Role of Polymeric Coating on the Phosphate Availability as a Fertilizer

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Role of polymeric coating on the phosphate availability as fertilizer: insight from phosphate release in soil castor polyurethane coatings Diego Cruz, Ricardo Bortoletto-Santos, Gelton Guimarães, Wagner Luiz Polito, and Caue Ribeiro J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b01686 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on July 5, 2017

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Journal of Agricultural and Food Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Journal of Agricultural and Food Chemistry

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Title: Role of polymeric coating on the phosphate availability as fertilizer: insight

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from phosphate release in soil castor polyurethane coatings

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Authors: Diego F. da Cruz¹; Ricardo Bortoletto-Santos; Gelton G. F.

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Guimarães; Wagner L. Polito; Caue Ribeiro*

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Number of Tables: 4

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Number of Figures: 5

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Corresponding author:

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Dr. Caue Ribeiro Embrapa Instrumentação Rua XV de Novembro, 1452 Phone: +55 (16) 2107 2915 Fax: +55 (16) 2107 2902 E-mail: [email protected]

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Role of polymeric coating on the phosphate availability as fertilizer:

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insight from phosphate release in soil castor polyurethane coatings

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Diego Fernandes da Cruz1, Ricardo Bortoletto-Santos2, Gelton Geraldo

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Fernandes Guimarães2, Wagner Luiz Polito1, and Caue Ribeiro2*

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(USP), Campus de São Carlos, Av. Trabalhador São-Carlense 400, Caixa

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Postal 780, Arnold Schimidt, São Carlos, São Paulo, 13566-590, Brasil.

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Phone/Fax: +55 (16) 3373 9976.

Instituto de Química de São Carlos (IQSC), Universidade de São Paulo

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Embrapa Instrumentação, Rua 15 de Novembro 1452, Centro, São Carlos, São

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Paulo, 13560-970, Brasil.

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Phone/Fax: +55 (16) 2107 2800.

Laboratório Nacional de Nanotecnologia para o Agronegócio (LNNA),

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* E-mail: [email protected]

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ABSTRACT

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The coating of fertilizers with polymers is an acknowledged strategy for

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controlling the release of nutrients and their availability in soil. However, its

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effectiveness in the case of soluble phosphate fertilizers is still uncertain, and

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information is lacking concerning the chemical properties and structures of such

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coatings. Here, an oil-based hydrophobic polymer system (polyurethane) is

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proposed for control of the release of phosphorus from diammonium phosphate

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(DAP) granules. This material was systematically characterized, with evaluation

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of the delivery mechanism and the availability of phosphate in an acid soil. The

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results indicated that thicker coatings can change the maximum nutrient

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availability towards longer periods like 4.5 to 7.5 wt% DAP coated that

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presented the highest concentrations at 336 h, compared to 168 h for uncoated

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DAP. In contrast, DAP treated with 9.0 wt% began to increase the concentration

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after 168 h until it results in maximum release at 672 h. These effects could be

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attributed to the homogeneity of the polymer and the porosity. The strategy

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successfully provided long-term availability of a phosphate source.

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Keywords: Castor oil; controlled release; fertilizer; phosphorus; polymers.

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INTRODUCTION

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Increased agricultural productivity to meet the demand for food, fiber, and

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energy production has driven the increased use of inputs such as mineral

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fertilizers.1 High doses of soluble fertilizers are required to supply the nutritional

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needs of plants grown on soils with low natural fertility, such as Oxisols.

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However, the acidity and high contents of iron and aluminum-based compounds

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in these soils reduce the efficiency of phosphate fertilizers, due to strong

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phosphorus (P) fixation, which decreases the availability of this nutrient to

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plants.2-4 Such losses can be minimized by controlling nutrient release and

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maintaining a temporally homogeneous availability of phosphate at moderate

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levels in soil.5-7

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One technique that can be employed to provide controlled delivery is the

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use of coatings that restrict the rate of solubilization of the fertilizer, hence

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altering the process of fixation in the soil.8-10 Considering the dynamics of

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phosphate assimilation by plants, the effect of such coatings can lead to the

