Particulate P Research Articles

Surface runoff and sub-surface drainage phosphorus losses under regular free drainage and controlled drainage with sub-irrigation systems in southern Ontario

Tan, C. S. and Zhang, T. Q. 2011. Surface runoff and sub-surface drainage phosphorus losses under regular free drainage and controlled drainage with sub-irrigation systems in Southern Ontario. Can. J. Soil Sci. 91: 349–359. Soil phosphorus (P) loss and its partition in various pathways may differ depending on water management practices. A study was conducted using large field plots equipped with automatic flow volume measurement and sampling systems over a 5-yr period to determine the effectiveness of regular free drainage (RFD) and controlled drainage with sub-irrigation (CDS) for mitigating soil P losses of various forms [dissolved reactive P (DRP), dissolved un-reactive P (DURP), and particulate P (PP)] and to identify the relative roles of surface runoff and sub-surface tile drainage in soil P loss. For RFD, flow weighted-means (FWM) of DRP, DURP, PP and the total P (TP) concentrations over the 5-yr period were averaged at 0.057, 0.057, 0.627, and 0.741mg P L−1 in surface runoff water and at 0.034, 0.053, 0.393, and 0.480 mg P L−1 in tile drainage water, respectively. CDS increased FWM of DRP, DURP and the TP concentrations in surface runoff water and DRP concentration in tile drainage, but decreased the FWM of DURP, PP and the TP concentrations in tile drainage water. The CDS produced similar annual total dissolved P (TDP) loss, the sum of DRP and DURP, but reduced losses of PP by 15% and of TP by 12%, relative to RFD. The PP loss accounted for over 80% of TP loss for both CDS and RFD. Of the total soil P loss, from 3 to 5% was accounted for in surface runoff water, while from 95 to 97% was accounted for in tile drainage water, for RFD. For CDS, from 29 to 35% of the total soil P loss was in surface runoff water, while 65 to 71% was in tile drainage water. Subsurface tile drainage played a predominant role in soil P loss. CDS can be considered a beneficial management practice to reduce soil P loss under the similar climate and relatively flat field conditions in Southern Ontario.

A Spatial Analysis of Phosphorus in the Mississippi River Basin

Phosphorus (P) in rivers in the Mississippi River basin (MRB) contributes to hypoxia in the Gulf of Mexico and impairs local water quality. We analyzed the spatial pattern of P in the MRB to determine the counties with the greatest January to June P riverine yields and the most critical factors related to this P loss. Using a database of P inputs and landscape characteristics from 1997 through 2006 for each county in the MRB, we created regression models relating riverine total P (TP), dissolved reactive P (DRP), and particulate P (PP) yields for watersheds within the MRB to these factors. Riverine yields of P were estimated from the average concentration of each form of P during January to June for the 10-yr period, multiplied by the average daily flow, and then summed for the 6-mo period. The fraction of land planted in crops, human consumption of P, and precipitation were found to best predict TP yields with a spatial error regression model ( = 0.48, = 101). Dissolved reactive P yields were predicted by fertilizer P inputs, human consumption of P, and precipitation in a multiple regression model ( = 0.42, = 73), whereas PP yields were explained by crop fraction, human consumption of P, and soil bulk density in a spatial error regression model ( = 0.49, = 61). Overall, the Upper Midwest's Cornbelt region and lower Mississippi basin had the counties with the greatest P yields. These results help to point out specific areas where agricultural conservation practices that reduce losses to streams and rivers and point source P removal might limit the intensity or spatial occurrence of Gulf of Mexico hypoxia and improve local water quality.

