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There were many phosphorus chemical enterprises and phosphogypsum slag fields in the Fuquan section of the Chong-an River in the Guizhou Province of China; therefore, the dissolved phosphorus in the Fengshanqiaobian water quality monitoring section of the area has exceeded 0.2 mg/L for a long time. This study analyzed the monitoring data of dissolved phosphorus, fluoride, nitrate–nitrogen, and chemical oxygen demand in the main stream and key tributaries of the Fuquan section of the Chong-an River from 2015 to 2021, in which the main exceeding factor was dissolved phosphorus of more than 0.2 mg/L; determined the variation law of phosphorus pollution in the water body of the Fuquan section of the Chong-an River, in which the exceedance of phosphorus was mainly concentrated in May of flood season and in January and March of the drought season; evaluated the sources of phosphorus pollution, in which the main pollution sources were from phosphogypsum slag field leakage, phosphorus chemical industry plant leakage, agricultural nonpoint source pollution; and presented measures to reduce phosphorus pollutants in rivers, such as strengthening phosphogypsum slag field and phosphorus chemical enterprise seepage prevention measures, and reducing the amount of phosphate fertilizer application. HIGHLIGHTS The monitoring data of dissolved phosphorus, fluoride, nitrate–nitrogen, and chemical oxygen demand were analyzed in the mainstream and key tributaries of the Fuquan section of the Chong-an River from 2015 to 2021.; The variation law of phosphorus pollution was determined in the water body of the Fuquan section of the Chong-an River.;

Phosphorus (P) losses from clay soils can be mitigated by introducing measures for improving soil structure. These include structure liming, where a mixture of CaO or Ca(OH)2 and CaCO3 is added to the soil. In a field experiment with separately tile-drained plots on a clay loam in Sweden, we examined the effects of structure liming on leaching of total-P, phosphate P (PO4-P) and total nitrogen (N) during three years after initial application. The treatments included two application rates (8 and 16 t ha−1) of a common product in comparison with a control (no lime). Effects of structure liming emerged during the second and third year after application, with 45 and 38% lower total-P leaching than in the unlimed control. A significant effect of the application rate was found in the third year. Nitrogen leaching and crop yield were not affected. As expected, soil pH raised following structure lime addition. Measurements of aggregate stability did not confirm the reduction in P leaching, indicating that it is important to measure P concentrations in drainage water directly when assessing the effect of structure liming.

Eutrophication of the Baltic Sea is driven by phosphorus and nitrogen. While the anthropogenic point source loads of both nutrients have decreased markedly, further reductions are needed. This is true particularly for phosphorus, as highlighted by its stringent abatement targets in HELCOM's Baltic Sea Action Plan. To meet the targets, more results need to be achieved in non-point source abatement, specifically from agricultural sources. The growing pressure for phosphorus abatement from agriculture may lead to environmentally and economically inefficient outcomes unless we account for the variability in how different forms of phosphorus respond to abatement measures, and how these forms contribute to eutrophication. The precautionary and efficiency improving way to advance policies is to either replace or supplement the Total Phosphorus metric with a metric more accurate in reflecting the biologically available phosphorus. This policy fix becomes more important as the relative share of agricultural emissions of total pollution increases. [ABSTRACT FROM AUTHOR]

Dredging activities cause sediment resuspension, which can change the bioavailability of nutrients such as phosphorus (P) in aquatic ecosystems due to remobilization. This study evaluated the remobilization of P in the solid and dissolved phase before and after sediment resuspension in the Meriti and Iguaçu River estuaries and the Rio de Janeiro and Niterói harbor in Guanabara Bay (Rio de Janeiro, Brazil). Three water and sediment samples were collected at each point. Dissolved phosphorus (DP), total phosphorus (TP), organic phosphorus (OP), and inorganic phosphorus (IP) were analyzed before and after resuspension. Resuspension directly impacted the fine-grained samples, causing the release of P into the water column after resuspension, increasing eutrophication of the estuary and risk to biota. The phosphorus enrichment index (PEI) was calculated in the four areas, and in all areas, the index was above 1, which means high ecological risk. The area with sandy granulometry and a lower percentage of organic matter showed an increase in the index after resuspension. The resuspension may impact the increase of eutrophication in some areas, due to the remobilization of the sediment and the adsorbed contaminants. [ABSTRACT FROM AUTHOR]

Study region: The study region was the Sakura River, a tributary of Lake Kasumigaura, Japan. Study focus: We asked whether particulate organic phosphorus (POP) contributed to the P cycle in rivers. Previous studies have focused on the interaction between PO43– and particulate inorganic P (PIP), but our study focused on whether POP could also be the source of PO43–. New hydrological insights for the region: After rain events during irrigation, concentrations of suspended particles and particulate P (PP) increased. POP accounted for 42.4 % of PP concentrations. The PIP and POP concentrations were both positively correlated with suspended particle concentrations. The PO43– concentrations were significantly higher during decomposition experiments when suspended particles were present in the water. Decomposition experiments revealed that the P composition changed and the proportion of organic P decreased as DNA-P disappeared from the PP. The fact that the DOP concentration was significantly lower in the treatment with suspended particles than without suspended particles suggested that DOP mineralization rates were increased by the presence of suspended particles. In addition to mineral-sorbed P, the POP derived from the irrigation of paddy fields may also be bioavailable.

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Denitrifying woodchip bioreactors are a practical nitrogen (N) mitigation technology but evaluating the potential for bioreactor phosphorus (P) removal is highly relevant given that (1) agricultural runoff often contains N and P, (2) very low P concentrations cause eutrophication, and (3) there are few options for removing dissolved P once it is in runoff. A series of batch tests evaluated P removal by woodchips that naturally contained a range of metals known to sorb P and then three design and environmental factors (water matrix, particle size, initial dissolved reactive phosphorus (DRP) concentration). Woodchips with the highest aluminum and iron content provided the most dissolved P removal (13±2.5 mg DRP removed/kg woodchip). However, poplar woodchips, which had low metals content, provided the second highest removal (12±0.4 mg/kg) when they were tested with P-dosed river water which had a relatively complex water matrix. Chemical P sorption due to woodchip elements may be possible, but it is likely one of a variety of P removal mechanisms in real-world bioreactor settings. Scaling the results indicated bioreactors could remove 0.40 to 13 g DRP/ha. Woodchip bioreactor dissolved P removal will likely be small in magnitude, but any such contribution is an added-value benefit of this denitrifying technology. [ABSTRACT FROM AUTHOR]