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delivery of smaller amounts of phosphate, with fixation in the soil components

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being reduced due to lower anion migration. This effect was studied by

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Figueiredo et al. (2012), who observed better performance in terms of yield and

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total dry matter production of maize, compared to the use of uncoated

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fertilizers.11 Garcia et al. (1997) reported that pine lignin coatings increased the

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availability of P in the soil and reduced fixation.12 The coating of diammonium

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phosphate (DAP) with rosin has also been shown to provide control of P fixation

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in the soil, with coated fertilizer enabling the maintenance of P availability for

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longer periods.13-14

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Previous studies have not described the structures of coatings, and have

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not correlated the chemical properties of coated materials with agronomic

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outcomes. Hence, there is a need for improved information concerning suitable

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coatings designed specifically for agricultural applications. Many of the

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materials currently available employ coatings developed for other applications,

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which have been tested for use with phosphate without detailed investigation of

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their chemical characteristics.

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A recent study about our group described the kinetics of N release in a

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biodegradable oil-based polymeric coating system.15 However, there are

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several concerns about the effectiveness of this strategy to non-volatile

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fertilizers, such as DAP. Especially for phosphate fertilizers, the main drawback

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for efficient use is related to the reaction against Fe or Al sources, widely

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present in tropical soils – which leads to very low soluble compounds

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immobilizing the phosphate. Our findings show that the slow DAP release has a

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role in the reduction of its phosphate immobilization by a continuous nutrient

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supply, avoiding local saturation and consequently optimizing the fertilizer

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usage. Therefore, despite the paper investigates the effect of different coating

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thickness in this fertilizer release, the main goal of this paper is show that

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polyurethane (PU) coatings may be an effective strategy for phosphate-based

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fertilizers management, which is a considerable advance for the chemical

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processing of such products.

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In addition, the present work also proposes the use of an oil-based

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hydrophobic polymer system to control the release of phosphorus from DAP

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granules. A systematic characterization of this material was performed, and

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evaluation was made of the delivery behavior and availability of phosphate in an

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Oxisol. The findings contribute to an improved understanding of the effects of

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coatings on phosphate fertilizers, supporting future strategies for better fertilizer

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management.

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MATERIALS AND METHODS

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Preparation of materials

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Granules of diammonium phosphate (DAP), typically 3 mm in diameter,

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were coated using a castor oil-based polyurethane resin system (here denoted

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PU). DAP was chosen due to its high phosphate content, high solubility, and

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granular morphology, which enabled it to be employed as a model for different

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soluble granular phosphate fertilizers. The preparation of the coating followed

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the methodology of Bortoletto-Santos et al. (2016), and the resin was

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synthesized using commercial MDI (4.4'-diphenylmethane diisocyanate, Bayer).

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The oil:MDI mass ratio used was 60:40, and the quantities were calculated from

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the mass of DAP. The resin was dispersed over the granules using a metal

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turntable coater with 25 cm inner side flaps, at 30 rpm rotation, under a flow of

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air heated at 145-155 ºC.

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The quality of the coatings was first evaluated by deposition onto glass

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slides, with selection of the conditions that resulted in the most visually

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homogeneous layers. The coatings were prepared at mass ratios from 1.5 wt%

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to 9.0 wt% (in steps of 1.5 wt%). For example, in the process employing 1.5

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wt% coating, 15 g of polymer were added to 1.0 kg of DAP. The coating

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process involved only one stage for each percentage prepared, with the

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required mass of resin being added to the metal dish coater containing the

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granules using a needleless syringe. The same procedure was performed for

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each of the coatings.

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After preparation of the material, the actual coating percentage was

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determined in order to check for any significant difference from the theoretical

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coating percentage. For this, a 0.5 g sample of the coated granules was

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comminuted and solubilized in distilled water for 15 min with the aid of an

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ultrasonic stirrer. The solution was then filtered to obtain only the coating, which

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was dried in an oven at 35 °C for 48 h, followed by weighing. The results are

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shown in Table 1.

