Particulate P Research Articles

Remineralization of phytoplankton-derived organic matter by natural populations of heterotrophic bacteria

The relative lability, elemental stoichiometry, and remineralization rates of various particulate organic matter (POM) substrates by natural heterotrophic marine microorganisms were investigated. POM was harvested from laboratory cultures of a marine diazotroph (Trichodesmium IMS101), a cosmopolitan diatom (Thalassiosira weissflogii), a common marine cyanobacteria (Prochlorococcus MED4), and from surface waters off the Oregon coast. These POM resources were used as inoculants in a field experiment conducted at the Hawaii Ocean Time-series Station ALOHA in the North Pacific Subtropical Gyre. POM from these various sources was added to seawater collected from below the surface mixed layer, incubated in the dark, and remineralization rates were quantified via high-frequency measurement of soluble phosphorus (P) and nitrogen (N) concentrations over a 6-d period. Rapid solubilization and near complete remineralization of particulate P (PP) occurred in all treatments where cultured POM was used, with lesser relative mobilization of P from a ‘natural’ POM sample isolated from surface seawater off the Oregon coast. Soluble P pools, likely consisting of surface-adsorbed inorganic P and inorganic P liberated from cells during harvesting of biomass accounted for 28% of natural PP pools and 80±32% of cultured PP. 31P nuclear magnetic resonance (NMR) confirmed that PP was predominately present as orthophosphate in all POM types. By the end of the incubation period, all added P from cultured material had been converted to dissolved inorganic P. This finding may be a caveat of our utilization of laboratory cultures and natural POM which has been exposed to high inorganic P concentrations (0.8–5.0μmolL−1), albeit it is consistent with previous reports of significant contributions of surface-adsorbed P to total particulate P. In contrast, over the course of these experiments, only 37–40% of added N had been remineralized to ammonium (NH4+). In general, N remineralization rates of cultured material increased with the amount of N added (per gram of dry material). The net yield of bacterial cells was also positively correlated to the initial amount of C and N added. Most notably, when corrected for non-biological turnover (i.e. removal of soluble pools), the N:P remineralization ratio of cultured material (8.5±1.3) was independent of the N:P of added organic material (5–23).

Phosphorus transport and retention in a channel draining an urban, tropical catchment with informal settlements

Abstract. Urban catchments in sub-Saharan Africa (SSA) are increasingly becoming a major source of phosphorus (P) to downstream ecosystems. This is primarily due to large inputs of untreated wastewater to urban drainage channels, especially in informal settlements (or slums). However, the processes governing the fate of P in these catchments are largely unknown. In this study, these processes are investigated. During high runoff events and a period of base flow, we collected hourly water samples (over 24 h) from a primary channel draining a 28 km2 slum-dominated catchment in Kampala, Uganda, and from a tertiary channel draining one of the contributing slum areas (0.54 km2). The samples were analysed for orthophosphate (PO4-P), particulate P (PP), total P (TP), suspended solids (SS) and hydrochemistry. We also collected channel bed and suspended sediments to determine their geo-available metals, sorption characteristics and the dominant phosphorus forms. Our results showed that the catchment exported high fluxes of P (0.3 kg km2 d−1 for PO4-P and 0.95 for TP), which were several orders of magnitude higher than values normally reported in literature. A large proportion of P exported was particulate (56% of TP) and we inferred that most of it was retained along the channel bed. The retained sediment P was predominantly inorganic (> 63% of total sediment P) and consisted of mostly Ca and Fe-bound P, which were present in almost equal proportions. Ca-bound sediment P was attributed to the adsorption of P to calcite because surface water was near saturation with respect to calcite in all the events sampled. Fe-bound sediment P was attributed to the adsorption of P to iron oxides in suspended sediment during runoff events given that surface water was undersaturated with respect to iron phosphates. We also found that the bed sediments were P-saturated and showed a tendency to release P by mineralisation and desorption. During rain events, there was a flushing of PP which we attributed to the resuspension of P-rich bed sediment that accumulated in the channel during low flows. However, first-flush effects were not observed. Our findings provide useful insights into the processes governing the fate and transport of P in urban slum catchments in SSA.

