Nitrogen release from polymer-coated urea: Effect of seasons and growing degree-days
Controlled-release nitrogen (CRN) fertilizers are designed to gradually release N at rates that closely match the N demand by plants while potentially reducing N losses to the environment. Polymer-coated urea (PCU) is the most widely used CRN. Actual N release patterns in the field are expected to be influenced by differences in weather conditions during different times of the year. Six field trials were carried out to estimate and model N release from PCUs during different times of the year (seasons) and to investigate a possible relationship that may exist between N release from PCU fertilizers and growing degree-days and whether such a relationship could be used as a reliable predictor of N release under field conditions.
Controlled-release nitrogen (CRN) fertilizers are designed to gradually release N at rates that closely match the N demand by plants (Goertz, 1993; Oertli, 1980) while potentially reducing N losses to the environment (Shaviv, 2001; Slater, 2010). As a result, use of CRN fertilizers is being considered as part of potential best management practices (BMPs) to reduce N losses to surface- and ground-water supplies (Bartnick et al., 2005; Carson & Ozores-Hampton, 2014; Obreza et al., 2006). These fertilizers are also gaining acceptance because of improved N use efficiencies, reduced labor and application costs, and increased yields (Goertz, 1993; Shaviv & Mikkelsen, 1993; Oertli, 1980). For example, Florida sugarcane growers are gradually using CRN fertilizers to reduce, or eliminate, supplemental applications of soluble N fertilizers on mineral soils during the rainy summer months when severe N losses can occur due to leaching, denitrification, and/or runoff.
Polymer-coated urea (PCU) is the most widely used CRN fertilizer (Du et al., 2006). Through manipulation of the polymer coating, manufacturers are offering a variety of PCU fertilizers with differing N release characteristics (Trenkel, 1997). Although suppliers do specify effective release periods for their PCU (e.g., 60, 90, 180 days, etc.), these periods are generally established under controlled laboratory conditions and constant temperature, such as at 70°F. Limited information exists on N release patterns from PCU under field conditions, which can be significantly different than those in a controlled laboratory environment. For example, actual N release patterns in the field are expected to be influenced by differences in weather conditions (e.g., temperature and rainfall) during different times of the year. Carson and Ozores-Hampton (2013) reported that N release is positively correlated with soil temperature and moisture, so an increase or decrease in soil temperature or moisture results in an increase or decrease in N release rate, but temperature usually has the greatest influence on N release rates (Oertli & Lunt, 1962a, 1962b; Tamimi et al., 1983; Lamont et al., 1987; Medina et al., 2014).
Growers often use a concept called growing degree-days (GDD), sometimes called heat units, to understand how daily temperatures and plant growth are related. Growing degree-days is based on the idea that the development of a plant will occur only when the temperature exceeds a specific base temperature (Tbase) for a certain number of days:
GDD = (Daily Maximum Air Temperature + Daily Minimum Temperature)/2 – Tbase
If the mean temperature is at or below Tbase, which is usually 50°F, then the GDD value is zero. If the mean temperature is above Tbase, then the GDD amount equals the mean temperature minus Tbase. Because GDD relates air temperature to plant growth, development, and maturity, the concept has been used to predict the growth stages of plants and to estimate when a crop (and/or variety) will be ready for harvest. Researchers have also developed charts relating growth stages of certain crops to GDD accumulation (Fraisse & Paula-Moraes, 2007).
If the GDD concept, which is used to predict crop growth stages (and, conceivably, crop N demand) is temperature based, and if temperature generally has the greatest influence on N release rates from PCU, then we should be able to find a meaningful relationship between N release from PCU fertilizers and GDD. A better understanding of N release patterns under field conditions should enable the formulation of fertilizer blends that can be better matched with N demand by specific crops, including sugarcane.
The objectives of this work were to estimate and model N release from PCUs with different N release durations during different times of the year (seasons) and to investigate a possible relationship that may exist between N release from these fertilizers and GDD and whether such a relationship could be used as a reliable predictor of N release under field conditions.
