Ground water and surface water interactions are of fundamental importance for the biogeochemical processes governing phosphorus (P) dynamics in riparian buffers. The four most important conceptual hydrological pathways for P losses from and P retention in riparian buffers are reviewed in this paper: (i) The diffuse flow path with ground water flow through the riparian aquifer, (ii) the overland flow path across the riparian buffer with water coming from adjacent agricultural fields, (iii) irrigation of the riparian buffer with tile drainage water from agricultural fields where disconnected tile drains irrigate the riparian buffer, and (iv) inundation of the riparian buffer (floodplain) with river water during short or longer periods. We have examined how the different flow paths in the riparian buffer influence P retention mechanisms theoretically and from empirical evidence. The different hydrological flow paths determine where and how water-borne P compounds meet and interact with iron and aluminum oxides or other minerals in the geochemical cycling of P in the complex and dynamic environment that constitutes a riparian buffer. The main physical process in the riparian buffer-sedimentation-is active along several flow paths and may account for P retention rates of up to 128 kg P ha(-1) yr(-1), while plant uptake may temporarily immobilize up to 15 kg P ha(-1) yr(-1). Retention of dissolved P in riparian buffers is not as pronounced as retention of particulate P and is often below 0.5 kg P ha(-1) yr(-1). Several studies show significant release of dissolved P (i.e., up to 8 kg P ha(-1) yr(-1)).

as a third phase, thus enhancing the transport of strongly sorbing contaminants (McCarthy and Zachara, 1989).Until some two decades ago, it was believed that only the soil liquid and gaseous phases were mobile and could facilitate the transport of chemicals and nutrients through the vadose zone. It is now generally SOURCES OF MOBILE SOIL COLLOIDS accepted that also part of the soil solid phase is mobile, and thatThe tendency of soil colloids to disperse from soil mobile organic and inorganic soil colloids may facilitate chemical transport. However, the magnitude and significance of these colloidal aggregates in response to infiltration of water is a natural transport processes are yet to be determined. It is essential to examine phenomenon, sometimes even leading to the developwhether current models for transport and fate of chemicals in soil ment of illuvial subsurface horizons with higher contents and groundwater need to be revised. The collection of papers in this of clay compared with the upper eluvial horizons. Microspecial section of Vadose Zone Journal mainly take their origin, morphological features showing deposits of clay skins but not exclusively, from an international workshop "Colloids and on ped faces and at the interface of water-conducting Colloid-Facilitated Transport of Contaminants in Soil and Sediments" pores represent evidence of such colloid translocation held at the Danish Institute of Agricultural Sciences, Denmark, 19-20 (Buol and Hole, 1961). Dispersion of colloids is also Sept. 2002. The workshop was organized to review our present knowlsuspected to be responsible for affecting soil physical edge of colloid behavior and transport in porous media and the possi-properties such as surface crusting, surface erosion, wability of colloid-bound transport of contaminants and nutrients in soil and groundwater. Here we will first give a brief introduction to the ter infiltration, and hydraulic conductivities (e.g., Miller topic of mobilization and transport of colloids in the vadose zone, and Baharuddin, 1986;Shainberg et al., 1992). The source and highlight previous evidence of colloid-facilitated transport. We of mobile colloids in the vadose zone is generally considthen introduce the review and technical papers in the special section. ered to be the in situ release of water-dispersible colWe hope that the information provided in this special section will loids. Colloids are operationally defined as particles belead to improvements in our understanding and associated conceptual tween 1 to 10 nm and 2 to 10 m in diameter (e.g., models of contaminant transport and fate in soil.

Preferential flow and particle-facilitated transport may be largely responsible for observed leaching patterns of strongly sorbing contaminants such as phosphorus. A series of field experiments was performed to investigate the effects of slurry application and plowing on the subsurface transport of colloids and P. Two 25-m 2 plots at a structured sandy loam site were irrigated on six occasions during 1 yr. Effluent sampled in tile drains below the plots was analyzed for turbidity and fractions of dissolved (,0.24 mm) and particulate inorganic and organic P. The observed flow conditions indicated macropore flow. The particle concentration in the effluent was initially high, peaking before flow peak, and later gradually decreased with flow rate. The colloid leaching pattern was attributed to an initial depletion of high colloid concentrations in the pore water and an eventual diffusion-limited release of colloids from immobile intra-aggregate water to mobile water. Seasonal variability and management practices caused significant variations in the leaching of P forms. After slurry application dissolved P dominated P loss to the drains. At the events in autumn and winter, particle-facilitated transport of P came to dominate over dissolved P transport, reaching a maximum of 80% of P loss. Results suggested that plowing increases the risk of particle-facilitated and dissolved P leaching in rainstorms shortly after the inversion of the soil. We observed an almost fourfold increase in the leaching of P per volume of leachate when comparing irrigation experiments before and after plowing. Amounts of P associated with particulate matter in drain water were constant within events, but varying between storms. For Danish structured clay soils, P density in leached particles was found to range between a maximum of 6 mg P g 21 for soils having recently been fertilized and an approximate minimum of 3 mg P g 21 for soils not recently fertilized.

