Sources and Pathways of Formation of Recalcitrant and Residual Phosphorus in an Agricultural Soil

The fate of fertilizer-derived phosphorus under different long-term fertilization regimes: A phosphate oxygen isotope study

Understanding the fate of exogenous fertilizer-derived inorganic phosphorus (P_i) is essential for effective P management. Hence, this study carried out a 180-day incubation experiment with or without KH₂P¹⁸O₄ in soils with four different fertilization regimes [without fertilizer (CK), mineral P and K fertilizer (PK), mineral N, P, and K fertilizer (NPK), compost (OM)]. We analyzed the atom % excess in phosphate oxygen isotope of sequentially extracted P_i pools (H₂O-P_i, NaHCO₃-P_i, NaOH-P_i, and HCl-P_i), soil respiration, potential phosphatase activities, and microbial biomass. Our results showed that exogenous phosphate fertilizer was immediately transformed into the H₂O-P_i and NaHCO₃-P_i pools and gradually partially immobilized in the HCl-P_i pool. Additionally, biotransformation plays an important role in the turnover of fertilizer-derived P_i. After the 180-day incubation, the biologically transformed H₂O-P_i content was significantly (P<0.05) reduced by 63.2 % on average, with the largest reduction in PK. The NaHCO₃-P_i gradually increased in both CK and OM through biotic processes. However, it continuously decreased in PK and NPK, likely due to the strong adsorption and microbial fixation. The NaOH-P_i fluctuated slightly in CK, NPK, and OM while gradually decreasing in PK. At the end of the incubation, 28.6 %, 37.0 %, 61.2 %, and 75.2 % of the P_i increment in CK, OM, NPK, and PK were stored in the HCl-P_i pool, respectively. Overall, these findings provide important information on the dynamics of fertilizer-derived P_i, delivering new insights into rational phosphate fertilizer management and sustainable agricultural development.

Analysis of oxygen isotopes of inorganic phosphate (δ18Op) in freshwater: A detailed method description for silver phosphate purification

The ability to identify the origin of phosphorus and understand processes controlling P cycling is essential for designing effective mitigation and restoration of eutrophic freshwater ecosystems. The oxygen isotope composition of orthophosphate (δ¹⁸O_p) has significant potential as a tracer for P entering freshwater ecosystems. However, methods of analysis of δ¹⁸O_p are still in their preliminary stages and have proven challenging to implement for new practitioners. In order to achieve progress in developing the application of δ¹⁸O_p signatures as a tracing tool, there is a need to eliminate the methodological challenges involved in accurately determining δ¹⁸O_p. This protocol article describes the various steps needed to concentrate and isolate orthophosphate in freshwater samples into an adequately pure Ag₃PO₄ analyte, without isotopic alteration during processing. The protocol compiles the disperse experiences from previous studies, combined with our own experience. The twofold aim of the paper is toprovide a baseline for an increasing standardisation of the silver phosphate purification method associated with analysis of the oxygen isotope composition of orthophosphate (δ¹⁸O_p), and to foster new research in the applicability of δ¹⁸O_p signatures for P source tracing in catchment science.

Tracing the sources of phosphorus along the salinity gradient in a coastal estuary using multi-isotope proxies

Eutrophication in coastal water has compromised ecosystem services. Identification of phosphorus (P) sources and their load contributions are required for the development of effective nutrient management plans. In this research, multi-isotope proxies were applied to track P sources and evaluate their relative contributions in Love Creek, a coastal estuary in Delaware. The isotope values of carbon (ca. -22‰), nitrogen (ca.+6‰), and phosphate oxygen (ca.+18‰) of agricultural soils under different agricultural practices are generally similar even though their concentrations are distinctly different from forest soils (δ¹³C: ca. -27‰; δ¹³N: ca.+2‰; δ¹⁸O_P: ca.+22‰). Comparison of these parameters among potential land sources (agricultural soils, forest soils, septic wastes, and plant debris) and sink (colloids in water) revealed that the plant debris and soils from forest sources are likely dominant sources of P in freshwater sites. The contribution of terrestrial P sources gradually decreased along the salinity gradient and agricultural soil sources gradually dominanted in the saline water portion of the creek. The variations of P loads due to weather-related discharge, changing land use and activities, and seasons were high and reflected the limitation of accurate estimation of sources. Overall, these results provide improved insights into potential sources and biogeochemical processes in the estuary, which are expected to be useful for water quality monitoring programs.

