Browsing by Author "Attanayake CP"

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    Exploring Phosphorus Dynamics in Submerged Soils and Its Implications on the Inconsistent Rice Yield Response to Added Inorganic Phosphorus Fertilisers in Paddy Soils in Sri Lanka
    (2024-03-01) Palihakkara J; Burkitt L; Jeyakumar P; Attanayake CP
    Rice is the primary energy source of more than half of the global population. Challenges persist in managing phosphorus (P) in paddy soils of tropical rice-growing countries. In Sri Lanka, one specific challenge is the inconsistent yield response observed when inorganic P fertilisers are applied to paddy soils. Previous research conducted in Sri Lanka has shown that the rice yield response to added P fertilisers cannot be adequately explained by factors such as soil available P, irrigation schemes, soil texture, pH, electrical conductivity, total carbon content and available Fe and Mg concentrations. Due to the submerged conditions in which rice is grown for a significant portion of its lifespan, a unique environment controlled by redox-driven processes is developed in paddy soils. Therefore, releasing P from submerged soils is an outcome influenced by complex hydrological and biogeochemical processes, strongly influenced by inherent soil characteristics. The present review paper aimed to critically examine existing literature on soil P behaviour in submerged paddy soils of Sri Lanka, to clarify the behaviour of P under submergence, identify the factors affecting such behaviour and highlight the research gaps that need to be addressed, in order to effectively manage P in the paddy soils of Sri Lanka.
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    Phosphorus Release and Transformations in Contrasting Tropical Paddy Soils Under Fertiliser Application
    (Springer Nature on behalf of the Sociedad Chilena de la Ciencia del Suelo, 2025-04-11) Palihakkara J; Attanayake CP; Burkitt L; Jeyakumar P
    Purpose: Inconsistent yield responses to inorganic phosphorus (P) fertilisers in tropical rice paddy soils remain a challenge. This study investigated the contributions of applied P fertilisers to soluble soil P and P transformation mechanisms in P-added paddy soils. Methods: An incubation study was conducted on three rice-growing soils (Ultisol, Alfisol, and Entisol) in Sri Lanka with and without single superphosphate (SSP), triple superphosphate (TSP), and urea. Dissolved reactive phosphorus (DRP) was measured over 112 days of submergence. Thermodynamic modelling and chemical P fractionation were employed to assess soil P transformations. Results: Phosphorus-fertilised soils had significantly higher DRP concentrations (1.1–8.0 mg L−1) compared to controls at 7 days after submergence but DRP declined beyond 21 days (0.024–0.300 mg L−1). Single superphosphate increased DRP more than TSP, short-term. Urea did not affect DRP concentration. Ultisols exhibited the lowest DRP, while Alfisols maintained higher DRP than Ultisol which was near or above the critical concentration for rice (0.1 mg L⁻1) after 28 days. In Entisol, only SSP maintained DRP above 0.1 mg L−1. Modelling suggested Ca phosphates and Fe oxy(hydr)oxides dissolved during submergence. Released P may be resorbed by Fe/Al oxy(hydr)oxides and Ca minerals, with evidence of downward movement of dissolved P and its resorption onto Fe/Al and Ca minerals possibly due to saturation of P sorption sites in the topsoil layer. Conclusion: Low dissolved P in porewater may be linked to inherent soil characteristics, including low organic matter and high amorphous Fe and Al oxides.
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    Phosphorus release under short-term submergence of pasture soils in critical source areas.
