Effects of deforestation on phosphorus pools in mountain soils of the Alay Range, Khyrgyzia

 The amount, quality and turnover of soil P is heavily influenced by changes in soil management. The objective of this study was to investigate the effects of deforestation and pasture establishment on the concentrations, forms and turnover rate of soil P in mountain soils of the Alay Range, Khyrgyzia. A sequential extraction was applied to distinguish soil P pools. We used particle-size fractionation to follow the dynamics of different P pools in soils under forest and pasture and ³¹P-NMR spectroscopy to investigate the structure of alkali-soluble P forms. In the A horizons of the forest soils, total soil P concentration was 1093 mg kg^–1, organic P (P_o) representing 46% of the total P. Deforestation followed by pasture establishment not only increased significantly (P<0.01) the total P concentration (1560 mg kg^–1) but also the contribution of P_o to total P was increased by 17%. Pasture soils had significantly higher P pools than forest soils except highly labile inorganic P (P_i NaHCO₃) and primary P_i (P_i HCl_dil). Both in forest and pasture soils stable P increased with decreasing particle size (coarse sand 50%, clay 80% of total P) and primary P decreased with decreasing particle size. Phosphate monoesters and diesters represented 80% of P identified by ³¹P NMR. Low monoester to diester ratios in the alkali extracts of forest and pasture soils indicate low microbial activity. This is consistent with high C/P_o ratios and high stable P_o concentrations in the fine earth of forest and pasture. Received: 10 March 1999     

Soil phosphorus dynamics in a long-term field experiment at Askov

Inorganic and organic soil P (P_i, P_o) fractions were followed monthly for 15 months in a 100-year-old, fertilization and crop-rotation experiment with the Rubaek-Sibbesen, macroporous resin method, the Olsen method, and the Hedley fractionation method. Resin P, and Olsen P had similar levels and variation patterns. They increased in spring after fertilization, decreased during summer and autumn, and increased again in winter after repeated slurry applications. Resin P_o decreased in spring and peaked in summer. The variation in time of the Hedley P_i and P_o fractions was relatively smaller and was neither related to season nor to fertilization. Unmanured soil contained much less total P than NPK and slurry-treated soils, but the differences in total P_i were greater than those in total P_o. Neither total P_i nor total P_o concentrations differed between NPK and slurry treatments, indicating that P_o in animal manure is quickly mineralized. All P_i and P_o fractions were smaller in unmanured than in fertilized treatments. These differences were relatively largest for resin P_i and resin P_o, i.e., the most labile fractions, and decreased for the medium and less labile P_i and P_o fractions. The reactions by resin P_i, Olsen P, and resin P_o to seasons and treatments indicate that these fractions are estimates of the most labile pools of P_i and P_o in soil, which make them relevant for shortterm studies. The medium and less labile P_i and P_o fractions of the Hedley fractionation method seem more relevant for long-term studies.     

Dynamics of phosphorus fractions in a savanna Alfisol under continuous cultivation

Abstract. We examined the dynamics of inorganic P (P_i) and organic P (P_o) pools of a savanna Alfisol under continuous cultivation complemented with nitrogen (N) and phosphorus (P) fertilizers with or without cow manure (D), using a modified Hedly fractionation. Continuous cultivation without P fertilizer decreased the concentration of P_i and P_o, pools including the residual P fraction compared with an unfertilized treatment with natural vegetation. Adding P fertilizers alone or in combination with D (P, NPK, D+P, D+NP and D+NPK) increased the concentration of labile P_i pools, but decreased the concentration of P_o, pools and the residual P fraction. The tendency of the residual P fraction to decrease along with P_o pools suggested that the residual P fraction was largely P_o. This is consistent with the correlation between the residual P fraction and total P_o (r=0.74^**) and the residual P fraction and organic carbon content (r=0.47^**). Analysis of the relationships between plant available P, estimated by resin-extractable P, and the more stable P_i and P_o, pools indicated that 85% of the variation in resin-P was atiributable to the hydroxide extractable P_i, (OH-P_i) and HCI-P_i which acted as sinks for fertilizer-P. The contribution of the residual P fraction to resin-P was indirect via bicarbonate-extractable P_o (HCO₃-P_o) and OH-P_o. These results also clarify why attempts to find P extraction methods which correlate well with response to P and to P uptake in the savanna have given inconsistent results.     

