Influence of Slow Release Urea Fertilizer on Growth , Yield and N Uptake on Maize under Calcareous Soil Conditions

Heavy N losses are always associated with urea fertilizer application therefore, improving its efficiency is crucial to reduce economic and environmental losses related with its application. The viable approach to reduce the N losses from the surface applied urea is to coat it with sulfur, urease inhibitor and other biodegradable materials. Field experiment were conducted to investigate the impact of super and urease coated urea on growth, yield, yield components and N uptake in maize under calcareous soil conditions of Khyber Pakhtunkhwa (KPK) during 2011-12. The experiment was conducted in a randomized complete block design (RCBD) with 4 replications each. Urea coated with urease (agrotain) and nitrification inhibitors (super urea) were applied at 90 kg and 120 kg N ha in 2 or 3 splits. The fertilizer treatments were applied at sowing, after 30 and 60 days of sowing (DAS), respectively. The results showed that inhibitors treated urea significantly improved the growth, yield and yield components of maize crop over granular urea and control treatment. Super urea shows its superiority over agrotain in terms of increased yield. Inhibitors application at 90 kg N ha has pronounced effect than 120 kg N ha. Three splits of coated urea performed better than 2 splits. The highest improvement of 38.06% in grain yield and 45.13% in total N uptake was obtained in treatment receiving super urea at 90 kg N ha compared with granular urea at 90 kg N ha. Agrotain treated urea at 90 kg N ha gave the next highest improvement of 30.55% in grain yield and 38.87% in total N uptake than granular urea and control plots at 90 kg N ha. These results suggest that the combined use of urease + nitrification inhibitors (super urea) was better than urease alone. It was concluded that both agrotain and super coated urea delayed the urea hydrolysis and made the maximum N availability to plants and increase the yield of maize crop.


Introduction
Urea has been shown to have lower N use efficiency in many cropping systems compared to other N fertilizers, meaning that a large percentage of the applied fertilizer N is not being used for productive purposes and is essentially lost [1].After surface application, urea is quickly hydrolyzed within 1 to 2 days by soil urease to NH4 + , hydroxyl (OH -) and carbonate (CO3 2-) ions, leading to a high pH and very high concentrations of NH4 + around the urea.This NH4 + reaches equilibrium with dissolved NH3.The sharp rise in soil pH increases the likelihood of gaseous NH3 losses to the atmosphere.A wide range of gaseous NH3 losses from 1.7% to 56% of the applied N have been reported which depend on soil moisture, temperature and pH, wind velocity, soil organic C and N fertilizer type [2].The temporary rise in soil pH and production of high NH4 + due to fast urea hydrolysis, or after application of NH4 + based fertilizers such as DAP, lower the activity of Nitrobacter, the genus of bacteria responsible for the oxidation of NO2 -to NO3 -.This results in an accumulation of NO2 -in the soil around the fertilizer granules, which can have harmful effects on germinating seeds and young plants, and increase the risk of N being lost as N2O, a greenhouse and ozone (O3 _ ) depleting gas, via nitrification [3].Similarly, nitrate is repelled from soil exchange sites because of its negative charge and is prone to both leaching losses if drainage occurs, and emission to the atmosphere as N2O via denitrification.Because of its very dynamic nature, the efficient use and management of N is always a challenging task in agricultural systems.Where there is substantial water input through irrigation or rainfall, total N losses can be very high up to 80%.Nitrogen losses through NO3 -leaching, NH3 volatilization and N2O emissions have both economic and environmental implications.It is very important therefore to minimize such losses by synchronizing plant N uptake with the available soil N (NH4 + and NO3 -) to optimize fertilizer N efficiency.Therefore, various attempts have been made to coat or treat N fertilizers with urease inhibitors, nitrification inhibitors, polymers and elemental S to reduce N losses and enhance fertilizer N efficiency.Among the urease inhibitors, N-(n-butyl) thiophosphoric triamide (NBPT), trade name "Agrotain" is one of the most efficient and widely available commercially [4].After application, Agrotain is quickly converted in soil to its more effective oxygen analogue N-(nbutyl) phosphoric triamide (NBPT), which then forms a tridentate ligand with the urease enzyme, slowing urea hydrolysis, reducing NH3 volatilization from surface applied urea, and increasing N uptake and yield in a range of crops.Treating urea with Agrotain enables plants to take up more N in either urea or NH4 + forms, both of which require less energy to metabolize and covert N into plant protein [5].Nitrification inhibitors such as dicyandiamide (DCD) and (DMPP) slow the activity of Nitrosomonas, the genus of nitrifying bacteria responsible for the oxidation of NH4 + to NO2 -and can there by reduce NO3 -leaching and N2O emissions.DCD is more widely used nitrification inhibitor because it is cheaper, less volatile and relatively soluble in water [6].However, the persistence of excess NH4 + in soil as a result of reduced activity of Nitrosomonas may lead to increased N losses via NH3 volatilization.Therefore, urea in combination with NBPT and NBPT + DCD can increase N recovery by crops by minimizing ammonia volatilization and thus increases nitrogen use efficiency.The soils of Pakistan are mostly alkaline and calcareous in nature and are favorable to N losses via NH3 volatilization.Limited work has been done in Pakistan to assess the impact of urease and nitrification inhibitors with urea fertilizer on minimizing N losses and increasing crop yields.This study was therefore undertaken to investigate the impact of urease and nitrification inhibitors with urea fertilizer on yield and N use efficiency and also to compare the effect of two and three split applications of urea on N recovery and yield of maize crop.

