Dissolved Reactive P Concentrations Research Articles

Preferential flow of phosphorus and nitrogen under steady‐state saturated conditions

AbstractRepeated broiler litter application on agricultural lands can cause nutrient enrichment of subsurface effluent, especially with the existence of preferential flow through soil macropores. Previous studies quantifying soil macropores have not attempted to establish a connection of soil macropore characteristics with the subsurface nutrient (nitrogen [N] and phosphorus [P]) losses, across different topographical locations in the field. This study investigated the effect of broiler litter application and preferential flow on subsurface nutrient transport (N and P) at different topographical positions (upslope, midslope, and downslope) in a no‐till pasture field located in Alabama, USA. Twelve intact soil columns (150 mm id and 500 mm length) were used, and the nutrient leaching measurements from laboratory experiments were linked to soil macropore characteristics quantified using X‐ray computed tomography image analysis and solute transport modeling. Treatments included surface broadcast broiler litter (5 Mg ha−1, on dry basis) and unamended control. Leachates were analyzed for dissolved reactive P (DRP), total P (TP), and nitrate + nitrite‐N (NO3− + NO2−–N). The bromide breakthrough curves provided evidence of preferential flow in all columns. Litter application significantly increased leachate P concentrations, and average TP and DRP concentrations were significantly higher in the leachate from upslope columns compared to those at downslope location. The NO3−–N concentrations in leachate exceeded the US EPA drinking water standard of 10 mg L−1 in all the treatment columns. The highest flow‐weighted mean concentrations of TP and DRP, at 2.7 and 2.5 mg L−1, respectively, were recorded in the upslope columns. Soil physicochemical properties and nutrient leaching losses varied substantially across topographical positions, indicating a need for variable litter application rates to reduce P build‐up and subsequent leaching in vulnerable locations within the field. The relevance of the effect of topographic position on nutrient leaching found in this study should be further tested by investigating a wider range of slopes and soil types in pastures.

Comparison of manure application methods on nutrient and metal loss to snowmelt

Manure injection is well documented to reduce rain-driven nutrient losses, but the effect on snowmelt-driven losses is less studied. This study examined the surface application versus injection of liquid swine manure on phosphorus (P), nitrate‑nitrogen (nitrate-N), zinc (Zn), manganese (Mn), iron (Fe), magnesium (Mg), and calcium (Ca) release from soils to snowmelt. Manure was applied in the fall of 2021 to four replicated plots of surface-applied or injected treatments, and boxes were installed to collect snowmelt in each plot, including four unmanured (control) plots. In the spring of 2022, the snowmelt was collected from each box for 10 days, volume was recorded, and analyzed for dissolved reactive P (DRP), nitrate-N, Zn, Mn, Fe, Mg, and Ca concentrations, and pH. The mean daily snowmelt volume and snowmelt depth ranged from 2.9 to 25.2 L, and 2.7 to 23.3 mm, respectively. The mean snowmelt DRP concentrations for manure injected, surface-applied, and control treatments were 0.66 ± 0.07, 0.89 ± 0.07, and 0.65 ± 0.07 mg L−1, respectively. The mean DRP concentration did not vary among the manure application methods for the first 5 days of snowmelt. However, DRP was significantly higher in the manure surface-applied treatment, compared to both control and manure-injected treatments on days 7 and 8, compared to the control on day 6, and compared to the manure-injected on day 9. The effect of the manure application method on nitrate-N (3.27 to 3.60 mg L−1) and metal concentrations in the snowmelt was not significant. The cumulative loads (mean across application methods and sampling days in kg ha−1) of DRP (0.56), nitrate-N (4.03), and metals [Zn (0.11), Mn (0.01), Fe (0.18), Mg (8.03), Ca (21.70)] mobilized with snowmelt flooding did not significantly vary among the manure application methods. Results suggest that DRP, nitrate, and metal loads were primarily controlled by the snowmelt volume rather than their concentrations. Therefore, land management practices that reduce the volume of snowmelt discharge could be more effective for reducing the P, nitrate-N, and metal loss to snowmelt from manured soils.

