Reducing options of ammonia volatilization and improving nitrogen use efficiency via organic and inorganic amendments in wheat (Triticum aestivum L.)

Introduction

Nitrogen (N) in the atmosphere is the principal source of all soil nitrogen. It naturally enters the soil through dead animal and plant residues, biological nitrogen fixation, and chemical N fertilizer applications. Nitrogen fertilizers have become essential to increase crop yield and enhance food quality (Leghari et al., 2016). Food grain crops account for more than 69% of India’s total N fertilizer intake, where wheat alone has a share of 24%. In the crop year 2020–21, India’s wheat production increased from 109 to 118 Mt to meet the growing population’s demand. However, in future, the wheat yield has to be increased by 1.5% per year to satisfy the growing population’s demand (Grain & Feed Annual report, 2021). Ammonia volatilization is a significant cause of nitrogen depletion in agricultural soil worldwide, contributing to low N fertilizer usage, crop production, and indirect nitrous oxide (N₂O) emissions. Globally, the average NH₃ volatilization risks range from 0.9% to 64% of the applied N (a mean of 17.6%) (Pan et al., 2016). Ammonia volatilization into the atmosphere negatively affects agriculture, ecosystems, and human health.

Further, it also increases the loss of nitrogen for plant growth, thus increasing the cost of cultivation (Brink & Van Grinsven, 2011). Secondary particulate matter (PM₁₀) is formed when NH₃ reacts with other air contaminants such as sulfuric acid and nitric acid. It flies long distances and remains in the air for several days, and causes respiratory diseases in humans (Bittman et al., 2014).

A study conducted by Cao et al. (2013) found that the significant loss of N from applied fertilizer was through NH₃ volatilization, which was estimated to be 10.0–19.5% of total N loss. Further, they concluded that NH₃ volatilization might be the dominant pathway of N loss. Ammonia volatilization favourably occurs in N fertilizers like urea and organic manure. However, ammonia emission is undesirable as it removes N from the soil/plant system and releases it into the atmosphere. Volatilized NH₃ is deposited back to the earth’s surface mainly through two processes which are (i) wet deposition through precipitation and (ii) dry deposition when combined with particulate matter. This seriously impacts the environment as it causes acidification of soil and water bodies and eutrophication of the natural ecosystem. It also acts as an indirect source of N₂O, a potent greenhouse gas. In some instances, it also directly affects plants under high concentrations and low temperatures. There are many other ways to control N loss, like avoiding the application of urea under high-risk conditions, deep placement, and using controlled-release N-fertilizers. However, the more effective way to control is by using organic and inorganic amendments; some of them are chemical inhibitors like N-(n-butyl) thiophosphoric triamide (NBPT), cyclohexyl phosphoric triamide (CHPT), ammonium thiosulphate (ATS), hydroquinone (HQ), and calcium carbide (CaC₂).

Among these, NBPT is used globally, being the most effective in a market that has expanded at 16% per year over the last 10 years. NBPT-treated urea reduces NH₃ loss by 53% (Cantarella et al., 2018). The application of 12 kg N ha⁻¹ HQ on an alluvial soil, in conjunction with 120 kg urea-N ha⁻¹, decreased N₂O emission by 5% in rice and 7% in wheat systems when compared to the crops grown solely in the presence of 120 kg N ha⁻¹ urea (Modolo et al., 2018).

