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Systematic Review

Slow-Release Fertilisers Control N Losses but Negatively Impact on Agronomic Performances of Pasture: Evidence from a Meta-Analysis

by
Gunaratnam Abhiram
Department of Export Agriculture, Faculty of Animal Science and Export Agriculture, Uva Wellassa University, Badulla 90000, Sri Lanka
Nitrogen 2024, 5(4), 1058-1073; https://doi.org/10.3390/nitrogen5040068
Submission received: 6 September 2024 / Revised: 7 November 2024 / Accepted: 14 November 2024 / Published: 17 November 2024
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Abstract

:
High nitrogen (N) losses and low nitrogen utilisation efficiency (NUE) of conventional-nitrogen fertilisers (CNFs) are due to a mismatch between N-delivery and plant demand; thus, slow-release N fertilisers (SRNFs) are designed to improve the match. A quantitative synthesis is lacking to provide the overall assessment of SRNFs on pasture. This meta-analysis analyses application rate and type of SRNFs on N losses and agronomic performances with 65 data points from 14 studies in seven countries. Standardized mean difference of SRNFs for nitrate leaching losses and N2O emission were −0.87 and −0.69, respectively, indicating their effectiveness in controlling losses. Undesirably, SRNFs had a more negative impact on dry matter (DM) yield and NUE than CNFs. Subgroup analysis showed that SRNF type and application rate had an impact on all tested parameters. The biodegradable coating-type of SRNF outperformed other types in controlling N losses and improving agronomic performances. High application rates (>100 kg N ha−1) of SRNFs are more effective in controlling N losses. In conclusion, SRNFs are more conducive to controlling N losses, but they showed a negative impact on yield and NUE in pasture. Further studies are recommended to assess the efficacy of SRNFs developed using advanced technologies to understand their impact on pastoral agriculture.

1. Introduction

Pastoral agriculture is the backbone of the economy of many countries including New Zealand, Australia, the Netherlands, and India, as well as many countries in Asia and Africa [1,2]. It involves raising livestock primarily for milk, wool, and other animal products. Pastoral agriculture plays a critical role in global food production, rural economies, and sustainable land management [3]. As the demand for food continues to rise, it is becoming increasingly challenging to strike a balance between food production and sustainable resource management [4,5]. Several strategies are being used to enhance the efficiency and sustainability of pastoral agriculture through innovative practices and the use of precision technologies [6,7]. The use of slow- or controlled-release fertilizers is a strategy employed to enhance environmental sustainability and improve nutrient management practices [8,9].
Nitrogen (N), being one of the primary nutrients essential for all plants, is particularly critical for pasture growth [10]. Since a significant portion of N is removed with the grazing or clipping, ensuring a consistent supply of N is essential for high-quality pasture production [11]. Nitrogen is primarily absorbed by the plants in nitrate form, although they are applied in various forms. However, N utilisation efficiency is typically low in agricultural systems, including pastoral agriculture, primarily due to the high losses to the environment [12,13]. Nearly half of the applied Nitrogen fertilizer is lost through processes such as nitrate leaching, ammonium volatilization, and emissions of nitrous and nitric oxide [14]. As a remedy to control these losses, employing slow- or controlled-release N fertilisers (SRNFs or CRNFs) has been implemented in agricultural practices for last few decades [15,16].
Different types of SRNF are developed with different features, including coating with polymers [17], the ability to polymerise the urea with other chemicals [18], coating with inorganic materials, [19] and coating with biodegradable materials [20]. The type of the SRNF has influence on controlling the N losses. These SRNFs restrict the contact of water with the nitrogen source in the core, reducing their solubility and effectively controlling the nutrient release from the fertilizers [9]. SRNFs are deigned to synchronise the nutrient supply with plant demand, aiming to reduce N losses and enhance nitrogen utilisation efficiency (NUE) [21]. However, achieving complete synchronization between nutrient supply and plant demand is challenging due to variations in plant demand influenced by species, climatic conditions, and soil factors [22]. Further, engineering the SRNFs to precisely align with plant demand remains a significant challenge. Nevertheless, SRNFs slow down the release and thereby substantially control the losses [23]. Furthermore, the inherent properties of a coating, such as cracks and pinholes on the surface, hydrophobicity of the coating material, membrane tensile strength, coating thickness, thermal stability, and chemical inertness, also influence the nutrient release characteristics of the SRNFs.
Application of SRNFs is common in field crops and horticultural crops. However, there is limited information available on the testing of SRNFs on pasture. Studies reported differential responses for N losses and agronomic performances. A few studies supported that SRNFs can control leaching losses [10,24], while others have reported null effects [25]. Dry matter yield and NUE also have shown mixed results in the literature [26,27]. Therefore, to understand the true impact of SRNFs on nitrogen loss control and agronomic performances, a meta-analysis is required. While a few meta-analyses have reported the impact of SRNFs on N losses [28,29] in other agricultural systems, a dedicated and comprehensive meta-analysis for pastoral systems is currently unavailable. Hence, this meta-analysis was conducted to provide insights into the application of SRNFs concerning controlling leaching and gaseous losses, DM yield, herbage nitrogen (HN), and NUE. This study offers insights into identifying the optimal application rate and type of Slow-Release Nitrogen Fertilizers (SRNFs) to achieve improved outcomes on pasture lands.

