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Article

Evaluating How Enhanced Efficiency Nitrogen Fertilizers Improve Agricultural Sustainability: Greenhouse Multi-Phase Tracking System

by
Ran Darzi
1,
Oz Kira
2,3,*,
Avi Shaviv
1 and
Yael Dubowski
1
1
Faculty of Civil and Environmental Engineering, Technion, Israel Institute of Technology, Haifa 3200003, Israel
2
Department of Civil and Environmental Engineering, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel
3
School of Sustainability and Climate Change, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel
*
Author to whom correspondence should be addressed.
Agriculture 2023, 13(7), 1384; https://doi.org/10.3390/agriculture13071384
Submission received: 7 June 2023 / Revised: 10 July 2023 / Accepted: 11 July 2023 / Published: 12 July 2023
(This article belongs to the Section Crop Production)
Browse Figures
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Abstract

:
The vast consumption of nitrogen-based fertilizers is a significant source of environmental pollutants in all phases. While enhanced-efficiency nitrogen fertilizers (EENFs) improve N-use efficiency and reduce environmental impact, their performance depends on local conditions. Here, we present a relatively simple soil–fertilizer–plants greenhouse setup for the multi-phase tracking of N species of the effects of EENFs. We compared the environmental and agronomic performance of three urea-based EENFs (urea embedded with urease inhibitors, 2-month, and 4-month controlled release urea) and the common split surface application of the granulated urea. We tested the fertilizer applications on basil seedlings for three consecutive growing cycles. The analyses carried out included nitrogen recovery in plant biomass, soil, drainage, and NH3 and N2O emissions. This multi-phase research indicates that tested EENFs perform better than the standard surface-applied urea. The four-month controlled-release urea fertilizer matched the basil N demand the best, significantly increasing yield and improving nitrogen use efficiency while reducing NO3− leaching and NH3 and N2O emissions. The presented soil–fertilizer–plants experimental setup provides a relatively easy-to-handle system for the comprehensive tracking of N dynamics, including plant response. It allows stakeholders to estimate and improve fertilization sustainability.

Graphical Abstract

1. Introduction

Enhancing nitrogen use efficiency (NUE) is crucial for maintaining food security while reducing agricultural and environmental impacts. Modern agriculture’s vast consumption of nitrogen (N) fertilizers increases the emission of pollutants, e.g., ammonia (NH3) and nitrate (NO3−), to the environment and is considered the primary anthropogenic source of the greenhouse gas nitrous oxide (N2O) present in the atmosphere [1]. Tracking the different pathways of applied N in the agroecological system can lead to improved agricultural practices and technological advances that will improve NUE, promote more sustainable agriculture, and create financial benefits [2].
One of the primary efforts to increase NUE when using urea, the most widely used N fertilizer [3], is developing enhanced-efficiency N fertilizers (EENFs), such as N fertilizer combined with urease inhibitors or controlled release coating. These fertilizers have demonstrated the ability to improve NUE and significantly reduce NH3 and N2O emissions to the atmosphere [1,4,5,6,7,8,9,10]. EENFs slow down urea hydrolysis, either through the enzymatic inhibition of the hydrolysis process (urease inhibitors) or by releasing the urea gradually into the soil (slow or controlled release), thus reducing nitrogen losses via NH3 volatilization [11,12,13,14], nitrification, and denitrification [15,16,17].
NUE is often evaluated based on simple measures carried out in the field: the amount of applied fertilizer, the amount of nitrogen in the plant, and the amount of soil nitrogen [18]. In the last decade, research has been extended to meta-analyses using significant data sources to provide insights and understand the effects of different N fertilization approaches and EENFs on NUE [16,19,20,21]. Such efforts offer directions for improvements but still emphasize the importance of specific local information, which may be critical for the best decision making regarding N fertilization. Thus, local integrative approaches are essential to fathom the complexity of the N transformations in cultivated soils, find ways to improve NUE, and provide enhanced data for biogeochemical models (e.g., [22]). By applying urea transforms in the soil into N mineral and organic species, some may leach down the soil column and to water sources (NO3− and nitrite (NO2−)) or be emitted as gases (nitrogen oxides (NOx), NH3, N2O, and molecular N (N2)) into the atmosphere [23,24]. To better account for all the different N pathways, these mineral N species should be tracked in all phases (soil, atmosphere, and drainage water) and crop response/N-uptake [16,21,25]. The complexity of N processes under differing conditions requires a relatively simple system to evaluate the contributions of different fertilization approaches under specific local conditions.
Closed soil chamber systems have the potential to track temporal changes in mineral N species, both in soil and in the chamber headspace, under various environmental conditions and agricultural applications [26,27]. Kira et al. (2019) tracked soil N species (NH4+, NO3−, and NO2−) and gaseous emissions (NH3 and N2O) in closed chambers under different urea fertilization rates and application modes. They gathered improved insights about N dynamics in the soil; however, a caveat of such a system is NH3 solubility in the moisture upon the chamber walls. Later, they modified their approach to a continuous flow chamber (steady state), which yielded improved estimations of NH3 volatilization and, thus, a better understanding of the soil N transformations [28]. While these later measurements yielded much more realistic results, this steady-state chamber still did not include a significant part of the agroecological system—the plant. The simultaneous monitoring of N loss pathways in all phases in a soil–fertilizer–plants system is crucial, especially when evaluating new EENFs or fertilization approaches to confront the need for sustainable intensification.
The information regarding the many pathways of the nitrogen cycle is crucial for many aspects of crop production. We can better track and understand the effects of different fertilizers and environmental conditions by tracking all phases, i.e., soil, drainage, and atmosphere. By carrying this out, we can optimize the plant’s NUE and reduce the emission of pollutants and greenhouse gases to the environment. This research presents a soil–fertilizer–plants experiment for NUE and an environmental performance evaluation of different urea-based EENFs compared to the common application of regular urea. Using such a simple experimental setup, based on a common laboratory tool (Fourier transform infrared spectroscopy—FTIR), which can carry out measurements in all phases, comparing different nutrient approaches and environmental conditions, e.g., the use of fertilizers, soils, precipitation rates, etc., is straightforward and yields more accurate insights. The main goal was to conduct multi-phase tracking (soil, drainage water, headspace, and plant) of the different N species of interest (NH3, N2O, NH4+, NO3−, and NO2−). The experimental setup will improve the evaluation and examination of various aspects affecting EENFs’ performance.

