Effect of the initial soil water saturation on the behaviour of a mixed LNAPL and heavy metal contaminated glaciofluvial deposit

Soil contamination by mixtures of petroleum hydrocarbons and heavy metals is common in urban and industrial localities. Interactions between these contaminants have an impact on the mobility and the management of contamination. We have characterized the modifications to the transport of heavy metals (Cd, Cu, Pb, Zn) in soil induced by residual light non‐aqueous phase liquid (LNAPL) for two conditions of trapping. Experiments on the elution of tracers and heavy metals in columns of soil were performed with a glaciofluvial material as the soil. Tracer experiments were modelled with the mobile–immobile (MIM) system of partial differential equations. The experiments were designed to compare water flow and metal transport in LNAPL‐contaminated soil with a control set. Residual LNAPL was trapped in water‐saturated and dry soil to ensure preferential wettability of soil surfaces, namely either water‐wet or LNAPL‐wet. In water‐wet soil columns, LNAPL decreased water flow by two orders of magnitude and increased the fraction of immobile water. Solute residence times (SRTs) suggested that heavy metals resided mainly in mobile water where the reaction time was sufficient to reach steady‐state retention. The SRTs also indicated that a fraction of the heavy metal flux diffused to the immobile water where its retention was limited by diffusion. Retention of heavy metals was significantly greater than in the control columns. In LNAPL‐wet soil columns, the obstruction of small pores and surface coating by residual LNAPL significantly decreased the attenuation capacity of the soil by decreasing the diffusion of heavy metals to immobile water and surface sites. Evidently, the individual behaviour of heavy metals can be significantly modified by non‐miscible organic contaminants. These modifications can have important implications for risk evaluation, contamination management and in situ remediation of soil that is contaminated by mixtures of petroleum hydrocarbons and heavy metals.     

Shrinkage of initially very wet soil blocks, cores and clods from a range of European Andosol horizons

In advanced stages of volcanic ash soil formation, when more clay is formed, soil porosity values and soil water retention capacities are large and the soils show pronounced shrinkage on drying. Soil shrinkage is a key issue in volcanic soil environments because it often occurs irreversibly when topsoils dry out after changes from permanent grassland or forest to agriculture. European Andosols have developed in a wide range of climatic conditions, leading to a wide range in intensity of both weathering and organo‐mineral interactions. The question arises as to whether these differences affect their shrinkage properties. We aimed to identify common physically based shrinkage laws which could be derived from soil structure, the analysis of soil constituents, the selected sampling size and the drying procedure. We found that the final volumetric shrinkage of the initially field‐wet (56–86% of total porosity) or capillary‐wet (87–100% of total porosity) undisturbed soil samples was negatively related to initial bulk density and positively related to initial capillary porosity (volumetric soil water content of soil cores after capillary rise). These relationships were linear for the soil clods of 3–8 cm³, with final shrinkage ranging from 21.2 to 52.2%. For soil blocks of 240 cm³ and soil cores of 28.6 cm³ we found polynomial and exponential relationships, respectively, with thresholds separating shrinkage and nearly non‐shrinkage domains, and larger shrinkage values for the soil cores than for the soil blocks. For a given sample size, shrinkage was more pronounced in the most weathered and most porous Andosol horizons, rich in Al‐humus, than in the less weathered and less porous Andosol horizons, poor in Al‐humus. The Bw horizons, being more weathered and more porous, shrank more than the Ah horizons. We showed that the structural approach combining drying kinetics under vacuum, soil water analysis and mercury porosimetry is useful for relating water loss and shrinkage to soil structure and its dynamics. We also found that the more shrinkage that occurred in the Andosol horizon, the more pronounced was its irreversible mechanical change.     

