The influence of radioactive phosphate level on the absorption of phosphate by plants and on the determination of labile soil phosphate

Summary Previous work has suggested that the presence of P³² in fertilizers inhibits the uptake of the applied phosphate from the soil by plants, and also that if the applied phosphate is not incorporated uniformly in the soil there will be preferential uptake from regions of low specific activity. This made it desirable to determine the effect of P³²-level on phosphate uptake and the determination ofL-values in pot experiments in which the labelled phosphate source is added as discrete particles of the phosphate form of an anion-exchange resin.Increasing the level of P³² from 0.05 to 1.25 mo per gram of phosphorus in the added phosphate did not have a significant effect on the fresh weight, dry weight or total phosphorus uptake of the ryegrass crop. The measuredL-value showed a significant increase, about 15 per cent for a five-fold increase in P³² level, on each of the four soil types used, as would be expected if P³² depressed the uptake of labelled fertilizer phosphate.Although a significant effect of P³² was observed this does not invalidate a comparison of soils with respect toL-value.     

Native soil and fresh fertilizer phosphorus uptake as affected by rate of application and P fertilizers

³²P labelled fertilizers were used to measure native soil and fresh fertilizer phosphorus uptake byLolium perenne L. in greenhouse experiments. The P source evaluation was carried out for multiple rates of application for a standard P fertilizer (DAP) on low and medium soil P levels and for North Carolina rock phosphate (RP) at the medium soil P level only. At the low soil P level, the native P uptake increase was independent of P-DAP applied, and represented 19% of the nil P uptake. At the medium soil P level, the variability of the native soil as a nutrient P source depended on the phosphate fertilizer applied, and the rate of application. Consequently the amount of total P uptake could conceal differences in P fertilizer evaluations as the nutrient P source. Fresh P uptake increased linearly with the rates of P applied as standard or tested P fertilizer. The comparison of various P sources by means of fresh P uptake ratio (i.e. fresh P uptake from tested phosphate divided by fresh P uptake from standard phosphate) was independent of the rate of application. It was therefore suggested that various phosphorus sources be evaluated by measuring the fresh P uptake for a single rate of application.     

Differences between Natural Populations of Trifolium repens L. in Response to Mineral Nutrients

The response to phosphate of samples of natural local populationsof T. repens, collected from soils of different phosphate contents,was determined over a wide range of phosphate levels in sandculture. A marked correlation (r = +0.96) was found betweenresponse of populations to phosphate at low phosphate levelsand the amount of phosphate in the native soil of the population;populations from soils high in phosphate showed larger reductionsin growth at low phosphate levels than did those from soilslow in phosphate. It is concluded that response to phosphateis one aspect of the physiological adaptation of the naturalpopulations to particular soil conditions. Populations alsodiffered in response to very high levels of phosphate, and inthe range of phosphate levels tolerated; the possible adaptivesignificance of this is discussed in relation to the phosphatebuffering effects of the native soils. Populations tolerantof low phosphate levels had higher concentrations of phosphorusin the shoot material, and a higher uptake of phosphorus perunit weight of roots, than did intolerant populations. The differencesbetween populations in response to phosphate are therefore believedto be due to difference in uptake rather than to differencein metabolic requirement of phosphate.     

Isoionic exchange of phosphate in paddy soils

Summary Isoionic exchange of phosphate was studied in aqueous suspensions of two British and four paddy soils under anaerobic and aerobic conditions. Linear relationships between the logarithms of the P³²-concentration in solution and the logarithm of time elapsed after application of P³² to the suspensions were observed under the two contrasting conditions. Using a rate equation for recrystallization, the calculated reaction orders ranged from 4.0 to 6.8. Based on these values it is believed that hydroxyapatite was present in at least some of the soils. Anaerobic treatment of the soils produced higher pH, phosphorus and iron in solution, isotopically exchangeable phosphorus, and reaction order values than did aerabic treatment.     

