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The study of the fractions and distribution characteristics of organic phosphorus in the sediment of the water level fluctuating zone of Nansi Lake is conducive to revealing the transformation of phosphorus in the lake, and has important scientific significance for controlling the eutrophication of Nansi Lake. Based on the sediment of the water level fluctuation zone of Nansi Lake. The improved Hedley continuous grading extraction, ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscope were used to characterize the structural characteristics and stability of organic molecules in the sediment, and to reflect the differences in the structure and stability of organophosphate in the water level fluctuating zone. Principal component analysis (PCA), Redundancy analysis (RDA) and correlation heat map analysis were used to analyze the correlation between phosphorus and physicochemical index. The results showed that the alternation between wet-dry conditions was more favorable for the release of phosphorus from sediment, compared to continuous inundation conditions. Moreover, the higher the frequency of wet-dry alternations, the greater the release of phosphorus in different forms from the sediment. Wet-dry alternation resulted in a reduction of substituent on the aromatic rings of sediment DOM (dissolved organic matter), and the continuous drying would increase the molecular weight and humidification degree of DOM in the sediment. Correlation analysis showed that NaOH-Po content in sediment was significantly negatively correlated with TP, IP, OP and various organophosphorus forms, indicating a close transformation relationship between phosphorus forms in sediment. The results can provide a scientific basis for controlling the release of endogenous phosphorus and the risk of eutrophication in Nansi Lake.

Phosphorus (P) is an essential nutrient for phytoplankton growth in water bodies and an important element in biochemical processes. Also, excessive P in river causes eutrophication which endangers aquatic organisms while sediment acts as sink for inorganic and organic pollutants and nutrients. To assess the availability of P and heavy metals (HMs) in Ogun River, surface sediment samples were collected at twelve anthropogenic sites for three months. The surface sediment samples were sequentially extracted to determine five phosphorus fractions [loosely-sorbed P (LS-P), iron-bound P (Fe-P), authigenic apatite, biogenic apatite and CaCO3-bound P (CFA-P), detrital apatite (Det-P) and other inorganic P (IP) as well as organic P (OP)] and levels of selected HMs (Pb, Cr and Zn). Phosphorus fractions were determined spectrophotometrically and HMs were determined using atomic absorption spectrophotometer (AAS) after wet digestion. The average percentages of fractions of P in surface sediments were Det-P (34.66±1.00%) > Fe-P (27.67± 0.02%) > OP (18.11±2.00%) > authigenic apatite, biogenic apatite and CaCO3-bound P (14.38±4.56%) > LS-P (5.18±0.78%). It was observed that IP was dominant (86.38%) of the total phosphorus (TP) compared to OP (13.62%). One of the sites, Garri processing, had highest TP (710 µg P g-1) which was above the standard values for Chinese Environmental Dredging (500 µg P g-1) and Canadian Province of Ontario (600 µg P g-1) Sediment Quality Guidelines (SQG). The mean concentrations of HMs determined in sediments were below the SQG values except Zn in some sites. The results revealed moderate pollution of P and Zn and also, accumulation of these pollutants overtime might pose threat to aquatic organisms. The results obtained would serve as baseline values for P speciation in Ogun River.

Understanding soil phosphorus (P) distribution and its key drivers is fundamental for sustainable P management. In this study, a 21-year fertilization experiment on black soil was carried out, setting up five fertilization treatments: unfertilized control (CK), nitrogen and potassium (NK), nitrogen, P and potassium (NPK), NPK plus straw (NPKS), and NPK plus manure (NPKM). The distribution and effecting factors of P pools within soil aggregates were investigated. Compared to CK, the NK and NPK treatments decreased calcium-associated P concentration in all aggregate fractions. Meanwhile, the NPK treatment significantly increased the organic P extracted from NaOH in unaggregated particles (3, and NaOH increased as aggregate size increased. This was mainly because straw or manure addition promoted soil organic carbon (SOC) storage in aggregates, creating more sorption sites via association with amorphous metallic minerals, and, thus, facilitating P accumulation. In conclusion, decreasing soil pH by chemical fertilizers is an effective strategy for mobilizing soil P, whereas increasing SOC by straw or manure facilitates P accumulation.

