Abstract
Urea cocrystal materials are a potential fertilizer source that has shown to decrease environmental nitrogen losses. Novel nitrogen (N)-containing urea cocrystal fertilizers, CaSO4·4urea (UC1) and Ca(H2PO4)2·4urea (UC2), were synthesized using the mechanochemical method to form stable urea cocrystals to be tested as a fertilizer source for turfgrass. The objectives of this study were to 1) evaluate the response of ‘Tifway’ hybrid bermudagrass (Cynodon dactylon × C. traansvalensis Burt Davy) to N fertilization by urea cocrystals and traditional coated urea products (MU·PCU, methylene urea, urea, polymer-coated urea; PCU, polymer-coated urea, urea) supplied at two rates at the beginning of two, 10-week study periods conducted under a greenhouse setting and 2) investigate N release behavior of urea and two cocrystal products using a rapid water release test. In the turfgrass response study conducted in the greenhouse, improved turfgrass quality above the minimum quality threshold was observed when averaging across all products. For normalized difference vegetation index (NDVI), cocrystal outperformed all other products in the summer study and both cocrystal products outperformed the traditional product (MU·PCU) in the winter study. Further, both cocrystal products showed favorable growth responses compared with the commercial products provided by positive clipping production and vertical extension rates. In the nitrogen release experiment, a rapid water release test revealed the N release peak of urea was significantly higher than both UC1 and UC2. Furthermore, significantly higher N was leached from urea (15% loss) compared with both UC1 and UC2 (≈8% loss). Results from both studies provide evidence supporting suitability of urea cocrystal application on bermudagrass and potential as a slow-release fertilizer source through sustained turfgrass vigor, growth, decreased N release peak, and decreased leaching losses.
Fertilization is a primary component for enhancing and maintaining turfgrass quality because most soils cannot supply sufficient amounts needed for healthy growth (Carey et al. 2012; Turner and Hummel 1992). N is needed in the greatest quantity compared with other essential nutrients, so managing N is among the most important cultural factors controlling turfgrass quality (Hull 1996; Munson 1998). However, the production of nitrogenous fertilizers requires significant amounts of energy, which results in significant CO2 emissions (Manthiram and Gribkoff 2021; Mudahar and Hignett 1985). Further, the application of low-stability nitrogen compounds to the environment has resulted in adverse ecological and climate impacts (Liu et al. 2010). Because of the current human impact on the global environment, it is important to decrease energy consumption during production life cycles and improve N fertilizer material stability (Eisa et al. 2022).
Urea is the most widely produced and used N compound, accounting for 50% to 60% of the world’s total N supply (Heffer and Prud’homme 2016; Serrano-Silva et al. 2010). High utilization is primarily due to low transportation costs from its high N content (46%) and relative stability during storage; however, its high water solubility gives rise to environmental concerns as research has shown nearly half of what is applied has been lost by reactive pathways (Chen et al. 2008; Coskun et al. 2017; Paulot and Jacob 2014). Physical or chemical transformations that decrease water solubility can aid in mitigating environmental losses by controlling the rate of N release from the formulation (Varadachari and Goertz 2010). Physical transformations to decrease water solubility include encapsulating the nitrogen source by coating the source with sulfur, a polymer, or a combination of both (Azeem et al. 2014). Chemical transformations include conversion to less-soluble forms or combining urea with a coforming molecule to synthesize a cocrystal (Alexander and Helm 1990). These formulations are broadly categorized as slow (i.e., controlled) release and are a preferred N formulation to use in turfgrass management (Soldat et al. 2008). Along with the environmental benefits, these formulations can supply the plant N in a sustained and more uniform manner compared with their fast (i.e., quick) release counterparts. Although sulfur- and polymer-coated products are widely used in turfgrass maintenance, no published research has reported the use of emerging slow-release N cocrystal fertilizer products on turfgrass.
Several urea cocrystal formulations have been developed that have been shown to reduce gaseous losses (NH3, ammonia) from soil. These include compacting urea with NH4Cl, ZnSO4, and MgSO4 along with newer formulations through mechanochemically synthesizing urea cocrystals using inorganic salts (Purakayastha and Katyalt 1998a; Purakayastha and Katyalt 1998b; von Rheinbaben 1987). The application of urea cocrystals has shown to provide increased grain yields in maize (Zea mays L.) and shown positive results in minimizing N loss and improved N use efficiency in sorghum (Sorghum bicolor L.) (Bista et al. 2023; Swify et al. 2022). The objectives of this study were to 1) evaluate the response of ‘Tifway’ hybrid bermudagrass to N fertilization by urea cocrystals and traditional coated urea products supplied at two rates at the beginning of two, 10-week study periods under a greenhouse setting and 2) investigate N release behavior of urea and two cocrystal products using a rapid water release test.