The supply of nutrient ions by diffusion to plant roots in soil

Summary Measurements were made of the diffusion of P³²-labelled phosphate to single roots of onion, leek and rye-grass growing in an Upper Greensand sandy loam (UGS) and a Coral Rag Clay (CRC) to which different amounts of phosphate had been added. Concentration-dependent diffusion coefficients for phosphate ions in the soils were calculated from phosphate desorption isotherms in calcium chloride. The experimental uptake by roots of known dimensions was compared with supply expected by diffusion to a cylindrical model root of the same dimensions. Allowance was made for absorption by the root hairs on rye-grass roots. Phosphate absorption by a cm length of intact root was found to continue for at least 16 days for onion, 10 days for leek and 5 days for rye-grass. Over a wide range of conditions (phosphate concentrations, soils, plant species), experimental uptake was close to the maximum calculated to be possible for the diffusion model except on one soil at a high level of phosphate. Although the concentration of phosphate in the soil solution at the root boundary appeared to be reduced to a small fraction of the initial concentration, because of the extreme non-linear form of the desorption isotherm less than 1/2 of the P³² exchangeable pool of P was considered to contribute to diffusion. Phosphate uptake by rye grass could only be accounted for if the root hairs were active. Although only a small fraction of the uptake is derived from inside the root hair cylinder, this increases the efficiency of the central root 2.3 fold by providing a zone close to the central root through which phosphate moves very readily.     

A study of the mechanism of soil-phosphate uptake in relation to plant species

Summary The concept of plant available and unavailable soil phosphate has been examined by growing a range of plant species in soils well mixed with carrier free P³².It was found that although activities of 5–100 µC per kg soil caused changes in dry matter and P uptake, they had no influence on the specific activity per unit dose.Only small and agriculturally insignificant differences have been found in the proportion of soil P to added P³² taken up by the different species in the acid and neutral soils employed. It is considered that such differences as there are may be due to exchange of phosphate between seed and soil. Marked differences, however, occurred in the total phosphate absorbed by the crops.These results support the view that a fraction of the soil phosphate exists in a labile pool which will exchange with added P³² and will maintain a definite equilibrium potential in the soil solution. Plants do not appear to have a means of increasing significantly the size of the labile pool under the experimental conditions described. The disparity, however, between the total phosphate uptake especially in very deficient soils does suggest that certain species are more efficient than others in the absorption of soil phosphate at low potential.     

Effect of different soil and nitrogen fertilizer conditions on 32P-labelled phosphate uptake by two grass species

In two years of trials, roots of ryegrass took up more ³²P-labelled phosphate than roots of fescue. Application of 672 kg N ha^-1 increased phosphate absorption compared with application of 112 kg N ha^-1. Roots in mineral soil absorbed more phosphate than those in peat soil. In both soils uptake decreased as depth of phosphate injection increased from 5 to 30 cm. An interaction occurred whereby roots in the intermediate depth (10–22-5 cm) in peat absorbed less phosphate than in mineral soil and this was apparently unrelated to the exchange or sorption properties of the soil.     

Direct evidence on participation of root hairs in phosphorus (32P) uptake from soil

Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer ³²P (185 kBq g^-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the ³²P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total ³²P uptake). The uptake of³² P from soil via the root hairs only was quantified by measuring activity of ³²P in the plant shoot (³²P uptake only via root hairs).The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the ³²P labelled soil, the quantity of ³²P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of ³²P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.     

In vitro studies of p32 uptake in mouse liver mitochondria

Isolated mouse liver mitochondria were incubated in two types of P³²-labelled sucrose-phosphate buffers. The first contained no added ATP or oxidizable substrate. The second contained added ATP. Samples were taken at specified times, up to 60 minutes, and analyses were made of the mitochondrial TCA-soluble inorganic P³² and the total mitochondrial residue P³¹ and P³². The results of the analyses showed that when the phosphorus inhibition index (the ratio of the amount of incubation inorganic phosphorus to the square of the amount of tyrosine in the mitochondria) was high, inorganic P³² uptake was low and vice versa. In accordance with established data, increased P³² uptake was obtained when ATP was added. ATP was found to stabilize the turnover of TCA-insoluble residue phosphorus as well as to maintain the TCA-soluble orthophosphate pool. These results support findings regarding the inhibitory and controlling effects of incubation medium phosphate in the regulation of inorganic phosphorus uptake.     

Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.     

Effects of soil density on yield and fertilizer requirement of sugar beet

Three experiments (1971–3) on medium-textured soils of low organic-matter content examined the effects of soil density on seedling emergence and yield of sugar beet, and the interactions between soil density and the requirement for nitrogen (N) and phosphate (P₂O₅) fertilizers. Three soil densities were produced by rolling and harrowing, dutch harrowing twice, and power harrowing once; each was tested with 75, 150 and 225 kg ha^-1 N and 0, 100 and 200 kg ha^-1 P₂O₅. Most seedlings emerged on the medium-density treatment produced by dutch harrowing twice. The crop in the least dense soil treatment, produced by power harrowing once, consistently yielded slightly more sugar than the medium-density treatment, which yielded significantly more than the most dense treatment. In dense soil, created by rolling and harrowing, there was a response to more fertilizer N than the less dense ones; the crop did not respond to phosphate fertilizer. Mechanical impedance, not toxic concentrations of ethylene in the soil atmosphere, appeared to be the cause of reduced yield on the dense soils; these reductions were considerable at dry soil bulk densities in excess of 1·5 g ml^-1.     