With the continuous improvement of surface water environmental quality in China, sediment has gradually become the focus of research on internal pollution in reservoirs. To investigate the forms, distribution, and migration patterns of phosphorus in karst canyon reservoir sediments, we employed an improved sequential graded extraction method to determine phosphorus content in the sediments of the Wanfeng Reservoir. We studied the spatial distribution characteristics and release risk of phosphorus form in the sediments. The results showed that the total phosphorus (TP) content ranged from 79.37 to 438.04 mg·kg−1, while inorganic phosphorus (IP) content ranged from 77.32 to 424.64 mg·kg−1. Iron–aluminum-bound inorganic phosphorus (Fe/Al-Pi) accounted for 36.41% and was found to be the dominant form of IP. Organic phosphorus (OP) content ranged from 1.84 to 13.59 mg·kg−1, with weakly adsorbed organic phosphorus (H2O-Po) being the dominant form of OP. Potentially active inorganic phosphorus (NaHCO3-Pi) showed a highly significant positive correlation (p < 0.01) with Fe/Al-Pi, residual phosphorus (Res-P), and TP. This indicates that potentially active phosphorus (NaHCO3-P) is a significant potential source of phosphorus (P) in the reservoir. Biologically active phosphorus (BAP) content ranged from 66.97 to 201.46 mg·kg−1, with BAP/TP ratios ranging from 55.6% to 59.6%. The risk of phosphorus release from Wanfeng Reservoir sediments is high. The release of various forms of phosphorus from sediments is one of the important factors leading to the deterioration of water quality in the reservoir area in the future. To effectively manage water ecology in karst canyon reservoirs, it is essential to reduce the risk of endogenous phosphorus release.

The pH value of lake water varies with the lake environment, which has an effect on the form of phosphorus in sediment, and then the release of sediment phosphorus. The form of phosphorus in sediments was analyzed using field sampling. The environmental conditions with pH values of 4.0, 7.0 and 10.0 were simulated indoors to estimate the effects of pH on phosphorus release from sediments and the content change of various forms of phosphorus was studied. The results showed that in Wuliangsuhai Lake, Ca-P accounted for 54.3%, which was the largest portion of the TP. Phosphorus release was favored under acidic and alkaline conditions, and the alkaline condition was more favorable. The proportion of Fe/Mn-P and Fe/Al-P in the TP decreased with an increase in the pH, while the proportion of Ca-P in the TP increased with an increase in the pH. Under the alkaline condition (pH = 10), Ca-P in the sediment increased significantly, with an increase of 22.5%. However, Fe/Mn-P and Fe/Al-P decreased significantly, with drops of 37.3% and 44.9%, respectively. Under the acidic condition (pH = 4), Fe/Mn-P and Fe/Al-P in the sediment increased significantly, and the increases were 63.1% and 37.1%, respectively. However, Ca-P decreased significantly, with a drop of 39.2%. In general, low pH promoted the release of Ca-P, and a high pH promoted the release of Fe/Mn-P and Fe/Al-P. Wuliangsuhai Lake water is characterized by weak alkaline characteristics throughout the year, and biological available phosphorus accounts for 13.3%–20.9% of the TP, with Fe/Mn-P being the dominant form. This study revealed that the risk of phosphorus release from sediments to the overlying water was greater under alkaline conditions. HIGHLIGHTS Evaluate the effect of pH change on different phosphorus forms content in sediments of lake.; Determine phosphorus forms and the effects of pH on their fraction transformation based on indoor simulation experiments.; To explore which part of phosphorus form was affected by pH change, and which resulted in the change of phosphorus content in sediments, which is the focus of this study.;