Conventional soil test phosphorus failed to accurately predict dissolved phosphorus release in agricultural hydromorphic soils in Brittany, Western France

Researches have proved that agricultural phosphorus (P) loss contributes significantly to surface water eutrophication. Various soil test P (STP) methods have been developed to assess the P loss risk from agricultural soils. In the intensively-cultivated Brittany region of Western France, hydromorphic soils in wetland domains exhibit high risks of leaching and transferring dissolved P - the most bio-available form of P - to surface waters. It remains unclear whether STP conventionally developed for well-drained soils can accurately predict the risk of dissolved P release from these hydromorphic soils. In this study, we measured the dissolved reactive P (DRP) concentrations in soil solutions sampled in situ from 26 hydromorphic soils in the Brittany region and examined their relationship with several STPs available on the corresponding soils, such as the degree of soil P saturation, the equilibrium soil P concentration, or the soil Olsen P, Dyer P, and water extractable P contents. DRP concentrations ranged from 0.01 to 0.310 mg P l−1 (mean = 0.075 mg P l−1), highlighting the potential of hydromorphic soils as hotspots for DRP release in agricultural landscapes. Correlations between DRP concentrations and STPs were relatively weak (0.09 < r2 < 0.64), indicating that conventional STPs are generally unable to accurately predict the DRP release risks in hydromorphic soils. Tentatively, Olsen P showed promises as a useful risk indicator, with a relatively high r2 value of 0.6 and wide inclusion in the current STP database, especially in the Brittany region. Nevertheless, this hypothesis requires further evaluation with additional data. This study confirms the high risk of dissolved P release from hydromorphic soils in agricultural wetland domains and emphasizes the need for developing specific risk assessment tools to these hydromorphic soils.

Alum reduced phosphorus release from flooded soils under cold spring weather conditions.

The effectiveness of amendments such as alum [Al2 (SO4 )3 ·18H2 O] in reducing phosphorus (P) loss to floodwater has been reported under summer conditions and laboratory-controlled environments, but not under actual spring weather conditions in cold climate regions with high diurnal temperature variations when potential for P losses is high. The effectiveness of alum in reducing P release under Manitoba spring weather conditions was evaluated in a 42-day experiment using 15-cm soil monoliths from eight agricultural soils, which were unamended or alum-amended (5 Mg ha-1 ) and flooded to a 10-cm head. Dissolved reactive P (DRP) concentrations and pH of porewater and floodwater were determined on flooding day and every 7 days after flooding (DAF). Porewater and floodwater DRP concentrations in unamended soils increased 1.4- to 4.5-fold, and 1.8- to 15.3-fold, respectively, from 7 to 42 DAF. In alum-amended soils, DRP concentrations averaged across soils was 43%-73% (1.0-2.0mg L-1 ) lower in porewater, and 27%-64% (0.1-1.2mg L-1 ) lower in floodwater than unamended soils during the flooding period. The reduction of DRP by alum was more pronounced under high fluctuating diurnal spring air temperature than with controlled air temperature (4°C) in a previous similar study. Acidic pH in porewater and floodwater due to alum did not persist over 7 days. This study showed that alum application is a viable option in reducing P released to floodwater in agricultural soils of cold regions where flooding-induced P loss is prevalent in the spring.

Evaluating fall application of soil amendments to mitigate phosphorus losses during spring snowmelt

The potential of soil amendments in reducing phosphorus (P) loss has been reported under simulated snowmelt, but not evaluated in the field, under snowmelt conditions. We examined the effectiveness of alum (Al2(SO4)3·18H2O), gypsum (CaSO4·2H2O), and Epsom salt (MgSO4·7H2O) in reducing P loss from two flood-prone agricultural fields (non-manured field with initial soil test P concentrations, STP, of 76 mg kg−1 and manured field with initial STP of 202 mg kg−1), during snowmelt on Canadian Prairies. Amendments were applied (2.5 Mg ha−1) in 2020 fall to four replicated plots (3 m × 1 m) with pre-installed runoff boxes (1.2 m × 0.9 m). In the 2021 spring, snowmelt water in each runoff box was pumped out, volume recorded for each sampling day, and analysed for dissolved reactive P (DRP). Snowmelt DRP concentrations was greater in manured soil (0.73 ± 0.23 mg L−1) with high STP, than in non-manured soil (0.28 ± 0.10 mg L−1.) with low STP, implying a greater risk of P loss to snowmelt runoff when STP is high. Cumulative snowmelt DRP loads in non-manured soil ranged from 3.1 to 3.8 mg (equivalent to 0.03–0.04 kg ha−1) and were not significantly different among treatments. In the control treatment of the manured soil, the cumulative snowmelt DRP load from runoff boxes was 20.8 mg (equivalent to 0.19 kg ha−1), while amended treatments had 42 to 68 % lower cumulative DRP load than control, with the difference being significant only in Epsom salt treatment. Cumulative snowmelt DRP load was significantly and positively correlated to cumulative snowmelt volume (R2 = 0.84 for each soil). This study shows the potential of fall application of soil amendments, particularly Epsom salt, to reduce P loss to snowmelt from soils with high STP.

