Bio-based Interpenetrating Network Polymer Composites from Locust

Jun 28, 2016 - Research Center for Slow and Controlled Release Fertilizers, College of Resources and Environment, Shandong Agricultural. University, T...
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Bio-based Interpenetrating Network Polymer Composites from Locust Sawdust as Coating Material for Environmentally Friendly Controlled-Release Urea Fertilizers Shugang Zhang,† Yuechao Yang,*,†,‡ Bin Gao,§ Yongshan Wan,†,‡ Yuncong C. Li,‡ and Chenhao Zhao† †

National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources, National Engineering & Technology Research Center for Slow and Controlled Release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Tai’an, Shandong 271018, People’s Republic of China ‡ Department of Soil and Water Science, Tropical Research and Education Center, Institute of Food and Agricultural Sciences (IFAS), University of Florida, Homestead, Florida 33031, United States § Agricultural and Biological Engineering, Institute of Food and Agricultural Sciences (IFAS), University of Florida, Gainesville, Florida 32611, United States ABSTRACT: A novel polymer-coated nitrogen (N) fertilizer was developed using bio-based polyurethane (PU) derived from liquefied locust sawdust as the coating material. The bio-based PU was successfully coated on the surface of the urea fertilizer prills to form polymer-coated urea (PCU) fertilizer for controlled N release. Epoxy resin (EP) was also used to further modify the bio-based PU to synthesize the interpenetrating network (IPN), enhancing the slow-release properties of the PCU. The N release characteristics of the EP-modified PCU (EMPCU) in water were determine at 25 °C and compared to that of PCU and EP-coated urea (ECU). The results showed that the EP modification reduced the N release rate and increased the longevity of the fertilizer coated with bio-based PU. A corn growth study was conducted to further evaluate the filed application of the EMPCU. In comparison to commercial PCU and conventional urea fertilizer, EMPCU was more effective and increased the yield and total dry matter accumulation of the corn. Findings from this work indicated that bio-based PU derived from sawdust can be used as coating materials for PCU, particularly after EP modification. The resulting EMPCU was more environmentally friendly and cost-effective than conventional urea fertilizers coated by EP. KEYWORDS: modified polymer-coated urea, bio-based coating, nitrogen release rate

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release fertilizers (CRFs).14 However, the high cost of these materials and complicated production processes limited their industrial production and large-scale applications for CRFs in agricultural fields.15 In addition, these petroleum-based synthetic coating materials are made from non-renewable resources. The residue shells are non-biodegradable and can be potentially harmful to the soil environment.16 Recent research has, thus, focused on bio-based and low-cost coating materials for CRFs to reduce environmental impacts.17 These renewable and biodegradable coating materials include lignin, chitin, keratin, cellulose, and starch, which can be used directly or after modification as the coating materials for PCUs.18−20 However, the longevity of those fertilizers is often less than 30 days, which greatly underscored the slow-release nature of these fertilizers.21−24 In this study, a polyurethane (PU) polymer was derived from liquefied locust sawdust (LLS) as the coating material for PCU. Epoxy resin (EP) was further used to modify the bio-based coating material to improve nutrient slow-release characteristics of PCU. As organic residues from the wood processing industry, locust sawdust (LS) is abundant, readily available,

INTRODUCTION The world population has now reached approximately 7 billion and will approach 9.5 billion by 2050, 1 requiring a corresponding increase in crop production and food supply. As a result, the agricultural sector is bound to use ever larger quantities of chemical fertilizers to increase food supply.1 The chemical fertilizer application rate is projected to increase by approximately 2.5 million tons per year, reaching 190.4 million tons in the world by the year 2015.2,3 In addition to the increasing fertilizer demand, low fertilizer use efficiency has also been a serious issue all over the world. This is especially true for nitrogen (N) fertilizer because of its N loses through volatilization, leaching, and runoff in the agricultural production system. About 45−55% of the total N fertilizer applied by farmers was lost as a result of low N fertilizer use efficiency.4,5 This may also result in inevitable environmental problems, such as water pollution and increased emission of hazardous gases (e.g., NH3 and N2O).6,7 Controlled-release fertilizer is put forward as a new technology to solve the problem of low fertilizer use efficiency and to satisfy the increasing demand.8−10 Polymer-coated urea (PCU) fertilizer is one of the effective techniques that can enhance N use efficiency and reduce environmental pollution.11 Petroleum-based synthetic materials, such as polyolefins,12 polysulfone,13 and glycerol ester, have been investigated as potential coating materials for PCU and other controlled © XXXX American Chemical Society