Relative efficiency of rock phosphate and superphosphate for cowpea (Vigna sinensis Savi) fodder

Summary Cowpea responded well to phosphorus and P application increased green-fodder production and P uptake. With respect to green-fodder production, the rock phosphate was found to be 50–55% as effective as superphosphate. Phosphorus when applied at 50 kg P₂O₅/ha as superphosphate or rock phosphate almost doubled the P uptake by the summer-season crop of cowpea as compared to no phosphorus control. The residual effect did not differ significantly on succeeding rainy-season crop of cowpea. Application of starter N increased the total green-fodder yield of two crops by 38 q/ha. re]19750710     

PHOSPHATE ACCUMULATION AND METABOLISM BY HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE) DURING DIEL VERTICAL MIGRATION IN A STRATIFIED MICROCOSM1

Diel vertical migration by Heterosigma akashiwo (Hada) Hada (Raphidophyceae) was monitored in a 1.5 in tall microcosm. Vertical stratification, with low salinity and low orthophosphate (P_i) concentration in the upper layer and high salinity and high P_i concentration in the lower layer, was simulated in the tank, analogous to summer stratification in the Seto Inland Sea. The phosphate metabolism of H. akashiwo during this vertical migration was studied using ³¹P-NMR spectroscopy. At night this species migrated to the lower phosphate-rich layer and took up inorganic phosphate (P_i) which then was accumulated as polyphosphate (PP_i) by an increase in the chain length of PP_i During the daytime this species migrated to the phosphate-depleted surface water and utilized the accumulated PP_i for photophosphorylation by decreasing the chain length of PP_i During the first night after the phosphorus was introduced to the previously impoverished waters, the cells took up inorganic phosphate, accumulating the new phosphorus nutrient internally as P_i But the cells did not convert P_i to PP_i presumably due to their lack of ATP. After the second day of the experiment, conversion of P_i to PP_i at night was much more rapid than on the first day, presumably due to increased ATP availability. Then the cycle continued, with uptake of P_i and conversion to PP_i at night at the bottom and its utilization during the day at the surface. These data suggest that the role of PP_i in the metabolism of this species appears to be as a phosphate pool which regulates the level of P_i and ATP in the cell. Diel vertical migration allows this red tide species to shuttle between the phosphate-rich lower layer and the photic upper layer in stratified waters. ³¹P-NMR is shown to be a valuable tool in studying the phosphorus metabolism in migrating organisms.     

Promotion of AM utilization through reduced P fertilization 2. Field studies

The hypothesis of this study was that cumulative P fertilization decreases the contribution of arbuscular mycorrhiza (AM) to crop growth and nutrient uptake in Northern European field conditions. The modes of action of P fertilization were evaluated through effects on mycorrhization, crop dependence on AM, and AM fungal (AMF) community. Field studies were carried out within long-term experiments on soils with low and intermediate initial content of extractable P, where no P fertilization and 45 kg ha^–1 a^–1 P were applied for 20 years. AM effectiveness in terms of growth and nutrient uptake of flax, red clover and barley, percentage root length colonized by AMF, P response of flax, and spore densities and species composition of the AMF communities, were assessed. In the soil with low initial P supply, cumulative P fertilization decreased AM contribution to crop growth and nutrient uptake. The higher AM effectiveness in soil with no added P compensated the cumulative P fertilization (soil P_H2O 2.5 v. 9.5 mg kg^–1) for flax, but not completely for clover. In contrast, barley obtained no benefit from AM at harvest and only a slight benefit from cumulated P. In the soil with intermediate initial P supply, AM reduced growth of flax and barley, especially with no added P, and no response to AM was obtained on clover due to retarded mycorrhization. Cumulative P fertilization reduced yield losses of flax by AM (P_H2O 18.8 v. 5.4 mg kg^–1), because fertilization inhibited mycorrhization. In both soils, root colonization and spore density were decreased by cumulative P fertilization, but no changes in AMF species composition were observed.     

The use of P32 in studies on the uptake of phosphorus by plants

Summary A fertilizer experiment with calcium phosphates labelled with P³² was carried out in pots with barley as a test plant. By determining the ratio of P³² in fertilizer and plant phosphorus after more than 7 weeks the content of exchangeable phosphate in the soil could be calculated. The results of the calculation were independent of the amount of phosphate added providing it was soluble.     

