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Phosphorus flux from agricultural landscapes to surface waters may cause eutrophication. In the northeastern United States, P transport largely depends on P sorption of soils in variable source areas or in land treatment systems. Soil redox fluctuation commonly occurs in these areas. Nevertheless, the effect of soil redox on P sorption has been variable in the literature. This study investigated P sorption of an organic-rich northeastern glaciated silt loam (Langford) under air-dried, field-wet, and reduced conditions using batch P sorption experiments. Additionally, the influence of farm wastewater on soil P sorption was studied. Major results indicated that soil reduction increased the maximum amount of P that can be sorbed ([S.sub.max]) and decreased the aqueous P concentration at which P sorption and desorption are equal ([EPC.sub.0]), both determined from a modified Langmuir isotherm model. The slightly reduced field-wet soils had no significant difference in [S.sub.max] due to limited soil reduction. Using the diluted wastewater as the sorption solution matrices instead of 0.01 mol [L.sup.-1] KCl solution, the soils generally exhibited greater [S.sub.max] and lower [EPC.sub.0] except for the [EPC.sub.0] of a reduced surface soil, implying more complex P sorption in the field. Identified P sorption mechanisms include phosphate precipitation, ligand exchange with organic matter, and adsorption onto Fe hydroxides. Transformation of Fe compounds during soil reduction is primarily responsible for the changes in soil P sorption. Abbreviations: DOC, dissolved organic carbon; ICP-AES, inductively coupled plasma-atomic emission spectrometry; OM, organic matter; SRP, soluble reactive phosphorus; TC, total carbon; TN, total nitrogen; VSA, variable source area; VTA, vegetative treatment area. doi:10.2136/sssaj2009.0123

Modeling the transport of chemicals is challenging due to the presence of preferential flow paths and the input data needed to describe these paths. We propose a simple equation that can predict the breakthrough of solutes without excessive data requirements under steady state flow conditions. In our generalized preferential flow model (GPFM), the soil is conceptually divided into a distribution zone and a conveyance zone. The distribution zone acts as a linear reservoir resulting in an exponential loss of solutes through preferential flow paths from this zone. In the conveyance zone, the transport of solutes is described with the convective-dispersive (CD) equation. Input data required are the apparent water content of the distribution zone, and solute velocities and dispersivities in the conveyance zone. The model is tested with chloride breakthrough curves (BTCs) resulting from three sets of experiments performed with steady state artificial acid rainfall at different intensities applied sequentially using duplicate columns filled with undisturbed soil or sand both exhibiting preferential flow. The model is able to describe the breakthrough of solutes with physically meaningful parameters through coarse sand with fingered flow and through undisturbed soil cores. In coarse sand, water and solutes flow only through the fingered flow paths in the conveyance zone. In undisturbed columns there is both flow through the matrix and preferential flow paths, and two velocities in the conveyance zone are needed to simulate the breakthrough curves.

While the rapid transport of nonadsorbed chemicals is relatively well understood, preferential transport of adsorbable chemicals including P requires further study. Our objective was to characterize subsurface P transport in glaciated soils of the northeastern USA. Large intact columns from five soils with differing structures were subjected in duplicate to both synthetic acid rainfall at a low rate simulating natural rainfall, and ponded water for observing saturated flow. The rain was enriched with inorganic and organic P after baseline conditions were established. Drainage water P concentrations were measured and adsorption isotherms were determined. Type of flow was characterized using Cl breakthrough curves and visualizations of blue dye patterns. Baseline P concentrations in drainage water were 0.02 to 0.04 mg [L.sup.-1]. At low flow rates, P appeared in the drainage water soon after application of either inorganic or organic P for the silt loam soil (firm, moderate coarse prismatic parting to moderate medium subangular blocky structure) in which preferential flow paths carried most of the water flow. In contrast, the soils in which matrix type flow dominated (weak fine granular or weak medium subangular blocky structures) had little or no increase in drainage water P. However, under ponded conditions all soils exhibited preferential flow and rapid P breakthrough. Elevated P concentration in the drainage water could not be explained by the P adsorption strength with the possible exception of the sandy loam soil, where the outflow P concentration was consistently low. Variation in flow rate in conjunction with soil structure satisfactorily explained elevated concentrations of dissolved inorganic and organic P in drainage water from surface applied P.