Received: April 14, 2016 Revised: June 19, 2016 Accepted: June 28, 2016

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DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry relatively inexpensive, and biodegradable.25−27 Cellulose, hemicellulose, and lignin are the primary constituents of LS. It can be liquefied to produce bio-based polyols for synthesis of PU polymer as the coating material of PCUs.28,29 The bio-based coating material also provides organic matter and nutrients (mainly N and P) to improve soil fertility after degradation in soil. The modifier, EP, is hydrophobic and can slow the process for moisture to enter the inner part of the PCUs through pin holes of the coating shell, thereby providing additional control of the N release.30 When EP and the bio-based PU are mixed together, an interpenetrating network (IPN) coating polymer can be formed.31 It is hypothesized that the IPN structure will make the new coating polymer more hydrophobic and wearable than the original bio-based PU polymer.32−38 To the best of the authors’ knowledge, however, none of the previous studies has attempted to use the bio-based polymer with EP modification as the coating material for PCUs. The objective of this study was to synthesize and evaluate the bio-based epoxy-resin-modified polymer-coated urea (EMPCU). First, LS was liquefied to produce bio-based polyols. Second, bio-based PU was synthesized using the polyols and diphenylmethane diisocyanate (MDI) to coat urea fertilizer prills. Finally, EMPCUs were prepared using EP to modify the bio-based PU to coat the urea prills.39,40 The relationships among the N release characteristics, coating content, and different EP contents in the coating material of EMPCUs were investigated.41−43 Finally, EMPCUs were further evaluated in a pot study for their application in crop production.44−48

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Scheme 1. (a) Liquidation Mechanism of Cellulose, Synthesis Mechanism of (b) PU and (c) EP, and (d) Synthesis Process Routing of EMPCU

MATERIALS AND METHODS

Materials. LS, collected from a wood-working factory in Tai’an, Shandong, China, was milled and dried at 105 °C in an oven for 24 h and then passed through a 60-mesh sieve. Sulfuric acid (97%, v/v), ethylene glycol, polyethylene glycol (molecular weight of 400), potassium persulfate (99.5%), N,N′-methylenebis(acrylamide) (MBA, 99%), glycol, polyethylene glycol (99%), and triethylene tetramine were purchased from Tianjin Kaitong Chemical Industry Co., Ltd. (Tianjin, China). Diphenylmethane diisocyanate (MDI) with 30.03 wt % NCO group was obtained from Yantai Wanhua Polyurethane Co., Ltd. (Shandong, China). E-44 (EP) was purchased from Shanhai Yugao Co., Ltd. (Shanghai China). Urea prills (3−5 mm in diameter and 46% N) were purchased from Shandong Hualu Hengsheng Chemical Industry Co., Ltd. (Shandong, China). Preparation of Liquefied Sawdust. The LLS was prepared by mixing ethylene glycol (1200 mL) and polyethylene glycol (800 mL) in a double-layer glass reaction kettle (3000 mL) equipped with a reflux condenser, thermometer, and motor-driven stirrer.3 After preheated to 100 °C, the reflux condenser was opened and the motor-driven stirrer was set to rotate at a speed of 800 rpm. The LS (400 g) was then poured into the reaction kettle, mixed with the solution, refluxed, and continuously stirred at 130 °C. At the same time, sulfuric acid (54 mL) was added to the reaction kettle. The mixture was allowed to react for 1 h under atmospheric pressure at 150 °C to convert cellulose into polyol (Scheme 1a).17 Finally, the LLS (biopolyols) was removed from the kettle for analysis and used as one of the coating materials. The LS can be liquefied for liquid (LLS) absolutely, which was complete to synthesize bio-PU without separating and purifying.27,28 As a result, the technology is easier and cheaper. Preparation of Liquefied Locust Sawdust Polymer-Coated Urea (BPCU). Fertilizers with LLS polymer coating were prepared at the laboratory scale from 1 kg of urea prills (3−5 mm in diameter and 46% N) for each coating sample. The prills were loaded into a rotating drum and preheated at 75 ± 2 °C for 10 min. After the preheating stage, 10.0 g of a mixture of coating materials (6.53 g of MDI, 3.45 g of

