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

Abstract

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.

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

“…By the end of the sampling campaign (42 DAF), Eh of the soils had declined by 176-460 mV resulting in Eh ranging from 59 to 329 mV for all soils, regardless of alum amendment. The Eh decrease was lower in the current study with widely fluctuating air temperatures, than was observed by Kumaragamage et al (2022) that used a single, controlled, air temperature. The rate at which saturated soils become reduced is greater at higher temperature (Vaughan et al, 2009).…”

“…Overall, the floodwater DRP concentrations in this study (averaged across soils for each sampling time) were 1.8-7.3-fold of the floodwater DRP concentrations measured in our previous study (Kumaragamage et al, 2022) conducted with flooded, intact monoliths from the same locations and time but incubated at a constant temperature of 4˚C. The higher DRP concentrations in this current study than Kumaragamage et al (2022) may be due to the multiple soil freeze-thaw events between day and night. Our results imply that there is a high risk of losing P to overlying floodwater during spring snowmelt in cold regions irrespective of the air temperature during the flooding event.…”

“…In addition, the wide fluctuations in air temperatures result in soil freezing when temperatures fall below zero at night, which might have decreased the microbial activity as previously reported (Watanabe & Misako, 2008). The decrease in microbial activity could be the reason for the lower reduction in Eh with flooding time in the current study than under a constant, but above zero temperature used by Kumaragamage et al (2022). As noted by Kumaragamage et al (2020), warm flooded soil conditions had a greater Eh decrease compared to cold flooded soils and consequently enhanced greater DRP release to porewater and floodwater due to soil reducing conditions.…”

“…Six replicate intact soil monoliths (diameter = 10 cm and height = 15 cm) were collected with cylindrical polyvinyl chloride tubes (length = 30 cm) from each of the eight selected fields as previously described (Kumaragamage et al, 2022;Vitharana et al, 2021). Details of the soil names, classifications, and soil properties (Table S1) have been previously reported (Kumaragamage et al, 2022;Vitharana et al, 2021). Alum was applied to the surface of three soil monoliths from each site at a rate of 2.5 g kg −1 based on soil mass, which is equivalent to 5 Mg ha −1 for a bulk density of 1.2 Mg m −3 at a 15-cm soil depth.…”

“…These warm temperature conditions are not typical in the spring of cold regions where flooding‐induced P losses following snowmelt are prevalent. To our knowledge, only one study (Kumaragamage et al., 2022) evaluated the effectiveness of alum under cold controlled temperature (4°C) without freeze‐thaw events in the laboratory, reporting maximum decreases of 34%–90% in floodwater DRP concentrations during the flooding period for eight alum‐amended soils.…”