18O enrichment in phosphorus pools extracted from soybean leaves

An optimized method for extraction and purification of inorganic phosphate from plant material for oxygen isotope ratio analysis

Compound-specific stable isotope ratio analysis of oxygen isotopes in inorganic phosphate can be used to study biological phosphorus cycling and the transformation processes controlling the fate of phosphorus. However, methods for extraction of inorganic phosphate from plant tissue for oxygen isotope ratio analysis are not consistent. Further, the purification into solid silver phosphate can be challenging and laborious. In this work, a detailed and optimized method to provide a more consistent, easily implementable and reproducible extraction using trichloroacetic acid and subsequent purification of inorganic phosphate from plant material for oxygen isotope ratio analysis is presented. Key focus points were: uniform extraction of inorganic phosphate from barley leaves, removal of dissolved organic material, flexibility in regards to the amount of inorganic phosphate extracted for the purification into silver phosphate, reduced use of chemicals and, removal of co-precipitated oxygen-bearing compounds before analysis. Most notable optimizations to the method and associated effects were:•Drying of plant material before inorganic phosphate extraction increases the method applicability to a broader range of plant sample types.•Removal of dissolved organic matter improves inorganic phosphate purification.•Sample volume adjustment according to inorganic phosphate content is vital for effective and quantitative precipitations.

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.

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.

A rapid ammonium fluoride method to determine the oxygen isotope ratio of available phosphorus in tropical soils

RATIONALE

The isotopic composition of oxygen bound to phosphorus (δ¹⁸ O_P value) offers an opportunity to gain insight into P cycling mechanisms. However, there is little information for tropical forest soils, which presents a challenge for δ¹⁸ O_P measurements due to low available P concentrations. Here we report the use of a rapid ammonium fluoride extraction method (Bray-1) as an alternative to the widely used anion-exchange membrane (AEM) method for quantification of δ¹⁸ O_P values of available P in tropical forest soils.

METHODS

We compared P concentrations and δ¹⁸ O_P values of available and microbial P determined by AEM and Bray-1 extraction for a series of tropical forest soils from Panama spanning a steep P gradient. This involved an assessment of the influence of extraction conditions, including temperature, extraction time, fumigation time and solution-to-soil ratio, on P concentrations and isotope ratios.

RESULTS

Depending on the extraction conditions, Bray-1 P concentrations ranged from 0.2 to 66.3 mg P kg^-1 across the soils. Extraction time and temperature had only minor effects on Bray-1 P, but concentrations increased markedly as the solution-to-soil ratio increased. In contrast, extraction conditions did not affect Bray-1 δ¹⁸ O_P values, indicating that Bray-1 provides a robust measure of the isotopic composition of available soil P. For a relatively high P soil, available and fumigation-released (microbial) δ¹⁸ O_P values determined by Bray-1 extraction (20‰ and 16‰, respectively) were higher than those determined by the AEM method (18‰ and 12‰, respectively), which we attribute to slightly different P pools extracted by the two methods and/or differences resulting from the longer extraction time needed for the AEM method.

CONCLUSIONS

The short extraction time, insensitivity to extraction conditions and smaller mass of soil required to extract sufficient P for isotopic analysis make Bray-1extraction a suitable alternative to the AEM method for the determination of δ¹⁸ O_P values of available P in tropical soils.

Changes of oxygen isotope values of soil P pools associated with changes in soil pH

Field data about the effect of soil pH on phosphorus (P) cycling is limited. A promising tool to study P cycling under field conditions is the ¹⁸O:¹⁶O ratio of phosphate (δ¹⁸O_P). In this study we investigate whether the δ¹⁸O_P can be used to elucidate the effect of soil pH on P cycling in grasslands. Soils and plants were sampled from different fertilisation and lime treatments of the Park Grass long term experiment at Rothamsted Research, UK. The soils were sequentially extracted to isolate different soil P pools, including available P and corresponding δ¹⁸O_P values were determined. We did not observe changes in plant δ¹⁸O_P value, but soil P δ¹⁸O_P values changed, and lower δ¹⁸O_P values were associated with higher soil pH values. At sites where P was not limiting, available P δ¹⁸O_P increased by up to 3‰ when lime was applied. We show that the δ¹⁸O_P method is a useful tool to investigate the effect of pH on soil P cycling under field conditions as it highlights that different soil processes must govern P availability as pH shifts. The next challenge is now to identify these underlying processes, enabling better management of soil P at different pH.