Watersheds, in many parts of the world, consist of sloping soils with a dense subsoil at shallow depth. Very few measurement techniques exist for realistically determining hydraulic properties in situ on these hillside soils. A hillslope infiltrometer, open at the bottom, top, and downhill sides, was developed that can measure the vertical saturated conductivity of the horizon sequence and lateral hydraulic conductivity of the horizon most conductive to water. The infiltrometer was tested on the steeply sloping soils in Honduras. Increasing rainfall intensities were applied and lateral flow rate was measured in several troughs at the downslope end of the hillslope infiltrometer. Vertical infiltration rates for the horizon sequence and lateral conductivity were calculated at each rainfall rate after steady state conditions were established. The hillslope infiltrometer proved useful in the characterization of subsurface flow on the steep slopes in Honduras.

Water-repellent soils occur all over the world and affect both groundwater pollution and crop yield. The finger-like wetting patterns in these soils have many similarities with unstable wetting fronts in coarse-grained sandy soils. Our objectives were to study the water movement in water-repellent sand and to examine how the theory for unstable wetting fronts applies to water-repellent sands. Infiltration experiments, in which moisture content and matric potentials were measured, were carried out in slab chambers with identical sands but with different levels of water repellence. Soil water characteristics were determined in separate experiments. Infiltration in the hydrophilic soil resulted in a uniform and horizontal front. All water-repellent sands showed a fingered flow pattern. For negative water-entry values, water infiltrated without delay. For positive values, water entered the soil only after the depth of the ponded water equaled or exceeded the water-entry pressure, which increased with increasing repellency. The finger widths predicted with the unstable flow theory agreed rather well with the observed values. In general, the research showed that the wetting patterns of water-repellent sands depended directly on the soil water characteristic curve. This implies that the type of wetting front and risk to groundwater pollution can be predicted based on laboratory-measured soil hydraulic properties.

Plant roots and soil biota require [O.sub.2] to function normally. When the soil [O.sub.2] is depleted, it must be replaced by [O.sub.2] from the atmosphere. The transport occurs primarily by gaseous diffusion. Although [O.sub.2] diffusion has been researched in homogeneous soils, and in tight clay soils with cracks, little is known about the effects of macropores on [O.sub.2] transport in typical agricultural soils. In this study we examined the impact of large pores on [O.sub.2] concentration in a sandy loam soil. Experiments were carried out with homogeneous and simulated-macropore columns with a steady-state rainfall of 3 cm [d.sup.-1] in which the [O.sub.2] concentration in the soil matrix and in the macropore was measured at 15-cm vertical intervals. The diameter of each column was 20 cm and the height was 105 cm. Groundwater depth was systematically varied between 30 and 105 cm. Soil water content did not vary with distance above the water table for the 30-, 60-, and 90-cm groundwater depths. While both the soil water for the entire column and the [O.sub.2] concentration from the surface to the 30-cm depth was the same for the homogeneous and macropore columns, the [O.sub.2] penetrated deeper in the macropore column and was 4% greater below the 30-cm depth in the macropore column than for the homogeneous column.

A new high-resolution quantitative method of examining quick alterations of the fluid amount present in porous media, was provided by synchrotron X-rays at the Cornell High Energy Synchrotron Source. This method is founded on controlling X-ray absorbance alterations in sand volumes crossed by an acute, highly collimated synchrotron X-ray beam. This method helps assess the physical and chemical characteristics of a rapid flow field in porous media.

Robust and noninvasive techniques for in situ measurement of soil water content are essential for monitoring and research of many hydrologic processes. Although in situ measurement of soil water content by means of a soil's electric and dielectric properties has been explored extensively, the use of ground-penetrating radar (GPR) for this purpose is not well established. We compared the soil's dielectric constant as measured by GPR and time domain reflectometry (TDR) and its relationship to water content determined independently in auger holes. We propose a calibration equation relating soil water to GPR-measured dielectric constants. The difference in calibration equations between GPR and TDR was partially attributed to the difference in frequency ranges in which TDR and GPR operate and the zero offset of the GPR.