Nitrogen, Phosphorus, and Snowmelt Runoff Losses after Application of Dairy Manure with Variable Solids Content

Snowmelt nutrient loss is an important but poorly understood process in cold climates. We measured nutrient losses at three sites after dairy manure was applied on top of an established snowpack. Treatments included no manure controls and three manure solids levels (12–19.4% solids = High; 7.5–8.0% = Medium; 2.9–5.5% = Low) applied at 26,670 L ha−1 to all treatments. Snowmelt runoff was monitored and analyzed for dissolved reactive P (DRP), total P (TP), total N (TN), ammonium-N, organic-N, and total solids (TS) concentrations. Results showed that manure application dramatically increased N and P loading compared to controls. Across site-years, manure application increased average runoff TP, DRP, and TN concentrations by 1.3- to 13.3-fold, 1.5- to 21-fold, and 1.4- to 14.2-fold, respectively, relative to controls. While cumulative N, P, and TS losses generally increased with manure solids, Medium/Low showed equal or greater nutrient transfer to runoff for some events. TN and TP lost in runoff were linearly related to manure solids concentration; however, N and P loss as a percent of applied showed the opposite trend. The results indicate that applying manure on top of snow resulted in high nutrient losses when runoff occurred regardless of manure solids content.

Assessing the concept of control points for dissolved reactive phosphorus losses in subsurface drainage.

Agricultural phosphorus (P) loss, which is highly variable in space and time, has been studied using the hot spot/hot moment concept, but increasing the rigor of these assessments through a relatively newer "ecosystem control point" framework may help better target management practices that provide a disproportionate water quality benefit. Sixteen relatively large (0.85ha) subsurface drainage plots in Illinois were used as individual observational units to assess dissolved reactive P (DRP) concentrations and losses within a given field over four study years. Three plot-months were identified as DRP control points (one export and two transport control points), where each plot-month contributed>10% of the annual DRP load from the field. These control points occurred on separate plots and in both the growing and nongrowing seasons but were likely related to agronomic P applications. Elevated soil test P, especially near a historic farmstead, and soil clay content were spatial drivers of P loss across the field. The nongrowing season was hypothesized to be the most significant period of P loss, but this was only documented in two of the four study years. A cereal rye (Secale cereale L.) cover crop did not significantly reduce DRP loss in any year, but there was also no evidence of increased drainage P losses due to freezing and thawing of the cover crop biomass. This work confirmed annual subsurface drainage DRP losses were agronomically small(<3% of P application rate), although the range of DRP concentrations relative to eutrophication criteria still demonstrated a potential for negative environmental impact. The control point concept may provide a new lens to view drainage DRP losses, but this framework should be refined through additional within-field studies because mechanisms of P export at this field were more nuanced than just the presence of tile drainage (i.e., a transport control point).

Ecohydrology of irrigated silage maize and alfalfa production systems in the upper midwest US

Increased reliance on maize silage as a primary dairy forage in place of perennial crops like alfalfa may result in tradeoffs with on-farm water balances and water quality. Our objective was to evaluate how modern dairy forage crop, manure, and irrigation management influence field-scale ecohydrology of silage maize and alfalfa production systems. This study utilized year-round measurements of precipitation, irrigation, evapotranspiration (ET), soil water storage, subsurface tile drainage, and drainage concentrations and loads of nitrate (NO3-N), dissolved reactive P (DRP), and total suspended solids (TSS) to calculate annual water balances, water-use efficiency, and water quality impacts of these crops. Mean annual ET (µ = 499 mm yr−1) and tile drainage (µ = 90 mm yr−1) were similar between crops, due in part to high variability, though cumulative drainage over three paired years was 326 mm for silage maize and 214 mm for alfalfa. Annual soil water storage was similar for alfalfa (25 ± 44 mm yr−1) and silage maize (27 ± 50 mm yr−1), and confidence bounds indicated water budgets were balanced. Inherent water use efficiency (IWUE*) was 70% greater for maize than alfalfa (30.8 versus 18.1 g C hPa kg H2O−1 d−1). Paired-watersheds analysis indicated alfalfa reduced NO3-N but increased DRP concentrations in drainage compared to silage maize. However, weekly loads of NO3-N and DRP were reduced by 72% and 33%, respectively, and annual TSS loads reduced by 37% with alfalfa compared to silage maize. Results highlight the value of alfalfa for managing on-farm water flows and water quality during critical periods of the year and suggest that increased usage of maize silage has facilitated gains in dairy productivity at the expense of water quality in the Upper Midwest US.