Nitrate Sorption in a Mexican Allophanic Andisol using Intact and Packed Columns

Abstract

Contamination of groundwater by nitrate is a worldwide environmental issue. A better knowledge of nitrate sorption characteristics by soils contributes to efficient fertilizer use and prevents aquifer contamination. In volcanic soils, nitrate sorption is induced by variable charges due to the presence of amorphous materials and aluminum (Al) and iron (Fe) oxides. Anion transport in packed and intact columns was investigated in a Mexican Allophanic Andisol, under different permanent flow regimes in unsaturated conditions and several NO₃ ^?‐N and Br^? input concentrations. In the packed columns, the NO₃ ^?‐N adsorption in the soil was nonlinear. In the intact columns, the retardation coefficient variation was directly correlated to the increase of amorphous material with depth. The presence of preferential flow in the intact columns significantly increased the mobility and velocity of nitrate moving through the columns, whereas in the packed columns, NO₃ ^?‐N fate was only affected by soil chemical composition and mineralogy.     

Soil‐aggregate formation as influenced by clay content and organic‐matter amendment

Naturally occurring wetting‐and‐drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic‐matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting‐and‐drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha^–1, 6.2 t ha^–1, and 12.4 t ha^–1) on the development of water‐stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water‐stable aggregates in those soils with 34% and 38% clay. Repeated wetting‐and‐drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha^–1 and 12.4 t ha^–1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha^–1 were required. However, rapid wetting of straw‐amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement.     

Different responses in bulk density and saturated hydraulic conductivity to soil deformation by logging machinery on a Ferralsol under native forest

Ferralsols have high structural stability, although structural degradation has been observed to result from forest to tillage or pasture conversion. An experimental series of forest skidder passes in an east Amazonian natural forest was performed for testing the effects of mechanical stress during selective logging operations on a clay‐rich Ferralsol under both dry and wet soil conditions. Distinct ruts formed up to 25 cm depth only under wet conditions. After nine passes the initially very low surface bulk density of between 0.69 and 0.80 g cm^?3 increased to 1.05 g cm^?3 in the wet soil and 0.92 g cm^?3 in the dry soil. Saturated hydraulic conductivities, initially >250 mm h^?1, declined to a minimum of around 10 mm h^?1 in the wet soil after the first pass, and in the dry soil more gradually after nine passes. The contrasting response of bulk density and saturated hydraulic conductivity is explained by exposure of subsoil material at the base of the ruts where macrostructure rapidly deteriorated under wet conditions. We attribute the resultant moderately high hydraulic conductivities to the formation of stable microaggregates with fine sand to coarse silt textures. We conclude that the topsoil macrostructure of Ferralsols is subject to similar deterioration to that of Luvisols in temperate zones. The stable microstructure prevents marked compaction and decrease in hydraulic conductivity under wetter and more plastic soil conditions. However, typical tropical storms may regularly exceed the infiltration capacity of the deformed soils. In the deeper ruts water may concentrate and cause surface run‐off, even in gently sloping areas. To avoid soil erosion, logging operations in sloping areas should therefore be restricted to dry soil conditions when rut formation is minimal.     

Determination of chemical transport properties for different textures of undisturbed soils

Agrichemicals usually contaminate groundwater via preferential flow, therefore determination of the preferential flow characteristics of soil is needed. One model that predicts solute transport due to preferential flow is the mobile–immobile (MIM) solute-transport model, which partitions total water content (θ; m³ m^?3) into mobile (θ_m) and immobile fractions (θ_im). In undisturbed soils, a method is proposed for determining the MIM model parameters, i.e. immobile water fraction (θ_im), mass transfer coefficient (α) and hydrodynamic dispersion coefficient (D _h). Breakthrough curves were obtained for five different soil textures in three replicates, by miscible displacement of Cl^? in undisturbed soil columns. Cl^? breakthrough curves were evaluated in terms of the MIM model. Analysis suggests that the values of D _h and α increased with lighter soil textures and θ_im increased with heavier soil textures. The values of θ_im ranged from 5.31 to 14.28% in different soil textures. Furthermore, values of θ_im were found to be related to soil clay content. Values of α ranged from 0.0257 to 0.32 h^?1 and values of D _h ranged from 0.36 to 11.2 cm² h^?1 in different soil textures. A significant linear correlation was obtained between α, θ_im, D _h and soil saturated hydraulic conductivity (K _s) and pore water velocity (v). A multivariate pedotransfer function was developed to estimate α, θ_im and D _h based on the geometric mean (d _g) and the standard deviation (σ_g) of the diameter of soil particles and soil organic matter content. The pedotransfer functions for D _h, θ_im and α were validated by independent data sets from other investigators.     