LLS, and 0.02 g of triethylene tetramine) was dropped onto the surfaces of the rotating urea prills. The heat-curing reaction of the mixed coating materials was finished in the rotating drum in 8−10 min, and the bio-based PU coating (Scheme 1b) was then synthesized and attached to the surface of the urea prills. The reaction mechanism for PU synthesis is illustrated in Scheme 1. The weight of bio-based PU coating accounted for approximately 1 wt % of that of the urea fertilizer. Three types of BPCUs (BPCU1, BPCU2, and BPCU3) were produced with different coating rates by repeating the coating process for 3, 5, and 7 times, respectively. Preparation of Epoxy-Resin-Coated Urea (EPCU). The same coating method was used to coat the prills with EP (Scheme 1c). EP was first heated to 80 °C in a water bath to soften it. A total of 10 g of coating material consisting of 8 g of E-44 and 2 g of triethylene tetramine were then used to coat urea prills in the drum. It took about 8 min to finish the reaction. The mechanism for EP synthesis is illustrated in Scheme 1c. The weight of EP coating accounted for approximately 1 wt % of that of the urea fertilizer. Similarly, three types of EPCUs (EPCU1, EPCU2, and EPCU3) were produced with different coating rates by repeating the coating process for 3, 5, and 7 times, respectively. Preparation of EMPCU. The same coating method was also used to prepare EMPCU, and its synthesis process routing was shown in Scheme 1d. The prills were preheated to 75 ± 2 °C in the drum, and the epoxy resin was heated at 80 °C to soften it for ease of pouring. Then, 10.0 g of three different mixtures of coating materials, (1) 1.6 g of E-44, 0.4 g of triethylene tetramine; 5.21 g of MDI, 2.59 g of LLS, 0.2 g of MBA, and 0.01 g of potassium persulfate, (2) 2.4 g of E-44, 0.6 g of triethylene tetramine, 4.5 g of MDI, 2.3 g of LLS, 0.2 g of MBA, and 0.01 g of potassium persulfate, and (3) 4 g of EP, 1 g of triethylene tetramine, 3.18 g of MDI, 1.82 g of LLS, 0.2 g of MBA, and 0.01 g of B

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry potassium persulfate, were poured onto the surfaces of the rotating urea prills. The EP contents were 20, 30, and 50% by weight in the mixed coating materials, respectively. The curing reaction of the mixed coating material was finished in the rotating drum in 8 min. The weight of EP-modified PU coating occupied approximately 1 wt % of that of the urea fertilizer in each coating process. Nine types of EMPCUs listed in Table 1 were produced with different coating rates by repeating the coating process for 3, 5, and 7 times for the three EP contents in the coating materials.

