The Influence of Two‐Stage Ditches with Constructed Floodplains on Water Column Nutrients and Sediments in Agricultural Streams

The two‐stage ditch is a novel management practice originally implemented to increase bank stability through floodplain restoration in channelized agricultural streams. To determine the effects of two‐stage construction on sediment and nutrient loads, we monitored turbidity, and also measured total suspended solids (TSS), dissolved inorganic nitrogen (N) species, and phosphorus (P) after two‐stage ditch construction in reference and manipulated reaches of four streams. Turbidity decreased during floodplain inundation at all sites, but TSS and P, soluble reactive phosphorus (SRP) and total phosphorus (TP) decreased only in the two‐stage ditches with longer duration of inundation. Both TSS and TP were positively correlated within individual streams, but neither were correlated with turbidity. Phosphorus was elevated in the stream to which manure was applied adjacent to the two‐stage reach, but not the reference reach, suggesting that landscape nutrient management plans could restrict nutrient transport to the stream, ultimately determining the efficacy of instream management practices. In addition, ammonium and nitrate decreased in two‐stage reaches with lower initial N concentrations. Overall, results suggest that turbidity, TSS, and TP were reduced during floodplain inundation, but the two‐stage alone may not be effective for managing high inorganic N loads.     

Impact of Dredging on Phosphorus Transport in Agricultural Drainage Ditches of the Atlantic Coastal Plain1

Abstract: Drainage ditches can be a key conduit of phosphorus (P) between agricultural soils of the Atlantic Coastal Plain and local surface waters, including the Chesapeake Bay. This study sought to quantify the effect of a common ditch management practice, sediment dredging, on fate of P in drainage ditches. Sediments from two drainage ditches that had been monitored for seven years and had similar characteristics (flow, P loadings, sediment properties) were sampled (0‐5 cm) after one of the ditches had been dredged, which removed fine textured sediments (clay = 41%) with high organic matter content (85 g/kg) and exposed coarse textured sediments (clay = 15%) with low organic matter content (2.2 g/kg). Sediments were subjected to a three‐phase experiment (equilibrium, uptake, and release) in recirculating 10‐m‐long, 0.2‐m‐wide, and 5‐cm‐deep flumes to evaluate their role as sources and sinks of P. Under conditions of low initial P concentrations in flume water, sediments from the dredged ditch released 13 times less P to the water than did sediments from the ditch that had not been dredged, equivalent to 24 mg dissolved P. However, the sediments from the dredged ditch removed 19% less P (76 mg) from the flume water when it was spiked with dissolved P to approximate long‐term runoff concentrations. Irradiation of sediments to destroy microorganisms revealed that biological processes accounted for up to 30% of P uptake in the coarse textured sediments of the dredged ditch and 18% in the fine textured sediments of the undredged ditch. Results indicate that dredging of coastal plain drainage ditches can potentially impact the P buffering capacity of ditches draining agricultural soils with a high potential for P runoff.     

Improvements in Fluvial Stability Associated with Two‐Stage Ditch Construction in Mower County,Minnesota

Water quality and stream habitat in agricultural watersheds are under greater scrutiny as hydrologic pathways are altered to increase crop production. Agricultural drainage ditches function to remove water quickly from farmed landscapes. Conventional ditch designs lack the form and function of natural stream systems and tend to be unstable and provide inadequate habitat. In October of 2009, 1.89 km of a conventional drainage ditch in Mower County, Minnesota, was converted to an alternative system with a two‐stage channel to investigate the improvements in water quality, stability, and habitat. Longitudinal surveys show a 12‐fold increase in the pool‐riffle formation. Cross‐sectional surveys show an average increase in bankfull width of approximately 10% and may be associated to an increased frequency in large storm events. The average increase in bankfull depth was estimated as 18% but is largely influenced by pool formation. Rosgen Stability Analyses show the channel to be highly stable and the banks at a low risk of erosion. The average bankfull recurrence interval was estimated to be approximately 0.30 years. Overall, the two‐stage ditch design demonstrates an increase in fluvial stability, creating a more consistent sediment budget, and increasing the frequency of important instream habitat features, making this best management practice a viable option for addressing issues of erosion, sediment imbalance, and poor habitat in agricultural drainage systems.     