The utility of Burns’s equation to describe solute movement through soil under various boundary and initial conditions

Burns’s equation for describing solute movement through soil is attractive because it is simple and predicts adequately in many instances. However, the assumptions implicit in it are not inconsistent with preferential solute flow. We have explored the consequences of this by leaching initially resident chloride and surface-applied tritium and nitrate through 250-mm-long intact cores of a silt loam soil. The applied flow rates of 3 and 5 mm h^?1 (realistic rainfall intensities) produced unsaturated soil conditions, except near the base where free water dripped out. Burns’s equation described the movement of the three solutes fairly successfully with the water content parameter having values between 0.29 and 0.48, similar to the actual volumetric water content of 0.47. The leaching of resident chloride to 450-mm-deep mole drains in the field was also successfully simulated using Burns’s equation. However, simulation of the leaching of bromide applied to the soil surface as a solid salt was problematic. This resulted from uncertainty as to whether to treat the application as a pulse input to the flux or resident concentration. The observed behaviour fell about midway between the simulations for these contrasting initial conditions.     

Movement of nitrogen‐15 labeled nitrate in large undisturbed columns of poorly drained soil

Abstract

A laboratory study was conducted with large (20‐cm i.d., 110‐cm long PVC pipe) intact soil columns to determine the movement of fertilizer NO₃ ^‐ in poorly drained, conventionally tilled soil under simulated low (7.6 cm) and heavy (15.2 cm) rainfall. Soil in the columns was brought to near‐maximum water‐holding capacity (9 kPa) to simulate the typical field soil moisture regime during the spring. A constant‐level water table was imposed at the base of the column to further simulate field conditions of the Drummer silty clay loam (mixed, mesic, Typic Haplaquoll) soil used. Fertilizer was applied in solution at a rate equivalent to 168 kg N ha^‐1 as ¹⁵N‐labeled KNO₃. Water was then applied in three applications, spaced one wk apart. To minimize the movement of water along the soil‐pipe interface, a 3 mm‐wide band of air‐dried disturbed soil was packed around the core to ensure a seal along the interface. Recovery of fertilizer NO₃ ^‐‐N below the water table at the end of the 28‐d study was < 0.06% (0.1 kg N ha^‐1) and 0.5% (0.9 kg N ha^‐1) of that applied for the low and high treatments, respectively. Denitrification losses were negligible for both water treatments (≤ 1 kg N ha^‐1). Fertilizer N distribution in the columns indicated significant movement of N beyond estimated water‐displacement depths, apparently caused by preferential flow. However, the majority of the N was restricted to the upper portions of the columns. The results indicate that preferential flow of water in poorly drained, conventionally tilled soils during high rainfall periods can lead to the movement of fertilizer N to shallow ground water, but that the amounts are apparently very small.     

Evaluation of leaching and runoff losses of selenium from seleniferous soils through simulated rainfall

Experiments were conducted to study drainage and runoff losses of selenium (Se) from two seleniferous soils (from Simbly containing total Se 850 μg [kg soil]^–1 and from Barwa containing 1310 μg [kg soil]^–1) under simulated rainfall (250–260 mm in three rainstorms) conditions. Rainfall intensities ranged from 56 to 120 mm h^–1 with uniformity coefficients ranging from 70.6% to 84.2%. Selenium lost through drainage (sum of drainage from initially saturated soil for 24 h and through dry and wet runs) was 0.15% and 0.11% of total Se content in the two soils. In soils having similar pH and organic‐C content, losses of Se through drainage as well as runoff were defined by total Se, water‐soluble Se, CaCO₃ content, and texture of the soils. The amount of runoff water was almost two times in the soil with fine texture and less infiltration rate than in the other and that same trend was observed with respect to loss of sediment. The soil with higher CaCO₃ content and water‐soluble Se lost more Se with moving water both through leaching and runoff, whereas the other soil with fine texture lost greater amount of Se with the sediment. Total Se lost through drainage as well as runoff was 0.29% of the native Se present in both the soils suggesting that significant amount of Se could be lost from seleniferous soils during irrigation and rainfall events.     