Soil phosphorus (P) can be divided into inorganic P (Pi) and organic P (Po). Microorganisms play essential roles in soil P transformation. However, there are many ways to detect P transformation, and the relationship between P forms and microorganisms under long-term fertilization is largely unclear. In this study, soil P forms were analyzed by a chemical sequential fractionation method and solution 31P nuclear magnetic resonance (31P-NMR) technique. Phospholipid fatty acid (PLFA) contents were measured by gas chromatography as the characterization of soil microbial community structures. The objective was to determine the changes of soil P forms and associated microbial community composition in mollisol with long-term fertilization. We sampled soil from a field experiment with 26-year-old continuous maize (Zea mays L.) cropping in Northeastern China. Three fertilization treatments were selected as chemical fertilization (NPK), NPK with crop straw (NPKS), and NPK with manure (NPKM). As shown in 31P-NMR spectra, orthophosphate accounted for 62.8–85.8% of total extract P. Comparison to NPK and NPKS treatments, NPKM application notably increased the concentrations of Po, Olsen-P, orthophosphate, orthophosphate monoester, and total P. Soil P fractions including resin-Pi, NaHCO3-P, NaOH-P, and HCl-P, especially Pi fractions, were enhanced by NPKM. The amounts of total PLFAs and PLFAs in bacteria, Gram-positive (G+) and Gram-negative (G−) bacteria, actinomycetes, and fungi were high in NPKM-treated soil. The percentages of PLFAs in bacteria and fungi in total soil PLFAs were 56.8% and 9.7%, respectively, which did not show any significant difference among the treatments. NPKM increased the proportions (%) of PLFAs in G+ bacteria, and NPKS increased the proportions (%) of G− bacteria in total PLFAs. The composition of soil microbial community was found to be significantly affected by soil total carbon and pH. There was a close relationship between HCl-Pi, NaHCO3-Po, orthophosphate, and pyrophosphate with anaerobe, aerobes, and G+. Manure addition directly increased soil available P concentrations, and indirectly acted through the alterations of anaerobe, aerobes, and G+. It is concluded that long-term NPKM application would lead to the accumulation of labile P and moderately labile P in mollisol through the activity of soil microbes.

The phosphorus (P) leaching from continuous fertilization is generally neglected in mollisol. The in situ leaching loss of phosphorus (P), especially dissolved organic P, is poorly quantified under organic amendments given its potential environmental risks. In this study, we conducted an in situ soil column experiment, instead of the traditional measuring of the soil layer, in the mollisol area of northeast China to investigate the seasonal variations in different P forms under three typical fertilization practices, including chemical fertilizer (CF), CF supplemented with straw (CFS), and CF co-applied with straw and manure (CFSM). Compared with the CF treatment, CFS treatment generally reduced the leaching loss of dissolved organic P by 57.3% to reduce the total P loss, while the CFSM treatment increased the leaching loss of dissolved inorganic P by 20.9% to increase the total P loss. Other than the effects of management practices, precipitation and temperature-oriented environmental factors significantly affected the seasonal variation in leaching loss of both the dissolved and particulate P forms. We conclude that straw incorporation into the mollisol of northeast China is recommended, considering its low leaching risk of P, while a co-amendment of straw and manure resulted in the opposite. Despite the slight environmental risk of P leaching loss (0.75–1.95% of external P input per year) practically quantified by in situ experiments, a proper reduction in chemical P input with organic amendments may be an effective P fertilizer management strategy in mollisol areas.

Microbes associated with phosphorus (P) cycling are intrinsic to soil P transformation and availability for plant use but are also influenced by the application of P fertilizer. Nevertheless, the variability in soil P in the field means that integrative analyses of soil P cycling, microbial composition, and microbial functional genes related to P cycling remain very challenging. In the present study in the North China Plain, we subjected the bacterial and fungal communities to amplicon sequencing analysis and characterized the alkaline phosphatase gene (phoD) encoding bacterial alkaline phosphatase in a long-term field experiment (10 years) with six mineral P fertilization rates up to 200 kg P ha–1. Long-term P fertilization increased soil available P, inorganic P, and total P, while soil organic P increased until the applied P rate reached 25 kg ha–1 and then decreased. The fungal alpha-diversity decreased as P rate increased, while there were no significant effects on bacterial alpha-diversity. Community compositions of bacteria and fungi were significantly affected by P rates at order and family levels. The number of keystone taxa decreased from 10 to 3 OTUs under increasing P rates from 0 to 200 kg ha–1. The gene copy numbers of the biomarker of the alkaline phosphatase phoD was higher at moderate P rates (25 and 50 kg ha–1) than at low (0 and 12.5 kg ha–1) and high (100 and 200 kg ha–1) rates of P fertilization, and was positively correlated with soil organic P concentration. One of the keystone taxa named BacOTU3771 belonging to Xanthomonadales was positively correlated with potential functional genes encoding enzymes such as glycerophosphoryl diester phosphodiesterase, acid phosphatase and negatively correlated with guinoprotein glucose dehydrogenase. Altogether, the results show the systematic effect of P gradient fertilization on P forms, the microbial community structure, keystone taxa, and functional genes associated with P cycling and highlight the potential of moderate rates of P fertilization to maintain microbial community composition, specific taxa, and levels of functional genes to achieve and sustain soil health.

