A review of phosphate oxygen isotope values in global bedrocks: Characterising a critical endmember to the soil phosphorus system

Understanding the phosphate oxygen isotope (δ¹⁸O‐PO₄) composition of bedrock phosphate sources is becoming ever more important, especially in areas of soil research which use this isotope signature as a proxy for biological cycling of phosphorus (P). For many of these studies, obtaining a sample of the source bedrock or applied mineral fertiliser for isotope analysis is impossible; meaning there is now a demand for a comprehensive characterisation of global bedrock δ¹⁸O‐PO₄ to support this work. Here we compile δ¹⁸O‐PO₄ data from a wide range of global bedrocks, including 56 new values produced as part of this study and a comprehensive overview of those within the previously existing literature. We present δ¹⁸O‐PO₄ data from the range of major phosphatic lithologies alongside as much metadata for the samples as could be gathered. Much of the data comes from bedrocks of marine sedimentary origin (< 1 Ma = > +22‰, > 540 Ma = ≈ +12‰), but we also present data from bedrocks associated with guano (range: +19.5 to +15‰) and igneous deposits (range: +12 to –0.8‰), both of which have distinct δ¹⁸O‐PO₄ signatures due to their formation mechanisms. We show that where repeat measurements of the same formation have been undertaken, regardless of method or exact sample location, there is an average within formation error of ± 1.25‰. This is important, as is constitutes a reasonable level of uncertainty for phosphorus cycling studies which need to estimate bedrock δ¹⁸O‐PO₄ composition based on the literature. In combination, this data set presents 284 δ¹⁸O‐PO₄ values from 56 countries; a comprehensive starting point for researchers interested in understanding bedrock end member δ¹⁸O‐PO₄.     

Effects of floating Azolla on phosphorus fluxes and recovery from former agricultural lands in wetland microcosms

The long-term fertilization results in accumulation of phosphorus especially in the top layer of the soils. Inundation of agricultural lands leads to a switch to anaerobic soil condition, causing reduction of iron and leaching of phosphate simultaneously. From the ecological and environmental perspective, high nutrients flux especially phosphorus will increase the possibility of eutrophication in aquatic system. The fern Azolla had a good potential to adsorb phosphorus, it also has distinctive nitrogen-fixing capacity. We conducted a 10-week aquarium experiment to investigate the phosphorus release capacity from two agricultural soils in the Netherlands with different Fe and P concentrations but comparable Fe/P ratios. Besides, the research questions rose to whether Azolla could use the mobilized phosphate released from the soils for growth. We also tried to find an effective indicator to estimate the actually phosphate mobilization from sediment to water layer. Results showed that the soils with high Fe and P concentrations had higher phosphate release rate compared with the soil with low Fe and P concentrations. Pore water Fe: PO₄^3? ratios were valid to identify P release to surface water, when the Fe: PO₄^3? ratios less than 8 mol mol^?1 substantial phosphorus mobilization occurred. The conclusions showed that the actual mobilization of phosphate is more important than the phosphorus retained in the sediments for the internal PO₄^3? fluxes. From 10-week experimental results, we found that Azolla can reuse the phosphate retained in soils thus removed the mobilized phosphate in a moderately low surface water nutrient loading.     

Quantification of individual phosphorus species in sediment: a sequential conversion and extraction method