The use of gypsum for preventing soil sodification: effect of gypsum particle size and location in the profile

Laboratory studies were conducted on a mixture of surface soils from the Nile Delta (Egypt). Twenty-two soil columns, initially saturated both with respect to their water-holding capacity and to their base exchange capacity with calcium, contained 0.0, 0.25, 0.5, 1.0 and 2.0 per cent solid gypsum in the total weight of the solid material in the column. Three particle sizes of gypsum (>0.5, 0.5–1 and 1–2 mm) were mixed either with the top layer or with the whole soil column. The result of leaching these columns with saline water (36 meq 1^?1 NaCl+4 meq 1^?1 CaCl₂) at 0.1 cm h^?1 was compared with a mathematical model based on thermodynamic equilibria. The three different particle sizes gave the same experimental results. Applying a given amount of gypsum to the surface soil was more effective in reducing the exchangeable sodium percentage (ESP) than mixing the same quantity through the soil. The mathematical model adequately predicted the changes in the soil column.     

Macropore flow estimations under no-till and till systems

The processes associated with water movement through silt loam soils involve both the flow through macropores as preferential flow or macropore flow and flow through the micropore as matrix flow. Macropore and matrix flow components were separated from total flow by a hydrograph-separation technique which used the assumption of dual porosity and a tracer mass balance. A mixture of potassium bromide was applied through a rain simulator to four plots in northern Mississippi in two rain events at 12.7 mm/h lasting 5 and 3 h separated by 6 h. The plots were either tilled or no-tilled with drains installed by two methods at the surface of the fragipan. The magnitude of water and bromide (Br⁻) transported by macropore flow to a drain line were estimated and the resulting hydrographs provided an indication of the potential significance of macropore flow in transporting water and non-reactive chemicals through macropores to the shallow groundwater system. Matrix flow appears to contribute the majority of the water moving to the drains even during the early stages of the drain flow hydrographs. The no-till plots produced more macropore flow than the tilled plots, independent of how the drains were installed. Macropore flow in the drainage at any time was small as compared to the matrix flow; however it contributed a disproportionate amount of Br⁻ tracer. These data support the concept that models used to predict mass balances using only the matrix (Darcian) flow will underestimate those chemicals that move like bromide into the soil profile.     

Reduced root water uptake after drying and rewetting

The ability of plants to extract water from soil is controlled by the water‐potential gradient between root and soil, by the hydraulic conductivity of roots, and, as the soil dries, by that of the soil near the roots (rhizosphere). Recent experiments showed that the rhizosphere turned hydrophobic after drying and it remained temporarily dry after rewetting. Our objective was to investigate whether rhizosphere hydrophobicity is associated with a reduction in root water uptake after drying and rewetting. We used neutron radiography to trace the transport of deuterated water (D₂O) in the roots of lupines growing in a sandy soil. The plants were grown in aluminum containers (28 × 28 × 1 cm³) filled with a sandy soil. The soil was initially partitioned into different compartments using a 1‐cm layer of coarse sand (three vertical × three horizontal compartments). We grew plants in relatively moist conditions (0.1 < θ < 0.2). Three weeks after planting, we let the upper left compartment of soil to dry for 2–3 d while we irrigated the rest of the soil. Then, we injected D₂O in this compartment and in the upper right compartment that was kept wet. We monitored D₂O transport in soil and roots with time‐series neutron radiography. From the changes of D₂O concentration inside roots, we estimated the root water uptake. We found that root water uptake in the soil region that was let dry and rewetted was 4–8 times smaller than that in the region that was kept moist. The reduced uptake persisted for > 1–0.5 h. We conclude that a reduction in hydraulic conductivity occurred during drying and persisted after rewetting. This reduction in conductivity could have occurred in roots, in the rhizosphere, or more likely in both of them.     

