Identification of Phosphorus Sources in a Watershed Using a Phosphate Oxygen Isoscape Approach

Tracing phosphorus sources in the river-lake system using the oxygen isotope of phosphate

The biogeochemical cycling of phosphorus (P) in river-lake systems presents significant challenges in tracing P sources, highlighting the importance of effective traceability approaches for formulating targeted management measures to mitigate lake eutrophication. In this study, we used the oxygen isotope of phosphate (δ18Op) as a tracer in the river-lake systems, establishing a tracing pathway from potential end-members, through inflow rivers, and eventually to the lake. Taking Dianshan Lake and its main inflow rivers as the study area, we measured δ18Op values of potential end-members, including domestic sewage treatment plant effluents, industrial effluents from phosphorus-related enterprises (printing and dyeing, electroplating, plastics, etc.), and farmland soils. Notably, the industrial effluent signatures ranged from 13.1 ‰ to 21.0 ‰ with an average of 16.8 ‰ ± 3.2 ‰, enriching the δ18Op threshold database. Using the MixSIAR model, it was found that phosphorus in the Jishuigang River primarily originated from agricultural non-point sources and domestic sewage in the dry season, while the Qiandengpu River, with a higher proportion of urban area, had a greater influence from domestic sewage and industrial effluents. Moreover, significant differences were observed between δ18Op values at the lake entrances of the inflow rivers (13.7 ‰ ± 1.0 ‰) and in acid-soluble phosphate of the lake sediments (9.9 ‰ ± 1.0 ‰). Isotopic tracing revealed that phosphorus in the lake originated from both external inputs (80.6 %) and internal release (19.4 %) in the dry season. Alongside pollutant flux calculations based on the hydrological conditions and water quality of the inflow rivers, our findings indicated that phosphorus in Dianshan Lake was mainly attributed to agricultural non-point sources, domestic sewage and sediment release in the dry season. This study provided novel insights into the identification of pollution sources in the river-lake systems, with broad implications for pollution control and environmental protection.

A novel pretreatment method for analysis the oxygen isotopic compositions of inorganic phosphorus pools in freshwater sediment

Identifying the sources and cycling of phosphorus (P) is particularly important for formulating effective P management strategies in inland water. The oxygen isotopic compositions of phosphate (δ18OP) are recognized as a promising tool to solve this problem. However, the application of δ18OP in freshwater sediment is currently constrained by multiple difficulties. In this study, we presented a novel pretreatment method for δ18OP analysis of sediment inorganic P pools. Our results showed that the new method has advantages of simple operation, less time-consuming, and high P recovery rates. Specifically, we replaced the traditional Mg-induced co-precipitation (MAGIC) method by introducing Zr-Oxides gels with high selective adsorption function for phosphate. This made subsequent processing simpler and reduced the time consumption to ∼10 days, and the range of P recovery rates were from 88 % to 104 %. Furthermore, we emphasized the necessity of vacuum roasting following lyophilized Ag3PO4 to eliminate residual oxygen-containing impurities (e.g., NO3-, Ag2O, and organic matter). Additionally, evidences from microscopy and spectroscopy confirmed that this method ultimately yielded high-purity Ag3PO4 with the Ag:P molar ratios of 3.35:1. Importantly, combining direct synthesis Ag3PO4 between KH2PO4 and AgNO3 with the Ag3PO4 obtained by the method revealed no stark oxygen isotopic fractionation of phosphate during the pretreatment processes. The newly established δ18OP pretreatment methods here can also be extended to broader studies of the biogeochemical cycling of P in aquatic ecosystems, potentially advancing the understanding of the global P cycle.

Impacts of land use on phosphorus and identification of phosphate sources in groundwater and surface water of karst watersheds

The thin soil layer with uneven distribution in karst areas facilitates the migration of phosphorus (P) to groundwater, threatening the safety of water sources seriously. To offer a scientific guidance for water pollution control and land use planning in karst areas, this study examined the relationships between land use and P in groundwater and surface water, and quantified the phosphate sources in Gaoping river basin, a small typical watershed in karst areas. Spatial distribution analysis revealed that the highest mean P concentrations in groundwater and surface water were in farmland and construction-farmland zones, respectively. Land use impact analysis showed that the concentration of P in groundwater was influenced positively by farmland but negatively by forest land. In contrast, the concentration of P in surface water was influenced positively by both farmland and construction land. The mixed end-element and Bayesian-based Stable Isotope Analysis in R (SIAR) model results showed that agricultural fertilizers were the main phosphate source for groundwater in farmland and forest-farmland zones, while urban sewage was the main source in the construction-farmland zone. For surface water, the main phosphate source was agricultural fertilizers in both farmland and construction-farmland zones. This study indicates that controlling P pollution in local water bodies should pay close attention to the management of land use related to human activities, including regulating sewage discharge from construction land and agricultural fertilizer usage.