Materials and Methods
Field experiment was conducted at the Agronomy Research Farm, University of Agriculture, K.P.K Peshawar to study the effect of urease and nitrification inhibitors on maize yield and nitrogen use efficiency during 2011-12.Soil of the experimental site was silt loam, alkaline in reaction (pH 8.06), non-saline (EC 0.16 dSm -1 ), strongly calcareous (lime 16.0%) and low in organic matter (Table 1).There were a total of 10 treatments were viz., (1) control (no urea), (2) urea at the rate of 90 kg N ha -1 (in two splits), (3) urea at the rate of 120 kg N ha -1 (in two splits), (4) urease inhibitor (agrotain) coated urea at the rate of 90 kg N ha -1 (in two splits), (5) urease inhibitor (agrotain) coated urea at the rate of 120 kg N ha -1 (in two splits), (6) urease plus nitrification inhibitor coated urea (super urea) at the rate of 90 kg N ha -1 (in two splits), (7) urease and nitrification inhibitors coated urea (super urea) at the rate of 120 kg N ha -1 (in two splits), ( 8) urea at the rate of 120 kg N ha -1 (in three splits), ( 9) urease inhibitor (agrotain) coated urea at the rate of 120 kg N ha -1 (in three splits) and ( 10) urease and nitrification inhibitors coated urea (super urea) at the rate of 120 kg N ha -1 (in three splits).Agrotain product was used as urease inhibitor while super-urea was used as urease and nitrification inhibitors.Experimental design was randomized complete block (RCBD) and each treatment was replicated four times.Net plot size of 5 m wide and 3 m long with 0.5 m distance between the treatments and 1 m distance between the blocks was used.All treatment plots received P205 at 90 kg ha -1 and K2O at 60 kg ha -1 as basal dose at sowing time.Urea was applied with and without inhibitors at sowing, 2 nd and 3 rd dose after 30 and 60 days of sowing (DAS) as per experimental plan.Maize hybrid cultivar "Baber" was sown in July and harvested in October.Recommended cultural practices were followed throughout the growing period.The crop was irrigated with canal water when needed.The crop was harvested at maturity and data was recorded on various agronomical traits (viz., plant height, ear length, number of ears plant -1 , number of rows ear -1 , number of grains row -1 , number of grains ear -1 , biological yield, grain yield, 100-grain weight) and total N uptake in crop.