Effects of snowmelt on phosphorus and sediment losses from agricultural watersheds in Eastern Canada

Snowmelt is the most important hydrological event in cold climates. However, snowmelt effects on suspended sediment (SS) and phosphorus (P) loss are poorly documented in Canada. Using two agricultural watersheds in Eastern Canada, this study aimed to quantify SS and P loss during the snowmelt period and to investigate how snowmelt contributes SS and P loss. Water samples were collected from the outlets of the Bras d’Henri watershed (BHW, 2007–2009) and Black Brook watershed (BBW, 2008–2009) and measured for SS and P concentrations. Hydrological parameters (precipitation, snow water equivalent, and runoff discharge), soil frozen status and soil temperature were also measured. Results revealed inter-annual variation of snowmelt conditions and SS and P losses in each watershed. The 2008 snowmelt in BHW and BBW mainly occurred on unfrozen soils, while the 2007 and 2009 snowmelts in BHW and 2009 snowmelt in BBW mainly on frozen soils. In BHW, 2008 snowmelt caused much higher median concentrations of SS, total P (TP), dissolved P (DP) and particulate P (PP) in stream water than 2007 and 2009; ratios of PP fractions in TP were variable with events but the median values were similar, suggesting both DP and PP important contrubutors to TP loss. In BBW, the median concentration of dissolved reactive phosphorus (DRP) in stream water was greater in 2008 snowmelt than in 2009 snowmelt; PP dominated TP loss. This study also suggests that soil state (i.e. frozen status) and rainfall were the most important factors influencing SS and P losses during snowmelt. Furthermore, snowmelt P export represented more than 20% of the total annual P export in BHW, and more than 12% of the annual DRP export in BBW. Thus, we strongly recommend adopting Best Management Practices (BMPs) that specifically target sediment and P loss during snowmelt.

Bioavailability of Phosphorus Fractions for Incepting of Chlorella pyrenoidosa in the Lower Reaches of the Hanjiang River in China

Using total phosphorus (TP) concentrations to estimate eutrophication risk is problematic for management purposes, as only some forms of P are biologically available for phytoplankton growth. This study estimated the bioavailability of P fractions of water and the floodplain sediment in the lower reaches of the Hanjiang River in China using multiple regressions. And the effects of concentrations of P fractions on algal growth were also studied. The results showed that the total soluble P (TSP) and the particulate P (PP) can well predict the growth of Chlorella pyrenoidosa in water samples. And the exchangeable P (exch-P) and the P bound to Al, Fe, and Mn oxides and hydroxides (Fe/Al–P) were adapted to predicting the growth of Chlorella pyrenoidosa in the floodplain sediment samples.

Surface and Subsurface Phosphorus Losses from Sugarcane Fields with Different Management Practices

Phosphorus losses in runoff from sugarcane fields can contribute to non-point source pollution of surface and subsurface waters. The objective of this study was to evaluate the effects of three different management practices on P losses in surface runoff and subsurface leaching from sugarcane (Saccharum officinarum L.) fields. Field experiments with treatments including conventional burning (CB), compost application with burning (COMB), and remaining green cane trash blanketing (GCTB) treatments were carried out to assess these management practice effects on P losses from sugarcane fields. In the CB treatment, sugarcane residue was burned after harvest. The COMB treatment consisted of compost applied at “off bar” with sugarcane residue burned immediately after harvest. Compost was applied in the amount of 13.4 Mg ha−1 annually, 8 weeks before planting. In the GCTB treatment, sugarcane residue was raked off from the row tops and remained in the wheel furrow after harvest. Surface runoff was collected with automatic refrigerated samplers, and subsurface leachate was collected with pan lysimeters over a period of 3 years. Measured concentrations of total P (TP), dissolved reactive P (DRP), and particulate P (PP) in surface runoff from the COMB treatment were significantly higher than concentrations from the CB and GCTB treatments. The mean losses of P (TP and DRP) after burning (postharvest, years 2 and 3) were significantly greater than the no-burn treatment (preharvest, year 1) in the CB, COMB, and CB/COMB/GCTB combined options. Additionally, the mean losses of total suspended solid and total combustible solids in residue burning were, on average, 2.7 and 2.2 times higher than the no-burn practices, respectively (preharvest and GCTB treatment). Annual P losses from surface runoff in the third year of study were 12.90%, 6.86%, and 10.23% of applied P in CB, COMB, and GCTB treatments, respectively. However, the percent of annual DRP losses from applied P in COMB and GCTB treatments was similar magnitude, and their values were less than 50% compared to the value from CB treatment. In the leaching study, percent of monthly mean TP and DRP losses in the COMB and GCTB treatments were greatly reduced. Based on these results, the COMB and GCTB procedures were equally recommended as sugarcane management practices that improve water quality in both surface runoff and subsurface leachate.

