Surrounding Soil Matrix Research Articles

An experimental investigation to characterise soil macroporosity under different land use and land covers of northeast India

Saturated macropore flow is the dominant hydrological process in tropical and subtropical hilly watersheds of northeast India. The process of infiltration into saturated macroporous soils is primarily controlled by size, network, density, connectivity, saturation of surrounding soil matrix, and depthwise distribution of macropores. To understand the effects of local land use, land cover and management practices on soil macroporosity, colour dye infiltration experiments were conducted with ten soil columns (25 × 25 × 50 cm) collected from different watersheds of the region under similar soil and agro-climatic zones. The sampling sites included two undisturbed forested hillslopes, two conventionally cultivated paddy fields, two forest lands abandoned after Jhum cultivation, and two paddy fields, one pineapple plot and one banana plot presently under active cultivation stage of the Jhum cycle. Digital image analyses of the obtained dye patterns showed that the infiltration patterns differed significantly for different sites with varying land use, land cover, and cultivation practices. Undisturbed forest soils showed high degree of soil macroporosity throughout the soil profile, paddy fields revealed sealing of macropores at the topsoil due to hard pan formation, and Jhum cultivated plots showed disconnected subsoil macropores. The important parameters related to soil macropores such as maximum and average size of macropores, number of active macropores, and depthwise distribution of macropores were estimated to characterise the soil macroporosity for the sites. These experimentally derived quantitative data of soil macroporosity can have wide range of applications in the region such as water quality monitoring and groundwater pollution assessment due to preferential leaching of solutes and pesticides, study of soil structural properties and infiltration behaviour of soils, investigation of flash floods in rivers, and hydrological modelling of the watersheds.

Dye staining and excavation of a lateral preferential flow network

Abstract. Preferential flow paths have been found to be important for runoff generation, solute transport, and slope stability in many areas around the world. Although many studies have identified the particular characteristics of individual features and measured the runoff generation and solute transport within hillslopes, very few studies have determined how individual features are hydraulically connected at a hillslope scale. In this study, we used dye staining and excavation to determine the morphology and spatial pattern of a preferential flow network over a large scale (30 m). We explore the feasibility of extending small-scale dye staining techniques to the hillslope scale. We determine the lateral preferential flow paths that are active during the steady-state flow conditions and their interaction with the surrounding soil matrix. We also calculate the velocities of the flow through each cross-section of the hillslope and compare them to hillslope scale applied tracer measurements. Finally, we investigate the relationship between the contributing area and the characteristics of the preferential flow paths. The experiment revealed that larger contributing areas coincided with highly developed and hydraulically connected preferential flow paths that had flow with little interaction with the surrounding soil matrix. We found evidence of subsurface erosion and deposition of soil and organic material laterally and vertically within the soil. These results are important because they add to the understanding of the runoff generation, solute transport, and slope stability of preferential flow-dominated hillslopes.

Different microhabitats affect soil macroinvertebrate assemblages in a Mediterranean arid ecosystem

Spatio-temporal variability is a key factor in conservation, management and restoration of ecosystems. Spatial heterogeneity is caused in many cases by organisms that are able to modify their environments. This is especially relevant in arid systems, where organisms such as shrubs and ants create patches of high nutrient availability (fertile islands) surrounded by a low-nutrient matrix. Although variations in structure and physiology among shrubs provoke differences in their effects as fertile islands, whether different microhabitats vary in their influence on animal communities is poorly known. The principal aim of our study is to analyse the effects of different shrub species and Messor harvester ant-nest mounds on the structure of soil macroarthropod assemblages in a strongly seasonal desert location of SE Spain. Shrub microhabitats and ant-nest mounds maintained higher species density, abundance and biomass of soil macroinvertebrates than the surrounding soil matrix. The different microhabitats differed in taxonomic and trophic composition, abundance, and biomass of soil macroinvertebrates, at both litter and belowground levels. Also, variations of invertebrate abundance among microhabitats changed throughout sampling periods. Thus the spatio-temporal heterogeneity of the study site affected the distribution and dynamics of the macroinvertebrate community. The above results indicate that the spatio-temporal mosaic created by microhabitat and seasonal variations on macroinvertebrate assemblages is a relevant issue to be considered in conservation, sustainable management and environmental restoration in heterogeneous arid systems to preserve their biodiversity and ecosystem functioning.