A laboratory experiment was conducted to determine the extent and the importance of phosphorus (P) fractions of some agriculturally important Sudanese soils on P availability. The soils were gathered from different locations in Sudan, to represent three different orders: Aridisols (Um Dum soil, North Kordofan state), Alfisols (Hagu soil, Senar state), and Vertisols (Hosh soil, Gezira state). The soil P was fractionated using an authenticated procedure. The results showed that the soils had low total, organic and available P contents. Most of the inorganic soil P was present in the Fe-Al fraction in Um Dum soil (> 50%), and about 40% of this fraction in Hagu soil, but, only 20% in alkaline Hosh soil. The Ca-P fraction constituted > 60% in Hosh soil, about 40% in Hagu soil and 20 % in Um Dum soil. The available P in the top soils studied was positively related to the percent sum of Al+Fe -P of the total P, and was negatively related to the percent Ca-P fraction content of the total P. It has been proposed that the P sorption starts by exchange with singly coordinate Fe or Al- OH clay (OH− edge group) and quickly reorganizes into more stable and less soluble ring forms especially at pH more than 7.0, Alkaline pH more than 8.0 in soil like Hosh, will in presence of Ca, favour with time, the formation of inactive less soluble form like octacalcium phosphates and apatites at expense of the initial Al and Fe-P forms which are less stable at alkaline pH.

Columnar sediment samples were collected from five representative river inflow areas of Dianchi Lake, China. The vertical distribution of each form of P were tested. Results showed that the concentration of TP in the sediments from areas A, B, C, D and E in the order of D > B > A > C > E, and the average concentration of D, B, A, C and E were 2991, 2064, 1308, 879, and 759 mg•kg−1, respectively. The concentration of Ex-P, Fe/Al-P, Ca-P and Org-P all decreased with increasing depth. The release of Ex-P was significantly related to TP whereas the Fe/Al-P was not significantly related to TP in the samples from areas polluted by domestic sewage. However, the release of Ex-P and Fe/Al-P were both significantly related to TP in the samples from areas polluted by phosphate mining and phosphate fertilizer application. The results of equilibrium P concentration (EPC0) analysis showed that P in the sediments of areas A, D and E were the source of P in Dianchi Lake, and the P in the sediments of areas B and C were in relative equilibrium with the overlying water.

This study investigates the behavioral characteristics and forms of phosphorus in the sediment according to the oxygen condition (aerobic/anaerobic). In the behavioral characteristics analysis, Al-P and Fe-P concentrations were the highest among the forms of inorganic phosphorus, and therefore had the strongest impact on sorption and release corresponding to environmental condition changes. In the experimental investigation of the inorganic forms of phosphorus in the sediment according to the oxygen condition, we determined that the forms of inorganic phosphorus did not greatly affect the sorption or release reaction because the distribution ratios of the inorganic forms remained constant corresponding to changes of dissolved oxygen (DO) conditions. In contrast, the forms of organic phosphorus in the sediments affected both sorption and release. Furthermore, labile-P and moderately labile-P forms were the major mechanisms of sorption in sediment. Moderately labile-P was the greatest contributor to phosphorus release action in sediment. As environmental changes are important for the behavioral characteristics of phosphorus in sediment, the forms of phosphorus should be considered to have a greater effect, especially in the organic phosphorus case. Therefore, based on the present study results, sediment evaluation aimed at controlling internal pollutants in reservoirs should include an examination of the forms of phosphorus present, as well as the release characteristics of environmental changes, which are influential factors of phosphorus control. Further research in this field is required.