Predicting mobilization of suspended sediments and phosphorus from soil properties: a case study from the north west Po valley, Piemonte, Italy

AbstractPedotransfer functions are useful for helping environmental agencies to identify soils vulnerable to both total suspended sediment (TSS) and phosphorus (P) mobilization and transport in storm runoff in order to establish strategies to control eutrophication. The DESPRAL test was applied to 90 different and representative agricultural soils of the Piemonte region in the west Po valley, Italy, to investigate the relationships between soil properties and potential TSS and P mobilization in land runoff. Sixty six percent of the variance in TSS is explained by clay, silt and Olsen P content using a multiple regression. Particulate P (PP) was the dominant fraction [89% of total P (TP)] dispersed by the test. The P enrichment of the dispersed particles is strongly related to the amount of TSS and soil clay content according to an exponential function, with a threshold of 20% clay below which sediment P content and PER (P enrichment ratio) increased. The ratio of PP to TSS is also significantly related to the P concentration of soil clay content (r2 = 0.69). The soluble reactive P (RP) fraction could be estimated by soil Olsen P (r2 = 0.74). Whilst the study suggests some key relationships between soil properties and potential mobilization of sediment and soluble P from cultivated soils of the Piemonte region, soil properties could not be used to estimate the amount of TP or particulate P dispersed by the test. Identification of the risk of TSS, PP and TP transfer within individual fields cannot simply rely on pedotransfer functions but must take account of local site conditions, crop cover and variable management.

Wildfire impacts on hillslope sediment and phosphorus yields

Purpose The impact of wildfire on (a) slope hydrological processes, (b) soil erodibility, and (c) post-fire hillslope sediment and phosphorus (P; dissolved and particulate) yields are quantified for natural forest areas of the burned Evrotas River basin, Peloponnese, Greece. Further, the geochemical partitioning of P in burned and unburned sediment is evaluated by sequential extraction to assess potential bioavailability of particulate P (PP) in downstream aquatic ecosystems.

Long-term monitoring of buffer zone efficiency under different cultivation techniques in boreal conditions

Vegetative buffer zones (BZs) between fields and watercourses are one way to minimize phosphorus (P) losses from fields to surface runoff and thus decrease eutrophication and blue-green algal blooms in lakes and coastal waters. A 6-plot experimental field was established in clay soil (Vertic Cambisol) in SW Finland in 1991 to study long-term changes in functioning of BZs and their retention capacity for total solids (TS), total P (TP), dissolved reactive P (DRP) and particulate P (PP) in different seasons. The steep slope in lower sections of four plots (18 m wide × 70 m long) was planted with 10-m-wide mowed grass buffer zones (GBZ) or unmowed vegetated buffer zones (VBZ) growing natural herbage and shrubs. Surface runoff water samples from the GBZ and VBZ plots were compared to samples from plots cultivated without a buffer (NBZ). The source field area in all plots and the steep slope (12–18%) on the NBZ were ploughed in autumn, and sown with barley ( Hordeum vulgare) or oats ( Avena sativa) in spring (conventional tillage, 1991–2001), sown with grass and grazed (72–234 cow grazing days ha −1 yr −1; 2003–2005) and direct drilled without tillage (2006–2008). Surface soil samples (0–2 cm) were taken from the BZs in autumn and spring to evaluate the level of plant-available P ( P Ac) extracted with 0.5 M NH 4–acetate–0.5 M acetic acid at pH 4.65. The BZs were most effective at decreasing TS, TP and PP with conventional tillage, less so with direct drilling and least effective with grazing. In a conventionally tilled field, the TS and TP removal efficiencies were over 50% and 27–36%, respectively, for the BZs as compared to the NBZ. In the VBZ plots, the DRP load was, however, 60% greater than in the NBZ or GBZ plots. In direct drilling, the surface runoff losses were smaller than in conventionally tilled NBZ plots. The lowest losses of TS, TP and PP were found during grazing for all plots, but with grazing the DRP load, 0.3–0.4 kg ha −1 yr −1, was higher than during grain growing in all treatments. The GBZ and VBZ were effective in retaining P in summer and autumn, whereas in spring their retention capacity was decreased. The reason for high DRP losses in spring was the high P Ac in surface soil and frozen broken plant tissues in the VBZ and the grazed source field. Mowing and removing of swathe from the GBZ decreased the DRP losses.