Fit-for-purpose phosphorus management: do riparian buffers qualify in catchments with sandy soils?

Hillslope runoff and leaching studies, catchment-scale water quality measurements and P retention and release characteristics of stream bank and catchment soils were used to better understand reasons behind the reported ineffectiveness of riparian buffers for phosphorus (P) management in catchments with sandy soils from south-west Western Australia (WA). Catchment-scale water quality measurements of 60 % particulate P (PP) suggest that riparian buffers should improve water quality; however, runoff and leaching studies show 20 times more water and 2 to 3 orders of magnitude more P are transported through leaching than runoff processes. The ratio of filterable reactive P (FRP) to total P (TP) in surface runoff from the plots was 60 %, and when combined with leachate, 96 to 99 % of P lost from hillslopes was FRP, in contrast with 40 % measured as FRP at the large catchment scale. Measurements of the P retention and release characteristics of catchment soils (<2 mm) compared with stream bank soil (<2 mm) and the <75-μm fraction of stream bank soils suggest that catchment soils contain more P, are more P saturated and are significantly more likely to deliver FRP and TP in excess of water quality targets than stream bank soils. Stream bank soils are much more likely to retain P than contribute P to streams, and the in-stream mixing of FRP from the landscape with particulates from stream banks or stream beds is a potential mechanism to explain the change in P form from hillslopes (96 to 99 % FRP) to large catchments (40 % FRP). When considered in the context of previous work reporting that riparian buffers were ineffective for P management in this environment, these studies reinforce the notion that (1) riparian buffers are unlikely to provide fit-for-purpose P management in catchments with sandy soils, (2) most P delivered to streams in sandy soil catchments is FRP and travels via subsurface and leaching pathways and (3) large catchment-scale water quality measurements are not good indicators of hillslope P mobilisation and transport processes.

Mitigation of phosphorus leaching losses via subsurface drains from a cracking marine clay soil

In Scandinavia, subsurface transport via tile drains contributes significantly to phosphorus (P) and nitrogen (N) leaching from arable land, which adds to the eutrophication of surface waters. Using flow-proportional water sampling, various options for mitigating subsurface P leaching losses (and N leaching) were examined in 28 experimental plots on a flat, tile-drained site with 60% marine clay. Two crop rotations and unfertilised fallow were monitored for a total of six years. In addition to topsoil management practices (different forms of tillage, structural liming and mineral P fertilisation), local spatial variations in subsurface transport were determined within the experimental area.Mean total P (TotP) leaching losses after conventional autumn ploughing and inverting the soil to a depth of 23cm were 0.79kgha−1year−1, with 87% occurring as particulate P (PP), and the corresponding mean total N leaching losses were 27kgha−1year−1, with 91% occurring as nitrate. The coefficient of variation in TotP leaching both in spring before the experiment started (64%) and during the six-year experiment (60%) was higher than the coefficient of variation in P-soil status (20%), or drainage (25%), illustrating the importance of local-scale subsurface transport in this cracking clay. However, TotP and PP leaching losses were significantly (pr>F<0.002) lower from plots with structural liming than from the other treatments grouped together. Different P fertilisation strategies (band-spreading/broadcasting of mineral P and applying a balanced amount/no P fertiliser) had no significant effect on P leaching losses. Nitrogen leaching was significantly (pr>F<0.001) lower from unfertilised fallow than from other treatments and was not significantly lower after shallow autumn tillage than after conventionally ploughing, whereas PP losses tended to be higher. Infiltration measurements with tension infiltrometers revealed a high variation in saturated hydraulic conductivity within plots. In view of the generally high PP losses, efforts to combat eutrophication of the nearby Baltic Sea should concentrate on soil structure improvements, while extensive tillage and totally omitting P fertilisation of cracking soils with moderate soil P status appears to be inefficient mitigation options.