Enhancing phosphorus source apportionment in watersheds through species-specific analysis

Phosphorus (P) is a pivotal element responsible for triggering watershed eutrophication, and accurate source apportionment is a prerequisite for achieving the targeted prevention and control of P pollution. Current research predominantly emphasizes the allocation of total phosphorus (TP) loads from watershed pollution sources, with limited integration of source apportionment considering P species and their specific implications for eutrophication. This article conducts a retrospective analysis of the current state of research on watershed P source apportionment models, providing a comprehensive evaluation of three source apportionment methods, inventory analysis, diffusion models, and receptor models. Furthermore, a quantitative analysis of the impact of P species on watersheds is carried out, followed by the relationship between P species and the P source apportionment being critically clarified within watersheds. The study reveals that the impact of P on watershed eutrophication is highly dependent on P species, rather than absolute concentration of TP. Current research overlooking P species composition of pollution sources may render the acquired results of source apportionment incapable of assessing the impact of P sources on eutrophication accurately. In order to enhance the accuracy of watershed P pollution source apportionment, the following prospectives are recommended: (1) quantifying the P species composition of typical pollution sources; (2) revealing the mechanisms governing the migration and transformation of P species in watersheds; (3) expanding the application of traditional models and introducing novel methods to achieve quantitative source apportionment specifically for P species. Conducting source apportionment of specific species within a watershed contributes to a deeper understanding of P migration and transformation, enhancing the precise of management of P pollution sources and facilitating the targeted recovery of P resources.

Evaluation of Phosphorus Enrichment in Groundwater by Legacy Phosphorus in Orchard Soils with High Phosphorus Adsorption Capacity Using Phosphate Oxygen Isotope Analysis

Long-term phosphorus (P) fertilization results in P accumulation in agricultural soil and increases the risk of P leaching into water bodies. However, evaluating P leaching into groundwater is challenging, especially in clay soil with a high P sorption capacity. This study examined whether the combination of PO4 oxygen isotope (δ18OPO4) analysis and the P saturation ratio (PSR) was useful to identify P enrichment mechanisms in groundwater. We investigated the groundwater and possible P sources in Kubi, western Japan, with intensive citrus cultivation. Shallow groundwater had oxic conditions with high PO4 concentrations, and orchard soil P accumulation was high compared with forest soil. Although the soil had a high P sorption capacity, the PSR was above the threshold, indicating a high risk of P leaching from the surface orchard soil. The shallow groundwater δ18OPO4 values were higher than the expected isotopic equilibrium with pyrophosphatase. The high PSR and δ18OPO4 orchard soil values indicated that P leaching from orchard soil was the major P enrichment mechanism. The Bayesian mixing model estimated that 76.6% of the P supplied from the orchard soil was recycled by microorganisms. This demonstrates the utility of δ18OPO4 and the PSR to evaluate the P source and biological recycling in groundwater.

Global scale identification of catchments phosphorus source shifts with urbanization: A phosphate oxygen isotope and Bayesian mixing model approach

Different scenarios of urban expansion can influence the dynamic characteristics of catchments in terms of phosphorus (P). It is important to identify the changes in P sources that occur during the process of urbanization to develop targeted policies for managing P in catchments. However, there is a knowledge gap in quantifying the variations of potential P sources associated with urbanization. By combining phosphate oxygen isotopes from global catchments with a Bayesian model and the urbanization process, we demonstrate that the characteristics of potential P sources (such as fertilizers, urban wastewater, faeces, and bedrock) change as urban areas expand. Our results indicate that using phosphate oxygen isotopes in conjunction with a Bayesian model provides direct evidence of the proportions of potential P sources. We classify catchment P loadings into three stages based on shifts in potential P sources during urban expansion. During the initial stage of urbanization (urban areas < 1.5 %), urban domestic and industrial wastewater are the main contributors to P loadings in catchments. In the mid-term acceleration stage (1.5 % ≤ urban areas < 3.5 %), efforts to improve wastewater treatment significantly reduce wastewater P input, but the increase in fertilizer P input offsets this reduction in sewage-derived P. In the high-level urbanization stage (urban areas ≥ 3.5 %), the proportions of the four potential P sources tend to stabilize. Remote areas bear the burden of excessive P loadings to meet the growing food demand and improved diets resulting from the increasing urban population. Our findings support the development of strategies for water quality management that better consider the driving forces of urbanization on catchment P loadings.

