Blended soil amendment effects on phosphorus loss from manured soils under simulated snowmelt flooding

Phosphorus (P) accumulation in agricultural soils from long-term fertilizer and manure applications increases the risk of P mobilization into freshwater systems, contributing to eutrophication. In the Canadian Prairies, spring snowmelt over frozen soils creates anaerobic conditions, exacerbating the transport of dissolved reactive phosphorus (DRP) to surface waters. This research investigated the effectiveness of single and blended soil amendments in reducing P losses from high legacy P soils under simulated snowmelt flooding conditions. Two complementary laboratory studies were conducted using agricultural soils from southern Manitoba. The first study employed packed soil incubations to evaluate fifteen treatments across six soils, including an unamended control, six single amendments at different rates of alum [KAl(SO4)2.12H2O], ferric chloride (FeCl3), gypsum (CaSO4.2H2O), and magnesium sulfate (MgSO4), plus eight blended combinations. The second study used intact soil monoliths from four sites to compare gypsum, ferric chloride, and their 1:1 combination. Results demonstrated that ferric chloride-based treatments were consistently the most effective across soil types. In packed soils, single amendment of ferric chloride achieved maximum DRP reductions of 64%, while blended amendments containing ferric chloride achieved reductions up to 89%. The monolith study confirmed these findings, with ferric chloride reducing floodwater DRP by 93-99%. Calcium and magnesium-based amendments showed soil-dependent effectiveness, with gypsum achieving 31-56% reductions in the monolith study. Blended amendments did not provide substantial advantages over single ferric chloride applications. These findings suggest that ferric chloride represents a viable single-amendment strategy for mitigating snowmelt-driven P losses from Prairie agricultural soils, offering practical implications for water quality protection in cold climatic regions.

Phosphorus (P) in snowmelt runoff from agricultural fields across the Canadian prairies is a major source of nutrient pollution to freshwater bodies, such as Lake Winnipeg. The use of soil amendments, such as alum (Al2(SO4)318H2O), gypsum (CaSO4·2H2O), and magnesium-sulphate (MgSO4) have previously shown to reduce P losses from soils with snowmelt in the field, and under simulated snowmelt flooding in the laboratory through converting P to less soluble forms; however, their long-term effectiveness has not been investigated. This thesis examined the effectiveness of alum, gypsum, and magnesium- sulphate after 12 - 18 months after amendment application in (a) reducing dissolved reactive P (DRP) loss with snowmelt runoff in field plots, (b) reducing DRP concentrations in floodwater of intact soil columns with simulated snowmelt flooding, and (c) reducing potential P mobility through changes to P fractions and speciation. Amendments were applied in the fall of 2020 at a rate of 2.5 Mg ha-1 in plots arranged in a randomized complete block design, consisting of 4 replicates of 4 amendments (alum, gypsum, magnesium sulphate, and unamended) for a total of 16 field plots. During the snowmelt period, concentrations of DRP and cations, as well as pH in snowmelt were determined. Using intact soil columns taken from the same field plots, a more controlled laboratory study was conducted, where soil columns were flooded and incubated at 4°C to simulate snowmelt conditions. Porewater and floodwater samples were extracted weekly and analyzed for DRP, cation concentrations, pH, and soil redox potential. Phosphorus species in porewater were predicted using Visual MINTEQ 3.1. on days 0, 28 and 49 during the laboratory study. Phosphorus fractions were also determined using a modified sequential fractionation method. In the field, snowmelt DRP concentrations increased over the sampling period regardless of amendment, with higher concentrations seen after the soil had thawed. In the latter days of sampling, the alum, gypsum, and magnesium sulphate-amendments decreased DRP concentrations in snowmelt by 9 –31% relative to the control. However, differences between treatments and the control were not statistically significant. Snowmelt DRP loads (calculated using DRP concentrations and snowmelt volume) showed a significant positive relationship with snowmelt volume, whereas the relationship with DRP concentration was not significant. During the laboratory study, redox potential in all treatments decreased with time of flooding. Soil columns taken from alum-amended field plots had significantly lower DRP concentrations in porewater when compared to all other treatments, but this effect was not observed for floodwater. Predicted P species showed slight changes in alum-amended soils, suggesting the potential for alum to delay the reductive dissolution of ferric phosphate (strengite, FePO4·2H2O), thereby delaying the P release. However, P fractionation analysis did not show significant differences in soil P fractions between treatments. The results of this research suggest that the effectiveness of these amendments in reducing P loss to snowmelt is very small to negligible one year after application, or after one snowmelt flooding event. This implies that re-application of amendments on a more frequent basis or application at a higher rate may be necessary for amendments to be effective in reducing P loss to snowmelt. However, more field scale research is necessary to provide recommendations to farmers regarding the use of soil amendments for this purpose.