Sequentially extracted phosphorus fractions in peat‐derived soils

Phosphorus (P) forms were sequentially extracted from peat derived soils (Eutric Histosols and Gleysols) at eight sites in Saxony‐Anhalt (Germany) to disclose general differences in P pools between mineral and organic soils and to investigate effects of peat humification and oxidation in conjunction with land use and soil management on the P status of soils. Overall 29 samples providing a wide variety of basic chemical properties were subjected to the Hedley fractionation. The Histosol topsoils contained more total P (P_t) (1345 ± 666 mg kg^—1) than the Gleysol topsoils (648 ± 237 mg kg^—1). The predominant extractable fractions were H₂SO₄‐P (36—63 % of P_t) in calcareous and NaOH‐P_o (0—46 % of P_t) in non‐calcareous Histosols. These soils had large pools of residual P (13—93 % of P_t). Larger contents and proportions of P_o and of labile P fractions generally distinguished organic from mineral soils. Regression analyses indicated that poorly crystalline pedogenic oxides and organic matter were binding partners for extractable and non‐extractable P. Intensive management that promotes peat humification and oxidation results in disproportional enrichments of labile P fractions (resin‐P, NaHCO₃‐P_i, and NaHCO₃‐P_o). These changes in P chemistry must be considered for a sustainable management of landscapes with Histosols and associated peat derived soils.     

Changes in P fractions and sorption in an Alfisol following crop residues application

With the emphasis on sustainable agriculture, attention has been increasingly turning to recycling of crop residues as a component of fertility management strategies for tropical soils. We assessed the effects of soybean residue (SR) and wheat residue (WR) applied either alone or in combination with fertilizer P (FP) on dynamics of labile P, distribution of P fractions, and P sorption in a semiarid tropical Alfisol by conducting a 16 w long incubation experiment. The amount of P added through crop residues, FP or their combinations was kept constant at 10 mg P (kg soil)^–1. Addition of SR or WR resulted in net increase of labile inorganic (P_i) and organic P (P_o) and microbial P throughout the incubation period, except that the WR decreased labile P_i during first 2 w due to P_i immobilization. The P immobilization associated with WR addition was, however, offset when fertilizer P was combined with WR. Generally, the increases in labile‐P fractions were larger with the SR and SR+FP than with the WR and WR+FP. The sequential fractionation of soil P at the end of 16 w indicated that a major part of added fertilizer P transformed into moderately labile and stable P fractions as evident from the increased NaOH‐P_i and HCl‐P in the FP treatment. In contrast, the addition of SR and WR alone or in combination with FP favored a build‐up in NaHCO₃‐P_i and ‐P_o and NaOH‐P_o fractions while causing a decrease in NaOH‐P_i and HCl‐P fractions. The addition of these crop residues also effectively decreased the P‐sorption capacity and hence reduced the standard P requirement of the soil (i.e., the amount of P required to maintain optimum solution P concentration of 0.2 mg P l^–1) by 24%–43%. Results of the study, thus, imply that soybean and wheat crop residues have the potential to improve P fertility of Alfisols by decreasing P‐sorption capacity and by redistributing soil P in favor of labile‐P fractions and promoting accretion of organic P.     

Long-Term Phosphorus Desorption Using Dialysis Membrane Tubes Filled with Iron Hydroxide and its Effect on Phosphorus Pools

Many agricultural fields that have received long-term applications of phosphorus (P) often contain levels of P exceeding those required for optimal crop production. Knowledge of the effect of the P remaining in the soil (residual effect) is of great importance for fertilization management. Plant P availability of residual P in soils is usually estimated using successive cropping experiments carried out in field or greenhouse studies. As this approach is very expensive and time consuming, more rapid soil test methods that can approximate this biological measure are required. The objective of this paper was to use a different approach to evaluate P availability (desorption) over a long period of time instead of the classical means of extraction. Thus, a modified sequential P extraction procedure using dialysis membrane tube filled with ferric hydrate solution (DMT-HFO) was used on the long-term P fertilized soils that received differential P treatments (PoLo, P₁L₁ and P₂L₁) to determine the changes in the different P pools and to relate these P fractions with maize yield. In this study, the contribution of both the labile and non-labile Pi fractions in replenishing the solution Pi was significant where as the organic fractions appeared to have limited contributions in replenishing the solution P. Highly significant correlations were observed between dry matter yield and the P pools extracted by HFO-P_i (0.997*), HCO₃-P_i (r = 0.994**), OH-P_i (r = 0.969**), OH-P_o (r = 0.944**), D/HCl-Pi (0.991**), and C/HCl-P_i (r = 0.997**). Strongly significant correlations were also observed between the different P fractions and plant P uptake. The C/HCl-Pi was the fraction that decreased most especially for the high P treatments indicating that this fraction contributed significantly to the P extracted by DMT-HFO. This suggested that this fraction might be a buffer to more labile P fractions. The combined method employed here could act as an analytical tool to approximate successive cropping experiments carried out under green house condition. But the applicability of this method at a field level should also be assessed. Data from a wider range of soils is also needed to evaluate the universality of this method.     

