Inorganic Phosphorus Fractions Research Articles

Seasonal variations and bioavailability of inorganic phosphorus in soils of Yeyahu Wetland in Beijing, China

Phosphorus (P) in soils has an important influence on the development and productivity of wetland. The objective of this study was to investigate the distribution, bioavailability and seasonal variations of inorganic phosphorus (IP) in soils of Yeyahu Wetland. The results showed that the concentration of Ca-bound P (Ca-P) remained the main occurrence in all IP fractions despite the season, at the same time the Al-bound P (Al-P) was always the lowest IP fraction despite the season. Al-P and Fe-bound P (Fe-P) concentration were all reached the highest value in May and the lowest in November, which was opposite to the occluded P (Oc-P) fraction. The notable higher Oc-P concentration in November observed in this study was probably due to the decomposition of plants and the subsequently releasing of oxides to soils. Ca-P didn't change much with season variations. Exchangeable P (Ex-P) and Oc-P were all positively correlated with soil moisture content and organic matter content, but Al-P was negatively correlated with these two soil properties. As a stable form, Ca-P was not associated with soil properties. Al-P, Fe-P and Ca-P might contribute to the storage of P in studied wetland with the principle component analysis results. The concentration rank order of the bio-available P in soils was generally as follows: Olsen-P > Algal available P (AAP) > Water soluble P (WSP) > Readily desorbed P (RDP). Olsen-P was positively correlated with Ex-P and Oc-P, and it was thought to be a more suitable indicator for bio-available P in soils.

Fractions and Bioavailability of Soil Inorganic Phosphorus in the Loess Plateau of China under Different Vegetations

Abstract:Plants play an important role in soil phosphorus nutrition. However, the effect of plants on phosphorus nutrition in soils of the Loess Plateau of China is not well understood. This study was conducted to reveal the relationships between plants and phosphorus' fractions and availability in the Loess Plateau of China. Twenty‐two plant communities were surveyed and soil samples under different plant canopies were collected for the determination of soil properties and inorganic phosphorus fractionation. The results showed that Leguminosae and Lilaceae reduced pH and increased organic matter, cation exchange capacity, total and Olsen phosphorus in soils under their canopies, while Labiatae and Rosaceae increased pH and decreased organic matter, cation exchange capacity, total and Olsen phosphorus in soils under their canopies. The contents of Ca2P, Ca8P, Al‐P and Fe‐P were highly related with soil Olsen phosphorus. They were all higher in soils under Leguminosae and Lilaceae and lower in soils under Labiatae and Rosaceae. The results of this study indicate that Leguminosae and Lilaceae improved phosphorus nutrition in soils, yet Labiatae and Rosaceae impeded the improvement of phosphorus nutrition in soils under their canopies, which will be of more help to instruct vegetation restoration in the region and provide information for soil development.

Inorganic Phosphorus Fractions and Phosphorus Availability in a Calcareous Soil Receiving 21-Year Superphosphate Application

A long-term (21-year) field experiment was performed to study the responses of soil inorganic P fractions and P availability to annual fertilizer P application in a calcareous soil on the Loess Plateau of China. Soil Olsen-P contents increased by 3.7, 5.2, 11.2 and 20.6 mg P kg −1 after 21-year annual fertilizer P application at 20, 39, 59, and 79 kg P ha −1, respectively. Long-term fertilizer P addition also increased soil total P and inorganic P (Pi) contents significantly. The contents of inorganic P fractions were in the order of Ca 10-P > Ca 8-P > Fe-P > Al-P > occluded P > Ca 2-P in the soil receiving annual fertilizer P application. Fertilizer P application increased Ca 8-P, Al-P and Ca 2-P contents as well as their percentages relative to Pi. Pi application increased Fe-P and occluded P contents but nor their percentages. Soil Ca 10-P content remained unchanged after fertilizer P application while its percentage relative to Pi declined with increasing fertilizer P rate. All Pi fractions but Ca 10-P were correlated with Olsen-P significantly. 90% of variations in Olsen-P could be explained by Pi fractions, and the direct contribution of Ca 8-P was predominant. Long-term annual superphosphate application would facilitate the accumulation of soil Ca 8-P, and thus improve soil P availability.