2. Materials and Methods

2.1. Experimental Setup

We conducted the multi-phase experiment in a glasshouse with a temperature range of 40–20 °C. Basil (Ocimum basilicum L.) was chosen for this study for two reasons: (1) it is a herb grown in the Mediterranean region, with intensive production in climate-controlled greenhouses [29]; and (2) it is fast growing, heat-resistant, and very suitable for the research design. Plants were grown in rectangular containers (40 cm length × 16 cm width × 16 cm depth) filled with 9 kg of sandy-loam soil (Table 1). The containers were fixed on custom-made growing tables, enabling N sampling in plant biomass, soil, drainage, and gaseous emissions (Figure 1A). We tested four fertilization treatments, each with nine containers arranged in three blocks (Figure 1B). We divided each experiment (105 days) into three successive growth cycles (days 0–41, 42–72, and 73–105). An additional visualization of the experimental setup is presented in Figure S1. Generally, experiment initialization included wetting the soil to field capacity, planting 14 seedlings in each container, and the application of fertilizer (we applied controlled-release fertilizers before wetting—see Section 2.2). Finally, we weighed each container. We reweighed the containers every 1–3 days during the experiment to determine water loss and irrigated to compensate for it (back to field capacity), using a hand pressure sprayer to mimic rain events.

2.2. Fertilizers

This study compared the environmental and agronomic performance of urea-based EENFs to the split surface application of regular urea. The tested treatments, summarized in Table 2, included the granulated urea (UR), urea amended with a mixture of N-(n-propyl) thiophosphoric triamide (NPPT) and N-(n-butyl) thiophosphoric triamide (NBPT) urease inhibitors (UI) [30], a 2-month controlled release of polymer-coated urea (CRF-2M), and a 4-month controlled release of polymer-coated urea (CRF-4M) (Haifa Chemicals Ltd., Israel). We applied all fertilizers at a rate of 5.70 g N container−1. CRF-2M and CRF-4M were incorporated into the top 5 cm of the soil before wetting and planting (on the same day). We applied UR and UI on the soil surface in three equal doses of 1.90 g per N container−1 (referred to hereafter as split application). We applied the first dose immediately after planting. The second and third applications followed the first and second harvesting, respectively. In addition, we added dipotassium hydrogen phosphate (K2HPO4, Sigma-Aldrich, Darmstadt, Germany) to all treatments before planting at a rate of 3 g per container−1 (corresponds to 200 kg-K ha−1 and 85 kg-P ha−1). We carried out fertilization efficiency estimations via a mass balance according to the multi-phase analyses of N species in plant biomass, soil, drainage, and gaseous emissions throughout the basil growth cycles. We determine NUE as the ratio of N content in plant/N supplied [31].

2.3. Sampling and Analyses

2.3.1. Plant Biomass

We performed harvesting at the end of each growth cycle (i.e., days 41, 72, and 105). We cut all plants above the lowest leaves, allowing for an adequate re-growth of a new cycle. We weighed the fresh yield immediately after each harvest and the dry yield after oven-drying at 105 °C. We determined the N content in the dry matter via a wet digestion process using H2O2-H2SO4 [32].