Phosphorus (P) losses from clay soils can be mitigated by introducing measures for improving soil structure. These include structure liming, where a mixture of CaO or Ca(OH)2 and CaCO3 is added to the soil. In a field experiment with separately tile-drained plots on a clay loam in Sweden, we examined the effects of structure liming on leaching of total-P, phosphate P (PO4-P) and total nitrogen (N) during three years after initial application. The treatments included two application rates (8 and 16 t ha−1) of a common product in comparison with a control (no lime). Effects of structure liming emerged during the second and third year after application, with 45 and 38% lower total-P leaching than in the unlimed control. A significant effect of the application rate was found in the third year. Nitrogen leaching and crop yield were not affected. As expected, soil pH raised following structure lime addition. Measurements of aggregate stability did not confirm the reduction in P leaching, indicating that it is important to measure P concentrations in drainage water directly when assessing the effect of structure liming. [ABSTRACT FROM AUTHOR]

Inputs of phosphorus (P) during baseflow period usually come from groundwater, bed sediments, and some other sources. Baseflow P can have critical effects on nutrient enrichment of surface waters in some intensively cultivated agricultural watersheds. This study was conducted to estimate the baseflow dissolved phosphorus (DP) export in a typical rainy agricultural watershed of eastern China using a recursive tracing source algorithm (RTSA) and reveal the rules and trends of baseflow DP loads and concentrations. Results indicated that RTSA provided a satisfactory prediction for baseflow DP load (Nash-Sutcliffe efficiency (NSE) = 0.72, R2 = 0.74). From 2003 to 2012, the annual baseflow DP loads ranged from 0.159 (2004) to 0.771 (2012) kg/ha which contributed about 64.3% of the mean total annual DP export in stream (0.597 kg/ha). The annual flow-weighted DP concentrations in streamflow (0.076–0.125 mg/L) and baseflow (0.076–0.137 mg/L) far exceeded the eutrophication threshold of DP (0.01 mg/L). Significantly increasing trends were obtained in the streamflow and baseflow DP loads and the flow-weighted concentrations (Mann-Kendall test, Zs > 2.56, p < 0.01) because of the changes of hydro-meteorological conditions. This indicates that, in the context of global climate change, baseflow DP export would be one of key issues for nonpoint source pollution control in the intensive agricultural watershed. [ABSTRACT FROM AUTHOR]

Drainage ditches are effective agricultural facilities because they can drain excess water from the field, and an eco-friendly drainage ditch can also eliminate agricultural nonpoint source pollution effectively. Field work has been conducted in four types of drainage ditches with different revetment techniques, i.e., a two-stage ditch with a woodpile bank, a single trapezoidal ditch with a geotextile bag, a hollow hexagonal brick, and a reinforced vegetative bank revetment. The corresponding retention capacity of phosphorus in these ditches has been investigated. Results show that two-stage ditches can retain dissolved phosphorus (DP) more effectively compared to the other three single trapezoidal drainage ditches, and the retention efficiency is as high as 60%. The concentration of total suspended sediment (TSS) is positively correlated with that of total phosphorus (TP), and the retention capacity of drainage ditches on phosphorus will decrease as discharge increases. Current work proves that a well-designed two-stage section ditch can effectively eliminate the nonpoint source pollution during practical agriculture drainage application. [ABSTRACT FROM AUTHOR]

Leaching of nitrogen (N) and phosphorus (P) from agricultural lands can cause serious environmental problems such as eutrophication. The objective of this study was to investigate the impacts of biochar application, tillage practices, and irrigation systems on nitrate and dissolved phosphorus (DP) concentrations in subsurface drainage water and grain yield of winter wheat using a strip-split plot design with 3 replications. Irrigation at three different levels (flood (Ifl), furrow (Ifu), and sprinkler (Is) systems) considered as main factor, tillage at two levels (reduced tillage (Tr) and conventional systems (Tc)) as subplot factor, and bagasse biochar at two levels (without biochar (B0) and 20 ton ha−1 biochar (B1)) as sub-subplot factor. Polyvinyl chloride (PVC) standpipes were used in each sub-subplot to collect leachate water at 100-cm depth. The results indicated that irrigation had significant effects on yield, collected water volume (CWV), nitrate, and DP concentrations (P < 0.01). Interaction of tillage and irrigation was significant for grain yield (P < 0.05). Biochar application only caused a significant decrease in nitrate concentration under sprinkler irrigation (P < 0.05), while no significant impact was observed under flood and furrow irrigation systems. Under sprinkler irrigation, the total nitrate collected in the PVC standpipes decreased by 37.51 and 34.29% compared with flood and furrow irrigations, respectively. Biochar application reduced the total nitrate collected by 16.84%, while difference among tillage treatments was negligible (4.51%). The total DP collected under sprinkler irrigation was lower in comparison with flood and furrow irrigations by 42.24 and 38.76%, respectively. Biochar application reduced the total DP collected by 10.84%, while reduced tillage increased the total DP collected by 8.90% compared with the conventional tillage. [ABSTRACT FROM AUTHOR]

Two-stage channels are effective drainage facilities because they can decrease soil erosion and nutrient loss effectively. The lateral distribution of streamwise velocity in two-stage channels has been substantially studied. However, knowledge about the lateral distribution of sediment and phosphorus in two-stage channels with emergent vegetation on the floodplain are limited. In this work, flume experiments were conducted to investigate the relationship between current and transport of sediment and phosphorus. Results show that lateral distributions of total suspended sediment (TSS), particulate phosphorus (PP), and dissolved phosphorus (DP) are affected by the lateral variation of streamwise velocity, i.e., larger TSS and DP were found in the main channel, while larger PP appeared in the floodplain. Besides, theoretical models for the lateral distribution of TSS and DP were presented. Three models were adopted to predict the depth-averaged streamwise velocity, and corresponding results have been used to predict the TSS and DP. Comparisons indicated that with a high-precision velocity model, the newly presented analytical models can give satisfied predictions of the lateral distributions of TSS and DP. [ABSTRACT FROM AUTHOR]