2. Materials and Methods

2.1. Data Collection

Search engines such as Web of Science (Thompson Reuters, Toronto, ON, Canada), Google scholar (Google Inc., Mountain View, CA, USA), and Scopus (Science Direct, Amsterdam, The Netherlands) were used to rigorously search the literature, and the search ended in April 2024. The following key terms were used alone and in combination; controlled-release fertiliser, slow-release fertiliser, enhanced-efficiency fertiliser, nitrogen fertiliser, pasture, grass, nitrogen losses, nitrate leaching, ammonium leaching, nitrous oxide emission, dry matter yield, yield, cutting, herbage nitrogen, nitrogen uptake, plant uptake, and nitrogen utilisation efficiency [30]. Out of a total of 118 articles downloaded using the keywords, 21 articles were removed due to duplication. Subsequently, 38 articles were omitted as they were not in English. Following this, 34 articles were excluded as they were out of focus on agricultural pasture. Additionally, 11 articles were excluded due to a lack of proper information and not following the correct methodology. Finally, a total of 14 articles were included in this meta-analysis (Figure 1).
The data from the graph were captured using the Webplotdigitizer Version 5.0 [31]. A data set was prepared including the following information: author name, year, fertiliser treatment, fertiliser application rate, slow-release fertiliser type, replicates and average and standard deviation for nitrate leaching, ammonium leaching, nitrous oxide emission, dry matter yield, herbage nitrogen, and nitrogen utilisation efficiency. The units of each parameter were standardized to maintain uniformity. The data were categorised based on types of SRNF and fertiliser application rate for each parameter. The risk of bias assessment was performed using Cochrane Risk of Bias (ROB) 2.0.
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Figure 1. Schematic diagram for inclusion criteria of articles for this systematic review and meta-analysis (PRISMA) [32].
Figure 1. Schematic diagram for inclusion criteria of articles for this systematic review and meta-analysis (PRISMA) [32].
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2.2. Meta-Analysis

Data analysis was conducted using the software Review Manager (RevMan 5.4) [33]. The standardized mean difference (SMD) was used to measure the overall effect size. An SMD value >0 suggests that the intervention is effective. The following values of SMD were classified as a rule of thumb: an SMD < 0.4 indicates a small effect, an SMD of 0.4–0.7 indicates a moderate effect, and an SMD > 0.7 indicates a large effect [34]. The random-effect model was used to analyse the pool data, assuming there is a distribution effect. In the case that there was no significant (p > 0.05) heterogeneity, data were analysed using a fixed-effect model. The heterogeneity of the pooled data was expressed using I2 and p values [30]. Publication bias and selective reporting of the studies are analysed using a funnel plot.
Subgroup analysis was performed for different SRNF types (polymer coating: PC, biodegradable coating: BC, inorganic coating: IC, and polymer chain: PCH) and fertiliser application rates (≤100, 101–200, and >200 kg/ha). Subgroup analysis was conducted only if a subgroup had data from two or more studies and more than 4 data points. Correlation analyses were conducted between effect size (SMD) of nitrate leaching loss, ammonium leaching loss, N2O emission, DM yield, HN and NUE, and SRNF application rates.