Spatial variation of soil phosphorus within a drainage ditch network

Agricultural drainage ditches serve as P transport pathways from fields to surface waters. Little is known about the spatial variation of P at the soil-water interface within ditch networks. We quantified the spatial variation of surficial (0-5 cm) soil P within vegetated agricultural ditches on a farm in Princess Anne, MD with an approximately 30-yr history of poultry litter application. Ditch soils from 10 ditches were sampled at 10-m intervals and analyzed for acid ammonium oxalate-extractable P, Fe, Al (P(ox), Fe(ox), Al(ox)), and pH. These variables were spatially autocorrelated. Oxalate-P (min = 135 mg kg(-1), max = 6919 mg kg(-1), mean = 700 mg kg(-1)) exhibited a high standard deviation across the study area (overall 580 mg kg(-1)) and within individual ditches (maximum 1383 mg kg(-1)). Several ditches contained distinct areas of high P(ox), which were associated with either point- or nonpoint-P sources. Phosphorus was correlated with Al(ox) or Fe(ox) within specific ditches. Across all ditches, Al(ox) (r = 0.80; p < 0.001) was better correlated with P(ox) than was Fe(ox) (r = 0.44; p < 0.001). The high level of spatial variation of soil P observed in this ditch network suggests that spatially distributed sampling may be necessary to target best management practices and to model P transport and fate in ditch networks.     

Evaluating Geomorphic Change in Constructed Two‐Stage Ditches

Straight, trapezoidal‐shaped surface drainage channels efficiently drain the soil profile, but their deviations from natural fluvial conditions drive the need for frequent maintenance. Ecological and socioeconomic impacts of drainage ditch maintenance activities can be significant, leading to harmful algal blooms and increased sedimentation. We developed a two‐stage ditch design that is more consistent with fluvial form and process. The approach has potential to enhance ecological services while meeting drainage needs essential for agricultural production. We studied geomorphic change of the inset channel, benches and banks of seven two‐stage ditches in Ohio, Indiana, and Michigan. Three to 10 years after construction, inset channel changes reflected natural adjustments, but not all ditches had reached their quasi‐equilibrium state. Ditches had experienced both degradation and aggradation on the benches at a rate of 0.5‐13 mm/yr. Aggradation on the benches was not likely to threaten tile drain outlets. Localized scour was observed on the banks at some sites, but at all but one site changes were not statistically significant. Except for the removal of woody vegetation, none of the ditches required routine maintenance since construction. Two‐stage ditches can be a stable, viable option for drainage ditch management if designed and installed properly on the landscape.     

Dredging of drainage ditches increases short-term transport of soluble phosphorus

Managed drainage ditches are common in the midwestern United States. These ditches are designed to remove water from fields as quickly as possible, and sediment buildup necessitates dredging, to ensure adequate water removal. This laboratory study was conducted to determine the impact of ditch dredging on soluble phosphorus (P) transport. Ditch sediments were collected from a drainage ditch in northeastern Indiana immediately before and after dredging. The sediments were placed in a stream simulator, and stream water was loaded with 0.55 mM P for 5 d (adsorption experiment). Water was then removed, and "clean" water (no P added) was used for a desorption experiment, lasting 1 d. During the adsorption experiment, pre-dredged sediments were able to remove P from the water column quicker, and P concentrations 120 h after introduction of high P water were lower for the pre-dredged sediments (0.075 mM P) than the dredged sediments (0.111 mM P). During the desorption experiment, P was released to the water column slower in the pre-dredged treatment than the dredged treatment (instantaneous flux at t = 0 was 0.205 microM P h(-1) for pre-dredged and 0.488 microM P h(-1) for dredged). This occurred despite higher Mehlich 3-extractable P in the pre-dredged sediments than the dredged sediments. Equilibrium phosphorus concentrations (EPCo) were lower in the pre-dredged sediments during both adsorption and desorption experiments. Transport of soluble P immediately after dredging will likely increase in drainage ditches; however, dredging is a necessary management tool to ensure adequate discharge of water from surrounding fields.     