Phosphorus forms in urban and agricultural runoff: Implications for management of Danish Lake Nordborg

The catchment of eutrophic Lake Nordborg, Denmark, consists of 25% urban areas with separate sewer systems and 63% agricultural areas with clayey soil and grain crops. Diffuse runoff is the only external phosphorus (P) source (∼ 550 kg/yr), but the lake suffers from high internal loading (∼ 1300 kg/yr). In-lake aluminum treatment is suggested if external P-loading can be brought down by ∼ 30% to an annual average inlet concentration of < 0.100 mg/L. We examined the share and bioavailability of particulate P (PP) in 14 tributaries during a winter season to evaluate if the reduction in bioavailable P-load can be obtained by construction of precipitation/retention ponds in major inlets. One-third of the P-load occurred as PP, independent of catchment type. For urban runoff, 62% of PP was surface adsorbed P, iron-bound P, and extractable organic P, all of which may be considered bioavailable. The corresponding value for agricultural tributaries was 76%. For both types of catchments more than 70% of total P (PP and dissolved P) was bioavailable, but total concentrations were much higher (0.174 ± 0.032 mg/L) in agricultural runoff than in urban runoff (0.082 ± 0.019 mg/L). Measurements of PP settling velocity revealed that ∼ 30% settled slower than 1 cm/h whereas ∼ 50% settled faster than 1 m/h. Therefore, water retention time in precipitation ponds to Lake Nordborg should exceed 8 h to reach the target reduction in P-load. Focus should be on the agricultural catchment that contributes 140 kg to PP load, rather than the urban catchment that only contributes 48 kg.

The effect of soil phosphorus on particulate phosphorus in land runoff

SummaryAccumulation of surplus phosphorus (P) in the soil and the resulting increased transport of P in land runoff contribute to freshwater eutrophication. The effects of increasing soil P (19–194 mg Olsen‐P (OP) kg−1) on the concentrations of particulate P (PP), and sorption properties (Qmax, k and EPCo) of suspended solids (SS) in overland flow from 15 unreplicated field plots established on a dispersive arable soil were measured over three monitoring periods under natural rainfall. Concentrations of PP in plot runoff increased linearly at a rate of 2.6 μg litre−1 per mg OP kg−1 of soil, but this rate was approximately 50% of the rate of increase in dissolved P (&lt; 0.45 μm). Concentrations of SS in runoff were similar across all plots and contained a greater P sorption capacity (mean + 57%) than the soil because of enrichment with fine silt and clay (0.45–20 μm). As soil P increased, the P enrichment ratio of the SS declined exponentially, and the values of P saturation (Psat; 15–42%) and equilibrium P concentration (EPCo; 0.7–5.5 mg litre−1) in the SS fell within narrower ranges compared with the soils (6–74% and 0.1–10 mg litre−1, respectively). When OP was &lt; 100 mg kg−1, Psat and EPCo values in the SS were smaller than those in the soil and vice‐versa, suggesting that eroding particles from soils with both average and high P fertility would release P on entering the local (Rosemaund) stream. Increasing soil OP from average to high P fertility increased the P content of the SS by approximately 10%, but had no significant (P &gt; 0.05) effect on the Psat, or EPCo, of the SS. Management options to reduce soil P status as a means of reducing P losses in land runoff and minimizing eutrophication risk may therefore have more limited effect than is currently assumed in catchment management.