Influence of Inundation Depth on Typha domingensis and its Implication for Phosphorus Removal in the Everglades Stormwater Treatment Area

The influence of inundation depth on phytomass, shoot density, photosynthesis, elongation, and nutrients of Typha domingensis in Stormwater Treatment Area-1East (STA-1East) was examined and results were evaluated relative to phosphorus (P) treatment from 2006 to 2011. Biomass and nutrient concentrations of T. domingensis were not related to inundation depth. Necromass and belowground/leaf biomass ratio increased with increasing inundation depth from 30 to 80 cm. Shoot density, photosynthesis, and elongation decreased with inundation depth. The contents of necromass nutrients and phytomass nitrogen increased with inundation depth. Annual total P (TP) concentration increase (−) or decrease (+) from inflow to outflow was −21 %, −3 %, +8 %, −54 %, +41 %, and +29 % in 2006–2011, respectively. Decreases in soluble reactive P (SRP), total dissolved P (TDP), dissolved organic P (DOP), and/or particulate P (PP) from inflow to outflow were low or did not occur annually. Prolonged inundation damaged T. domingensis community and thus adversely impacted the capacity of the system to treat P. Inundation depths in the STAs can be categorized into optimal, subtle stress, and lethal stress zones. This study stressed the importance in maintaining vegetation within an optimal zone of inundation for a sustainable system.

Relation between soil P test values and mobilization of dissolved and particulate P from the plough layer of typical Danish soils from a long‐term field experiment with applied P fertilizers

AbstractAccumulation 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 &lt; 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.

The effect of dams on phosphorus in the middle and lower Yangtze river

[2] We investigated the effect of the Three Gorges Project and other dams on the load of phosphorus (P) to the middle and lower Yangtze River (MLY) and discussed the alteration of P on the ecosystem of the MLY. We collected data for continuous flow and sediment over the past 60 years and observed the concentrations of total P (TP) and particulate P (PP) in the pool reaches of the Three Gorges Reservoir (TGR), both before and after the impoundment in 2003. As a result, we obtained highly positive correlations between P and sediment and revealed two changes that were caused by the impoundments: (1) the sediment load to the MLY decreases by 91% and the river becomes almost clear; and (2) the loads of TP and PP to the MLY are sequestered by 77% and 83.5% annually and 75% and 92% in dry seasons, respectively. Because P was the limiting nutrient for bioactivity in the MLY before 2003, such significant reductions, along with the many other consequences of the dams, will not only further reduce the bioavailability of P but also increase the existing high ratio of nitrogen (N) to P. Therefore, it is quite possible to alter the nutrient regime and reduce the aquatic primary productivity of the MLY. Given that many large dams with huge reservoirs are under construction or planned upstream and elsewhere, studies focused on the long-term effects of sediment and P reduction deserve a high priority for the protection of lowland rivers and aquatic ecosystems.

Characterization of Colloidal Phosphorus Species in Drainage Waters from a Clay Soil Using Asymmetric Flow Field-Flow Fractionation

Phosphorus transport from agricultural land contributes to eutrophication of surface waters. Pipe drain and trench waters from a grassland field on a heavy clay soil in the Netherlands were sampled before and after manure application. Phosphorus speciation was analyzed by physicochemical P fractionation, and the colloidal P fraction in the dissolved fraction (<0.45 μm) was analyzed by asymmetric flow field-flow fractionation (AF4) coupled to high-resolution inductively coupled plasma-mass spectrometry and ultraviolet diode array detector. When no manure was applied for almost 7 mo, total P (TP) concentrations were low (<21 μmol L), and TP was almost evenly distributed among dissolved reactive P (DRP), dissolved unreactive P (DUP), and particulate P (PP). Total P concentrations increased by a factor of 60 and 4 when rainfall followed shortly after application of cattle slurry or its solid fraction, respectively. Under these conditions, DRP contributed 50% or more to TP. The P speciation within the DUP and PP fractions varied among the different sampling times. Phosphorus associated with dissolved organic matter, probably via cation bridging, comprised a small fraction of DUP at all sampling times. Colloidal P coeluted with clay particles when P application was withheld for almost 7 mo and after application of the solid cattle slurry fraction. At these sampling times, PP correlated well with particulate Fe, Al, and Si, indicating that P is associated with colloidal clay particles. After cattle slurry application, part of DUP was probably present as phospholipids. Physicochemical fractionation combined with AF4 analysis is a promising tool to unravel the speciation of colloidal P in environmental water samples.