Modelling the impact of within-storm variability of rainfall on the loading of solutes to preferential flow pathways

Rainfall variability within a storm can have a significant impact on the amount of chemical transported by surface runoff and preferential flow. Previously, studies have evaluated only a few slowly varying rainfall patterns and related sorption capacities. We use a bounded random cascade approach to generate 50 000 realizations of realistic rainfall intensity patterns within a single storm event (96 minutes duration, mean intensity of 18.75 mm hour^?1) to explore the effects on the partitioning of rainfall and linearly sorbing solutes between fast preferential flow (loading) and slow flow in the soil matrix for a silt loam and a sandy clay. Loading and infiltration are modelled by a near‐surface mixing model and Green–Ampt infiltration. The statistical properties of loading were evaluated from these simulations. For this storm the mean total of resident solute mixing from the soil to preferential flow reached a maximum for a retardation factor R ～ 5. Much smaller loadings occurred for more weakly sorbing and more strongly sorbing solutes. The variability of loading tended to decrease with increasing R. Ensemble averaged rainfall patterns were derived which related to the magnitude of loading. The patterns of rainfall generating large preferential flows did not necessarily lead to large solute loading. Early peaking, mid‐storm peaking and late peaking rainfall contributed to large solute loadings, depending upon soil and chemical properties. These patterns result from a balance between the amount of preferential flow generated and the amount of solute available when preferential flow is triggered. The results suggest that the use of R as a measure of the mobility of resident solutes depends on the flow pathway considered. In addition, characterization of flux distributions in soil with weakly sorbing, resident tracers, may underestimate the potential for rapid transport of strongly sorbing solutes subject to natural variations in rainfall.     

Temperature dependence of thermal conductivity of soil over a wide range of temperature (5–75°C)

The coupled heat and mass transfer in soil can be analysed by examining the temperature dependence of thermal conductivity. We have measured the thermal conductivity of two kinds of soil (Ando soil and Red Yellow soil) as a function of both temperature (5–75°C) and water content by the twin heat probe method. From our results we concluded that the thermal conductivity resulting from the latent heat transfer can be separated from the apparent thermal conductivity by subtracting the thermal conductivity at a temperature near 0°C from that at a higher temperature. The relation between the phenomenological enhancement factor (β) and the volumetric air‐filled porosity was divided into two parts: β increases linearly as the volumetric air‐filled porosity increases from zero (that is, water saturation), to the point at which soil water potential corresponds to ?320 J kg^?1; from that point to oven‐dry condition, β decreased logistically with the volumetric air‐filled porosity. From these results, we could generalize the behaviour of β.     

Determination of total organic‐C in soils by an improved chromic acid digestion and spectrophotometric procedure

Abstract

An improvement to the Walkley‐Black wet digestion method for the rapid determination of organic carbon over the range 0.2–5.5% in air‐dry soil is described. It permits total recovery of the organic‐C in finely ground soil samples digested with the heat of dilution from mixing N K₂ Cr₂ O₇ with concentrated H₂SO₄. in test tubes followed by external heating from a hot‐plate digestor. The organic‐C concentrations are determined directly, as the Cr product in diluted soil digests, by absorptiometry at 600 nm with calibration against similarly treated sucrose standards in solution. For the soils tested, there were negligible interferences from carbonates, wood charcoal, coke, Fe⁺² and readily reducible Mn; Cl does not interfere with the organic‐C assay in non‐saline soils but for saline soils a correction based on 1/12 Cl^‐ assay of the soil is necessary. The present method is compared with Tabatabai and Bremner's dry combustion procedure and Allison's manometric adaptation for calcareous soils. The procedure described here does not require carbonate to be determined and is therefore simpler. In addition it is cheaper, faster and more effective in controlling interferences than dry combustion procedures.     