The slow-N-release coating technology is also suitable for consistent N supply to the plants and reducing loss and contamination effects. We have explored various organic and inorganic amendments to minimize NH₃ volatilization losses and enhance NUE and wheat crop productivity. The application of N inhibitors along with urea has increased the average grain yield by 6.8% (Školníková et al., 2022). Nitrification and urease inhibitors are being suggested to decrease N losses and thus increase crop nitrogen usage efficiency (Abalos et al., 2014). Different mechanisms are involved in reducing NH₃ volatilization in that NBPT retards the activity of the urease enzyme by competitive inhibition (Fan et al., 2018). Hydroquinone and calcium carbide is also effective in reducing urease activity. Applying HQ to the soil inhibited urease enzyme activity and inhibited or enhanced the activity of other enzymes like polyphenol oxidase, dehydrogenase, protease and phosphatase (Wołejko et al., 2020). The CaC₂ has a negative impact on ammonia-oxidizing bacteria, which are present in the soil, and also reduces the activity of dehydrogenase and nitrate reductase enzymes, thereby reducing the N losses (Mahmood et al., 2014). Pongamia oil (karanjin) works as a nitrification inhibitor by reducing the Nitrosomonas activity without affecting Nitrobacter spp. Activity. The treatment with garlic extract, which is rich in the compound thiosulfinate, works on the mechanism of competitive inhibition of urease enzyme as it contains an organosulphur group (-S(O)-S) which is similar to urea which helps in inhibiting the hydrolysis process (Mathialagan et al., 2017).

The world’s population will surpass 9.7 billion by 2050, posing a significant obstacle to achieving food sustainability. The projected increase in the world’s population demands at least a 70% increase in agriculture production in developed countries and 100% in developing countries (Mahmud et al., 2021). Food security of the country and N-based global warming and environmental degradation are interlinked (Bilal & Aziz, 2022). The use of urease inhibitors in agricultural activities has already been investigated as one of the best methods for ensuring adequate nutritional security (Modolo et al., 2018). N-(n-butyl) thiophosphoric triamide treated urea lowers NH₃ loss by approximately 53%. The yield benefits from NBPT application on an average of 6.0% and ranges from 0.8% to 10.2% depending on crop types (Cantarella et al., 2018). The effectiveness of NBPT in reducing NH₃ loss is well known, but there is still room for progress to increase the amount of inhibition and hence the retention of NBPT-treated urea in the field. However, these inorganic amendments are not economically viable, and most of them restricted their usage in the research areas due to high costs. Upadhyay, Tewari & Patra (2011) investigated that these chemicals are harmful and inhibit the growth of specific beneficial soil microorganisms that indirectly affects crop growth and development. Therefore, the best option that stands before us is using organic amendments to reduce NH₃ losses.

Organic amendments are eco-friendly, economically viable, and, most importantly, biodegradable. Therefore, they act as promising N inhibitors. There are many plant-derived inhibitors like Neem (Azadirachta indica) oil, Pongamia (Pongamia glabra) oil, linseed (Linum usitatissimum) oil, garlic extract, and mint (Mentha spicata) as N amendments in inhibiting N losses. Thiosulfinates (TS) present in fresh garlic extract act as a bio-inhibitor of urease enzyme activity and can be used as a potential urease inhibitor in agriculture. Thiosulfinates have the potential to inhibit the urease enzyme, and it takes 3 h and 30 min to start acting on the urease enzyme after its application (Ramli et al., 2014). Applying coated urea fertilizers such as neem-coated urea and pine oleoresin-coated urea in a vertisol reduced the NH₃ volatilization by 27.5% and 41.1%, respectively (Jadon et al., 2018). However, these organic amendments lack commercialization and efficient utilization. Therefore, this study concentrates mainly on stressing the importance and comparative study of these organic and inorganic amendments to decrease ammonia volatilization loss and improve increasing NUE and productivity of wheat (Triticum aestivum L.) crops.