release of fertilizers reached over 80%. The N concentration was determined using the Kjeldahl method.35 The N release longevity of the coated fertilizers is defined as the time when the cumulative N release reached 80% of the total N. N Use Efficiency and Leaching. The influence of EMPCU on plant growth was investigated using sweet corn (Zea mays var. rugosa; the seed was named Bainuo No. 1, bought from Denghai Co., Ltd., China). The growth study of sweet corn was conducted outdoor at the new fertilizer test station of Shandong Agricultural University (SDAU), Tai’an, Shandong, China. The corn seeds were sown on June 21, 2015, and the plants were harvested on September 29, 2015. Plastic pots were used to plant the corn. Soil samples were taken locally from 0 to 10 cm in the field at the Experimental Station of SDAU. The soil is Typic Hapli-Udic Argosols. Each pot (diameter of 50 cm and height of 50 cm) was filled with 25 kg of soil. Three types of fertilizers (conventional urea, KPCU bought from Kingenta Co., Ltd., China, with N release longevity of 3 months, and EMPCU6) and two N rates (0 and 2.5 g of N pot−1) were used. The same amount of phosphorus (2 g of P2O5 pot−1, as potassium dihydrogen phosphate) and potassium (2 g of K2O pot−1, as potassium sulfate) were applied to each pot as base fertilizers. For the conventional urea, the N application was split 2 times. About half of the total conventional N fertilizer was used as the base fertilizer before planting seeds, and the remaining half was applied on the 30th day after planting. For the two controlled-release fertilizers (i.e., KPCU and EMPCU6), the fertilizer was applied in a single dose at the time when the seeds were planted. Three seeds were planted in each pot, but only one seedling was kept in each pot after the 7th day of germination. The planting and management practices, such as irrigation cultivation, fertilizer application, and pest and weed control, were closely followed by local agronomic practices.3 During the growth experiment, soil samples were collected from the root zone in the six-leaf collar stage, blossom stage, and maturity stage. All soil samples were air-dried and passed through a 10-mesh sieve. To determine effective N content in soil, 2 g of soil sample was put into a 50 mL centrifuge tube containing 20 mL of 0.01 mol/L calcium chloride solution. The sample were shaken in a mechanical shaker at 200 rpm for 1 h. The solution was passed through a quantitative filter paper (aperture of 30−50 μm) for determination of NH4+ N and NO3− N using an AQ2 auto analyzer.3 Leachate samples were also collected during the experiment. Ammonia and nitrate in the leachate were analyzed with the AQ2 auto analyzer. After plants were harvested, plant dry biomass was determined by oven drying at 80 °C. The plant samples were ground to pass through a 425 μm sieve. The N concentration was measured using the automated chemistry analyzer (AMS Smartchem 200, Italy). The N uptake, corn yield, and N use efficiency were calculated and evaluated. Statistical Analysis. All statistical analyses were conducted using Statistical Analysis System (SAS) package, version 9.2 (SAS Institute, Cary, NC). Comparisons of various treatments were evaluated using the analysis of variance (ANOVA). Mean separation tests (Duncan’s multiple range test) were used to determine significant difference among treatments. Regression equations and coefficients were calculated between N release longevity and coating rate for BPCU, EPCU, and EMPCU. The differences among means and correlation coefficients were considered significant when p < 0.05.

Table 1. Composition of Various Coated Fertilizersa fertilizers

design of the coating content (%)

actual coating content (%)

total N (%)

BPCU1 BPCU2 BPCU3 EPCU1 EPCU2 EPCU3 EMPCU1 EMPCU2 EMPCU3 EMPCU4 EMPCU5 EMPCU6 EMPCU7 EMPCU8 EMPCU9

3 5 7 3 5 7 3 5 7 3 5 7 3 5 7

2.91 4.92 6.87 2.89 4.91 6.89 2.90 4.91 6.94 2.85 4.88 6.89 2.90 4.95 6.91

44.66 43.74 42.84 44.67 43.74 42.83 44.67 43.74 42.81 44.69 43.76 42.83 44.67 43.72 42.82

a

The content of total N was determined from calculation. The EP was 20% by the weight of mixed coating materials in EMPCU1, EMPCU2, and EMPCU3. The EP was 30% by the weight of mixed coating materials in EMPCU4, EMPCU5, and EMPCU6. The EP was 50% by the weight of mixed coating materials in EMPCU7, EMPCU8, and EMPCU9.

