Comparative effect of various organic extracts coated urea fertilizer on the release pattern of Ammonium and Nitrate in the soil at different time intervals


 Nitrogen (N) fertilizer application is a very important commodity in agricultural systems. However, due to the losses of applied N from the soil microenvironment its efficiency is too low. Different strategies like the use of polymer coating and use of chemical nitrification inhibitors had been employed to reduce N losses. But these chemical nitrification inhibitors are very expensive. Thus, a study was conducted to investigate the effects of different concentrations of parthenium extract, neem oil and acidulated cow dung compost extract on N dynamics in the soil. Three concentrations of parthenium extract (5, 10 and 15 %) and neem oil (1, 2 and 3 %) were coated on urea granules after mixing with the polymer material. Three pH (2, 4 & 6 pH) based acidulated cow dung compost extracts were also coated on urea granules in the same pattern. These coated fertilizers and uncoated urea were applied in jars filled with soil (100g per jar) at the rate of 1g of fertilizer per jar. One treatment was kept as control (without any fertilizer). Treatments along three replications were arranged according to the completely randomized design (CRD). Results depicted that all coating materials caused the release of N consistently from applied fertilizers compared to uncoated treatment. In addition, percent nitrified N was also reduced significantly in coated treatments in comparison to the uncoated urea and control treatments. However, the level of concentration effect was not obvious as lower concentrations of these extracts and oil also performed almost equal to that of higher concentrations.


Introduction
The increasing population demands healthy food with more production of crops for which plant nutrients are vitally essential. Chemical-based inorganic fertilizers are of prime importance under soil management strategies to enhance the quality and quantity of agricultural outputs (Itelima et al. 2018). Nitrogen (N) plays an important role in plant growth and development as a prime mineral nutrient. Higher plants require large amounts of N for physiological and biochemical processes that lead to a higher quality of produce and increased yield. Low organic matter contents and several other factors are responsible for the de ciency of N in Pakistani soils. According to an estimate, 78-79 % N is present in the atmosphere but it is in an inert structure. Thus, technical nutrient management practices are required (Leghari et al. 2016;Zhang et al. 2021). In modern agriculture, large amounts of N fertilizers are applied for higher production, but less than 50 % of these applied fertilizers are utilized by the crops (Zhang et al. 2020).
Recently, different strategies had been employed to enhance the e ciency of N fertilizers (Gagnon and Zaidi 2010). Urea is the ammoniacal fertilizer that is the cheapest and easily available N fertilizer (Nehring 2013). The manufacturing, storage, transport, distribution and handling of urea is also easier than other N fertilizers (Hauk 1984).
However quick hydrolysis of urea releases plant-available forms of N in the soil suddenly after exposure to moisture in the presence of the urease enzyme. Firstly, NH 4 + is formed which is a plant available form and immobile in the soil (Lei et al. 2018). Excessive application of N fertilizer at an early stage of the plant induces low use e ciency of applied fertilizer because this excess fertilizer got lost via volatilization, nitri cation, denitri cation and runoff (Jain and Abrol 2017; Anas et al. 2020). To reduce such losses different strategies had been employed like the 4-R strategy (i.e. considering right source, right rate, right time and right place for fertilizer application), split application of urea and controlledrelease urea (Tao et al. 2018;Freedman 2018;Yaseen et al. 2016;Noor et al. 2017). Biodegradable polymer coated urea slows the release kinetics of available forms of N in the soil (Beig et al. 2020). Thus slow-release N fertilizers are very crucial to reduce the losses of N to the environment and groundwater.
But N released in the form of NH 4 + in the soil got converted to the nitrate form via nitri cation process if not up-taken by the plants or lost to the atmosphere via volatilization (Sahrawat 2008;Lasisi et al. 2019).
During the process of nitri cation and subsequent denitri cation, nitrous oxide gas got escaped to the atmosphere, in addition to the nitrate leaching, thus nitri cation is a less desirable process (Wrage et al. 2001).
To overcome the nitri cation losses from agricultural activities different nitri cation inhibitors had been used in recent years (Byrne et al. 2020). But commercial nitri cation inhibitors are much expensive for small farmers (Hatano et al. 2019). On the other hand, organic extracts and oils are much cost-effective and good nitri cation inhibitors in comparison to commercial nitri cation inhibitors. As neem oil reduced nitri cation losses more effectively than the lower dose of Dicyandiamide (Opoku et al. 2014). Parthenium hysterophorus is a well-known invasive weed with plenty of negative roles as agricultural, medicinal and environmental hazards (Kaur et al. 2014). In addition to its other roles, its plant extract is a very good nitri cation inhibitor (Mahmood et al. 2014). Kanchan and Jayachandra (1981) also showed that root and leaf extract of Parthenium hysterophorus effectively reduced the Nitrosomonas and Nitrobacter population in the soil resulting in inhibited nitrite production. Low pH also contributes to the inhibition of the nitri cation process (Rose et al. 2020). Thus based on these studies, it is clear that coating of organic extracts mixed with the polymer on urea could be bene cial for N management. In this way, an additional bene t of optimization of organic extracts will also be achieved. Thus, it was hypothesized that polymer mixed neem oil, parthenium extract and acidulated cow dung extract may reduce the nitri cation losses of nitrogen in addition to slow release of available N from urea fertilizer. So, in the present study effect of polymer mixed neem oil, parthenium extract and acidulated cow dung extract coated urea is compared with the uncoated urea for N release, NH 4 + and NO 3 − concentrations in the soil and percent nitri ed N at different time intervals. Neem oil was extracted by using a solvent extraction method (Puri 1999), while parthenium extract was collected by following Anteneh and Mendesil method (Netsere and Mendesil 2011). A 10 % Cow dung manure (with different pH) extract was collected from the Soil and Environmental Microbiology Laboratory, ISES, UAF. Oil and organic extracts were coated on urea granules with polymer as a binding agent. To avoid any contamination, controlled Laboratory conditions were maintained for all activities.