Materials and Methods

Results

Ammonia volatilization losses during the wheat growth period

The addition of organic and inorganic amendments in wheat crop have shown effective controlled of ammonia volatilization losses during entire cropping season as shown in Fig. 1. Results revealed that significant losses of N through NH₃ volatilization were observed in the initial 5–6 days after N fertilizer application. The highest N volatilization losses were observed in T₂ (20.4 kg ha⁻¹ season⁻¹) and the lowest losses were found in T₁ (6.4 kg ha⁻¹ season⁻¹). T₃ performed well in reducing the NH₃ losses by 40% as compared to T₂. NH₃ losses in T₃, T₄ and T₈ were recorded 12.4, 14.6, and 16.6 kg ha⁻¹ season⁻¹, respectively (Fig. 2). Duncan’s multiple range test revealed that T₃ (NBPT) and T₄ (HQ) effectively reduced the NH₃ losses. T₅ (CaC₂) and T₈ (garlic powder) showed no significant difference among their means. However, among plant-based amendments, T₈ with garlic powder-treated urea showed better results by reducing NH₃ volatilization losses by 17% compared to only RDF-treated urea (T₂). Other treatments were not so effective in reducing the NH₃ losses.

Initial and final soil nutrient status

Available nitrogen content in soil after wheat crop harvest was found highest in treatment T₇ (i.e., Azotobacter treated plot) which was 195 kg ha⁻¹. Detailed observations on available N content for different treatments is given in Table 1. The highest SOC was observed in T₇, treated with Azotobacter spp., i.e., 0.63%. It significantly increased the SOC content compared to the initial SOC (0.42%) before sowing the wheat crop, as shown in Fig. 3. The highest level of available P was observed in T₆ treated with VAM, i.e., 30.4 kg ha⁻¹, which is more than double the initial value of available P, as shown in Table 1. The initial level of available K was 196 kg ha⁻¹, while in the final analysis after harvesting the crop, it increased to 229 kg ha⁻¹ in T₁₀, as depicted in Table 1.

Table 1:

Effect of different amendments on available nitrogen, phosphorous, potassium and soil organic carbon in soil, N concentration in wheat grains and straw.

(Statistically significant at p = 0.05. Means followed by common alphabets are not significantly different among themselves by DMRT.

Treatments	Available N (kg ha−1) after harvesting of the crop	Available P (kg ha−1) after harvesting of the crop	Available K (kg ha−1) after harvesting of the crop	Nitrogen content in grain (%)	Nitrogen content in straw (%)	Agronomic efficiency of N (kg ha−1)	Production efficiency of N (kg kg−1)	Apparent N recovery (%)
T-1	141g	17.9g	214c	1.5g	0.32h	0	0	0
T-2	173def	19.7f	221b	1.91f	0.39g	9.1	25.8	35.2
T-3	172ef	22.5e	222b	2.09c	0.65a	12.9	19.8	65.2
T-4	171f	23.7d	222b	2.06cde	0.62b	11.6	18.5	62.8
T-5	177cde	23.7de	223b	2.02de	0.56c	9.8	19.8	49.4
T-6	187b	30.4a	228a	1.97b	0.53d	9.1	16.5	55.4
T-7	195a	27.8b	229a	1.98a	0.63ab	12.5	15.4	80.8
T-8	180c	26.9cb	229a	1.92b	0.54d	10.1	17.4	57.7
T-9	181cb	27.58bc	227a	2.02cd	0.41f	9.6	21.5	44.8
T-10	178cd	26.8c	229a	1.91e	0.44e	10.7	24	44.5
Statistical significance (F test)	Significant	Significant	Significant	Significant	Significant	–	–	–
C.D.	5.55	1.58	3.30	0.10	0.02	–	–	–
SE (m)	1.85	0.53	1.10	0.03	0.01	–	–	–
SE (d)	2.62	0.75	1.56	0.05	0.01	–	–	–
C.V.	1.83	3.70	0.85	2.88	2.20	–	–	–