Materials and Methods
Polymer-coated urea fertilizers with differing manufacturer’s release periods, (i.e., 60, 90, 120, and 180 days) were used in a series of field trials that spanned the period of Oct. 6, 2015 to May 22, 2019 (Table 1). The soil at the trials’ site has been classified as Immokalee Sand (Sandy, siliceous, hyperthermic Arenic Haplaquods). In each trial, three replicates of 5–6 g (0.18–0.21 oz) of each PCU were weighed to the nearest 0.01 g and placed in fiberglass mesh bags. The mesh bags were folded and stapled (to prevent any loss of the PCU material). Parallel trenches were opened in the field to a depth of 3–6 inches in an east-to-west orientation, and the bags were then placed in the trenches and covered with soil. Air and soil temperature sensors, and rain gauges, were placed in the field to record daily air (2-ft elevation) and soil (4-inch depth) temperature fluctuations as well as daily rainfall amounts.
Table 1. Establishment and termination dates of the six field trials
| Field trial | Establishment datea | Termination dateb |
|---|---|---|
| 1 | Oct. 6, 2015 | Mar. 29, 2016 |
| 2 | Apr. 20, 2016 | Oct. 12, 2016 |
| 3 | July 13, 2016 | Dec. 28, 2016 |
| 4 | Oct. 10, 2017 | Apr. 10, 2018 |
| 5 | Mar. 5, 2018 | Sept. 4, 2018 |
| 6 | Nov. 21, 2018 | May 22, 2019 |
Future recovery dates of the mesh bags were randomized within location for each trial, and they were subsequently collected at specific intervals (e.g., 7, 14, 28, …., 182 days) after placement. After recovery, PCU granules in the mesh bags were gently rinsed with cold tap water to remove debris; transferred to labeled, pre-weighed (nearest 0.01 g) 20-ml glass vials; and then placed in an oven for 24 h at 167°F (75°C). The vials containing the dry granules were weighed, and the weights were recorded. Then the previously determined weight of empty vials was subtracted from the weight of the vials containing the dry granules to obtain the weight of the dried granules, or the “ending weight.”
Percent N release from the PCU was calculated using the weight loss method (Salman et al., 1989; Savant et al., 1982). Wilson et al. (2009) found this method to be a good predictor of N release from PCU, thus suggesting it can be reliably used for determining PCU N release characteristics. Porous mesh bags and the weight loss method have been used in field research to estimate N release for different crops in Florida (Obreza et al., 2006; Carson & Ozores-Hampton, 2014; Ozores-Hampton, 2017).
Nitrogen release was assumed to be zero on the day the mesh bags were placed in the field. Thereafter, percent N release with time after placement was calculated according to the following formula:
Percent N release = (1 – a/b) × 46/c × 100,
where
- a = ending weight
- b = beginning weight
- c = percent N in the PCU product (e.g., 44%, 43%, etc.), and
- 46 = percent N in urea (46–0–0)
The mean percent N release (and standard error of the mean) for each PCU at each sampling date within each trial was calculated, and the results were summarized and presented graphically. This enabled specific comparisons of the effective N release from these PCU fertilizers during different times of the year (seasons). Regression models, representing in most cases an exponential rise to maximum, were fit to the N release data with time after placement for each trial, and with GDD for N release data pooled from all trials.
Results and Discussion
Weather Data
As would be expected, air and soil temperatures and rainfall distribution and amounts were quite different during the six field trials (Figure 1). Temperature and rainfall are the major factors affecting N release from PCU with temperature being the primary factor. Except for the months of December through February, air and soil temperatures exceeded 70°F (the temperature at which release curves are normally established by manufacturers) for most of the seasons during which these trials were taking place.
Rainfall amount and distribution were also different, reflecting the influence of the dry and wet seasons of Florida. A mere 7.2 inches of rainfall were recorded at the field trials’ site during the October 2017 trial, whereas 22.9, 26.4, and 22.1 inches of rainfall were recorded during the span of the October 2015, July 2016, and November 2018 trials, respectively. Wet conditions prevailed during the April 2016 and March 2018 trials with recorded rainfall amounts of 48.6 and 43.6 inches of rainfall, respectively. Rainfall distribution was also highly variable as can be seen in Figure 1. These notable differences in weather conditions undoubtedly influenced the pattern of N release from the PCU fertilizers.
Nitrogen Release Data
Figure 2 shows the observed and predicted N release from a 60-day (A), 90-day (B), and 180-day PCU during the six field trials. Nitrogen release was generally characterized by three distinct phases: an initial rapid rate of release, followed by a steadily decreasing rate, and finally a plateau, or a decay phase (Shaviv et al., 2003). This trend was more pronounced in the shorter-duration material (i.e., the 60- and 90-day PCU) than in the longer-duration one (180-day PCU) where N release often tended to approach linearity.