3. Results and Discussion

3.1. The Overview of the Dataset

In total, the dataset included 14 studies from 7 different countries. There are four studies from New Zealand, two studies from Australia, Portugal, the USA, and Spain, and one study from the United Kingdom and Scotland. Australia and the USA have both temperate and tropical climates, while the other countries mentioned belong to the temperate climate zone. Even within Australia and the USA, pasture production predominantly occurs in temperate regions. The majority of the studies primarily focused on dry matter yield, nitrogen use efficiency, and herbage nitrogen content. However, only three studies reported on ammonium leaching losses, and four studies reported on nitrous oxide emissions from SRNFs (Figure 2). Perennial ryegrass is commonly cultivated in pasture production systems across most of these countries, and slow-release fertilizers have been evaluated in perennial ryegrass in all the studies included in this meta-analysis.

3.2. Nitrogen Losses

3.2.1. Nitrate Leaching Losses

A total of 27 studies were analysed in this meta-analysis regarding nitrate leaching (Table 1). The SMD value for overall studies was −0.87, with a 95% CI of −1.3 to −0.43. The SRNF controlled the median nitrate loss by 36% compared to CNF (Table 2). This shows that applying SRNFs instead of conventional fertilisers in pasture has effectively controlled the nitrate leaching losses (Figure 3). The heterogeneity between studies is low with I2 = 30%, but it is not statistically significant (p = 0.07). A significant moderate correlation was found between nitrate leaching control and SRNF application rates (R2 = 0.23, p < 0.05, n = 27) (Figure 4a).
The subgroup analysis for different SRNF types exhibited that polymer coating (PC)-, bridgeable coating (BC)-, and polymer chain (PCH)-type fertilisers showed positive impact in controlling nitrate leaching losses. The corresponding SMD values were −0.75 (±0.59), −2.19 (±1.54), and −1.33 (±0.88). The BC-type SRNF outperformed other types of SRNF, followed by PCH and PC (Figure 3). The median nitrate loss control by BC, PCH, and PC were 41%, 34% and 80%, respectively (Table 2). In contrast, inorganic coating provided unfavourable results or marginal positive results in controlling nitrate leaching losses, with an SMD of 0.13 (±0.83). This meta-analysis primarily included sulphur-coated urea as a type of inorganic fertilizer. Studies have shown sulphur-coated SRNFs had subpar performance in both greenhouse [45,46] and field [36,47] studies, likely due to issues with cracks on coating and failing to release nutrients effectively [48]. This factor may have contributed to the observed limitations in controlling nitrate leaching losses. The results indicate that the BC-type of SRNF is more effective in controlling nitrate loss. Additionally, it is considered more environmentally friendly compared to other SRNF options. Therefore, it is recommended to use the BC-type SRNF over other varieties for its environmental benefits and effectiveness in managing nitrate loss.
With respect to the fertiliser application rate, higher application rates (101–200 and >200 kg N/ha) of SRNF were found to be more effective in controlling nitrate leaching losses. In contrast, lower application rates (<100 kg N/ha) had only a marginal effect on controlling nitrate leaching losses (Figure 3). The median nitrate loss control at application rate ≤100, 101–200, and >200 kg N/ ha are 5%, 85%, and 33%, respectively (Table 2). At lower application rates of CNF, plants absorb a significant portion of the available nitrate, leaving only a small amount for potential leaching after rainfall or irrigation [49,50]. On the other hand, at higher application rates of CNF, only a small portion is taken up by the plants, resulting in a larger amount of nitrate remaining in the soil [51]. Therefore, applying SRNF instead of CNF at higher application rates potentially controls the leaching losses. This meta-analysis suggests that at lower application rates, choosing CNF may be more beneficial than selecting SRNF in controlling nitrate leaching losses. Conversely, at higher application rates, opting for SRNF could be more advantageous in managing nitrate leaching losses.