DOI: 10.7717/peerj.14965/table-1

Ammonical N (NH₄⁺-N) and Nitrate (NO₃⁻N) in the soil

The available nitrogen forms, mostly NH₄⁺-N and NO₃⁻-N in the soil, decreased from tillering to the grain filling stage (Fig. 4). The mean quantities of NH₄ ⁺-N in the soil at various crop growth stages (tillering, flowering, grain filling, and physiological maturity stage) were 78.6, 65.6, 57.9, and 53.5 kg ha^−1, respectively. The mean of NO₃⁻-N in the soil at all four growth stages was 70, 63.4, 56.5, and 51.1 kg ha^−1, respectively. During the tillering stage, the highest NH₄⁺-N (108 kg ha⁻¹) was observed in T₃, i.e., treatment with NBPT amended urea, and the highest NO₃⁻-N (58.5 kg ha⁻¹) was found in RDN treated plot (T₂). In all the stages of plant growth, T₃ showed significantly higher values of NH₄⁺-N, which was 47% higher than T₂. Duncan Multiple Range Test also showed that the mean of treatments T₄, T₇, and T₈ was also significantly at par for the available NH₄⁺-N content in the soil, and in the case of NO₃⁻-N availability, the mean of treatments T₄, T₇, and T₅ also showed significant difference during all the stages of the wheat crop growth.

Nitrous oxide flux and denitrification losses from the soil

Nitrous oxide emission is directly related to the amount of N fertilizer available in the soil. The highest cumulative N₂O flux was observed in T₂ (RDF) plots with 6.52 kg N₂O-N ha⁻¹ season⁻¹. There were no significant differences between T₃, T₄, and T₅ in the case of N₂O emission reduction (Fig. 3). But all of them were significant when compared with T₂. Among plant-based inhibitor treatments, T₈, having urea amended with garlic powder, showed effective results. On the other hand, the highest denitrification losses were observed in T₂ (3.66 kg-N ha⁻¹) (Fig. 5). N-(n-butyl) thiophosphoric triamide treated urea was comparatively effective by reducing the denitrification losses by 35%.

Soil microbial biomass carbon and nitrogen in the soil

Soil biomass carbon and soil biomass nitrogen (found within living organisms like fungi and bacteria) were observed at the 50% flowering stage of the crop. The highest MBN content was reported in T_7, i.e., Azotobacter spp. treated plot with 37 mg kg^−1, followed by T₆, having VAM treated plot. The highest MBC (136 mg kg⁻¹) was observed in T₇, Azotobacter spp. treated plot was 63.9% more than T₂ (Fig. 6).

Soil urease enzyme activity in the soil

As shown in Fig. 7, under the 50% flowering stage in T_2, i.e., only RDN treated plot showed the highest urease activity (18.9 mg urea g⁻¹ soil h⁻¹) and the lowest urease activity (7.71 mg urea g⁻¹ soil h⁻¹) was observed in T₃ treatment containing NBPT which was indicated by the effectiveness of urease inhibitor.

Discussion

Our experimental findings in the case of NH₃ volatilization losses depicted that T₃ outperformed all other treatments because of competitive inhibition of urease enzyme activity by NBPT in the soil, which has a significant role in urea hydrolysis process where urea molecule is converted into ammonium ion. Next to NBPT, the other inorganic amendments significantly decreased the NH₃ volatilization losses. The allicin (thiosulfinate) naturally present in garlic is a bio-inhibitor of urease activity, as it contains an organosulfur functional group like that of the ureas. This might help allicin to decrease urease activity. This is in agreement with the research study conducted by Mathialagan et al. (2017). These amendments have practical implications in reducing the quantity of urea applied to the field, thereby reducing the N losses, even though many studies have been taken on the case of NBPT.

There is still a need to improve the duration of inhibition of urease activity, the shelf life of urea fertilizer coated with NBPT, and the economic viability of these compounds.

Agriculture alone contributes 80−90% of NH₃ emissions globally, mainly through volatilization from livestock and synthetic nitrogen fertilizer (Xu et al., 2019). It acts as a pollutant influencing the biosphere through haze formation and soil acidification. N₂O, a potent greenhouse gas, is also of great concern, contributes to global warming, and affects human and environmental health. Li et al. (2015) reported that the Limus^® (a new urease inhibitor consisting of 75% NBPT and 25% N-(n-propyl) thiophosphoric triamide (NPPT)), showed an average 83% decrease in NH₃ losses during winter wheat season in China. Only the addition of NBPT can reduce NH₃ volatilization losses by 61% to 74%, as reported by Lasisi, Akinremi & Kumaragamage (2019).

