Yield Response of Upland Rice as Influenced by Enhanced-Efficiency Nitrogen Fertilizers in the Brazilian Cerrado

Nitrogen (N) fertilizers have their use efficiency adversely affected by the rate and source of N. A two-year field experiment was conducted to examine the yield response of upland rice by using NBPT (urease inhibitor), PCU (polymer-coated urea) and uncoated urea under different N application rates. It was hypothesized that either NPBT or PCU may result in increased yield components of upland rice when compared to conventional urea. The experiment was set up in a randomized block design in a 3 × 4 + 1 factorial scheme, with four replicates. Treatments comprised three sources (conventional uncoated urea, NBPT-treated urea, and polymer-coated urea) and four rates (30, 60, 90 and 120 kg ha) of N, in addition to a control treatment (no fertilizer application). Nitrogen fertilizers were applied in two split doses: 50% at the seedling stage, and 50% at the tillering stage (~80 days after planting). The results revealed that the use of enhanced-efficiency N sources increased the productivity and plant height of upland rice crop when compared to conventional urea. As compared to when it is untreated or polymner-coated, treating urea with NBPT resulted in increased 100-grain weight.


Introduction
Rice (Oryza sativa L.) is one of the major food crops for more than 3.5 billion (> 50%) people in the world (CGIAR, 2016). It is a staple food in the diet of many developing countries, particularly in Asia, Latin America and Africa . Based on FAO's forecasts for cereal production, 516.8 million tonnes of rice was harvested in 2019, led by China and India (FAO, 2019). Brazil s among the ten largest rice producers worldwide, accounting for 7.2 million tonnes, down 11.7% from the previous year's output because of the reduction in the planted area (FAO, 2019). The traditional methods for cultivating rice are known as upland rice and irrigated lowland rice, which are distinguished by the soil's water availability. Due to lower yield as compared with flooded rice, upland rice contributes less than 40% of total rice production in Brazil (Barbosa Filho & Yamada, 2002).
To produce high grain yields, modern rice cultivars require adequate amount of essential nutrients. Nitrrogen (N) fertilizers are extensively used by farmers to enhance rice crop production, as N is usually the most limiting nutrient except in soils containing high content of organic matter (V. B. Singh & V. K. Singh, 2017). Rice crops use 1 kg of N to produce 68 kg of grain (Witt et al., 1999). Deficiency of N in plants rapidly slows shoot growth and lead to severe nutritional disorders.
Fertilizer N-use efficiency by the crops is typically low. In general, plants assimilate less than 50% of the N applied (Tilman et al., 2002;Dobermann & Cassman, 2004), turning the N losses into a potential source of environment pollution. The supply of to crop plants comes from various sources, including native soil N crop residues, animal manure, and inorganic or mineral fertilizers (Ladha et al., 2005). In the case of rice, N fertilizers have their use efficiency adversely affected by the rate and source of N. Inadequate rate of N fertilizer application, as well as poor N management strategies lead to substantial losses of N due to inherently low N uptake capacity and absence of extensive root system in rice (Sinclair & Rufty, 2012).
The major contributors to N losses in rice systems are nitrification-denitrification, ammonia (NH 3 ) volatilization (Buresh et al., 2008), and to a less extent, leaching (Linquist et al., 2013). In non-flooded soils, NH 3 volatilization is of primary concern, particularly when N is applied as urea in alkaline soils. Urea is quickly hydrolysed to NH 3 and CO 2 after it is added to the soil, resulting in an increase of the soil pH and NH 4 + around the fertilizer granule (Francis et al., 2008). Under alkaline conditions, the equilibrium of NH 3 -NH 4 is shifted more to NH 3 , increasing volatilization losses that lead to lower fertilizer N use efficiencies (Ladha et al., 2005).
Several approaches have been adopted for reducing N losses and enhancing the N use efficiency by the crops. The use of formulated forms of fertilizer containing urease and nitrification inhibitors to reduce NH 3 volatilization from urea hydrolysis has emerged as an effective strategy.
Urease inhibitor NBPT [N-(n-butyl) thiophosphorictriamide] has been reported to significantly inhibit the activity of the urease enzyme, which reduces NH 3 volatilization losses due to urea application to rice (Buresh et al. 1988;Norman et al., 2009). Polymer-coated urea (PCU) is another important alternative to uncoated urea for improving N-use efficiency since it synchronizes N release and crop N uptake with minimum side effects (Patil et al. 2010).
Many researchers have recorded significant increase in grain yield of flood rice due to combined application of either NBPT + urea or PCU + urea over application of urea alone (Dillon et al., 2012;Norman et al., 2009;Pang & Peng 2010;Rogers et al., 2015). However, only a few studies discuss their utility for lowland rice systems. Hence, a two-year field experiment was conducted to examine the yield response of upland rice by using NBPT, PCU and uncoated urea under different N application rates. It was hypothesized that either NPBT or PCU may result in increased yield components of upland rice when compared to conventional urea.