Iron bound phosphorus predominates the contribution of phosphorus to lake system from terrigenous source: The evidence from the small watershed scale

The reduction of exogenous emissions of phosphorus (P) is a crucial measure for resolving eutrophication in lakes. However, the input of terrigenous materials still potentially contributes to an increase of P load in lake systems. In this study, we examined the phosphate oxygen isotope (δ18OP) of various P fractions in soils and sediments in a small lake watershed, namely, Shijiuhu watershed. The high-resolution in-situ diffusive gradients in thin films (DGT) technology was also used to survey the dynamic processes of P diffusion from sediment particles to the water. The results demonstrated that lighter δ18OP values (16.2-19.5‰) for individual P fractions in lake sediments were detected compared to other land-use patterns, indicating the cumulative biological P recycling on anaerobic condition. Fe bound P (Fe-P) overall had heavier δ18OP values (17.3-24.8‰) than some of Ca bound P (Ca-P) and equilibrium values, suggesting that Fe-P conserved the parental isotope signatures from terrigenous source and could act as the ideal tracer for the lake sediments. The mixing effect of terrigenous detrital input and biological mineralization made the source identification uncertain by using Ca-P, which had a wider range of δ18OP values (13.0-26.6‰). Additionally, significantly positive correlation (r = 0.551-0.913, p<0.05) between soluble reactive P (SRP) and Fe2+ in interstitial water obtained using DGT measurement revealed the conspicuous release and desorption of solid Fe-P toward the water. High diffusion fluxes from the sediments toward the overlying water further demonstrated that the desorption of Fe-P in the soil-originated sediments toward the solution conspicuously facilitated the accumulation of SRP in lake water. The first-time application of δ18OP isotope combined with in-situ DGT techniques certified that it's feasible for the contribution confirmation from terrigenous to lacustrine environments, and presented the direct evidence for management strategy making about P control and eutrophication restoration at the catchment scale of lakes.

Optimization framework of sediment phosphate oxygen isotope pretreatment method based on large-scale application: A case study of Fuyang River basin

Phosphate oxygen isotope (δ18OP) technique is an effective tool to identify the source and transformation process of phosphorus. The poor applicability of existing δ18OP pretreatment methods for sediments hindered the large-scale application of δ18OP technology. This paper presents an optimization framework for the pretreatment of sediment δ18OP samples based on large-scale applications, using the Fuyang River Basin as a case study. The typical channel landscape outflow lake, South Lake, was selected as the most favorable point for assessing the applicability and optimizing the mainstream δ18OP pretreatment method, which was achieved by clarifying the sediment environmental characteristics of South Lake. To evaluate the suitability of the Blake and McLaughlin methods in South Lake, a comparative study was carried out based on five dimensions: phosphorus recovery rate, removal efficiency of organic matter, removal efficiency of extraction liquid impurity ion, experimental time, and reagent consumption cost. The findings demonstrated that the Blake method outperformed the McLaughlin method across all five dimensions. Based on the environmental characteristics of the sediments of South Lake, the Blake method was optimized from two perspectives, namely the substitution of reagents and adjustment and optimization of experimental procedures. This resulted in an enhancement of phosphorus recovery and organic matter removal efficiency, while also reducing the experimental time required. The optimized method also yielded satisfactory results when applied to the entire watershed. This research paper can thus offer valuable technical support for the widespread application of sediment δ18OP technology.