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.

In the Canadian prairies, spring snowmelt occurs rapidly and causes flooding in low-lying areas, inducing anaerobic soil conditions and exacerbating phosphorus (P) release to meltwater. Soil amendments can mitigate P loss from flooded soils soon after amendment application; however, their residual benefits are less understood. We examined the initial and residual benefits of alum (Al2(SO4)3·18H2O), gypsum (CaSO4·2H2O), and Epsom salt (MgSO4·7H2O) in a simulated snowmelt flooding experiment. Intact soil columns were taken from amended and unamended field plots in the same year and 1 year after the amendment application. The soil columns were flooded and incubated at a cold temperature. Porewater and floodwater samples were analyzed for dissolved reactive P (DRP), calcium (Ca), magnesium (Mg), iron (Fe), and manganese (Mn) concentrations, and pH. During the year of application, alum, gypsum, and Epsom salt decreased the mean porewater DRP by 68%, 29%, and 19%, and floodwater DRP by 69%, 51%, and 31%, respectively, relative to unamended treatment, with only alum showing significant differences. One year after applications, alum significantly decreased porewater DRP by 35%, but not floodwater DRP, whereas gypsum or Epsom salt did not decrease porewater or floodwater DRP. Correlation and principal component analysis revealed that porewater and floodwater DRP are positively related to pH and Fe, but only in alum-amended treatment, suggesting the influence of pH and Fe in stabilizing P. While alum was effective in mitigating P loss from flooded soils, its effectiveness decreased over time, with negligible residual benefits a year later.

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

: Phosphorus (P) losses from tile drained agricultural lands may differ with tile depth and spacing. Studies were conducted on clay loam soils using large field plots equipped with automatic flow volume measurement and sampling systems over a 4-year period to evaluate tile depth (0.65 m vs. 0.85 m) and spacing (4.2 m vs. 7.5 m) on P losses under two drainage water management systems (regular free drainage, RFD, vs. controlled drainage/subirrigation, CDS) with a corn-soybean rotation. Under RFD with the 4.2 m tile spacing, soil P losses with the tile depth of 0.65 m were 0.021 kg ha -1 for dissolved reactive P (DRP) and 0.275 kg ha -1 for total P (TP) in runoff, and 0.382 kg ha -1 for DRP and 5.43 kg ha -1 for TP in subsurface drainage. With the tile depth of 0.85 m, soil P losses increased by 92 fold for DRP and 28 fold for TP in runoff and by 8.8 fold for DRP and by 1.5 fold for TP in subsurface drainage, because of the increased of both discharge and flow weighted mean DRP and TP concentrations. Effects of tile depth on soil DRP and TP losses under CDS followed similar patterns to those under RFD in runoff, except for the greater increases, while in subsurface drainage there were slight increases in DRP and decreases on TP with increased tile depth, related to redirection of discharges from subsurface to surface. Regardless of water management, increased tile depth increased soil DRP loss in both runoff and subsurface drainage at similar extents. However, CDS reduced soil TP loss in combined runoff and subsurface drainage by 15.7 %, relative to RFD. Effects of tile spacing on soil TP loss were less pronounced than those of tile depth. With tile depth of 0.65 m and increased tile spacing from 4.2 m to 7.5 m, Soil TP increased in runoff at similar extents for both RFD and CDS. In contrast, increased tile spacing decreased TP loss in subsurface drainage under both RFD and CDS. Consequently, increased tile spacing increased soil TP loss in combined surface and subsurface drainage, regardless of water management. There appeared to be a greater effect of tile depth than tile spacing on both DRP and TP losses, of which the effects were predominately attributed to the redirection of field water discharge, followed by changes in P concentration. Installing shallower tile depths, or managing the outlet at shallower levels, may be an effective practice to mitigate soil P loss from tile drained agricultural lands.