Materials And Methods
Shade drying of coated urea was done and stored at room temperature (25°C).
The experiment was conducted in the incubator by using disposable cups (250 cm 3 ), and each cup was lled with 100 g soil after determining the physicochemical properties of the soil (Table 1). In all the cups eld capacity was maintained after saturation percentage measurement. Urea fertilizer was added in cups at the rate of 1g in each cup (having 460 mg N). After the addition of soil, fertilizer and water, the weight of cups was maintained at an interval of 24 h with distilled water. The whole experiment was carried out at 25 ± 1°C and NH 4 + and NO 3 − were measured after 20, 40 and 60 days of incubation through the indophenol blue method and phenoldisulphonic acid method respectively (Keeney and Nelson 1989).
Sum of NH 4 + and NO 3 − was taken to determine cumulative nitrogen (N), while nitrate to ammonium ratio, percent nitri ed N and N release e ciency (Tong et al. 2018) were calculated by using the following formulas: All collected data were analyzed according to completely randomized design (CRD) with factorial arrangements following Fisher's analysis of variance (Steel et al. 1997). Mean comparison was done with the Tuckey HSD test.

Results
Ammonium concentration in the soil Ammonium concentration in the soil was signi cantly controlled by all three types of extracts used for coating the urea fertilizer (Table 2). Uncoated urea gave the highest concentration of ammonium in the soil at the rst interval that was signi cantly reduced at later intervals. While in the case of coated fertilizers, ammonium concentration showed an opposite trend as ammonium concentration was lower at the rst interval, but this release was increased at later intervals. In the case of coated fertilizers, the release was maximum at the second interval in all types of coating materials, even differences were nonsigni cant between organic amendments but in close view Parthenium extract and acidulated cow dung extract showed slightly higher release than neem oil. At the third interval release of ammonium was reduced in comparison to the second interval but still release was signi cantly higher in coated fertilizers than uncoated fertilizer. Ammonium release was higher at a lower concentration of organic amendments but was reduced with an increase in the concentration.

Nitrate concentration in the soil
The uncoated urea showed maximum nitrate concentration at the rst interval that was lowered at latter intervals but still higher in comparison to the coated treatments. In coated fertilizer treated jars nitrate concentration was signi cantly lower in comparison to uncoated urea. At latter intervals, the concentration of nitrate was slightly increased in coated treatments but still lower in comparison to uncoated urea that shows the inhibition of the nitri cation process in the soil. Among concentrations of organic amendments lower concentrations shown more reliable results. Among the type of organic amendments, acidulated cow dung extract showed slightly higher nitrate concentration in the soil, while other amendments showed slightly lower concentrations (Table 3).  Table 4 showed that the concentration of cumulative N was signi cantly lower in the coated treatments in comparison to the uncoated treatment at the rst interval. While the release was increased with the passage of time in coated treatments but decreased in uncoated treatment. All organic amendments performed almost equally in N release, though acidulated cow dung extract coated urea showed a slight increase in the N release. At the last interval, N release was decreased in all treatments but the decrease in the release was signi cantly more in uncoated treatment than coated treatments. It was also observed that lower concentrations of organic extracts gave more trustworthy results as cumulative N release was lower at the rst interval but increased at the second interval and this release was more in comparison to higher concentrations, even at third interval release was higher with lower concentrations. Nitrate to ammonium ratios given in Fig. 1 depicted that nitrate to ammonium ratio was higher in both control treatments. While in organic extracts and oil-coated fertilizer treatments nitrate to ammonium ratios were signi cantly lower in comparison to treatments without extracts. Though at all intervals nitrate to ammonium ratio was higher in uncoated urea treatment, the highest ratio was observed at the second interval. In coated treatments nitrate to ammonium ratios were higher at the rst and last interval in comparison to second intervals. Among extracts, non-signi cant results were seen between parthenium extract, neem oil and acidulated cow dung extract, though acidulated cow dung showed a slightly higher ratio.
Percent nitri ed N (%) Figure 2 illustrated the percent nitri ed N for all treatments at all intervals. These results clearly stated that the nitri cation process was signi cantly reduced with the use of organic extracts coated urea in comparison to treatments not receiving extracts. Though all the extracts performed signi cantly to lower the nitri cation process, the results of parthenium extract were much better. Among the concentrations of these extracts, all concentrations showed signi cant results, even lower concentrations also performed equally to that of higher concentrations.
N release e ciency (%) N release e ciency was highest at the rst interval for uncoated urea but it was reduced signi cantly at latter intervals even at 60 days interval its release e ciency was just 12 % (Fig. 3). In the case of coated treatments release e ciency was lower at the rst interval but at the second interval, it was increased signi cantly. At the third interval, a slight decline was seen in N release e ciency by the coated treatments but the release was still much higher than uncoated treatment. Among different types of coating materials, N release e ciency was slightly higher in parthenium extract coated urea than other coating materials.