Phosphorus fractions and oxygen isotope composition of inorganic phosphate in typical agricultural soils

Phosphorus (P), despite being an essential nutrient element for plants growth in agricultural ecosystem, the low utilization rate of soil P and the environmental problems caused by soil P losses are serious. Therefore, scoping knowledge of the possible sources and utilization extent of soil P by microorganisms is very helpful for better understanding of promoting P utilization for sustainable agriculture. Oxygen isotope of phosphate technology is an effective tool to trace the sources of P. In this study, P contents and oxygen isotope composition of inorganic phosphate (δ¹⁸O_P) of different pools (H₂O-P, NaHCO₃-P, NaOH-P, and HCl-P) in typical agricultural soil from Northeast China and Central China were analyzed and quantified. The results showed that fertilizer and land use were important factors influencing the contents of H₂O-P_t and NaHCO₃-P_t and the soil TP contents from different types of soils were greatly affected by soil weathering degree. The δ¹⁸O_P of different P pools indicated that the difference in utilization extent of different P fractions by microorganisms and the δ¹⁸O_P values of different P fractions could be due to accumulation of multiple factors. The results will provide effective information for further study on sources and effective utilization of different P fractions in soil.

Measurements of 18 O-Pi uptake indicate fast metabolism of phosphate in tree roots

Phosphorus (P) nutrition of beech ecosystems depends on soil processes, plant internal P cycling and P acquisition. P uptake of trees in the field is currently not validated due to the lack of an experimental approach applicable in natural forests. Application of radiolabelled tracers such as ³³ P and ³² P is limited to special research sites and not allowed in natural environments. Moreover, only one stable isotope of P, namely ³¹ P, exists. One alternative tool to measure P acquisition in the field could be the use of ¹⁸ O-labelled ³¹ P-phosphate (³¹ P¹⁸ O₄ ^3- ). Phosphate (P_i ) uptake rates calculated from the ¹⁸ O enrichment of dried root material after application of ³¹ P_i ¹⁸ O₄ ^3- via nutrient solution was always lower compared to ³³ P incorporation, did not show increasing rates of P_i uptake at P deficiency under controlled conditions, and did not reveal seasonal fluctuations in the field. Consequently, a clear correlation between ³³ P-based and ¹⁸ O-based P_i uptake by roots could not be established. Comparison of P_i  uptake rates achieved from ³³ P-P_i and ¹⁸ O-P_i application led to the conclusion of high P_i metabolism in roots after P_i uptake. The replacement of ¹⁸ O by ¹⁶ O from water in ¹⁸ O-P_i during root influx, but most probably after P_i uptake into roots, due to metabolic activities, indicates high and fast turnover of P_i . Hence, the use of ¹⁸ O-P_i as an alternative tool to estimate P_i acquisition of trees in the field must consider the increase of ¹⁸ O abundance in root water that was disregarded in dried root material.

18O-labeled phosphate applied to soil appears in the shoots of maize after uptake by roots but not after uptake by an arbuscular mycorrhizal fungus

The application of ³³P or ³²P isotopes to directly trace phosphorus (P) uptake during arbuscular mycorrhizal (AM) symbiosis is limited by the radioactivity of the two P isotopes, especially under field conditions. A potential alternative method for tracing P uptake in plant-soil systems relies on the analysis of the stable oxygen (O) isotopes of ortho-phosphate (Pi); however, little is known about the fate of the P-O bond during Pi uptake in AM symbioses. This study investigated whether the abundance of ¹⁸O in Pi extracted from the shoots of maize increased after ¹⁸O-labeled Pi added to soil was taken up by either roots of maize or AM extraradical hyphae. A two-compartment culture system, consisting of a root and AM hyphal compartment (RHC, including both roots and AM hyphae) and an AM hyphal compartment (HC, including only hyphae) was designed, and the AM fungus Funneliformis mosseae was used to inoculate the roots of maize. Our results indicated that the abundance of ¹⁸O in Pi extracted from the maize shoots increased significantly 3 months after the addition of ¹⁸O-labeled Pi to the soil in the pots which only contained roots. The abundance of ¹⁸O was much lower than expected, however, which suggests a great majority of ¹⁸O in labeled Pi was lost in the soil or during Pi metabolism in the shoots of maize. The abundance of ¹⁸O in Pi extracted from the maize shoots did not increase 3 months after ¹⁸O-labeled Pi was added to the HC, and therefore, loss of ¹⁸O in labeled Pi may also occur during Pi metabolism in AM hyphae. Use of ¹⁸O-labeled Pi as a qualitative tracer of P uptake during AM symbiosis appears unfeasible for such a long-term (3 months) experiment, although it should be investigated in a short-term labeling experiment.