Effects of goat manure and inorganic phosphate addition on soil inorganic and microbial biomass phosphorus fractions under laboratory incubation conditions

Changes in inorganic phosphorus (P) and soil microbial biomass P following the addition of goat manure and different amounts of inorganic P to an Umbric Ferralsol were assessed over 12 weeks under laboratory incubation conditions using a sequential fractionation procedure. Triple superphosphate was added at rates equivalent to 0, 45, 90, 135 and 180 mg P kg−1, with or without goat manure at a rate of 10 g kg−1soil on a dry-weight basis and incubated moist. Resin P, 0.5 mol L−1 NaHCO3 extractable inorganic P (NaHCO3-Pi), 0.1 mol L−1 NaOH extractable inorganic P (NaOH Pi) and soil microbial biomass P concentrations were determined on days 7, 14, 28, 56 and 84. The magnitudes of the inorganic P fractions extracted were: NaOH Pi > Resin P > NaHCO3 Pi. Thus, NaOH Pi was the major sink for the applied P. The dynamics of the three labile P fractions (resin P, NaHCO3 Pi and microbial biomass P) varied considerably during the incubation period. The resin P fraction consistently declined with time in all treatments, whereas the NaHCO3 Pi fraction changed little with time in the control and goat manure amended soil, but increased rapidly with time when inorganic P was applied alone or in combination with goat manure. Microbial biomass P increased with time in all treatments, peaking on day 28 and declining thereafter. The co-application of inorganic P with goat manure produced up to twofold more microbial biomass P than either inorganic P or goat manure applied alone. Therefore, the combined application of manure with low rates of P fertilizers may be a cost effective strategy for increasing the efficiency of fertilizer P use through enhanced biological cycling of P in small-holder farms in South Africa.

Organic and Inorganic Forms of Phosphorus in a Calcareous Soil Planted with Four Species of Eucalyptus in Southern Iran

The objectives of the present study were to evaluate the effect of four eucalyptus species on (i) selected surface soil properties and (ii) the distribution of inorganic and organic phosphorus (Pi and Po) fractions. Soil samples were collected from soil 0–20 cm deep beneath and between trees. The Pi forms were determined by sequential extraction with sodium bicarbonate (NaHCO3‐P), ammonium acetate (NH4OAc) (OAc‐P), ammonium fluoride (NH4F‐P), sodium hydroxide (NaOH)–sodium carbonate (Na2CO3) (HC‐P), citrate dithionite (CD‐P), and sulfuric acid (H2SO4) (H2SO4‐P). The Po forms were sequentially extracted with NaHCO3 (NaHCO3‐Po), NaOH (NaOH‐Po), and H2SO4 (H2SO4‐Po). The NaOH‐Po was subdivided into moderately stable (NaOH‐Pom) and highly stable Po (NaOH‐Pos). Organic matter, clay and silt contents, total nitrogen, and available potassium of the soil beneath the trees increased. The OAc‐P and HC‐P forms beneath the trees were less than of that between them, which shows that these fractions probably are labile inorganic P pools. The NaHCO3‐Po and NaOH‐Pos forms were greater beneath the trees than those of interspaces, whereas NaOH‐Pom and H2SO4‐Po were not affected by plantation.

Interrelation of Inorganic Phosphorus Fractions and Sorghum‐Available Phosphorus in Calcareous Soils of Southern Khorasan

The objectives of this research were to determine inorganic phosphorus (P) fractions in calcareous soils of southern Khorasan and find their relationship with sorghum‐available P. Eighty soil samples were obtained and analyzed for some physical and chemical characteristics, among them 24 samples that varied in plant‐available P and soil properties were used for soil testing. From 24, 8 samples were selected for P fractionation as well. Five extraction procedures were used for soil testing. Results indicate that the extracted plant‐available P by the Olsen et al. (1954) and Paauw (1971) methods show the greatest correlation coefficients with plant P uptake and sorghum dry matter. The sequential inorganic P extraction analyzing indicated that the abundance of various inorganic fractions was in the order Ca10‐P > Al‐P > Ca8‐P > Ca2‐P > Oc‐P > Fe‐P. The results also indicate Olsen P correlates positively and significantly with Ca2‐P and Fe‐P fractions and positively but not significantly with the Al‐P fraction.