Common sequential phosphorus (P) extraction methods are not specific to particular chemical species and have several limitations. This work presents the first chemical method for quantification of individual mineral and sorbed P species. It was developed by combining a conversion technique with a sequential extraction procedure. Mangrove sediments with different characteristics were incubated in pH‐adjusted 0.01 M CaCl₂ with and without reference material additions of octacalcium phosphate (Ca₈H₂(PO₄)₆·5H₂O; OCP), hydroxyapatite (Ca₅(PO₄)₃OH), strengite (FePO₄·2H₂O) or variscite (AlPO₄·2H₂O). The changes in soluble phosphate concentration were measured in the supernatant solution, while pH‐induced variations in P composition were determined by subsequent sequential extraction of the sediments. Dissolved phosphate concentration was controlled by adsorption below pH 7.8. Above this pH, soluble phosphate concentration was governed by OCP, which was qualitatively determined by plotting the experimental values of pH + pH₂PO₄ and pH – 0.5 pCa on a solubility diagram including the isotherms of known crystalline phosphate compounds. In contrast to the often‐predicted slow dissolution rate of crystalline phosphates in soils or sediments, drastic changes in P composition by dissolution, precipitation and adsorption processes were detected after 7 days. These were mainly not observed indirectly by changes in dissolved phosphate through adsorption effects, but were determined quantitatively by subsequent sequential extraction, thus enabling the quantification of individual species. Evaluation of the method was performed by standard addition experiments. Besides P species quantification, the method provides the means for other applications, such as the determination of P mineral dissolution kinetics in soils and sediments, the prediction of P composition in changing environmental settings and the refinement of theoretical models of phosphate solubility in soil and sedimentary environments.     

Short‐Term Effect of Lime,Phosphorus, and Iron Amendments on Water‐Extractable Lead and Arsenic in Orchard Soils

Abstract

Lead arsenate was extensively used to control insects in apple and plum orchards in the 1900s. Continuous use of lead arsenate resulted in elevated soil levels of lead (Pb) and arsenic (As). There are concerns that As and Pb will become solubilized upon a change in land use. In situ chemical stabilization practices, such as the use of phosphate‐phosphorus (P), have been investigated as a possible method for reducing the solubility, mobility, and potential toxicity of Pb and As in these soils. The objective of this study was to determine the effectiveness of calcium carbonate (lime), P, and iron (Fe) amendments in reducing the solubility of As and Pb in lead‐arsenate‐treated soils over time. Under controlled conditions, two orchard soils, Thurmont loam (Hapludults) and Burch loam (Haploxerolls), were amended with reagent‐grade calcium carbonate (CaCO₃), iron hydroxide [Fe(OH)₃], and potassium phosphate (KH₂PO₄) and incubated for 16 weeks at 26°C. The experimental results suggested that the inorganic P increased competitive sorption between H₂PO₄ ^? and dihydrogen arsenate (H₂AsO₄ ^?), resulting in greater desorption of As in both Thurmont and Burch soils. Therefore, addition of lime, potassium phosphate, and Fe to lead‐arsenate‐contaminated soils could increase the risk of loss of soluble As and Pb from surface soil and potentially increase these metal species in runoff and movement to groundwater.     

Oxygen isotope ratios of plant available phosphate in lowland tropical forest soils

Phosphorus (P) cycles rapidly in lowland tropical forest soils, but the process have been proven difficult to quantify. Recently it was demonstrated that valuable data on soil P transformations can be derived from the natural abundance of stable oxygen isotopes in phosphate (δ¹⁸O_P). Here, we measured the δ¹⁸O_P of soils that had received long-term nutrient additions (P, nitrogen, and potassium) or litter manipulations in lowland tropical forest in Panama and performed controlled incubations of fresh soils amended with a single pulse of P. To detect whether δ¹⁸O_P values measured in the incubations apply also for soils in the field, we examined the δ¹⁸O_P values after rewetting dry soils. In the incubations, resin-P δ¹⁸O_P values converged to ∼3.5‰ above the expected isotopic equilibrium with soil water. This contrasts with extra-tropical soils in which the δ¹⁸O_P of resin-P matches the expected equilibrium with soil water. Identical above-equilibrium resin-P δ¹⁸O_P values were also found in field soils that did not receive P additions or extra litter. We suggest that the 3.5‰ above-equilibrium δ¹⁸O_P values reflect a steady state between microbial uptake of phosphate (which enriches the remaining phosphate with the heavier isotopologues) and the release of isotopically equilibrated cell internal phosphate back to the soil. We also found that soil nutrient status affected the microbial turnover rate because in soils that had received chronic P addition, the original δ¹⁸O_P signature of the fertilizer was preserved for at least eight weeks, indicating that the off-equilibrium δ¹⁸O_P values produced during microbial phosphate turnover was not imprinted in these soils. Overall, our results demonstrate that ongoing microbial turnover of phosphate mediates its biological availability in lowland tropical soils.     

