Performance of UREAstabil in the Nitisols and Vertisols of North-Western Ethiopia

Abstract Nitrogen is a critical yield-limiting plant nutrient for crop production in Ethiopia. The demand for synthetic fertilizer is significantly increasing. Urea is the main source of synthetic nitrogen fertilizer that mainly applied in the surface resulting in significant nitrogen loss. UREAstabil fertilizer is urea with N-(n-butyl) thiophosphoric triamide (nNBPT) that reduce the rate of urea hydrolysis by urease to reduce nitrogen loss and increase crop productivity. The research was conducted in Yilmana Densa district, Ethiopia for two years to evaluate the performance of UREAstabil compared to urea. The research evaluated the effect of UREAstabil fertilizer technology on bread wheat and tef at Nitisol and Vertisol. The finding of this research denies our prior hypothesis that UREAstabil could give better yields of tef and wheat with lower rates of nitrogen at single application rate compared to urea. Reducing the amounts of nitrogen by one third using UREAstabil resulted in an intolerable significant yield penalty for all the study sites and both years on wheat. Both the grain and straw yields were increased by splitting the UREAstabil, indicating that the enzyme that hydrolysis urea was merely inhibited. Considering a non-significant yield difference between the conventional urea and UREAstabil for all crops, soils, rates, and forms of applications UREAstabil is not promising. Further research with different rates of nNBPT as well as different nitrification inhibitors needs to be evaluated for their efficiency to improve crop yield and reduce nitrogen loss for different crops, different soils, and agro-ecologies.

Abstract: Nitrogen is a critical yield-limiting plant nutrient for crop production in Ethiopia. The demand for synthetic fertilizer is significantly increasing. Urea is the main source of synthetic nitrogen fertilizer that mainly applied in the surface resulting in significant nitrogen loss. UREA stabil fertilizer is urea with N-(n-butyl) thiophosphoric triamide (nNBPT) that reduce the rate of urea hydrolysis by urease to reduce nitrogen loss and increase crop productivity. The research was conducted in Yilmana Densa district, Ethiopia for two years to evaluate the performance of UREA stabil compared to urea. The research evaluated the effect of UREA stabil fertilizer technology on bread wheat and tef at Nitisol and Vertisol. The finding of this research denies our prior hypothesis that UREA stabil could give better yields of tef and wheat with lower rates of nitrogen at single application rate compared to urea. Reducing the amounts of nitrogen by one third using UREA stabil resulted in an intolerable significant yield penalty for all the study sites and both years on wheat. Both the grain and straw yields were increased by splitting the UREA stabil , indicating that the enzyme that hydrolysis urea was merely inhibited. Considering a nonsignificant yield difference between the conventional urea and UREA stabil for all crops, soils, rates, and forms of applications UREA stabil is not promising. Further research with different rates of nNBPT as well as different nitrification inhibitors Tadele Amare ABOUT THE AUTHOR Tadele Amare (PhD) is a Senior Researcher in Soil and Water Management at Adet Agricultural Research Center, Ethiopia. He engaged in several research projects as principal investigator and project leader. He has a number of publications. His researches focus on sustainable land management, increasing crop productivity through prudent nutrient management, soil organic carbon, modeling on plant-nutrient interactions, integrated soil fertility management, and spectroscopic based soil analysis, rehabilitation of degraded-acid soils through integrated management measures. He also involved in outreaching of the community services. He is a member of editorial boards for Ethiopian Soil Science Society and Blue Nile Journal of Agricultural Research of ARARI.

PUBLIC INTEREST STATEMENT
Synthetic nitrogen fertilizer accounts for more than 50% yield increased in Ethiopia. The use of synthetic nitrogen fertilizer has been drastically increased from time then and its contribution to food security remains one of the most priorities. The global nitrogen fertilizer recovery is less than 50%. Technologies that enhance the recovery of synthetic nitrogen could be summarized as: 1) Urease inhibitors that reduce fast hydrolysis of urea by urease, 2) Nitrification inhibitors, and 3) Technologies that allow controlled release of nitrogen. Our research was designed to find solutions to this critical public interest with UREA stabil a fertilizer enhanced with urease inhibitor (nBTPT) for the two major soils and two major cereal crops in northwest Ethiopia. The overall findings of the research showed no evidence of yield advantage of UREA stabil over the conventional urea and hence further adjustment of nBTPT content is needed to the Ethiopian farming environment.