Changes in phosphorus fractions in three tropical soils amended with corn cob and rice husk biochars

ABSTRACT

The formation of phosphorus (P) compounds including iron-P, aluminum-P and calcium-P in highly weathered tropical soils can be altered upon biochar addition. We investigated the effect of corn cob biochar (CC) and rice husk biochar (RH) pyrolyzed at three temperatures (300°C, 450°C and 650°C) on phosphorus (P) fractions of three contrasting soils. A 90d incubation study was conducted by mixing biochar with soil at a rate of 1% w/w and at 70% field capacity. Sequential P fraction was performed on biochar, soil and soil-biochar mixtures. Increase in most labile P (resin-P_i, NaHCO₃-P_i) and organic P fraction (NaHCO₃-P_o + NaOH-P_o) in CC and RH biochars were inversely related to increasing temperature. HCl-P_i and residual P increased with increasing temperature. Interaction of CC and RH with soils resulted in an increase in most labile P as well as moderately labile P (NaOH-P_i) fractions in the soils. CC increased most labile P in the soils more than RH. The increase in most labile P fraction in soils was more significant at relatively lower temperatures (300°C and 450°C) than 650°C. However, the increase in HCl-P_i and residual P of the soils was more predominant at high temperature (650°C). The study suggested that biochar pyrolyzed at 300–450°C could be used to increase P bioavailability in tropical soils.     

Phosphorus fractions and profile distribution in newly formed wetland soils along a salinity gradient in the Yellow River Delta in China

Soil P availability has been identified as one of the key factors controlling wetland productivity, structure, and function. Soil P fractions at different depths in newly formed wetlands along a salinity gradient in Yellow River Delta (China) were studied using a modified Hedley fraction method. The total P (P_t) content ranged from 471.1 to 694.9 mg kg^–1, and diluted HCl‐extractable inorganic P (Dil‐HCl‐P_i) ranged from 324 to 524.2 mg kg^–1. The Dil‐HCl‐P_i is the predominant P form in all profiles, with on average 70% of the P_t extracted as P_i. Organic P (P_o) comprised (4.2 ± 2.0)% (mean ± SD) of the P_t, due to low organic‐matter content in coastal salt marsh ecosystems. The labile P (resin‐P, NaHCO₃‐P_i, and NaHCO₃‐P_o) and moderately labile P (NaOH‐P_i and NaOH‐P_o) concentrations were both low, ranged from 11.6 to 38.1 and 2.8 to 21.3 mg kg^–1, respectively, constituting (3.7 ± 1.1)% and (2.0 ± 0.7)%, respectively, of P_t, suggesting low availability of P to plants in these soils. Our results suggested that vegetation cover significantly influenced soil P dynamics and availability. In particular, the labile P content under Tamarix chinensis increased significantly by 23.2%–145.5% compared with adjacent soils. These findings have important implications for wetland conservation or restoration and long‐term sustainable management of newly formed wetland ecosystems in the Yellow River Delta.     