Combining spectroscopic and isotopic techniques gives a dynamic view of phosphorus cycling in soil

Current understanding of phosphorus (P) cycling in soils can be enhanced by integrating previously discrete findings concerning P speciation, exchange kinetics, and the underlying biological and geochemical processes. Here, we combine sequential extraction with P K-edge X-ray absorption spectroscopy and isotopic methods (³³P and ¹⁸O in phosphate) to characterize P cycling on a climatic gradient in Hawaii. We link P pools to P species and estimate the turnover times for commonly considered P pools. Dissolved P turned over in seconds, resin-extractable P in minutes, NaOH-extractable inorganic P in weeks to months, and HCl-extractable P in years to millennia. Furthermore, we show that in arid-zone soils, some primary mineral P remains even after 150 ky of soil development, whereas in humid-zone soils of the same age, all P in all pools has been biologically cycled. The integrative information we provide makes possible a more dynamic, process-oriented conceptual model of P cycling in soils.

Our understanding of phosphorus (P) cycling in soils, a basis for many ecosystem services, has been limited by the complexity of P forms and processes. Here the authors use spectroscopic and isotopic techniques to estimate turnover times of P pools and tease apart biologically-driven and geochemically-driven P fluxes.

Assessing Cumulative Effects of Climate Change Manipulations on Phosphorus Limitation in a Californian Grassland

Grasslands throughout the world are responding in diverse ways to changing climate and environmental conditions. In this study we analyze indicators of phosphorus limitation including phosphorus concentrations, phosphorus to nitrogen, and carbon ratios, oxygen isotope ratios of phosphate in vegetation, and phosphatase enzyme activity in soil to shed light on potential effects of climate change on phosphorus availability to grassland vegetation. The study was conducted at the Jasper Ridge Global Change Experiment (JRGCE), California where manipulations mimicking increases in temperature, water, nitrogen and carbon-dioxide have been maintained for over 15 years. We compare our results to an earlier study conducted 3 years after the start of the experiment, in order to assess any change in the response of phosphorus over time. Our results suggest that a decade later the measured indicators show similar or only slightly stronger responses. Specifically, addition of nitrogen, the principle parameter controlling biomass growth, increased phosphorus demand but thresholds that suggest P limitation were not reached. A study documenting changes in net primary productivity (NPP) over time at the JRGCE also could not identify a progressive effect of the manipulations on NPP. Combined these results indicate that the vegetation in these grassland systems is not very sensitive to the range of climate parameters tested.

Handling the phosphorus paradox in agriculture and natural ecosystems: Scarcity, necessity, and burden of P

This special issue of Ambio compiles a series of contributions made at the 8th International Phosphorus Workshop (IPW8), held in September 2016 in Rostock, Germany. The introducing overview article summarizes major published scientific findings in the time period from IPW7 (2015) until recently, including presentations from IPW8. The P issue was subdivided into four themes along the logical sequence of P utilization in production, environmental, and societal systems: (1) Sufficiency and efficiency of P utilization, especially in animal husbandry and crop production; (2) P recycling: technologies and product applications; (3) P fluxes and cycling in the environment; and (4) P governance. The latter two themes had separate sessions for the first time in the International Phosphorus Workshops series; thus, this overview presents a scene-setting rather than an overview of the latest research for these themes. In summary, this paper details new findings in agricultural and environmental P research, which indicate reduced P inputs, improved management options, and provide translations into governance options for a more sustainable P use.

A dual isotopic approach using radioactive phosphorus and the isotopic composition of oxygen associated to phosphorus to understand plant reaction to a change in P nutrition

BACKGROUND

Changing the phosphorus (P) nutrition leads to changes in plant metabolism. The aim of this study was to investigate how these changes are reflected in the distribution of ³³P and the isotopic composition of oxygen associated to P (δ¹⁸O_P) in different plant parts of soybean (Glycine max cv. Toliman). Two P pools were extracted sequentially with 0.3 M trichloroacetic acid (TCA P) and 10 M nitric acid (HNO₃; residual P).

RESULTS

The δ¹⁸O_P of TCA P in the old leaves of the - P plants (23.8‰) significantly decreased compared to the + P plants (27.4‰). The ³³P data point to an enhanced mobilisation of P from residual P in the old leaves of the - P plants compared to the + P plants.

CONCLUSIONS

Omitting P for 10 days lead to a translocation of P from source to sink organs in soybeans. This was accompanied by a significant lowering of the δ¹⁸O_P of TCA P in the source organs due to the enzymatic hydrolysis of organic P. Combining ³³P and δ¹⁸O_P can provide useful insights in plant responses to P omission at an early stage.