Bioretention systems have been found to be potential candidates for the removal of various pollutants/nutrients from rainfall or stormwater runoff. Despite bioretention has been widely developed for the removal of nutrients from stormwater, effective removal of both phosphorus and nitrogen is still a challenge. Hence, in this study, bioretention systems modified by alkaline solid waste media have been reported for the effective removal of nutrients. Six different types of solid wastes were first assessed using leaching and adsorption tests, and then the bottom ash from a refuse incineration plant was selected as a modifier. The bottom ash was mixed with soil to form a special media as the filter layer in the bioretention systems. The nutrient removal efficiencies of the modified bioretention systems were evaluated and also compared with those of the unmodified control. For this purpose, the design of the modified filter media with a saturated zone was combined to enhance the simultaneous removal of nitrogen and phosphorus. The effect of different rainfall intensities and nutrient concentrations in stormwater runoff on the removal efficiency of nutrients was evaluated. The results indicated that the modified bioretention with bottom ash modified soil media and saturated zone could exhibit the excellent removal efficiency of nitrogen and phosphorus from stormwater runoff. The extent of removal of total nitrogen, total Kjeldahl nitrogen, and total phosphorus was found to be 58–70%, 66–82% and 82–97%, respectively. The performed correlation analysis showed that the bioretention cell using the special media could simultaneously enhance the removal of phosphorus and nitrogen. As a part of this study, the adsorption isotherms of phosphorus removal by the modified bioretention systems have also been determined. Finally, the implications and opportunities for deploying modified bioretention systems for optimizing water-energy nexus and stormwater management were illustrated. In overall, this study demonstrated that the modified bioretention systems could substantially enhance the removal efficiencies of nutrients from stormwater runoff. Image 1 • The use of bottom ash as modified media greatly improved P/N removal efficiency. • Combined saturated zone and modified media is an effective design in P/N removal. • Effect of rainfall intensity and nutrient concentration on P/N removal is assessed. • Nutrient concentration has a great influence on P/N removal in bioretention. • Isotherm results reveal monolayer sorption for P removal by modified bioretention. [ABSTRACT FROM AUTHOR]

Abstract The development of anoxic conditions in riparian wetland (RW) soils is widely known to release dissolved phosphorus (DP), but the respective roles of reductive dissolution of Fe-oxyhydroxide, pH changes and sediment inputs in this release remain debated. This study aimed to identify and quantify these respective roles via laboratory anaerobic/aerobic incubation of RW soils with and without the addition of sediment. The investigated soils came from two RWs with contrasting P status and organic matter (OM) content in their soils, while the added sediment came from an adjacent cultivated field. Results showed that the amount and speciation of the DP released during anaerobic/aerobic incubations were controlled by soil P status and soil OM content. During anaerobic incubation, DP release in the soil with high extractable P and low OM contents was controlled by reductive dissolution of Fe-oxyhydroxides (83%), whereas that released in the soil with low extractable P and high OM contents was controlled by an increase in pH (88%). Anaerobic incubation of a mixture of eroded sediments and RW soils increased the release of DP, dissolved organic carbon and Fe(II) (by 16%, 4% and 18%, respectively) compared to the simple addition of the amounts released during their separate incubations. Management practices should decrease soil erosion from upland fields to avoid deposition of P-rich sediments on RW soils. Management efforts should focus preferentially on RWs whose Fe:P molar ratios in the soil solution during reduction are the lowest, since they indicate a high risk that the DP released will be transferred to watercourses. Graphical abstract Unlabelled Image Highlights • Dissolved P release increased in wetland soils under anoxic conditions. • Soil properties controlled the amount and speciation of the DP released. • FeO(OH) reduction controlled DP release in high extractable-P but low OM soils. • pH increase controlled DP release in low extractable-P but high OM soils. • Sediment input into RW soils increased its DP release under anoxic conditions. [ABSTRACT FROM AUTHOR]

Information on dissolved phosphorus (DP) and particulate phosphorus (PP) is essential to evaluate the P dynamics and control eutrophication. In this work, DP and PP in 10 freshwater lakes representing various trophic statuses were analyzed by solution 31P nuclear magnetic resonance spectroscopy. The results indicated that the predominant forms of DP and PP were orthophosphate (Ortho‐P) and monoester phosphate (Mono‐P). There was a greater concentration of Ortho‐P and Mono‐P in the water and particulate matter of medium‐eutriphic lakes than in lightly eutriphic or mesotrophic lakes. α‐Glycerophophate (2.7–32.5%), β‐glycerophosphate (1.3–23.4%), guanosine 2′Mono‐P (20.2–29.3%), inositol hexakisphosphate (IHP) (8.3–36.4%) and nicotinamide adenine dinucleotide (NAD) (17.3–35.9%) were identified as the major chemical forms of Mono‐P in water and particulate matter, which originate mainly from the degradation of labile diesters, aquatic/microbial sources and a combination of terrestrial and aquatic/microbial sources. Diester phosphate (Di‐P) was dominated by teichoic acid (0.5–14.8%) and deoxyribonucleic acid (DNA) (0.6–17.7%), which originated from aquatic/microbial sources. Moreover, correlation analysis showed that dissolved Mono‐P in water and particulate matter had a positive correlation with chlorophyll‐a (Chl‐a), which indicated their potential bioavailability for algal activity. pH was a crucial parameter to control Di‐P, pyrophosphate (Pyro‐P) and polyphosphate (Poly‐P) in water. Mono‐P/PPs showed a positive correlation with Chl‐a (R2 = 0.459), total phosphorus (TP) (R2 = 0.586) and the trophic status index (R2 = 0.588), which suggested that particulate Mono‐P can potentially contribute to lake eutrophication. Both Ortho‐P and Mono‐P were major contributors of P nutrients for algae, and therefore source control and new techniques are needed for reducing eutrophication. • Dissolved phosphorus and particulate phosphorus forms in lakes representing various trophic status were investigated. • Ortho‐P and Mono‐P in water and particulate matters showed a greater concentration in mid‐eutrophic lakes. • Dissolved and Particulate Mono‐P processed a potential contribution to lake eutrophication. [ABSTRACT FROM AUTHOR]