Dye tracer and infiltration experiments to investigate macropore flow

Dye tracer experiments provide qualitative pictures to illustrate the flow pathways in soil. Combined to the laboratory analysis and in situ irrigation experiments they provide better understanding of hydrodynamic aspect of flow processes in soil. This study was conducted to assess the impact of compaction and tillage on soil structure of two different soils. Lateral exchange LE from macropores into the surrounding soil matrix was investigated to show the efficiency of macropores in conducting water. Two sites were compared with each other. Site 1 was used as grassland and site 2 as barley. The highest concentration values of the dye tracer most frequent at the topsoil of grassland and the lowest value of LE show that more tortuous macropores were destroyed by compaction. Consequently, only the larger less tortuous and efficient ones remain. The surface density diagram shows a significant raise of dye coverage below 0.30 m. This is due to an increased network of macropores with decreasing diameters which leads to an enhanced penetration of tracer into the matrix. The only significant difference between topsoil pore volumes of the two sites concerns pores with a diameter larger than 50 µm. In fact, these pores are observed more frequently in the barley site than in the grassland site. The measured porosities and the hydrodynamic variation of water content confirm the loosening tillage effect of the topsoil as well.

Estimation of Phosphorus Status of Soil Fe‐Enriched Concretions with the Acid Ammonium Oxalate Method

Iron (Fe)‐enriched concretions, a complex natural matrix with high chemical heterogeneity and phosphate‐sorption capacity, is widespread in soils with restrictive drainage in Greece. However, the phosphorus (P) status and related characteristics of Fe‐enriched concretions in agricultural soils in areas where P fertilization is mainly inorganic are relatively unknown. Active noncrystalline Fe and aluminum (Al) oxides (Feox, Alox), oxalate extractable P (Pox), P sorption capacity (PSC), and the degree of P saturation (DPS) of Fe‐enriched concretions from agricultural imperfectly drained soils in central Greece were determined using the acid ammonium oxalate method. The concretions contain 13 times as much Feox, twice as much Alox, and almost 15 times as much Pox than the surrounding soil matrix. Pox accounted for 50–80% of total P of the soil concretions, indicating strong accumulation of noncrystalline P components (Al‐ and Fe‐P). The PSC, expressed as a 0.5 (Alox+Feox), ranged from 184.7 to 314 mmol kg−1, demonstrating the strong affinity of the Fe‐enriched concretions for P. The DPS, which represents the fraction of concretion sorbent surface coverage by P, was computed as 100 (Pox/PSC) with values ranging from 6 to 13% (mean=8%). The results of this study indicate that the Fe‐enriched concretions, due to their high noncrystalline Fe and Al oxides content, act as major sink of phosphate, controlling the location, mobility, and dynamics of P in agricultural soils with restrictive drainage.

Development of a simple lateral preferential flow model with steady state application in hillslope soils

A general model describing three‐dimensional lateral preferential water flow in a hillslope with soil pipes was developed. Matrix flow and pipe flow were regarded as separate flow systems and computed using the governing equations (Richards' and Manning's equations, respectively), while simultaneously considering the interaction between these two flow systems. The model accommodates both partially filled and full pipe flow, seepage into the pipe, and backflow from the pipe into the surrounding soil matrix. Simulations were conducted for conditions outlined in an earlier bench‐scale experiment, including differing internal roughness configurations within the pipe. Both groundwater levels and preferential flow under steady state conditions were simulated for different roughness elements within the soil pipe; previous models have not simulated such conditions. Six different pipe arrangements were also simulated: no pipe (uniform matrix flow), single straight pipe, two sets of discontinuous pipes, branched pipes, and unopened pipe. Branched pipes had the highest discharge, and even the unopened pipe contributed to an enhanced total discharge compared to the no pipe simulation. These simulations demonstrated the versatility of the model under the steady state conditions, although the model can also be applied to transient conditions.