Shortcut Approach Alternative to the Step‐by‐Step Conventional Soil Phosphorus Fractionation Method

Abstract

In an attempt to characterize the phosphorus (P)–supplying capacity of a soil and to understand the dynamics of soil P, a procedure was followed whereby consecutive extraction procedures were carried out on a soil sample, first by dialysis membrane tubes filled with hydrous ferric oxide (DMT‐HFO), followed by subsequent P fractionation procedure. However, this combined method is lengthy and time‐consuming, and an approach to shorten these P desorption studies in soils was important. The major objective of this article, therefore, was to present a shortcut method as an alternative approach to the combined fractionation method. Comparison of the sum of DMT‐HFO‐P_i, sodium bicarbonate (NaHCO₃)‐P_i, sodium hydroxide (NaOH)‐P_i, D/hydrochloric acid (HCl)‐P_i, and C/HCl‐P_i extracted by a conventional step‐by‐step method with the sum of DMT‐HFO‐P_i and a single D/HCl‐P_i extraction as a shortcut approach for all extraction periods resulted in a very strong and significant correlations. Both these methods were correlated with maize grain yield, and it was found to be highly significant. This study revealed that this shortcut approach could be a simplified and economically viable option to study the P dynamics of soils especially for soils where the P pool acting as a source in replenishing the labile portion of P is already identified.     

Phosphorus pools in soil after land conversion from silvopasture to arable and grassland use

Differences in soil P among silvopasture, grassland, and arable lands have been well established. Nevertheless, most of the reports compare soil properties under long‐term sites. Thus, there exists little information on the effect of the conversion of silvopasture to arable or grassland use on soil P pools. The objective of the study was to determine the impact of converting silvopasture system (SP) into arable cropping and grassland system on the distribution of P pools and potential P bioavailability. We compared the following systems: SP system, SP converted to arable cropland (SP‐AL), SP converted to grassland (SP‐GL), and for comparative purposes, a long‐term arable cropland (AL). The P fractionation was performed by a sequential extraction scheme, using acid and alkaline extractants on samples collected from the 0–10 and 10–20 cm soil layers. It was assumed that the large variations in soil‐P fractionations are caused by the different management practices associated with land conversion. The results of P fractionation showed a dominance of calcium‐bound P, HCl‐extractable Pi constituted up to 36% of the soil total P (TP). However, the type of land use did not affect this P fraction. On the other hand, the reduction in labile‐P_i and NaOH‐P_i fractions observed at the SP‐AL site may have led to the decline in readily available P. The soil total organic P (TP_o) content was 8% and 17% lower at SP‐AL compared to SP and SP‐GL site, respectively. Labile organic‐P (labile‐P_o) content was markedly higher at SP site compared to arable soils, and was ≈ 10% of TP_o. The NaOH‐P_o constituted the highest fraction of the organic‐P pool (55%–79% of TP_o) across all the study systems, and was positively correlated with TP_o (p < 0.01). The study indicates that conversion of SP system in temperate regions to arable cropping with conventional tillage seems to result in the reduction of P availability compared to SP, indicating SP as an important land‐use practice.     

Soil phosphorus fractions after seven decades of fertilizer application in the Rengen Grassland Experiment

Declining global P reserves require a better understanding of P cycling in soil and related plant uptake. On managed grasslands, application of lime and fertilizer affects not only soil nutrient status, but also plant‐species composition of the sward. We examined the P fractionation in the Rengen Grassland Experiment (RGE) on a naturally acid Stagnic Cambisol in the Eifel Mts. (Germany) 69 y after the setup of the experiment. A modified sequential Hedley fractionation was carried out for samples from 30 plots at 0–10 cm depth. Application of inorganic phosphorus fertilizer had diverse effects on inorganic (P_i) and organic P (P_o) fractions. Resin‐P_i, NaHCO₃‐P_i, NaHCO₃‐P_o, NaOH‐P_i, HCl_dil‐P_i, HCl_conc‐P_i, and HCl_conc‐P_o contents increased, while NaOH‐P_o significantly decreased and residual‐P remained unaffected. Strongest enrichment occurred in the HCl_dil‐P_i fraction, probably due to the chemical nature of the basic Thomas slag applied as P fertilizer. Without P fertilization, all fractions except residual‐P were more or less depleted. Strong P limitation of the vegetation in the limed treatments without P led to lowered contents also for NaOH‐P_i and NaOH‐P_o. However, NaOH‐P_o was largest in the Control and even exceeded the respective content in the treatments with P. It remained unclear why species adapted to a low soil P status did not access this P fraction though being P‐limited. Published theory on the availability of Hedley P fractions does neither match P exploitation nor P nutritional status of the vegetation in the RGE. Regarding NaOH‐P_o as stable and HCl_dil‐P_i as moderately labile led to a more realistic evaluation of plant P uptake. Evaluation of P availability on the basis of chemical extractions alone is questionable for conditions like in the RGE. On long‐term grassland, plant‐species composition has to be taken into account to estimate access of plants to soil P.     