Context The dissolved nitrogen (DN) and dissolved phosphorus (DP) released from fresh litter are important pathways by which total nitrogen and phosphorus are transferred from the vegetation to soil in forest ecosystems. However, few studies have paid attention to the DN and DP in fresh litter, which affects our understanding of the nitrogen and phosphorus cycles. Aims The objectives of this study were to elucidate the dynamic characteristics of the concentrations and storage of DN and DP, and to analyze how DN and DP are affected by different plant species and organs, and climate factors. Methods Fresh litter was collected in three plots in a spruce-fir forest and classified by different plant species and organs. Concentration and storage of DN and DP in fresh litter were determined and related to the climatic variables that were monthly recorded. Results The concentration of DP was higher than that of DN in fresh litter, and the concentrations of both elements were determined by plant organs and species. Moreover, The DN and DP concentration was positively related to mean monthly temperature, while DN and DP storage was negatively correlated with mean monthly temperature and monthly precipitation. The storage of DN and DP was determined by litter biomass, which the order in litter from different plant organs was leaves>twigs>miscellaneous>flowers and fruits. The storage of DN and DP in leaves showed two peaks in April and October, but that in twigs and the miscellaneous showed only one peak in October. Conclusion Our results indicated that dissolved nitrogen (DN) is transferred and reabsorbed more than dissolved phosphorus (DP) before plant leaf senescence and other organs fall. Furthermore, DN and DP were associated with climate, plant organs and species in a subalpine forest on the eastern Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Excess phosphorus (P) is generally responsible for the eutrophication of Midwestern surface waters and causes algal blooms, fish kills, and other detrimental ecological effects. In-stream transport of dissolved and particulate P (PP), under both stormflow and baseflow conditions, was studied in an agricultural Wisconsin watershed with diverse sizes of animal farm operations. Trends in P losses were compared both on an individual event and seasonal (spring, summer, fall) basis. In-stream P transport was characterized by high PP losses in the spring during periods of relatively little crop cover and high dissolved P losses in the fall, when surface coverage was still present. The majority of P (64-71%) was transported in the spring season during periods of high flow. Presence of high animal density at lower portions of the watershed resulted in increasing downstream P concentration and load (i.e., concentration dilution effects with increasing drainage area were absent). The baseflow TP concentrations were always above the USEPA eutrophication threshold [Ecoregion VII, level III-53: 0.08 mg L; (USEPA in Ambient water quality criteria recommendations. Information supporting the development of state and tribal nutrient criteria. Rivers and stream in nutrient ecoregion VII. EPA 822-B-00-018. , 2000)]. While upland conservation practices are necessary for non-point source pollution control, it is important to ensure that these practices afford protection during the spring time periods when agricultural fields are most vulnerable for offsite movement of sediments and associated constituents. [ABSTRACT FROM AUTHOR]

This paper presents a newly-derived method for directly determining the amount of transported dissolved phosphorus by water erosion. The results of the method are compared to prediction based on enrichment ratio (as proposed by Sharpley) and average share of dissolved phosphorus (DP) in total transported phosphorus (5%) that is widely used in the Czech Republic. Four study areas (catchments of dozens of sq. kilometer) were chosen for their different characteristics (land use, average slope, average elevation, phosphorus concentration in the soil) which influence their rainfall-runoff behavior. The modeled results are compared with data measured in situ. The two methods provide similar results in intensively agriculturally used regions. Agreement among the methods was observed for three study areas with significant erosion intensity (above 4 t/ha/year). In the catchment with significantly lower erosion intensity (0.5 t/ha/year), the indirect method (Sharpley) underestimates the amount of DP transported in the watercourses. The sum of transports of suspended solids into watercourses and the average available phosphorus content in the soil determined by the Mehlich 3 method (P) are the main factors influencing the results provided by the two methods. An analysis of the impact of these factors on the difference between the results of the methods was provided. Transport of suspended solids is related to the method difference ( R range from 0.37 to 0.71). However, no significant relationship was found between the difference in the results and the average P content in the soil ( R range from 0.15 to 0.36). [ABSTRACT FROM AUTHOR]

Previous work has shown that the drying/rewetting (D/W) of soils mobilizes phosphorus (P), and that the effect of D/W on P release likely depends on the soil microbial community composition. We tested the hypotheses that (i) P release after D/W from fungi is lower than from bacteria and that (ii) gram-positive bacteria are less susceptible to D/W than gram-negative bacteria. We investigated the release of dissolved organic (DOP) and inorganic phosphorus (DIP) from bacterial and fungal biomass after rewetting of an artificial soil that was desiccated to different degrees. For this purpose, sterilized soil amended with growth medium was inoculated separately with one of two bacterial strains ( Pseudomonas fluorescens, gram-negative and Micrococcus luteus, gram-positive) or with one fungal strain ( Penicillium chrysogenum ). The bacterial strains were grown for 7 days, the fungus for 25 days at 50% soil water holding capacity. After the pre-incubation period, microbial biomass P (Pmic) was determined by chloroform fumigation extraction, and soils were desiccated at 20 °C for 5–8 days until pF 6 (−100 MPa) was reached, while the controls were kept permanently at 50% water holding capacity. At different degrees of desiccation, samples were destructively harvested and soils were extracted with water to measure the release of DIP and DOP. The net release of total dissolved P per unit Pmic following D/W was in the order P. fluorescens » M. luteus = P. chrysogenum. In case of P. fluorescens , net release started already after desiccation to pF 4 (−1.0 MPa) and increased with further desiccation. For M. luteus and P. chrysogenum , a tendency for net release was only observed after severe desiccation up to pF 6. Our results suggest that the effect of D/W on P release from microbial biomass depends largely on the microbial community composition, with fungi and gram-positive bacteria being less susceptible to D/W than gram-negative bacteria. [ABSTRACT FROM AUTHOR]

Wet ponds have often been installed to detain runoff with the intention of mitigating peak flow. However, significant current drivers exist to improve their ability to mitigate pollutants, and simple retrofits to existing ponds could help to meet total maximum daily load regulations. A review of past studies showed that ponds struggle to treat dissolved pollutants, which are often the most bioavailable. The present study evaluated the nutrient and sediment treatment performance of a retrofitted upflow filter (UF) attached to the drawdown orifice of an existing wet retention pond in Durham, North Carolina. The pond was studied during three periods: (1) control, where baseline pond performance was established; (2) after retrofit with floating treatment wetlands (FTWs); and (3) after retrofit with FTWs and an UF on the pond's drawdown orifice. The added UF was unable to improve the pond's retention of nitrogen. Further reduction of the TSS effluent concentrations by the UF may have been hampered by the low effluent concentrations during the FTW period (13 mg=L); however, the variability of TSS effluent concentrations was substantially reduced following UF retrofit. The UF significantly reduced mean effluent concentrations of orthophosphate (OP) to less than 0.01 mg=L. Effluent concentrations of OP and total phosphorus from the UF were similar to those from bioretention systems. Clogging of the filter media occurred in intervals of 5 to 9 months prior to breakthrough of OP. Future designs of UFs should incorporate microscale pretreatment and/or filter media cartridges that (1) are easy to remove and (2) provide a method to flush accumulated sediment from the media. [ABSTRACT FROM AUTHOR]