2.3.2. Soil Cores

We collected soil samples three times during the experiment to minimize system disruption (on days 13, 83, and 105). We obtained the samples with a tubular soil sampler from all the containers and filled the sampling location with fresh soil. We later sectioned the obtained soil cores (2 cm in diameter and 15 cm long) into two sub-samples (0–7.5 cm and 7.5–15 cm), representing the upper and lower parts of the soil. We extracted the sample by adding 1 N KCl to the soil samples at a 1:10 ratio (2.5 gsoil with 25 mL solution) and shaking them for one hour at room temperature. After filtration, we determined NH4+, NO2−, and NO3− concentrations using an auto-analyzer (Lachat Quikchem 8500, Hach Company, Loveland, CO, USA).

2.3.3. Leaching and Drainage Collection

We equipped each container with three drainage tubes filled with porous fabric and connected to a 2.0 L drainage collection tank. We almost did not see any drainage during daily irrigations to field capacity. We intentionally leached soils eight times on days 7, 13, 18, 41, 53, 72, 90, and 104, using excessive irrigation of 500–1000 mL per container−1. Although the amount of excessive water differed between the eight leaching tests, all four treatments received equal amounts of water each time, simulating similar rain events.
Five hours after excessive irrigation, we recorded and sampled the drained water amount for each container. An auto-analyzer determined the NH4+, NO3−, and NO2− concentrations in the drainage samples collected (Lachat Quikchem 8500, Hach Company, Loveland, CO, USA). To quantify the leached N of a specific container, all measured species of a sample were summarized (g-N L−1) and multiplied by the drained water amount (L container−1). The total accumulative mineral N for each container is the sum of eight leaching processes. The accumulative mineral N for each of the four treatments (CRF-2M, CRF-4M, UI, and UR) is thus an average of nine containers.

2.3.4. Gaseous Emission

We used specially built cover chambers (135 cm length × 25 cm width × 60 cm height) for soil-induced emission sampling, with each chamber covering three adjacent containers of a given treatment only before sampling (Figure 1A). We placed a small electric fan inside each chamber to enhance air mixing. Then, we measured NH3 and N2O content in gas samples using laboratory FTIR spectroscopy. Emitted gases accumulate during closure times, which we determined according to preliminary tests. During high NH3 emission rates, we shortened the accumulation periods to 30 min (i.e., on the days following fertilizer application) to minimize NH3 dissolution in the condensed water on the chamber walls. To reduce the quantification uncertainty due to the low N2O emission rates, we extended the accumulation periods to 120 min when the NH3 emission rate decreased and allowed for it. We withdrew air samples from the chamber headspace using a peristaltic pump to 1.0 L multi-layer foil bags (Jensen Inert Products, Coral Springs, FL, USA). N2O and NH3 concentrations were then analyzed using a laboratory FTIR spectrometer (Tensor 27, Bruker Optics GmbH, Ettlingen, Germany). We transferred each gas sample to a 0.5 L Long-Path IR gas measurement cell (Infrared Analysis Inc., Anaheim, CA, USA) with a 4.2 m optical path. Then, we recorded IR spectra at the spectral range of 850–5000 cm−1, at 1 cm−1 resolution, using 32 co-added scans per spectrum, and determined the concentrations based on the spectral regions 910–970 cm−1 and 2160–2260 cm−1 for NH3 and N2O, respectively. We quantified their concentrations using (1) a calibration curve of standard N2O mixtures and (2) spectral library data for NH3 (Pacific Northwest National Laboratory database). The emission rates were calculated by dividing the measured concentration by the accumulation period. To summarize the daily loss via gaseous emission, we multiplied the rate by 24 h for each measurement day.

2.3.5. Statistical Analyses

Mean values and the standard deviation for each fertilizer treatment were calculated using nine replicates (i.e., containers) for plant, drainage, and soil samples. The values for gas emission were calculated using three replicates (i.e., three cover chambers). We performed an analysis of variance using JMP® pro 16.0.0 software. To determine which specific treatments differ significantly from each other, post hoc Tukey HSD analysis was conducted (α = 0.05, p < 0.0001).

3. Results and Discussion

3.1. Plant Biomass

Fresh yield weight for each harvest and total accumulative fresh and dry yields are presented in Table 3. The total fresh yield generally followed the order CRF-4M > CRF-2M > UI > UR. The total fresh yield from the UR treatment was significantly lower than that of all other treatments and had the lowest yield in growth cycles 2 and 3. CRF-2M provided the lowest yield at the first cut among all of the treatments. According to the CRF-2M release pattern provided by the fertilizer supplier (Figure S2), 45% of the fertilizer was expected to be released during the first two weeks. The single-dose application of this fertilizer before planting and its relatively fast N-release rate probably resulted in plant stress in the first growth cycle. Indeed, CRF-2M yield increased in cycles 2 and 3. CRF-4M provided the highest total fresh and dry yields of all treatments in all three growth cycles.