Results and Discussion
The results obtained on the effect of urease and nitrification inhibitors on yield and N uptake of maize are presented and discussed as under: Yield Components: The results obtained on plant height, ear length, number of ears plant -1 , number of rows ear -1 , number of grains rows -1 , number of grains ear -1 and 100 grain (wt) as influenced by N inhibitors and split application of urea are reported in Table 2 and 3.It is revealed that N inhibitors and split application of coated urea significantly improved the growth, yield and yield components of maize.Maximum plant height of 136.70 (cm) was obtained for treatment receiving super-urea (urease + nitrification inhibitor treated urea) at 120 kg N ha -1 in three splits (Table 2).This was however statistically at par with that (135 cm) obtained with agrotain treated urea at 120 kg N ha -1 in three splits.Maximum ear length of 24 cm were obtained from plots receiving super-urea (urease + nitrification inhibitor treated urea) at 120 kg N ha -1 in three splits (Table 2) followed by agrotain coated urea at 120 kg N ha -1 in three splits.The next highest number of ears plant -1 (2.53) was obtained from plots receiving super urea at 120 kg N ha -1 in three splits.The lowest number of ears plant -1 (1.17) was obtained in the control treatment and this was significantly lowest in all fertilizer treatments.The minimum number of rows ears -1 (17.33) was obtained in control treatment.The results showed that number of ears plant -1 of maize hybrid cultivar increased with increasing level of N. Furthermore, number of ears plant -1 of maize was generally greater for three than for two splits of coated urea application.Moreover, the number of grains row -1 increased with increasing level and splits of urea application.The significantly highest number of grains row -1 (36 grains row -1 ) were obtained for treatment receiving super urea (urease + nitrification inhibitor treated urea) at 120 kg N ha -1 in three splits.This was however statistically at par with that (34.21 grains row -1 ) obtained with agrotain treated urea at 120 kg N ha -1 in three splits.The lowest number of grains row -1 (27.67) were obtained in the control treatment and this was significantly lowest than all fertilizer treatments.The results obtained on number of grains ear -1 of maize revealed that maximum number of grains ear -1 of 636.30 was obtained in the treatment receiving super urea (urease + nitrification inhibitors treated urea) at 120 kg N ha -1 in three splits and this was statistically at par with those obtained with super urea at 90 kg N ha -1 in two splits (585.70) or agrotain (urease inhibitor treated urea) at 90 kg N ha -1 (557.30 1 ) (Table 3).As with other yield components, the significantly highest 100-grain weight of 42 g was obtained for treatment receiving super urea (urease + nitrification inhibitors treated urea) at 120 kg N ha -1 in three splits (Table 3).The lowest 100-grain weight of 30 g was obtained in the control treatment and this was significantly (p<0.05)lowest than all fertilizer treatments.It was however noticed that 100-grain weight was generally increased with increasing level and number of split application of urea.Almost similar results have been reported for the influence of urease and nitrification inhibitors on yield and yield components of wheat and other crops [1, 13].Biological Yield: The results obtained on biological yield of maize as influenced by N inhibitors treated and untreated urea treatments are presented in Table 4.The results showed that on average the maximum biological yield of 13744 kg ha - 1 was obtained in the treatment which had received super-urea (urease + nitrification inhibitors treated urea) at 120 kg N ha -1 .The super-urea improved the biological yield by 21.30% with 90 kg N and 17.73% with 120 kg N ha -1 , whereas agrotain (urease treated urea) improved biological yield by 16.59% with 90 kg N and 10.67% with 120 kg N ha -1 (Table 4).Zaman et al.
[1, 14-16] also reported that agrotain and dicyandiamide (DCD) in combination were more effective in minimizing NH3 losses and N2O emission, and in controlling urea hydrolysis, improving pasture production and retaining N in NH4 + form.Other researchers had also observed similar results whereas the combined use of nitrification and urease inhibitors gave better results in terms of increasing crop yields than their use alone [2, 14 ].Grain Yield: The results obtained on grain yield of maize showed that on average the highest grain yield of 4517 kg ha -1 was obtained in the treatment receiving super-urea (urease + nitrification inhibitor treated urea) at 120 kg N ha -1 followed by 4201 kg N ha -1 obtained in the treatment receiving agrotain (urease inhibitor treated urea) at 120 kg N ha -1 (Table 5).Super urea improved the grain yield of maize by 38.06% with 90 kg N and 25.23% with 120 kg N ha -1 , whereas agrotain improved the grain yield by 30.55% with 90 kg N and 19.61% with 120 kg N ha -1 as compared to treatments  receiving untreated urea at 90 or 120 kg N ha -1 (Table 5).These results are consistent with those of [17,18] who reported that combination of urea with urease inhibitors, N-(n-butyl) thiophos phorictriamide (NBPT), and a nitrification inhibitor, dicyandiamide (DCD), significantly reduced urea hydrolysis and in turn increase the grain yield of wheat by 27.8%.Nitrogen Uptake: On average, the highest N uptake of 101.2 kg ha -1 was obtained in treatment receiving super-urea (urease + nitrification inhibitor treated urea) at 120 kg N ha -1 followed by 98.6 kg ha -1 in treatment receiving super urea at 90 kg N ha -1 (Table 6).Super-urea increased the N uptake of maize by 45.13% with 90 kg N and 32.6 with 120 kg N ha -1 .However, agrotain (urease treated urea) increased N uptake of maize by 38.87 % with 90 kg N and 32.44% with 120 kg N ha -1 .The results also showed that N uptake increased with increasing the number of split application of urea both at 90 and 120 kg N ha -1 (data not shown).[18] reported that urea in combination with dicyandiamide plus hydroquinone (U + DCD + HQ) recovered 69% of urea-N in plant and 73% of the recovered N was transformed into grain protein by spring wheat.Other workers also reported that the use of urease and nitrification inhibitors reduced N losses and increased N use efficiency by various crops [18, 19].These results revealed that the use of nitrification inhibitors with urea did improve the yield and yield components of maize planted on a calcareous alkaline soil.Using both urease and nitrification inhibitors with urea performed much better than the use of urease inhibitor alone [19, 20].Thus it could be concluded that strongly calcareous alkaline soils are highly conducive to rapid urea hydrolysis and nitrification processes which lead to N losses.Curtailing such processes with urease or nitrification inhibitors prolong the existence of nitrogen in soil giving greater opportunity to plants for its utilization and hence reducing chances of its volatilization.These results therefore suggest that the use of urea on highly calcareous alkaline soils need special management to reduce its loss and maximize its efficiency or plant utilization.The use of nitrogen inhibitors seems one of the promising strategies to reduce N losses from urea on calcareous alkaline soils provided other conditions are favorable such as soil organic matter and temperature.Conclusions Three split applications of controlled release urea fertilizers are fruitful for improving growth, yield and soil fertility as compared to single or pre-plant application.Moreover, coated urea fertilizers are less susceptible to various N losses which occur in coarse textured soils.Combined application of urease + nitrification inhibitors (super-urea) was superior to urease inhibitor alone (agrotain) in increasing the yield and yield components of maize.The superiority of these controlled release urea fertilizer can be affirmed by giving significantly higher dry matter yield and N uptake by maize crop.
hydroquinone, dicyandiamide and encapsulated calcium carbide on urea-N uptake by spring wheat, soil mineral N content and N2O emission.Soil Use and Management.

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Total mineral N in soil samples was determined by the steam distillation method as given in Mulvaney [6].Total N in soil and plant samples was determined by Kjeldhal method as described in Bremner [7].Soil pH and EC were determined in the saturated soil extract.Soil extract was read for pH on pH meter [8] and for EC on EC meter [9].

Table - 1
. Some important soil characteristics of the experimental site Table -2.Effect of urease inhibitor (Agrotain) and urease plus nitrification inhibitor (Super-urea) on yield and yield components of maize.Means followed by same letter(s) within columns are statistically non-significant (P<0.05).**Agrotain contain urease inhibitor; ***Super urea contain both urease and nitrification inhibitor Table-3, Effect of urease inhibitor (Agrotain) and urease plus nitrification inhibitor (Super-urea) on yield and yield components of maize.
**Means followed by same letter(s) within columns are statistically non-significant (P<0.05).

Table - 4
, Percent increase in biological yield of maize by urease (Agrotain) or urease plus nitrification (Super-urea) inhibitors treatments over non-inhibitors treatments.

Table - 5
, Percent increase in grain yield of maize by urease (Agrotain) or urease plus nitrification (Super-urea) inhibitors treatments over non-inhibitors treatments.