Adequacy of transport parameters obtained in soil column experiments for selected chemicals

The transport parameters were determined for the ¹⁸O isotope (in the form of H₂ ¹⁸O), the Br^? ion, and atrazine in intact columns of allophanic Andosol (Mexico State, Mexico). A one-dimensional model for the convective-dispersive transport of chemicals with account for the decomposition and equilibrium adsorption (HYDRUS-1D), which is widely applied for assessing the risk of the chemical and bacterial contamination of natural waters, was used. The model parameters were obtained by solving the inverse problem on the basis of laboratory experiments on the transport of the ¹⁸O isotope, the Br^? ion, and atrazine in intact soil columns at a fixed filtration velocity. The hydrodynamic dispersion parameters determined for the ¹⁸O and Br^? ions in one column were of the same order of magnitude, and those for atrazine were higher by 3?C4 times. The obtained parameters were used to calculate the transport of these substances in another column with different values of the water content and filtration velocity. The transport process was adequately described only for the ¹⁸O isotope. In the case of the Br^? ion, the model significantly underestimated the transport velocity; for atrazine, its peak concentration in the column was overestimated. The column study of the transport of the three chemical compounds showed that transport parameters could not be reliably predicted from the results of a single experiment, even when several compounds were used in this experiment.     

Interaction Between Plant Roots and Soil Water Flow in Response to Preferential Flow Paths in Northern China

Preferential flow is expected to provide preferential channels for plant root growth and variations in soil water flow, but few studies were conducted to imply the impacts of these changes, particularly for preferential flow in stony soils. This study aimed to characterize soil water flow and plant root distribution in response to preferential flow paths and quantitatively describe the relation between plant root distribution and soil water flow. Field dye‐tracing experiments centered on experimental plants were conducted to determine the root length density and soil water flow process. Laboratory analyses were performed to characterize changes in the relative concentration of the accumulated effluent and the degree of interaction between plant roots and soil water flow. The amount of fine plant roots with preferential flow paths decreased with increasing soil depth for all experimental plots. The largest plant roots were recorded in the upper soil layers to a depth of 20 cm. The relative concentration of the accumulated effluent increased with time and decreased with soil depth under saturated soil conditions, whereas a distinct early turning point for the relative concentration of the accumulated effluent was observed in the 0–20‐cm soil columns, and the relative concentration of the accumulated effluent initially decreased and then increased with time under unsaturated soil conditions. This study provides quantitative information with which to characterize the interaction between plant roots and soil water flow in response to preferential flow paths in soil–plant–water systems. Copyright © 2016 John Wiley & Sons, Ltd.     

Seasonal soil nitrogen dynamics affect yields of lowland rice in the savanna zone of West Africa