Materials and methods
Cocrystal synthesis
In this study, urea [CO(NH2)2 (Sigma-Aldrich, St. Louis, MO, USA); assay 99.0% to 100.5%], calcium sulfate dihydrate (purity 98+%, ACROS) and triple superphosphate [purity 99+%, (J.T. Baker, Phillipsburg, NJ, USA)] were purchased as powders and used as received. CaSO4·4urea and Ca(H2PO4)2·4urea cocrystals were synthesized using mechanochemical methods described previously by Honer et al. (2017). Briefly, the raw materials were initially subjected to milling using a Retsch MM300 ball mill (Retsch, Haan, Germany) equipped with a stainless steel 15-mL milling jar (Form-Tech Scientific, Montreal, Canada) with a milling time of 10 min at 25 Hz using two 8-mm zircona balls and a total sample weight of 0.200 g. After the conversion was confirmed using X-ray powder diffraction, the process was scaled up using a Retsch PM100 planetary mill equipped with a 250-mL stainless steel jar operating at 500 rpm for 5 min containing 20 stainless steel balls to produce 200-g quantities for soil experiments in 50-g batches. Each 50-g batch contained the same ratio of the precursors, including 26 g of triple superphosphate (on a dry basis) and 24 g of urea or 21 g of calcium sulfate dihydrate and 29 g of urea.
Greenhouse experiment
Studies were conducted in a greenhouse at the US Arid Land Agricultural Research Center in Maricopa, AZ, USA, over 10-week periods in the summer, and then were repeated in the winter. Summer and winter studies were initiated on 8 Aug 2022 and 9 Jan 2023, respectively. Air temperatures in the greenhouse were set to 33/26°C (day/night) for both studies. Seven to 10 weeks before each study, 14-cm-diameter washed sod plugs of ‘Tifway’ hybrid bermudagrass were established in pots constructed from PVC pipe (PVC) (15.2 cm diameter × 30.5 cm depth). Pots were filled with a US Golf Association specification sand [90:10 (vol:vol) sand:peatmoss]. A 10-mm hole was drilled at the bottom of each pot to allow water drainage and a plant and seed guard cloth (DeWitt, Sikeston, MO, USA) was then laid at each base to avoid sand spillage. During the establishment period, all lysimeters were maintained in a well-watered state to ensure rapid establishment and trimmed one to two times weekly to a height of 2.5 cm.
Both studies were arranged in a completely randomized design to support four fertilizer source × two application rate factorial experiments. Each pot received one of the four fertilizer treatments (Table 1) applied at a rate of 4.9 g N/m2 (low rate) or 9.8 g N/m2 (high rate) on the initial date of each study. The traditional product MU·PCU contained 7.3% slowly available N from methyleneureas and polymer-coated sulfur-coated urea. Polymer-coated urea (PCU) contained 7.5% slow-release N from PCU. Cocrystals were applied in powder form. All products were applied by hand to ensure even application over the turfgrass surface. Three days before each trial initiation, all lysimeters received 15 mL of a N-free Hoagland solution (Plantmedia, Dublin, OH, USA). No additional nutrient applications were made between establishment and treatment application. Further, no nutrient applications were made after the treatments were applied. Pots were well-watered during each establishment period and were watered to a weekly depth of 40 mL during each study period.
List of fertilizers used in both summer and winter studies including nitrogen (N) source and fertilizer analysis applied to ‘Tifway’ hybrid bermudagrass.
Pots were evaluated weekly for turfgrass quality according to a modified National Turfgrass Evaluation Program (NTEP) visual quality ranking system (scale 1 to 9; minimum quality = 5) (Morris and Shearman, 1998). For reference, a rating of 1 indicated completely brown or dead turf and a rating of 9 would represent perfectly green turf that is fully dense and dark green. Turfgrass vigor NDVI was also collected weekly using a RapidScan CS-45 NDVI meter (Holland Scientific, Lincoln, NE, USA) that contains an internal lighting source held 0.7 m above the turf canopy. Pots were spaced apart evenly to avoid any background materials from adjacent pots.