Managing phosphorus (P) losses in soil leachate folllowing land application of manure is key to curbing eutrophication in many regions. We compared P leaching from columns of variably textured, intact soils (20 cm diam., 20 cm high) subjected to surface application or injection of dairy cattle (Bos taurus L.) manure slurry. Surface application of slurry increased P leaching losses relative to baseline losses, but losses declined with increasing active flow volume. After elution of one pore volume, leaching averaged 0.54 kg P ha(-1) from the loam, 0.38 kg P ha(-1) from the sandy loam, and 0.22 kg P ha(-1) from the loamy sand following surface application. Injection decreased leaching of all P forms compared with surface application by an average of 0.26 kg P ha(-1) in loam and 0.23 kg P ha(-1) in sandy loam, but only by 0.03 kg P ha(-1) in loamy sand. Lower leaching losses were attributed to physical retention of particulate P and dissolved organic P, caused by placing slurry away from active flow paths in the fine-textured soil columns, as well as to chemical retention of dissolved inorganic P, caused by better contact between slurry P and soil adsorption sites. Dissolved organic P was less retained in soil after slurry application than other P forms. On these soils with low to intermediate P status, slurry injection lowered P leaching losses from clay-rich soil, but not from the sandy soils, highlighting the importance of soil texture in manageing P losses following slurry application.

No abstract

Land application of manure can exacerbate nutrient and contaminant transfers to the aquatic environment. This study examined the effect of injecting a dairy cattle (Bostaurus L.) manure slurry on mobilization and leaching of dissolved, nonreactive slurry components across a range of agricultural soils. We compared leaching of slurry-applied bromide through intact soil columns (20 cm diam., 20 cm high) of differing textures following surface application or injection of slurry. The volumetric fraction of soil pores >30 microm ranged from 43% in a loamy sand to 28% in a sandy loam and 15% in a loam-textured soil. Smaller active flow volumes and higher proportions of preferential flow were observed with increasing soil clay content. Injection of slurry in the loam soil significantly enhanced diffusion of applied bromide into the large fraction of small pores compared with surface application. The resulting physical protection against leaching of bromide was reflected by 60.2% of the bromide tracer was recovered in the effluent after injection, compared with 80.6% recovery after surface application. No effect of slurry injection was observed in the loamy sand and sandy loam soils. Our findings point to soil texture as an important factor influencing leaching of dissolved, nonreactive slurry components in soils amended with manure slurry.

While it is recognized that preferential flow may increase the transport of colloids, less is known about the actual influence of preferential flow on colloid mobilization in situ. Changes in pore structure upon soil exposure to drying and rewetting may additionally affect colloid mobilization. Information about the pore structure and the active flow volume, as well as the changes in these properties, are therefore important when investigating colloid mobilization. We investigate the pore structure characteristics and the transport of tritium (3H2O) during steady unsaturated flow conditions. A total of 54 soil columns sampled along a natural clay gradient representing six clay contents (12, 18, 24, 28, 37, and 43% clay) were equilibrated to three different initial matric potentials (IMP), ψ = −2.5, −100, and −15500 hPa. Pore structure characteristics were deduced from water retention characteristics and measurements of air‐filled porosity and air permeability. Tracer experiments were conducted at 1 mm h−1 and with a suction of 5 hPa. A mobile–immobile region model (MIM) and a three‐region model (2MIM) with two mobile and one immobile region were used for describing the breakthrough curves (BTCs). The 2MIM model was able to fit the data well and predicted the existence of two mobile flow regions, most pronounced at higher clay content. The 12% clay soil exhibited matrix‐dominated flow behavior, which is probably attributable to a large fraction of drained pores disconnecting the rapidly conducting flow system. Soils with ≥18% clay exhibited asymmetrical BTCs with early breakthrough and tailing and an increasing amount of immobile water, indicating preferential flow. Drying and rewetting, because of associated changes in the pore structure, significantly reduced the degree of preferential flow.

This study investigated in situ colloid mobilization and leaching from unsaturated, undisturbed soil columns to evaluate the processes controlling colloid mobilization in structured soils. A total of 54 soil columns sampled along a natural clay gradient representing six clay contents (12, 18, 24, 28, 37, and 43% clay) were equilibrated to three different initial matric potentials (IMP) ψ = −2.5 (wet), −100 (moderately wet), and −15500 hPa (dry) followed by 5 d of irrigation at 1 mm h−1 and applying a suction of 5 hPa at the lower boundary. The results revealed that (i) colloid leaching from the initially wet and moderately wet soils decreased with increasing clay content, (ii) colloid leaching from the initially dry soils was low and independent of clay content, and (iii) the leaching of total organic C (TOC) consisted mainly of dissolved organic C (DOC), and drying to −15500 hPa increased the leaching of C. In situ colloid mobilization and leaching was related to measurements of low‐energy water‐dispersible colloids (LE‐WDC). Results indicate that in situ colloid mobilization from the initially wet and moderately wet 12% clay soils subjected to matrix‐dominated flow behavior was controlled mainly by the time‐dependent increase in colloid dispersion, while colloid mobilization from the initially dry soils was limited by the strong and persistent association created during the drying. In the more clayey soils, which were dominated by preferential flow, a lower displacement of high–ionic strength soil water with low–ionic strength rainwater may contribute to the inherently lower dispersibility in controlling colloid mobilization.