Simplified modeling of phosphorus removal by vegetative filter strips to control runoff pollution from phosphate mining areas

Runoff non-point source pollution from phosphate mining areas poses a potential risk to ecosystems in many parts of the world. Mining sand tailings in Central Florida, which still contain apatite (phosphate rock), have shaped the landscape in reclaimed lands at the upper Peace River basin. The objective of this study is to model the efficiency of vegetative filter strips for controlling surface runoff pollution from phosphate mining sand tailings. The numerical model VFSMOD-W is used to predict overland flow and sediment trapping within the filter and is linked to a simplified phosphorous (P) transport algorithm based on experimental data to predict total P (TP), particulate P (PP) and dissolved P (DP) fractions in the filter outflow. An advanced global inverse optimization technique is used for the model calibration process, and the uncertainty of the measured data is considered in goodness-of-fit indicators. The VFSMOD-W can predict hydrology and sediment transport well (Nash–Sutcliffe coefficient of efficiency >0.6) for calibration and validation events with peak outflow rate from VFS greater than 0.0004 m3/s. The good prediction in runoff and sediment resulted also in good predictions of PP and TP transport since apatite is a main component of sediment. A good prediction of DP was found by considering the rainfall impact on DP dissolved from apatite in surface soil. The uncertainty of measured data included in the goodness-of-fit indicators is a more realistic method to evaluate model performance and data sets. VFSMOD-W combined with the simplified P modeling approach successfully predicted runoff, sediment, and P transport in phosphate mining sand tailings, which provides management agencies a design tool for controlling runoff and P transport using vegetative filter strips.

The effect of farming practices on phosphorus transfer to a headwater stream in England

Characterization of phosphorus (P) sources in catchments is critical to the restoration of stream integrity following eutrophication. To better understand the effects of farming practices on P transfer in a small rural mixed farming catchment (Rosemaund), spatial and temporal patterns in P delivered from a field-drain (Foxbridge, 6 ha), and to two downstream stations (Jubilee, 31 ha and Belmont, 150 ha) were investigated in relation to detailed records on cultivation practices and fertilizer/manure P inputs over a 3-year period (1997–2000) with above-average rainfall. The Belmont catchment also included a farmstead and a small sewage treatment works (STW). The main source of P in drainflow (December–April) was the soil with flow-weighted concentrations of particulate P (PP) up to 2 mg L −1 and concentrations of dissolved-reactive P (DRP) (0.1–0.4 mg DRP L −1) above current riverine targets to limit eutrophication. Agricultural operations carried out in autumn/early winter sporadically increased concentrations of DRP (fertilizer/manure applications) and PP (cultivations) in drain/stream discharge, but only during events that produced small amounts of stormflow. Occasionally large DRP and PP concentrations in drainflow were also recorded during the ecologically sensitive summer months, but this was unrelated to any farming practice. Stream P concentrations were further influenced by processes of baseflow dilution, in-stream retention/release and discharges from the STW and farmstead. The results highlight the disparity between the timing of farming operations at this site (autumn/early winter), the main period of land drainage (winter/spring) and the main period of biological activity (spring and summer). A greater understanding of the ecological significance of variably timed P inputs to headwater streams is needed to develop effective catchment management.