Clipping production and vertical extension rates were assessed through weekly clipping collections and height measurements. The turfgrass canopy on each pot was trimmed to a 2.5-cm height using scissors and a ruler every 7 d. After each collection, the clippings were oven dried for 72 h at 65°C and weighed to obtain dry weight values. For each period, clipping dry weights and height differences were divided by the number of growing days to calculate daily growth (mg·d−1) and vertical extension rate (mm·d−1).
Data for all parameters were subjected to analysis of variance (ANOVA) in JMP 15.2.0 (SAS Institute, Cary, NC, USA). Mean separation procedures were performed usings Tukey’s honestly significant difference test (comparing means of ≥ 3 groups) or Student’s t test (comparing means of two groups) at P ≤ 0.05 level.
Column experiment
This experiment was conducted in a laboratory at the US Arid Land Agricultural Research Center in Maricopa, AZ, USA, in Oct and Nov 2023. A column experiment was used to investigate N release behavior using a rapid water release test setup shown in Fig. 1 of the two cocrystal products (Table 1), urea, and unamended soil. This portion only focused on the comparison of N release of the cocrystal products and urea for which further analysis of the traditional products (MU·PCU and PCU) was omitted. Previous studies have demonstrated notable distinctions between conventional urea and slow-release materials within a mere 320-s timeframe (Kottegoda et al. 2017). In this study, we conducted an accelerated test spanning 1600 s to further investigate these differences.
Schematic representation of experimental setup for the dissolution experiment.
Citation: HortTechnology 34, 4; 10.21273/HORTTECH05423-24
Treatments consisted of 4.92 mg-N from each N source and an unamended control with no additional N. By using a single rate, authors were able to focus on the effects of urea release and isolate the impacts of dose. Further, by using the lowest N dose, authors were able to best address the research question and observe differences in nitrification. Nitrogen amounts were chosen to match N concentration of the 4.9 mg·m−2 used in the turfgrass response experiment. The column was composed of 0.5 g of glass wool, 100 g of Ottawa sand, N source to be tested (0.0049 g), 215 g of sandy loam soil, 100 g of sand, and 0.5 g of glass wool (from bottom to top).
Distilled water was pumped from the bottom of the column at a rate of 3.75 mL·min−1, with a linear flux of 0.23 cm·min−1. The eluted water was collected at 20-s intervals continuously. Immediately after collecting, ammonia N (N-NH3) and nitrate N (N-NO3−), concentrations were determined using an AutoAnalyzer (AutoAnalyzer 3; SEAL, Norderstedt, Germany).
For the column experiment, polynomial regression was performed using linear mixed-effects ANOVA in Rstudio 1.4.1717 (RStudio PBC, 2021).
Results and discussion
Greenhouse experiment
For visual quality, NDVI, clipping production, and vertical extension ANOVA found a significant study effect. For this reason, results are presented separately for each study (Table 2). These differences are likely due to a combination of factors including lower quality of the turfgrass at trial initiation in the summer compared with winter (Table 3) and longer days and more intense solar radiation (data not shown) during the summer period. The lower quality at trial initiation for the turfgrass in the summer study could possibly be explained by the shorter recovery time of 7 weeks from planting to trial initiation, compared with 10 weeks in the winter study.
Analysis of variance table for measuring date, fertilizer source, and application rate effects on visual quality, normalized difference vegetation index (NDVI), clipping production, and vertical extension rate during the summer and winter studies.
Turfgrass visual quality, normalized difference vegetation index (NDVI), daily clipping production, and vertical extension rate as affected by either fertilizer source, application rate, or date (week of experiment). Fertilizer sources include the cocrystal products CaSO4·4urea (UC1) and Ca(H2PO4)2·4urea (UC2) along with the traditional products methylene urea, urea, polymer-coated urea (MU·PCU), and polymer-coated urea, urea (PCU). Means in the same column followed by the same letter within a given study period are not significantly different based on Tukey’s honestly significant difference or Student’s t test at P ≤ 0.05.
For visual quality, there was a significant two-way interaction for fertilizer source × application rate in both study periods (Table 2). In the summer study, visual quality of MU·PCU applied at the low rate was significantly higher than MU·PCU and PCU applied at the high rate and UC2 applied at the low rate (Table 4). In the winter study, PCU applied at the low rate produced a significantly higher visual quality rating compared with MU·PCU and PCU applied at the high rate and UC1 applied at the low rate. In both study periods, all averages were above the minimum quality threshold (≥5) (Table 4).