Uncertainties in estimated phosphorus loads as a function of different sampling frequencies and common calculation methods

Water quality monitoring programs are often based upon low-frequency regular sampling regimes from which loads are estimated. In this study, stream flow (Q) and phosphorus concentrations (C) were measured at 2-hourly intervals over a 10-week period between October and December 2006 in a tributary of Loch Leven, Scotland. The dataset was deconstructed to emulate different weekly, daily and composite sampling strategies, the aim being to highlight the large amount of uncertainty and imprecision in estimating total (TP) and soluble reactive (SRP) phosphorus loads on the basis of commonly applied sampling strategies and calculation methods. When based on the full dataset, phosphorus (P) loads estimated from the 2-hourly data were 459 kg TP, 351 kg particulate P (PP) and 78 kg SRP. In contrast, P loads estimated from different weekly, daily and composite sampling regimes and determined by applying seven different calculation methods ranged from 22 to 5028 kg TP, 13 to 4588 kg PP and 7 to 286 kg SRP. The results of this study highlight the large amount of uncertainty and imprecision associated with estimating P loads and contributes to the body of evidence that high-frequency monitoring is necessary if P loads to standing water bodies are to be quantified accurately and the effects of nutrient management programs interpreted correctly.

Mersin Körfezi-Erdemli Kıta Sahanlığı Yüzey ve Dip Sularında İnorganik ve Organik Fosfor Formlarının Zaman ve Mekan Ölçekli Değişimleri

The aim of this study was to understand spatial and temporal variations of inorganic and organic forms of phosphorus in the surface and deep waters of Erdemli shelf zone of the Mersin Bay. For this goal, time- series hydrographical and phosphorus data were obtained monthly at the three selected stations between December 2001 and December 2003. The present results have shown that the concentrations of different fractions of total phosphorus displayed a decreasing trend from nearshore to shelfbreak zone of the bay. Total phosphorus (dissolved+particulate) in the shelf water was dominated by dissolved-P fraction (TDP:56-97%) and dissolved organic-P (DOP) was principally the major constituent of the TDP in the shelf water. Similarly, particulate organic-P (POP) dominates the particulate-P (PP) pool; however, the contribution of particulate inorganic-P (PIP) to PP pool was more pronounced in the nearshore zone fed by riverine inputs.

Long-term temporal dynamics and trends of particle-bound phosphorus and nitrate in agricultural stream waters