3.2. Drainage

Accumulative mineral N in the drainage from all tested treatments is presented in Figure 2A. In all treatments except CRF-4M, the plants exhibited a visual growth inhibition in the first two weeks. Hence, leaching was performed four times in the first growing cycle: in the middle and at the end of the growth cycle (days 13 and 41), and two additional times were needed for plant stress relief and better adaptation (days 7 and 18). An accumulation trend of leached mineral N can reflect N availability in the soil. While N leaching in UR and UI treatments gradually increased after every split application, the CRF-4M treatment resulted in a classical sigmoidal release pattern, in line with the expectations from efficient polymer-coated fertilizers [33]. The faster release rate of CRF-2M compared to that of CRF-4M is well reflected in the observed N-leaching pattern, starting from day 18.
N species analysis of the leachate showed that NO3− accounted for 50–100% of the leached mineral N (Figure S3). The NO2− concentration in the collected drainage is presented in Figure 2B. Under the CRF-4M treatment, the NO2− concentrations were negligible throughout the experiment. For CRF-2M, UI, and UR, the concentration increased during the first and second growth cycles, which was expected considering the fast urea supply and its hydrolysis, which increases the pH and can thus slow NO2− oxidation (e.g., [34]). The observed plant growth inhibition in the first cycle under the CRF-2M, UI, and UR treatments seems to correlate with the high NO2− levels [26,35]. NO2− concentration under the UI and UR treatments decreased at the end of the first growth cycle but increased again after applying the second urea dose in the UR treatment. With CRF-2M, which has a relatively high initial rate of urea release, NO2− concentration remained high and decreased only toward the end of the second growth cycle.

3.3. Soil Mineral Nitrogen

We analyzed soil mineral N in the middle of the first and third growth cycles and at the end of the experiment (Figure 3). The most notable differences occurred two weeks after the initial fertilizer application of the first growth cycle (orange diamond shape in Figure 3, day 13). In this sample, total soil mineral N was 1.5 ± 0.4, 0.5 ± 0.1, 2.0 ± 0.2, and 2.4 ± 0.7 g per N container−1 for CRF-2M, CRF-4M, UI, and UR treatments, respectively. Expectedly, soil N availability was lower in the controlled-release treatments (CRF-2M and 4M) even though the total N dose initially applied was higher than in the split application of UR and UI (5.7 vs. 1.9 g container−1, respectively). The CRF-2M treatment resulted in three times higher mineral N availability than that of CRF-4M due to a faster release pattern, particularly in the initial stage. For the split-applied UR and UI, a mineral N concentration higher than the applied dose indicates that the whole amount initially applied was available after two weeks as mineral N (for UR, probably even before). The high level of mineral N for UR treatment (2.4 g per N container−1 after application of 1.9 g) indicates a significant mineralization enhancement [36]. Plants treated with CRF-4M grew better than those with other treatments in the first growth cycle, suggesting that the lower mineral N in the soil was the most efficient. The higher mineral N in all other treatments surpasses plant needs and possibly induces plant stress and environmental loss. As expected, in the third growth cycle (blue square shape, Figure 3), there was a higher soil mineral N in the split-applied treatments (UI and UR) after the third fertilizer dose. At the end of the experiment (i.e., at the end of the third growth cycle), the soil’s fertilizer-induced N remained only in the CRF-4M treatment.
We further analyzed soil mineral N species (NH4+, NO3−, and NO2−) for days 13 and 83 samples. Figure 4 summarizes the mineral N analysis in the top and bottom parts of the containers (on day 13). In the CRF treatments, N availability and NO2− accumulation predominate in the upper soil layer (adjacent to the fertilizer application location). In contrast, N concentrations were higher in the bottom of the container for UI and UR treatments. This is unsurprising because urea is more mobile than ammonium and prone to moving downwards via irrigation/leaching. It also explains the very high ammonium concentrations at the bottom of the containers receiving the UR and UI treatments after 13 days (Figure 4). NO2− accumulation under the latter two treatments was significant. It occurred in both the top and bottom of the container. NO2− formation under CRF-4M was negligible, while under CRF-2M, its build-up in the upper soil layer was as high as that observed with the UR and UI treatments. The soil NO2− analysis on day 13 is consistent with the trend of UR > UI > CRF-2M > CRF-4M viewed in the leaching sampling of the same day (Figure 2B). It is worth noting that the root system was not well-developed at this stage; hence, the mineral N located at the bottom of the soil was probably consumed less by the plants.
The soil analysis on day 83 (Figure 5) yielded noteworthy differences between the split-applied treatments (UI and UR) and the CRF treatments (CRF-2M and CRF-4M). In the split-applied treatments, the available mineral N is mostly NO3−. In the UR treatment, the nitrate was evenly distributed between the top and bottom layers, while in the UI treatment, the bottom of the container contained higher NO3− than the top. These results align with the higher accumulation rates seen in the leaching on day 90 (Figure 2A). In contrast, the main available N species of the CRF treatments was NH4+, which accumulated mainly in the top layer. At this stage, three months after fertilization, the higher NH4+ concentration in CRF-4M treatment and the relatively low availability in CRF-2M coincide with their release pattern (Figure S2).