Sediment and water-column phosphorus chemistry in streams at baseflow across varying catchment geologies

ABSTRACT Streams can attenuate inputs of phosphorus (P) and therefore dampen the likelihood of ecosystem eutrophication. This P attenuation derives from many processes and remains poorly understood, particularly in reference to the geochemical mechanisms involved. We studied P attenuation in the form of (1) potential for mineral (co-)precipitation from the water column and (2) P sorption with benthic sediments. We hypothesized that both mechanisms vary with catchment geology and that P sorption depends on sediment reactive iron (Fe). We sampled 31 streams at baseflow, covering a gradient of P inputs (via land use), hydrological characteristics, and catchment geologies. Geochemical equilibria in the water column were measured and benthic sediments were analyzed for sorption properties and P and Fe fractions. Neither P-containing minerals nor calcite-phosphate co-precipitation had the potential to form. However, in-stream dissolved reactive P (DRP) correlated with labile sediment P (water soluble and easily reduced Fe-P) for streams where hyporheic exchange between the water column and sediment porewaters was likely sufficient. Because this labile P was associated with poorly crystalline Fe oxides, which determined P sorption capacity, we observed that more sorptive sediments were positively related to DRP concentrations. Our results suggest that DRP attenuation in these streams at baseflow depends in part on the combination of biogeochemical Fe and P cycles and the hydrological exchange with the hyporheic zone. Such combinations will likely vary spatiotemporally within a catchment and must be considered alongside inputs of P and sediment if the P concentrations at baseflow, and eutrophication risk, are to be well managed.

Ferric chloride amendment reduces phosphorus losses from flooded soil monoliths to overlying floodwater

The accumulation of phosphorus (P) in agricultural soils and subsequent losses to waterways contribute to eutrophication in surface water bodies. In agricultural lands prone to prolonged flooding during spring snowmelt, P may be released to overlying floodwater and transported to lakes downstream. Ferric chloride (FeCl3) is a potential soil amendment to mitigate P losses, but its effectiveness for flooded soils with snowmelt is not well documented. Thirty-six intact soil monoliths taken from four agricultural fields in Manitoba's Red River Valley region were surface-amended with FeCl3 at three rates (0, 2.5, and 5 Mg ha–1) to evaluate the effectiveness of FeCl3 in minimizing P losses to porewater and floodwater. Over 8 weeks of simulated snowmelt flooding, porewater, and floodwater samples taken weekly were analyzed for concentrations of dissolved reactive P (DRP), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), and pH. Change in the redox potential was also measured weekly. With time of flooding, redox potential decreased in all soil monoliths. At early stages of flooding, the porewater pH values were significantly lower in FeCl3-amended monoliths but increased with flooding time. Porewater and floodwater DRP concentrations increased in all soils when flooded, but the magnitudes varied. Amendment of FeCl3 decreased the DRP concentrations from 17% to 97% in porewater and 26% to 99% in floodwater, with the effectiveness varying depending on the soil, FeCl3 rate, and flooding time. Amendment of FeCl3 increased porewater concentrations of Ca, Mg, Fe, and Mn. Soil amendment with FeCl3 at both rates shows promise in mitigating redox-induced P losses from flooded soils.