Introduction
The agricultural productivity of Ethiopia depends mainly on the amount and distribution of the rainfall as well as on the state of soil fertility. The state of soil fertility has been the major reason for wealth disparities among and between Ethiopian farmers. Synthetic fertilizers have been used to improve the state of soil fertility and to increase crop productivity since the 1970s in the country; and yet with very high potential for further yield gap closings (Dercon & Hill, 2009). Nitrogen is the most yield-limiting nutrient under all soils, landscapes, agro-ecologies, and regions of the region (Amare et al., 2018). It is also a universal yield-limiting nutrient (Hirel et al., 2007). Synthetic nitrogen fertilizer accounted for about 50% of food increased in the world (Yang et al., 2016). The primary sources for synthetic nitrogen fertilizers in Ethiopia is urea as it has high contents of nitrogen (46%), its low cost, ease of handling, storage, and transport makes urea to be used worldwide for the agricultural production that accounts about 56% (Mira et al., 2017). However, the recovery of nitrogen from urea is only about 30-40% (Zhou et al., 2003), 30-50% (Abalos et al., 2014), 50% (Janssen et al., 1990), 30-65% (Herrera et al., 2016). According to Zaman et al. (2013) the key nitrogen loses could be summarized as NH 3 volatilization, NO 3 − leaching and N 2 O emission.
Integrated soil fertility management, selection of fertilizer sources, identifying and applying at a critical time, improving the reaction of urea fertilizer through various modifications increase the nitrogen fertilizer recovery and efficiency as well as reduces environmental impacts. Among the measures, a split application of urea is reducing the nitrogen losses (Hinton et al., 2015). New technologies that reduce the loss of nitrogen by modifying the conventional urea are getting the attention of researchers and development practitioners. These technologies release nitrogen more slowly than the conventional urea and hence improve the recovery of fertilizers (Feng et al., 2016;Trenkel, 2010). UREA stabil is one of the slow-releasing nitrogen fertilizers and hydrolysis of urea is reduced by the presence of N-(nbutyl) thio-phosphoric-triamide (nBTPT) that slows down urease activity of urea hydrolysis thereby improve recovery of nitrogen applied (Abalos et al., 2014;Krajewska, 2009;Watson et al., 2008). This fertilizer technology increased crop productivity (Abalos et al., 2014;Qiao et al., 2015). However, the effect of urease inhibitor (nBTPT) depends on the climate, soil, crop and management (Thapa et al., 2016). Therefore, the present research was conducted to evaluate the advantages of UREA stabil Compared to the conventional urea in increasing the productivity of two major cereal crops: Bread wheat (Triticum aestivum) and tef (Eragrostis tef) for Nitisols and Vertisols of northwestern Ethiopia.

Description of the study areas
The research was conducted in Yilmana Densa district of the west Gojjam zone of the Amhara National Regional State of Ethiopia ( Figure 1). Yilmana Densa is located at about 42 km from Bahir Dar; the capital city of the Amhara National Regional State on the way to Addis Abeba through Mota.
The mid-altitude takes the lion share of the district. The district receives a uni-modal type of rainfall that begins in May-June and ends in October with an annual average rainfall that ranges from 1100 to 1270 mm and temperature range of 8.8 to 25.2 O c (Damtie & Mekonnen, 2015). The dominant soils according to the FAO/UNESCO classification system (IUSS Working Group WRB, 2015) of the study sites are Nitisols and Vertisols. The farming system is characterized by a mixed livestock raising and crop production. Tef (Eragrostis tef) and bread wheat (Triticum aestivum) are the leading cereal crops in the study area. Yilmana Densa is one of the highly populated districts of the Amhara National Regional State (CSA (Central Statistical Agency of Ethiopia), 2007).

Experimental set-up
The experiment was conducted for two consecutive rainy seasons with the treatment setups shown in Table 1. This on-farm research was conducted on multi-locations of Nitisols and Vertisols. The test crops were bread wheat and tef with varieties TAY and Kuncho, respectively.
A randomized completed block design (RCBD) with three replications was used. Both crops were planted in rows with seed rates of 125 kg/ha and 10 kg/ha for wheat and tef; respectively. Major agronomic data including grain and biomass yields were collected. The grain weight and moisture content of wheat were simultaneously taken and finally adjusted to 12.5% moisture content. Collected data were subjected to the analysis of variance (ANOVA) using SAS software (Statistical Analysis System (SAS), 2003). The ratio of yield response was calculated by dividing the yield of treatments to the yield of the recommended nitrogen for each crop.

Soil sampling, preparations, and analysis
Composite soil samples were collected at depth of 0-20 cm before planting for each site. Samples were air-dried, ground sieved. Soil pH was determined in a 1:2.5 soil to water suspension following the procedure outlined by Sertsu and Bekele (2000). Soil organic carbon content was determined by the wet digestion method using the Walkley and Black procedure (Nelson & Sommers, 1982). Total nitrogen was determined using the Kjeldahl method (Bremner & Mulvaney, 1982) while the available phosphorus was determined following the Olsen procedure (Olsen & Sommers, 1982). The exchangeable potassium was measured by flame photometer after extraction of the samples with ammonium acetate solution at pH-7 following the procedures described by Sertsu and Bekele (2000).