Biological decay of 18O-labeled phosphate in soils

There is an increasing demand to develop a means to trace phosphorus (P) movement through the environment as excessive inputs of P have led to the eutrophication of many fresh water bodies. ¹⁸O labeled phosphate has been suggested as a potential tool for tracing P, and other researchers are using information from natural abundance ¹⁸O levels of phosphate to study phosphorus cycling. The objective of this research was to determine the rate of biological de-labeling of ¹⁸O in soils. This objective was achieved using a laboratory incubation study in which three silt-loam textured soils were incubated with 250 mg kg⁻¹ P¹⁸O₄-P for a period of 3, 10, 30, or 50 d. The incubations were conducted on both sterilized and unsterilized soils. Following incubation, phosphate from soils was extracted with a modified Bray extractant and analyzed using electrospray ionization mass spectrometry to determine the distribution of labeled phosphate species. The half-life of P¹⁸O₄ in the non-sterile soils ranged from 15 to 22 d, while there was no observed P¹⁸O₄ de-labeling in sterile soils after 50 d. A parameterized numerical model was developed which provided insight into the dynamics of the individual labeled phosphate species, including their half-lives and relative concentrations across the incubation period. The use of P¹⁸O₄ may be useful in areas where use of radioisotopes of P is restricted, and P¹⁸O₄ has potential to be useful to elucidate the dynamics of the P cycle in soils.     

An anion resin membrane technique to overcome detection limits of isotopically exchanged P in P-sorbing soils

Isotopically exchanged phosphorus is difficult to determine in soils that strongly sorb P (so that there is little P in solution) and in soils with large concentrations of colloidal P in soil suspensions. A method is proposed in which anion exchange membranes (AEM) are added to the soil suspension after an initial period of isotopic exchange with ³²P‐labelled phosphate ions. Isotopically exchanged P, termed E_AEM, is calculated from the ratio of labelled phosphate ions to the total phosphate ions on the membrane. The E_AEM was compared with the E value measured in an aqueous soil extract (E_{Water extract}) for 14 soils with different degrees of P sorption. The two methods gave similar results in soils with large P concentrations in an aqueous soil extract. However, E_{Water extract} values significantly exceeded the E_AEM values by up to 18‐fold when soluble P was near the determination limit (0.008 mg P l^?1). In a second experiment, two Ferralsols received further P from inorganic and plant sources and were incubated for 7 days. Treatment effects on labile P were erroneous as detected by the E_{Water extract} but were significant as detected with the AEM method. Third, E_AEM values were followed in a Lixisol and a Ferralsol which received labelled phosphate ions with carrier just before the beginning of a 23‐day incubation. The approximate recovery of added inorganic P in the E_AEM value suggested that this method adequately samples labile P in P‐sorbing soils. All these results showed that errors in the determination of E values for soils with very small concentrations of P in the soil solution are reduced using the proposed method.     

Dual isotope and isotopomer measurements for the understanding of N2O production and consumption during denitrification in an arable soil

The aim of our research was to obtain information on the isotopic fingerprint of nitrous oxide (N₂O) associated with its production and consumption during denitrification. An arable soil was preincubated at high moisture content and subsequently amended with glucose (400 kg C ha^?1) and KNO₃ (80 kg N ha^?1) and kept at 85% water‐filled pore space. Twelve replicate samples of the soil were incubated for 13 days under a helium‐oxygen atmosphere, simultaneously measuring gas fluxes (N₂O, N₂ and CO₂) and isotope signatures (δ¹⁸O‐N₂O, δ¹⁵N^bulk‐N₂O, δ¹⁵N^α, δ¹⁵N^β and ¹⁵N site preference) of emitted N₂O. The maximum N₂O flux (6.9 ± 1.8 kg N ha^?1 day^?1) occurred 3 days after amendment application, followed by the maximum N₂ flux on day 4 (6.6 ± 3.0 kg N ha^?1 day^?1). The δ¹⁵N^bulk was initially ?34.4‰ and increased to +4.5‰ during the periods of maximum N₂ flux, demonstrating fractionation during N₂O reduction, and then decreased. The δ¹⁸O‐N₂O also increased, peaking with the maximum N₂ flux and remaining stable afterwards. The site preference (SP) decreased from the initial +7.5 to ?2.1‰ when the N₂O flux peaked, and then simultaneously increased with the appearance of the N₂ peak to +8.6‰ and remained stable thereafter, even when the O₂ supply was removed. We suggest that this results from a non‐homogenous distribution of NO in the soil, possibly linked to the KNO₃ amendments to the soil, causing the creation of several NO pools, which affected differently the isotopic signature of N₂O and the N₂O and N₂ fluxes during the various stages of the process. The N₂O isotopologue values reflected the temporal patterns observed in N₂O and N₂ fluxes. A concurrent increase in ¹⁵N site preference and δ¹⁸O of N₂O was found to be indicative of N₂O reduction to N₂.     