Bioavailability and Fate of Phosphorus in Constructed Wetlands Receiving Agricultural Runoff in the San Joaquin Valley, California

Elevated nutrient concentrations in agricultural runoff contribute to seasonal eutrophication and hypoxia in the lower portion of the San Joaquin River, California. Interception and filtration of agricultural runoff by constructed wetlands may improve water quality of return flows ultimately destined for major water bodies. This study evaluated the efficacy of two small flow-through wetlands (2.3 and 7.3 ha; hydraulic residence time = 11 and 31 h) for attenuating various forms of P from irrigation tailwaters during the 2005 irrigation season (May to September). Our goal was to examine transformations and removal efficiencies for bioavailable P in constructed wetlands. Inflow and outflow water volumes were monitored continuously and weekly water samples were collected to measure total P (TP), dissolved-reactive P (DRP), and bioavailable P (BAP). Suspended sediment was characterized and fractionated into five operationally-defined P fractions (i.e., NH4Cl, bicarbonate-dithionite, NaOH, HCl, residual) to evaluate particulate P (PP) transformations. DRP was the major source of BAP with the particulate fraction contributing from 11 to 26%. On a seasonal basis, wetlands removed 55 to 65% of PP, 61 to 63% of DRP, 57 to 62% of BAP, and 88 to 91% of TSS. Sequential fractionation indicated that the bioavailable fraction of PP was largely associated with clay-sized particles that remain in suspension, while less labile P forms preferentially settle with coarser sediment. Thus, removal of potentially bioavailable PP is dependent on factors that promote particle settling and allow for the removal of colloids. This study suggests that treatment of tailwaters in small, flow-through wetlands can effectively remove BAP. Wetland design and management strategies that enhance sedimentation of colloids can improve BAP retention efficiency.

Phosphorus losses from a subdrained clayey soil as affected by cultivation practices

Conservation tillage practices are included in the Finnish Agri-Environmental Program as phosphorus (P) loss control measures, but only few experiments have been performed to check their effectiveness in the local conditions. We studied surface and subsurface losses of P from a clayey underdrained field (Jokioinen/ Kotkanoja; Vertic Cambisol/Typic Cryaquept), with 2% mean slope, during two separate experimental periods. Primary tillage treatments of the first experimental period of three years were moldboard ploughing (to 20–23 cm depth) vs. no autumn tillage (wintertime stubble). During the second experimental period of five years, the treatments were moldboard ploughing (20–23 cm) vs. shallow (to 5–8 cm) autumn tillage. The stubble treatment of the first experimental period produced higher dissolved reactive P (DRP) losses (104–259 g ha–1 yr–1) than autumn ploughing (77–96 g ha–1 yr–1), and equally high particulate P (PP) losses (mean 660, 235–1300 g ha–1 yr–1). During the second experimental period, shallow autumn tillage produced 28% higher DRP losses (mean 120, 107–136 g ha–1 yr–1) than ploughing (83–117 g ha–1 yr–1) and 11% higher PP losses (mean 1090, 686–1336 g ha–1 yr–1) than ploughing (783–1253 g ha–1 yr–1). Surface runoff made up 28% and 16% of the total flow from the ploughed soil during the first and the second experimental period, respectively, as compared to 50% for the stubble and 44% for the shallow autumn tillage. Routing of flow between surface and subsurface pathways had a major influence on the P losses. In the relatively flat landscapes of the main agricultural areas of southern Finland, the potential for decreasing agricultural P losses by reduced tillage appears limited.;

Particulate phosphorus transformations in south Florida stormwater treatment areas used for Everglades protection

Six large treatment wetlands (352–6698 ha), designated Stormwater Treatment Areas (STAs), have been constructed to sequester phosphorus (P) in drainage waters before entering the Everglades. We performed oxic and anoxic laboratory incubations to assess the stability of P in suspended particles from inflow and outflow waters for selected flow-paths of STA-2, a “well-performing” wetland, and STA-1 West (W), an “under-performing” wetland. Particles were concentrated by filtration prior to incubation. STA-2 flow-paths exhibited a reduction in the particulate P (PP) concentration (e.g., 28 to 3–8 μg/L), as well as an increase in particle stability (i.e., reduced conversion of PP to soluble reactive P [SRP]) with passage through the wetland. By contrast, while the STA-1W flow-path exhibited a PP reduction of 68–46 μg/L under anoxic incubations, the particles exiting the wetland were more labile than those entering. For PP collected from both STAs, anoxic incubations produced higher conversion rates to SRP than did oxic conditions. This redox influence on SRP release was more pronounced for outflow PP from STA-1W than for PP exiting the STA-2 flow-paths.