    (Elsevier, 2025-01-29) Palihakkara J; Burkitt L; Jeyakumar P; Attanayake CP
    Critical source areas (CSAs) can act as a source of phosphorus (P) in surface waters by releasing soil P to porewater during frequent rainfall events. The extent of P release under short-term, frequent submergence has not been systematically studied in CSAs in New Zealand. A study was conducted to explore the potential of three contrasting dairy and sheep/beef farm soils (Recent, Pallic and Allophanic soils) to release P to porewater and pondwater under short-term and frequent submergence. Five undisturbed soil blocks (20 × 20 × 15 cm) were sampled from each soil. Porewater samplers and half-cell platinum electrodes for in-situ redox potential measurements were installed at 2 and 10 cm depths from the soil surface. Six submerged events were created by maintaining a 5 cm waterhead. Porewater and pondwater samples were collected immediately and three days after each submergence event. After three days of submergence, the soil blocks were drained and maintained at 70% of water holding capacity for 10 days before the next submergence event. Dissolved reactive phosphorus (DRP), pH, dissolved organic carbon, cations, anions, and alkalinity of the water samples were measured. Soil chemical P fractions were assessed in initial soils and soils in the middle and end of the experiment. Thermodynamic modelling was used to infer dissolution and formation of P and P-associated minerals. The Recent soil released P to porewater at both depths and to pondwater. The Pallic soil released P to porewater at both depths but did not change pondwater DRP. Allophanic soil sorbed P and did not increase DRP either in porewater or pondwater. The average pondwater DRP of the three soils during submergence were 17 to 65-fold higher than the New Zealand lowland river target DRP concentration (0.01 mg/L). The mechanisms of P release from the Recent and Pallic soils were desorption and reductive dissolution of Mn(Ⅱ) minerals. Reductive dissolution of Fe(Ⅱ) minerals was not supported by fractionation or modelling results. Decreases in labile, moderately labile and stable P fractions contributed to P release in the Recent soil, while increases in the labile and moderately labile P fractions contributed to P retention in the Pallic and Allophanic soils. This study highlighted that the Recent soil is riskier than the other two soils in releasing P upon short-term submergence and the potential use of Allophanic soil as a P sorbing material in CSAs to mitigate P loss.
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    Redox-induced phosphorus release from critical source areas following rainfall events in New Zealand
    (Elsevier Ltd, United Kingdom, 2025-02) Palihakkara J; Burkitt L; Jeyakumar P; Attanayake CP
    Critical source areas (CSAs) can act as a source of phosphorus (P) during intermittent rainfall events and contribute to dissolved P loss via runoff. Dissolved forms of P are readily accessible for plant and algal uptake; hence it is a concern in terms of the eutrophication of freshwater bodies. The potential of CSAs to release dissolved P to surface runoff upon intermittent short-term submergence caused by different rainfall events has not been studied at a field-scale in New Zealand previously. A field study was conducted to investigate the potential of two different pastoral soil CSAs (Recent and Pallic soil) to release soil P over five rainfall events during winter and to explore the mechanisms of P release in these soils. Ten sampling stations were installed within each CSA in an area of 6 × 2 m2. Each sampling station had two porewater samplers installed at two depths (2 and 10 cm) below the soil surface. Two platinum half-cell electrodes were installed at the same two depths. Porewater and floodwater samples were collected following five rainfall events. Redox potentials were measured in-situ. Dissolved reactive phosphorus (DRP), pH, dissolved organic carbon, cations, anions, and alkalinity of the water samples were measured. Soil chemical P fractions were assessed at the beginning, middle and end of the experiment. Thermodynamic modelling was used to infer dissolution and formation of P and P-associated minerals. The average porewater DRP at the two depths during the rainfall events of the Recent and Pallic soils were 0.32-1.3 mg L-1 and 0.26-2.31 mg L-1, respectively. The average floodwater DRP concentrations of the Recent and Pallic soils were 35 and 43-fold higher than the target DRP concentration (0.01 mg L-1) for the Manawatū River. The study highlights the substantial risk of P loss from CSAs to surface water, driven primarily by the reductive dissolution of Fe and Mn oxy(hydr)oxides. The findings underscore the importance of targeted management strategies to mitigate dissolved P runoff, particularly in high-risk CSAs frequent submerged during rainfall events. This study developed an effective method for monitoring soil porewater P and redox conditions, offering valuable insights and practical tools for resource managers seeking to reduce P contamination.