Distribution of phosphorus fractionations in calcareous soils of the Loess Plateau

以黄土高原自北向南采集的12个0～20 cm耕层土壤为供试土样，采用Tiessen和Moir修正的Hedley土壤有机无机磷分级方法研究了黄土高原石灰性土壤中不同形态磷组分的分布特征。结果表明：供试土壤各形态P总体分布特征为：HCl-P＞Residual-P＞NaHCO₃-Po＞NaHCO₃-Pi＞NaOH-Po＞NaOH-Pi＞H₂O-P，以HCl-P和Residual-P为主，分别占土壤全磷的54.00%～88.96%和0～39.11%。黄土高原土壤磷含量总体分布表现为南高北低。在各土壤类型间，NaOH-Po、Residual-P和全磷平均含量表现为干润砂质新成土＜黄土正常新成土＜简育干润均腐土＜土垫旱耕人为土，自北向南依次增加；H₂O-P和HCl-P表现为简育干润均腐土＜黄土正常新成土＜干润砂质新成土＜土垫旱耕人为土，自北向南先降后升，且上升幅度较大。黄土高原土壤全氮与全磷及各形态磷含量相关性均达显著水平，其中与NaOH-Pi、NaOH-Po、HCl-P及全磷含量达到极显著水平。C/N、pH及砂粒与全磷及各形态磷含量呈负相关关系，其中pH与NaHCO₃-Po呈显著负相关，与H₂O-P、NaHCO₃-Pi、NaOH-Pi及HCl-P呈极显著负相关；砂粒与NaHCO₃-Pi、Residual-P及全磷呈显著负相关。除NaHCO₃-Po、NaOH-Po及Residual-P，CaCO₃与其他各形态磷含量具有一定程度负相关。除NaOH-Pi和NaOH-Po外黏粒与其他各形态磷及全磷也具有一定程度负相关。土壤各形态磷组分和有效磷的多元回归分析发现，各形态磷中对有效磷贡献最大的是H₂O-P，其次为NaOH-Pi和NaHCO₃-Po；土壤各形态磷组分和有效磷逐步回归分析结果，进一步说明H₂O-P的有效性最高。

Inorganic phosphorus fractions in the rhizosphere of xerophytic shrubs in the Alxa Desert

In spite of the extremely low phosphorus (P) availability in soils of the Alxa Desert region (Inner-Mongolia, China), many shrubs grow well there. The objective of this study was to investigate the status of inorganic phosphorus (Pi) fractions in the rhizosphere of xerophytic shrubs. For all five selected shrubs, soil pH in the rhizosphere decreased by 0.4–0.8 units and the total P concentrations were greater than those in the corresponding bulk soils. In the rhizosphere of Reaumuria soongorica, Olsen P concentration was lower than that in the bulk soil. In general, the order of Pi fractions was Ca10–P>Ca8–P>Al–P>Fe–P>Ca2–P>O–P. The concentrations of all Pi fractions varied significantly with rhizosphere and corresponding bulk soils. The results of stepwise regression showed that Olsen P concentration in rhizosphere and bulk soils was contributed by different Pi fractions. The correlation analysis showed that the relationships between Pi fractions differed between rhizosphere and bulk soils. These results indicated that the soil Pi fractions are significantly affected by root activities, differing between the rhizosphere and bulk soils, and different xerophytic shrubs exhibited significantly different effect on soil Pi fractions.