The supplemented scheme for determination of phosphorus forms in natural waters was tested for the River Prut.

Monitoring of phosphorus content in “water-particulatematerials-bottom sediments system” for river Prut. Seasonal and spatialdynamics of phosphorus forms in water, particulate materials and bottomsediments of river Prut was elucidated. The scheme for determination ofphosphorus forms in water and particulate materials according to World HealthOrganization classification was evaluated. Additionally, this scheme was tested forestimation of phosphorus content in bottom sediments. The supplemented schemeallows the analysis of the phosphorus forms for the entirely system “water –particulate materials – bottom sediments”, extending possibilities for interpretationof phosphorus dynamics in natural waters.

Mehlich-3 (M3) is one of the most common agronomic and environmental phosphorus (P) extractants for determining P fertilizer requirements and the potential for non-point source pollution. Understanding how soil properties impact M3 extractability can improve our ability to properly use this soil test. The objectives of this study were to investigate the impact of soil pH on P extractability by M3 and water in different soils containing equal total P, and to ascertain information about mechanisms of M3-P extraction. Soil pH at four field sites was previously adjusted to a range of approximately 4.5–7.5. Soils (Grant, Dale, Teller, Easpur) were characterized, and P was extracted with M3 and water. Extraction of Mehlich-3 P decreased 40% to 55% with increasing pH, which was potentially due to changing P forms, partial neutralization of extractant pH, and consumption of extractant fluoride (F−) by non P-containing calcium (Ca) minerals. Water-soluble P (WSP) increased with increasing pH up to pH 6–7. Mehlich-3 P and WSP were not positively correlated except for one soil type. Mehlich-3 P is best utilized with WSP as indicators of quantity and intensity, respectively. Use of M3-P alone at pH < 5.5 may overestimate solubility. Further research should examine the suitability of M3-P at pH > 7.

Este trabalho objetivou comparar o crescimento, a absorção de fósforo (P) e a distribuição das frações fosfatadas em mudas de sete espécies florestais classificadas como: pioneiras [aroeira (Lithraea molleoides), aroeirinha (Shcinus terebinthifolius), sabiá (Mimosa caesalpiniaefolia), sesbânia (Sesbania virgata)] ou clímax [jatobá (Hymenaea courbaril), guanandi (Calophyllum brasiliensis) e óleo-bálsamo (Myroxylon peruiferum)]. As plantas foram cultivadas com cinco doses de P (0, 100, 250, 500 e 800 mg dm-3 de P) em casa de vegetação. Aos 90 dias, as plantas foram colhidas, determinando-se a produção de matéria seca e o conteúdo de P da parte aérea e as frações de P total, P inorgânico e P orgânico presentes nas folhas. As espécies apresentaram grande variação no comportamento diante da adubação fosfatada. De modo geral, as espécies pioneiras foram mais eficientes em produzir matéria seca da parte aérea e na absorção de P do que as climácicas. Entre as espécies pioneiras, a aroeira apresentou produção de massa estreitamente relacionada ao acúmulo de P, e as proporções de fósforo inorgânico e orgânico permanecem inalteradas com o aumento da disponibilidade do nutriente. O crescimento de aroeirinha, sesbânia e jatobá não acompanhou a absorção do nutriente que é acumulado na forma de fósforo inorgânico na primeira e fósforo orgânico nas últimas. As espécies clímácicas guanandi e óleo-bálsamo apresentaram maior presença de fósforo orgânico, porém a absorção do nutriente e o crescimento das plantas sofreram pouca interferência da adubação fosfatada.This work aimed to compare growth, phosphorus (P) uptake and distribution of P-fractions in seedlings of seven tree species classified as pioneers (Lithraea molleoides, Shcinus terebinthifolius, Mimosa caesalpiniaefolia, Sesbania virgata) or climaxes (Hymenaea courbaril, Calophyllum brasiliensis, Myroxylon peruiferum) and cultivated under five doses of P (0, 100, 250, 500 and 800 mg dm-3 of P) in a greenhouse. Up to ninety days after germination, the plants were harvested, and dry matter, P-content and P-fractions (total-P, inorganic-P and organic- P) in the leaves were determined. The tree species presented a wide behavior variation regarding P-fertilization, with the pioneer species being more efficient in producing dry matter and absorbing P. The biomass production of the pioneer species Lithraea molleoides was directly related to P accumulation. Its proportions of inorganic-P and organic-P remained unaffected by the increased availability of P. The growth of the Shcinus terebinthifolius, Sesbania virgata and Hymenaea courbaril did not keep up with P uptake, which was mainly accumulated as inorganic-P in the first species and as organic-P in the two last ones. Climax species Calophyllum brasiliensis and Myroxylon peruiferum presented a larger proportion of organic-P, however, the P uptake and plant growth were little affected by P-fertilization.