Agricultural drainage ditches mitigate phosphorus loads as a function of hydrological variability

Phosphorus (P) loading from nonpoint sources, such as agricultural landscapes, contributes to downstream aquatic ecosystem degradation. Specifically, within the Mississippi watershed, enriched runoff contributions have far-reaching consequences for coastal water eutrophication and Gulf of Mexico hypoxia. Through storm events, the P mitigation capacity of agricultural drainage ditches under no-till cotton was determined for natural and variable rainfall conditions in north Mississippi. Over 2 yr, two experimental ditches were sampled monthly for total inorganic P concentrations in baseflow and on an event-driven basis for stormflows. Phosphorus concentrations, Manning's equations with a range of roughness coefficients for changes in vegetative densities within the ditches, and discharge volumes from Natural Resources Conservation Service dimensionless hydrographs combined to determine ranges in maximum and outflow storm P loads from the farms. Baseflow regressions and percentage reductions with P concentrations illustrated that the ditches alternated between being a sink and source for dissolved inorganic P and particulate P concentrations throughout the year. Storm event loads resulted in 5.5% of the annual applied fertilizer to be transported into the drainage ditches. The ditches annually reduced 43.92 +/- 3.12% of the maximum inorganic effluent P load before receiving waters. Agricultural drainage ditches exhibited a fair potential for P mitigation and thus warrant future work on controlled drainage to improve mitigation capacity.     

Mitigation assessment of vegetated drainage ditches for collecting irrigation runoff in California

Widespread contamination of California water bodies by the organophosphate insecticides diazinon and chlorpyrifos is well documented. While their usage has decreased over the last few years, a concomitant increase in pyrethroid usage (e.g., permethrin) (replacement insecticides) has occurred. Vegetated agricultural drainage ditches (VADD) have been proposed as a potential economical and environmentally efficient management practice to mitigate the effects of pesticides in irrigation and storm runoff. Three ditches were constructed in Yolo County, California for a field trial. A U-shaped vegetated ditch, a V-shaped vegetated ditch, and a V-shaped unvegetated ditch were each amended for 8 h with a mixture of diazinon, permethrin, and suspended sediment simulating an irrigation runoff event. Water, sediment, and plant samples were collected spatially and temporally and analyzed for diazinon and permethrin concentrations. Pesticide half-lives were similar between ditches and pesticides, ranging from 2.4 to 6.4 h. Differences in half-distances (distance required to reduce initial pesticide concentration by 50%) among pesticides and ditches were present, indicating importance of vegetation in mitigation. Cis-permethrin half-distances in V ditches ranged from 22 m (V-vegetated) to 50 m (V-unvegetated). Half-distances for trans-permethrin were similar, ranging from 21 m (V-vegetated) to 55 m (V-unvegetated). Diazinon half-distances demonstrated the greatest differences (55 m for V-vegetated and 158 m for V-unvegetated). Such economical and environmentally successful management practices will offer farmers, ranchers, and landowners a viable alternative to more conventional (and sometimes expensive) practices.     