Source partitioning using phosphate oxygen isotopes and multiple models in a large catchment

Excessive phosphorus (P) loadings cause major pollution concerns in large catchments. Quantifying the point and nonpoint P sources of large catchments is essential for catchment P management. Although phosphate oxygen isotopes (δ18O(PO4)) can reveal P sources and P cycling in catchments, quantifying multiple P sources in a whole catchment should be a research focus. Therefore, this study aimed to quantitatively identify the proportions of multiple potential end members in a typical large catchment (the Yangtze River Catchment) by combining the phosphate oxygen isotopes, land use type, mixed end-element model, and a Bayesian model. The δ18O(PO4) values of river water varied spatially from 4.9‰ to18.3‰ in the wet season and 6.0‰ to 20.9‰ in the dry season. Minor seasonal differences but obvious spatial changes in δ18O(PO4) values could illustrate how human activity changed the functioning of the system. The results of isotopic mass balance and the Bayesian model confirmed that controlling agricultural P from fertilizers was the key to achieving P emission reduction goals by reducing P inputs. Additionally, the effective rural domestic sewage treatment, development of composting technology, and resource utilization of phosphogypsum waste could also contribute to catchment P control. P sources in catchment ecosystems can be assessed by coupling an isotope approach and multiple-models.

Using a Multi-isotope Approach and Isotope Mixing Models to Trace and Quantify Phosphorus Sources in the Tuojiang River, Southwest China

Identifying phosphorus (P) sources is critical for solving eutrophication and controlling P in aquatic environments. Phosphate oxygen isotopes (δ¹⁸O_p) have been used to trace P sources. However, the application of this method has been greatly restricted due to δ¹⁸O_P values from the potential source having wide and overlapping ranges. In this research, P sources were traced by combining δ¹⁸O_p with multiple stable isotopes of nitrogen (δ¹⁵N), hydrogen (δD), and dissolved inorganic carbon (δ¹³C). Then, a Bayesian-based Stable Isotope Analysis in R (SIAR) model and IsoSource model were used to estimate the proportional contributions of the potential sources in the Tuojiang River. δ¹⁸O_p was not in equilibrium with ambient water, and statistically significant differences in the δ¹⁸O_p values were found between the potential sources, indicating that δ¹⁸O_p can be used to trace the P sources. δ¹⁵N, δD, and δ¹³C could assist δ¹⁸O_p in identifying the main sources of P. The SIAR and IsoSource models suggested that industrial and domestic sewage was the largest contributor, followed by phosphate rock and phosphogypsum and agricultural sewage. The uncertainty of the calculation results of the SIAR model was lower than that of the IsoSource model. These findings provide new insights into tracing P sources using multiple stable isotopes in watersheds.

First Measurements on Triple Oxygen Isotopes of Dissolved Inorganic Phosphate in the Hydrosphere

Although dissolved inorganic phosphate (DIP) is an important nutrient in the hydrosphere, it is difficult to quantitatively clarify the dynamics of DIP in the hydrosphere using the δ¹⁸O value of DIP as a tracer. In this study, we quantified the triple oxygen isotopic compositions (Δ'¹⁷O) of DIP relative to VSMOW with the reference line with a slope of 0.528 as an additional tracer to clarify the sources and dynamics of DIP in the hydrosphere. We found significant variation in the Δ'¹⁷O values of riverine DIP in urban areas, ranging from -107 × 10^-6 to +3 × 10^-6, while those of DIP in the effluents from wastewater treatment plants (WWTP) and DIP extracted from the chemical fertilizers showed -56 ± 5 × 10^-6 (1SD) and -98 ± 5 × 10^-6, respectively. We conclude that both the DIP supplied directly from the artificial loads (the WWTP effluent and chemical fertilizers) showing ¹⁷O-depleted Δ'¹⁷O values and the DIP turned over via the aquatic biosphere showing ¹⁷O-enriched Δ'¹⁷O values similar to ambient H₂O were the major sources of riverine DIP. High-precision determination of the Δ'¹⁷O value of DIP can contribute to quantitative clarification of the dynamics of DIP in the hydrosphere.

A new method for phosphate purification for oxygen isotope ratio analysis in freshwater and soil extracts using solid-phase extraction with zirconium-loaded resin

RATIONALE

Phosphate (PO₄ ) oxygen isotope (δ¹⁸ O_PO4 ) analysis is increasingly applied to elucidate phosphorus cycling. Due to its usefulness, analytical methods continue to be developed and improved to increase processing efficiency and applicability to various sample types. A new pretreatment procedure to obtain clean Ag₃ PO₄ using solid-phase extraction (SPE) with zirconium-loaded resin (ZrME), which can selectively adsorb PO₄ , is presented and evaluated here.