Spatial Distribution and Speciation of Lead around Corroding Bullets in a Shooting Range Soil Studied by Micro-X-ray Fluorescence and Absorption Spectroscopy

We investigated the spatial distribution and speciation of Pb in the weathering crust and soil surrounding corroding metallic Pb bullets in a shooting range soil. The soil had a neutral pH, loamy texture, and was highly contaminated with Pb, with total Pb concentrations in the surface soil up to 68 000 mg kg(-1). Undisturbed soil samples containing corroding bullets were collected and embedded in resin, and polished sections were prepared for micro-X-ray fluorescence (micro-XRF) elemental mapping and micro-X-ray absorption near edge structure (micro-XANES) spectroscopy. Bullet weathering crust material was separated from the metallic Pb cores and analyzed by powder X-ray diffraction analysis. Our results show a steep decrease in total Pb concentrations from the bullet weathering crust into the surrounding soil matrix. The weathering crust consisted of a mixture of litharge [alpha-PbO], hydrocerussite [Pb3(CO3)2-(OH)2], and cerussite [PbCO3], with litharge dominating near the metallic Pb core and cerussite dominating in the outer crust, which is in contact with the soil matrix. On the basis of these results and thermodynamic considerations, we propose that the transition of Pb species after oxidation of Pb(O) to Pb(II) follows the sequence litharge --> hydrocerussite --> cerussite. Consequently, the solubility of cerussite limits the activity of Pb2+ in the soil solution in contact with weathering bullets to < or =1.28 x 10(-6) at pH 7, assuming that the CO2 partial pressure (PCO2) in the soil is equal or larger than in the atmosphere (PCO2 > or = 0.000 35 atm).

An infiltration model based on flow variability in macropores: development, sensitivity analysis and applications

Simulating infiltration in soils containing macropores still provides unsatisfactory results, as existing models seem not to capture all relevant processes. Recent studies of macropore flow initiation in natural soils containing earthworm channels revealed a distinct flow rate variability in the macropores depending on the initiation process. When macropore flow was initiated at the soil surface, most of the macropores received very little water while a few macropores received a large proportion of the total inflow. In contrast, when macropore flow was initiated from a saturated or nearly saturated soil layer, macropore flow rate variation was much lower. The objective of this study was to develop, evaluate, and test a model, which combines macropore flow variability with several established approaches to model dual permeability soils. We then evaluate the INfiltration–INitiation–INteraction Model (IN 3M) to explore the influence of macropore flow variability on infiltration behavior by performing a sensitivity analysis and applying IN 3M to sprinkling and dye tracer experiments at three field sites with different macropore and soil matrix properties. The sensitivity analysis showed that the flow variability in macropores reduces interaction between the macropores and the surrounding soil matrix and thus increases bypass flow, especially for surface initiation of macropore flow and at higher rainfall intensities. The model application shows reasonable agreement between IN 3M simulations and field data in terms of water balance, water content change, and dye patterns. The influence of macropore flow variability on the hydrological response of the soil was considerable and especially pronounced for soils where initiation occurs at the soil surface. In future, the model could be applied to explore other types of preferential flow and hence to get a generally better understanding of macropore flow.

Numerical calculation of soil pipe flow and its effect on water dynamics in a slope