Occurrence of arsenic and phosphorus in ditch flow from litter-amended soils and barn areas

Little is known about the fate of arsenic (As) in land-applied litter from chickens that have been fed roxarsone, an organic feed additive containing As. This study seeks to elucidate the transfer of As in runoff from ditch-drained soils of the poultry-producing region of the Delmarva Peninsula by tracking As and phosphorus (P) export from seven drainage ditches over two water-years (1 July 2005 to 30 June 2007). Annual losses of As from ditches ranged from 0.004 to 0.071 kg ha(-1) while P losses ranged from 0.33 to 18.56 kg ha(-1), with the largest loads associated with a litter storage shed that served as a point source. Event-based As and P losses in ditch flow fluctuated by a factor of 162 and 1882, respectively. The two elements were correlated in flow from the ditch draining a litter storage shed (r = 0.99), and in sediment extracts in soils near the litter shed (r = 0.73), pointing to similar behavior under point source conditions. Indeed, As and P exhibited similar behavior within storms for all ditches, characterized by relatively high initial concentrations subject to rapid concentration declines before peak flow, consistent with dilution of a finite source. However, As and P concentrations varied significantly between ditches and showed considerable temporal variability within ditches, with no clear seasonal trends or associations with current management strategies. The results suggest that similar management strategies might be effective for As and P point sources, but that field management practices geared toward controlling nonpoint source P losses may not readily transfer to the control of As losses.     

Effectiveness of Constructed Overland Flow Areas in Decreasing Diffuse Pollution from Forest Drainages

Forestry is the largest scale human impact affecting catchments in Finland and a prominent source of diffuse pollution in many water courses. Among the forestry activities, draining of wetlands had the most pronounced impacts on sediment, nutrient, and metal loading in the past. At present, renovation of old ditches and fertilization of peatlands constitute the major risk of forestry-induced diffuse pollution. Contemporary forestry aims at decreasing this risk with various riparian buffer strip designs. Among such designs, creation of overland flow areas by plugging the outlet ditches is increasingly used. Our objectives were to evaluate the potential of constructed overland flow areas to function as riparian buffers and estimate the quality and quantity of diffuse pollution from old versus recent forest drainages. We studied retention and release of pollutants from 20 constructed, 2- to 10-m-wide overland flow areas receiving drainage water from forested peatlands. Drainage waters were sampled above and below the plugged ditches three times per year from 1998 to 1999. Chemical oxygen demand and nutrient and metal loads and concentrations varied strongly between seasons, years, and drainage areas. Areas subjected to recent ditch renovations and fertilizations had clearly elevated seasonal loads and concentrations of total phosphorus (TP), PO₄, Fe, and Al in comparison to old treatment areas. Especially TP loads were high above the national average values measured for forestry-induced diffuse pollution. In general, water quality above and below the buffer strips did not differ significantly. Our results indicate that plugged outlet ditches and associated narrow overland flow areas do not function as proper buffers in peatland areas. We suggest that wider buffers with extensive overland flow areas are needed in order to control diffuse pollution from forested and drained peatlands.     

Vertical distribution of phosphorus in agricultural drainage ditch soils

Pedological processes such as gleization and organic matter accumulation may affect the vertical distribution of P within agricultural drainage ditch soils. The objective of this study was to assess the vertical distribution of P as a function of horizonation in ditch soils at the University of Maryland Eastern Shore Research Farm in Princess Anne, Maryland. Twenty-one profiles were sampled from 10 agricultural ditches ranging in length from 225 to 550 m. Horizon samples were analyzed for total P; water-extractable P; Mehlich-3 P; acid ammonium oxalate-extractable P, Fe, and Al (P ox, Fe ox, Al ox); pH; and organic C (n = 126). Total P ranged from 27 to 4882 mg kg(-1), P ox from 4 to 4631 mg kg(-1), Mehlich-3 P from 2 to 401 mg kg(-1), and water-extractable P from 0 to 17 mg kg(-1). Soil-forming processes that result in differences between horizons had a strong relationship with various P fractions and P sorption capacity. Fibric organic horizons at the ditch soil surface had the greatest mean P ox, Fe ox, and Al ox concentrations of any horizon class. Gleyed A horizons had a mean Fe ox concentrations 2.6 times lower than dark A horizons and were significantly lower in total P and P ox. Variation in P due to organic matter accumulation and gleization provide critical insight into short- and long-term dynamics of P in ditch soils and should be accounted for when applying ditch management practices.     