METHODS

Our method comprises (1) PO₄ concentration, (2) PO₄ separation by SPE, (3) cation removal, (4) Cl^- removal, and (5) formation of Ag₃ PO₄ . The method was tested by comparing the resulting δ¹⁸ O_PO4 of KH₂ PO₄ reagent, soil extracts (NaHCO₃ , NaOH, and HCl), freshwater, and seawater with data obtained using a conventional pretreatment method.

RESULTS

PO₄ recovery of our method ranged from 79.2% to 97.8% for KH₂ PO₄ , soil extracts, and freshwater. Although the recovery rate indicated incomplete desorption of PO₄ from the ZrME columns, our method produced high-purity Ag₃ PO₄ and accurate δ¹⁸ O_PO4 values (i.e., consistent with those obtained using conventional pretreatment methods). However, for seawater, the PO₄ recovery was low (1.1%), probably due to the high concentrations of F^- and SO₄ ^2- which interfere with PO₄ adsorption on the columns. Experiments indicate that the ZrME columns could be regenerated and used repeatedly at least three times.

CONCLUSIONS

We demonstrated the utility of ZrME for purification of PO₄ from freshwater and soil extracts for δ¹⁸ O_PO4 analysis. Multiple samples could be processed in three days using this method, increasing sample throughput and potentially facilitating more widespread use of δ¹⁸ O_PO4 analysis to deepen our understanding of phosphorus cycling in natural environments.

A new sampling method with zirconium-loaded resin for phosphate oxygen isotope analysis in oligotrophic freshwater systems

RATIONALE

The phosphate oxygen isotope ratio ( δ 18 O PO 4 ) is a useful technique to trace the sources and biogeochemical cycles of phosphorus (P) in aquatic ecosystems. However, δ 18 O PO 4 has not been widely used in oligotrophic freshwater systems due to technical and methodological difficulties in collecting sufficient phosphate (PO₄ ) for the δ 18 O PO 4 analysis, which sometimes requires hundreds of liters of the water sample. In this study, a new approach (PaS-Zir) was developed for the δ 18 O PO 4 analysis in oligotrophic freshwater systems using zirconium (Zr)-loaded (ZrIRC) resin, which has a high affinity for PO₄ .

METHODS

ZrClO₂ was added to Amberlite IRC748 to obtain the ZrIRC resin. The adsorption/desorption experiment using KH₂ PO₄ with a known value of δ 18 O PO 4 was conducted to determine the adsorption/desorption properties of the resin and the likelihood of isotopic fractionation. By installing mesh bags filled with the resin, the PaS-Zir approach was used in two rivers with low PO₄ concentrations (0.2 and 5.3 μmol/L). A conventional sampling method was also performed in the study river with a higher PO₄ concentration to validate the efficacy of the PaS-Zir method.

RESULTS

The adsorption/desorption experiment demonstrated that the ZrIRC resin possessed a sufficient adsorption capacity (153 μmol/resin-mL) and exhibited little isotopic fractionation during the adsorption/desorption processes. Using the PaS-Zir method, we were able to collect sufficient PO₄ samples for the δ 18 O PO 4 analysis from the rivers within at least 4 days of mesh bag installation. The δ 18 O PO 4 values (14.2‰ ± 0.2‰) obtained using the PaS-Zir method were comparable to those obtained using the conventional method (14.0‰ ± 0.03‰).

CONCLUSION

We proved that the PaS-Zir method is applicable to oligotrophic freshwater systems and is generally more efficient than the conventional method. In addition, our method is useful for improving the understanding of the P dynamics of oligotrophic ecosystems because of the extremely low concentration of PO₄ commonly found in them, which are often prone to P pollution.

Assessment of long-term phosphorus budget changes influenced by anthropogenic factors in a coastal catchment of Osaka Bay

Phosphorus usage is irreplaceable in agriculture; however, its excessive use leads to wastage of invaluable resources and significant soil surplus. Agronomic soil phosphorus surplus in Asian regions has a much higher level than the global average. And with rapid urbanization and population growth in the recent decades, Asian countries have seen a rise in environmental pollution levels also. This study assessed the detailed phosphorus budget in the Yamato River catchment, an urbanized coastal catchment in Asia, from 1940s to 2010s using Soil and Water Assessment Tool, comprehensively analyzed the effect of anthropogenic factors on long-term phosphorus loading and agronomic soil phosphorus balance. The results showed the peak period of total phosphorus loading and agronomic soil phosphorus surplus occurred in 1970s, at 895 tons/year and 36.6 kg/ha, respectively. The major reasons for increased phosphorus loading and soil surplus during 1940-1970 were rapid population growth and increased fertilizer usage, respectively. Since the 1980s, the construction of wastewater treatment system and reduction in agricultural land contributed to environmental improvement. These anthropogenic factors had a much stronger impact on phosphorus budget than climate change in the study catchment. Soil phosphorus balance is affected by a combination of factors, such as soil properties, fertilizer usage and applied schedule, precipitation event, and crop types. And soil phosphorus surplus may be severely overestimated if the non-point source loss due to precipitation factor is not fully considered.