3.4. Gaseous Emissions

Figure 6 shows the NH3 emission rate during the three growth cycles. NH3 emission rate peaked immediately after each split application of UR treatment and decreased 3–5 days after it. The short time gap between urea application and the NH3 emission peak reflects rapid urea hydrolysis and NH4+ build-up under increased pH. In the following days, the emissions decreased following NH4+ plant uptake and the nitrification process. In contrast to UR, the three EENF treatments mitigated the NH3 burst during all growth cycles, in line with previous studies [11,30,37].
N2O emissions showed a longer time lag after fertilizer application (Figure 7) than NH3 emissions. Generally, N2O emissions peaked after NH3 emissions decreased, implying nitrification-induced N2O emissions. We assume that the high pH levels associated with high NH3 emissions inhibit microbial activity [34]; only after NH3 emissions decline do the nitrification processes intensify, resulting in increased N2O emissions. During the first growth cycle, we detected N2O emissions in all treatments; however, emission rates were much lower for the CRF-4M treatment. The temporary decrease in N2O emissions on day 14 (Figure 7) is attributed to the leaching process performed on day 13. While emissions gradually decreased toward the end of the first growth cycle, they peaked again in the second cycle, especially in UR treatment. As expected, in the third growth cycle, CRF fertilizers delivered low amounts of N, generating lower N2O emission rates for CRF-2M and CRF-4M. At this stage, the emission rate increased again in the UR treatment following the third fertilizer dose, unlike the lower emissions for the UI treatment.

3.5. Nitrogen Balance

Table 4 summarizes the total accumulative N load considering all tested phases. The CRF-4M treatment showed the highest NUE based on N content in plant shoots. Combined with reduced N losses via gaseous emissions and leaching, it demonstrates the advantages of using controlled-release fertilizer. CRF-2M and UI had higher NUE and lower gaseous emissions than UR; however, their N losses via leaching were higher than those of UR, accounting for 20–40% of N loss in our experiment. Leaching is a well-known problem regarding groundwater contamination [38,39], especially in areas of sandy (coarse) soils [40].
Tracked gaseous emissions followed the general trend UR > CRF-2M > UI > CRF-4M. NH3 was the main gaseous loss pathway for UR treatment, while the emissions from tested EENFs were equally distributed between NH3 and N2O. The higher NH3 loss in the UR treatment via volatilization (5.1% under the current setting), is a significant loss pathway in standard urea fertilizer applications. All EENFs mitigated the NH3 burst in the UR treatment (Table 4 and Figure 6). Although the application method and N-release mechanism of UI and CRF-4M differ, both treatments provided a similar NH3 loss of 0.5%. N2O emissions ranged between 0.2% and 1.2% of applied N. Although N2O emissions may vary broadly [41], the emissions in the UR treatment accounted for 0.9% of the applied N, similar to the mean global fertilizer-induced emission reported in [42]. CRF-4M and UI had reduced N2O emissions compared with the UR treatment, in agreement with previous studies [4,43]. In contrast, and mainly due to the emissions measured in the first growth cycle, CRF-2M had slightly higher total N2O emissions than those of the UR treatment. A similar trend was previously reported for N2O emissions from polymer-coated urea treatment with a short release time (e.g., [44]).

3.6. Difference between Growth Cycles

The multi-phase experiment shows inner differences between the tested treatments and changes occurring during growth cycles, which can be observed in the yield per harvest, accumulation patterns of mineral N in drainage, and gaseous emission variations. The differences between growth cycles were relatively similar for all treatments.
As newly planted seedlings have a less-developed rooting system and a thin stem, their nutrient uptake rate was probably lower during the first growth cycle. Furthermore, the soil microbial population was perhaps underdeveloped at that point. A combination of lower plant N demand and possibly lower microbial activity at the initial stage of the experiment with high N availability can lead to NO2− accumulation and higher N loss via N2O emissions and lower yields. Indeed, we measured lower yields in the first cycle despite its longer duration relative to cycles two and three. Additionally, the first cycle depicted more substantial fluctuations in N2O emissions and the drainage’s highest NO2 levels. During the second growth cycle, the agrosystem was more mature. Compared to the first cycle, the plant yields improved, suggesting a more developed rooting system and possibly improved growth conditions. The reduced N2O fluctuations and NO2− concentrations in the later cycle probably reflect the plants’ more intensive microbial activity and higher active N uptake. The third growth cycle represents the most developed cropping system. A significant amount of the applied N in the CRF treatments had been released before this cycle, resulting in negligible gaseous losses and leached mineral N. After the third applied dose of UR and UI, the drainage N loss was similar to that of the second growth cycle, although no NO2− was present. Additionally, the N2O emission rates were lower, presumably due to the intensification of microbial activity.