Affendi, Mansor & Samiri (2020) conducted a study by adding various chemical and natural urease inhibitors to reduce ammonia and nitrous oxide losses from soil. They reported similar findings in a combination of thiosulfate with urea. However, they found NBPT was more effective than the combined use of thiosulfate with urea to reduce NH₃ volatilization losses. Similar results were reported by Eduardo et al. (2016), reported a decrease of 31.6% N, which used to be lost if not applied with thiosulfinates, and concluded it was an upcoming natural urease inhibitor.

The final analysis of TNC, available N, P, and K, reported an increase in all these nutrient contents to the initial analysis, which helps the next sown crop in nutrition. T₇ treated with Azotobacter spp. performed well in increasing N and K content in the soil, and T₆ was treated with VAM to increase P content in the soil. An increase in the N content of the soil could be due to adding these bio-fertilizers as amendments that significantly increased residual N and reduced the N fertilizer application for the next crop, which will be sown in that plots. Vesicular Arbuscular Mycorrhiza improves P mobilization; hence uptake by wheat crops and P content in soil increases. The addition of bio-fertilizers as an amendments source to reduce N losses has improved the soil SOC.

The highest SOC was observed in T_7, which was treated with Azotobacter spp. i.e., 0.63%. It has doubled the SOC content compared to the initial SOC before sowing the wheat crop, i.e., 0.42%. A similar effect of these bio-fertilizers in increasing total nitrogen content was observed in a study on bio-fertilizer affecting structural dynamics, function, and network patterns of the sugarcane rhizospheric microbiota by Liu et al. (2021). Kader, Mian & Hoque (2002) also found similar results while working on the effect of Azotobacter spp. Inoculants on wheat yield and nitrogen uptake. This result is supported by Suri et al. (2011) while working on the influence of VAM and applied P on root colonization in wheat.

The available N forms, mostly NH₄⁺-N and NO₃⁻-N, decreased from the tillering stage to the grain-filling stage due to uptake and losses. During the tillering stage, the highest NH₄⁺-N and NO₃⁻-N was observed in T₃, i.e., treatment with NBPT amended urea due to a slowdown of urease activity, thereby enhancing the accumulation of NH₄⁺ and NO₃⁻ ions, and this increased the plant uptake. The urease activity was slowed down when NBPT-amended urea was applied to the soil, thus, enhancing the accumulation of NH₄⁺ ions and increasing the plant uptake of NH₄⁺ and NO₃⁻ ions. During N mineralization from the applied N fertilizers, most of the N remains as NH₄⁺ ions, then rapidly converting into NO₃⁻ due to the inhibitory effect of NBPT. That also helps in reducing NO₃⁻ leaching. A similar result was found by Dhakar et al. (2015) while working on the impact of nitrification inhibitors and various nitrogen sources on soil nitrogen distribution in Kinnow orchards.

As NH₄⁺ availability was initially low for the conversion into N₂O, the losses were low in the initial 7 days due to the application of these inhibitors. The application of urease inhibitors as amendments and urea regulates the concentration of NH₄⁺ and NO₃⁻ in the soil, thereby commanding NH₃ and N₂O emissions (Ding et al., 2015). Denitrification losses depend on soil moisture, soil temperature, and N-ions availability. Even though NBPT has no direct effect on lowering denitrification losses, it was observed that it had influenced the reduction of the emission of N₂O (Cassim et al., 2021). The reduction in the availability of NO₃⁻ has controlled the processes of denitrification and N₂O emission, mainly in the summer. The primary controller of denitrification processes in winter wheat is soil aeration and temperature (Aulakh et al., 2001).

