Modeling of biotic and abiotic processes affecting phosphate oxygen isotope ratios in a mineral-water-biota system

New insight into identifying sediment phosphorus sources in multi-source polluted urban river: Effect of environmental-induced microbial community succession on stability of microbial source tracking results

Identifying sediment phosphorus sources, the key to control eutrophication, is hindered in multi-source polluted urban rivers by the lack of appropriate methods and data resolution. Community-based microbial source tracking (MST) offers new insight, but the bacterial communities could be affected by environmental fluctuations during the migration with sediments, which might induce instability of MST results. Therefore, the effects of environmental-induced community succession on the stability of MST were compared in this study. Liangxi River, a highly eutrophic urban river, was selected as the study area where sediment phosphorus sources are difficult to track because of multi-source pollution and complicated hydrodynamic conditions. Spearman correlation analysis (P < 0.05) was conducted to recognize a close relationship between sediment, bacterial communities and phosphorus, verifying the feasibility of MST for identify sediment phosphorus sources. Two distinct microbial community fingerprints were constructed based on whether excluded 113 vulnerable species, which were identified by analyzing the differences of microorganisms across a concentration gradient of exogenous phosphorus input in microbial environmental response experiment. Because of the lower unknown proportion and relative standard deviations, MST results were more stable and reliable when based on the fingerprints excluding species vulnerable to phosphorus. This study presents a novel insight on how to identify sediment phosphorus sources in multi-source polluted urban river, and would help to develop preferential control strategies for eutrophication management.

Simplification of the pretreatment method for phosphate oxygen isotope measurement in phosphogypsum leachate

The stacking of phosphogypsum has caused considerable phosphorus pollution in water bodies near phosphogypsum yards through surface runoff and underground infiltration. The phosphate oxygen isotope (δ18Op) tracing method has served as a valuable tool for tracing phosphorus pollution in water. However, the existing δ18Op enrichment and purification methods are complex, costly, and inefficient for phosphate recovery, particularly for phosphogypsum leachate with complex compositions. Herein, a simplified and optimized pretreatment method for δ18Op measurement in phosphogypsum leachate was developed. Zirconium/polyvinyl alcohol (Zr/PVA) gel beads showed good selectivity for phosphate enrichment from water at different initial phosphate concentrations with appropriate Zr/PVA dosage. The optimal enrichment pH value was <7, and the concentrated phosphate on the Zr/PVA gel beads could be effectively eluted in an alkaline environment. Compared with the traditional Fe or Mg coprecipitation enrichment methods, impurities in the solution showed no obvious adverse effects on the phosphate enrichment process. Further, the phosphate solution eluted from the Zr/PVA gel beads was purified by a simple adjustment of the pH instead of cation exchange in the traditional purification process. Magnesium ions in the solution could be completely removed when the pH ranged from 3.17 to 6.15, and the phosphate recovery rate could reach 98.66% when the eluent pH was 5.02. Fourier-transform infrared spectroscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy revealed that similar to traditional pretreatment method, the proposed method can obtain high-purity Ag3PO4 solids for δ18OP measurement and no isotope fractionation of δ18OP was observed. Therefore, this study provides a promising and reliable pretreatment method for δ18OP measurement, especially in complex phosphogypsum leachate.

Optimization of phosphate oxygen isotope pretreatment measurement method based on phosphate in situ enrichment blanket

Phosphate oxygen isotope analysis is an effective tool for investigating phosphorus migration and transformation in water bodies. Unfortunately, current pretreatment methods for this technology are significantly limited due to their demanding sample amount requirements, complex operation, and limited scope of application. In order to enhance the efficiency of the pretreatment process, hydrated zirconia was synthesized through liquid-phase precipitation. Zeolite, D001 macroporous resin, activated carbon, and ceramsite were chosen as possible candidate materials for loading purposes. The optimal zirconium loading material was identified through a combination of field enrichment and laboratory elution experiments. The ideal in situ enrichment duration, material dosages, and elution time were ascertained using response surface methodology. The findings showed that D001 resin exhibited superior selective adsorption and elution capacity for phosphate. The response surface optimization yielded the optimal parameters for the in situ phosphate-enrichment blanket: a mass of 13 g for zirconium-loaded D001 resin, an enrichment period of 360 min, and an elution period of 853 min. The attainment of a bright yellow Ag3PO4 solid after purification served as proof of the reliability of the optimization method. The obtained results provide a fundamental basis for the preparation and application of phosphate oxygen isotope analysis in freshwater ecosystem.

Rethinking the biotic and abiotic remineralization of complex phosphate molecules in soils and sediments

Phosphorus (P) is an essential macronutrient for all living organisms. Despite a diversity of P compounds in the environment, orthophosphate is the most bioavailable form of P. Remineralization of complex P molecules (e.g., organic P and phosphoanhydrides) into orthophosphate is traditionally considered to be carried out primarily by enzymes. Natural minerals are recently viewed to be abiotic catalysts (as compared to the organic phosphatases) to facilitate the cleavage of terminal P-O-C/P bonds and remineralization of complex P compounds. However, quantitative comparison between biotic and abiotic remineralization pathways of complex P molecules is still missing, impeding our capability to assess the importance and contribution of abiotic P remineralization in the environment. This study compares the hydrolysis rates of six organic phosphates and three inorganic phosphoanhydrides by representative enzymes (acid and alkaline phosphatases) and natural oxide minerals (hematite, birnessite, and boehmite). The results show that enzymes and minerals have different substrate preferences. Specifically, alkaline phosphatase hydrolyzes phosphate monoesters faster than phosphoanhydrides, whereas acid phosphatase and minerals show higher hydrolysis rates toward phosphoanhydrides than phosphate monoesters. Although the hydrolysis rates by enzymes (~μM hr^-1) are orders of magnitude higher than those by minerals (~μM d^-1), normalization of the rates by the natural abundance of enzymes and minerals leads to comparable contributions of both processes in soils and sediments. These results highlight the significance of natural minerals in the remineralization of complex P compounds, a process that was traditionally overlooked but with important implications for constraining P biogeochemical cycling in the environment.