A suinocultura tem como inconveniente a grande produção de dejetos, os quais são reutilizados como fertilizantes agrícolas, porém, se utilizados em doses excessivas, tornam-se severos poluentes ambientais. O objetivo deste trabalho foi avaliar as formas de acumulação de P em um solo submetido a sucessivas aplicações de doses de dejeto líquido de suínos. O experimento foi instalado em 1995, em uma área cultivada com pastagem natural, que recebeu doses de 0, 20 e 40 m³ ha-1 de dejeto líquido de suínos (DLS), aplicadas em intervalos de 45 a 60 dias durante cinco anos. Em 1999, após 28 aplicações de DLS, que totalizaram 0, 560 e 1.120 m³ ha-1, foram coletadas amostras de solo nas profundidades de 0-2,5, 2,5-5, 5-10, 10-15 e 15-20 cm. A amostra foi seca, moída, passada em peneira de malha de 2 mm e submetida ao fracionamento químico do P pelo método de Hedley. No tratamento testemunha, foi determinada a capacidade máxima de adsorção de fósforo (CMAP), utilizando-se isotermas de adsorção e ajuste matemático pela equação de Langmuir. Os resultados mostraram que o P adicionado ao solo via DLS é acumulado essencialmente em formas inorgânicas, sendo as principais as extraídas por resina trocadora de ânions, NaHCO3 0,5 mol L-1, NaOH 0,1 mol L-1 e HCl 1,0 mol L-1.The swine industry generates large amounts of pig slurry (PS), which is reused as agricultural fertilizer. However, if applied at excessive rates, pig slurry becomes a severe environmental pollutant. The objective of this study was to evaluate forms of soil phosphorus accumulation after successive swine manure applications. The experiment was installed in 1995, in an area of native pasture. Rates of 0, 20 and 40 m³ha-1 of PS were applied periodically (at 45 to 60 days intervals) during five years. In 1999, after 28 PS applications totaling 0, 560 and 1.120 m³ha-1 of PS, soil samples were collected from the layers 0-2.5, 2.5-5.0, 5-10, 10-15, and 15-20 cm. Soil phosphorus in the samples was fractionated using Hedley's fractionation scheme (Hedley et al., 1982). The maximum phosphorus adsorption capacity (MPAC) was determined in soil samples without PS application using the Langmuir model. Results showed that phosphorus added as PS is accumulated essentially in inorganic forms, mainly those extractable by anion exchange resin, NaHCO3 0.5 mol L-1, NaOH 0.1 mol L-1 and HCl 1.0 mol L-1.