Aerosol phosphorus (P) dissolvable in seawater is a crucial nutrient in aerosol particles to fertilize marine ecosystem. However, the contents of dissolvable P in aerosol particles are usually quantified with water as the leaching medium. In this study, we collected 211 sample pairs of PM 2.5 and total suspended particles (TSP) at Qingdao—a coastal city of China, and quantified and compared the contents of soluble P in PM 2.5 and particles larger than 2.5 μm (PM > 2.5 = TSP-PM 2.5) extracted with Milli-Q water (DP water) and prepared seawater (DP seawater). Overall, DP water and DP seawater linearly correlated in both TSP and PM 2.5 samples with the confidence of 99% (r = 0.82–0.86), and the aerosol P solubility quantified with water extraction was approximately 20% more than that with seawater extraction. The correlation between the dissolved inorganic P in water (DIP water) and in seawater (DIP seawater) was more significant than that between the dissolved organic P in water (DOP water) and seawater (DOP seawater). Moreover, DIP seawater was approximately 91% of DIP water in PM 2.5 , and 74% in PM > 2.5 , whereas DOP seawater was approximately 45% of DOP water in both PM 2.5 and PM > 2.5. The fluxes of the DIP and DOP to sea surface estimated based on the results of water extraction were approximately 10% and 50% larger than those of seawater extraction, respectively. Our results provide effective coefficients for converting aerosol inorganic and organic soluble P quantified with water extraction to that soluble in seawater, and enable better assessments of aerosol P fertilization to the marine ecosystem with data from water extraction. [Display omitted] • Dissolved P (DP) measured by water extraction is 25% overestimated than by seawater. • DPs quantified by water and seawater extractions are proportional. • Overestimation of dissolved organic P (DOP) exceeds that of inorganic P (DIP). • Deposition by water extraction could be overestimated 10% for DIP and 50% for DOP. • P-associated marine primary productivity might be 27% enlarged on water extraction. [ABSTRACT FROM AUTHOR]

Effler SW, Prestigiacomo AR, Hairston NG, Auer MT, Kuczynski A, Chapra SC.  2016. Dissolved phosphorus concentrations in Cayuga Lake system and differences from two analytical protocols. Lake Reserve Manage. 32:392–401. Differences in the concentrations of dissolved forms of phosphorus (P) measured with 2 widely used spectral protocols were documented and evaluated for Cayuga Lake, New York, and 4 of its primary tributaries. The analysis focuses on 2 operationally defined forms of dissolved P, soluble reactive P (SRP) and soluble unreactive P (SUP), which together constitute dissolved P (TDP). Direct comparisons were based on analysis of the results from the 2 protocols of split samples of year-round deep water representative of the entire water column during turnover and the respective dependencies of tributary concentrations on stream flow. Although the TDP concentrations converged for the 2 protocols, there were systematic differences for the contributions of SRP versus SUP (i.e., one protocol yielding lower SRP but higher SUP). The interpretive implications of the differences in the operationally defined concentrations from the 2 analytical protocols were considered in the context of common limnological and bioavailable paradigms for these forms of P, and the needs and structures of mechanistic P-eutrophication models. The lower SRP analytical protocol was favored because of its greater consistencies with the independent bioavailability results, limnological paradigms for dissolved forms of P, and contemporary mechanistic modeling. The differences between the 2 protocols are particularly problematic where contemporary mechanistic models are to be applied, requiring compensating differences in kinetic representations, and likely structure, for cases of higher SRP datasets attributable to the protocol. [ABSTRACT FROM AUTHOR]

We investigated leaching of dissolved phosphorus (P) from 45 tile-drains representing animal husbandry farms in all regions of Denmark. Leaching of P via tile-drains exhibits a high degree of spatial heterogeneity with a low concentration in the majority of tile-drains and few tile-drains (15% in our investigation) having high to very high concentration of dissolved P. The share of dissolved organic P (DOP) was high (up to 96%). Leaching of DOP has hitherto been a somewhat overlooked P loss pathway in Danish soils and the mechanisms of mobilization and transport of DOP needs more investigation. We found a high correlation between Olsen-P and water extractable P. Water extractable P is regarded as an indicator of risk of loss of dissolved P. Our findings indicate that Olsen-P, which is measured routinely in Danish agricultural soils, may be a useful proxy for the P leaching potential of soils. However, we found no straight-forward correlation between leaching potential of the top soil layer (expressed as either degree of P saturation, Olsen-P or water extractable P) and the measured concentration of dissolved P in the tile-drain. This underlines that not only the source of P but also the P loss pathway must be taken into account when evaluating the risk of P loss. [ABSTRACT FROM AUTHOR]

Phosphorus loss from land can impair surface water quality. Losses via subsurface flow can be substantial, but most strategies to mitigate P losses focus on surface runoff. Aluminium sulphate (alum) was applied at 25 and 50 kg Al ha −1 to a flat, podzol soil under pasture regularly grazed by dairy cattle. Over a year, losses of filtered (<0.45 μm) reactive P (FRP) and total filtered P (TFP) intercepted at 35-cm depth by Teflon suction cups were c . 0.6 and 1.0 kg P ha −1 , respectively for the control treatment. The 50 kg Al ha −1 treatment decreased FRP and TFP by 26 and 27%, respectively: no significant difference to the control was noted for alum applied at 25 kg Al ha −1 . The cost-effectiveness was estimated at 190–952 USD kg −1 P mitigated. While more cost-effective strategies should be practised first, surface applying alum may provide an option where sub-surface P losses must be lowered further especially if applied to a small area of high P loss. [ABSTRACT FROM AUTHOR]