Discussion
The use of chemical fertilizers is a basic commodity in today's agricultural systems (Yan et al. 2008).
Especially nitrogen (N) fertilizers are much important as nitrogen is an essential macronutrient for all plants (Rochester et al. 1994;Dong et al. 2012). But with the addition of N fertilizer in the soil, several problems originate, as much of the applied N is lost via nitri cation and subsequent denitri cation processes (Cai et al. 2002;Chen et al. 2008;Artola et al. 2010). It is stated in many studies that applied N undergoes several interactions with loss pathways, chemical reactions, soil microbes and plant roots (Shaviv and Mikkelsen 1993). On oxidation, by the nitrifying bacteria, ammonium rst converts to nitrite then to nitrate in two consecutive steps. During this process, nitrogen might be lost nitrous oxide or nitrite form (Lunt 1968;Xu et al. 2013). The nal product of this reaction that is NO 3 − might also be lost via nitrate leaching or denitri cation process resulting in groundwater, surface water and atmospheric contamination (Dave et al. 1999). Thus, it is very crucial to control the nitrogen dynamics in the soil. A laboratory experiment was conducted at SFPN, ISES, UAF to check the effect of polymer mixed organic extracts and oil-coated urea on the N release e ciency, Percent nitri ed N and respective ammonium and nitrate concentrations in the soil.
Results depicted that all the coating materials signi cantly reduced the release of ammonium, nitrate and cumulative N in the soil in comparison to uncoated urea. The delay in the release of N might be due to the controlled release properties of polymer mixed extracts and oil coating. As Shivay et al. (2016) stated that sulfur coated urea slowed the N release due to its control release properties. The dissolution rate of urea was reduced by coating with gypsum-sulfur (Ibrahim et al. 2014). Trinh et al. (2015) established a model to state the release kinetics of N from coated urea which clearly described that the release of N was lower in coated fertilizer than uncoated. N release e ciency was also reduced in our experiment that was consistent with the results of Tong et al. (2018).
Results of nitrate to ammonium ratio and percent nitri ed N demonstrated the reduced nitri cation process in the organic extracts and oil-coated urea treatments. The reduction in nitri cation might be due to the nitri cation inhibiting compound present in these organic amendments (Irigoyen et al. 2003;Di and Cameron 2004), as parthenium extract contain parthenin, phenolics and terpenes that are having nitri cation inhibiting potential (Raina et al. 2003). Similarly, neem oil contains melacin and other compounds to inhibit the nitri cation process (Malla et al. 2005). Acidulated cow dung compost extract at low pH reduced percent nitri ed N that could be due to its low pH character, as low pH is itself a nitri cation inhibitor. With a decrease in pH, nitri cation reduces and even ceases below pH 6 (Hofman and Lee 1973;Harmsen 1987). Low concentrations of these extracts and oil reduced nitri ed N percentage in consistence with that of higher concentrations. Thus, 5 % parthenium extract, 1 % neem oil and 2 pH acidulated cow dung compost extract was found su cient for this purpose in consistence with the ndings of Kumar et al. (2011).

Conclusion
Coated fertilizers controlled the release of N very effectively. As all coating materials slowed the cumulative N release at the initial interval that was increased at later intervals. Thus, N release e ciency could be correlated with the crop demand in the eld, as crop demand at the initial stage is much lower but after about 40 days of germination, crops need more N. Decrease in percent nitri ed N is also very important to control N losses in the form of nitrate leaching and nitrous oxide. All organic extracts and oil performed almost equally in controlling the nitri cation process. However, lower concentrations of parthenium extract and neem oil performed very effectively, thus 5 % parthenium extract and 1 % neem oil are su cient to control the nitri cation process instead of applying higher concentrations. Similarly, 2 pH acidulated cow dung compost extract is suitable for coating on the urea granules to control nitri cation loss. Conclusively, the coating of polymer mixed organic extracts on the commercial N fertilizers is an effective technique to control both N release and nitri cation losses.
Declarations I am thankful to Higher Education Commission (HEC) Pakistan for providing funds for my research activities.

Data availability statement
All data of this manuscript is available with corresponding author on request.

Figure 1
Nitrate to ammonium ratios for treatments at 20, 40 and 60 days interval Percent nitri ed N (%) for treatments at 20, 40 and 60 days interval