Phosphorus dynamics and loading in the turbid Minnesota River (USA): controls and recycling potential

Phosphorus (P) dynamics in the agriculturally-dominated Minnesota River (USA) were examined in the lower 40 mile reach in relation to hydrology, loading sources, suspended sediment, and chlorophyll to identify potential biotic and abiotic controls over concentrations of soluble P and the recycling potential of particulate P during transport to the Upper Mississippi River. Within this reach, wastewater treatment plant (WWTP) contributions as soluble reactive P (SRP) were greatest during very low discharge and declined with increasing discharge and nonpoint source P loading. Concentrations of SRP declined during low discharge in conjunction with increases in chlorophyll, suggesting biotic transformation to particulate P via phytoplankton uptake. During higher discharge periods, SRP was constant at ~0.115 mg l−1 and coincided with an independently measured equilibrium P concentration (EPC) for suspended sediment in the river, suggesting abiotic control over SRP via phosphate buffering. Particulate P (PP) accounted for 66% of the annual total P load. Redox-sensitive PP, estimated using extraction procedures, represented 43% of the PP. Recycling potential of this load via diffusive sediment P flux under anoxic conditions was conservatively estimated as ~17 mg m−2 d−1 using published regression equations. The reactive nature and high P recycling potential of suspended sediment loads in the Minnesota River has important consequences for eutrophication of the Upper Mississippi River.

Seasonal changes in nitrogen and phosphorus transport in the lower Changjiang River before the construction of the Three Gorges Dam

Water and sediment samples were collected at Datong from June 1998 to March 1999 to examine seasonal changes in the transports of nitrogen (N) and phosphorus (P) from the Changjiang River (Yangtze River) to the East China Sea (ECS). Dissolved inorganic nitrogen (DIN; dominated by nitrate) concentration exhibited small seasonality, and DIN flux was largely controlled by water discharge. Dissolved inorganic phosphorus (DIP) concentration was inversely correlated with water discharge, and DIP was evenly delivered throughout a year. The transports of DIN and DIP from the Changjiang River were consistent with seasonal changes in nutrient distributions and P limitation in the Changjiang Estuary and the adjacent ECS. Dissolved organic and particulate N (DON and PN) and P (DOP and PP) varied parallel to water discharge, and were dominantly transported during a summer flood. The fluxes of DOP and particulate bioavailable P (PBAP) were 2.5 and 4 times that of DIP during this period, respectively. PBAP accounted for 12–16% of total particulate P (PP), and was positively correlated with the summation of adsorbed P, Al–P and Fe–P. Ca–P, the major fraction of PP, increased with increasing percent of CaCO 3. The remobilization of riverine DOP and PBAP likely accounted for the summer elevated primary production in DIP-depleted waters in the Changjiang Estuary and the adjacent ECS. The Changjiang River delivered approximately 6% of DIN (1459 × 10 6 kg), 1% of DIP (12 × 10 6 kg), and 2% of dissolved organic and particulate N and P to the totals of global rivers. The construction of the Three Gorges Dam might have substantially reduced the particulate nutrient loads, thereby augmenting P limitation in the Changjiang Estuary and ECS.