AbstractA chain of connected macropores, which develops nearly parallel to the impermeable subsurface layer, is commonly found in hillslopes and referred to as a soil pipe. Field observations have revealed that water flow in such a pipe (pipe flow) has large hydrogeomorphic effects, since it contributes to rapid transfer of storm water and subsurface soil erosion. This study proposed a simple simulation method to describe water dynamics in a piped hillslope. The applicability of the method was examined using data obtained from a bench‐scale experiment using pseudo soil pipes. Results of the experiment showed that (1) the flume‐bed pore water pressure became small around the upper ends of the pipes; (2) when pipe outlets were connected to a stream channel (the open pipe condition), the flume‐bed pore water pressures in the whole slope were much lower than those under the ‘no pipe’ condition; (3) when pipe outlets were not connected to a stream channel (the closed pipe condition), the flume‐bed pore water pressure at the pipe outlets was higher than that under the ‘no pipe’ condition. These results were reproduced by solving the two‐dimensional Richards equation assuming an equivalent soil matrix layer for the original ‘pipes plus soil matrix’ part, which had a larger saturated hydraulic conductivity than the surrounding soil matrix. This simple simulation method has an advantage over previous pipe flow models: both the pipe flow and the interaction between the soil pipes and the surrounding soil matrix can be computed by solving the Richards equation without using any further assumptions for the boundary conditions between the soil pipes and the soil matrix. Results of numerical experiments using the simple method showed that (1) under the closed pipe condition, the flume‐bed pore water pressure at the pipe outlets increased as the pipe length increased, and (2) once the open pipes clogged, flume‐bed pore water pressures in the whole slope increased, and the greatest increase occurred at the boundary between the clogging and non‐clogging parts. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Formation of iron nodules in a Hapludult of central Taiwan

This study compared iron nodules with their matrix materials in a Hapludult from central Taiwan. Mineralogy and 14C ages of the iron nodules were compared with their associated matrix materials. The samples were analyzed by X-ray diffraction (XRD) and accelerated mass spectroscopy (AMS). Hydroxy-interlayered vermiculite (HIV) and illite were the major clay minerals in the soil matrix, but illite and kaolinite were dominant in the iron nodules. Clay mineralogy of iron nodules revealed more intense weathering process than the surrounding soil matrix. 14C dates showed that the age of saprolite and iron nodules were similar, and were both much older than those of the soil matrix. It is suggested that iron nodules developed from residuum of the Tertiary shale (saprolite). During pedogenesis, saprolite acted as a nucleus, where Fe coated on its surface was transferred to form iron nodules. Key words: 14C dates, iron nodules, saprolite, soil matrix, Ultisol

The identification of Roman buildings from the air: recent discoveries in Western Transylvania

AbstractAn ongoing programme of archaeological aerial reconnaissance in southwestern Transylvania, the first time such a programme has been conducted in Romania, revealed quite large numbers of negative cropmarks or parchmarks, particularly in the dry summer of 2000. These areas of restricted crop growth indicated the presence of buried stone‐walled buildings, predominantly of Roman date, which were previously unknown. Important discoveries included much of the internal plan of the auxiliary fort at Cigmau; an extensive civil settlement to the east of that fort; buildings within the civil settlements outside the forts at Vetel (Micia) and Razboieni; buildings and property boundaries within the municipium at Apulum (Alba Iulia); and villas at Oarda and Vintu de Jos. By contrast, positive cropmarks, enhanced growth reflecting the existence of buried pits or enclosure ditches, were observed only rarely. Explanations for this phenomenon linked to a bias in the reconnaissance programme or to a general absence of enclosed sites within the archaeological record in the area are rejected. The preferred explanation relates to local soil conditions, whereby the widespread, deep alluvial soils reduce the contrast in moisture content between buried archaeological ditches or pits and the surrounding soil matrix so that positive cropmarks are rarely formed. Copyright © 2003 John Wiley &amp; Sons, Ltd.

An experimental tracer study of the role of macropores in infiltration in grassland soils