Soil properties of the study sites
The pH of the soil ranged between 4.82 to 5.48 for the Nitisols and 5.01 to 7 for the Vertisols. The soil organic carbon was below the critical level of 2% (Murphy, 2014). It was varying between 1.11 to 1.67% for the Nitisol and 0.57% to 1.35% for Vertisols. The total nitrogen was lower for both soils that ranged from 0.15% to 0.21% for the Nitisol and 0.09 to 0.15% for the Vertisol. The available phosphorus was also in the ranges of 8.24 to 14 mg/kg for the Nitisol and 13 to 17 for the Vertisol. The exchangeable potassium ranged from 0.83 to 1.31 cmol (+)/kg of soil for the Nitisol and 0.86 to 1.16 cmol (+)/kg of soil for the Vertisol and all of them are above the critical levels of 0.25 cmol (+)/kg of soil (IPI (International Potash Institute), 2016).

Yield response of crops to UREA stabil
Yields from treatments without nitrogen (control) were significantly (P < 0.05) lower than other treatments (Table 2), indicating nitrogen is still the major yield-limiting plant nutrient in the farming system of northwestern Ethiopia as stated by Amare et al. (2018). Reducing the rate of nitrogen to 67% (61 kg/ha) using UREA stabil without splitting significantly decreased both grain and straw yields for all sites in both years (Table 2) as compared to the recommended rate (92 kg/ha) using conventional urea. The grain yield penalty ranged between 700 and 900 kg/ha (mean 889 kg/ ha). The highest straw yield penalty (1514 kg/ha) was observed for the second year ( Table 2). The overall implication of the finding for wheat indicated that UREA stabil was not better than the conventional urea to improve wheat productivity with rates less than the conventional urea. Application of the same rates of nitrogen (92 kg N/ha) from conventional urea and UREA stabil (without split) resulted in insignificant yield differences (Table 2) with better yields from conventional urea. However, the split application of UREA stabil resulted in a better but insignificant yield compared to the same rates of nitrogen with conventional urea ( Table 2). Application of recommended nitrogen (92 kg/ha) from UREA stabil with and without split indicated a non-significant yield difference but yield increased (278 kg/ha on average) by splitting than a single dose. The grain yield using 92 kg N/ha from UREA stabil by splitting was only slightly lower than from 122 kg N/ha *Numbers in bracket indicated nitrogen rates (Kg/ha) applied as 1/3 at planting and 2/3 at the tillering stage while those rates without brackets indicated that nitrogen was not splitted; 46, 60 and 40 P 2 O 5 kg/ha were applied as a basal at planting for wheat, tef (Nitisols) and tef (Vertsisols); respectively.

Table 2. Comparisons of grain and straw yields of wheat response (kg/ha) to urea and UREAstable
Grain yield (kg/ha)