3.7. Fertilizer Efficiency Evaluation

The current multi-phase analysis indicates that the tested EENFs performed better than the standard surface-applied urea. The results imply that CRF-4M matched the basil N demand, significantly increasing the yield and improving NUE while reducing NO3− leaching and NH3 and N2O emissions. The other controlled-release fertilizer, CRF-2M, exhibits environmental disadvantages similar to those of UR: NO2− accumulation, increased N2O emission, and high mineral N leaching. Although it performed better than UR in crop yields and NUE, the CRF-2M fast-releasing rate may still have undesirable environmental effects when used on freshly planted seedlings of this type. The UI treatment also performed better than UR, improving NUE and mitigating gaseous N emissions. Yet, both the CRF-2M and the UI results suggest that the mitigation of gaseous emissions alone may leave more N in the soil, which tends to be leached. Although the UR treatment is the most popular globally, our experiment revealed agronomic inefficiency and environmental effects that suggest that the fertilizing procedure does not fit the N demand profile of the plants.

3.8. Advantages and Limitations of the Experimental Setting

The greenhouse setup allows for easy multi-phase plant, soil, drainage, and gas emission sampling. We compared fertilizer efficiency, plant NUE, and potential environmental loss mitigation according to several parameters: plant yield mass and N content, mineral N species in drainage, their distribution in soil, and N2O(g) and NH3(g) emissions. Such multi-phase monitoring provides an improved mass-balancing tool for N resources (inputs, outputs, and losses) and better insight for tracking N dynamics. Moreover, it enables a better evaluation of the N fertilizer’s performance under various conditions that affect the complexity of N dynamics and are essential for improving N fertilization. The multi-phase table design can be used for the simultaneous evaluation of fertilizers’ performance with different soil types, environmental conditions, water regimes, and crops. This setting provides a relatively simple yet comprehensive frame that can improve the assessment of NUE of EENFs for various agroecosystems.
It is worth noting that while using a static system for measuring NH3 emissions is valid for comparison between treatments, this is likely to underestimate the emission fluxes obtained under dynamic and steady-state systems (e.g., [26] vs. [28]). Thus, we recommend designing similar experimental techniques based on steady-state chambers for tracking N gaseous emissions in future research.
The mass balance of the current multi-phase N analyses is incomplete, with unaccounted N ranging between 19.3% and 36.5% (Table 4). One main reason is the underestimation discussed above without continuous NH3 measurements. Additionally, we did not account for N2(g) emissions formed via soil anaerobic denitrification processes. N2(g) cannot be easily monitored [45], and commonly used indirect methods may underestimate the denitrification rates [46]. Another potential bias may result from not including plant roots in our measurements, causing us to underestimate the assimilated N in the yields.
The UR treatment had higher values of unaccounted N in comparison to other treatments. Compared to all other treatments, the relatively high NH3(g) emissions support the assumption that a considerable percentage of the UR treatment’s unaccounted N may be associated with additional unmeasured NH3(g) emissions. Surprisingly, CRF-4M also had high unaccounted N. Overall, this treatment had a higher water consumption due to a higher biomass. Thus, it is likely that in the CRF-4M treatment, more N accumulated in the roots, making the underestimation of plant N uptake more significant than in the other treatments. Additionally, part of the protected urea in the CRF-4M was presumably still unreleased at the end of the experiment and not extracted via the extractions used.

4. Conclusions

We tested three urea-based EENF treatments and compared them to standard granulated urea in a controlled environment simulating field conditions. The soil–fertilizer–plants experimental setup provides a relatively easy-to-handle and efficient system for tracking N dynamics, including plant response. In agreement with previous knowledge, the current experiment clearly showed the disadvantages of surface-applied urea. In the present study, the evaluation of fertilizers’ efficiency followed the overall order of CRF-4M > UI ≊ CRF-2M > UR. This signified EENFs’ potential to reduce environmental impacts and improve agricultural and economic benefits.
In this research, we presented a relatively simple experimental setup to track N species in the soil, plant, water drainage, and atmosphere, which are part of the agroecological N cycle. Following the various N species allows for a better evaluation of NUE and the environmental impact of tested fertilizers. Such a system enables us to trace which processes generate higher losses and plan mitigating actions that also consider local conditions effects in order to reduce N losses in field applications and increase agricultural sustainability. This experimental system has limitations as it does not fully represent field conditions. However, performing such holistic tracking in the field is highly challenging. If carried out correctly, an N-tracking system like this may introduce significant insights for improving sustainable field applications. Yet, further research should be carried out to optimize the system, primarily comparing these results to data gathered in the field. This research used only one soil type, and more studies are needed to examine other soils.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture13071384/s1, Figure S1: Experimental setup and growing tables arrangement. Multi-phase sampling and nitrogen analyses were done to evaluate fertilizer efficiency.; Figure S2: N-release pattern for 2 months controlled release polymer-coated urea (CRF-2M) and 4 months controlled release polymer-coated urea (CRF-4M).; Figure S3: Mineral N distribution (bars) and total mineral N (diamonds) in the leachate.