Two‐Stage Ditch Floodplains Enhance N‐Removal Capacity and Reduce Turbidity and Dissolved P in Agricultural Streams

Two‐stage ditches represent an emerging management strategy in artificially drained agricultural landscapes that mimics natural floodplains and has the potential to improve water quality. We assessed the potential for the two‐stage ditch to reduce sediment and nutrient export by measuring water column turbidity, nitrate (NO₃^?), ammonium (NH₄⁺), and soluble reactive phosphorus (SRP) concentrations, and denitrification rates. During 2009‐2010, we compared reaches with two‐stage floodplains to upstream reaches with conventional trapezoid design in six agricultural streams. At base flow, these short two‐stage reaches (<600 m) reduced SRP concentrations by 3‐53%, but did not significantly reduce NO₃^? concentrations due to very high NO₃^? loads. The two‐stage also decreased turbidity by 15‐82%, suggesting reduced suspended sediment export during floodplain inundation. Reach‐scale N‐removal increased 3‐24 fold during inundation due to increased bioreactive surface area with high floodplain denitrification rates. Inundation frequency varied with bench height, with lower benches being flooded more frequently, resulting in higher annual N‐removal. We also found both soil organic matter and denitrification rates were higher on older floodplains. Finally, influence of the two‐stage varied among streams and years due to variation in stream discharge, nutrient loads, and denitrification rates, which should be considered during implementation to optimize potential water quality benefits.     

Hydrological variability and agricultural drainage ditch inorganic nitrogen reduction capacity

The application of inorganic nitrogen fertilizers on agricultural landscapes has the potential to generate concerns of environmental degradation at fine to coarse scales across the catchment and landscape. Inorganic nitrogen species (NO3*, NO2*, and NH3) are typically associated with subsurface flow processes; however, surface runoff from rainfall events in no-till agriculture with inorganic surface fertilizers might contribute to downstream eutrophication. Inorganic nitrogen reduction capacity of agricultural drainage ditches under no-till cotton was determined under natural, variable rainfall conditions in northern Mississippi. Monthly grab baseflow samples and storm-generated flow samples were variably sampled temporally within two experimental farm ditches over 2 yr. Inorganic nitrogen concentrations, in conjunction with Manning's equation and Natural Resources Conservation Service dimensionless hydrographs, provided individual water volumes per storm event and thus maximum effluent and outflow nitrogen loads. Base and stormflow regression results indicate drainage ditches reducing NO3* and NH3 over the length of the ditch for growing and dormant seasons. Overall, maximum storm loads of dissolved inorganic nitrogen (DIN) from the farm over the 2-yr sampling period accounted for 2.2% of the initial fertilizer application, of which 1.1% left the ditch (0.84 kg ha(-1) yr(-1)) (a 57% ditch reduction of DIN load over 2 yr). Long-term sampling incorporating data on application and loss of fertilizers and farm management will provide critical information for farmers and scientists on the potential of economic gains and downstream ecosystem eutrophication, respectively.     

Habitat Improvements and Fish Community Response Associated with an Agricultural Two‐Stage Ditch in Mower County,Minnesota

Water quality and stream habitat in agricultural watersheds are under greater scrutiny as hydrologic pathways are altered to increase crop production. Ditches have been traditionally constructed to remove water from agricultural lands. Little attention has been placed on alternative ditch designs that are more stable and provide greater habitat diversity for wildlife and aquatic species. In 2009, 1.89 km of a conventional drainage ditch in Mower County, Minnesota, was converted to a two‐stage ditch (TSD) with small, adjacent floodplains to mimic a natural system. Cross section surveys, conducted pre‐ and post‐construction, generally indicate a stable channel with minor adjustments over time. Vegetation surveys showed differences in species composition and biomass between the slopes and the benches, with changes ongoing. Longitudinal surveys demonstrated a 12‐fold increase in depth variability. Fish habitat quality improved with well‐sorted gravel riffles and deeper pool habitat. The biological response to improved habitat quality was investigated using a Fish Index of Biological Integrity (FIBI). Our results show higher FIBI scores post‐construction with scores more similar to natural streams. In summary, the TSD demonstrated improvements in riparian and instream habitat quality and fish communities, which showed greater fish species richness, higher percentages of gravel spawning fish, and better FIBI scores. This type of management tool could benefit ditches in other regions where gradient and geology allow.     