Sediment phosphorus characteristics in the clearwater state of Lake Mogan, Turkey

This study examined the vertical distributions of total phosphorus (TP) and phosphorus fractions, and the iron and organic matter, in the littoral sediment in a macrophyte-dominated, clearwater state in Lake Mogan between September 2005 and August 2006. Benthic macroinvertebrates and total bacteria in the sediment also were determined. No clear seasonal or depth-related (0‐20 cm) patterns were found in sediment concentrations for the measured parameters. The phosphorus release was quantitatively very low, and a negative phosphorus release (‐0.132 μg m ‐2 day ‐1 ) was measured during the summer months. The TP concentrations of the sediment samples ranged between 675.00 and 1463.80 μg g ‐1 dry weight (DW), and the trophic level of the lake was eutrophic. On average, inorganic phosphorus fractions comprised the largest fraction (63%), while organic-bound phosphorus (Org ≈ P) constituted 37% of the TP in Lake Mogan. The most important phosphorus-immobilizing factors are high iron content (14 200‐47 750 μg g ‐1 DW), the sediment’s clay content (47.80‐ 51.80%), and an abundance of macrophytes at the sampling station. The low abundance of benthic macroinvertebrates (510‐ 850 individuals m ‐2 ), which depend on sediments with high iron and low organic matter (5.42‐13.30%), played a role in the sediment phosphorus retention. Although bacterial abundance in the surficial sediment appeared to be positively correlated to temperature, the overlying water did not experience anoxic conditions, supporting a state in which bacteria were able to retain phosphorus in their cell structures. Long-term changes in the sediments of Lake Mogan must be monitored lake. In order to optimize the management of the lake, and to determine the longevity of a clearwater state following management measures and continued external phosphorus loading, long-term changes in the sediments of Lake Morgan must be monitored.

Availability of Organic and Inorganic Phosphorus Fractions to Wheat in Toposequences of Calcareous Soils

Information on the availability of different soil phosphorus (P) forms is useful for crop production. Phosphorus contents of 12 Iranian calcareous soils from upper‐, mid‐, and lower‐slope positions of two arid and two semiarid toposequences were fractionated to various organic and inorganic pools, and correlations of the P fractions with wheat responses were investigated. Among the inorganic P (IP) fractions, apatite type (Ca10‐P) and dicalcium phosphate equivalents (Ca2‐P) possessed the highest and the lowest amounts of P reserve in the soils, respectively. On average, about 20% of the total P was found in organic form (OP), of which 32% was labile (LOP), 51% was moderately labile (MLOP), and 17% was nonlabile (NLOP). The amounts of the soil P fractions were considerably influenced by the positions of the soils on the landscapes. The maximum contents of soil IP, Ca2‐P, Fe‐P (iron‐bound P), and Ca10‐P were observed in the lower‐slope positions. The amount of soil available [0.5 M sodium bicarbonate (NaHCO3) extractable] P was significantly correlated with Ca2P (r=0.895), Fe‐P (r=0.760), and Occl‐P (iron‐occluded P) (r=0.897). Direct correlation studies, however, showed that wheat shoot dry‐matter yield (DMY) was significantly affected by the amounts of Ca2‐P, Fe‐P, OP, LOP, and MLOP fractions both at early (4 weeks) and late (10 weeks) stages of growth. All organic and inorganic P fractions, except Al‐P (aluminum‐bound P), Ca8‐P (octacalcium phosphate equivalents), and NLOP, also showed significant relations to the amount and/or concentration of P in wheat tissues at 4 and 10 weeks after sowing. Among the measured soil properties, the amount of organic carbon was the most affecting factor on the size of the P fractions.

Phosphorus fractions and adsorption characteristics of floodplain sediments in the lower reaches of the Hanjiang River, China

The phosphorus fractions and adsorption characteristics of seven floodplain sediment samples collected in the lower reaches of China's Hanjiang River were studied. Most phosphorus fractions showed a marked downstream increase in response to point-source inputs from urban areas. Total phosphorus (TP) contents in the sediments ranged from 603.68 to 945.25 mg x kg(-1). Inorganic phosphorus (IP) was the major component of TP, and calcium-bound phosphorus (Ca-P) was the major fraction of IP. The distribution characteristics of the phosphorus contents were affected by sediment grain size and hydrodynamic conditions. The maximum phosphorus adsorption capacities (Q (max)) and the half-saturation concentration (k) were obtained using an improved Langmuir model. Native adsorbed exchangeable phosphorus content (w (NAP)) and the zero-equilibrium phosphorus concentration value (c (EPC0)) were subsequently calculated. The effects of sediment grain size, temperature, and disturbance on the phosphorus adsorption isotherms were also studied. The results showed that phosphorus adsorption on floodplain sediments was primarily chemisorption; the particle concentration effect played a more important role at a disturbance intensity of 150 r x min(-1) (on a shaker table) than at 100 r x min(-1).