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Physical and chemical characteristics of the coatings

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Scanning electron microscopy (SEM)

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Morphological characterization of DAP and the polymer coatings was

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performed by SEM, using a JEOL JSM 6510 microscope coupled to an energy

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dispersive X-ray (EDX) analysis system (Thermo Scientific). For analysis, the

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samples were dispersed on a carbon ribbon attached to the surface of a metal

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disc (stub), followed by coating with gold in an ionization chamber (MED 020,

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Bal-Tec). The images were acquired using the following conditions: 200 or 500

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magnification levels, 10 kv tension and secondary detector.

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Water release assays

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Water release assays were used to determine the rate of fertilizer release

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according to time, at room temperature. The experimental conditions followed

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the methodology described by Bortoletto-Santos and Ribeiro (2014), in which a

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beaker containing a known mass of the coated material was immersed in a

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larger beaker containing water, under constant stirring, hence ensuring

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homogeneous release of DAP. This arrangement ensured that the release was

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only due to diffusion through the coating, and not the mechanical action of the

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stirrer.16

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Phosphorus determination

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The determination of phosphorus followed the methodology presented by

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Plotegher and Ribeiro (2016), involving the preparation of a mixed reagent (25

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mL of a 25% solution of sulfuric acid + 5.5 mL of 9.5% ammonium molybdate

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solution + 0.6 mL of 3.25% antimony and potassium tartrate solution), together

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with solutions of ascorbic acid (7%) and citric acid (2.2%).

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A 5 mL volume of the diluted sample was mixed with 0.2 mL of citric acid

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solution + 2 mL of ascorbic acid solution + 2 mL of the mixed reagent solution.

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This mixture was heated at 50 °C for 10 min, forming a blue complex that was

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analyzed at a wavelength of 800-900 nm using a UV-Vis spectrophotometer

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(1601PC, Shimadzu).17

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Phosphorus release in soil

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The release of phosphorus into soil from the DAP coated with different

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proportions of PU polymer was evaluated in an incubation experiment. DAP

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granules, with or without coating, were applied to an Oxisol collected from the

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surface layer (0-20 cm) of an agricultural soil in São Carlos (São Paulo State,

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Brazil). Samples of soil with particle size smaller than 2 mm showed the

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following characteristics: 433 g.kg-1 (sand), 35 g.kg-1 (silt), and 532 g.kg-1 (clay),

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according to soil texture analysis using the pipette method; equivalent moisture

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200 g.kg-1; pH (H2O) 5.18, measured with a glass electrode; potential

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extractable acidity 2.6 cmolc H + Al kg-1; available P 0.7 mg.kg-1; remaining

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phosphorus (P-rem) 2.1 mg.L-1; organic carbon 7.56 g.kg-1, measured by the

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Walkley-Black method; and cation exchange capacity (CEC) 4.23 cmolc.kg-1.18-

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Samples (20 g) of soil were incubated in Petri dishes (5.5 cm diameter)

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with DAP granules, with or without a polymeric coating, using a P:soil ratio of

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1:2000 g.g-1. After incorporation of the granules, soil moisture was standardized

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to 80% of water-holding capacity and was maintained constant during the

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incubation period, with the addition of groundwater whenever necessary.

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The samples were incubated at a controlled temperature of 25 °C for 24,

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72, 168, 336, and 662 h. After each incubation period, the remainder of the DAP

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granules was removed and the soil was dried at 40 °C. The phosphorus

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remaining in the granules was indirectly estimated from the remaining N

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content, which was measured by mass spectrometry (CHN-S/O 2400 Series II

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Elemental Analyzer, PerkinElmer). The available phosphorus in the soil was

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determined by Mehlich-1 extraction using a soil:extractant ratio of 1:50 (g.mL-1),

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with quantification as described by Embrapa (1979).