Tracing the sources and cycling of phosphorus in river sediments using oxygen isotopes: Methodological adaptations and first results from a case study in France

An essential aspect of eutrophication studies is to trace the ultimate origin of phosphate ions (P-PO₄) associated with the solid phase of river sediments, as certain processes can make these ions available for algae. However, this is not a straightforward task because of the diversity of allochthonous and autochthonous sources that can supply P-PO₄ to river sediments as well as the existence of in-stream processes that can change the speciation of these inputs and obscure the original sources. Here, we present the results of a study designed to explore the potentials, limitations and conditions for the use of the oxygen isotope composition of phosphate (δ¹⁸Op) extracted from river sediments for this type of tracing. We first tested if the method commonly applied to soils to purify P-PO₄ and to measure their δ¹⁸Op concentrations could be adapted to sediments. We then applied this method to a set of sediments collected in a river along a gradient of anthropogenic pressure and compared their isotopic signatures with those from samples that are representative of the potential P-PO₄ inputs to the river system (soils and riverbank material). The results showed that following some adaptations, the purification method could be successfully transposed to river sediments with a high level of P-PO₄ purification (>97%) and high δ¹⁸Op measurement repeatability and accuracy (<0.4‰). The values for the potential allochthonous sources varied from 11.8 to 18.3‰, while the δ¹⁸Op value for the river sediments ranged from 12.2 to 15.8‰. Moreover, a sharp increase (>3‰) in the sediment δ¹⁸Op value immediately downstream from the discharge point revealed the strong impact of municipal wastewater. The calculation of the theoretical equilibrium δ¹⁸O_p values using the river water temperature and δ¹⁸O_w showed that the downstream sediments were in equilibrium, which was not the case for the upstream sediments. This difference could be related to the contrast between the short residence time of the transfer system in the catchment head, which can preserve the isotopic variability of the source materials, and the longer residence times and higher P bioavailability in the lower catchment, possibly fostering the recycling of P-PO₄ by the biota and the equilibration of the oxygen isotope signature in P-PO₄. These results demonstrate the potential of the isotopic approach to assess the sources and in-stream turnover of sedimentary P in river systems.

Internal loading of phosphate in Lake Erie Central Basin

After significant reductions in external phosphorus (P) loads, and subsequent water quality improvements in the early 1980s, the water quality of Lake Erie has declined considerably over the past decade. The frequency and magnitude of harmful algal blooms (primarily in the western basin) and the extent of hypoxic bottom waters in the central basin have increased. The decline in ecosystem health, despite meeting goals for external P loads, has sparked a renewed effort to understand P cycling in the lake. We use pore-water P concentration profiles and sediment cores incubation experiments to quantify the P flux from Lake Erie central basin sediments. In addition, the oxygen isotopes of phosphate were investigated to assess the isotopic signature of sedimentary phosphate inputs relative to the isotopic signature of phosphate in lake water. Extrapolating the total P sediment flux based on the pore-water profiles to the whole area of the central basin ranged from 300 to 1250metric tons per year and using the flux based on core incubation experiments an annual flux of roughly 2400metric tons of P is calculated. These estimates amount to 8-20% of the total external input of P to Lake Erie. The isotopic signature of phosphate in the extractable fraction of the sediments (~18‰) can explain the non-equilibrium isotope values of dissolved phosphate in the deep water of the central basin of Lake Erie, and this is consistent with sediments as an important internal source of P in the Lake.

Phosphate stable oxygen isotope variability within a temperate agricultural soil

In this study, we conduct a spatial analysis of soil total phosphorus (TP), acid extractable phosphate (PO₄) and the stable oxygen (O) isotope ratio within the PO₄ molecule (δ¹⁸O_PO4 ) from an intensively managed agricultural grassland site. Total P in the soil was found to range from 736 to 1952 mg P kg^- 1, of which between 12 and 48% was extractable using a 1 M HCl (HCl_PO4 ) solution with the two variables exhibiting a strong positive correlation. The δ¹⁸O_PO4 of the extracted PO₄ ranged from 17.0 to 21.6‰ with a mean of 18.8‰ (± 0.8). While the spatial variability of Total P has been researched at various scales, this is the first study to assess the variability of soil δ¹⁸O_PO4 at a field-scale resolution. We investigate whether or not δ¹⁸O_PO4 variability has any significant relationship with: (i) itself with respect to spatial autocorrelation effects; and (ii) HCl_PO4 , elevation and slope - both globally and locally. Results indicate that δ¹⁸O_PO4 was not spatially autocorrelated; and that δ¹⁸O_PO4 was only weakly related to HCl_PO4 , elevation and slope, when considering the study field as a whole. Interestingly, the latter relationships appear to vary in strength locally. In particular, the δ¹⁸O_PO4 to HCl_PO4 relationship may depend on the underlying soil class and/or on different field managements that had operated across an historical north-south field division of the study field, a division that had been removed four years prior to this study.

Multi-factorial in vivo stable isotope fractionation: causes, correlations, consequences and applications

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.