Tracing phosphorus cycle in global watershed using phosphate oxygen isotopes

Fractionation of phosphate oxygen isotope in the leachate of phosphogypsum during its infiltration

The oxygen isotope composition of phosphate (δ18O-PO43-) is a valuable tool for tracing the origins of phosphate (PO43-) in the environment. However, the presence of microorganisms can pose challenges in identifying potential sources of PO43- in complex environment, such as groundwater. Given that phosphogypsum leachate (PGL) significantly contributes to groundwater P pollution, the infiltration of PGL into aquifers may alter the fractionation of δ18O-PO43-, thereby impacting the application of δ18O-PO43- techniques in tracing the origin of PO43- within groundwater. The present study investigated the influence of environmental conditions and the properties of PGL, such as low pH and high concentrations of fluoride and sulfate, on the fractionation of δ18O-PO43-. The results revealed that that the presence of microorganisms led to an increase in the δ18O-PO43- from 10.13 ‰ to 10.92 ‰, while no significant change was observed in a sterilized environment. The fractionation of δ18O-PO43- adhered to the Rayleigh fractionation model with a fractionation coefficient of -2.97 ‰ (R2 = 0.977). Furthermore, high concentrations of fluoride ions were observed to impede the kinetic fractionation of δ18O-PO43- by suppressing microbial activities, which was consistent with the Rayleigh fractionation model. Additionally, low pH inhibited pyrophosphatase activity, thereby impeding equilibrium fractionation of δ18O-PO43-. Under anaerobic conditions, sulfate ions contributed to transient fractionations through sulfate reduction-mediated Fe/Al-P release, however, this effect disappeared upon reaching adsorption equilibrium. Overall, this study provides valuable insights into understanding the factors influencing δ18O-PO43- fractionations during the infiltration of PGL from soil to groundwater.

The overlooked contribution of aquaculture to phosphorus pollution in estuary water with phosphate oxygen isotope

Phosphorus is one of the key contributors to the eutrophication of aquatic ecosystems. With the rapid growth of the aquaculture industry, aquaculture effluent has gradually become a significant source of phosphorus pollution. However, this important phosphorous source has often been overlooked in previous studies. In this study, we utilized phosphate oxygen isotope (δ18OP) combined with the Bayesian isotope mixing model in R (MixSIAR) to identify and quantify the major phosphorus sources in Xijiang Estuary, China. The results show that the average concentration of total phosphorus (TP) in the receiving waters ranged from 0.03 to 0.34 mg/L, with aquaculture effluent indeed contributing the most to phosphorous loading (30.5 %), generally higher than other phosphorus sources such as livestock feces (29.6 %), Wastewater Treatment Plants (WWTPs) effluent (24.0 %) and agricultural soil (15.8 %). Additionally, significant spatial differences were observed in the contribution of different phosphorous sources. This study highlights the previously underestimated role of aquaculture in phosphorus loading in estuarine regions and provides valuable insights for water quality management strategies.

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.

Different patterns of bacterioplankton in response to inorganic and organic phosphorus inputs in freshwater lakes - a microcosmic study

Phosphorus (P) is a limiting factor in fresh waters and is also the main cause of water eutrophication and deterioration, However, the practical effect of elevated P level on bacterioplankton is less evaluated. In this study, we investigated the bacterioplankton in a 96 hours microcosm experiment with P additions in two forms (organic/inorganic P, OP/IP) and three levels (final conc., 0.040, 0.065 and 0.125 g/L), aiming to find out the response pattern of bacterioplankton in coping with the increasing P levels. Results showed a more dramatic change of water properties and bacterioplankton between P forms (OP and IP) than among the addition levels, and a more remarkable effect of OP addition than the IP. Both OP and IP treatments significantly decreased the water pH, dissolved oxygen (DO), Electrical Conductivity (EC), Nitrate Nitrogen (NO3--N) and Total Organic Carbon (TOC), and reduced the α-diversity of bacterioplankton and relative abundance of Cyanobacteria, but increased the abundance of Proteobacteria. The IP addition decreased Actinobacteria abundance (especially for HgcI) and showed higher denitrification potentials, while the OP addition depressed the Bateroidota and exhibited lowed methylotrophic functions, but such trends decreased with increasing addition concentrations. The network analysis showed that both IP and OP additions increased the proportion of positively correlated edges and reduced the network complexity and stability, but the OP network was more stable than the IP network. The study clarifies the response pattern of bacterioplankton to the P input with different forms and levels, and deepens our understanding of the eutrophication process, which provides a scientific basis for the management and control of freshwater lakes facing eutrophication.