Lake Erie has undergone re-eutrophication beginning in the 1990s, even though total phosphorus (TP) loads to the lake continued to slowly decline. Using our 1982 and 2007-10 studies of the bioavailability of dissolved and particulate phosphorus export from major Ohio tributaries, together with our long-term TP and dissolved reactive phosphorus (DRP) loading data, we estimated long-term annual export of dissolved and particulate bioavailable phosphorus. DRP was found to adequately represent dissolved bioavailable export while 26-30% of the particulate phosphorus (PP) was extractable by 0.1 N NaOH, a frequently used indicator of PP bioavailability. During the period of re-eutrophication (1991-2012), DRP export from nonpoint sources in the Maumee and Sandusky rivers increased dramatically while NaOH-PP export had a slight decline for the Maumee and a small increase in the Sandusky. For the Cuyahoga River, both DRP and NaOH-PP increased, but these changes were small in relation to those of the Maumee and Sandusky. During this period, whole lake loading of both nonpoint and point sources of phosphorus declined. This study indicates that increased nonpoint loading of DRP is an important contributing factor to re-eutrophication. Although nonpoint control programs in the Maumee and Sandusky have been effective in reducing erosion and PP export, these programs have been accompanied by increased DRP export. Future target loads for Lake Erie should focus on reducing bioavailable phosphorus, especially DRP from nonpoint sources. Agricultural P load reduction programs should address both DRP and PP, and take into account the lower bioavailability of PP. [ABSTRACT FROM AUTHOR]

Total phosphorus (P) in soil is classified as molybdate-reactive P (MRP) and molybdate-unreactive P (MUP) based on bioavailability, and P tests are generally focused on MRP fraction of soil, despite the fact that MUP can contribute significantly to total extracted P. This survey study indicated that water-soluble P (WP) contributed as much as 9% to total P (TP) in two intensively fertilized (rice–onion cropping upland and greenhouse) soils, and most (ca. 80%) of WP was present in MUP form. The total P lost through runoff and leaching was high (>5 ppm), due to rotating greenhouse into submerged paddy soil, especially at the initial stage of rice cultivation, and MUP contributed 43% and 77% to total P lost through runoff and leaching, respectively. Therefore, it is necessary to monitor MUP to develop suitable soil-management strategies to reduce MUP release and P loss from high-P-containing soils. [ABSTRACT FROM PUBLISHER]

Accumulation of phosphorus ( P) in agricultural topsoils can contribute to leaching of P which may cause eutrophication of surface waters. An understanding of P mobilization processes in the plough layer is needed to improve agricultural management strategies. We compare leaching of total dissolved and particulate P through the plough layer of a typical Danish sandy loam soil subjected to three different P fertilizer regimes in a long-term field experiment established in 1975. The leaching experiment used intact soil columns (20 cm diameter, 20 cm high) during unsaturated conditions. The three soils had small to moderate labile P contents, expressed by water-extractable P (3.6-10.7 mg/kg), Olsen P (11-28 mg/kg) and degree of P saturation ( DPS) (25-34%). Mobilization of total dissolved P ( TDP) increased significantly ( P < 0.05) from the intact soil columns with increasing labile P, whereas the increase in particulate P ( PP) with increasing labile P content was modest and statistically insignificant. We found concentrations up to 1.5 mg TP/L for the plough layer of this typical Danish sandy loam soil. This highlights that even a moderate labile P content can be a potential source of TDP from the plough layer, and that a lower concentration margin of optimum agronomic P levels should be considered. [ABSTRACT FROM AUTHOR]

Gaspar, F.L; Montes, M. de J.F; Nascimento-Filho, G.A.;Lins, I.C.; Paulo, J.G.; Longo, A.P. 2013. Spatial and seasonal sediment phosphorus species and its relation with granulometry, organic matter and CaCO3 in a tropical estuary In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.) The identification of phosphorus species in sediments is a useful approach for the assessment of potential sources or sinks of phosphorus in estuarine regions, and helps to understand the biogeochemical processes that control the availability of phosphorus in tropical areas. This study aimed to identify the spatial and seasonal distribution of the occluded, bioavailable and organic phosphorus fractions, in sediments of a polluted estuary (Botafogo river), and other considered as a background area (Carrapicho river), and determine the correlations between the concentration of phosphorus in sediments with the grain size, calcium carbonate, organic matter, dissolved phosphorus and salinity. No significant seasonal variation of phosphorus fractions concentrations was observed. The highest concentrations of occluded phosphorus was registered at Santa Cruz Channel sampling station with median value of 558,80 μg.g−1. In the Botafogo river estuary were found the highest concentrations of organic, and bioavailable phosphorus with median values of 209,19 μg.g−1 and 398,09 μg.g−1 respectively, due to the high content of fine sediments in this area. The organic matter content was positively correlated with the organic, bioavailable and occluded phosphorus fractions r2 = 0.9335; 0.8882; and 0.7750 respectively. The occluded phosphorus was the major fraction of the total phosphorus in the whole study site and showed a high correlation (r = 0.8218) with the percentage of CaCO3 in sediments, unlike the others phosphorus fractions with r < 0.6.Weak correlations between the phosphorus fractions and the dissolved phosphorus were found, the bioavailable phosphorus showed the higher correlation with the water column concentrations (r = 0,5362). [ABSTRACT FROM AUTHOR]

Phosphorus (P) leaching from agricultural soils is a serious environmental concern. Application of aluminium water treatment residuals (Al-WTR5) at a rate of 20 Mg ha-1 to clay soils from central Sweden significantly increased mean topsoil P sorption index (PSI) from 4.6 to 5.5 imol kg-1 soil. Mean degree of P saturation in ammonium lactate extract (DPS-AL) significantly decreased from 17 to 13%, as did plant-available P (P-AL). Concentrations of dissolved reactive P (DRP) decreased by 10-85% in leaching water with Al-WTR treatments after exposure of topsoil lysimeters to simulated rain. Soil aggregate stability (AgS) for 15 test soils rarely improved. Three soils (clay loam, silty loam and loam sand) were tested in greenhouse pot experiments. Aluminium-WTR application of 15 or 30 ton ha-1 to loam sand and a clay loam with P-AL values of 80-100mg kg-1 soil significantly increased growth of Italian ryegrass when fertilised with P but did not significantly affect growth of spring barley on any soil. Al-WVR should only be applied to soils with high P fertility where improved crop production is not required. [ABSTRACT FROM AUTHOR]

Abstract: Eutrophication and algal bloom has been occurring in the Three Gorges Reservoir ever since the reservoir''s impoundment in 2003. The concentration of dissolved phosphorus (DP) is one of the main controlling factors for algal bloom. Fine sediment adsorbs DP thereby affecting the concentration of DP. The effect of the sediment reduction on the DP concentration of the Three Gorges Reservoir is discussed in this paper. Samples were taken from the Yangtze River and its tributaries, i.e. the Minjiang River and the Tuojiang River. The DP concentration of the water samples and the available phosphorus, total phosphorus, and phosphorus-fixing capacity of the sediment samples were analyzed. The amount of phosphorus adsorbed by the sediment was about 3% of the total DP load in the Yangtze River. An experiment was done in the lab simulating the DP concentration, sediment concentration, and turbulence. The results showed that quite a high amount of DP was adsorbed by the sediment. The adsorption capacity of the sediment was 135 mg/kg and the highest rate of adsorption was 29 mg/kg·h. The Xiluodu Dam is located at nearly 1,100 km upstream of the Three Gorges Dam. The Xiloudu Dam will trap sediment and thence reduce the sediment load entering the Three Gorges Reservoir after impoundment in 2013. It is estimated that the sediment reduction due to the Xiluodu Dam may result in an increase of the DP concentration in the Three Gorges Reservoir by 6.8%. [Copyright &y& Elsevier]