Nitrous oxide (N2O) emissions were monitored using the micrometeorologi- cal eddy covariance technique from manure-fertilized cropland on a large dairy farm in New York State in 2006 to 2009. Nitrous oxide emissions demonstrated episodic behavior with intermittent short-duration peak fluxes up to 39.7 mg N2O-N m-2 d-1, whereas most of background fluxes during the annual agricultural cycle were below 6.5 mg N2O-N m-2 d-1. This paper discusses temporal variability of measured N2O emissions using a "hot moment" approach. To identify and quantify peak events as potential hot moments and to determine whether or not they could be treated statistically as outliners, N2O daily fluxes were analyzed by the box plot method using multiple thresholds. Peak events exceeding outlier thresholds contributed up to 51% of cumulative annual N2O emissions, although they represented <7% of the total observation time. Individual N2O peaks were also categorized by their duration, as single day spikes and multiday events. The highest contributing instances were multiday N2O peaks during summer precipitation and early spring thaw, largely enhanced by manure fertilization. These high-intensity emission events demonstrated repetitive seasonal responses to a combination of environmental factors and were therefore identified as hot moments. Abrupt rises in both temperature and soil moisture appeared to trigger major hot moments, whereas the availability of manure N controlled their magnitude. In the absence of strong correlations between time-series of individual environmental factors and N2O emissions, the hot moment approach can be a promising tool for the integrated analysis of most significant N2O events in cultivated fields receiving manure applications. [ABSTRACT FROM AUTHOR]

Phosphorus flux from agricultural landscapes to surface waters may cause eutrophication. In the northeastern United States, P transport largely depends on P sorption of soils in variable source areas or in land treatment systems. Soil redox fluctuation commonly occurs in these areas. Nevertheless, the effect of soil redox on P sorption has been variable in the literature. This study investigated P sorption of an organic-rich northeastern glaciated silt loam (Langford) under air-dried, field-wet, and reduced conditions using batch P sorption experiments. Additionally, the influence of farm wastewater on soil P sorption was studied. Major results indicated that soil reduction increased the maximum amount of P that can be sorbed (Smax) and decreased the aqueous P concentration at which P sorption and desorption are equal (EPC0), both determined from a modified Langmuir isotherm model. The slightly reduced field-wet soils had no significant difference in Smax due to limited soil reduction. Using the diluted wastewater as the sorption solution matrices instead of 0.01 mol L-1 KCI solution, the soils generally exhibited greater Smax and lower EPC0 except for the EPC∅ of a reduced surface soil, implying more complex P sorption in the field. Identified P sorption mechanisms include phosphate precipitation, ligand exchange with organic matter, and adsorption onto Fe hydroxides. Transformation of Fe compounds during soil reduction is primarily responsible for the changes in soil P sorption. [ABSTRACT FROM AUTHOR]

Modeling the transport of chemicals is challenging due to the presence of preferential flow paths and the input data needed to describe these paths. We propose a simple equation that can predict the breakthrough of solutes without excessive data requirements under steady state flow conditions. In our generalized preferential flow model (GPFM), the soil is conceptually divided into a distribution zone and a conveyance zone. The distribution zone acts as a linear reservoir resulting in an exponential loss of solutes through preferential flow paths from this zone. In the conveyance zone, the transport of solutes is described with the convective-dispersive (CD) equation. Input data required are the apparent water content of the distribution zone, and solute velocities and dispersivities in the conveyance zone. The model is tested with chloride breakthrough curves (BTCs) resulting from three sets of experiments performed with steady state artificial acid rainfall at different intensities applied sequentially using duplicate columns filled with undisturbed soil or sand both exhibiting preferential flow. The model is able to describe the breakthrough of solutes with physically meaningful parameters through coarse sand with fingered flow and through undisturbed soil cores. In coarse sand, water and solutes flow only through the fingered flow paths in the conveyance zone. In undisturbed columns there is both flow through the matrix and preferential flow paths, and two velocities in the conveyance zone are needed to simulate the breakthrough curves. [ABSTRACT FROM AUTHOR]