The influence of soil volume on the absorption of soil phosphorus by plants and on the determination of labile soil phosphorus

Summary In a pot experiment the soil volume available for ryegrass growth was varied. There was relatively greater uptake of phosphorus from the smaller amounts of soil and an increase in the pool of labile phosphorus (the L-value). It appears that an equilibrium exists in the soil between non-labile and labile phosphorus, and this equilibrium may be disturbed by the removal of phosphorus by plants.The variations of L-value with time followed a pattern indicating the initial influence of seed-borne phosphorus and slow isotopic exchange of the added carrier-free P³² with soil phosphorus.     

Exocellular and intracellular accumulation of lead in Pseudomonas fluorescens ATCC 13525 is mediated by the phosphate content of the growth medium

Pseudomonas fluorescens appears to elicit disparate lead detoxification mechanisms in phosphate-rich and phosphate-deficient media. When grown in the presence of 0.1 mM Pb2+ complexed to citrate, the sole source of carbon, only a slight diminution in cellular yield was observed in the former medium. However, in a phosphate-deficient milieu, lead imposed approximately a 30% reduction in bacterial multiplication. At stationary phase of growth, 72% of the metal was found in the bacterial cells from the phosphate-deficient medium, while that from phosphate-rich broth contained only 12.5%. The latter medium was characterized by an insoluble pellet that accounted for 73.5% of the lead. Although no citrate was detected in the phosphate-rich media after 40 h of incubation, only 72% of citrate was consumed even after 70 h of growth in the phosphate-deficient cultures. The inclusion of lead did not appear to enhance the production of either extracellular proteins or carbohydrates.     

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl₂ solution) varied ten-fold (averaging 2.3 mol g^–1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl₂ solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.     

A chemical and biological approach towards the definition of calcareous soils

Summary Several agricultural problems are associated with the presence of certain levels of CaCO₃ in soils. The level of CaCO₃ at which the phosphate fixation becomes an apparent agricultural problem, is thought to be an appropriate margine at which the soil can be considered calcareous. Thus, a set of soil mixtures, ranging in CaCO₃ content from 1 to 96% was prepared and used in a column study to determine the level at which the CaCO₃ fraction becomes a dominant factor controlling. P³² movement and distribution.Increasing the percentage of oolitic sand, in the soil mixture, from 1 to 10% caused a sharp drop in P³² movement with soil solution and any increase in CaCO₃ content above 10% did not show any further drop in P³² movement. The amount of P³² removed with the soil solution was generally low compared to that retained in soil columns. Studying the distribution of P³² in soil columns, after five displacements, has indicated that the migration of P³² from the top soil increased by increasing CaCO₃ from 1 and 2 to 6%. The amount of P³² removed was however retained in lower sections. A very sharp decrease in P³² migration from the top soil was observed when CaCO₃ content was raised from 8 to 10%.A similar picture was shown when the CaCO₃ material used was in clay size fraction. However the sharp increase in phosphate retention in top soil sections took place at CaCO₃ content of 8% rather than at 10%. A limit of 8 to 10% CaCO₃ was proposed as an appropriate margine for defining calcareous soils.     

紫色丘陵区玉米/大豆套作系统土壤磷吸附-解吸动力学

利用田间小区试验研究了玉米/大豆套作(M/S)、玉米单作(M)、大豆单作(S)3种栽培模式下施磷对土壤磷吸附解吸特征的影响.结果表明： 在不施磷处理下,M/S系统产量较M、S分别提高9.8%、79.1%,土地当量比为1.58;在施磷处理下,M/S系统产量较M、S分别提高10.4%、80.3%,土地当量比为1.62.与不施磷处理相比,施磷使M/S、M、S 3种栽培模式下的系统总产量分别提高12.7%、12.2%、17.6%.无论施磷与否,3种栽培模式下土壤最大缓冲容量(SBC)值均表现为M/S>M>S;但与施磷处理相比,不施磷处理下M/S、M、S栽培模式下土壤SBC值分别降低19.6%、30.3%、12.0%.各栽培模式在施磷处理下土壤单位吸附量的解吸量b值大小为M/S>M>S;与不施磷处理相比,施磷使M/S、M、S模式下b值分别增加10.9%、39.1%、-9.6%.土壤最大吸磷量(Q_m)及土壤磷解吸率也表现出相同的趋势.