Agricultural Operations in the Sacramento Valley, California

AbstractAir quality in California's intensively farmed Central Valley violates regulatory particulate matter limits nearly year‐round. Little is known about the amount of fugitive dust produced by individual agronomic operations. This study measured the respirable fraction, covering particles ≤4 µm in aerodynamic diameter and the influences of climate, soil, and implement conditions on dust production. Respirable dust was collected at the implement from 18 cultural operations for a complete annual crop cycle on 22, 4097‐m2 plots, 11 km west of Davis, CA. Plots included conventional and organic corn (Zea mays L.) and tomato [Lycopersicon lycopersicum (L.) Karsten] fields as well as fields with a winter cover crop or fallow plots followed by wheat (Triticum aestivum L.). The 266 samples were collected with standard cyclone samplers mounted directly on each implement as the dust source (average height of 94 cm above ground). Sampling near the dust source measured the relative respirable‐particulate‐matter production from each operation. The highest average respirable‐dust concentration was 10.3 mg/m3 air from soil ripping and land planing, carried out on dry surface soil. The lowest dust concentration was from disking of corn stubble (0.3 mg/m3) into soil during the wet season. Approximately 64% of all operations were performed during hot and dry weather, producing 83% of the annual respirable dust for this three‐crop system. Among the measured environmental and implement conditions, air temperature, soil moisture, wind speed, and tractor speed were significantly correlated with the dust concentration in an exponential model.

Polyacrylamide Effect on Infiltration and Erosion in Furrows

Surface sealing and crusting are important factors affecting runoff and erosion in many cultivated soils. The objective of this study was to evaluate the effects of low rates of surface-applied anionic polyacrylamide (PAM) on surface sealing and crusting, water infiltration, and soil erosion. A series of experiments was conducted on a Cecil sandy loam soil (clayey, kaolinitic, thermic Typic Kanhapludult) in ridge-furrow-type field plots (3.5 by 0.92 m). Polyacrylamide solutions containing 1 kg solids m−3 and 2.5 mol CaSO4 m−3 were sprayed onto dry soil surfaces at 15 and 30 kg ha−1 rates with two replicates each. Three simulated rains, 31 min each, were applied at an intensity of 85 mm h−1 at 2-wk intervals. Final infiltration rates were 30 mm h−1 for the control treatment for all three rains and were >85 mm h−1 for the first rain and 45 mm h−1 for the two following rains for the PAM treatments. Total soil loss of the control was 1.62 kg m−2 for the first rain, while the average soil loss of the two PAM treatments was only 0.03 kg m−2. For two subsequent rains, although the differences were narrowed, soil loss from PAM was 48 to 66% less than from the control. The percentage of sediments >0.5 mm in the PAM treatments was 45% greater than in the control, indicating high aggregate stability with PAM addition. The sediment reduction was attributed to the significant reduction of runoff and the prevention of rill formation due to PAM addition. Based on this study, surface application of PAM at a rate of 15 kg ha−1 controlled surface sealing and crusting and therefore reduced runoff and erosion.

Spatial and temporal patterns of soil erosion rates in an agricultural catchment, central Belgium

This paper presents results on the temporal and spatial variability of the erosional system in two adjacent cultivated catchments located in the Belgian Loam belt. Annual catchment erosion rates, calculated for the whole catchment area, range between 1 and 15 m 3/ha · y. Mean annual catchment erosion rates calculated for a three year period, were 5.4 m 3/ha · y and 8.2 m 3/ha · y despite relatively low slope gradients. About 60 to 70% of the total soil loss due to rill and ephemeral gully erosion occurred in late spring and early summer. This is attributed to the higher frequency of high-intensity low-frequency storms falling on a very dry soil surface in late spring and early summer. The relative importance of ephemeral gully erosion is strongly influenced by the season: mean ratio of ephemeral gully erosion rates to rill erosion rates equals ca. 1.7 in winter and ca. 1.0 in summer. Ephemeral gully erosion in topographically defined flow paths is the dominant sediment source and represents 41% of the total soil loss. During the survey period, about 90% of the material eroded upstream was transported outside the catchment. As a result, villages situated downstream were severily damaged by flooding and major sedimentation events.

Dissipation and Water Activation of UCC-C4243

Field experiments were conducted in 1992 and 1993 to determine the timing and amount of rainfall required to activate UCC-C4243 applied preemergence. UCC-C4243 at 0, 70, and 140 g ai ha−1was applied 1, 7, 14, and 21 d before 0.5 or 2 cm of simulated rainfall. Temporary rainshelters protected field plots from natural rainfall during the 21 d dry period. Herbicide activity was determined in the field by seeding lentil, wheat, common lambsquarters, and field pennycress and in the greenhouse by a sugarbeet bioassay of soil samples (0 to 3 cm depth) taken from all plots immediately before irrigation. UCC-C4243 did not injure wheat; however, lentil population was reduced when simulated rainfall occurred within 7 d after application. Lentil injury was greater with higher herbicide rate and higher water level. UCC-C4243 at 70 and 140 g ha−1reduced populations of both weed species by 75 and 90%, respectively, when either 0.5 or 2 cm simulated rainfall was received within 1 d after herbicide application. Weed control was reduced with a 21 d delay between herbicide application and water activation. The sugarbeet bioassay showed a linear decrease of herbicide activity over time and also with accumulated photosynthetically active radiation. After 17.9 d, herbicide activity on a dry soil surface decreased 50%. Laboratory investigations show that [14C]-UCC-C4243 on glass slides was photodegraded by near ultraviolet light (290 to 400 nm). Volatilization of14C-labeled herbicide from glass slides was less than 5% after exposure to turbulent air for 48 h.

Root growth and nitrate uptake by wheat (Triticum aestivumL.) following wetting of dry surface soil

The nitrate uptake capacity of surface roots of spring wheat (Triticum aestivum L. cv. Kulin) was investigated following wetting of dry surface soil. Plants were grown to stem elongation stage with adequate watering at depth while the surface soil was allowed to dry. Eight weeks after sowing, water or a l5 N-nitrate solution was added to the surface soil to simulate rainfall. Root growth and nitrate uptake were measured up to 4 d after wetting on plants with unconfined nodal root growth and on plants with the majority of nodal roots confined within small vials. Prior to wetting, plants from both nodal treatments had seminal roots with collapsed cortices along the upper 10 cm and many short, viable lateral roots. Nodal roots, where present, were only a few cm long and unbranched. Only plants with unconfined nodal roots were able to take up nitrate within the 24 h before any new root growth. By 2 d after wetting there was significant growth of the seminal lateral roots, and rapid growth and branching of nodal roots. From 2d after wetting, plants with confined nodal roots also took up nitrate, presumably due to the growth of the seminal lateral roots. Hence it appears as though the nodal roots in the unconfined treatment could immediately take up nitrate, but the seminal roots required new lateral root growth to become active in nitrate uptake. The plants with confined nodal roots had a lower nitrate uptake than those with unconfined nodal roots 4d after wetting, indicating that the seminal root system was not able to compensate for lack of nodal roots. Insufficient nitrate was taken up after 4 d, by plants from either nodal root treatment, to increase the shoot N concentration significantly.