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The P-rem value of each sample was expressed as a percentage relative

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to the P applied as DAP. The recovery profile of available phosphorus in the soil

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was calculated using means and standard deviations. Differences between

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mean values were determined by analysis of variance (ANOVA), and

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differences between treatments were compared using Fisher’s test (p 1, we assume that this indicates a strong influence of the

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coating on retarding the release, which is gradually allowing the diffusion

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(accelerating the process) by opening pores or degrading the structure. The n

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values close to 1 to uncoated and 1.5 wt% coated sample indicate that these

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samples are typically solubilizing and diffusing in the medium, i.e., the 1.5 wt%

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sample probably have significant uncoated parts where the dissolution takes

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place without any barrier. The similar n values for the other PU coated samples

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(≈ 2) indicate that the coating is probably complete in the samples, imposing a

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continuous diffusion barrier for phosphate dissolution. It is noteworthy that these

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samples differ mainly in k values, indicating that this process velocity is

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governed by coating thickness; and in t0, which may be interpreted as the

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necessary time to the coating attain equilibrium to the solution. These results

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also show that PU coated samples are changing their structure during release,

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probably by the interaction with water opening pores or void spaces.

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In this case, the similar n and k values for 6.0 and 7.5 wt% PU coated

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samples indicate that the diffusion is continuously occurring through the coating,

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whereas the strong deviation observed for 9.5 wt% PU coated sample indicate

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that the coating is changing its structure during release. Similar results were

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observed for urea-coated granules, showing that the chemical interaction of PU

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coating to water is almost the same. The adjust constants in modified Ritger-

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Peppas model were depicted in Table 2.

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A soil incubation experiment was conducted to evaluate the effectiveness

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of the castor oil-based PU resin for DAP coating in order to control the release

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of phosphorus. Figure 4 and Table 3 shows a plot of the remaining P content of

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the DAP granules after different incubation periods. The results indicated that

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the treatments could be classified into three groups. The first "fast release"

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group showed total P release in the first 24 h after incorporation of the fertilizer

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in the soil, and included the untreated DAP and the treatments using 1.5 wt%

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and 3.0 wt% coatings (Table 3). The second "slow release" group showed

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release of around 60% during periods of between 168 h and 672 h, and

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included the DAP treatments using 4.5 wt%, 6.0 wt%, and 7.5 wt% PU. The

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third "extended release" group consisted of the DAP coated with 9.0 wt% PU,

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which showed only 6.0% release of P in the first 168 h, and 95% release after

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672 h. These results were in agreement with the release of P into water from

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coated DAP (Figure 2), reflecting a positive correlation between the release

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tests performed using water and soil. As suggested by the release tests using

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water (Figure 2), the poor release control exhibited by DAP coated with 1.5 wt%

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or 3.0 wt% PU could be attributed to the presence of imperfections (cracks) in

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the coating, showing that 3.0 wt% PU was not sufficient to adequately cover the

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granules. However, further increase in the proportion of PU increased the

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thickness of the coating, thereby reducing the imperfections. Hence, the 9.0

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wt% PU treatment, which resulted in the greatest thickness of coating,

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prevented any significant release of P for up to 168 h after incorporation into the

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soil. However, after this time, the degradation of the coating by the action of

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microorganisms present in the soil was intensified, leading to an increase in the

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quantity of pores (cracks) in the coating and simultaneously increasing the

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release of phosphorus.

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Figure 5 shows the recovery profile of available phosphorus in the soil for

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different periods of incubation of the DAP granules. The soil phosphorus

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concentration reflected the profile of release from the granules, and the

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treatments could be grouped in the same way as for the data shown in Figure 6.

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For the uncoated DAP and the granules coated with 1.5 wt% and 3.0 wt% PU,

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the maximum concentration of available phosphorus in the soil was observed at

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168 h. However, the DAP treated with 4.5 wt%, 6.0 wt%, and 7.5 wt% PU

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presented the highest concentrations at 336 h, reflecting slower release. On the

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other hand, in the case of the DAP treated with 9.0 wt% PU, the concentration

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started to increase after 168 h and then continued to increase until the end of

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the incubation period.

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Table 4 shows the average values for P available in the soil during the

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incubation period. Due to the rapid and similar release of P from the uncoated

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DAP and the granules coated with 1.5 wt% and 3.0 wt% PU, the available P

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content of the soil exceeded 53 mg.kg-1 up to 24 h. For the treatment using DAP

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coated with 4.5 wt% PU, there was a significant increase in available P content

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from 72 h of incubation. In the case of the treatments with DAP coated with 6.0

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wt% and 7.5 wt% PU, there were increases in the soil P content from 168 h,

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while the use of DAP coated with 9.0 wt% PU resulted in an increase from 336

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h of incubation (after 14 days).