Phosphate solubilizing bacteria and fungi in desert soils: species,limitations and mechanisms

Desert soils are infertile, and the ability to improve them by P-fertilization is limited by the solubility of phosphate. We aimed to understand the function of phosphate solubilizing bacteria and the mechanisms behind phosphate solubilization in desert soils. Vegetated and barren desert soils, mine spoil and a fertile temperate grassland loam were sampled. Bacteria and fungi were isolated and identified, and their phosphate-solubilizing abilities were measured in vitro. The release of plant available PO₄, SO₄, NO₃ and NH₄ from desert soils did not compare with that of a grassland soil. Desert soils had substantially lower solubilization than grassland, 162 and 99–121 µg PO₄-P g^?1 dry soil, respectively. Phosphate-solubilizing bacteria and fungi were inhabiting the soils. Si addition increased phosphate solubilization of fungi by 50%. The isolated microbes were shown, using ³¹P nuclear magnetic resonance (NMR) analysis, to rapidly take-up both intracellular and extracellular phosphate during the phosphate solubilizing process. Desert soil had potentially active microbial populations that are capable to solubilize inorganic phosphorus; S and Si as the limiting factors. Acidification as the main mechanism to solubilize mineral phosphate was not as evident in our desert soils as in former studies dealing more fertile soils.     

Remediation of a Magnesium‐Contaminated Soil by Chemical Amendments and Leaching

The deposition of magnesium (Mg)‐rich dust from magnesite mining activities has resulted in serious land degradation. However, the main factors limiting plant growth in Mg‐contaminated soils are unclear. Moreover, little information is available on the remediation of Mg‐contaminated soils. In this study, remediation of soils contaminated with Mg‐rich dust was investigated in a pot experiment using maize as the indicator plant. There were five treatments: (i) control; (ii) leaching; (iii) application of CaCl₂; (iv) leaching + CaCl₂ application; and (v) application of Ca(H₂PO₄)₂ · H₂O. Soil properties and growth of maize (Zea mays L.) seedlings were measured. Leaching alone significantly decreased soluble Mg concentration. Leaching + CaCl₂ application greatly increased exchangeable Ca concentration and decreased soil pH by 0·3 units. Application of CaCl₂ alone increased soluble Mg concentration sharply, which directly inhibited the germination of maize seeds. Application of Ca(H₂PO₄)₂ · H₂O significantly increased the concentrations of exchangeable Ca and available phosphorus and decreased soil pH by 1·7 units. The biomass of maize seedlings increased in the order of control = leaching < leaching + CaCl₂ < < Ca(H₂PO₄)₂ · H₂O. These results suggested that the plant growth in Mg‐contaminated soils was limited primarily by Ca deficiency and secondarily by high soil pH when exchangeable Ca was sufficient. High soil pH suppressed plant growth probably mainly by inhibiting phosphate uptake from the soil. Applying acid Ca salt with low solubility is an attractive option for the remediation of Mg‐contaminated soils. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of phosphite phosphorus on alfalfa growth

