Phosphorus speciation in submerged agricultural soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
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Date
2024
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Massey University
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Abstract
Phosphorus (P) management in submerged agricultural soils is challenging as release of P in such soils occurs due to complex hydrological and biogeochemical processes that are influenced by inherent soil characteristics and external factors such as climate and agronomic practices. Thus, in depth understanding of P speciation in submerged agricultural soils is crucial to optimise P management and mitigate environmental risks, ensuring the sustainability of agricultural systems in diverse climatic regions. This thesis explores the dynamics of P speciation in submerged agricultural soils via three studies: one focusing on transformation of P in inorganic P fertiliser applied tropical paddy soils under long-term (>2 months) submergence to provide fundamental understanding of P dynamics in those soils to support inconsistent response of rice yields for applied P, and the others focusing on the potential risk of dissolved P release in critical source areas (CSAs) in temperate soils under short-term (hours to few days) submergence during rainfall events.
In tropical regions, long-term submergence of paddy soils leads to unique P dynamics due to the alternative oxic/anoxic conditions and high levels of iron (Fe) and aluminium (Al) oxy(hydr)oxides present in these soils. The challenge lies in understanding P dynamics in such soils to optimise fertiliser management strategies and enhance rice productivity sustainably. An incubation study was conducted in Sri Lanka to investigate P release and transformations in three contrasting paddy soils (Ultisols, Alfisols and Entisols) with applied two types of inorganic P fertilisers. This study revealed that the P fertilisers did not increase dissolved reactive P (DRP) into porewater in all soils, except immediately after fertiliser application because the P released by dissolution of calcium (Ca) phosphates and P associated ferrihydrite under reduced conditions were translocated to deeper soil layers and resorbed onto abundantly available Fe/Al oxy(hydr)oxides and reprecipitated as Ca minerals. Further, it was revealed that during submergence, moderately labile P pools (ie: sodium hydroxide extractable P) increased in P applied soils, which can be available for plant adsorption under unique micro-environment of the rice plant rhizosphere.
Contrastingly, in temperate regions, short-term submergence events, such as heavy rainfall, or flooding, pose risks of dissolved P release from soils, and subsequently diffuse P loss to overlying floodwater leading to freshwater quality concerns. The CSAs are nutrient hotspots in a farm which have active hydrological connectivity to surface waters. These areas often saturated/submerged during rainfall events. Hence, P release from CSAs can contribute to eutrophication in nearby freshwater bodies, posing environmental and agricultural sustainability challenges. Two studies were conducted to explore P release and transformations from three contrasting soils upon short-term submergence of CSAs during rainfall in New Zealand. A glasshouse study revealed that two soils (Recent and Pallic soils) released P to porewater while the other soil (Allophanic soil) sorbed P during short-term submergence suggesting the potential use of the Allophanic soil as a P sorbing material to mitigate P loss. A field study conducted in two CSAs (Recent and Pallic soils) connected to the Manawatū River revealed elevated DRP in porewater and floodwater during winter aligned with the findings of the glasshouse study. The findings of these studies can be applied to mitigate P losses from these CSAs during periods of high-risk for surface runoff such as in winter and to select suitable sites/soils for edge of farm mitigation practices such as wetlands and detainment bund constructions.
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Keywords
critical source areas, dissolved reactive phosphorus, phosphorus, submergence