Abstract
Phosphorus (P) is an essential nutrient for sustaining life and agricultural production. Transformation of readily available P into forms that are unavailable to plants adds costs to P replenishment, which eventually translates into lower agronomic benefits and potential loss of soil P into runoff may degrade water quality. Therefore, understanding the sources and pathways of the formation of residual P pools in soils is useful information needed for the development of any technological or management efforts to minimize or inhibit the formation of such P pool and thus maximize availability to plants. In this research, we paired phosphate oxygen isotope ratios (δ18OP) with solid-state 31P NMR and quantitative XRD techniques along with general soil chemistry methods to identify the precipitation pathways of acid-extracted inorganic P (Pi) pools in an agricultural soil. Based on the comparison of isotope values of 0.5 mol L−1 NaOH-Pi, 1 mol L−1 HCl-Pi, and 10 mol L−1 HNO3-Pi pools and correlations of associated elements (Ca, Fe, and Al) in these pools, the HNO3-Pi pool appears most likely to be transformed from the NaOH-Pi pool. A narrow range of isotope values of acid-Pi pools in shallow (tilling depth) and below (where physical mixing is absent) is intriguing but likely suggests leaching of particle-bound P in deeper soils. Overall, these findings provide an improved understanding of the sources, transport, and transformation of acid-Pi pools in agricultural soils and further insights into the buildup of legacy P in soils.
Highlights
Phosphorus (P) is a limited (~0.09 wt. % of Earth’s crust) non-renewable resource and an essential component of fertilizer for food production
A narrow range of isotope values of acid-Pi pools in shallow and below is intriguing but likely suggests leaching of particle-bound P in deeper soils. These findings provide an improved understanding of the sources, transport, and transformation of acid-Pi pools in agricultural soils and further insights into the buildup of legacy P in soils
A series of research efforts and technological developments are being undertaken towards recycling P from human waste and sewage sludge in wastewater treatment plants, incentivizing recycled P products, rationing P application in agricultural soils based on soil P concentration, reducing leaching and runoff
Summary
Phosphorus (P) is a limited (~0.09 wt. % of Earth’s crust) non-renewable resource and an essential component of fertilizer for food production. Multiple studies have raised concern over the sustainable use of P because the increased exploitation of P, after the Green Revolution, has reduced its reserves on Earth and the unidirectional transport—from mines to fertilizers for crop production and to foods and fodders [1]—has caused a series of unintended environmental problems [2]. Various methods to enhance P uptake from soils including manipulation of symbiotic fungi and microorganisms on plant roots and genetic modification on plants are being investigated [4,5].
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