3.2.2. Ammonium Leaching Losses

The meta-analysis for assessing ammonium leaching losses included only three studies with a total of 10 data points. Results show that application of SRNF does not significantly contribute to controlling ammonium leaching losses (SMD = −0.34 ± 0.5) (Figure 5). However, SRNF helps to control median ammonium losses by 10% (Table 2). The heterogeneity of the studies for ammonium leaching was low, with an I2 value of 31%, and it was not statistically significant (p = 0.16). Due to its positive charge, the ammonium ion has a high affinity for clay minerals in the soil [52]. As a result, only a small amount of the ion is lost through leaching [8,53]. Hence, there might not be a substantial difference in the loss of ammonium between SRNF and CNF. Similar findings were observed in various studies across different agricultural systems [54,55], including pasture production systems [10,36]. The correlation between ammonium leaching control and SRNF application rate was negative, but not significant (R2 = −0.11, p > 0.05, n = 10) (Figure 4b). Subgroup analysis was not conducted due to the limited number of available studies on ammonium leaching in pasture systems.

3.2.3. Nitrous Oxide Emission

A total of four studies with 12 data points were included in the meta-analysis on nitrous oxide emissions. While the application of SRNF did not show a significant control of N2O emissions (SMD = −0.69 ± 0.76) compared to CNF, a marginal decrease was observed. The heterogeneity was high, with an I2 value of 70%, and it was statistically significant (p < 0.05) (Figure 6). The median reduction in N2O emissions when using SRNF was 16% compared to CNF. This percentage is very similar to the 19% reduction in N2O emission control reported in a meta-analysis for rice, corn, and wheat crops [28]. Most of the studies included in this meta-analysis reported a non-significant impact of SRNF application on controlling N2O emissions [10,27,44]. This could be the reason for the above observation. Several other studies have also reported a similar finding where SRNF marginally controlled the loss of N via N2O emissions [56]. Several other factors including soil moisture level, management practises, and soil factors influenced the N2O emission [57,58]. Nevertheless, subgroup analysis was not conducted to explore potential reasons for this observation as there were a limited number of data points available for nitrous oxide emissions in pasture systems. Therefore, further studies are required to understand the impact of SRNF on pasture under different soil and climatic conditions. A weak and positive correlation was found between N2O emission and SRNF application rate, but it was not statistically significant (R2 = 0.13, p > 0.05, n = 12) (Figure 4c).

3.3. Dry Matter Yield of Pasture

In the meta-analysis for dry matter (DM) yield of pasture, a total of 12 studies with 63 data points were included. Application of the SRNF significantly decreased (SMD = −0.45 ± 0.29) the DM yield of pasture compared to the CNF (Figure 7). Median DM yield was decreased by 25% for SRNF compared to CNF (Table 2). The correlation between DM yield and SRNF application rates was negative and significant (R2 = 0.1, p < 0.05, n = 61) (Figure 4d). Among the 63 data points included in this meta-analysis, 41 data points demonstrated a negative response in DM yield with the application of SRNF. This is evidence that SRNFs are not conducive for increasing DM yield of pasture [59]. In other crops, the nutrient requirement of the plant follows a sigmoid curve, where the nutrient demand is initially lower, then increases linearly, and eventually declines [48,58]. However, pasture root systems are developed fully even during grazing and therefore they require relatively constant nutrient supply [8,60]. After every grazing, the nutrient demand of the pasture declines and then increases with the growth [61] but varies within a small range, as shown in Figure 8. This observation may be attributed to the controlled release of SRNF, which limits the nutrient supply necessary for the regeneration of the pasture (Figure 8). A subgroup analysis was conducted to better understand the behaviour of different types of SRNF and fertilizer rates.
SRNF type has significant influence on the DM yield of pasture. SRNF types such as PC (SMD = −0.38 ± 0.32) and IC (SMD = −2.16 ± 1.47) significantly decreased the DM yield whereas BC (SMD = 1.565 ± 1.13) showed a significantly positive impact on the DM yield (Figure 7). The median DM yield declined by 7% and 13% for PC and IC, respectively, while BC and PCH increased the median DM yield by 26% and 5%, respectively. Several studies have reported that PC SRNF significantly decreased the DM yield in pasture systems [24,25] and other crops [62,63]. Delayed release of nutrients [63] and lock-off of nutrients in the coating [9,48] are major reasons for this observation in PC SRNFs. In this study, S-coated urea is mainly included as IC SRNF. These S-coated urea showed cracks on the coating surfaces, which caused the failure to release nutrients (i.e., release of nutrients soon after application before being utilized by the plants) [48,64]. This could be the possible reason for low performance of IC on DM yield of pasture. Similarly, PCH SRNFs also slowly degrade in the soil [65], which delays the nutrient supply for the pasture [35]. However, recently developed nanocomposite-based PCH SRNFs are showing promising results [16,66] which can be tested on pasture systems. Interestingly, BC SRNFs have good control over nutrient release [67] and are environmentally friendly; therefore, they can be encouraged for use in pasture systems [68].
Application rate of SRNFs has significant impact on the DM yield of pasture. At an application rate of ≤100 and 101–200 kg N/ha, CNF performed significantly better than SRNF (Figure 7). Nevertheless, an application rate of <200 kg N/ha showed comparable results for both CNF and SRNF. The median DM yield decreased by 5% and 9% for ≤100 and 101–200 kg N/ha, respectively. Meanwhile, median DM yield increased by 17% at a <200 kg N/ha application rate. Similar observations were reported in other meta-analyses [28,69].