Quantifying Uncertainties in Sequential Chemical Extraction of Soil Phosphorus Using XANES Spectroscopy

Sequential chemical extraction has been widely used to study soil phosphorus (P) dynamics and inform nutrient management, but its efficacy for assigning P into biologically meaningful pools remains unknown. Here, we evaluated the accuracy of the modified Hedley extraction scheme using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy for nine carbonate-free soil samples with diverse chemical and mineralogical properties resulting from different degrees of soil development. For most samples, the extraction markedly overestimated the pool size of calcium-bound P (Ca-P, extracted by 1 M HCl) due to (1) P redistribution during the alkaline extractions (0.5 M NaHCO₃ and then 0.1 M NaOH), creating new Ca-P via formation of Ca phosphates between NaOH-desorbed phosphate and exchangeable Ca²⁺ and/or (2) dissolution of poorly crystalline Fe and Al oxides by 1 M HCl, releasing P occluded by these oxides into solution. The first mechanism may occur in soils rich in well-crystallized minerals and exchangeable Ca²⁺ regardless of the presence or absence of CaCO₃, whereas the second mechanism likely operates in soils rich in poorly crystalline Fe and Al minerals. The overestimation of Ca-P simultaneously caused underestimation of the pools extracted by the alkaline solutions. Our findings identify key edaphic parameters that remarkably influenced the extractions, which will strengthen our understanding of soil P dynamics using this widely accepted procedure.

Degradation of glyphosate and bioavailability of phosphorus derived from glyphosate in a soil-water system

Glyphosate, the most commonly used herbicide in the world, can be degraded into more toxic and persistent products such as aminomethylphosphonic acid (AMPA) or non-toxic products such as sarcosine and glycine. In this study, we used liquid chromatography mass spectrometry (LC-MS) and electrospray ionization (ESI) source Q Extractive Orbitrap mass spectrometry (ESI-Orbitrap MS) to identify glyphosate degradation products and combined with sequential extraction and stable isotopes to investigate the degradation of glyphosate and transformation of phosphorous (P) product in a soil-water system. The LC-MS and ESI-Orbitrap MS results showed that glycine formed during the early stage but was rapidly utilized by soil microorganisms. AMPA started to accumulate at the late stage and was found to be 3-6 times more resistant than glyphosate against degradation; while no sarcosine was formed. The 18O labeling and phosphate oxygen isotope results allowed a clear distinction of the fraction of inorganic P (Pi) derived from glyphosate, about half of which was then rapidly taken up and recycled by soil microorganisms. Our results provide the first evidence of the preferential utilization of glyphosate-derived Pi by microorganisms in the soil-water system. The rapid cycling of Pi derived from this disregarded source has important implications on nutrient management as well as water quality.

Identifying sources and cycling of phosphorus in the sediment of a shallow freshwater lake in China using phosphate oxygen isotopes

Biotic and abiotic pathways for the transformation of phosphorus (P) in the sediment of Taihu Lake, a eutrophic shallow freshwater lake in southeastern China, were studied using the oxygen isotope ratios of phosphate (δ¹⁸O_P) along with sediment chemistry, X-ray diffraction, and ⁵⁷Fe-Mössbauer spectroscopic methods. The results showed that δ¹⁸O_P values of sediment P pools significantly deviated from equilibrium and thus allowed distinguishing potential P sources or pathways of transformation. Isotope values of authigenic P being lighter than equilibrium suggests the re-mineralization of organic matter and subsequent precipitation of apatite as the major pathway of formation of authigenic P. The δ¹⁸O_P values of the Al-bound P pool (18.9-23.5‰) and ferric Fe-bound P (16.79-19.86‰) could indicate potential terrestrial sources, but the latter being closer to equilibrium values implies partial overprinting of potential source signature, most likely due to reductive dissolution and release of P and followed by partial biological cycling before re-sorption/re-precipitation with newly formed ferric Fe minerals. Oxic/anoxic oscillation and dissolution/re-precipitation reactions and expected isotope excursion are corroborated by sediment chemistry and Mössbauer spectroscopic results. These findings provide improved insights for better understanding the origin and biogeochemical cycling of P associated with eutrophication in shallow freshwater lakes.

Stable Isotopes and Bayesian Modeling Methods of Tracking Sources and Differentiating Bioavailable and Recalcitrant Phosphorus Pools in Suspended Particulate Matter

Understanding the sources of different phosphorus (P) pools and their bioavailability under imposed biogeochemical environments in a watershed is limited largely due to the lack of appropriate methods. In this research, phosphate oxygen isotope ratios and Bayesian modeling on fingerprinting elements were applied as two novel methods to identify sources and relative recalcitrancy of particulate P pools suspended in water in the continuum of sources from land to the mouth of a coastal estuary to the Chesapeake Bay. Comparative analyses of sizes, relative ratios, and oxygen isotope values of particulate P pools in the creek water suggested that the NaHCO₃-P pool was bioavailable, whereas NaOH-P and HCl-P pools were recalcitrant during P transport along the creek. Agricultural field soil, streambank, and river bottom sediments were major sources of particulate P and their contributions varied significantly at the headwater and downstream regions of the creek. Bayesian modeling based on fingerprinting elements suggested that tides played a major role in forming particulate matter from estuarine sources at the creek mouth region and importing it upstream. These findings provide new insights into the origin and fate of particulate P and the fidelity of isotope and fingerprinting methods in source tracking of P in tidally influenced watersheds.