Abstract: Livestock grazing and treading is strongly linked to a decrease in freshwater quality and promotes eutrophication. A two-year field trial was carried out to investigate the influence cattle, sheep and deer have on soil physical quality and the loss of phosphorus (P) and suspended sediment (SS) in surface runoff. Surface runoff plots (4m long by 1m wide) were installed within areas designated as stock or ungrazed (control). Surface runoff was collected and analysed for concentrations and loads of P fractions (dissolved reactive P-DRP, dissolved unreactive P-DUP, total dissolved P-TDP, particulate P-PP and total P-TP) and SS. Grazed at equivalent stocking density, soil physical samples (macroporosity, bulk density, and saturated hydraulic conductivity K sat) were taken after each grazing event (n =11). Soil physical data indicated differences between cattle, sheep and deer with cattle having greater negative effects. However, these differences had no impact on P and SS losses between stock types in surface runoff. Significant relationships showed that an increase in macroporosity, K sat, and time (days) since grazing decreased concentrations and loads of P and SS losses. A separate rainfall simulation study also revealed that an increase in simulated cattle treading intensity increased the volume of surface runoff and SS losses. Most surface runoff (> 90%) occurred in winter when soil moisture was at or above field capacity. A seasonal effect was observed and showed that although the greatest P loads occurred in winter, the greatest P concentrations occurred in summer months, under infiltration-excess conditions. These summer losses could pose a risk to receiving waterways because increased light and warmth may induce an algal response compared to winter. While there is limited scope to manage for infiltration-excess surface runoff losses from pasture, with most runoff occurring in winter, these findings reinforce the use of mitigation strategies such as restricted or nil grazing in winter when soil moisture has reached field capacity to minimise P and SS loss to surface water, regardless of stock type. [Copyright &y& Elsevier]

Cattle treading is known to influence surface runoff contaminants such as phosphorus (P) and suspended sediment (SS). The objective of this work was to determine how simulated cattle treading affects losses of P and SS in surface runoff from four soils (Brown, Melanic, Pallic and Recent Gley) with contrasting resistance to treading damage and drainage properties. Soils were excavated, placed in boxes (0.8 m long and 0.20 m wide) and one of three moisture treatments were imposed (10, 50 and 90% of available water holding capacity) before surface runoff was induced via simulated rainfall; P fraction and SS were determined in the resulting runoff. The Pallic and Recent Gley soils lost the most P and SS compared with the better structured Brown and Melanic soils. Greater losses of particulate phosphorous (PP) and SS were observed at higher soil moisture contents. In contrast, dissolved P in runoff was greater at lower soil moisture contents, especially dissolved reactive phosphorus (DRP), which accounted for up to 90% of total P losses at 10% soil moisture in the Recent Gley soil. Measurement of soil water soluble P, a surrogate for DRP at a range of soil moisture contents, suggests this could be due to lysis of soil microbial biomass upon drying. Although concentrations and loads of DRP in surface runoff were greatest at 10% moisture, the risk of surface runoff is less than the risk posed by PP and SS losses with treading on wet soils, where surface runoff is more likely. This was particularly true for the Pallic and Recent Gley soils. It is therefore recommended that, to minimise SS and P losses to surface water, care be taken when grazing these soils under wet winter-spring conditions. Although collected under controlled conditions, these data help to explain the wide range of potential SS and P losses that may occur in a field due to hydrologic and soil variation. [ABSTRACT FROM AUTHOR]

Phosphorus (P) in runoff can pose a water quality risk in phosphate mine reclamation areas. High dissolved P (DP) concentrations (about 0.4–3.0 mg L−1) in runoff from these areas and high equilibrium P concentrations for the soils led us to hypothesize that P release is controlled by dissolution of apatite rather than by desorption mechanisms. Objectives were to (a) verify via chemical- and solid-state assessments that P in the reclamation soils is mainly in the form of apatite and (b) examine evidence that DP concentrations in runoff water from these soils is controlled by apatite dissolution. Soil analyses included total P (TP), P sorption isotherms, P fractionation, mineralogy, and P distribution by particle size classes. Runoff samples were chemically characterized and modeled for speciation. Results showed high TP concentrations and the presence of apatite. The Ca- and Mg-bound P accounted for about 95% of TP. Runoff samples were undersaturated with respect to apatite. A strong relationship between calculated apatite specific surface area and measured DP concentration in water extracts is supportive of other evidence that apatite dissolution is a major factor controlling P release from these soils. Data indicate that these soils will be a long-term P source rather than sink. Results are applicable to other phosphate mine reclamation sites and illustrate the need to account for compositional differences between reconstructed soils on reclaimed mining sites and their indigenous soil analogues. [ABSTRACT FROM AUTHOR]

A reduction in surface water quality in urban watersheds due to nonpoint source phosphorus (P) loading has prompted municipalities to consider management practices to reduce P loss from landscapes. However, locating P source areas can be time consuming and expensive. Use of distributed models allows delineation of P source areas and focused management strategies. Using the spatially distributed soil moisture distribution and routing model, we adapt and validate a dissolved P (DP) loading model for application to an urban watershed, in Ithaca, New York, to identify P source areas. The model calculates DP loss separately for base flow, impervious surfaces, plant-soil complex, and fertilized areas. The load at the outlet is the sum of P loss from the four components distributed throughout the watershed. Both stream and distributed DP loss were well predicted as indicated by comparison with measured data. The model predicted the largest contribution from plant-soil complexes (36%). Impervious surfaces contributed 10% of the total load but as much as 17% in the winter. More important, the impervious surfaces increased DP losses from the adjacent areas due to runoff from the impervious surfaces saturating the soil, thus increasing runoff losses. Fertilizer contributed substantially following application but decreased rapidly thereafter, a result of conversion from soluble to insoluble P. However, fertilization increased soil P levels, and thus DP losses were higher as a whole (19%). Results demonstrate that correctly predicting the coincidence of P and runoff source areas can be a powerful tool to identify and mitigate contamination of surface waters. [ABSTRACT FROM AUTHOR]