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.

Genome-wide association studies dissect low-phosphorus stress response genes underling field and seedling traits in maize

Phosphorus (P) is an essential element for plant growth, and its deficiency can cause decreased crop yield. This study systematically evaluated the low-phosphate (Pi) response traits in a large population at maturity and seedling stages, and explored candidate genes and their interrelationships with specific traits. The results revealed a greater sensitivity of seedling maize to low-Pi stress compared to that at maturity stage. The phenotypic response patterns to low-Pi stress at different stages were independent. Chlorophyll content was found to be a potential indicator for screening low-Pi-tolerant materials in the field. A total of 2900 and 1446 significantly associated genes at the maturity and seedling stages were identified, respectively. Among these genes, 972 were uniquely associated with maturity traits, while 330 were specifically detected at the seedling stage under low-Pi stress. Moreover, 768 and 733 genes were specifically associated with index values (low-Pi trait/normal-Pi trait) at maturity and seedling stage, respectively. Genetic network diagrams showed that the low-Pi response gene Zm00001d022226 was specifically associated with multiple primary P-related traits under low-Pi conditions. A total of 963 out of 2966 genes specifically associated with traits under low-Pi conditions or index values were found to be induced by low-Pi stress. Notably, ZmSPX4.1 and ZmSPX2 were sharply up-regulated in response to low-Pi stress across different lines or tissues. These findings advance our understanding of maize's response to low-Pi stress at different developmental stages, shedding light on the genes and pathways implicated in this response.

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.

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.

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.

The Effect of Rainfall on Aquatic Nitrogen and Phosphorus in a Semi-Humid Area Catchment, Northern China

The impact of rainfall on water quality may be more important in semi-arid regions, where rainfall is concentrated over a couple of months. To explore the impact of rainfall changes on water quality, e.g., nitrogen (TN) and phosphorous (TP), the diversion from Luan River to Tianjin Watershed in the northern semi-humid area was selected as the study area. TN and TP concentrations in rivers and the Yuqiao Reservoir during the three-year high-flow season (2019-2021) were analyzed. The response relationship and influencing factors among the watershed's biogeochemical process, rainfall, and water quality were clarified. The results showed that rainfall in the high flow season mainly controlled the river flow. The concentration of TN and TP in the inflow rivers is regulated by rainfall/flow, while the concentration of TN and TP in the water diversion river has different variation characteristics in the water diversion period and other periods. The lowest annual concentrations of TN and TP were observed in the normal year, while the highest annual concentration was observed in the wet year, indicating that the hydrological process drove the nutrient transport in the watershed. For the tributaries, the Li River catchment contributed a large amount of N and P to the aquatic environment. For the reservoir, the extreme TN concentrations were the same as the tributaries, while the extremes of TP concentrations decreased from the dry year to wet year, which was in contrast to the tributaries. The spatial variation of TN and TP concentrations in the reservoir showed that the concentration decreased following the flow direction from the river estuary to the reservoir outlet. Considering climate change, with the increase of rainfall in North China in the future, the TN and TP transport fluxes in the watershed may continue to increase, leading to the nitrogen and phosphorus load of the downstream reservoir. To ensure the impact of the increase of potential N and P output fluxes in the watershed on the water quality of the reservoir area, it is necessary to strengthen the effective prevention and control of non-point source pollution in the watershed.