AbstractWater flow in macropores is an important mechanism of infiltration in natural soils and, as such, is crucial for the prediction of runoff generation. The major flow processes controlling macropore flow are the initiation of macropore flow (water supply into macropores) and the water transfer from the macropores into the surrounding soil matrix (interaction). The water movement during infiltration and the resulting flow paths were studied with combined sprinkling and dye tracer experiments under different rainfall intensities and initial soil moisture conditions. The dye tracer was continuously applied with the sprinkling water on 1 m2 plots. After the sprinkling, horizontal and vertical soil sections were prepared for surveying dye patterns, which showed the cumulated flow pathways in the soils. These experiments were carried out on four hillslope sites covered with grassland, where earthworms mainly built the macropore system. The evaluation of the flow processes in the soil was based on classified dye patterns and measurements of water content and matric potential. The results illustrate how flow in earthworm channels influences general hydrological flow processes during extreme rainfall events. Macropore flow was initiated from the soil surface or from a saturated or partially saturated soil layer. Transfer of water from the macropores into the soil matrix was mainly influenced by the soil properties and soil water content. The permeability of the underlying bedrock in combination with this transfer of water controlled the drainage of the macropores. Finally, major effects of macropore flow processes on the hydrological response were extracted. Infiltration excess overland flow was reduced if water bypassed the less permeable layer through macropores, saturation excess overland flow was less affected by macropores, and subsurface flow was activated very rapidly because the infiltrated water bypassed the soil matrix. This study highlights the most important processes that have to be considered in order to understand better and to model infiltration in natural soils in the future. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Simulating surface and subsurface initiation of macropore flow

Initiation of macropore flow either from the soil surface or from a saturated soil layer at depth is a first order control on water flow in macropores and water transfer from macropores into the surrounding soil matrix. Nevertheless, these initiation processes have not been well documented. We surveyed surface topography at four field sites with permanent grass vegetation with grid spacing of 10 cm and applied Kriging to derive the spatial correlation structure. We then simulated the water flux into macropores based on different combinations of surveyed surface micro-topographies, spatial earthworm burrow distributions, and the soil properties, to examine more fully the role of macropore drainage area (MDA) on macropore flow initiation. The spatial distributions of the earthworm burrows were derived from horizontal soil sections extracted from each study profile. The MDA was calculated for different sets of surface topography and macropore density using a flow accumulation algorithm. The resulting MDA of each macropore was used to calculate the total relative MDA, which is equal to the proportion of overland flow draining into macropores, and the MDA probability distribution. The results showed that the macropore density primarily controlled the total MDA and that surface micro-topography strongly influenced the probability distribution of the MDA. Only a few macropores contributed significantly to the total macropore flow whereas the majority of macropores received little water; a phenomenon especially pronounced for a rough surface topography and for a low soil surface gradient. The simulated probability distribution of subsurface initiation was very different from the distribution derived for surface initiation; more symmetrical, less variable and slightly influenced by the roughness and the gradient of the interface between the saturated and the low permeable soil layer. We conclude that the different amount of water supplied to each macropore further alters the percolation depth and transport of solutes in macroporous soils and should be considered for modelling infiltration in macroporous soils.

Characterisation of iron nodules in a Ultisol of central Taiwan

The chemical and physical properties, clay mineralogy, and 14C radiocarbon age of iron nodules found in a Ultisol located on a low elevation terrace of Taiwan were investigated. The results indicated that the amount and size of iron nodules increase with soil depth. Free iron contents in nodules were markedly higher than in the surrounding soil matrix. Feo/Fed ratio was low in all iron nodules, indicating that the predominant form of free iron was crystalline. Al-substituted goethite was found as the predominant oxide mineral and Al substitution ranged from 16 to 24% mole. Illite and hydroxy-interlayered vermiculite and kaolinite were the dominant clay minerals in the iron nodules. X-ray fluorescence (XRF) analysis of the iron nodules are composed primarily of Fe, Si, and Al, and minor amounts of Mn. Based on the similarity of clay mineralogical compositions and radiocarbon dating ages, it is suggested that the major driving force promoting the formation of iron nodules was in situ weathering of saprolite by the frequent wetting/drying cycles. We speculate that illuviated iron played only a minor role in their formation.