One problem in evaluating efforts to reduce phosphorus (P) and nitrogen (N) losses to waters is that variations in weather conditions cause nutrient concentrations and waterflow to vary. Analyses of biweekly stream water samples collected manually from two small, neighbouring Swedish agricultural catchments with clay soil (E23 and E24) demonstrated unpredictability in P and N concentrations. However, particulate P (PP) concentrations in the two separate catchments, usually sampled within 2–3 hours on the same day, were clearly correlated to each other (Spearman correlation coefficient r=0.70). Corresponding nitrate–nitrogen (NO3–N) concentrations were also correlated to each other (r=0.79). Particulate P concentrations could reasonably be predicted from suspended solid (SS) concentrations above base flow (BF) in both catchments (regression coefficient R 2=0.84 and 0.86, respectively). In the period 1993–2009, before eutrophication control programmes were introduced in catchment E23, there was no general trend in PP or SS in either catchment. Mean PP (0.13 mg L−1) predicted (R 2=0.88) from high-resolution (15 minute) turbidity concentrations was significantly higher than flow-weighted mean PP concentration estimated from discrete samples (0.10 mg L−1) collected manually at the catchment E23 outlet. Mean PP concentration estimated directly from flow-proportional sampling was also higher. High synoptic concentrations of PP (up to 0.65 mg L−1) were recorded along the open reach of the stream in the ascending limb of high-flow pulses. Using high-resolution monitoring at the catchment outlet, episodes with a clear clockwise hysteresis effect for PP concentration (seen as turbidity) were frequently observed. By contrast, the NO3–N peak appeared 4–7 hours after the flow peak and anticlockwise hysteresis was observed. Significant erosion along stream banks may take place, and the degree of erosion was estimated based both on farmers’ observations and on results from a distributed erosion model (USPED). Monitoring and erosion mapping are currently being used in practical remedial work.

Testing the Wisconsin Phosphorus Index with Year-Round, Field-Scale Runoff Monitoring

The Wisconsin Phosphorus Index (WPI) is one of several P indices in the United States that use equations to describe actual P loss processes. Although for nutrient management planning the WPI is reported as a dimensionless whole number, it is calculated as average annual dissolved P (DP) and particulate P (PP) mass delivered per unit area. The WPI calculations use soil P concentration, applied manure and fertilizer P, and estimates of average annual erosion and average annual runoff. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field-years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from the average annual estimates used in the WPI, the WPI and measured loads were not well correlated. However, when measured runoff and erosion were used in the WPI field loss calculations, the WPI accurately estimated annual total P loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that the WPI methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment tool for managing runoff P losses.

Effectiveness of Grass Filters in Reducing Phosphorus and Sediment Runoff

Surface water contamination can often be reduced by passing runoff water through perennial grass filters. Research was conducted in 2006 to 2008 to evaluate the size of cool season grass filters consisting primarily of tall fescue (Festuca arundinacea Schreb) with some orchard grass (Dactylis glomerata L.) relative to drainage area size in reducing runoff sediment and phosphorus (P). The soil was Pohocco silt loam Typic Eutro- chrepts with a median slope of 5.5 %. The grass filters occupying 1.1 and 4.3 % of the plot area were compared with no filter with four replica- tions. The filters were planted in the V-shaped plot outlets which were 3.7×11.0 m in size. The filter effect on sediment and P concentration was deter- mined from four natural runoff events when nearly all plots had runoff. Filter effect on runoff volume and contaminant load was determined using total runoff and composites of samples collected from 12 runoff events. Sediment concentration was re- duced by 25 %w ith filters compared with no filter (from 1.10 to 1.47 gL −1 ), but P concentration was not affected. The 1.1 and 4.3 % filters, respective- ly, compared with having no grass filter, reduced: runoff volume by 54 and 79 %; sediment load by 67 and 84 % (357 to 58 kgha −1 ); total P load by 68 and 76 %( 0.58 to 0.14 kgha −1 ); particulate P (PP) load by 66 and 82 %( 0.39 to 0.07 kgha −1 ); and dissolved reactive P (DRP) load by 73 and 66 % (0.2 to 0.07 kgha −1 ), respectfully. A snowmelt runoff event had 56 % greater DRP concentration compared with rainfall-induced runoff events. Grass filters re- duced sediment and P load largely by reducing runoff volume rather than reducing concentration. Well- designed and well-placed grass filters that occupy 1.0 to 1.5 % of the drainage area and intercept a uniform