3.4. Herbage Nitrogen of Pasture

Herbage N, the nitrogen level in a pasture, is an important quality parameter of a pasture and the higher the N content, the better the quality [70]. A pasture with high nitrogen content is highly valued, as it helps maintain the health status and improve the productivity of grazing animals [71]. A total of 52 data points from 10 studies were included in the meta-analysis for herbage nitrogen. The overall effect of herbage nitrogen (HN) was similar for both CNFs and SRNFs with an SMD of −0.135 ± 0.43 (Figure 9). The median HN was increased by 6.4% by the application, but it was not significant. Among the 52 data points, 23 observations showed a positive impact on HN, 28 observations showed a negative impact, and one observation showed a null effect on HN. This indicates that the nutrient release from the SRNF did not fully synchronize with the pasture nitrogen demand, resulting in a limited increase in HN levels. As a result, application of SRNF is not conducive for increasing the HN levels in pastures. A significant negative correlation was found between HN and SRNF application rate (R2 = 0.33, p < 0.05, n = 52) (Figure 4f).
Among the different types of SRNF, the IC-type exhibited a significantly negative impact on HN (SMD = −2.31 ± 1.92), while the BC-type showed a significantly positive impact (SMD = 1.295 ± 0.95). The median HN decreased by 32.8% with the application of IC-type SRNF and increased by 52.5% with the application of BC-type SRNF (Table 2). On the other hand, the PC (SMD = −0.335 ± 0.44) and PCH (SMD = 0.05 ± 0.79) types had comparable HN levels to CNF (Figure 9). Application of PC and PCH decreased the median HN by 4.3% and 9.6%, respectively (Table 2). These observations could be attributed to factors such as the lock-off effect [72], failure to release, [73] and environmental factors such as soil moisture and mean air temperature [74], which have been reported to negatively impact the performance of SRNFs [38,75].
Fertiliser application has significant impact on the HN of pasture. Application of SRNFs at a rate of 101–200 kg N/ha significantly decreased the HN of pasture (SMD = −0.59 ± 0.53) compared to CNFs. In contrast, the application of SRNFs at rates below 100 kg N/ha and above 200 kg N/ha had a neutral effect on HN. The corresponding SMD values are −0.07 ± 0.83 and 0.37 ± 0.8, respectively (Figure 9). This indicates that the application of SRNFs at lower or higher rates is not conducive to increasing HN in pastures. This finding is in agreement with other studies on ryegrass [10,38], rice [76], and maize [77].