Tracing the sources of phosphorus in lake at watershed scale using phosphate oxygen isotope (δ18OP)

Phosphorus (P) is normally considered as the limited nutrient for shallow freshwater lakes and can potentially trigger eutrophication on account of high concentrations. Due to the various transportation and transformation processes, P source apportionment and management in lake ecosystems have become more and more difficult. Combining with sequential extraction of P fractions and mineralogical analysis, the isotopic compositions of oxygen in phosphate (δ¹⁸O_P) of resin-extractable P from the different samples including soil, estuary sediments, pond sediments, and lake sediments in the Shijiuhu Lake catchment, China, were investigated. The results showed that δ¹⁸O_P values ranged from +15.23 to +21.92‰ in agricultural soil, +16.53 to +24.10‰ in estuary sediments, +18.90 to +20.90‰ in pond sediments, and +17.42 to +19.70‰ in lake sediments. Isotopic signatures indicated that chemical fertilizers with heavier δ¹⁸O_P values (+20.70 to +26.50‰) were the predominant contributors of P in the soil. The river transportation together with Fe/Al-P desorption on anaerobic condition simultaneously stimulated the enrichment of P in the lake sediments, even though the biotic activity regulated the isotope values moving toward the equilibrium. Eroded soil was the important source of P in lake and pond sediments via drainage and runoff, and conserved the source isotope signal in the samples. Stronger biotic activity in the aquatic environments dragged δ¹⁸O_P values toward the equilibrium. However, conspicuous off-equilibrium isotope signature suggested the terrestrial sources in the aquatic ecosystems. The calculation of two end-member linear mixing models suggested that soils also predominantly controlled the P occurrence in the lake sediments with contribution higher than 80%, indicating that decreasing inputs from the agricultural activities is important in P reduction on catchment scale. Generally, δ¹⁸O_P from different sources can provide indirect and important evidences for the identification and management of P sources in the lake catchment.

δ18O as a tracer of PO43- losses from agricultural landscapes

Accurately tracing the sources and fate of excess PO₄^3- in waterways is necessary for sustainable catchment management. The natural abundance isotopic composition of O in PO₄^3- (δ¹⁸O_P) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that δ¹⁸O_P could distinguish between agricultural PO₄^3- sources by measuring the integrated δ¹⁸O_P composition and P speciation of contrasting inorganic fertilisers (compound vs rock) and soil textures (sand, loam, clay) in southwestern Australia. δ¹⁸O_P composition differed between the three soil textures sampled across six livestock farms: sandy soils had lower overall δ¹⁸O_P values (21 ± 1‰) than the loams (23 ± 1‰), which corresponded with a smaller, but more readily leachable, PO₄^3- pool. Fertilisers had greater δ¹⁸O_P variability (∼8‰), with fluctuations due to type and manufacturing year. Consequently, catchment 'agricultural soil leaching' δ¹⁸O_P signatures could span from 18 to 25‰ depending on both fertiliser type and timing (lag between application and leaching). These findings emphasise the potential of δ¹⁸O_P to untangle soil-fertiliser P dynamics under controlled conditions, but that its use to trace catchment-scale agricultural PO₄^3- losses is limited by uncertainties in soil biological P cycling and its associated isotopic fractionation.