Abstract

Phosphite (PO^‐3‐P) was compared to phosphate (PO₄ ^‐3‐P) at 2.5, 5, 10, 20, and 40 mg ? kg^‐1) in a P‐deficient soil in a greenhouse pot study with alfalfa (Medicago sativa L.). Alfalfa growth (g dry matter pot^‐1) and ? accumulation (g ? pot^‐1) were measured by clipping each pot 3 times on 30‐day intervals. Although there was a significant (P<.05) growth response to ? during the first 30‐day growth period, it was significantly less from PO₃ ^‐3 than from PO₄ ^‐3‐P at the 10, 20, and 40 mg kg^‐1 rate. The growth response to ? continued through the next 60 days with only the 40 mg kg^‐1 PO₃ ^‐3‐P being significantly lower than PO₄ ^‐3‐P. During the first and second growth periods, there were no significant effects of PO₃ ^‐3‐P on total ? uptake, as compared to PO₄ ^‐3‐P, except at 40 mg kg^‐1. Tissue concentrations of PO₃ ^‐3‐P at 40 mg kg^‐1 dropped successively from 150, to 50, to 10 ppm during the first, second, and third growth periods. This suggests that 40 mg kg^‐1 of soil‐applied PO₃ ^‐3‐P had largely oxidized to PO₄ ^‐3‐P by 90 days after application.     

Influence of phosphorus application on growth and cadmium uptake of spinach in two cadmium‐contaminated soils

A pot experiment was conducted to investigate the influence of phosphate (P) application on diethylene triamine pentaacetic acid (DTPA)–extractable cadmium (Cd) in soil and on growth and uptake of Cd by spinach (Spinacia oleracea L.). Two soils varying in texture were contaminated by application of five levels of Cd (NO₃)₂ (0, 20, 30, 40, and 60 mg Cd kg^–1). Three levels of KH₂PO₄ (0, 12, and 24 mg P kg^–1) were applied to determine immobilization of Cd by P. Spinach was grown for 60 d after seeding. Progressive contamination of soils through application of Cd affected dry‐matter yield (DMY) of spinach shoot differently in the two soils, with 67% reduction of DMY in the sandy soil and 34% in the silty‐loam soil. The application of P increased DMY of spinach from 4.53 to 6.06 g pot^–1 (34%) in silty‐loam soil and from 3.54 to 5.12 g pot^–1 (45%) in sandy soil. The contamination of soils increased Cd concentration in spinach shoots by 34 times in the sandy soil and 18 times in the silty‐loam soil. The application of P decreased Cd concentration in shoot. The decrease of Cd concentration was higher in the sandy soil in comparison to the silty‐loam soil. Phosphorus application enhanced DMY of spinach by decreasing Cd concentration in soil as well as in plants. The results indicate that Cd toxicity in soil can be alleviated by P application.     

Uptake of residual phosphate and freshly applied diammonium phosphate by Lolium perenne and Trifolium repens

Residual fertilizer phosphorus (residual P) may significantly contribute to crop P nutrition. To test this hypothesis, a pot experiment was conducted with ryegrass (Lolium perenne) and clover (Trifolium repens) grown separately on three different soils which either had not received P fertilizer for at least nine years (0F) or had received P fertilizer equivalent to crop P off‐take (F). Soils in the pot experiment were given either none (0F, F) or a single rate of 15 mg P (kg soil)^–1 as diammonium phosphate (0F+DAP, F+DAP). In the treatments 0F+DAP and F+DAP DAP had been labeled with ³³PO₄ while in the treatments 0F and F the pool of available soil P had been labeled with carrier‐free ³³PO₄. This allowed estimating the quantities of P in plant dry matter that derived from native soil P, residual fertilizer P or fresh fertilizer P. Fourteen to 62 % of the P in the above ground biomass of white clover or perennial ryegrass were derived from residual P whereas 7 to 28 % were derived from freshly applied DAP. The proportion of P derived from residual P was correlated to the total amount of P fertilizer added to the soils, while the proportion of P derived from DAP was correlated to the concentration of P in the soil solution of the 0F and F soils.     

Perspectives from the Fritz‐Scheffer Awardee 2019. Oxygen isotopes in phosphate—The key to phosphorus tracing?