Transformation and recovery of fertilizer phosphorus applied to five Quebec Humaquepts

Abstract

Improving phosphorus (P) fertilizer efficiency while minimizing environmental impacts requires better understanding of the dynamics of applied P in soils. This study assessed the fate of fertilizer P applied in Quebec Humaquepts. A pot experiment with five textural Humaquepts, each receiving 0 (P0), 10 (P10), 20 (P20) and 40 (P40) mg P kg^?1 soil was conducted under barley (Hordeum vulgare L.)-soybean (Glycine max L.) rotations. A modified Hedley procedure was used for soil P fractionation. The clayey soils reached a plateau of dry matter at less P applied than the coarser-textured soils. Plant P uptake, soil labile inorganic P (resin-P?+?NaHCO₃-P_i) and moderately labile inorganic P (NaOH-P_i) increased proportionally with P rate. The coarser-textured soils had lower contents of labile and moderately labile P_i, but a larger increase in labile P_i than the finer-textured soils after receiving P additions. The applied P was retained primarily as soil labile P_i, accounting for 43–69% of total soil recovery of applied P, compared to 20–30% recovered as moderately labile P_i, and 7–29% assumed to be sparingly soluble P (HCl-P?+?H₂SO₄-P). The labile P_i recovery of applied P was linearly depressed with clay content, compared to a quadratic relation for the moderately labile P_i recovery. The results suggest the importance of accounting for soil texture along with soil P adsorption capacity when assessing the efficiency of applied P, P accumulation in soils and subsequently P nutrient management.     

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 (P_i and P_o) fractions. Soil samples were collected from soil 0–20 cm deep beneath and between trees. The P_i 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 P_o forms were sequentially extracted with NaHCO₃ (NaHCO₃‐P_o), NaOH (NaOH‐P_o), and H₂SO₄ (H₂SO4‐P_o). The NaOH‐P_o was subdivided into moderately stable (NaOH‐P_om) and highly stable P_o (NaOH‐P_os). 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‐P_o and NaOH‐P_os forms were greater beneath the trees than those of interspaces, whereas NaOH‐P_om and H₂SO₄‐P_o were not affected by plantation.     

Phosphorus status of a Humic Cryaquept profile in a frigid continental climate as influenced by cropping practices

Most previous studies have limited the assessments of soil phosphorus (P) status within the plow layer. This study was to assess the impacts of crop sequences and nutrient sources on P status of a Labarre silty clay (Humic Cryaquept) profile in a frigid continental climate. Soil of the 0- to 15-, 15- to 30-, 30- to 60-, and 60- to 90-cm layers was sampled from a split-plot experiment comprising a barley (Hordeum vulgare L.) monoculture and a 3-year barley-forage rotation as main plots, and receiving mineral fertilizers (MIN) or liquid dairy manure (LDM) as subplots. A modified Hedley sequential fractionation was used to characterize soil P status. Labile P pools were more affected than stable ones by the investigated treatments. After 10 years, the MIN resulted in larger resin-P and NaHCO₃-P_i, and lower NaHCO₃-P_o and NaOH-P_o pools than the LDM in the top 30 cm of soil. The rotation resulted in larger labile P_i and P_o pools than the monoculture in the 30- to 60-cm layer. The rotation associated with LDM produced the largest total labile P pool, whereas the LDM resulted in an about 20% higher degree of soil P saturation as expressed by the P_ox/(Fe_ox + Al_ox) molar ratio than the MIN in the 0- to 30-cm layer. Our observations stressed that the impacts of crop sequences and nutrient sources on soil labile P extended deeper into the profile than the disturbance caused by primary tillage.     