Tracing phosphorus cycle in global watershed using phosphate oxygen isotopes

The Phosphorus (P) cycle is a crucial biochemical process in the earth system. However, an extensive increase of P input into watersheds destroyed the ecosystem. To explore the effects of internal P loading and external P input in global watersheds, we reviewed the research progress and synthesized the isotope data of experimental results from literatures. An integrated result of the observational and experimental studies revealed that both internal P and external P largely contribute to watershed P loadings in watersheds. Internal P can be released to the overlying water during sediment resuspension process and change of redox conditions near the sediment-water interface. Growing fertilizer application on farmlands to meet food demand with population rise and diet improvement contributed to an huge increase of external P input to watersheds. Therefore, water quality cannot be improved by only reducing internal P or external P loadings. In addition, we found that phosphate oxygen isotope technology is an effectively way to trace the P biogeochemical cycle in watersheds. To better predict the dynamic of P in watersheds, future research integrating oxygen isotope fractionation mechanisms and phosphate oxygen isotope technology would be more effective.

Riverscape approaches in practice: perspectives and applications

Landscape perspectives in riverine ecology have been undertaken increasingly in the last 30 years, leading aquatic ecologists to develop a diverse set of approaches for conceptualizing, mapping and understanding 'riverscapes'. Spatiotemporally explicit perspectives of rivers and their biota nested within the socio-ecological landscape now provide guiding principles and approaches in inland fisheries and watershed management. During the last two decades, scientific literature on riverscapes has increased rapidly, indicating that the term and associated approaches are serving an important purpose in freshwater science and management. We trace the origins and theoretical foundations of riverscape perspectives and approaches and examine trends in the published literature to assess the state of the science and demonstrate how they are being applied to address recent challenges in the management of riverine ecosystems. We focus on approaches for studying and visualizing rivers and streams with remote sensing, modelling and sampling designs that enable pattern detection as seen from above (e.g. river channel, floodplain, and riparian areas) but also into the water itself (e.g. aquatic organisms and the aqueous environment). Key concepts from landscape ecology that are central to riverscape approaches are heterogeneity, scale (resolution, extent and scope) and connectivity (structural and functional), which underpin spatial and temporal aspects of study design, data collection and analysis. Mapping of physical and biological characteristics of rivers and floodplains with high-resolution, spatially intensive techniques improves understanding of the causes and ecological consequences of spatial patterns at multiple scales. This information is crucial for managing river ecosystems, especially for the successful implementation of conservation, restoration and monitoring programs. Recent advances in remote sensing, field-sampling approaches and geospatial technology are making it increasingly feasible to collect high-resolution data over larger scales in space and time. We highlight challenges and opportunities and discuss future avenues of research with emerging tools that can potentially help to overcome obstacles to collecting, analysing and displaying these data. This synthesis is intended to help researchers and resource managers understand and apply these concepts and approaches to address real-world problems in freshwater management.

Phosphate distribution and sources in the waters of Huangbai River, China: using oxygen isotope composition of phosphate as a tracer

We investigated the distributions of phosphate (PO₄-P) and used the oxygen isotope composition of phosphate (δ¹⁸O_P) to quantify PO₄-P sources in the waters of Huangbai River. According to the environmental characteristics of Huangbai River basin, the sampling stations in the Huangbai River were divided into three groups: sampling stations in the phosphate mining area, in the outcrop area of phosphate rock, and in the residential/agricultural area. The average PO₄-P concentration was highest (2.34 ± 1.00 μmol/L) in the outcrop area of phosphate ore, intermediate in the residential/agricultural area (1.06 ± 1.21 μmol/L), and lowest in the phosphate mining area (0.58 ± 0.31 μmol/L). The δ¹⁸O_P measured in the Huangbai River waters ranged from 6.0 to 20.9‰, with the highest average value in the outcrop area of phosphate rock (average: 14.6‰ ± 3.1‰). The majority of the measured δ¹⁸O_P values in the Huangbai River deviated greatly from the expected equilibrium values, indicating that δ¹⁸O_P in this area could be used to trace PO₄-P sources. We used two end-member mixing models to quantify the contribution of PO₄-P from different sources. In the phosphate mining area, the average fractions of PO₄-P from phosphate ore and sewage were 49.5% ± 23.8% and 50.5% ± 23.8%, respectively. In the outcrop area of phosphate rock, the average fractions of PO₄-P from phosphate ore and sewage were 60.1% ± 21.7% and 39.9% ± 21.7%, respectively. In the residential/agricultural area, the average fractions of PO4-P from fertilizer and sewage were 49.2% ± 23.2% and 50.8% ± 23.2%, respectively. These results indicate that phosphate mining activities was not an important source for PO₄-P in the waters of Huangbai River. The natural weathering of phosphate rock, fertilization, and domestic sewage contributed more to the high PO₄-P concentrations in the Huangbai River waters.

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.