Water Absorption Rate of the Coating Material. The procedure for measuring the water absorption rate in this study is similar to the method used by Tomaszewska et al.13 for testing the porosity of the coating films. Briefly, each of the five coating materials synthesized as above was pasted on a glass plate with a glass bar. The sample was dropped evenly on the glass plate at about 3 mm thick and was allowed to cure at room temperature for 48 h. After that, all of the cured sample was removed from the glass plate and oven-dried at 60 °C until reaching a constant weight 5.0000 g of each coating material submerged in 200 mL of deionized water in a container on a shaker. Samples were taken out at different time intervals (1, 3, 5, 16, and 24 h), and the surface was dried with absorbent paper. Then, each coating material was weighed, and its water absorbent rate was calculated on the basis of the amount of water absorbed. Characterization of BPCU, EPCU, and EMPCU Coatings. The coatings of BPCU, LLS, EMPCU, and EPCU were dried at 40 °C for 24 h. Then the coating materials were powdered, mixed with KBr powder, and then compressed to make pellets for Fourier transform infrared spectroscopy (FTIR) characterization at the wavenumber range from 500 to 4000 cm−1. The morphologies of the coating layer were examined using scanning electron microscopy (SEM, Zeiss microscope EVO LS15, Germany). To examine the cross-sections of the coating layers, the coated fertilizer pellets were cut into two halves and the cross-sections were coated with a gold layer prior to SEM analysis. N Release Characteristics of BPCU, EPCU, and EMPCU. The percentages of N release from BPCU, EPCU, and EMPCU in the first 24 h were measured in water at 25 °C. With three replicates, 10 g of the coated fertilizer was placed in a glass bottle containing 200 mL of deionized water and kept in an electroheating, standing-temperature incubator at 25 ± 0.5 °C. The N released from each of the coated fertilizers at 1, 3, 5, 7, 10, 14, 28, and 63 days or until the cumulative N

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RESULTS AND DISCUSSION Morphologies of BPCU, EPCU, and EMPCU. The SEM micrographs showed that the surface of the coating shell of BPCU (i.e., coated only with bio-based PU) was very coarse with layered superposition phenomenon (panel A1 of Figure 1). Many pin holes and an irregular structure were observed in the cross-section micrographs of the BPCU shell (panel A2 of Figure 1). These pin holes and an irregular structure might be easily permeated by water, causing quick release of nutrients from the coated fertilizer. The surface and cross-section of the coating shell of EPCU (i.e., coated only with EP) appeared C

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Figure 1. Scanning electron microscope images of BPCU, EPCU, and EMPCU. Panels A1, B1, C1, D1, and E1 showed the surface of the BPCU, EPCU, 20% EP EMPCU, 30% EP EMPCU, and 50% EP EMPCU coating shell, respectively. All image magnification is 1000×. Panels A2, B2, C2, D2, and E2 showed the cross-section of the BPCU, EPCU, 20% EP EMPCU, 30% EP EMPCU, and 50% EP EMPCU coating shell, respectively. All image magnification is 20000×.

much more smooth, compact, and uniform than that of the BPCU (panel B1 of Figure 1). When EP and curing agent were added to the bio-based PU coating material, the surface of the

coating shell of EMPCUs (panels C1, D1, and E1 of Figure 1) was also much smoother than that of BPCU (panel A1 of Figure 1). Furthermore, the cross-section of coating shells of D

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry EMPCUs (panels C2, D2, and E2 of Figure 1) was much more compact than that of BPCU (panel A2 of Figure 1), suggesting that EP was an effective amendment for the bio-based PU for fertilizer coating. When different amounts of EP were used in the coating, the EMPCU shells also exhibited different surfaces and cross-section structures. In all of the coating shells of the EMPCUs, the one with 30% EP (panels D1 and D2 of Figure 1) showed a much smoother and more compact surface and cross-section than the others (panels C1, E1, C2, and E2 of Figure 1). Most of the fertilizer prills were then separated when the coating materials solidified on the surface of the prills. However, too much EP in the coating tended to create high viscosity, resulting in clusters of fertilizer prills being bound together (Figure 2b). For example, when 50% EP was added, the fertilizer prills were bound together during the rapid curing process.

Figure 3. FTIR spectra of PCU, ECU, EMPCU, LLS, and LS.