Alum and Gypsum Amendments Decrease Phosphorus Losses from Soil Monoliths to Overlying Floodwater under Simulated Snowmelt Flooding

Phosphorus (P) loss from soils poses a threat of eutrophication to downstream waterbodies. Alum (Al2(SO4)3·18H2O) and gypsum (CaSO4·2H2O) are effective in reducing P loss from soils; however, knowledge on their effectiveness under cold temperatures is limited. This study examined the reduction of P loss from soils with alum and gypsum amendment under simulated snowmelt flooding. Intact soil monoliths (15 cm depth) collected from eight agricultural fields in flood-prone areas of Manitoba, Canada, were surface amended with alum or gypsum, pre-incubated for 2 weeks, then flooded and incubated at 4 °C for 8 weeks. Porewater and floodwater samples collected weekly were analyzed for dissolved reactive P (DRP), dominant cations and anions. An enhanced P release with flooding time was observed in all soils whether amended or unamended; however, alum/gypsum amendment reduced DRP concentrations in porewater and floodwater in general, with alum showing a more consistent effect across soils. The reduction in floodwater DRP concentrations (maximum DRP concentration during flooding) with alum and gypsum ranged from 34–90% and 1–66%, respectively. Based on Visual MINTEQ thermodynamic model predictions, precipitation of P and formation of P-sorbing mineral species with alum and gypsum amendment reduced DRP concentrations at latter stages of flooding.

Flooding-induced inorganic phosphorus transformations in two soils, with and without gypsum amendment.

Anaerobic conditions developed during flooding can increase phosphorus (P) losses from soils to waterways. Soil amendment with gypsum (CaSO4 ·2H2 O) can effectively reduce flooding-induced P release, but its effectiveness is soil dependent, and the reasons are poorly understood. The objectives of this study were to reveal the possible inorganic P transformations during flooding of two soils (acidic-Neuenberg sandy loam [NBG-SL] and alkaline-Fyala clay [FYL-Cl]), with and without gypsum amendment prior to flooding. Porewater samples collected at 0, 35, and 70 d after flooding (DAF) from soils incubated in vessels were analyzed for dissolved reactive P (DRP); pH; and concentrations of calcium (Ca), magnesium, iron (Fe), manganese, chloride, nitrate, sulfate, and fluoride. Thermodynamic modeling using Visual MINTEQ software and chemical fractionation of soil P were used to infer P transformations. Soil redox potential (Eh) decreased with flooding and favored reductive dissolution of Fe-associated P increasing porewater DRP concentrations. Greater solubility of Ca-P under acidic pH maintained a higher DRP concentration in NBG-SL during early stages of flooding. A subsequent increase in pH with flooding and higher Ca concentration with added gypsum enhanced the stability of Ca-P (β-tricalcium phosphate and octacalcium phosphate), reducing the DRP concentration in gypsum-amended NBG-SL. Stability of Ca-P was less affected with flooding and gypsum amendment in FYL-Cl soil because it had an alkaline pH and inherently higher Ca concentration. The FYL-Cl, with a more rapid decrease in Eh than NBG-SL, became severely reduced, releasing more P and Fe by 70 DAF. These conditions favored vivianite formation in FYL-Cl but not in NBG-SL.

Effects of drying and simulated flooding on soil phosphorus dynamics from two contrasting UK grassland soils

AbstractFlooding is known to mobilise soil phosphorus (P). However, it is still not clear how climate change‐driven extended periods of soil drying followed by flooding will affect soil‐P dynamics. We tested the hypothesis under laboratory conditions that soil antecedent conditions (moist/dry) determine the amount of P mobilised upon flooding. A series of controlled laboratory experiments were carried out by flooding samples of two contrasting soils (a Dystric Cambisol [Crediton series] and a Stagni‐Vertic Cambisol [Hallsworth series]), which had each been either dried (40°C for 10 days) or kept at field moisture conditions (25% moisture content). Flooding was simulated by maintaining a 10‐cm water column depth in mesocosms. Periodically collected water samples were analysed for dissolved reactive P (DRP), total dissolved P (TDP) and dissolved unreactive P (DUP). The onset of flooding significantly (p &lt; 0.001) increased dissolved concentrations of all forms of P. The release of TDP coincided with a reduction in redox potential, suggesting reductive dissolution of P bearing iron/manganese (Fe/Mn) minerals as indicated by a significant positive correlation between TDP and dissolved Fe (r = 0.430, p &lt; 0.001) and TDP and dissolved Mn (r = 0.622, p &lt; 0.001). Flooding of the dried soils caused a significantly greater increase in the dissolved P concentrations of all forms of P relative to their moist‐flooded counterparts. This could be due to a combination of factors which are associated with soil drying and flooding. The Crediton dry‐flooded soils released higher concentrations of DRP upon flooding (e.g. 0.14 mg P L−1 on day 1 after flooding) perhaps due to its higher concentrations of water‐ and NaHCO3‐extractable P than the Hallsworth dry‐flooded (HDF) soil (0.03 mg P L−1 on day 1 after flooding). However, most of the P in the water column of the dry‐flooded soils was unreactive, with the HDF soil releasing higher concentrations of DUP, likely due to its higher organic matter and microbial biomass P contents. The results suggest that flooding of dried soils has greater potential to enhance mobilisation of soil‐P than flooding of moist soils and thus has potential implications for soil fertility and surface water quality.Highlights Flooding significantly increased dissolved concentrations of all forms of P. Dried soils released much greater P than their moist counterparts upon flooding. Increased amounts of P in the floodwater coincided with reductions in redox potential and biomass P. Soil with greater amount of biomass‐P and organic matter released greater amounts of dissolved unreactive P.