Phosphorus fractions of floodplain sediments and phosphorus exchange on the sediment–water interface in the lower reaches of the Han River in China

Phosphorus fractions and phosphate adsorption characteristics of the floodplain sediments in the lower reaches of the Han River in China were investigated in order to provide the environmental managers with essential data on the Han River ecosystem. The results showed that the total phosphorus (TP) contents ranged from 643.86 to 985 mg kg −1; inorganic phosphorus (IP) was the major fraction of TP, and calcium–bound phosphorus (Ca–P) was the main fraction of IP. Immersion played an important role in bioavailable phosphorus (exch-P and Fe/Al–P) desorption. The phosphorus adsorption process was completed within about 16 h. The desorption amount of phosphorus increased within 7 h and then decreased after 7 h. The contents of native adsorbed phosphorus ( w NAP ), zero equilibrium phosphorus concentration ( c EPC), total maximal adsorption amount of phosphorus (T Q max) and partitioning coefficient value ( K p) were calculated by corresponding formulae. w NAP ranged from 5.96 to 8.48 mg kg −1 and c EPC ranged from 0.15 to 0.24 mg L −1. w NAP was positively related to the exch-P and Ca–P contents ( w exch-p and w Ca – P ) and negatively related to the Fe/Al–P contents ( w Fe / Al – P ). A more significant linear relationship between T Q max and K p was observed. The floodplain sediments would release phosphorus into the overlying water. The Han River ecosystem should be restored in response to the development of the sustainable Han River ecosystem.

Inorganic Phosphorus Fractionation of Highly Calcareous Soils of Iran

Highly calcareous soils are abundant in Iran. The calcium carbonate equivalent (CCE) of these soils reach up to 650 g kg−1. Although phosphorus (P) fertilizer is being widely used in these soils, little information, if any, is available about P status in such soils. The objectives of this study were to 1) determine inorganic P forms in 18 surface soils of southern Iran, 2) study P readsorption during different stages of fractionation schemes, 3) assess the ability of NaOH to extract aluminum (Al)‐P, and 4) evaluate the relationships between P availability indices and inorganic P forms. Eighteen soil samples with a wide range of physicochemical properties were selected for this study. Inorganic P forms was determined by sequential extraction with NaHCO3, NH4OAc, NH4F, NaOH, citrate dithionite (CD), and H2SO4, which are referred to as Ca2‐P, Ca8‐P, Al‐P, Fe‐P, occluded P (O‐P), and Ca10‐P. Phosphorus readsorption in different stages was determined by 1 M MgCl2. Furthermore, a fractionation scheme without an NH4F step was used to evaluate the ability of NaOH to extract Al‐P. NaHCO3 (Olsen‐P) and MgCl2‐extractable P (Exch‐P) were regarded as P-availability indices. The abundance of different P forms was in the order Ca2‐P<Fe‐P<Al‐P<O‐P<Ca8‐P<Ca10‐P. Ca2‐P was highly correlated with Olsen‐P and Exch‐P. Ca2‐P, Olsen‐P, and Exch‐P showed a relationship with CCE, citrate–bicarbonate–dithionite extractable Fe (Fed), and Al (Ald). Phosphorus readsorption appeared to be important only in the Ca8‐P step, and the content of readsorbed P was related to Ca8‐P, CCE, and clay content of the soils. In the present study, Al‐P and Fe‐P accounted for 10 and 5% of the sum of the inorganic P fractions, respectively, and Fe‐P showed a strong relationship with Feo, whereas Al‐P showed a significant relationship with oxalate‐extractable Al (Alo) and Ald. It was found that one extraction with NaOH is not a good indicator for Fe‐ and Al‐P, and the ability of NaOH to extract Al‐P was reduced with increase in Al‐P content.