3.5. Nitrogen Utilization Efficiency

Nitrogen utilisation efficiency (NUE) reflects the ability of plants to efficiently uptake, assimilate, and utilize nitrogen for growth and development, playing a crucial role in crop productivity and sustainability in agriculture [16]. In total, 13 studies with 65 data points were considered for the meta-analysis. The results showed that application of SRNF significantly (SMD = −0.39 ± 0.3) decreased the NUE of pastures compared to CNF (Figure 10). Interestingly, the median NUE value was increased by 21% more for SRNF than CNF. The SMD values for NUE exhibit a similar pattern to the SMD values for DM yield of pasture. Correlation between NUE and SRNF application rate was significantly negative (R2 = 0.14, p < 0.05, n = 61) (Figure 4e). A large variation in NUE, between 5 and 95%, was observed for using SRNFs on pasture. Several factors contributed to this variation, including soil variation [28,78], climatic variation [79], different types of study (field and pot trials), and grass species [80] used in the pasture. A sub-group analysis was conducted in order to find the impact of SRNF types and application rates.
Similar to DM yield, NUE was also significantly impacted by the type of SRNF used. Among the SRNF types, BC significantly (SMD = 1.565 ± 1.13) increased the NUE compared to CNF. However, IC exhibited a significant (SMD = −1.765 ± 1.35) negative impact on NUE compared to CNF. A null effect was shown by other two SRNF types; PC (SMD = −0.225 ± 0.42) and PCH (SMD = −0.445 ± 0.54) (Figure 10). Fertilisation with PC, PCH, and BC increased the median NUE by 5, 10 and 26% compared to CNF, respectively (Table 2). Nevertheless, the median NUE was decreased by 19% with the application of IC-type SRNF in comparison to CNF. These results suggest that other than BC, other types of SRNFs are not suitable to increase the NUE of pasture systems. Several studies on pasture [27] and other crops such as maize [81] and rice [82] support the finding that SRNFs are not effective in increasing NUE.
High (>200 kg N/ha) and low (≤100 kg N/ha) fertilisation rates of SRNFs did not show any significant difference in NUE compared to CNFs. The corresponding SMD values were −0.265 ± 0.84 and −0.33 ± 0.41, respectively. The moderate application rate (100–200 kg N/ha) of SRNFs significantly (SMD = −0.58 ± 0.4) decreased the NUE compared to CNFs (Figure 10). These results suggest that any application rate of SRNFs is not conducive to increasing NUE on pasture.

4. Limitations of This Study

The number of studies included in a meta-analysis can influence the outcomes. In this particular meta-analysis, only 14 studies met the inclusion criteria and were incorporated. Moreover, there are a limited number of studies focusing on the comprehensive analysis of nitrogen fate when applied to pasture. Few studies reported nitrogen dioxide (N2O) emissions, and none reported ammonium volatilization from slow-release nitrogen fertilizers in pastoral agriculture. In recent times, many biodegradable membrane-coated and nanotechnology-based SRNFs have been developed, but they have not been tested on pasture. Therefore, further studies are needed to conduct a comprehensive analysis of the impact of slow-release nitrogen fertilizers (SRNFs) on pasture. The studies included in this meta-analysis were conducted in different climatic conditions, soil types, and with different pasture species. All these factors influence the performance of the SRNFs [57,83]. However, the influence of these factors was not considered in this meta-analysis.

5. Conclusions

This study offers a comprehensive global overview of the effects of slow-release nitrogen fertilizers (SRNFs) on managing nitrogen losses and agronomic performances in pasture, drawing from 14 selected articles. SRNFs effectively controlled nitrate leaching losses and nitrous oxide emission from pasture lands. Nevertheless, dry matter (DM) yield and nitrogen utilization efficiencies (NUE) are negatively affected by SRNFs as compared to conventional nitrogen fertilisers (CNFs). SRNF type and application rate have significant impact on both N losses and agronomic performance. Biodegradable-coated SRNFs outperformed other types in controlling N losses and DM yield; however, data points were insufficient to make a confident claim. The N demand of pasture differed from other plants; thus, SRNFs performing well for other plants may not necessarily perform well on pasture. In recent years, several advanced technologies, including nanotechnology, have been used to formulate SRNFs. However, they have not been tested in pasture fields. Hence, further studies are recommended to comprehensively assess the impact of SRNFs on pasture and to provide a generalized outcome through meta-analysis. Analysis based on soil and climate conditions was not conducted due to the limited number of articles included in this study. However, in future research, evaluating the effectiveness of SRNFs across diverse climatic regions and soil types can offer valuable insights into their performance, aiding farmers in decision-making based on these criteria. This study primarily focuses on the performance of SRNFs in pasture settings and suggests using these fertilizers cautiously, considering their cost-effectiveness compared to CNFs as they may offer limited advantages.