Amendments like bio-fertilisers Azotobacter spp. and VAM positively affect soil biomass carbon and soil biomass nitrogen. These microbes help decompose organic matter and release the essential nutrients for plant uptake, increasing the MBC and MBN of the soil. Similar results were observed by Faujdar (2011) while working on the effect of FYM, bio-fertilizers, and zinc on nutrient transformations, soil properties, and yield of maize, and their residual effect on wheat.

The soil urease enzyme was suppressed by N inhibitors by a competitive inhibition mechanism, thereby reducing or slowing down the enzyme’s activity. Rapid losses in N are mainly due to urea hydrolysis by the urease enzyme, which drastically increases pH and NH₄⁺ ion concentration in the soil (Liu et al., 2018). So, this rise in pH and ion concentration increases NH₃ volatilization losses. In the entire growth period of wheat, the urease activity in the soil was always at a peak, especially during tillering stage; as it grew to maturity, its’ activity dropped. Similar results were reported by Fu et al. (2020) while working on the effects of urease and nitrification inhibitors on the soil.

The physiological characteristics of wheat analyzed (photosynthesis rate, chlorophyll content, LAI, and stomatal conductance) were also significantly influenced by organic and inorganic amendments. In this study, photosynthesis rate, chlorophyll content, LAI, and stomatal conductance were slightly increased in T₇ treatment with azotobacter because of an increase in nitrogen availability, which positively affects these physiological parameters. Furthermore, followed by Azotobacter treated plot, NBPT treated plot showed significant improvement in all these parameters.

The increase in the N content in wheat straw and grain might be due to improved nitrogen availability due to a reduced N loss and better NUE. As nitrogen is the most limiting nutrient in the growth and development of the wheat crop, the better uptake of the nutrient N will affect plant growth which will help to attain global food security by increasing the wheat yield by 1.5% per year. These results were similar to those obtained by Singh et al. (2018) while working on certain microorganisms like Bacillus spp. and Azotobacter spp. Mukhtar, Bashir & Nawaz (2018) found that microorganisms enhance crop growth by making nutrients available by fixing nitrogen and phosphate solubilization. In addition, azotobacter spp. can produce a few metabolites like phytohormones and exopolysaccharides, which help crops absorb nutrients and develop roots (Hindersah et al., 2020). Both are inorganic amendments that have effectively improved the plant’s nutrient availability, thereby increasing yield. These findings were similar to those of Kumar et al. (2015), who reported an increase in grain and biological yield by 22.6% and 17.4%, respectively. It was also reported by Galindo et al. (2020) that NBPT has some effect on metabolic pathways in decreasing urease enzyme activity hence increasing the NUE.

Furthermore, slowing down nutrient release has improved the period of availability of N and increased the crop’s N uptake (Liu et al., 2020). These positive effects, in turn, enhanced wheat’s growth, development, and yield. However, in reducing NH₃ volatilization NBPT, HQ amendments performed much better than other amendments, but these amendments have some adverse effects on soil microorganism activity as per the studies by Upadhyay, Tewari & Patra (2011).

Conclusions

The current study concludes that both organic and inorganic amendments significantly reduce NH₃ losses, thus increasing available soil N and enhancing the NUE of the crops. Treatment containing NBPT reduced the losses by 40% compared to only RDN without any amendments. These results may be due to strong inhibition of urease activity by NBPT in the soil. Among plant-based amendments, garlic powder-treated urea showed better results in decreasing NH₃ volatilization losses by 17% compared to only RDN-treated urea. These positive effects, in turn, enhanced the growth, development, and yields of wheat. Even though inorganic amendments performed much better than organic amendments in reducing NH₃ volatilization, opting for organic amendments gives way to sustainable agriculture. Thus, the focus should be more on organic than inorganic amendments to reduce NH₃ emissions from agricultural fields.

Supplemental Information