022 Delivery of Gases to the Soil Matrix via Buried Drip Irrigation Tubing

The impending worldwide restrictions on the use of methyl bromide (MeBr) as a soil fumigant have prompted an intensive search for more-effective methods for delivering MeBr or replacement compounds. Although the majority of agrochemicals are applied in the solid phase or the liquid phase at ambient pressure and temperature, some chemicals, including certain soil fumigants such as MeBr, are gases under normal field conditions. Experiments were conducted to evaluate use of two types of commercial drip irrigation tubing to deliver gases to nontarped planting beds. Air moved through each tubing type immediately after burial; water was not necessary for inflation. Air was also able to move through 40 m of buried rigid drip tubing and through 90 m of buried flat tape that had been used for subsurface drip irrigation for more than 1 year. Mixtures of known ratios of propane and air were introduced into the buried tubing over several time intervals to evaluate gas movement from buried drip tubing into the surrounding soil matrix. Samples were collected from sets of three soil gas sampling tubes placed 15, 30, and 45 cm to the side of the buried tubing and at regular intervals along the length of the tubing, and propane concentrations were quantified by gas chromatography. Tubing lengths and run times affected the magnitudes and uniformity of propane concentrations. Results suggest gas-phase chemicals can be delivered via buried drip-irrigation tubing, but effective distances from the point of introduction will be limited by the low densities and viscosities of gases, and corresponding high rates of escape through tubing emitters.

Integrating Phytoliths within Use-Wear/Residue Studies of Stone Tools

Analyses of phytolith assemblages extracted from residues on obsidian artefacts and from the surrounding soil matrix of two sites in Papua New Guinea address two important methodological issues. First, multivariate analysis of phytolith assemblages extracted from the edges of stone artefacts, from soil directly in contact with the artefact, and from soil derived from the same archaeological context demonstrates the integrity of phytolith residues. Second, reconstructions of tool use based on the analysis of phytoliths are compared with results from an independent use-wear/residue study. The interpretation of phytolith assemblage variability enhances and strengthens the use-wear/residue results. Finally, integrating the phytolith data within the broader use-wear/residue study produces significant new findings about changes in obsidian tool-use in prehistoric Papua New Guinea.

Introduction to the Special Issue: ecological significance of open burrow systems

Krotovinas, burrows of small mammals, occur widely in the soils of the steppes. The distribution and effect of krotovinas on soil organic carbon (SOC) stocks were studied in a deep profile (450 cm) at an archaeological site of the Palaeolithic period at Shenyang Agricultural University of China. It was found krotovinas occurred at all depths, and there was variation in their distribution and size. The density of krotovinas increased with depth, and was the highest (10/m2) at 250 cm depth. The average area of krotovinas' sections in most part of the profile ranged from 100 to 160 cm2. Krotovinas covered an area of 7 to 17% of the profile wall. Average SOC concentration in the krotovinas was 5.3 g/kg, whereas it was 4.5 g/kg in the surrounding soil matrix. SOC stocks at 100 to 450 cm accounted for 72% of total SOC stocks at 0 to 450 cm depth. Total SOC stock in the whole profile (0 to 450 cm) was 295 t/ha considering the presence of krotovinas, whereas it was 274 t/ha when the krotovinas were excluded. This study shows how krotovinas can be quantified, and that krotovinas can affect SOC concentration and change patterns of SOC distribution. The amount of SOC stored in deep soils should be taken into consideration for evaluating SOC stocks.

Microhabitat to landscape impacts: crested porcupine digs in the Negev Desert highlands

Indian crested porcupines (Hystrix indica) are adaptable, generalist herbivores with wide environmental tolerances and a large geographic range. In the arid Negev Desert highlands of southern Israel, porcupines excavate and consume subterranean organs of 18 species of geophytes and hemicryptophytes in a variety of habitats. Their foraging generates a network of direct and indirect impacts on ecological processes. Porcupine digs varied in size according to depth of forage and substrate, and new digs averaged about 8 cm in depth, and 400 cm2and 940 cm3in surface area and volume, respectively. Digging rates also differed among habitats. In hilly watersheds, highest digging rates were in soil belts supporting the greatest cover of shrubs and geophytes. Dig densities peaked at > -031 dig m−2, impacting up to 4% of soil surfaces. Digs are temporary soil pockets (longevities of < -032 to > -0310 years) that trap soil, water, organic matter, and seeds. As highly productive microhabitats compared to surrounding soil matrix, they promote the germination and growth of annual plants including some porcupine forage species. Loose soil excavated by porcupines enhances soil erosion and transport, and may substantially enhance ecosystem flows of water, soil, and nutrients. Porcupine digging may help to maintain bare rock outcrops in Negev watersheds, thereby promoting large runoff flows that produce mesic patches and plant habitats. Runoff dynamics have greatly influenced past and present human exploitation of Negev landscapes. Recent experiments indicate that porcupine digs have important properties as small-scale landscape disturbances, and as sources and sinks of critical environmental resources. Additional research is needed to better define and predict porcupine digging impacts at several ecological levels, and as a basis for realistic management of Negev habitats and landscapes.