Funding

This research received no external funding.

Data Availability Statement

All the data are used in this manuscript.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 2. The summary of the reported parameters from each study included in this meta-analysis [10,24,25,27,35,36,37,38,39,40,41,42,43,44].
Figure 2. The summary of the reported parameters from each study included in this meta-analysis [10,24,25,27,35,36,37,38,39,40,41,42,43,44].
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Figure 3. The nitrate leaching losses of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha) and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
Figure 3. The nitrate leaching losses of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha) and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
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Figure 4. The correlation between effect size (standardized mean difference: SMD) and application rate of SRNFs for (a) nitrate leaching losses, (b) ammonium leaching losses, (c) N2O emission, (d) dry matter yield, (e) nitrogen utilisation efficiency (NUE) and (f) herbage nitrogen. Dark shade and light shades indicate a 95% confidence interval and a 95% prediction level, respectively.
Figure 4. The correlation between effect size (standardized mean difference: SMD) and application rate of SRNFs for (a) nitrate leaching losses, (b) ammonium leaching losses, (c) N2O emission, (d) dry matter yield, (e) nitrogen utilisation efficiency (NUE) and (f) herbage nitrogen. Dark shade and light shades indicate a 95% confidence interval and a 95% prediction level, respectively.
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Figure 5. The effect of SRNF on ammonium leaching losses. SRNF and CNF refer to slow-release nitrogen fertiliser and conventional nitrogen fertiliser, respectively [10,27,36].
Figure 5. The effect of SRNF on ammonium leaching losses. SRNF and CNF refer to slow-release nitrogen fertiliser and conventional nitrogen fertiliser, respectively [10,27,36].
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Figure 6. The effect of SRNF on N2O emission. SRNF and CNF refer to slow-release nitrogen fertiliser and conventional nitrogen fertiliser, respectively [10,27,42,44].
Figure 6. The effect of SRNF on N2O emission. SRNF and CNF refer to slow-release nitrogen fertiliser and conventional nitrogen fertiliser, respectively [10,27,42,44].
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Figure 7. Dry matter yield of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha), and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
Figure 7. Dry matter yield of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha), and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
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Figure 8. The plant nutrient demand (PND) and nutrient delivery by slow-release nitrogen fertiliser (NDSRNF) for (a) other crops and (b) pasture. The blue shaded area shows the PND of pasture during continuous grass grazing.
Figure 8. The plant nutrient demand (PND) and nutrient delivery by slow-release nitrogen fertiliser (NDSRNF) for (a) other crops and (b) pasture. The blue shaded area shows the PND of pasture during continuous grass grazing.
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Figure 9. Herbage nitrogen (HN) in a pasture of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha), and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
Figure 9. Herbage nitrogen (HN) in a pasture of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha), and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
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Figure 10. Nitrogen utilisation efficiency (NUE) in a pasture of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha), and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
Figure 10. Nitrogen utilisation efficiency (NUE) in a pasture of conventional nitrogen fertilisers (CNFs) and slow-release nitrogen fertilisers (SRNFs) based on (a) SRNF types (PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain), (b) fertiliser application rates (kg N/ha), and (c) overall studies. SMD stands for standard mean difference. Numbers next to range graph indicate the number of studies included for analysis.