Effects of Lime and Phosphorus Application on Phosphorus Runoff Risk

Lime was investigated as a soil amendment to decrease phosphorus (P) loss in runoff from two Delaware sandy loam soils, one high and one low in P. Soils were limed at three rates (control and target pH values of 6 and 6.8, respectively), packed into runoff boxes (2,000 cm2) and received simulated rainfall (80 mm h−1 for 30 min). Lime showed potential to decrease P loss in runoff, but its effectiveness was soil specific and dependant on other management factors also. Lime decreased dissolved reactive P (DRP) and dissolved organic P (DOP) loss by 20–25 and 52–93 %, respectively, for the high-P soil and particulate P (PP) by 13 % for the low-P soil. The majority of P lost in runoff was DOP (3–29 %) or PP (64–96 %). Lime increased PP losses from the finer-textured soil following P application, indicating that increased P sorption can lead to increased losses if P is sorbed to more erodable particles. Initial soil P status was more important than liming in determining P loss. While amendments may decrease P losses in the short term, addressing nutrient imbalances at the field scale is clearly necessary in the long term. Losses increased significantly following inorganic P application. Although P was sorbed rapidly, with less than 2 % of added P removed in runoff, mean concentrations in excess of 700 μg l−1 DRP, 2,500 μg l−1 OP and 6,500 μg l−1 PP were recorded for both soils immediately following P application.

Influence of hydropattern and vegetation on phosphorus reduction in a constructed wetland under high and low mass loading rates

Large constructed wetlands are extensively used in south Florida for surface water quality improvement as well as for flood control. Due to the periodic wet–dry climate of this region, it is likely that soils in these large constructed wetlands may become dry or partially dry (experiencing drawdown), which represent a challenge to managing and optimizing phosphorus (P) reductions in these systems. Therefore, we designed an experiment to study the interactive effect of hydropattern (batch vs. continuous flow) and the presence or absence of emergent vegetation on P exchange between surface water and organic soil in two sets of 12 mesocosms. These 24 mesocosms were filled with 30-cm deep peat soil, and each set were subjected to either high (12.5gm−2year−1) or low (3.4gm−2year−1) P loading from surface water. All treatments performed similarly prior to surface water drawdown with P reductions being relatively high for high P loading rates and being low for low P loading rates. Mesocosms subject to wet–dry–wet cycles exhibited a three- to four-fold increase in surface water effluent P concentrations immediately following soil re-flooding, which lasted 1–3 and 3–10 weeks for low and high P loading rates, respectively. The magnitude of P flux from sediment to surface water and the time period over which P release took place were P loading-dependent (higher loading led to higher P flux compared to lower P loading rates), and season-dependent (a longer duration of higher P flux experienced during dry- compared to wet-season drawdowns). Results indicated that hydropattern was the dominant factor affecting P flux to overlying surface water for the high P loading rate, while the presence or absence of emergent vegetation was the dominant factor influencing P release for the low P loading rate. Treatments lacking emergent vegetation generated the most particulate P (PP) for both high and low P loading rates. All P fractions were correlated to either hydropattern or inflow concentrations, for both low and high P loading rates, and, with the exception of PP, correlated to vegetation at low-P loading rate. Our results indicated that the presence or absence of emergent vegetation is a critical factor in the management of large constructed wetlands receiving low P loadings while hydropattern should be the focus in managing treatment systems receiving high loads of P. Regardless of the P loading rates, maintaining moist soils in large constructed wetlands is a good management strategy, particularly during dry climatic periods, to minimize soil P oxidation and P flux to surface water after soil re-flooding.

Efficacy of a large-scale constructed wetland to remove phosphorus and suspended solids from Lake Apopka, Florida