Hydrologic and Water Quality Functions of a Disturbed Wetland in an Agricultural Setting1

Abstract: In 2006, we collected flow, sediment, and phosphorus (P) data at stream locations upstream and downstream of a small degraded wetland in south‐central Wisconsin traversed by a stream draining a predominantly agricultural watershed. The amount of sediment that left the wetland in the two largest storms, which accounted for 96% of the exported sediment during the observation period, was twice the amount that entered the wetland, even though only 50% of the wetland had been inundated. This apparently anomalous result is due to erosion of sediment that had accumulated in the low‐gradient channel and to the role of drainage ditches, which trapped sediment during the wetland‐filling phase. In the case of total P, the inflow to the wetland approximately equaled the outflow, although the wetland sequestered 30% of the incoming dissolved reactive P. The discrepancy is almost certainly due to net export of sediment. Many wetlands in the glaciated midwestern United States are ditched and traversed by low‐gradient channels draining predominantly agricultural areas, so the processes observed in this wetland are likely to be common in that region. Knowledge of this behavior presents opportunities to improve water quality in this and similar regions.     

Implementing Innovative Drainage Management Practices in the Mississippi River Basin to Enhance Nutrient Reductions

In the Mississippi River Basin (MRB), practices that enhance drainage (e.g., channelization, tile drainage) are necessary management tools in order to maintain optimal agricultural production in modern farming systems. However, these practices facilitate, and may speed the delivery of excess nutrients and sediments to downstream water bodies via agricultural streams and ditches. These nonpoint sources contribute to elevated nutrient loading in the Gulf of Mexico, which has been linked to widespread hypoxia and associated ecological and economic problems. Research suggests agricultural drainage ditches are important links between farm fields and downstream ecosystems, and application of new management practices may play an important role in the mitigation of water quality impairments from agricultural watersheds. In this article, we describe how researchers and producers in the MRB are implementing and validating novel best management practices (BMPs) that if used in tandem could provide producers with continued cropping success combined with improved environmental protection. We discuss three BMPs — low‐grade weirs, slotted inlet pipes, and the two‐stage ditch. While these new BMPs have improved the quality of water leaving agricultural landscapes, they have been validated solely in isolation, at opposite ends of the MRB. These BMPs have similar function and would greatly benefit from stacked incorporation across the MRB to the benefit of the basin as a whole.     

Evaluating agricultural best management practices in tile-drained subwatersheds of the Mackinaw River, Illinois

Best management practices (BMPs) are widely promoted in agricultural watersheds as a means of improving water quality and ameliorating altered hydrology. We used a paired watershed approach to evaluate whether focused outreach could increase BMP implementation rates and whether BMPs could induce watershed-scale (4000 ha) changes in nutrients, suspended sediment concentrations, or hydrology in an agricultural watershed in central Illinois. Land use was >90% row crop agriculture with extensive subsurface tile drainage. Outreach successfully increased BMP implementation rates for grassed waterways, stream buffers, and strip-tillage within the treatment watershed, which are designed to reduce surface runoff and soil erosion. No significant changes in nitrate-nitrogen (NO-N), total phosphorus (TP), dissolved reactive phosphorus, total suspended sediment (TSS), or hydrology were observed after implementation of these BMPs over 7 yr of monitoring. Annual NO-N export (39-299 Mg) in the two watersheds was equally exported during baseflow and stormflow. Mean annual TP export was similar between the watersheds (3.8 Mg) and was greater for TSS in the treatment (1626 ± 497 Mg) than in the reference (940 ± 327 Mg) watershed. Export of TP and TSS was primarily due to stormflow (>85%). Results suggest that the BMPs established during this study were not adequate to override nutrient export from subsurface drainage tiles. Conservation planning in tile-drained agricultural watersheds will require a combination of surface-water BMPs and conservation practices that intercept and retain subsurface agricultural runoff. Our study emphasizes the need to measure conservation outcomes and not just implementation rates of conservation practices.     