observed at 1227 cm−1 were assigned to the stretching vibration of the C−O−C bond. The results confirmed that PU was created by the reaction between LLS and MDI. Three characteristic absorption peaks of the EPCU shell (EPCU in Figure 3) at 3417, 1613, and 1255 cm−1 corresponded to the stretching vibration of −OH, CC of the aromatic ring, and the C−O−C bond, respectively. In EMPCU, the peak at 3418 cm−1 was assigned to the stretching vibration of the N−H bond and the 1635 cm−1 peak was assigned to the stretching vibration of the βN−H bonding (EMPCU in Figure 3). The two peaks at 3417 and 1635 cm−1 observed in EPC and LS were also shown in EMPCU, indicating that EMPCU might carry the property of both EP and PU.2,3 N Release Characteristics of BPCU, EPCU, and EMPCU. The release curves of different fertilizers coated with 3% coating materials showed that the N initial release rate of BPCU1 reached more than 80% during the first day of incubation (Figure 4). The corresponding N release rates were 46% for EPCU1, 15% for 20% EMPCU1, 4% for 30% EMPCU1, and 12% for 50% EMPCU1. At the 10th day of incubation, the N release rates were 100% for BPCU1, 93% for EPCU1, 78% for 20% EMPCU1, 80% for 30% EMPCU1, and 84% for 50%

Figure 2. (a) Photograph of various coated fertilizers and (b) fertilizer prills bound together.

All kinds of feitilizer were shown (Figure 2a). FTIR Analysis of Coating Shells. The FTIR spectra of LS, LLS, EPCU, EMPCU, and BPCU revealed chemical shifts during biomass liquefaction and the synthesis process of PU (Figure 3). For example, in LS, the existence of −OH, CC, and C−O bonds corresponded to three characteristic absorption peaks for cellulose and lignin at 3415, 1637, and 1122 cm−1, respectively (LS in Figure 3). In LLS, however, a right shift was observed for −OH at 3296 cm−1, CC of the aromatic ring at 1570 and 1475 cm−1, and two characteristic absorption peaks at 1312 cm−1 representing −COO bonds (LLS in Figure 3). The wide peaks around 3200 cm−1 were indicative of the stretching vibration of −OH bonds from both LS and LLS. The absorption peaks of the BPCU shell (BPCU in Figure 3) observed at 3419 cm−1 were assigned to the stretching vibration of the N−H bond, observed at 1539 and 1616 cm−1 were assigned to stretching vibration of the βN−H bonding, and

Figure 4. Release curves of N for fertilizers coated with different materials (3% coating content) at 25 °C in water. E

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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the coating material was increased from 3 to 5 and 7%, the N release longevity of the BPCU increased from 3 to 14 and 28 days, respectively (Figures 4−6). Correspondingly, the N release longevity increased from 7 to 28 and 80 days for EPCU, from 10 to 21 and 56 days for 20% EMPCU, from 14 to 28 and 80 days for 30% EMPCU, and from 10 to 21 and 56 days for 50% EMPCU (Figures 4−6). The results indicated that the LLS modified by EP significantly improved the release longevity of coated fertilizers. Because the materials (triethylene tetramine and E-44) that are required for synthesizing 1 g of EPCU are more expensive than that of 1 g of EMPCU, EMPCU can be a cheaper replacement of EPCU, and EP is sticky, so that the prills were easily coherent (Figure 2b). Findings from this work indicated that the 30% EMPCU provided the best controlled release effect. In a previous study, corn straw or wheat straw was also used to synthesis BPCU.14 However, the BPCU synthesis by LS in this work was better. For example, the longevity of BPCU synthesis by LS was 10 days, but the longevity of BPCU synthesis by corn straw or wheat straw was only 3−5 days in a 3% coating ratio. Water Absorption Rate of Five Coating Materials. Within 1 h, the water absorption rate was 12, 1.5, 9, 1, and 3.7% for BPCU, EPCU, 20% EMPCU, 30% EMPCU, and 50% EMPCU, respectively (Figure 7). When the incubation time

EMPCU1. Overall, the release rates of EMPCU1 and EPCU1 were slower than that of PCU1. The N initial release rate (on the first day) of fertilizers coated with 5% coating materials were 21, 7, 9, 0.5, and 4% BPCU2, EPCU2, 20% EPCU2, 30% EMPCU2, and 50% EMPCU2, respectively (Figure 5). When the incubation time

Figure 5. Release curves of N from fertilizers coated with different materials (5% coating content) at 25 °C in water.