Visual quality responses, clipping production, and vertical extension rates of applied fertilizer sources as affected by applied rates during the summer and winter experiments. Fertilizer sources include the cocrystal products CaSO4·4urea (UC1) and Ca(H2PO4)2·4urea (UC2) along with the traditional products methylene urea, urea, polymer-coated urea (MU·PCU) and polymer-coated urea, urea (PCU). Means in the same column followed by the same letter within a given study period are not significantly different based on Tukey’s honestly significant difference at P ≤ 0.05.
For NDVI, there was a significant fertilizer source effect in both study periods (Table 2). Average NDVI across application rate and rating date was highest for UC2 compared with all other treatments in the summer study, whereas MU·PCU had the lowest NDVI average compared with the other treatments in the winter study (Table 3). ANOVA also revealed a significant date effect in both study periods for visual quality and NDVI (Table 2). For rating dates there was the same general trend for average visual quality and NDVI in the summer study in which values increased from trial initiation to week 4 (5.75 to 6.95 and 0.59 to 0.72, respectively) but declined between weeks 4 and 6 (Table 3). After week 6, values generally increased and remained level for visual quality and NDVI (7.08 to 7.12 and 0.65 to 0.68, respectively) (Table 3). In the winter study, visual quality and NDVI remained relatively steady (7.25 to 7.75 and 0.80 to 0.86, respectively) throughout the study period (Table 3).
For clipping production, ANOVA detected significant fertilizer source × application rate interactions for clipping production in the summer and winter study periods, respectively (Table 2). In the summer, clipping production for UC2 applied at the low rate was significantly higher than PCU applied at the high rate (Table 4). Clipping production results for the fertilizer source × application rate interaction was more variable in the winter study in which UC2 applied at the high rate had the highest clipping production compared with all others fertilizer source and application rate combinations. Further, UC1 and UC2 applied at the low rate had higher clipping production than MU·PCU applied at the low rate. Positive growth responses of ‘Tifway’ hybrid bermudagrass from UC1 and UC2 application seen in this study are consistent with productivity responses seen in crop species like maize and sorghum (Bista et al. 2023; Swify et al. 2022).
Cocrystal products also showed promising results in terms of vertical extension rate due to a significant fertilizer source × rate interaction detected in the winter study (Table 2). In this study period when averaging across rating date, vertical extension of UC2 applied at either rate was significantly higher than MU·PCU at both application rates, PCU at the low rate, and UC1 at the high rate (Table 4). Further, MU·PCU applied at the low rate had the lowest vertical extension rate to all source and rate combinations except for PCU applied at the low rate and UC2 applied at the high rate (Table 4).
In both study periods, ANOVA detected a significant date effect for vertical extension rate (Table 2). Although analysis is clearer for the summer study, there was a general trend of decreased vertical extension rates at week 6 to trial end (Table 3). In the summer study, vertical extension from rating dates 1 to 4 (≈8.5 mm·d−1) were ≈ 45% higher compared with rating dates 6 to 9 (4.6–5.1 mm·d−1) and vertical extension at rating date 5 was intermediate (6.8 mm·d−1) (Table 3).
Although no published data currently exist regarding the effect of cocrystal fertilizers on turfgrass performance, UC1 was previously tested by Barčauskaitė et al. (2020) and was done using drywall gypsum, and manufacturing waste via mechanochemical grinding. Besides N, UC1 cocrystal provides Ca and S, which are essential to plant growth (Hewitt 1951). Furthermore, N uptake increases when S is applied due to increased N use efficiency mainly by increasing the N recovery from the soil (Salvagiotti et al. 2009). The UC2 was prepared by Honer et al. (2018) with other ionic cocrystals of abundant low-solubility minerals, using mechanochemistry. This is an efficient and environmentally viable route for fertilizer production, by mechanically milling urea components, and with different possibilities of ionic cocrystals in terms of additional macronutrients (P, K, Ca, Mg, and S).
Given the positive responses of ‘Tifway’ hybrid bermudagrass to both applied rates, further studies could include field testing and the investigation of turfgrass response from cocrystal fertilizer sources on other commonly used warm-season turfgrasses or cool-season turfgrass species. Future studies could also consider mowing requirements and production costs from producing cocrystal products from waste sources.