Controlling runoff from subtropical pastures has differential effects on nitrogen and phosphorus loads

A 4-yr (2005-2008) study was conducted to evaluate the potential of pasture water management for controlling nutrient losses in surface runoff in the Northern Everglades. Two pasture water management treatments were investigated on Bahia grass ( Flüggé) pastures: reduced flow and unobstructed flow. The reduced flow treatment was applied to four of eight 20.23-ha pastures by installing water control structures in pasture drainage ditches with flashboards set at a predetermined height. Four other pastures received the unobstructed-flow treatment, in which surface runoff exited pastures unimpeded. Automated instruments measured runoff volume and collected surface water samples for nutrient analysis. In analyzing data for before-after treatment analysis, the 2005 results were removed because of structural failure in water control structures and the 2007 results were removed because of drought conditions. Pasture water retention significantly reduced annual total nitrogen (TN) loads, which were 11.28 kg ha and 6.28 kg ha, respectively, in pastures with unobstructed and reduced flow. Total phosphorus (TP) loads were 27% lower in pastures with reduced flow than in pastures with unobstructed flow, but this difference was not statistically significant. Concentrations of available soil P were significantly greater in pastures with reduced flow. Pasture water retention appears to be an effective approach for reducing runoff volume and TN loads from cattle pastures in the Northern Everglades, but the potential to reduce TP loads may be diminished if higher water table conditions cause increased P release from soils, which could result in higher P concentration in surface runoff.     

Regional scale variability in sediment and nutrient delivery from small agricultural watersheds

Although many studies have pointed out the various controlling factors of sediment and nutrient delivery on a plot or watershed scale, little is known on the spatial variability of sediment and nutrient delivery on a regional scale. This study was conducted to reveal regional variations in sediment-associated nutrient delivery in central Belgium. Sediment deposited in 13 small retention ponds was sampled and analyzed for total phosphorus (TP), K, Mg, and Ca content. The TP content of the sediment deposits varied from 510 to 2001 mg P per kg sediment. Nutrients are predominantly fixed on the very fine sediment fraction (<16 microm), which is the reason why the nutrient trap efficiency of the ponds is only a fraction of the sediment trap efficiency. Average nutrient trap efficiency of the studied ponds varies between 4 and 31%, whereas sediment trap efficiency varies between 10 and 72%. For watersheds ranging from 7 to 4873 ha, sediment yield ranged between 1.2 and 20.6 Mg ha(-1) yr(-1), whereas TP export varied from 1.8 to 39.7 kg ha(-1) yr(-1). The observed spatial variability in nutrient losses is primarily attributed to regional variations in erosion and sediment yield values and to a far lesser degree to the spatial variations in fertilizer application. Redistribution of manure in the framework of an agricultural policy may increase the rate of nutrient delivery by ways of erosion and sediment transport.     

Longitudinal and Seasonal Patterns of Phosphorus in Riverbed Sediments

Seasonal variability in particulate phosphorus (P) was characterized in riverbed sediment along the longitudinal gradient of an agricultural watershed in southern Ontario. Particulate P forms, nonapatite P (NAIP), apatite P (AP) and organic P (P), were determined by sequential extraction. Increased concentrations of NAIP and OP in fine-grained sediment ( 63 mu m) in upper reaches of the watershed are attributed to agricultural activity in the watershed. From a management perspective, this study provides a locational focus for P abatement programmes where a combination of traditional best management practices and improved nutrient management practices should be targeted.