Impact of water source dynamics on dissolved reactive phosphorus loadings in heterogenous karst agroecosystems with phosphatic limestones

AbstractKarst landscapes underlain with phosphatic limestones are now recognized to be an important contributor of fluvial phosphorus (P) to coastal waters. Specifically, karst agroecosystems may be a hotspot for dissolved reactive P (DRP) due to chronic over‐application of organic and inorganic fertilizers that create legacy P accumulation in surface soils. Nevertheless, few studies have assessed the hydrologic controls on DRP transport in these systems at the watershed scale, which is the focus of this study. We analysed soil moisture, soil water extractable P, and storm event hydrologic and water quality data from a small heterogenous karst watershed (10.7 km2) in the Inner‐Bluegrass Region of Central Kentucky, USA. Four storm events were sampled in winter, 2020 and were analysed for flow pathways using hydrograph recession analysis and water source connectivity using a tracer‐based unmixing model. Based on hydrograph separation results, multiple linear regression analysis was performed to assess drivers of DRP concentrations and loadings. Soil water extractable P results showed stark vertical gradients with greater concentrations at both the surface and deeper soil zones, and minimum concentrations in the root zone. Results for the storm event analysis showed that water source connectivity provided superior prediction of DRP concentrations over the flow pathway analysis, which reflected the heterogeneity of karst maturity masking intermediate flow pathways. Findings from the MLR and loading analysis suggest waters sourced from the soil/epikarst produced significantly higher loadings compared with phreatic and precipitation water source in the three largest events, although concentrations fell between the phreatic (low) and precipitation (high) sources. Findings highlight variable activation of matrix‐macropore exchange at different depths throughout the event. Collectively these results suggest existing models and approaches to assess karst hydrology need revision to improve management strategies in this critical landscape.

Quantifying hydrologic pathway and source connectivity dynamics in tile drainage: Implications for phosphorus concentrations

AbstractFlowpathways and source water connectivity dynamics are widely recognized to affect tile‐drainage water quality. In this study, we developed and evaluated a framework that couples event‐based hydrograph recession and specific conductance end‐member mixing analysis (SC‐EMMA) to provide a more robust framework for quantifying both flow pathway dynamics and source connectivity of drainage water in tile‐drained landscapes. High‐frequency (30‐min) flow and conductivity data were collected from an edge‐of‐field tile main located in northwestern Ohio, and the newly developed framework was applied for data collected in water year 2019. Multiple linear regression (MLR) analysis was used to evaluate the impact of pathway‐connectivity dynamics on flow‐weighted mean dissolved reactive P (DRP) concentrations, which were collected as part of the USDA‐ARS edge‐of‐field monitoring network. The hydrograph recession and SC‐EMMA results highlighted intra‐ and interevent differences between quick (preferential) flow and new (precipitation) water transported during events, challenging a common assumption that new water reflects drainage through preferential flow paths. The analysis of hydrologic flow pathways demonstrated matrix–macropore exchange (Qquick‐old), preferential flow of new water (Qquick‐new), slow flow of old water (Qslow‐old), and slow flow of new water (Qslow‐new) contributed 9, 39, 42, and 10% to tile discharge, on average, with interevent variability. Matrix water that is transported to tile drains via macropore flowpaths was found to be activated throughout the year, even under drier antecedent conditions, suggesting that matrix–macropore exchange was more sensitive to within‐event hydrological processes as compared with antecedent conditions. The MLR results highlighted that pathway‐connectivity hydrograph fractions improved prediction of DRP concentrations, although improvement may be more pronounced in landscapes with higher rates of matrix–macropore exchange.