Distributed physically based erosion and phosphorus (P) transport models, run by the overland flow model described in Taskinen and Bruen (2006. Hydrological Processes 21(1)), are described. In the erosion model, the additional components to the basic model were the outflow of the particles by infiltration and a new model component, i.e. deposition when rainfall stops. Two ways of calculating the shielding factor due to the flow depth were compared. The P transport model had both dissolved P (DP) and particulate P (PP) components. The processes included in the DP model were desorption from the soil surface, advection, storage in the overland flow and infiltration. The PP model accounted for advection, storage in the flow, infiltration, detachment from the soil surface by flow and rainfall and deposition both when transport capacity of suspended solids (SS) is exceeded and when rainfall ceases. When the models were developed and validated in small agricultural fields of cohesive soil types in southern Finland. comparisons were made between corresponding processes and the significance of added components were estimated in order to find out whether increased model complexity improves the model performance. The sedigraphs were found to follow the dynamics of rainfall, emphasizing the importance of the rainfall splash component. The basic model was too slow to react to changes in rainfall and flow rates, but infiltration and deposition that acts during the cessation in rainfall improved the model significantly by enabling the modelled SS to fall sharply enough. The shielding effect of flow depth from the splash detachment was found to play a significant role. Transport capacity should also be included in erosion models when they are applied to cohesive soils. In this study, the Yalin method worked well. A strong correlation was obvious between the measured SS and total P concentrations, indicating that the main form of P in runoff is PP. This emphasizes the importance of a good sediment transport model in P transport modelling. The submodel used for DP resorption from the soil surface produced plausible results without any calibration. [ABSTRACT FROM AUTHOR]

Phosphorus (P) losses through overland flow (surface runoff) may contribute to eutrophication of water bodies. The main purpose of this work was to study P forms in overland flow (dissolved and particulate) to identify which can be potentially used by algae. To this end, rainfall on 17 representative soils from Mediterranean areas was simulated, and P forms in overland flow studied by chemical and sink (resin, iron oxide–impregnated paper strip) extraction; sequential chemical fractionation of the suspended sediments was also used to establish “operational pools” with a differential capacity of P release. Total P (TP) in runoff ranged from 0.089 to 0.765 mg L -1 and was mainly related to suspended sediment (particulate P, 86% of TP on average). Iron oxide strip P, which is taken to be an estimate of algal‐available P, accounted for 34% of TP on average in runoff samples; most of the P extracted by this sink was particulate P (68%). In most cases, FeO strip P was equivalent to dissolved reactive P (DRP) plus P extracted by NaOH and citrate‐bicarbonate in suspended sediment (the more labile P fractions) ( Y=X, R 2 =0.82; P ). One can thus assume that Fe oxide extracts DRP, adsorbed P on sediments, and P related to highly soluble precipitated Ca phosphates, but it does not extract releasable P through reduction of sorbent surfaces or the organic P that can be mineralized in the bottom of water reservoirs, which must be taken in account to estimate the long‐term algal‐available P in runoff. [ABSTRACT FROM AUTHOR]

Abstract: Phosphorus flux models show that the removal tends to be a common feature in polluted estuarine systems whereas the release of P to the adjacent coastal area occurs in pristine environments. This study analyzes the distribution of P in water and sediments along six rivers in the south coast of Brazil. Three rivers located inside protected areas were considered non-polluted and used as a control of urbanization. The other three, situated within urbanized areas, were considered as having highly elevated concentrations of P. Results showed a different behavior of P in water and sediments located in urbanized and non-urbanized areas. The concentrations of dissolved organic (P-org) and inorganic (P-inorg) phosphorus in water, and the total phosphorus (P-tot) and polyphosphate (P-poly) in sediments where higher in the urbanized rivers compared to the non-urbanized ones. Both P of punctual origin and of diffuse origin contributed to the maintenance of elevated concentrations and disturbed the natural fluxes along the polluted rivers. The minimum and maximum concentrations in urbanized areas varied from 0.39 to 12.45 (μM) for P-org and 0.00 to 5.92 (μM) for P-inorg in water, and in sediments from 89.90 to 808.16 (μMg−1) for P-tot and 0.00 to 76.51 (μMg−1) for P-poly. In non-urbanized areas concentrations in water varied from 0.22 to 1.20 (μM) for P-inorg and 4.43 to 5.56 (μM) for P-org, and in sediments from 45.91 to 652.26 (μMg−1) for P-tot and 0.00 to 8.61 (μMg−1) for P-poly. Using a hierarchical sampling design and a simple model of variation of P (K d model) it was possible to demonstrate that urbanized and non-urbanized estuaries may act as sinks or sources of P. The variation of P analysis in different points along each of the six rivers showed that release and removal areas may occur within a same river independently of its urbanization. [Copyright &y& Elsevier]

Four surface flow constructed wetlands (CWs) have been intensively investigated for phosphorus retention, from 3 to 7 years in the cold temperate climate of Norway. The aim of this study was to identify factors that affect phosphorus retention from non-point sources. The wetlands were located in first order streams, with surface areas of 0.06–0.4% of the watershed (CW-area 350–900 m2). Volume proportional composite samples were taken from inlet and outlet, and sedimentation plates were used in selected areas. The average retention of total phosphorus for the individual CWs was 21–44% of input, despite the high hydraulic load (mean load was 0.7–1.8 m per day). This equals a retention of 26–71 g phosphorus m−2 surface area per year. A first-order model was fitted to the data giving an average removal constant, k, of 214 m per year. However, the constant increased with increasing hydraulic load due to the simultaneous increase particle settling velocity. Hence, retention increased in spite of increasing hydraulic loads. Moreover, linear multiple regression models showed that retention was influenced by several external variables, e.g. input of phosphorus, season, phosphorus content on suspended solids and phosphorus settling velocity. The results suggest that the first-order model is less suitable to estimate phosphorus retention in similar gravity fed wetlands. The best of the proposed statistical prediction models, reproduced observed data from two independent test-CWs with a deviation of 0.1%. The investigation shows that small wetlands are a useful supplement to best management practice on arable fields. However, the present study focuses on the necessity to investigate how pollutants enter wetlands. Such knowledge can then be used to suggest improvements of wetland layout. [Copyright &y& Elsevier]