Effect of Mixed Urease Inhibitors on N Losses From Surface-applied Urea

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1 Effect of Mixed Urease Inhibitors on N Losses From Surface-applied Urea Sifang Li *, Jingjing Li, Jing Lu, Zhijuan Wang Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen , China * sfli@xmu.edu.cn Abstract The effect of mixed urease inhibitors on NH3 volatilization from surface-applied urea was studied. The results showed that mixed urease inhibitors could effectively reduce NH3 volatilization. Mixed urease inhibitors of 0.05% N-(nbutyl)thiophosphoric triamide (NBPT) and 0.05% N-(n-propyl) thiophosphoric triamide (NPPT) reduced NH3 volatilization of23.8% or 28.8% compared with the treatments with single NBPT or NPPT. Mixed inhibitors showed more effectiveness on slowing urea hydrolysis and maintaining lower soil NH4 + concentration for a long time than single inhibitor. Keywords Ammonia Volatilization; Mixed Urease Inhibitors; NBPT; NPPT Introduction Urea is the most widely-used and economical nitrogen fertilizer in the world due to its high content of N, ease of handling and storaging and relatively low manufacturing cost (Grant and Bailey 1999). However, the utilization rate of urea is as low as 30~50% when it is applied to the surface of soil. The use of urease inhibitor has been the most useful strategy to reduce NH3 volatilization in urea-fertilized soil. Urease inhibitor could abate N losses effectively by inhibiting urease activity and slow down the hydrolysis of urea and thereby avoiding nitrate leaching and reducing gaseous N emissions (Abalos et al. 2012; Dharani et al. 2009; Francisco et al. 2011; Saioa et al. 2011; Zaman et al. 2008). Various kinds of urease inhibitors have been identified and tested. N-(n-butyl)thiophosphoric triamide (NBPT) was one of the most effective compound of urease inhibitors and has been most widely used so far (Antisari et al. 1996; Cantarella et al. 2008; Henning et al. 2013; Xiang et al. 2008). NBPT inhibits urease after it is converted to N- (n-butyl) phosphorictriamide (NBPTO) in soil (McCarty et al. 1989; Creason et al. 1990). However, fast urea hydrolysis was reported to occur within hours after urea was applied to soil by catalyzed of soil urease (Dawar et al. 2011). Before NBPT is converted to NBPTO, it could hardly abate urea hydrolysis. In order to solve this problem, a few researches combined phenylphosphorodiamidate (PPD) and NBPT to slow urea hydrolysis and reduce NH3 emission (Phongpan et al. 1995). It appears that initially PPD inhibited urease actively. During this period, NBPT was converted into NBPTO and inhibited the urea hydrolysis while PPD declined. The aim of this study was to evaluate the effect of mixed urease inhibitors NBPT and N-(n-propyl) thiophosphoric triamide (NPPT) on NH3 volatilization, the soil ph and soil NH4 + concentration from urea application andsurface of soil. Materials and Methods Characterization of The Soil Sample The soil for this research was collected from the 0-20 cm surface layer of a silt (sandy) loam in Jiading district, Shanghai. The collected soil sample was air-dried and sieved with a 2 mm sieve. The main physico-chemical properties of the soil were shown in Table 1. Experiment Experiment was conducted under controlled laboratory conditions. The temperature was maintained at 25. The International Journal of Agricultural Science and Technology, Vol. 3, No. 1 February /15/ DEStech Publications, Inc. doi: /ijast