Quantification of phosphorus entrainment in a lowland river by in situ and laboratory resuspension experiments

Hydrodynamics at the sediment water interface initiate and control the exchange of particles and associated phosphorus (P) between river sediment and the water column. Currently, no general analytical theory for cohesive sediment resuspension is available, so these properties must be inferred from direct measurements. In a moderately slow-flowing (annual mean velocity 0.1–0.3 m s-1) stretch of the lowland River Spree, Germany, an in situ experiment (INS) and a laboratory experiment (LAB) with surface sediment (0–3 cm) were conducted concurrently in May 2005 using an erosion chamber. In both approaches, the entrainment of particles and particulate P (PP) increased significantly as shear velocities were incrementally increased from 0.57 to 1.67 cm s-1. In repetitive INS runs, particle and PP entrainment rates at the lowest shear velocity applied differed by a factor of 5 and 11 (0.25 – 1.18 g m-2 h-1 and 4.1 – 45.9 mg m-2 h-1), respectively, due to in situ river bed heterogeneities, specifically the presence or absence of a high-porosity fluff layer. These rates were on average 48 and 3 times higher, respectively, than those of LAB runs (0.01– 0.02 g m-2 h-1 and 3.1– 14 mg m-2 h-1, respectively), suggesting the temporary availability of local, in situ, fluffy surface layers on the river bed that are not preserved during sediment preparation for LAB runs. Such a transient storage feature has not been described previously in the literature for river sediments. The entrainment of this easily resuspendable and P rich material at a low flow-generated shear velocity can lead to batch-wise P burdens in the water column and consequently to a displacement of eutrophication potential. Those events would be underestimated in LAB experiments devoid of this fluffy layer.

Nutrient Transport as Affected by Rate of Overland Flow

Little information is currently available concerning the effects of varying flow rate on nutrient transport by overland flow. The objective of this study was to measure the effects of overland flow rate on nutrient transport following the application of beef cattle or swine manure to plots containing 0, 2, 4, or 8 Mg ha-1 of corn residue. After addition of residue materials to 0.75 m wide by 2.0 m long plots, beef cattle or swine manure was added and the plots were then either disked or maintained in a no-till condition. Three 30 min simulated rainfall events, separated by 24 h intervals, were applied at an intensity of approximately 70 mm h-1. The transport of dissolved phosphorus (DP), particulate P (PP), total phosphorus (TP), NO3-N, NH4-N, total nitrogen (TN), and soil loss was measured. Nutrient load from the plots on which manure was applied was not significantly affected by the amount of corn residue on the soil surface. Transport of DP in runoff was greater under no-till than till conditions. Rate of overland flow significantly affected PP and TP load. The transport of NO3-N and TN was affected by runoff rate but was not significantly influenced by tillage. Both tillage and runoff rate were found to affect the transport of NH4-N in runoff. Soil loss was significantly influenced by the amount of residue on the soil surface and runoff rate. Tillage condition and runoff rate should be considered when nutrient transport from land application areas is estimated.

Tracing suspended sediment and particulate phosphorus sources in catchments

Summary Information on suspended sediment and particulate P (PP) sources is an important requirement in many catchment-based diffuse source pollution studies, in order to assist with model validation and to provide information to support the development of effective sediment and phosphorus control strategies. Such information is, however, frequently unavailable or difficult to assemble. In the study reported, source fingerprinting procedures were successfully used to assemble this information for seven sub-catchments in the Hampshire Avon catchment and five sub-catchments in the Middle Herefordshire Wye catchment. The results provide important new information on the relative importance of the contributions from surface and channel/subsurface sources to the suspended sediment and PP fluxes from the catchments. In the Wye sub-catchments channel/subsurface sources contributed 40–55% of the overall suspended sediment flux and 21–43% of the PP flux from the catchments. Equivalent values for the Avon were 1–41% and 1–54%, respectively. Combination of the information on the relative importance of surface and channel/subsurface sources with measured suspended sediment fluxes has provided the first estimates of the specific fluxes of sediment and PP attributable to channel/subsurface sources for UK catchments. The former are as high as 15–20 t km−2 year−1 in some of the Wye sub-catchments, whereas the latter exceeded 0.1 kgP ha−1 year−1 in the same sub-catchments. The results emphasize the need to take account of potential contributions from channel/subsurface sources when using measured suspended sediment and PP flux data to validate predictions derived from models incorporating only surface contributions.