reached 28 days, the cumulative N release rates were 100, 83, 91, 85, and 97% for BPCU2, EPCU2, 20% EMPCU2, 30% EMPCU2, and 50% EMPCU2, respectively. The release rates of 30% EMPCU2 and EPCU2 were similar and lower than the others. Similarly, the initial N release rates of fertilizers coated with 7% coating materials were 12, 4, 4, 0, and 2% for BPCU3, EPCU3, 20% EMPCU3, 30% EMPCU3, and 50% EMPCU3, respectively (Figure 6). When the incubation time reached 80 days, N cumulative release rates were 92, 75, 87, 71, and 86% for BPCU3, EPCU3, 20% EMPCU3, 30% EMPCU3, and 50% EMPCU3, respectively. The N release characteristics of the coated fertilizers were significantly affected by the coating rate of the coating material.1 The cumulative N release rate also decreased with an increase in the weight percent of the coating material. When

Figure 7. Water absorption rate of five coating materials at 25 °C in water.

reached 24 h, the water absorption rate was 20.5, 3.2, 16, 8.5, and 11% for BPCU, EPCU, 20% EMPCU, 30% EMPCU, and 50% EMPCU, respectively. In general, EPCU absorbed the least water, BPCU absorbed the most water, and 30% EMPCU absorbed less water than 20 and 50% EMPCU. Note that the more water being absorbed by coating materials, the shorter the longevity of coated fertilizers (Figures 4−6). This was probably because water sorbed within the coating shells can promote the release of fertilizer through diffusion. Relationships among the Coating Ratio, EP Additive Amount, and Nutrient Release Characteristics. The N initial release rate had a very close relationship (R2 ≥ 0.94) with the coating rate and amount of EP used (Figure 8A). The close relationship indicated that the N initial release rate of coated fertilizers could be well-predicted with the three-dimensional fitting equation. Note that when the additive amount of EP is 30%, the controlled release effect is the best (Figure 8B). The reason may be that, when EP and PU form IPNs, the cross-

Figure 6. Release curves of N from fertilizers coated with different materials (7% coating content) at 25 °C in water. F

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Figure 8. Relationship between the N initial release rate of the coated fertilizer, coating content, and different EP contents in the coating material (A, z = 67.35 − 6.6333x − 2.5278y + 0.45x2 + 0.0344y2, with r2 = 0.94; B, z = −51.8611 − 7.3333x + 4.8333y + 1.4167x2 − 0.07y2, with r2 = 0.80).

Table 2. Dry Biomass, Yield, N Uptake, and Total N Efficiency of Sweet Corn under Different N Fertilizer Treatmentsa N uptake (mg plant−1) fertilizer treatment CK urea EMPCU6 KPCU a

dry biomass (g plant−1) 44.34 71.68 97.54 97.09

actual yield (g plant−1)

c b a a

78.19 129.36 180.79 165.22

1000 kernel weight (g plant−1)

c b a a

189.61 215.71 281.96 275.83

plant N uptake

c b a a

380.9 514.8 877.3 806.7

ear N uptake

c b a a

909.3 1732.4 2108.9 2024.6

total N uptake

c b a a

1289.2 2247.2 2986.2 2831.3

total N use efficiency (%)

c b a a

38.32 b 67.88 a 61.68 a

Means within the same column followed by different letters are significantly different (the Duncan multiple range tests; p < 0.05).

Table 3. Concentrations of NO3− N and NH4+ N in the Root Zone Soil during Different Stages of Corn Development under Different Fertilizer Treatmentsa NO3− N (mg/kg soil)

fertilizer treatment

b

CK urea EMPCU6 KPCU

SLF 1.65 2.64 7.86 9.54

b b a a

BS 2.20 2.21 9.60 4.60

MS b b a ab

NO3− N + NH4+ N (mg/kg soil)

NH4+ N (mg/kg soil) 0.66 0.57 1.41 1.67

SLF a a a a

1.07 1.14 1.24 1.65

BS

b ab ab a

1.04 1.08 1.42 1.22

MS a a a a

0.90 1.05 1.31 1.40

SLF a a a a

2.72 3.78 9.10 11.20

BS b b a a

3.24 3.29 11.02 5.82

MS b b a ab

1.57 1.62 2.71 3.07

a a a a

a

Means within the same column followed by different letters are significantly different (the Duncan multiple range tests; p < 0.05). bSLF, six-leaf collar stage; BS, blossom stage; and MS, maturity stage.