Author Contributions

R.D., O.K., A.S. and Y.D. contributed to the study’s conception and design. R.D. and O.K. performed material preparation, data collection, and analysis. R.D. and O.K. wrote the first draft of the manuscript, and O.K., A.S. and Y.D. commented on previous versions. R.D., O.K., A.S. and Y.D. All authors have read and agreed to the published version of the manuscript.

Funding

Thanks are given to Haifa Chemicals Ltd. for supporting this research.

Data Availability Statement

Data will be given on request.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Agriculture 13 01384 g001 550
Figure 1. (A) growing table arrangement for multi-phase sampling; (B) a mixed pattern of four fertilization treatments (details in Section 2.2), each including nine containers arranged in three separate blocks.
Figure 1. (A) growing table arrangement for multi-phase sampling; (B) a mixed pattern of four fertilization treatments (details in Section 2.2), each including nine containers arranged in three separate blocks.
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Figure 2. (A) Accumulative mineral N in the drainage. (B) NO2− concentrations in drainage. During the first growth cycle, we performed soil leaching four times. We conducted soil leaching in each cycle’s middle and end in the second and third growth cycles. Error bars represent standard deviation (n = 9).
Figure 2. (A) Accumulative mineral N in the drainage. (B) NO2− concentrations in drainage. During the first growth cycle, we performed soil leaching four times. We conducted soil leaching in each cycle’s middle and end in the second and third growth cycles. Error bars represent standard deviation (n = 9).
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Figure 3. Total soil mineral N (without the native soil mineral N) in the middle of the first and third growth cycles and at the end of the experiment. Negative values represent lower soil mineral N content than the initial native N concentration.
Figure 3. Total soil mineral N (without the native soil mineral N) in the middle of the first and third growth cycles and at the end of the experiment. Negative values represent lower soil mineral N content than the initial native N concentration.
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Figure 4. Mineral N analysis at the top (0–7.5 cm) and bottom (7.5–15 cm) container layers in the middle of the first growth cycle, 13 days after the initial fertilizer application. Error bars represent standard deviation (n = 9). Statistical differences are indicated by different letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
Figure 4. Mineral N analysis at the top (0–7.5 cm) and bottom (7.5–15 cm) container layers in the middle of the first growth cycle, 13 days after the initial fertilizer application. Error bars represent standard deviation (n = 9). Statistical differences are indicated by different letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
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Figure 5. Mineral N analysis at the top (0–7.5 cm) and bottom (7.5–15 cm) container layers in the middle of the third growth cycle, 83 days after the CRFs application and 8 days after the third UI and UR split application. Nitrite (NO2−) levels were under the detection limit for all treatments and are not presented. Error bars represent standard deviation (n = 9). Statistical differences are indicated by different letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
Figure 5. Mineral N analysis at the top (0–7.5 cm) and bottom (7.5–15 cm) container layers in the middle of the third growth cycle, 83 days after the CRFs application and 8 days after the third UI and UR split application. Nitrite (NO2−) levels were under the detection limit for all treatments and are not presented. Error bars represent standard deviation (n = 9). Statistical differences are indicated by different letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
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Figure 6. NH3 emission rate (mg per N container−1 hr−1) for the (A) first, (B) second, and (C) third growth cycles.
Figure 6. NH3 emission rate (mg per N container−1 hr−1) for the (A) first, (B) second, and (C) third growth cycles.
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Figure 7. N2O emission rate (mg per N container−1 hr−1) for the (A) first, (B) second, and (C) third growth cycles.
Figure 7. N2O emission rate (mg per N container−1 hr−1) for the (A) first, (B) second, and (C) third growth cycles.
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Table