O objetivo deste trabalho foi avaliar o conteúdo de fósforo armazenado na biomassa microbiana e a atividade de fosfatases ácidas, durante a diminuição dos teores de fósforo disponível no solo, causado por cultivos sucessivos com plantas. Foram utilizadas amostras de Latossolo Vermelho distroférrico típico, com adição prévia de fosfatos solúveis (0, 180, 360, 540 e 720 kg ha-1 de P2O5), aplicados em seis anos consecutivos. Efetuaram-se 15 cultivos sucessivos com diferentes plantas, em casa de vegetação, sem a reposição do fósforo absorvido pelas plantas. Após cada três cultivos sucessivos, foram determinados: o teor de fósforo disponível por resina trocadora de ânions, o fósforo microbiano e a atividade de fosfatases ácidas. Com a diminuição da disponibilidade de fósforo do solo, a quantidade de fósforo armazenada na biomassa microbiana do solo diminuiu, e a atividade de fosfatases ácidas aumentou. Em solos com baixo teor de fósforo e de resíduos de plantas, o P microbiano tem pouca importância para a nutrição das plantas.The objective of this work was to evaluate the content of phosphorus stored in the soil microbial biomass and the activity of acid fosfatases, during the decline of soil available phosphorus, caused by successive crops in pot experiment. Samples of Oxisol were utilized with previous addition of soluble phosphates (0, 180, 360, 540, and 720 kg ha-1 of P2O5), applied in six consecutive years. The soil samples were submitted to 15 successive crops in greenhouse, without replacement of absorbed phosphorus by plants. After each three successive crops, soil was sampled, and the following variables were determined: the available phosphorus by anion exchange resin, phosphorus stored in the soil microbial biomass and the activity of acid phosphatases. As a consequence of the reduction of the soil available phosphorus, the amount of microbial phosphorus decreased, and the activity of phosphatases increased. In soils with low phophorus content and no plant residues, the microbial P has little importance for plant nutrition.

O histórico de uso e o manejo da adubação influenciam a capacidade de suprimento de P pelo solo. Neste trabalho, foram avaliadas as frações de P num Argissolo Vermelho da região do Cerrado, adubado com fosfatos de reatividade distinta, em diferentes modos de aplicação e cultivado com milho durante três safras. A área, que já havia sido adubada em épocas passadas, estava coberta por braquiária. O delineamento experimental foi o de blocos casualizados com quatro repetições, num esquema fatorial 4 x 3 + 1, envolvendo quatro fontes de P, na quantidade de 180 kg ha-1 de P2O5 (superfosfato triplo, termofosfato magnesiano, fosfato natural reativo de Arad e fosfato natural de Araxá), três formas de aplicação (a lanço em área total no primeiro ano, localizada no sulco de plantio no primeiro ano e parcelada anualmente no sulco) e uma testemunha (sem P) como tratamento adicional. O P foi fornecido considerando-se os teores totais do nutriente nas fontes. Após três cultivos de milho, foram retiradas amostras de solo na profundidade de 0-20 cm para o fracionamento do P. A interação entre fontes e modos de aplicação de P interfere nas frações do nutriente no solo no final do cultivo de milho. De modo geral, o parcelamento da adubação condiciona maior teor do nutriente nas frações inorgânica (todas as fontes) e orgânica (fontes mais reativas). O fato de a área ter sido anteriormente adubada e ocupada por braquiária influenciou os efeitos dos tratamentos, pois a produção de milho foi relativamente elevada onde não houve aplicação de P. Houve evidências da participação de frações consideradas pouco lábeis no suprimento de P para o milho, em função da diminuição dessas frações no solo.The land use history and management of fertilization influence the soil phosphorus supply capacity. In this study, the P fractions in a Red Argisol (Ultissol) of the cerrado region were evaluated under different application methods and with phosphorus sources of different solubility, after three successive corn crops. The area had already been fertilized and cultivated in the past and remained covered with Brachiaria grass. A randomized block design with four replications was used, in a 4 x 3 + 1 factorial scheme, involving four P sources at a rate of 180 kg ha-1 P2O5 (triple superphosphate, magnesium thermophosphate, Arad reactive rock phosphate, and Araxá rock phosphate), three application methods (broadcasted in the first year, in-furrow application in the first year, and annually in-furrow split applications), and a control plot (without P) as additional treatment. The applied P rate was based on the total P2O5 content of each fertilizer. After three corn cropping seasons, soil samples were collected at a depth of 0-20 cm to determine the P fractions. The interaction between P sources and application methods interferes with the P fractions in the soil towards the end of the growing season. In general, split fertilization resulted in a higher values of inorganic (all sources) and organic P fractions (soluble sources). The fact that the area had previously been fertilized and cultivated with Brachiaria grass influenced the treatment effects, as shown by the relatively high crop yield of the control treatment not fertilized with P. There are evidences for the participation of less-labile P fractions in P supply to corn due to the observed reduction of these fractions in the soil.