The concentrations of NO3− N, NH4+ N, and total (NO3− N + NH4+ N) of the root zone soil were different in the different plant growth stages and for different treatments (Table 3). The different fertilizer treatments had significant impacts on the mean concentrations of NO3− N, NH4+ N, and total in the sixleaf collar stage and blossom stage but not in the maturity stage. In the last growth stage of the plant, N in EMPCU6 and KPCU was almost completely released and, thus, showed less impact on the last growth stage. In comparison to the urea treatment, EMPCU6 and KPCU significantly increased mean N concentrations (NO3− N, NH4+ N, and total) in the root zone soil during the growth stage. The available N concentrations of both EMPCU6 and KPCU were 2−4 times of that of U3 during the growth stage, indicating that coated fertilizer increased N concentration (NO3− N, NH4+ N, and total) bioavailability. The N leaching rates of the conventional urea, KPCU, and EMPCU were 40.25, 12.48, and 10.53%, respectively (Figure 9), confirming that coated fertilizers greatly improved the N use efficiency and reduced N leaching.

linking density of two materials is highest with 30% EP added (see Figure 1). As a result, the materials had less gaps or pin holes, providing the longest longevity of coated fertilizers (Figures 4−6). Performance of EMPCU as Fertilizer for Corn Growth. Total N use efficiencies of sweet corn in the urea (conventional fertilizer), KPCU, and EMPCU treatments were 38.32, 61.68, and 67.88%, respectively (Table 2), and the performance of EMPCU was the best. The N use efficiency of the plants applied EMPCU was 77.14% greater than the conventional urea treatment, although not significantly greater than that of KPCU application. In comparison to that of the conventional urea treatment, the actual corn yields of KPCU and EMPCU were significantly higher. The corn yield of the EMPCU treatment was 39.76% greater than the conventional urea treatment, and the KPCU treatment was 27.72% greater. There were no significant differences between EMPCU and KPCU treatments on N use efficiency, ear yields, and total dry matter accumulation. G

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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REFERENCES

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Figure 9. Total N leaching rate from different fertilizer treatments. The bar values indicate mean and standard error. Different letters indicate significant differences (p < 0.05).

A bio-based PU polymer for fertilizer coating was derived from the LLS, a readily available byproduct of the wood processing industry. EP was used to modify the bio-based coating material to improve its controlled-release properties. The EMPCU exhibited excellent controlled-release characteristics with the N release longevity of more than 2 months. In comparison to the unmodified PCU coating, the EMPCU coating showed lower porosity and water absorption rate and, thus, reduced the N release rate. Results from the pot growth study suggested that EMPCU showed comparable or even better performance to a commercially available polymer-coated fertilizer in improving N use efficiency and reduction of N leaching. This novel coating technology, thus, has great potential for large-scale applications to satisfy the increasing demand for fertilizers because it is economical and environmentally friendly.

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Article

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-538-824-2900. Fax: +86-538-824-2250. Email: [email protected]. Funding

This research was funded by the National Natural Science Foundation of China (Grant No. 31572201), the Natural Science Foundation of Shandong Province (Grant No. ZR2015CM035), Shandong agricultural innovation team (SDAIT-17-04), the projects of commercialization of research findings of Shandong Province (Grant No. [2014] 183), the Great innovation projects in agriculture of Shandong Province (Grant No. [2013] 136). Notes

The authors declare no competing financial interest.

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ABBREVIATIONS USED PU, polyurethane; LS, locust sawdust; LLS, liquefied locust sawdust; IPN, interpenetrating network; MDI, diphenylmethane diisocyanate; MBA, N,N′-methylenebis(acrylamide); EP, epoxy resin; BPCU, liquefied locust sawdust polymercoated urea; EPCU, epoxy-resin-coated urea; EMPCU, epoxyresin-modified polymer-coated urea; FTIR, Fourier transform infrared spectroscopy H

DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jafc.6b01688 J. Agric. Food Chem. XXXX, XXX, XXX−XXX