Phosphorus Fractionations in Acidic Piedmont Rice Soils

Piedmont lands in Bangladesh, India, Nepal, and many other Asian countries are important rice-growing soils, but most of the soils are potentially phosphorus (P) deficient because of low pH. Phosphorus fractions of rice-growing acidic piedmont soils were determined. Soil samples were amended with 100 and 200 mg P kg^?1 soil, and a control soil without P amendment was maintained. The samples were analyzed for the following fractions: solution P, labile pool [sodium bicarbonate (NaHCO₃) P], alkali-extracted inorganic pool [sodium hydroxide (NaOH) P_i], organic pool (NaOH P_o), acidic pool [sulfuric acid (H₂SO₄)?hydrochloric acid (HCl) P], and residual P. About 98% of the applied P in soils was extracted by the sequential extraction employed in the present experiment. The mean total P concentration in 10 acidic Piedmont soils was 247 ppm, of which only 0.12% was in solution, 8% labile (NaHCO₃), 16% NaOH-extracted inorganic, 32% resistant organic, 18% relatively recalcitrant acidic, and 25% residual. Application of P fertilizer increased mainly the labile P fraction, which would be easily available to wetland rice. Solution P was positively and significantly correlated with pH_KCl (r = 0.64, P < 0.05) and negatively correlated with clay (r = ?0.77, P < 0.01). A negative and significant correlation of NaHCO₃-P was observed with pH_H2O (r = ?0.62, P < 0.05). Solution P showed a negative and significant relationship with NaOH-P_i (r = ?0.63, P < 0.05). A significant and negative relationship of solution P was also observed with acid P (r = ?0.78, P < 0.01) and residual P (r = ?0.82, P < 0.01). The relationship of NaHCO₃-P with NaOH-P_i was positive (r = 0.70, P < 0.05) and significant. Similarly, a positive and significant relationship (r = 0.89, P < 0.01) between NaOH-P_i and acid P was observed, and acid P was positively and significantly correlated with residual P (r = 0.84, P < 0.01).     

Land-use and fertilization effects on P forms in two European soils: resin extraction and 31P-NMR analysis

Macroporous anion-exchange resin extraction and ³¹P-NMR spectroscopy of dialysed NaOH extracts were used to investigate the effects of land use (Taubenberg, Bavaria: spruce forest, deciduous forest, permanent grass, arable) and fertilization (Askov, Denmark: unmanured, mineral fertilizer, animal manure) on forms of phosphorus in soil with emphasis on the potentially labile organic (P_o) and inorganic (P_i pools. Carbon content ranged from 12.5–118.1 g kg^?1 and total P (P_i) content from 511 to 2063 mg kg^?1. For all soils, the C:P_o ratios of SOM decreased in the order: whole soil, 150: 1–44:1; alkali extract, 57:1–41: 1; resin extract, 20:1–9:1; suggesting an increasing P functionality of the OM with increasing P_o lability. Analysis of functional relation showed a close relation between resin P_o and ³¹P-NMR estimates for diester-P including teichoic acid-P, indicating that these species contributed significantly to the labile P_o pool as determined by the resin method. The most marked effects of land-use were an increase in P_i under grass and arable, a concurrent sequestration of P_o and SOM under grass, and a depletion of P_o under arable. The amount of resin P_j appeared to be a function primarily of fertilization, and amounted to around 100 mg kg^?1 in the fertilized soils irrespective of the SOM content and P source. The forest soil and the unfertilized agricultural soil had much smaller resin P_j values. The soil under grass had the largest amounts in resin P_o and diester-P including teichoic acid-P, indicating a rapid turnover of P_o with build-up of a large potentially labile, microbially derived P_o pool. ³¹P NMR also showed large proportions of labile P_o species in soils where microbial activity is restrained by acidity (Taubenberg spruce forest, phosphonates) or where highly microbially altered OM is relatively enriched (Taubenberg arable, diester-P including teichoic acid-P). We conclude that the resin used in this study isolates a structurally and functionally reasonably uniform pool of potentially labile soil P_o.     

Phosphorus fractions in Andisol and Ultisol inoculated with Bacillus thuringiensis and phosphorus uptake by wheat

Abstract

Only a small portion of soil phosphorus (P) is available to plants on a short-term basis, and therefore, P taken up by crops in one growing season is small compared to total P (Pt) content of the soil. A group of soil microorganisms capable of transforming insoluble P into soluble and plant accessible forms. The objective of this study was to evaluate the changes in soil P-pools and P uptake by wheat crops as influenced by inoculation with Bacillus thuringiensis in two soils (Andisol and Ultisol), the experiment was conducted in pots under greenhouse conditions using a completely randomized design. Wheat plants were inoculated and re-inoculated at 20 and 46?days after sowing, respectively, with B. thuringiensis; and, plant sampling were performed after 46, 66 and 87 and soil at 87?days based on the Zadoks growth scale, and the soil was submitted by Hedley’s P fractions. The inoculation with B. thuringiensis affected significantly some P organic P (P_o) and inorganic P (P_i) fractions in both soils (Andisol and Ultisol), improved P uptake by wheat crop in (Ultisol) and decreased significantly in (Andisol). The positive effect was more consistent in Ultisol than in Andisol, this strain can be solubilized P fraction extracted with conc. HCl-P_o and HCl 1?mol.     