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Table 1. Performance of selected controlled-release or slow-release nitrogen fertilisers (CRNFs or SRNFs) on pasture against conventional N fertiliser.
Table 1. Performance of selected controlled-release or slow-release nitrogen fertilisers (CRNFs or SRNFs) on pasture against conventional N fertiliser.
CRNF/SRNFExperiment TypeApplication Rate DM Yield Agronomic N Efficiency
(kg DM kg−1)		Leaching Loss ControlN2O Loss ControlCountryReference
Polyurethane coated urea (5 and 7%) Field trial50 and 150NSI22–24 SIN/ANew Zealand[24]
Dicyandiamide coated Urea (10% coating)Field trial50 and 150NSI12–15 SIN/ANew Zealand[24]
CoronGreenhouse
(NSCS)		84SD (30%)N/AN/AN/AUSA[35]
N-SureGreenhouse
(NSCS)		84SD (30%)N/AN/AN/AUSA[35]
NitrazineGreenhouse
(NSCS)		84SD (40%)N/AN/AN/AUSA[35]
Lesco sulfur-coated ureaField trial50 and 100N/AN/ANSIN/AUSA[36]
Floranid Komplett (FK) Pot experiment300 mg N/potSISISIN/ASpain[25]
CDU
(crotonylidendiurea)		Pot experiment300 mg N/potNSINSINSIN/ASpain[25]
Triabon (CDU+Urea)Pot experiment300 mg N/pot SISISIN/ASpain[25]
Lignin coated urea CRFs (16–34%)Pot experiment SISISIN/ASpain[37]
Ureaform Field plot trial3 kg-N/seasonN/ASIN/AN/AUSA[26]
IBDUField plot trial3 kg-N/seasonN/ASIN/AN/AUSA[26]
SCUField plot trial3 kg-N/seasonN/ASIN/AN/AUSA[26]
IBDUField plot trial120NSINSIN/AN/APortugal[38]
Ammonia-treated vermiculite (ATV)Pot trial500NSINSIN/AN/AScotland[39]
D-CODERField100NSINSIN/AN/APortugal[40]
SmartFert 1 TMField25 and 50NSINSIN/AN/ANew Zealand[41]
SmartFert 2TMField30 and 90NSISIN/AN/ANew Zealand[41]
Polymer coated ureaField85, 170 and 250NSINSIN/AN/AAustralia[43]
Polymer coated ureaField50NSINSIN/AN/AAustralia[42]
Polymer coated ureaField100N/AN/AN/ASIUnited Kingdom[44]
Epoxy-lignite coated ureaLysimeter50NSINSINSINSINew Zealand[27]
Polyester-lignite coated ureaLysimeter50NSINSINSINSINew Zealand[27]
Epoxy-lignite coated ureaLysimeter200NSINSISINSINew Zealand[10]
Urea impregnated ligniteLysimeter200NSINSISINSINew Zealand[10]
SI indicates significantly improved, SD indicates significantly decreased, NSI indicates no significant improvement, NSCS indicates nutrient solution culture system and N/A indicates not available.
Table 2. Summary of application of SRNF in controlling N losses and agronomic performances.
Table 2. Summary of application of SRNF in controlling N losses and agronomic performances.
ParameterMedian Nitrate Loss Control (%)Median Ammonium Loss Control (%)Median N2O Loss Control (%)Median DM Yield Increment (%)Median Herbage N Increment (%)Median NUE Increment (%)
SRNF Type
PC80--−7−4.35
BC41--2652.526
IC−25--−13−32.8−19
PCH34--5−9.610
Application Rate
≤1005--−519.8−12
101−20085--−9−32.2−9
>20033--1716.217
Over All361016−256.421
PC—polymer coating, BC—biodegradable coating, IC—inorganic coating, and PCH—polymer chain.
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Abhiram, G. Slow-Release Fertilisers Control N Losses but Negatively Impact on Agronomic Performances of Pasture: Evidence from a Meta-Analysis. Nitrogen 2024, 5, 1058-1073. https://doi.org/10.3390/nitrogen5040068

AMA Style

Abhiram G. Slow-Release Fertilisers Control N Losses but Negatively Impact on Agronomic Performances of Pasture: Evidence from a Meta-Analysis. Nitrogen. 2024; 5(4):1058-1073. https://doi.org/10.3390/nitrogen5040068

Chicago/Turabian Style

Abhiram, Gunaratnam. 2024. "Slow-Release Fertilisers Control N Losses but Negatively Impact on Agronomic Performances of Pasture: Evidence from a Meta-Analysis" Nitrogen 5, no. 4: 1058-1073. https://doi.org/10.3390/nitrogen5040068

APA Style

Abhiram, G. (2024). Slow-Release Fertilisers Control N Losses but Negatively Impact on Agronomic Performances of Pasture: Evidence from a Meta-Analysis. Nitrogen, 5(4), 1058-1073. https://doi.org/10.3390/nitrogen5040068

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