Influence of Flue Gas Desulfurization Gypsum on Phosphorus Loss from a Horticultural Growth Medium

In response to agriculture’s contribution to surface water quality, considerable effort is being made to develop best management practices to reduce nutrient loss. To evaluate the efficacy of gypsum as a horticultural media amendment for controlling phosphorus (P) leaching, flue gas desulfurization (FGD) gypsum was added to a standard horticultural growth medium at 0, 2.5, 5, 10 or 15% (v/v). FGD gypsum was either mixed with the growing medium or placed at the bottom of the containers. A fast-release or a control-release fertilizer was top-dressed to containers. The greatest P leaching occurred with the fertilizer-only treatments (no gypsum). Dissolved reactive P (DRP) losses were highest on the initial day of measurement for the fast-release fertilizer and then decreased rapidly. There was a delayed release of DRP from the controlled-release fertilizer. Increasing rates of FGD gypsum addition resulted in decreasing DRP leaching concentration loss and load. The FGD gypsum decreased leachate DRP concentration loss by a maximum of 75%, with an average decrease of 46%. Mixing the FGD gypsum with the medium (an easier/less expensive means of incorporation) was most effective with the fast-release fertilizer. These preliminary results indicate that less gypsum may be needed to reduce P loss from fast-released fertilizer as opposed to control-release fertilizer. FGD gypsum remained effective in reducing DRP loss throughout the experiment.

Phosphorus mobilization in unamended and magnesium sulfate-amended soil monoliths under simulated snowmelt flooding

Enhanced release of phosphorus (P) from soils with snowmelt flooding poses a threat of eutrophication to waterbodies in cold climatic regions. Reductions in P losses with various soil amendments has been reported, however effectiveness of MgSO4 has not been studied under snowmelt flooding. This study examined (a) the P release enhancement with flooding in relation to initial soil P status and (b) the effectiveness of MgSO4 at two rates in reducing P release to floodwater under simulated snowmelt flooding. Intact soil monoliths were collected from eight agricultural fields from Southern Manitoba, Canada. Unamended and MgSO4 surface-amended monoliths (2.5 and 5.0 Mg ha−1) in triplicates were pre-incubated for 7 days, then flooded and incubated (4 °C) for 56 days. Pore water and floodwater samples collected at 7-day intervals were analyzed for dissolved reactive P (DRP), pH, Ca, Mg, Fe and Mn. Redox potential (Eh) was measured on each day of sampling. Representative soil samples collected from each field were analyzed for Olsen and Mehlich 3-P. Simulated snowmelt flooding enhanced the mobility of soil P with approximately 1.2–1.6 -fold increase in pore water DRP concentration from 0 to 21 days after flooding. Mehlich-3 P content showed a strong relationship with the pore water DRP concentrations suggesting its potential as a predictor of P loss risk during prolonged flooding. Surface application of MgSO4 reduced the P release to pore water and floodwater. The 2.5 Mg ha−1 rate was more effective than the higher rate with a 21–75% reduction in average pore water DRP, across soils. Soil monoliths amended with MgSO4 maintained a higher Eh, and had greater pore water Ca and Mg concentrations, which may have reduced redox-induced P release and favored re-precipitation of P with Ca and Mg, thus decreasing DRP concentrations in pore water and floodwater.

Evidence of Phosphate Mining and Agriculture Influence on Concentrations, Forms, and Ratios of Nitrogen and Phosphorus in a Florida River

Florida has a long history of phosphate-mining, but less is known about how mining affects nutrient exports to coastal waters. Here, we investigated the transport of inorganic and organic forms of nitrogen (N) and phosphorus (P) over 23 sampling events during a wet season (June–September) in primary tributaries and mainstem of Alafia River that drains into the Tampa Bay Estuary. Results showed that a tributary draining the largest phosphate-mining area (South Prong) had less flashy peaks, and nutrients were more evenly exported relative to an adjacent tributary (North Prong), highlighting the effectiveness of the mining reclamation on stream hydrology. Tributaries draining &gt; 10% phosphate-mining area had significantly higher specific conductance (SC), pH, dissolved reactive P (DRP), and total P (TP) than tributaries without phosphate-mining. Further, mean SC, pH, and particulate reactive P were positively correlated with the percent phosphate-mining area. As phosphate-mining occurred in the upper part of the watershed, the SC, pH, DRP, and TP concentrations increased downstream along the mainstem. For example, the upper watershed contributed 91% of TP compared to 59% water discharge to the Alafia River. In contrast to P, the highest concentrations of total N (TN), especially nitrate + nitrite (NOx–N) occurred in agricultural tributaries, where the mean NOx–N was positively correlated with the percent agricultural land. Dissolved organic N was dominant in all streamwaters and showed minor variability across sites. As a result of N depletion and P enrichment, the phosphate-mining tributaries had significantly lower molar ratios of TN:TP and NOx–N:DRP than other tributaries. Bi-weekly monitoring data showed consistent increases in SC and DRP and a decrease in NOx–N at the South Prong tributary (highest phosphate-mining area) throughout the wet season, and different responses of dissolved inorganic nutrients (negative) and particulate nutrients (positive) to water discharge. We conclude that (1) watersheds with active and reclaimed phosphate-mining and agriculture lands are important sources of streamwater P and N, respectively, and (2) elevated P inputs from the phosphate-mining areas altered the N:P ratios in streamwaters of the Alafia River.