Phosphorus (P) tracing in natural environments is challenging, lacking stable P isotopes Oxygen isotope ratios in phosphate (δ¹⁸O_P) represent a novel tool for tracing the biological cycling of P from the global scale down to hotspots at the micro‐scale and within particular soil compartments such as aggregates or pores. Despite the small number of studies available so far, existing data indicate that δ¹⁸O_P values point to where, at what extent and how efficiently P is recycled in soils.     

Comparison of different extractants for the determination of inorganic sulphate in gypsum‐free agricultural soils

Four frequently used extractants (H₂O, 0.1 M NaCl, 0.016 M KH₂PO₄, and 0.5 M NaHCO₃) as well as different extraction conditions have been tested for sulphate extraction from gypsum‐free agricultural soils. Water is the preferable extractant for soils with pH > 6. Two extraction steps have to be carried out for complete extraction (> 95%). A 0.016 M KH₂PO₄ solution was found to be the most efficient extractant for soils with a pH < 6 within a single extraction step. A shaking frequency of 170 min^‐1 and a duration of extraction of 4 hours are the optimized conditions for the sulphate extraction with H₂O and KH₂PO₄ solution.     

Evaluation of the factors affecting direct polarization solid state 31P-NMR spectroscopy of bulk soils

³¹P‐NMR spectroscopy on bulk soils is a powerful tool for the identification of the different phosphorus forms in soils and for the evaluation of the dynamics of soil P. Up to now the majority of the papers dealt with liquid state ³¹P‐NMR spectroscopy on soluble soil organic substances. Only few papers were addressed to the study of the different phosphorus forms directly in bulk soils. In the present paper, some organic and inorganic phosphates of known structures, which are likely to be present in soil systems, were studied by direct polarization (DP) magic angle spinning (MAS) ³¹P‐NMR spectroscopy in order to understand the electronic factors responsible for chemical shifts of the phosphorus (P) nucleus and to serve as guidelines to assign P resonances in soil spectra. Number of hydrating water molecules, type of counter‐cation, degree of covalence, and spatial conformation of P in phosphate structures were found to affect signal positions in ³¹P‐NMR spectra. Both hydrating water and increase in degree of covalence of the X‐O‐P bonds (X=H, Na) enhanced the electronic density (E_D) around P, thereby producing up‐field shifts in ³¹P‐NMR spectra. The exchange of the Na⁺ counter‐cation with NH₄⁺ resulted in an increase of the cation potential (P_C) that is a measure of the cation polarizing power, and induced a down‐field shift of P signals, due to a corresponding reduction in E_D around the P nucleus. Both NMR down‐ and up‐field shifts were observed in organic phosphates, and were dependent on the spatial orientation of the phosphate groups that may have been fixed anisotropically in the solid state. Based on the factors that influence P chemical shifts for standard phosphates, attempts to assign ³¹P‐NMR signals in the spectra of five different unperturbed bulk soils were made.     

Fractionation and distribution of selenium in soils

Abstract

A modified selenium (Se) fractionation procedure was used to study Se distribution in three soils (two silt loams and one silty clay). This sequential procedure consisted of: i) 0.2 M potassium sulfate (K₂SO₄)‐soluble fraction, ii) 0.1 M potassium dihydrogen phosphate (KH₂PO₄)‐exchangeable fraction, iii) 0.5 M ammonium hydroxide (NH₃H₂O)‐soluble fraction, iv) 6 M hydrochloric acid (HCl)‐extractable fraction, and v) residual fraction digested with perchloric (HClO₄) and sulfuric (H₂SO₄) acids. The fractionation procedure had high recovery rates (92.5 to 106%). The Se distribution in soil was controlled by soil properties, such as pH, oxide, clay, and calcium carbonate (CaCO₃) contents. In the untreated soil samples, residual Se fraction was dominant. In the Se‐enriched soils, the silty clay had significantly more Se in the NH₃H₂O and residual fractions while in the two silt loams the largest were KH₂PO₄ and residual fractions. The Se availability in the two silt loams was higher than in the silty clay. The Se availability pattern in the untreated soils was: unavailable (HCl + residual fractions) >> potentially available (KH₂PO₄ + NH₃H₂O fractions) > available (K₂SO₄ fraction), while in the Se‐enriched soils it was potentially available > unavailable > available.     