A multisyringe flow-through sequential extraction system for on-line monitoring of orthophosphate in soils and sediments

A fully automated flow-through microcolumn fractionation system with on-line post-extraction derivatization is proposed for monitoring of orthophosphate in solid samples of environmental relevance. The system integrates dynamic sequential extraction using 1.0 mol l −1 NH 4Cl, 0.1 mol l −1 NaOH and 0.5 mol l −1 HCl as extractants according to the Hieltjes–Lijklema (HL) scheme for fractionation of phosphorus associated with different geological phases, and on-line processing of the extracts via the Molybdenum Blue (MB) reaction by exploiting multisyringe flow injection as the interface between the solid containing microcolumn and the flow-through detector. The proposed flow assembly, capitalizing on the features of the multicommutation concept, implies several advantages as compared to fractionation analysis in the batch mode in terms of saving of extractants and MB reagents, shortening of the operational times from days to hours, highly temporal resolution of the leaching process and the capability for immediate decision for stopping or proceeding with the ongoing extraction. Very importantly, accurate determination of the various orthophosphate pools is ensured by minimization of the hydrolysis of extracted organic phosphorus and condensed inorganic phosphates within the time frame of the assay. The potential of the novel system for accommodation of the harmonized protocol from the Standards, Measurement and Testing (SMT) Program of the Commission of the European Communities for inorganic phosphorus fractionation was also addressed. Under the optimized conditions, the lowest detectable concentration at the 3 σ level was ≤0.02 mg P l −1 for both the HL and SMT schemes regardless of the extracting media. The repeatability of the MB assay was better than 2.5% and the dynamic linear range extended up to 7.0 mg P l −1 in NH 4Cl and NaOH media and 15 mg P l −1 whenever HCl is utilized as extractant for both the HL and SMT protocols.

Comparison of Organic and Inorganic Phosphorus Fractions in an Established Buffer and Adjacent Production Field

Since the U.S. Department of Agriculture's Conservation Reserve Program was established in 1985, thousands of acres of cropland have been converted to conservation buffers. The distributions of soil phosphorus (P) in various organic and inorganic fractions in a buffer and an adjacent crop production field were compared. Most of the extractable inorganic P (32 to 39%) in both the crop field and the buffer was present in the calcium (Ca)‐P fraction. Levels of the most labile P fractions were higher in the cropped area; however, more P was in the iron (Fe)‐P fraction in the buffer (23 vs. 18%). There were few differences among organic P fractions between the buffer and crop field. Soil sampling depth had a significant effect on the distribution of P. Differences between the cropped area and the buffer were less significant as depth increased. These results suggest that care should be taken in choosing sampling depths when relating P fraction distribution to potential P loss.

Bioavailability of Inorganic Phosphorus Fractions in Calcareous Soils Estimated by Neubauer Technique, Iron‐Impregnated Filter Paper, and Chemical Tests

Plants commonly suffer from phosphorus (P) deficiency in calcareous soils. Plant responses to P application on such soils mostly show poor correlation with their soil test P values. Experiments were conducted on 24 different soil samples under laboratory and greenhouse conditions to illustrate the relationship of various inorganic P fractions in different calcareous soils with P uptake by plants, P extraction by iron‐impregnated filter paper, and P soil test values estimated by 0.5M NaHCO3 and ammonium bicarbonate diethylene triamine penta‐acetic acid. Total P in the 24 soils ranged from 652 to 1245 mgkg−1 with a mean of 922 mgkg−1. A major proportion (98%) of inorganic P was in HCl‐P (Ca‐bound) form. The HCl‐P (Ca‐bound) ranged from 296 to 729 with a mean of 480 mgkg−1. The iron (Fe) and aluminum (Al)‐P (NaOH‐P) ranged from 0.92 to 12 mgkg−1 with a mean of 1.57 mgkg−1. The Fe‐P (citrate‐dithionite bicarbonate) ranged from 0.22 to 4.40 mgkg−1 with a mean of 5.99 mgkg−1. Data regarding P release from the soil matrix obtained by desorption with iron‐impregnated filter paper was best described by the Elovich equation. Range of slope and intercept values were found to be 5.48 to 17.3 and 17.23 to 56.27 mgkg−1, respectively. Intercept values calculated for the Elovich equation may be related to labile P initially available for plant uptake in soils. Intercept values calculated for the Elovich equation correlated (r=0.77) significantly (p<0.01) with NaHCO3 extractable (Olsen‐P)P. Significant correlation (p<0.05) of intercept with CDB‐P (r=0.44) and of slope with HCl‐P (0.43) suggested that the initially available P, regulated through CDB‐P, is replenished by HCl‐P [calcium (Ca) bound].