Both the N release rate (slope of the regression lines) and plateau (final percent N released) were dramatically different in the six field trials, which is most likely an outcome of the differences in temperature and rainfall that prevailed during these trials. The number of days after establishment to achieve 80% N release, used here as a basis for comparison, can be estimated from the intersection points of the N release regression lines and the 80% N release threshold line parallel to the x-axis. For example, the regression model predicted that, for the 60-day PCU, 80% of the N was released in 16 days after establishment of the April 2016 trial and 61 days after establishment of the March 2018 trial (Figure 2A). For the 90-day PCU, the model predicted that 80% of the N was released in 31 and 81 days after establishment of the April 2016 and March 2018 trials, respectively (Figure 2B).
As for the longer-duration material (180-day PCU), 80% N release was predicted at 149 and 168 days after establishment of the April 2016 and October 2015 trials, respectively (Figure 2C). In the four other trials involving this PCU, the regression lines did not cross the 80% N release threshold line, indicating that N release was less than 80% during the measurement period of 182 days after establishment of these trials, as shown in Figure 2C.
The differences in N release patterns observed during these trials warranted a closer examination of N release from PCU fertilizers with different theoretical release durations. A comparison of N release from a 60-, 120-, and 180-day PCU with time after establishment of the October 2015 and April 2016 trials on one hand, and the October 2017 and March 2018 trials on the other, showed that seasonal differences in N release tended to decrease with an increase in the projected release period of the PCU (Figure 3). This is presumably an outcome of increased thickness of the polymer coating of the fertilizer. Nitrogen release from PCU with time is primarily controlled by temperature, coating thickness, and coating composition (Carson & Ozores-Hampton, 2013; Trenkel, 1997; Oertli & Lunt, 1962a).
The observed differences in N release patterns from PCU fertilizers during different seasons complicates our ability to predict N release with time after establishment of the trials, which is synonymous with days after application of the PCU fertilizer. Hence, a better way to predict this release is needed. Since GDD is temperature based, and since temperature generally has the greatest influence on N release rates from PCU, then it is conceivable that a meaningful relationship could be found between N release from PCU fertilizers and GDD and that N release could be better predicted as a function of cumulative GDD.
Figure 4 (A, B, and C), which shows N release from a 60-, 90, and 180-day PCU as a function of cumulative GDD, reveals an excellent relationship between the two parameters as can be judged from the scatter of the data points and the r2 statistic of the regression lines. According to the regression models, N release from the 60- and 90-day PCU reached 80% at 790 and 1,302 cumulative GDD, respectively (Figure 4A and 4B). However, the regression line describing N release from the 180-day PCU did not cross the 80% N release threshold line, indicating, as mentioned earlier, that less than 80% N was released from this PCU during the period of measurement (Figure 4C). Nevertheless, extrapolating the regression line shows that 80% N release would be reached at 5,944 cumulative GDD.
These results indicate that the short- and intermediate-duration material are suited for fast growing crops with a relatively short growing season, such as vegetables, whereas the longer-duration material is better suited for long-season crops or perennials, such as, for example, sugarcane, citrus, and/or other fruit trees. The results also show that, at times, mixing PCU fertilizer with different release rates may be useful to achieve a desired N release profile that better matches the N demand and uptake patterns by different crops.
Summary and Conclusions
- Air and soil temperatures and rainfall distribution and amounts were quite different during the six field trials. As a result, N release patterns varied significantly with time after establishment of individual trials.
- Generally, faster N release rates and higher N release plateaus were obtained during times of the year when higher temperatures and rainfall amounts prevailed. However, differences in seasonal N release patterns seemed to decrease with an increase in the projected release duration (and, presumably, increased thickness of the polymer coating) of the PCU fertilizers.
- The PCU fertilizers evaluated herein displayed N release patterns consistent with short-duration material generally releasing faster than the intermediate material, which, in turn, tended to release faster than the long-duration material.
- Regression models enabled us to obtain accurate estimates of percent N release by different PCU materials with time (days after establishment). The models also shed some light on remarkable differences that may exist in N release patterns by the different PCU materials within and across different times of the year.
- However, N release across different seasons could be more reliably predicted as a function of growing degree-days rather than time after establishment. Such information is critical towards formulating fertilizer mixes that can be better matched with N demand and uptake patterns by sugarcane and other crops.
Dig deeper
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