Comparison of Mehlich 3‐ and bray 1‐extractable phosphorus levels in a Starr clay loam amended with poultry litter

Abstract

Two soil tests commonly used to characterize the availability of soil phosphorus (P) are Bray P1 (B1) and Mehlich 3 (M3) extradants. The objective of this investigation was to compare M3‐ and B1‐extractable P levels in a Starr clay loam (fine‐loamy mixed thermic Fluventic Dystrochrepts) amended with relatively low to high surface applications of poultry litter (PL). The following eight treatments were applied to the soil for pasture renovation in 1991 and 1992: 1) a control, 2) P application as monocalcium phosphate [Ca(H₂PO₄)₂‐H₂O], 3) P application as Ca(H₂PO₄)₂‐H₂O and nitrogen (N) application as ammonium nitrate (NH₄NO₃) and urea, and 4) five levels of poultry litter (PL) based on N content. The five P levels as PL were 56.6, 113.2, 169.8, 226.4, and 283.0 kg ha^‐1 in 1991, and 49.2, 98.5, 147.7, 196.9, and 246.1 kg ha^‐1 in 1992, respectively. Soil samples were obtained from the 0‐ to 5‐cm layer in the spring of 1992 and 1993 for extractable P determination. Levels of P extracted from all treatments by the M3 and B1 extractants were linearly correlated both years (r²=0.96 in 1991 and r²=0.99 in 1992). However, M3 extracted more P from the PL treatments, whereas B1 extracted more P from the control, P application, and N and P application treatments. Curvilinear relationships were obtained between P uptake and levels of either M3‐ or B1‐extractable P for the PL treatments (r²=0.713 for M3 and 0.663 for B1 in 1991 and r²=0.925 for M3 and 0.933 for B1 in 1992). These close relationships in 1992 between extractable P and uptake of P for the PL‐ treated soils indicate that both the B1 and M3 extractants could be used to evaluate excess P in PL‐amended soils.     

The Effect of Vermicompost on Transformation Rate of Available P Applied as Chemical Fertilizer in a Calcareous Clay Soil

The effects of vermicompost (VC) (0% and 1% w/w) on treated calcareous clay soil with 0 and 50 mg phosphorus (P) kg^?1 as calcium phosphate [Ca(H₂PO₄)₂.H₂O] was investigated. The soil samples were incubated for 7, 30, 60, 120, and 150 d at 25 ± 1°C and Olsen-P was measured after each incubation time. Results showed that Olsen-P increased 36% and 38% after VC addition in treated soil with 0 and 50 mg P kg^?1, respectively. Recovery of Olsen-P in treated soils with VC, combined fertilizer VC + P, and fertilizer P was 42%, 42%, and 17%, respectively. The rate coefficient in treated soils with fertilizer, VC, and combined fertilizer VC + P was 0.033, 0.026, and 0.023 mg kg^?1 d^?1/2, respectively. It seems that the process that leads to the decrease in available P in amended soils, is controlled by P diffusion into sorption sites in micropores of aggregates.     

pH对磷钾铝石形成的影响

Effects of column temperature and flow rate on separation of organic acids were studied by determining nine low-molecular-weight organic acids on reversed-phase C18 column using high performace liquid chromatography(HPLC) with a wavelength of UV(ultraviolet)214 nm and a mobile phase of 18 mmol L^-1 KH2PO4 buffer solution (pH2.1).The thermal stabiltiy of organic acids was determined by comparing the recoveries of organic acids in different temperature treatments.The relationships between column temperature,flow rate or solvent pH and retention time were analyzed.At low solvent pH,separatioin efficiency of organic acids was increased by raising the flow rate of the solvent because of lowering the retention time or organic acids.High column temperature was unfavorable for the separation of organic acids.The separating effect can be enhanced through reducing column temperature in organic acid determination due to increasing retention time.High thermal stability of organic acids with low concentrations was observed at temperature of 40℃-45℃,Sensitivity and separation effect of organic acid determination by HPLC were clearly improved by a combination of raising flow rate and lowering column temperature at low solvent pH.