Phosphorus characterization of manure composts and combined organic fertilizers by a sequential‐fractionation method

Characterization of the forms of phosphorus (P) in organic soil amendments was conducted by sequential P fractionation. More than 60% of total P was inorganic P (P_i). The major P_i forms in the cattle‐manure composts were NaHCO₃‐ and HCl‐extractable P fractions. HCl‐extractable P_i was the predominant P form and a considerable proportion of the total P was present in the HCl‐extractable organic P fraction in the poultry manure composts and combined organic fertilizers.     

Distribution and Sorption of Phosphate in a Savanna Soil Under Improved Pastures in Northern Nigeria

Abstract

Land use patterns affect soil nutrient transformation and availability. The study determined the distribution of phosphorus (P) fractions and sorption in five pasture fields composed of Andropogon gayanus, Brachiaria decumbens, Chloris gayana, Digitaria smutsii, and Stylosanthes guianensis. The objectives were to characterize P fractions in improved pastures and to determine the effect of forage species on soil P lability. Total P (P_t) across the pastures was not significantly different. Organic P (P_o) accounted, on the average, for 64% of P_t. Resin‐P, considered the plant‐available P, ranged from 4 to 10 mg kg^?1, suggesting acute P deficiency in the pastures. The sum of P fractions extracted by 0.5 M NaHCO₃, 0.1 M NaOH, and 1.0 M HCl, together with the resin‐P, accounted for less than 35% of P_t. Factor analysis indicated that plant‐available P approximated by resin‐P was furnished by ?HCO₃‐P_o mineralization and HCl‐P. The highest concentrations of ?HCO₃‐P_o and ?OH‐P_o were maintained by Brachiaria decumbens. Grouping P_i and P_o fractions into labile and nonlabile fraction showed that Brachiaria decumbens maintained the greatest concentration of labile P as a proportion of its P_t. The pasture soils sorbed between 31 and 65% of added P from a standard concentration of 50 mmol kg^?1. Phosphorus sorbed by soils from the pasture fields was in the order: Digitaria smutsii=Stylosanthes guianensis>Brachiaria decumbens=Chloris gayana>Andropogon gayanus, whereas resin recovery of sorbed P was greater in Brachiaria decumbens than other pastures. Between 82 and 92% of sorbed P was bound irreversibly. It was concluded that the relatively high concentration of labile P maintained by soil under Brachiaria decumbens was probably related to its capacity to sequester more carbon than the other pastures.     

Can tropical grasses grown as cover crops improve soil phosphorus availability?

Tropical grasses grown as cover crops can mobilize phosphorus (P) in soil and have been suggested as a tool to increase soil P cycling and bioavailability. The objective of this study was to evaluate the effect of tropical grasses on soil P dynamics, lability, desorption kinetics and bioavailability to soya bean, specifically to test the hypothesis that introducing grass species in the cropping system may affect soil P availability and soya bean development according to soil P concentration. Three grass species, ruzi grass (Urochloa ruziziensis ), palisade grass (Urochloa brizantha ) and Guinea grass (Megathyrsus maximus ), were grown in soils with contrasting P status. Soya bean was grown after grasses to assess soil P bioavailability. Hedley P fractionation, microbial biomass P, phytase‐labile P and the diffusive gradient in thin films were determined, before and after cultivation. It was found that grasses remobilized soil P, reducing the concentration of recalcitrant P forms. The effect of grasses on changing the P desorption kinetics parameters did not directly explain the observed variation on P bioavailability to soya bean. Grasses and microorganisms solubilize recalcitrant organic P (P_o) forms and tropical grasses grown as cover crops increased P bioavailability to soya bean mainly due to the supply of P by decomposition of grass residues in low‐P soil. However, no clear advantages in soya bean P nutrition were observed when in rotation with these grasses in high‐P soil. This study indicates that further advantages in soya bean P nutrition after tropical grasses may be impeded by phytate, which is not readily available to plants.