Table 1. Soil properties. The values in the table are the mean ± standard deviation of three replicates.
Table 1. Soil properties. The values in the table are the mean ± standard deviation of three replicates.
Soil Type apH bEC b
dS m−1		N-NH4+ c
mg N kg−1		N-NO3− and N-NO2− c
mg N kg−1		Cation-Exchange Capacity
meq 100 g−1		Moisture Content
g Water g−1 Dry Soil
(Field Capacity)
Sandy Loam6.9 ± 0.10.2 ± 0.29.2 ± 1.57.6 ± 2.17.9 ± 0.213.9 ± 3.3
a “Hamra” soil from the central district of Israel, previously used in an avocado grove. b EC and pH were measured after 1:2 water extraction. c NH4+, NO3− and NO2− measured following 1:10 1N KCl extraction. The initial NO2− concentration was below the detection limit.
Table
Table 2. Fertilization treatments of the experiment.
Table 2. Fertilization treatments of the experiment.
Treatment AcronymFertilizer Type (Granulated)Mode of ApplicationN Applied
CRF-2MControlled release fertilizer—
polymer-coated urea with a 2-month release pattern 		Incorporated into the top 5 cm of the soil5.70 g per N container−1 (900 kg-N ha−1).
Single application at the beginning of the experiment, before planting.
The total amount is distributed along three growth cycles.
CRF-4MControlled release fertilizer—
polymer-coated urea with a 4-month release pattern		Incorporated into the top 5 cm of the soil5.70 g per N container−1 (900 kg-N ha−1).
Single application at the beginning of the experiment, before planting.
The total amount is distributed along three growth cycles.
UIUrease inhibited—
urea amended with a mixture of NPPT and NBPT urease inhibitors		Split surface application1.90 g per N container−1 (300 kg-N ha−1).
Applied three times at the beginning of each growth cycle (i.e., a total of 5.70 g per N container−1 for the three growth cycles).
URUreaSplit surface application1.90 g per N container−1 (300 kg-N ha−1).
Applied three times at the beginning of each growth cycle (i.e., a total of 5.70 g per N container−1 for the three growth cycles).
Table
Table 3. Fresh and dry yield. The values in the table are the mean ± standard deviation. Statistical differences are indicated by different superscript letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
Table 3. Fresh and dry yield. The values in the table are the mean ± standard deviation. Statistical differences are indicated by different superscript letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
TreatmentTotal Fresh YieldTotal Dry Yield1st Cut Day 41 (Fresh) 2nd Cut Day 72 (Fresh) 3rd Cut Day 105 (Fresh)
g Container−1g Container−1g Container−1g Container−1g Container−1
CRF-2M554 ± 19 b49 ± 3 b101 ± 10 d218 ± 14 a235 ± 9 b
CRF-4M674 ± 13 a65 ± 2 a170 ± 9 a219 ± 6 a285 ± 7 a
UI519 ± 15 c48 ± 2 b146 ± 9 b145 ± 8 b228 ± 9 b
UR421 ± 29 d39 ± 2 c134 ± 5 c117 ± 11 c170 ± 16 c
Table
Table 4. Multi-phase experiment N balance. The values in the table are the mean ± standard deviation. Statistical differences are indicated by different superscript letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
Table 4. Multi-phase experiment N balance. The values in the table are the mean ± standard deviation. Statistical differences are indicated by different superscript letters (Tukey HSD analysis, n = 9, α = 0.05, p < 0.0001).
Treatment (Container)N AppliedNUE—PLANT Uptake *Mineral N DrainageN Gaseous EmissionsSoil-N **Total N AccountedUnaccounted N
N2ONH3
g Container−1%%%%%%%
CRF-2M5.732.5 ± 3.0 a33.5 ± 7.5 a1.21.2068.431.6
CRF-4M5.745.5 ± 3.5 b20.0 ± 4.2 b0.20.54.270.429.6
UI5.739.6 ± 2.1 c40.0 ± 7.6 c0.50.5080.719.3
UR5.730.4 ± 1.9 d27.2 ± 6.0 d 0.95.1063.536.5
* Based on N uptake in plant shoots. ** Nitrogen left in the soil (mainly as nitrate) at the end of the experiment. The initial native concentration was subtracted.
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Darzi, R.; Kira, O.; Shaviv, A.; Dubowski, Y. Evaluating How Enhanced Efficiency Nitrogen Fertilizers Improve Agricultural Sustainability: Greenhouse Multi-Phase Tracking System. Agriculture 2023, 13, 1384. https://doi.org/10.3390/agriculture13071384

AMA Style

Darzi R, Kira O, Shaviv A, Dubowski Y. Evaluating How Enhanced Efficiency Nitrogen Fertilizers Improve Agricultural Sustainability: Greenhouse Multi-Phase Tracking System. Agriculture. 2023; 13(7):1384. https://doi.org/10.3390/agriculture13071384

Chicago/Turabian Style

Darzi, Ran, Oz Kira, Avi Shaviv, and Yael Dubowski. 2023. "Evaluating How Enhanced Efficiency Nitrogen Fertilizers Improve Agricultural Sustainability: Greenhouse Multi-Phase Tracking System" Agriculture 13, no. 7: 1384. https://doi.org/10.3390/agriculture13071384

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