On-line dynamic fractionation and automatic determination of inorganic phosphorus in environmental solid substrates exploiting sequential injection microcolumn extraction and flow injection analysis

Sequential injection microcolumn extraction (SI-MCE) based on the implementation of a soil-containing microcartridge as external reactor in a sequential injection network is, for the first time, proposed for dynamic fractionation of macronutrients in environmental solids, as exemplified by the partitioning of inorganic phosphorus in agricultural soils. The on-line fractionation method capitalises on the accurate metering and sequential exposure of the various extractants to the solid sample by application of programmable flow as precisely coordinated by a syringe pump. Three different soil phase associations for phosphorus, that is, exchangeable, Al- and Fe-bound, and Ca-bound fractions, were elucidated by accommodation in the flow manifold of the three steps of the Hieltjes–Lijklema (HL) scheme involving the use of 1.0 M NH 4Cl, 0.1 M NaOH and 0.5 M HCl, respectively, as sequential leaching reagents. The precise timing and versatility of SI for tailoring various operational extraction modes were utilized for investigating the extractability and the extent of phosphorus re-distribution for variable partitioning times. Automatic spectrophotometric determination of soluble reactive phosphorus in soil extracts was performed by a flow injection (FI) analyser based on the Molybdenum Blue (MB) chemistry. The 3 σ detection limit was 0.02 mg P L −1 while the linear dynamic range extended up to 20 mg P L −1 regardless of the extracting media. Despite the variable chemical composition of the HL extracts, a single FI set-up was assembled with no need for either manifold re-configuration or modification of chemical composition of reagents. The mobilization of trace elements, such as Cd, often present in grazed pastures as a result of the application of phosphate fertilizers, was also explored in the HL fractions by electrothermal atomic absorption spectrometry.

Distribution of Carboxylates and Acid Phosphatase and Depletion of Different Phosphorus Fractions in the Rhizosphere of a Cereal and Three Grain Legumes

This study investigates the distribution of carboxylates and acid phosphatases as well as the depletion of different phosphorus (P) fractions in the rhizosphere of three legume crop species and a cereal, grown in a soil with two different levels of residual P. White lupin (Lupinus albus L.), field pea (Pisum sativum L.), faba bean (Vicia faba L.) and spring wheat (Triticum aestivum L.) were grown in small sand-filled PVC tubes to create a dense root mat against a 38-μm mesh nylon cloth at the bottom, where it was in contact with the soil of interest contained in another tube. The soil had either not been fertilised (P0) or fertilised with 15 (P15) kg P ha−1 in previous years. The mesh size did not allow roots to grow into the soil, but penetration of root hairs and diffusion of nutrients and root exudates was possible, and a rhizosphere was established. At harvest, thin (1 mm) slices of this rhizosphere soil were cut, down to a 10-mm distance from the mesh surface. The rhizosphere of white lupin, particularly in the P0 treatment, contained citrate, mostly in the first 3 mm, with concentrations decreasing with distance from the root. Acid phosphatase activity was enhanced in the rhizosphere of all species, as compared with bulk soil, up to a distance of 4 mm. Phosphatase activity was highest in the rhizosphere of white lupin, followed by faba bean, field pea and wheat. Both citrate concentrations and phosphatase activities were higher in P0 compared with P15. The depletion of both inorganic (Pi) and organic (Po) phosphorus fractions was greatest at the root surface, and decreased gradually with distance from the root. The soil P fractions that were most depleted as a result of root activity were the bicarbonate-extractable (0.5 M) and sodium hydroxide-extractable (0.1 M) pools, irrespective of plant species. This study suggests that differences among the studied species in use of different P pools and in the width of the rhizosphere are relatively small.