2 24 Sifang Li, Jingjing Li, Jing Lu, Zhijuan Wang experiment included six treatments with three replicates: 1) UR, 2) UR + 0.1% NBPT, 3) UR % NBPT % NPPT, 4) UR % NBPT % NPPT, 5) UR % NBPT % NPPT, 6) UR + 0.1% NPPT. The fertilizer applied to the soil at a rate of 460 mg N per kg soil. The volatilization chambers were cylindrical plastic vessels with 15 cm height and a bottom area of 154 cm 2. The bottle top of the chamber should be air-tight and open the lid for 10 minutes to keep soil respiration every 12 hours. The vessels were filled with 300 g air-dried soil, the thick of the soil was 7-8 cm, then adjusted the soil to 40% of the maximum water holding capacity. TABLE 1 CHEMICAL AND PHYSICAL PROPERTIES OF THE SOIL SAMPLES USED Property Values ph 6.62 Total N (g Kg -1 ) 1.40 Total P (g Kg -1 ) 0.76 Total K (g Kg -1 ) 20.8 Sand (g Kg -1 ) 181 Silt (g Kg -1 ) 593 Clay (g Kg -1 ) 226 Statistical Analysis The statistical software SPSS 19.0 was used. Results were analyzed by one-way ANOVA to test the effects of each treatment. The least significant difference (LSD) test at 5% level was used to analysis the statistical significance of the mean differences. Results and Discussion NH3 Volatilization The maximum daily NH3 volatilization for the treatment without inhibitor reached 7.2% of the total N applied on 5 days after fertilizer applied(figure 1). As urease inhibitor NBPT or NPPT was added to urea, the peaks of NH3 losses was delayed until day 9 for 0.1% NBPT treatment and day 8 for 0.1% NPPT treatment. The treatments with mixed urease inhibitors showed better effect, the peaks of NH3 volatilization were 1.8, 1.7 and 1.9% of the total N applied on day 9, 8 and 8, respectively. These were correspond to 25.0, 29.2 and 20.8% reduction in NH3 volatilization compared with the treatments with single inhibitor NBPT. The total amounts of NH3 volatilization at the end of the incubation period was 32.7% of the applied N for urea without inhibitor, while there were 14.3% and 15.3% of cumulative NH3 volatilization for treatments with 0.1% NBPT and 0.1% NPPT respectively (Figure 2), which correspond to 56.3% and 53.2% reduction compared with the treatment with urea alone. The treatments with mixed urease inhibitors significant reduction NH3 emission compared with the treatment without any inhibitor, the values were 11.8, 10.9 and 11.6% of the total N applied for treatments UR % NBPT % NPPT, UR % NBPT % NPPT, and UR % NBPT % NPPT, which correspond to 63.9, 66.7 and 64.5% reduction compared with treatment with urea alone. The cumulative NH3 volatilization for treatments with mixed urease inhibitors were also lower than the treatments with single inhibitor. The treatment with 0.05% NBPT % NPPT corresponds to 28.8% reduction compared with the treatment with 0.1% NPPT. For the treatment with urea alone, relative high NH3 volatilization occurred during 14 days after fertilizer applied, due to the excessive accumulation of soil NH4 +. A number of researches had also reported that the majority of NH3 losses occurred during the first few days after fertilizer application (Asing et al. 2008; Zaman and Blennerhassett 2010; Singh et al. 2008). Although NBPT could significantly reduce NH3 emission, it could not be effective immediately after application. NBPT must be converted into NBPTO for inhibiting urease activity, and this process always took 1-2 days while urea was hydrolyzed rapidly during this time (Manunza et al. 1999). NPPT and NBPT have similar molecular structure and inhibitory mechanism by strongly block three urease active sites to inhibit

3 Effect of Mixed Urease Inhibitors on N Losses From Surface-applied Urea 25 urease activity (Manunza et al. 1999). Mixed urease inhibitors of NPPT and NBPT indicate reduction of NH3 volatilization compared with single inhibitor. FIGURE 1. DAILY LOSSES OF NH3 AFTER SURFACE APPLICATION OF UREA WITH SINGLE OR MIXED UREASE INHIBITORS FIGURE 2. TOTAL VOLATILIZATION OF AMMONIA AFTER SURFACE APPLICATION OF UREA WITH SINGLE OR MIXED UREASE INHIBITORS. VERTICAL BARS INDICATE LSD VALUES AT P=0.05 Soil PH The surface layer (0-2cm) soil ph declined all the incubation period for the treatment with urea alone (Figure 3). The treatments with mixed urease inhibitors also declined at a slower rate than the control treatment after an initial rise. There were no significant differences (P=0.05) among the mixed urease inhibitors and the treatment with 0.1% NBPT. The high soil ph in the treatment with urea alone indicates fast urea hydrolysis, the significant declined after a few days fertilizer application indicated that a amount of N losses as NH3 volatilization. The treatments with mixed urease inhibitors could effectively inhibit urease activity, slow urea hydrolysis and reduce the range of soil ph change compared with the treatment with single inhibitor. The Soil NH4 + -N The soil NH4 + concentration of the treatment without inhibitor was higher than other treatments after fertilizer applied(figure 4). As urease inhibitors were applied, the contents of soil NH4 + in the soil were significantly decreased, but there were no significant differences (P=0.05) among the treatments with different inhibitors. Compared with the treatment with urea alone, 0.1% NBPT has a slight increase on day 5 and then began to decline. It was indicated that urease inhibitor could effectively slow urea hydrolysis and reduce the accumulation of soil NH4 +. Consequently, the urease inhibitor could extend the validity of the fertilizer for crops and increase the utilization rate. On day 11 after fertilizer applied, the content of soil NH4 + for the treatment with 0.1% NPPT was reached highest concentration of mg N kg -1, which corresponds to 51.7% reduction compared with the treatment without any inhibitor. The content of soil NH4 + for the treatments with mixed urease inhibitors were no significant differences (P=0.05) compared with the treatments with single inhibitor. However, mixed urease inhibitors delayed 4 days to reach the maximum of soil NH4 + concentration compared with the treatment with 0.1% NBPT alone. It was indicated that mixed urease inhibitors have more potential to extend of validity date than single inhibitor. Conclusion Combination of two kinds of urease inhibitors NBPT and NPPT was more effective to minimize NH3 volatilization than single inhibitor. Mixed urease inhibitors of NBPT and NPPT could reduce the range of soil ph change and maintain the soil NH4 + concentration on a stable level during the period compared with treatments with single inhibitor or without inhibitor. It is suggested that mixed urease inhibitors were more potential than single inhibitor on delaying urea hydrolysis and reducing N losses.