Constructed wetlands treat various types of waters. We examined the efficacy of a large-scale constructed wetland (the marsh flow-way), which was designed to treat lake water by maximizing the removal rate of total phosphorus (TP), and total suspended solids (TSS) from Lake Apopka. The flow-way was constructed on former agricultural land and has four independently operated wetland cells (treatment area is ∼276ha). During the operating period (November 2003–March 2007), hydraulic loading rate (HLR) was high, with annual median values between 32 and 49myr−1. On average, 87% of the treated water returned to the lake, and the remainder flowed downstream. The wetland released soluble P (mostly soluble reactive P [SRP]), which was probably caused by P release from soils and/or senescing vegetation. This release was high initially and high during summers, but declined overall with time. Despite SRP release, the system removed TP over annual periods, with maximum net TP removal in 2007 (1.8gm−2yr−1). Most of the incoming and retained P was particulate. Median percent mass removal was 58% for particulate P (PP) and about 30% for TP. Total suspended solids were always removed (median removal rate=1.4kgm−2yr−1) and the percent mass removal was high (93%). Median first-order rate constants (P-k-C* model) were 43myr−1 for TP and 228myr−1 for TSS. Removal rates of TP and PP appeared to increase linearly up to the highest loading (5–6gm−2yr−1), whereas removal rate of TSS may have approached an asymptote at loading >3kgm−2yr−1. We found that C* declined from 0.070 in the previous pilot project to 0.045mgL−1 in the current flow-way system. Continued reduction in C* as TP declines in inflow lake water would assist P removal. Otherwise, selective wetland operation during periods of high lakewater TP concentrations may be required in the future to maintain net P removal.

Bioavailable Phosphorus in Fine‐Sized Sediments Transported from Agricultural Fields

Sediments transported from agricultural fields in surface and subsurface waters contain particulate P (PP) that could be partitioned into two pools—one available for aquatic organisms including cyanobacteria, namely bioavailable particulate P (BAPP, determined by 0.1 mol L−1 NaOH extraction), and the remainder not bioavailable (non‐BAPP). This study aimed to quantify the PP and BAPP concentrations in surface runoff and tile drainage water from eight agricultural fields with clay and sandy soils in the Missisquoi Bay region of Quebec, Canada. Particulate P in surface and drainage water varied spatially (among fields) and temporally, with concentrations as high as 3181 μg P L−1 in surface runoff and 1346 μg P L−1 in tile drainage. About 30% of PP was BAPP regardless of the drainage pathway. The PP and BAPP concentrations were related linearly (R2 = 0.86) to total suspended solids (TSS) in fine (0.05–1‐μm) and coarse (1–100‐μm) fractions. About 68% of the PP in clay soils and 50% of the PP in sandy soils were associated with the 0.05‐ to 1‐μm size particles, which had more BAPP, on average 0.46 g P kg−1, than the coarser 1‐ to 100‐μm fraction (0.22 g P kg−1). Soil parameters such as Mehlich‐3 extractable (M3) P, M3Fe, and the degree of soil P saturation were related to the PP and BAPP concentrations in TSS and particle size fractions. We concluded that sediments with particle size &lt;1 μm contained more BAPP and their loss from agricultural fields could contribute to the eutrophication downstream.

Hot moments and hot spots of nutrient losses from a mixed land use watershed

Summary Non-point nitrogen (N) and phosphorus (P) pollution from agriculture has increasingly received more public attention. In this study, NO 3 –N, dissolved P (DP) and particulate P (PP) concentrations and loads were investigated for four sub-basins (labeled 1–4 going up the watershed) within a mixed land use watershed (39.5 ha) in the Appalachian Valley and Ridge Physiographic Province. The hot moments of NO 3 –N concentration and load occurred in base flow and during the non-growing season. Great and temporally variable DP and PP concentrations were observed in storm flow. The hot moments of DP concentration and load were in storm flow from May to December and from September to Nov, respectively, while the hot moments of PP concentration and load were in storm flow from January to June. The NO 3 –N, DP, and PP loads were compared for all four sub-basins on a loss per length of stream reach basis to determine the hot spots and their corresponding losses. The hot spots and hot moments of NO 3 –N loads were in Sub-basins 1 and 4 during the non-growing season base flow period and Sub-basin 2 during the post-growing season base flow period (>110 g m −1 mo −1 ). The hot spots of DP loads were also in Sub-basins 1 and 4, but during the growing and post-growing season storm flow period (>1.4 g m −1 mo −1 ). In contrast, the hot spots and hot moments of PP load were in Sub-basin 3 during the pre-growing and growing season storm flow, as much as 13.4 and 14.1 g m −1 mo −1 , respectively. Controlling factors of nutrient export were discussed in this study, including season, hydrology (base flow, storm flow, surface and subsurface runoff), and land use. Although different hot moments and hot spots within the watershed were identified for NO 3 –N, DP, and PP losses, the implementation of a couple of management practices (cover crops and no-till) might be sufficient to effectively reduce nutrient losses from this and similar Valley and Ridge watersheds.