Sorption and Retention of Herbicides in Vertically Oriented Earthworm and Artificial Burrows

AbstractIn many temperate region no‐tillage agro‐ecosystems, the vertically oriented burrows of the nightcrawler earthworm (Lumbricus terrestris L.) function as preferential flow paths and may promote the rapid downward transport of surface‐applied chemicals. The burrows of nightcrawlers, however, are lined with a material that is enriched in organic C relative to the surrounding soil matrix that may affect transport of organic chemicals. Vie investigated the effect of this lining on the sorption and retention of four herbicides: atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine], metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N(2‐methoxy‐1‐methylethyl)acetamide], dicamba (3,6‐dichloro‐2‐methoxybenzoic acid), and primisulfuron [3‐[4,6‐bis(difluoromethoxy)‐pyrimidin‐2yl]‐1‐(2‐methoxy carbonylphenyl sulfonyl) urea]. Batch sorption isotherms were determined using burrow lining and bulk soil collected from seven depth intervals from 0 to 50 cm. Sorption of the more strongly sorbed herbicides (atrazine and metolachlor) was enhanced by a factor of up to three on burrow lining relative to bulk soil, while there was little or no enhancement of the weakly sorbed herbicides (dicamba and primisulfuron). Transport and retention of these herbicides was measured in intact burrows in undisturbed soil blocks. Concentrations of the two strongly sorbed herbicides decreased during flow through nightcrawler and artificial (unlined) burrows, with the largest decreases occurring in earthworm burrows. Concentrations of the two weakly sorbed herbicides decreased very little during flow through either type of burrow. Retained herbicide concentrations were higher in nightcrawler than in artificial burrow walls, and decreases in concentration from the surface to the second 1‐mm wall layer were greater in nightcrawler than in artificial burrow walls. These results indicate the material lining nightcrawler burrows may significantly retard herbicide transport during lateral flow into and out of burrows.

Throughflow and solute transport in an isolated sloping soil block in a forested catchment

A 3m wide by 9m long by 1m deep soil block on a forested hillslope near Orono, ME, was isolated by excavation of encircling trenches. A sprinkler system for application of water and potassium bromide tracer was constructed over the plot. Outflow was collected at six locations with troughs. Experiments were conducted at application rates of 2.5, 5, and 10 cm h −1. Pulses of tracer were applied subsequent to attainment of steady flow and breakthrough curves were measured at all outflow points. Recession limbs of outflow hydrographs exhibited distinct breaks when plotted on semilogarithmic axes, indicating drainage from at least two distinguishable pore size classes or flow pathways. Solute breakthrough curves were dominated by a single peak; travel times of solute were inversely related to the application rate. A secondary peak in the outflow curve, which is inconsistent with transport theories for a homogeneous soil, was observed in all cases. This second peak is unexplained, but is conceptually consistent with the notion of transport in at least two pore size classes. An undisturbed soil core (diameter of 30 cm and length of 40 cm) was sprinkled at the same rates as was the soil block, using Methylene Blue as tracer in the last run. Drainage hydrographs and visual examination of dye stains in the block indicated also at this smaller scale that flow and transport are controlled by preferred paths in the soil, paths that cannot be morphologically distinguished from the surrounding soil matrix. Theoretical explanations of processes on such hillslopes need to account for this fact.