4 26 Sifang Li, Jingjing Li, Jing Lu, Zhijuan Wang FIGURE 3. SOIL ph ON DAY 2,7,11,17 AND 23 AFTER SURFACE APPLICATION OF UREA WITH SINGLE OR MIXED UREASE INHIBITORS. VERTICAL BARS INDICATE LSD VALUES AT P=0.05 FIGURE 4. SOIL NH4 + -N CONTENT ON DAY 2,7,11,17 AND 23 AFTER SURFACE APPLICATION OF UREA WITH SINGLE OR MIXED UREASE INHIBITORS. VERTICAL BARS INDICATE LSD VALUES AT P=0.05 ACKNOWLEDGEMENT The authors thank Shanghai Wintong Chemicals Company for the financial support of this study. REFERENCES [1] Abalos, D et al., Effectiveness of urease inhibitor on the abatement of ammonia, nitrous oxide and nitric oxide emissions in a non-irrigated Mediterranean barley field, Chemosphere, 89 (2012): [2] Antisari, L. V et al. Effects of the urease inhibitor N-(n-butyl)phosphorothioic triamide in low concentrations on ammonia volatilization and evolution of mineral nitrogen, Biol fertil Soils, 22 (1996): [3] Asing, J et al., Assessment of nitrogen losses from urea and an organic manure with and without nitrification inhibitor, dicyandiamide applied to lettuce under glasshouse conditions, Aust J Soil Res, 46 (2008): [4] Cantarella, H et al., Ammonia volatilization from urease inhibitor-treated urea applied to sugarcane trash blankets Scientia Agricola, 65 (2008): [5] Creason, G. L et al., Urease inhibitory activity associated with N-(n-butyl) thiophosphoric triamide is due to formation of its oxon analog Soil Bio Biochem, 22 (1990): [6] Dawar, K et al., Urea hydrolysis and lateral and vertical movement in the soil: effects of urease inhibitor and irrigation Biol Fert Soils, 47 (2011): [7] Dharani, D et al. Use of urea coated with natural products to inhibit urea hydrolysis and nitrification in soil Biol Fertil Soils, 45 (2009): [8] Francisco, S. S et al., Efficiency of urease and nitrification inhibitors in reducing ammonia volatilization from diverse nitrogen fertilizers applied to different soil types and wheat straw mulching Journal of the Science of Food and Agriculture, 91 (2011): [9] Grant, C. A et al., Effect of seed-placed urea fertilizer and N-(n-butyl)thiophosphoric triamide (NBPT) on emergence and grain yield of barley Can J Plant Sci, 79 (1999): [10] Henning, S. W et al., Response of turfgrass to urea-based fertilizers formulated to reduce ammonia volatilization and nitrate conversion, Biol Fertil Soils, 49 (2013): [11] Manunza, B et al., The binding mechanism of urea, hydroxamic acid and N-(n-butyl)-phosphoric triamide to the urease active site. A comparative molecular dynamics study Soil Bio Biochem, 31 (1999): [12] McCarty, G. W et al., Effect of N-(n-butyl) thiophosphoric triamide on hydrolysis of urea by plant, microbial, and soil urease Biol Fert Soils, 8 (1989):

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