Phosphorus geochemistry of recent sediments in the South Basin of Lake Winnipeg

Lake Winnipeg supports the largest commercial fishery on Canadian Prairies. It has been influenced by a variety of environmental forces and anthropogenic activities. To gain a better understanding of recent changes in nutrient status of the lake, it is important to reconstruct its previous history from sedimentary records. Lacustrine sediments are known to be an important sink of many dissolved and suspended substances, including phosphorus, hence, they provide a permanent historical record of changes occurring in the lake. These changes may be induced by natural factors or by anthropogenic activities in the watershed. Phosphorus profiles from dated sediment cores collected in 1999 and 1994 from the South Basin of Lake Winnipeg were investigated to determine phosphorus enrichment in recent sediments. To interpret the nutrient status and depositional conditions responsible for the trends in total phosphorus, three operationally defined forms of phosphorus (P) were determined: non-apatite inorganic P, apatite P, and organic P. Significant increases in sediment phosphorus concentrations were observed in the uppermost 20 cm of the cores and several anomalies were observed at depth. A doubling in total phosphorus relative to aluminum over the last fifty years is largely due to increases in the non-apatite inorganic fraction, suggesting that much of sedimentary phosphorus increase is attributable to changes in the nutrient status of the water column related to anthropogenic inputs. Organic phosphorus exhibits a subtle increase in the upper 20 cm of the gravity cores, likely due to increases in the primary productivity of the lake. Except for the slight increase in deeper sediments, apatite phosphorus, which is thought to be of detrital origin, remained fairly constant over the length of the cores. Anomalous spikes in phosphorus concentrations deeper in the cores, comprised mainly of the non-apatite inorganic phosphorus fraction, likely resulted from natural variation in local oxidizing conditions, possibly induced by changes in water circulation and/or changes in sediment deposition rates due to climatic variation. The results of this investigation contribute to increased understanding of the depositional history of phosphorus in the lake over the last millennium.

Phosphorus fractions and phosphate sorption characteristics in relation to the sediment compositions of shallow lakes in the middle and lower reaches of Yangtze River region, China

Phosphorus is recognized as the most critical nutrient limiting lake productivity. The trophic status and development of lake systems are also influenced by the phosphorus content and fractions and phosphate sorption characteristics of the lake sediments. The phosphorus fractions and phosphate sorption characteristics of sediments in shallow lakes from the middle and lower reaches of Yangtze River region in China were investigated. The results show that the phosphorus contents in the sediments ranged from 217.8 to 1640 mg kg −1; inorganic phosphorus (IP) was the major fraction of total phosphorus (TP); phosphorus bound to Al, Fe, Mn oxides, and hydroxides (Fe/Al–P), and calcium bound phosphorus (Ca–P) were the main fractions of IP. Phosphate sorption on the sediments mainly occurred within 2 h and then reached equilibrium in 10 h. The phosphate sorption rate was closely related to the concentration of fine particles. The phosphate sorption capacity ranged from 128.21 to 833.33 mg kg −1, showing a significant correlation with the contents of Fe, Fe + Al, total organic carbon (TOC), cationic exchange capacity, total nitrogen, TP, Ca, IP, and the ratio of P/(Al + Fe), and it was higher in the sediments of eutrophic lakes than in mesotrophic lakes. Phosphate was mainly sorbed onto Fe and Al particles. The phosphate sorption efficiency ranged from 26.74 to 312.50 L kg −1, and had a strong positive correlation with Fe content. For the eutrophic lake sediments, there were no significant relationships between the phosphate sorption efficiency and the selected physical and chemical parameters. But for the mesotrophic lake sediments, the phosphate sorption efficiency was found to be positively related to the contents of Al and Fe + Al.