New insights into identifying sediment phosphorus sources in river-lake coupled system: A framework for optimizing microbial community fingerprints

Identifying sediment phosphorus sources in river-lake coupled system is a question in developing preferential control strategies for phosphorus. As sediments adsorbed phosphorus and microbes would be transported with changing hydrodynamic, the phosphorus source-specific microbial community fingerprints shed light on determining the major sediment phosphorus sources. However, the identification of microbial community fingerprints is a challenge because both microbial succession and hydrological characteristics of river-lake systems would affect the stability of fingerprints. Therefore, this study provided a framework for optimizing phosphorus source-specific microbial community fingerprints, and attempted to identify the major sources of sediment phosphorus in river-lake coupled ecosystem. Meiliang Lake is one of the highly eutrophic area in Taihu Lake, where the sediments, bacterial communities, and phosphorus had a close relationship. Through analyzing the connectivity of microbes along water continuum, a microbial fingerprints candidate database was constructed. The phosphorus-related bacterial communities were screened and optimized by comparing the difference of predicted results between upstream and downstream, forming the stable microbial community fingerprints which consisted of Bacteroidia, Bacilli, Clostridia, and other species at the class level. SourceTracker results that based on the optimized phosphorus source-specific microbial community fingerprints indicated that the major sediment phosphorus sources to Meiliang Lake were Liangxi River, Wujingang River, and Donghuandi River, with the relative standard deviations ranging from 2.59% to 27.56%. The accuracy of phosphorus source apportionments was further confirmed based on the composite pollution index and hydrodynamic condition. This study put forward suggestions on how to improve the stability of microbial community fingerprints, and would help to improve the understanding of applying microbial source tracking method to identify the sources of abiotic pollution like sediment phosphorus.

Tracing uptake and translocation of phosphorus in wheat using oxygen isotopes and mathematical modelling

Understanding P uptake in soil-plant systems requires suitable P tracers. The stable oxygen isotope ratio in phosphate (expressed as δ¹⁸ O_P ) is an alternative to radioactive labelling, but the degree to which plants preserve the δ¹⁸ O_P value of the P source is unclear. We hypothesised that the source signal will be preserved in roots rather than shoots. In soil and hydroponic experiments with spring wheat (Triticum aestivum), we replaced irrigation water by ¹⁸ O-labelled water for up to 10 d. We extracted plant inorganic phosphates with trichloroacetic acid (TCA), assessed temporal dynamics of δ¹⁸ O_TCA-P values after changing to ¹⁸ O-labelled water and combined the results with a mathematical model. Within 1 wk, full equilibration of δ¹⁸ O_TCA-P values with the isotope value of the water in the growth medium occurred in shoots but not in roots. Model results further indicated that root δ¹⁸ O_TCA-P values were affected by back transport of phosphate from shoots to roots, with a greater contribution of source P at higher temperatures when back transport was reduced. Root δ¹⁸ O_TCA-P partially preserved the source signal, providing an indicator of P uptake sources. This now needs to be tested extensively for different species, soil and climate conditions to enable application in future ecosystem studies.

Method for phosphate oxygen isotopes analysis in water based on in situ enrichment, elution, and purification

The phosphate oxygen isotope (δ¹⁸O_P) ratio has been proven to be an effective tool to trace the sources and biogeochemical cycles of phosphorus (P) in aquatic ecosystems. However, the enrichment of phosphate (PO₄) and the removal of impurities are quite complex and easy to cause PO₄ loss in current δ¹⁸O_P analytical methods. Moreover, the δ¹⁸O_P value obtained by the commonly-used instantaneous sampling method is more of the instantaneous information of P, which is accidental or uncertain for accurate identification of the P source. In this study, a new method of in situ enrichment, elution, and purification of PO₄ (ISEEP) was developed for δ¹⁸O_P analysis in waters. This method utilized a PO₄ binding phase (Zr-Oxide gel) to selectively in situ adsorb PO₄ in water and exhibited an adsorption capacity per unit area of up to 789.3 μg P/cm². The PO₄ on the gel was eluted easily with a 1 M NaOH solution. More than 99.7% of the common anions, cations, and dissolved organic matter (DOM), as well as more than 90% of the trace elements were removed synchronously after adsorption and elution of PO₄. The recovery rate of PO₄ in the whole procedure was as high as 92.8%. The XRD and SEM examinations showed that the ISEEP can obtain high-purity Ag₃PO₄ solid for the δ¹⁸O_P measurement. The reliability of the ISEEP method is confirmed by the measured δ¹⁸O_P value and standard deviation of parallel samples from different types of natural waters obtained by both the ISEEP and the current popular McLaughlin (2004) method. It provides a good prospect of this new method for tracing the P sources and their biogeochemical cycling in aquatic ecosystems.

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.