Column experiment
Nitrogen release behavior of urea (U), UC1, and UC2 in water was investigated by a rapid water release test. The results from these studies are shown in (Figs. 2 and 3). N release in water proceeded at a similar rate for UC1 and UC2. Urea peak occurred at 500 s, and it was significantly higher than UC1 and UC2. UC1 peak was at 400 s and the UC2 peak was ≈600 s (Fig. 2). UC1 initially had a faster N release rate as compared with UC2 (from 200 to 1000 s), not differing at 1600 s. Barčauskaitė et al. (2020) observed higher N release rate in U as compared with UC1 during the first 24 h of a sand bed column experiment, similar to our results. However, after the first 24 h, UC1 had a higher cumulative N dissolved (Barčauskaitė et al. 2020).
Relative N release rate of urea (U), CaSO4·4urea (UC1) and Ca(H2PO4)2·4urea (UC2). Means with the same letter at the same evaluation time are not significantly different based on Tukey’s honestly significant difference at P ≤ 0.05.
Citation: HortTechnology 34, 4; 10.21273/HORTTECH05423-24
Urea leaching experiments from sandy loam soil. Measured N accumulated in the leachate is shown for the background, urea (U), CaSO4·4urea (UC1), and Ca(H2PO4)2·4urea (UC2). Vertical line indicates cumulative N release in 20-s intervals.
Citation: HortTechnology 34, 4; 10.21273/HORTTECH05423-24
The most important change can be inferred from the cumulative N (measured in the leachate) after 1600 s of the experiment. Approximately 15% (0.8 mg) of the N from U had leached when background values (0.2 mg) were discounted from the global cumulative. Approximately 8% of UC1 and UC2 N was accounted for in the leachate after 1600 s (Fig. 3). In addition in that same study, effectively from all the N from UC1, only 0.6% loss was detected. On the other hand, 8.7% of N from U was lost, probably as NH3 gas (Barčauskaitė et al. 2020). The reduced losses may be an important piece to keep UC1 urea release at a more consistent rate as compared with U. Historically, the standard way of obtaining these urea ionic cocrystal materials has been from precipitation from saturated aqueous solutions via heating and slow evaporation. Mechanochemical synthesis provides numerous prospects in green engineering technology due to its solvent-free nature and easy scale up (Adassooriya et al. 2022).
The mechanochemical preparation of CaSO4·4CO(NH2)2 ionic cocrystals of urea and calcium sulfate was first investigated by Malinowski et al. (2007). Honer et al. (2017) used mechanochemical methods to prepare ionic cocrystals of urea with inorganic salts such as CaSO4·2H2O, Ca(H2PO4)2, Ca(NO3)2·4H2O, MgSO4·H2O, Mg(H2PO4)2·2H2O, and Mg(NO3)2·2H2O, and reported lower NH3 emission in synthesized ionic cocrystals, which were more stable and could potentially decrease N loss to the environment due to decomposition of urea (Honer et al. 2017).
This column experiment was designed to quantify the hydrolysis release process and the diffusion method described allows for the fractionation of hydrolysable soil N with precision and specificity. When urea is applied to the soil, it reacts with water and the soil enzyme urease, leading to a rapid conversion to ammonium (Sigurdarson et al. 2018). In the case of slow-release urea materials, urea diffuses slowly through the polymer layer (Naz and Sulaiman 2016). As urease requires contact with urea to promote hydrolysis, the reaction occurs at a slower rate over time. The aim in the N release experiment was to determine whether the cocrystals would exhibit a similar response pattern, resulting in a slower N release compared with regular urea. However, our focus was not on comparing the hydrolysis rate with other slow-release urea products on the market, as those have already been well described. By studying this, we sought to enhance our understanding of plant responses to these materials and the potential use of these materials as a substitute to other commercial fertilizers, maintaining turfgrass quality along with environmental benefits.
Conclusions
Application of N in turfgrass systems is an integral part of turfgrass quality and turfgrass function maintenance; however, the application of low-stability N products has led to adverse ecological and climate impacts. Although urea cocrystals have emerged as an improved N compound with the capability of minimizing N losses, their effects on turfgrass have not been explored. Turfgrass vigor was improved by the urea cocrystals used in this study, along with favorable growth responses as seen with clipping production and vertical extension rates. Further, the column experiment revealed decreased N release peaks with both cocrystal products accompanied by decreased N losses in leachate compared with urea. Overall, this study revealed promising results pertaining to urea cocrystal suitability as an N fertilizer source and a potential slow-release source. These findings highlight the need for field experiments to evaluate longer-term application effects of urea cocrystals on bermudagrass and other grass species. Last, mowing requirements and production costs from cocrystal synthesis from waste sources also should be evaluated.
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