Treatments
Year 1 (2017) Year 2 (2018)  UREA stabil without splitting (70 kg/ha). However, upon splitting, their difference increased to 374.2 kg/ha. This again justifies the importance of splitting and the weakness of UREA stabil against fast hydrolysis. Increasing recommended nitrogen from UREA stabil up to 122 kg N/ha (130%) gave higher yield over a single application of the same rate but the equivalent yield was found with conventional of the same rate The response ratio of the grain yield and straw yield clearly showed that the overall implications of using UREA stabil (Figure 2).
The findings of the research for tef showed lower yields from treatments without nitrogen than other treatments (Tables 3 and Table 4). Unlike the yield response of wheat, the yield response of tef to conventional urea and UREA stabil was irregular (3, 4) and hence difficult to draw conclusions that might be due to a smaller rate used (40 kg N/ha) for the Nitisols that needs future considerations. In 2017 applying 27 kg N/ha (67% of the recommended nitrogen) from UREA stabil on the Nitisol showed a yield reduction; pronounced on the straw yield than the grain yield while in 2018 there were little yield differences among and between treatments that received nitrogen and there was no uniform trend of increase or decrease as a result of UREA stabil . The significant yield difference (P < 0.05) was only observed between the control (without nitrogen fertilizer) and other treatments.
The yield response of tef on Vertisol did not support our hypothesis that the rate of nitrogen from UREA stabil could be reduced significantly to bring about equivalent yield to the recommended rate of nitrogen using conventional urea (60 kg N/ha). Both grain and straw yields were reduced when the rate of nitrogen reduced from 60 to 40 kg N/ha (Table 4 and Figure 3).
Combined analysis of the grain yield showed that the application of a full dose of UREA stabil at planting (no spilt) surpassed the splitted application with no significant differences (P > 0.05), (Table 4 and Figure 3). The response ratio of the yield compared to the recommended nitrogen from urea (60 kg N/ha) was above one except for 40 kg N/ha (67% recommended N from UREA stabil ), indicating that the recommended rate was biological optimum (Figure 3). The grain yield of tef from 60 kg N/ha (1448.9 kg/ha) with single application from UREA stabil was equivalent to the maximum yield (1512 kg/ha) observed from the application of 80 kg N/ha from conventional urea. Application of 80 kg N/ha from UREA stabil by split or single dose was not better than nitrogen from conventional urea with similar rates. The yield was proportionally increased with increasing rates of nitrogen from conventional urea.  The finding of this research is in line with the finding from a one year experiment in the Northern parts of Ethiopia (Dargie et al., 2018). They reported a yield advantage of 655.7 kg/ha (13.62%) grain wheat using 64 kg N/ha from conventional urea over the same rate of nitrogen from UREA stabil . They made further research recommendations on the split application of UREA stabil . The finding of our research did not support our key interest to avoid split nitrogen application and reduce associated costs using UREA stabil than using the conventional urea. This finding is not in line with the findings of authors (Hinton et al., 2015;Huérfano et al., 2015;Thapa et al., 2016;Trenkel, 2010). The additional cost of UREA stabil over the convention urea ranges from 6% (Dargie et al., 2018) and 20% (Růžek et al., 2014). Zaman et al. (2013) also indicated that for 25 kg N/ha, the cost of recommended nBTPT is 3.2 US dollars/ha, higher than the cost of urea per hectare for that area  (Drapal et al., 2013) where the conventional urea at a rate of 90 kg N/ha gave a yield advantage of 10.2% tuber yield than UREA stabil with the same rate of nitrogen. Rose et al. (2018b) also did not get any evidence of nitrogen recovery and yield advantage from enhanced fertilizers including urease inhibitors on rice. The overall yield advantage of UREA stabil observed from the study opposed to other findings (Abalos et al., 2014;Dawar et al., 2011Dawar et al., , 2012Qianqian et al., 2017;Qiao et al., 2015;Rose et al., 2018a;Thapa et al., 2016;Zaman et al., 2008Zaman et al., , 2013. Zaman et al. (2013) found a 16.1% yield advantage using urease inhibitor (nBTPT) over the conventional urea with the same rates of nitrogen (30 kg N/ha). Moreover, many authors proved that the recovery of nitrogen improved using nitrification inhibitors and urease inhibitors (nBTPT) (Abalos et al., 2012(Abalos et al., , 2014Krajewska, 2009;Drapal et al., 2013;Ni et al., 2014, Rose et al., 2018aRůžek et al., 2014;Thapa et al., 2016;Watson et al., 2008). The poor performance of UREA stabil to our finding could be related to the quantity of urease inhibitor (nBTPT), and the efficiency of nBTPT depends on factors including soil properties (McGeough et al., 2016;Watson et al., 2008Watson et al., , 1994, temperature (Abalos et al., 2014(Abalos et al., , 2017Watson et al., 2008), rainfall (Abalos et al., 2017) and management (Abalos et al., 2014). In Brazil, Mira et al. (2017) found reduced loss of nitrogen from urea in the ammonia using urease inhibitor nBTPT up to 1000 mg/kg urea. According to the authors, rates of nBTPT in Brazil were based on the findings in the temperate region (240 to 500 nBTPT mg/kg urea) that necessarily could not reflect the tropical regions. Improving the efficiency of UREA stabil by increasing the rate of nBTPT should be considered in the farming system of Ethiopia.

Conclusion and recommendation
A two year field experiment was carried in the northwestern parts of Ethiopia for wheat and tef under Nitisols and Vertisols to evaluate performance of UREA stabil compared to the conventional urea. The findings of this research deny our prior hypothesis that UREA stabi could give better yields of tef and wheat at low rates of nitrogen with single applications compared to the conventional urea. Under both soils and for both crops, application of UREA stabil did not show a significant yield advantage over the conventional urea. Split application of UREA stabil showed better yields over single application. Therefore, there was no evidence in our research that supports the advantage of UREA stabil over the conventional urea. Nevertheless, nNBPT is a proven technology to inhibit the activity of urease and hence reduce the loss of nitrogen. However, the rate of the inhibitor under different environment including Ethiopia needs further research considerations. Moreover, further research on quantifying the nitrogen losses, grain and straw qualities (protein content) and nitrogen recovery using urease inhibitors, nitrification inhibitors and controlled releasing fertilizers are needed.