Manure Composition and Incorporation Effects on Phosphorus in Runoff Following Corn Biomass Removal

Greater demand for corn ( L.) stover for bioenergy use may lead to increased corn production acreage with minimal surface residue cover, resulting in greater risk for soil erosion and phosphorus (P) losses in runoff. A rainfall simulation study was conducted to determine the effects of spring-applied dairy cow () manure (none, in-barn composted, and exterior walled-enclosure pit) with >200 g kg organic solids content following fall corn biomass removal with and without incorporation (chisel plow [CP] and no-till [NT]) on sediment and P in runoff. Runoff was collected from a 0.83-m area for 60 min following the onset of rainfall simulation (76 mm h), once in spring and once in fall. Runoff dissolved reactive P (DRP) and dissolved organic P (DOP) concentrations were positively correlated with manure P rate and were higher in NT compared with CP. Conversely, sediment and particulate P (PP) concentrations in runoff were inversely correlated with manure P rate (and manure solids) and were higher in CP compared with NT. Runoff volume where no manure was applied was higher in NT than in CP in spring but similar in fall. The addition of manure reduced runoff volumes by an average of 82% in NT and 42% in CP over spring and fall. Results from this study indicate that surface application of dairy manure with relatively high solids content may reduce sediment and PP losses in runoff without increasing the risk of increased DRP and DOP losses in the year of application where corn biomass is harvested.

Seasonal phosphorus depletion and microbial responses to the change in phosphorus availability in a subarctic coastal environment

Temporal variations of phosphorus (P) pools (soluble reactive P (SRP), dissolved organic P (DOP), and particulate P (PP)), alkaline phosphatase activity (APA), and orthophosphate uptake were measured in Funka Bay to describe the annual P cycle in the bay and determine the phytoplankton and bacterial responses to the change in P availability. Observations over 4 years demonstrated that SRP concentrations in the surface waters decreased from the winter maximum of 1.0 μmol L −1 to 0.3 μmol L −1 at the end of the spring phytoplankton bloom, and then to under the detection limit by around August. The DOP concentration was relatively constant at 0.1 μmol L −1 throughout the year with higher concentrations up to 0.3 μmol L −1 just after the peak of the spring bloom. PP concentrations varied around 0.1 μmol L −1 with higher concentrations up to 0.6 μmol L −1 during the high chlorophyll a concentration period. SRP comprised 85% of the total P pool prior to the spring bloom, but the contribution decreased to less than 30% in the post-bloom period and DOP dominated the P pool in the surface. Most of the measured APA was less than 0.1 nmol L −1 min −1 in the whole water column, but exceptionally high values up to 1.4 nmol L −1 min −1 were observed in low SRP surface waters during summer. When SRP decreased from 0.3 μmol L −1 in spring to under the detection limit in summer, turnover time for orthophosphate at 10 m depth decreased from 61 days to 4.2 h. The high APA and short turnover time for orthophosphate suggested that microbes in the surface mixed layer experienced severe P stress in summer. Under severely P stressed conditions, APA was detected mostly in the > 10 μm fraction, suggesting that large phytoplankton would acquire P from DOP to overcome the P stress. This was supported by the result that large phytoplankton were outcompeted by the 0.2–1 μm sized microbes for orthophosphate uptake.