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The soils treated with uncoated DAP or DAP coated with 1.5 wt% and

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3.0 wt% PU showed maximum contents of available phosphorus after 168 h,

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with values exceeding 100 mg.kg-1. However, these contents decreased to

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about 70 mg.kg-1 by the end of the incubation period, corresponding to

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reductions of between 38.4 and 43.3 mg.kg-1. This could be attributed to the

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high P adsorption potential of the colloids present in the Oxisol used in the

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incubation experiment, as indicated by its low pH (5.18), high acidity potential

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(2.6 cmolc.kg-1), low available P content (0.7 mg.kg-1), and low P-rem value (2.1

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mg.L-1).23

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Smaller decreases of available phosphorus were observed for the

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treatments using DAP coated with 4.5 wt%, 6.0 wt%, and 7.5 wt% PU, with

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reductions of between 13.8 and 23.3 mg.kg-1. The lower phosphorus adsorption

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associated with these treatments could be explained by the shorter contact time

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of P with the soil, due to its slow release from the DAP granules. In contrast, the

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treatment using DAP coated with 9.0 wt% PU, with greater thickness of the

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coating, showed an increase in P release up to 672 h of incubation, with an

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available P content of 95 mg.kg-1. The latter finding could be attributed to the

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extended release of P from the DAP granules, which provided higher levels of P

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for longer periods after fertilization of the soil.

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In summary, the findings demonstrated that the coating of a soluble P

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source (DAP) with PU had an appreciable effect on the availability of

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phosphorus in an acid tropical soil. This effect was influenced by the coating

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thickness, since the use of thin coatings resulted in poor surface quality due to

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the irregularity of the granules, with the formation of cracks in the coating

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causing accelerated release of phosphorus. The results showed that thicker and

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more homogeneous coatings provided satisfactory control of P release, leading

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to extended availability of P in the soil over longer periods.

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ACKNOWLEDGMENTS

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The authors thank to financial support given by CAPES, FAPESP (grant

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#2016/09343-6),

CNPq

(402287/2013-4),

SISNANO/MCTI,

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Embrapa AgroNano research network for the support provided.

FINEP

and

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CORRESPONDING AUTHOR

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*Embrapa Instrumentação (CNPDIA), Rua XV de Novembro 1452, São Carlos,

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SP, 13560-970, Brazil.

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Phone: +55 16 2107 2915; E-mail: [email protected]

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TABLES

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Table 1. Actual coating amount as a percentage of the total weight, after drying. Sample DAP 1.5 wt% DAP 3.0 wt% DAP 4.5 wt% DAP 6.0 wt% DAP 7.5 wt% DAP 9.0 wt%

Experimental percentage (%) 1.9 ± 0.2 3.2 ± 0.3 4.5 ± 0.2 6.0 ± 0.2 7.5 ± 0.3 8.8 ± 0.4

406 407

Table 2. Release kinetic parameters according to the Ritger-Peppas model for

408

phosphorus released. Material

k (h-1) x 10-3

n

t0 (hours)

50.9 34.7 0.36 0.39 0.07 0.07 0.005

0.98±0.01 0.96±0.09 2.24±0.10 1.90±0.05 1.82±0.20 1.79±0.13 1.88±0.06

0 0 2 3 5 5 24

DAP (uncoated) DAP 1.5 wt% DAP 3.0 wt% DAP 4.5 wt% DAP 6.0 wt% DAP 7.5 wt% DAP 9.0 wt% 409 410

Table 3. Fertilizer phosphorus recovery as P-remaining from granules of DAP

411

or DAP-coated applied to the soil after different incubation periods. Treatment

P-remaining (%) after incubation period (hours) 0 24 168 672

DAP (uncoated) DAP 1.5 wt% DAP 3.0 wt% DAP 4.5 wt% DAP 6.0 wt% DAP 7.5 wt% DAP 9.0 wt% 412

*

413

(Tukey’s test, P