Excessive phosphorus (P) from agricultural soils poses a significant environmental threat due to its contribution to eutrophication in water bodies. Soil amendments have been proposed to reduce soil P solubility and decrease losses in runoff, but their effectiveness, especially as blended amendments, and underlying mechanisms remain underexplored. This thesis investigated the effects of single and blended applications of alum (AlK(SO4)2·12H₂O), ferric chloride (FeCl3), gypsum (CaSO4·2H2O), and magnesium sulphate (MgSO4) on soil P dynamics and transformations in six agricultural soils from Manitoba, and separately examined the effects of single and blended gypsum and ferric chloride in a simplified model soil (artificial soil) system composed of sand, silt, clay, humic acid, and 1000 mg kg⁻¹ total P. In both experiments, soils were incubated for up to 84 days at 22 ±1°C with periodic measurements of water-extractable P (WEP) concentrations and Olsen P concentrations to evaluate potential P loss and available P. Sequential P fractionation was used in both studies after 84 days to identify shifts in P pools. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) was conducted only in the natural soil study to identify elemental co-localization. In the natural soil study, all amendments significantly reduced WEP concentrations relative to unamended controls except in soil 1. In soil 1, only a few treatments were effective in significantly reduced the WEP concentrations on one or more sampling days. The blended treatments, particularly gypsum or magnesium sulphate combined with ferric chloride, produced the greatest reductions (up to 85%). Decreases in Olsen P were comparatively modest (average of 9.5%), indicating that treatments reduced labile P without substantially lowering agronomically available P. Sequential fractionation revealed that amendments increased recalcitrant P forms and decreased NaHCO3-extracted P and NH4Ac-extracted P. In the controlled model soil experiment, all amendments significantly reduced WEP concentrations compared to the unamended control, with the gypsum + ferric chloride blend showing the most substantial decrease (47.6–58.9%). Olsen P concentrations initially increased in all amended treatments, but by 84 days, only soils amended with ferric chloride or its blend maintained higher Olsen P than the control. Sequential P fractionation revealed a shift from labile to more stable P pools, indicating increased P retention in the soil matrix. Collectively, these findings demonstrate that blended amendments, especially combinations with ferric chloride, enhance P retention by promoting P stabilization in agricultural and model soils.

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

Supplementing phosphorus (P) is essential for crop production, but excessive P becomes a pollutant through soil-to-waterway losses. Soil amendments can stabilize P and reduce dissolved P losses in runoff. While single-amendment applications have been extensively studied, the effectiveness of blended amendments in decreasing soil P loss remains largely unexplored. This study evaluated the effects of single and blended applications of alum [KAl(SO4)2·12H2O], ferric chloride (FeCl3), gypsum [CaSO4·2H2O], and magnesium sulphate (MgSO4) on soil P status and transformations in six agricultural soils from southern Manitoba. Fresh soils treated with fifteen amendment treatments, including an unamended control, were incubated at 22 ±1 ○C and analyzed for water extractable P (WEP) and Olsen P concentrations at 0, 28, and 84 days of incubation. Samples taken on day 84 were used for sequential P fractionation to quantify changes in P concentrations in different fractions and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) to examine co-localization of P with other elements. All amendments significantly reduced WEP concentrations compared to the unamended control. The blended amendments showed a superior impact, especially gypsum or magnesium sulphate blended with ferric chloride than single amendments. The decrease in Olsen P concentrations with amendments, on average (9.5%), was much less than in WEP concentrations (53.0%). The sequential fractionation revealed that amendments significantly reduced readily and moderately soluble P fractions with a corresponding increase in recalcitrant-P, while SEM-EDX results indicated co-localization of P with Ca and Fe, suggesting P transformations contributing to more stable forms.

Adoption of waste-derived soil conditioners and refined water management can improve soil physical quality and crop productivity of fine-textured soils. However, the impacts of these practices on water quality must be assessed to ensure environmental sustainability. We conducted a study to determine phosphorus (P) loss in tile drainage as affected by two types of soil conditioners (yard waste compost and swine manure compost) and water table management (free drainage and controlled drainage with subirrigation) in a clay loam soil under corn-soybean rotation in a 4-yr period from 1999 to 2003. Tile drainage flows were monitored and sampled on a year-round continuous basis using on-site auto-sampling systems. Water samples were analyzed for dissolved reactive P (DRP), particulate P (PP), and total P (TP). Substantially greater concentrations and losses of DRP, PP, and TP occurred with swine manure compost than with control and yard waste compost regardless of water table management. Compared with free drainage, controlled drainage with subirrigation was an effective way to reduce annual and cumulative losses of DRP, PP, and TP in tile drainage through reductions in flow volume and P concentration with control and yard waste compost but not with swine manure compost. Both DRP and TP concentrations in tile drainage were well above the water quality guideline for P, affirming that subsurface loss of P from fine-textured soils can be one critical source for freshwater eutrophication. Swine manure compost applied as a soil conditioner must be optimized by taking water quality impacts into consideration.