Background : Rice production in low‐input systems of West Africa relies largely on nitrogen supply from the soil. Especially in the dry savanna agro‐ecological zone, soil organic N is mineralized during the transition period between the dry and the wet seasons. In addition, in the inland valley landscape, soil N that is mineralized on slopes may be translocated as nitrate into the lowlands. There, both in‐situ mineralized as well as the laterally translocated nitrate‐N will be exposed to anaerobic conditions and is thus prone to losses. Aim : We determined the dynamics of soil NO₃‐N along a valley toposequence during the dry‐to‐wet season transition period and the effects of soil N‐conserving production strategies on the grain yield of rainfed lowland rice grown during the subsequent wet season. Methods : Field experiments in Dano (Burkina Faso) assessed during two consecutive years the temporal dynamics and spatial fluxes of soil nitrate along a toposequence. We applied sequential and depth‐stratified soil nitrate analysis and nitrate absorption in ion exchange resin capsules in lowlands that were open to subsurface interflow and in those where the interflow from the was intercepted. During one year only we also assessed the effect of pre‐rice vegetation on conserving this NO₃‐N as well as on N addition by biological N₂ fixation in legumes using δ¹⁵N isotope dilution. Finally, we determined the impact of soil N fluxes and their differential management during the transition season on growth, yield and N use of rainfed lowland rice. Results : Following the first rainfall event of the season, soil NO₃‐N initially accumulated and subsequently decreased gradually in the soil of the valley slope. Much of this nitrate N was translocated by lateral sub‐surface flow into the valley bottom wetland. There, pre‐rice vegetation was able to absorb much of the in‐situ mineralized and the laterally‐translocated soil NO₃‐N, reducing its accumulation in the soil from 40–43 kg N ha^?1 under a bare fallow to 1–23 kg N ha^?1 in soils covered by vegetation. Nitrogen accumulation in the biomass of the transition season crops ranged from 44 to 79 kg N ha^?1 with a 36–39% contribution from biological N₂ fixation in the case of legumes. Rice agronomic performance improved following the incorporation as green manure of this “nitrate catching” vegetation, with yields increasing up to 3.5 t ha^?1 with N₂‐fixing transition seasons crops. Conclusion : Thus, integrating transition season legumes during the pre‐rice cropping niche in the prevailing low‐input systems in inland valleys of the dry savanna zone of West Africa can temporarily conserve substantial amounts of soil NO₃‐N. It can also add biologically‐fixed N, thus contributing to increase rice yields in the short‐term and, in the long‐term, possibly maintaining or improving soil fertility in the lowland.     

Effect of water flow on the mobility of the root-knot nematode Meloidogyne incognita in columns filled with glass beads, sand or andisol

In the present study, the migration of nematodes was studied in columns filled with three materials of different textures and chemical properties. The role of soil pores that enable root-knot nematode (Meloidogyne incognita) second stage juveniles (J2) to escape rapid water flow in soil was demonstrated using columns filled with glass beads, sand or andisol that maintained a constant water flow. Under a constant flow flux of 36 cm h⁻¹, living J2, dead J2 or anion bromine tracer (Br⁻) was injected in the middle of the column and then drainage water equivalent to two pore volumes (PV) was collected. The passive transport of the anion tracer in water flow could be explained by a convection dispersion equation. The dead J2 showed a pattern similar to that of Br⁻. However, the living J2 resisted movement in the water flow and remained in the column even at the highest water flow rate of 93.3 cm h⁻¹ in glass beads. The mobility of living J2 was affected by the filling materials; the number of J2 passing through the column was much lower in the andisol-filled column than in the other two columns but the total number of J2 in drainage water was 5% or less of the number injected for all columns. We suggest that J2 were affected not only by soil water flow but also by soil pore structure and have the ability of withstanding or avoiding movement in soil water flow.     

Nitrate retention and leaching in variable charge soils of a watershed in São Paulo state,Brazil

Abstract

Two Oxisols [Dusky Latosol (LR) and Dark‐Red Latosol (LE)] and one Entisol [Quartzous Sand (AQ)] of a watershed situated at the boundaries of the most important Brazilian aquifer, and intensively cropped to sugarcane, were studied in relation to their nitrate retention and leaching potential, and related electrochemical properties. Nitrate leaching tests carried out with repacked soil columns showed that deep layers of the subsoils, in which electrochemical conditions favor nitrate retention (e.g., net charge near zero, and ZPC larger than pH in water) required a larger number of pore volumes (npv) of leaching solutions for nitrate peak elution and nitrate depletion than surface layers, in which nitrate retention is not favored. Well defined regions of nitrate accumulation were detected in the 220–460 cm layer in the LE and 40–600 cm layer in the AQ. In these regions nitrate concentrations varied from 0.06 to 0.22 and 0.08 to 0.20 cmol_c kg^‐1, against 0.03 to 0.05 cmol_c kg^‐1 in the upper and underlayers. Nitrate distribution curves in the LE profile in the years 1995 and 1997 presented similar shapes and positions, indicating that the accumulation region remained almost unaltered, and stationary, in this period. Nitrate accumulation was related to soil electrochemical conditions favorable to nitrate retention.