Sediment phosphorus buffering in streams at baseflow: A meta-analysis.

Phosphorus (P) pollution of surface waters remains a challenge for protecting and improving water quality. Central to the challenge is understanding what regulates P concentrations in streams. This quantitative review synthesizes the literature on a major control of P concentrations in streams at baseflow-the sediment P buffer-to better understand streamwater-sediment P interactions. We conducted a global meta-analysis of sediment equilibrium phosphate concentrations at net zero sorption (EPC0 ), which is the dissolved reactive P (DRP) concentration toward which sediments buffer solution DRP. Our analysis of 45 studies and>900 paired observations of DRP and EPC0 showed that sediments often have potential to remove or release P to the streamwater (83% of observations), meaning that "equilibrium" between sediment and streamwater is rare. This potential for P exchange is moderated by sediment and stream characteristics, including sorption affinity, stream pH, exchangeable P concentration, and particle sizes. The potential for sediments to modify streamwater DRP concentrations is often not realized owing to other factors (e.g., hydrologic interactions). Sediment surface chemistry, hyporheic exchange, and biota can also influence the potential exchange of P between sediments and the streamwater. Methodological choices significantly influenced EPC0 determination and thus the estimated potential for P exchange; we therefore discuss how to measure and report EPC0 to best suit research objectives and aid in interstudy comparison. Our results enhance understanding of the sediment P buffer and inform how EPC0 can be effectively applied to improve management of aquatic P pollution and eutrophication.

Runoff and leaching of dissolved phosphorus in streams from a rainfed mixed cropping and grazing catchment under a Mediterranean climate in Australia

Managing phosphorus (P) is a global priority for environmental water quality due to P lost from agricultural land through leaching, runoff and subsurface flow. In Western Australia (WA), following decades of P fertiliser application to crops and pastures in low rainfall regions, questions have been raised about this region's contribution to environmental P loss. This study was conducted on the Fitzgerald River catchment in the south Western Australia (WA) with mixed cropping and grazing land uses and a Mediterranean climate with low mean rainfall (~350 mm yr−1). Phosphorus forms were monitored continuously over a three-year period in five separate streams, each draining a defined sub-catchment. The P concentrations in streams consistently exceeded Australian and New Zealand Environment Conservation Council (ANZECC) trigger values throughout the monitoring period. Of the measured total P concentration, ~75% was dissolved P (DRP; <0.45 μm) and 80% of that fraction was in the filterable reactive form (FRP). These water quality measurements and other independent soil investigations at this site, suggest that transport of dissolved P rather than erosion of sediment-bound P was dominant in this environment. Based on extractable soil P (Colwell P) and the P buffering index (PBI), predicted concentrations of dissolved reactive P (DRP) in soil solution in topsoils (0–10 cm) across this catchment, generally exceeded ANZECC's values of 0.07 mg PL−1. The level of exceedance was spatially variable. Streams draining areas with the lowest predicted DRP concentrations also had the lowest measured FRP concentrations. Elsewhere stream water FRP concentrations depended on both DRP concentration and the PBI of the land being drained. Our findings suggest that deployment of practices that physically filter runoff, for example riparian vegetation, would be ineffective in restricting P transport into stream in this environment. This conclusion is consistent with previous findings of the ineffectiveness of riparian buffers on coarse textured sandy soils in higher rainfall areas of southwest WA. A reduction in DRP losses without yield loss could be achieved by following evidence-based fertiliser advice from soil testing to limit losses of legacy P”.