Cover crops differentially influenced nitrogen and phosphorus loss in tile drainage and surface runoff from agricultural fields in Ohio, USA

Reclamation of Guadalquivir river marshes (SW Spain) constitutes a representative example of wetland reclamation in Southern Europe. Nowadays, this is an important area of tile-drained soils (40,000 ha) with an intensive irrigated agricultural production where high fertilizer rates are usually applied. In tile-drained soils, flow through macropores or cracks, which connect the nutrient rich topsoil with drain lines, can be an important pathway for nutrient transfer from soil. In order to study P loss in these soils and how it is affected by soil amendment usually applied in the zone (phosphogypsum and manure) an experiment was performed during two consecutive growing seasons on a reclaimed marsh soil from the Guadalquivir Valley. In the first season (1998–1999), sugar beet (Beta vulgaris L.) was grown under sprinkler irrigation at a rate of 2.5 mm h−1; in the second (2000), cotton (Gossypium hirsutum L.) was grown under furrow irrigation at 8–10 mm h−1. The amendments applied included manure (30 Mg ha−1), and phosphogypsum (13 and 26 Mg ha−1). Drainage events were recorded, and water samples collected and analyzed for total P (TP), dissolved total P (DTP), and dissolved reactive P (DRP). Total P in drainflow ranged from 0 to 0.818 mg l−1 in the 1998–1999 season and from 0 to 0.565 mg l−1 in the 2000 season. The major P form in drainflow was DRP, which accounted for about 50% of TP in the two growing seasons (the mean DRP concentration was 0.068 mg l−1 in 1998–1999 and 0.043 mg l−1 in 2000). Dissolved organic P accounted for a higher portion of DTP in the first season (37%) than in the second (13%). A larger load of phosphorus was observed on plots receiving manure. This treatment significantly increased (P<0.05) the cumulative drainflow during the 1998–1999 growing season (sprinkler irrigation, low drainflow rates). This is consistent with the increased losses of TP, DTP, DAHP, and DRP resulting from this treatment in this growing season. In the following season, DTP loading were significantly increased by manure (P<0.05). This seems to be related mainly to significantly increased DOP losses (P<0.01), particularly during the first drainage event. The higher fraction of applied water was lost by drainage under furrow irrigation (high drainflow rates) is consistent with the high TP load during the 2000 growing season (199–285 g ha−1) relative to the 1998–1999 season (20–59 g ha−1). This difference in P losses was much greater than those resulting from amendment of the soil.

The potential for phosphorus (P) loss from New Zealand grassland soils was assessed using a combination of measured soil chemical properties and concentrations of dissolved reactive P (DRP) determined in drainage and overland flow from simulated rainfall experiments. Soil analyses included Olsen P, calcium chloride (0.01M CaCl2) and water extractable DRP, P sorption index (PSI), and % P retention. Results confirmed that DRP concentrations in drainage and overland flow were closely related to CaCl2‐ and water‐extractable DRP in soil, respectively. The preliminary data indicated that the potential concentration of DRP in subsurface and overland flow from pasture soils that have not been recently grazed and at a small scale (e.g., overland flow from 1‐m lengths) could be estimated from Olsen P and PSI (or P retention) data according to the following equations: DRP (subsurface flow) = 1.480 (Olsen P/PSI) = 0.069 (Olsen P/P retention) + 0.007 DRP (overland flow) = 0.495 (Olsen P/PSI) = 0.024 (Olsen P/P retention) + 0.024.

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

Agricultural phosphorus (P) losses are harmful to water quality, but knowledge gaps about the importance of fertilizer management practices on new (recently applied) sources of P may limit P loss mitigation efforts. Weighted regression models applied to subsurface tile drainage water quality data enabled estimating the new P losses associated with 155 P applications in Ohio and Indiana, USA. Daily discharge and dissolved reactive P (DRP) and total P (TP) loads were used to detect increases in P loss following each application which was considered new P. The magnitude of new P losses was small relative to fertilizer application rates, averaging 79.3g DRP ha-1 and 96.1g TP ha-1 , or<3% of P applied. The eight largest new P losses surpassed 330g DRP ha-1 or 575g TP ha-1 . New P loss mitigation strategies should focus on broadcast liquid manure applications; on average, manure applications caused greater new P losses than inorganic fertilizers, and surface broadcast applications were associated with greater new P losses than injected or incorporated applications. Late fall applications risked having large new P losses applications. On an annual basis, new P contributed an average of 14% of DRP and 5% of TP losses from tile drains, which is much less than previous studies that included surface runoff, suggesting that tile drainage is relatively buffered with regard to new P losses. Therefore old (preexisting soil P) P sources dominated tile drain P losses, and P loss reduction efforts will need to address this source.