Experiment Site
The field experiment was carried out in two growing seasons (2013/14 and 2014/15) in an area (16°35′50″S; 49°16′40″W; 735 m a.s.l.) located in the Agronomy College at the Federal University of Goiás, State of Goiás, Brazil. The local climate is classified as Aw (seasonal tropical savanna), with a humid season from October to April and a dry one from May to September according to the Köppen classification. The average annual precipitation is 1500 mm, and the mean annual temperature is around 22.5 °C. The soil was classified as typic dystrophic Red Latosol (LVd) in the Brazilian Soil Classification System (Santos et al., 2013), which corresponded to an Oxisol in the US Soil Taxonomy System (Soil Survey Staff, 2003). Prior to characterization, soil samples were air dried and sieved through a 2-mm mesh and then analysed following methodologies as proposed by Embrapa (1997Embrapa ( , 2009. Some selected chemical properties and particle size distribution of the top-layer soil (0-20 cm) at the beginning of the experiment in 2013 are given in Table 1. Table 1. Selected chemical properties and particle size distribution of the soil at the experimental site Chemical analysis Note. Ca 2+ , Mg 2+ , and Al 3+ were extracted by 1 mol L -1 KCl; P and K were extracted by 0.05 mol L -1 HCl + 0125 mol L -1 H 2 SO 4 (Mehlich-1 extractor); H+Al was extracted by 0.5 mol L -1 calcium acetate buffered at pH 7; CEC (cation exchange capacity): ∑ (K, Ca, Mg)/∑ (K, Ca, Mg, H+Al) × 100; V (base saturation): ∑ (K, Ca, Mg)/CEC] × 100; OM (organic matter) was estimated from the organic carbon (C) extracted by the Walkley-Black method. Textual Analysis conducted using the pipette method.

Experimental Design and Treatments
The experiment was set up in a randomized block design in a 3 × 4 + 1 factorial scheme, with four replicates. Treatments comprised three sources (conventional uncoated urea, NBPT-treated urea, and polymer-coated urea) and four rates (30, 60, 90 and 120 kg ha -1 ) of N, in addition to a control treatment (no fertilizer application).

Field Experiment
Field was ploughed at 20 cm depth prior to seeding. Plots consisted of four 5 meters long rows, spaced 0.5 m apart, using the rice cultivar BRS Esmeralda, which has a moderate resistance to major diseases and a certain tolerance to water stress (Castro et al., 2014). Additionally, BRS Esmeralda is a relatively recent upland rice cultivar developed by the breeding program coordinated by the Brazilian Corporation for Agricultural Research (EMBRAPA). The higher performance of BRS Esmeralda compared to other current cultivars is due to its high grain quality, good drought tolerance, high disease resistance and lodging resistance (Colombari Filho et al., 2013). Further, this cultivar has a great stability and adaptability to a large range of soils, climates, and crop management on the Cerrado region, wich may lead to a satisfactory yield performance in this study.
The useful area of the plot was composed of the two central rows, considering the lateral rows as borders. The soil was prepared in both years by one plowing and one disk harrow leveling. The seeds of rice were sown manually 20 cm spaced apart in rows, with two or three seeds per hole. Nitrogen fertilizers were applied in two split doses: 50% at the seedling stage, and 50% at the tillering stage (~80 days after sowing). All treatments received 400 kg ha -1 of the formula 00-20-20 as a basal fertilizer to supply phosphorus and potassium. Weed management consisted of hand weeding plots two times during the growing season. Rice was harvested in every growing season at the end of maturing stage (between 103 and 108 d after sowing).

Measurements
At harvest, plots were evaluated for the following yield components: plant height, which consisted in the length of the central culm; number of panicles per linear meter; 100-grain weight, which was randomly evaluated by collecting and weighing 100 fertile spikelets, and corrected to 13% of water content; and the grain yield, which was determined by weighing the harvested grain of each useful plot, corrected to 13% of the water content and converted to kg ha -1 as productivity.

Statistical Analysis
Data from both growing seasons were subjected to an analysis of variance. The statistical model used included sources of variation due to replication, growing season, N source, N rate and the interaction of growing season × N source, growing season × N rate, N source × N rate, and growing season × N source × N rate. For qualitative factors (growing seasons and N sources), the means were compared by the Tukey test at the P < 0.05 level when the F test proved significant, whereas the quantitative factors (N rates) were submitted to regression analysis. Sigmaplot 10.0 was used to create figures.

Results and Discussion
Statisitical analysis show the differences in plant height, number of panicles, 100-grain weight, and productivity of rice between or among growing seasons, N sources and N rates (Table 2). However, interaction effects of the factors growing season, N sources and N rates on the yield responses of upland rice were not significant (P > 0.05).
Plant height significantly increased with the addition of N rates of all the three N sources (Figure 1). The increase was in a quadratic form when N rates were increased in the range of 0 to 120 kg ha -1 , and varied from 83 to 101cm in 2013/14, and from 85 to 102 cm in 2014/15 on average across the N rates regardless of the N sources ( Figure 1). When the effect of N rates was analysed for each N source, maximum plant height was obtained with the application of N at a rate of 119 kg ha -1 by UU, 102 kg ha -1 by PCU, and 81.25 kg ha -1 by NBPT in the 2013/14 season. In the 2014/15 season, however, higher rates of UU (180 kg ha -1 ), PCU (125 kg ha -1 ) and NBPT (95 kg ha -1 ) were needed to achieve the maximum height of rice plants. Improvements in plant height with the addition of N in rice grown on Brazilian soils has also been reported by other authors (Fageria & Santos, 2018;. In our study, the increase in plant height in response to the application of N rates is probably due to enhanced availability of N with all the N sources, thereby indicating that high N inputs inhibited the effect of fertilizers. In a two-growing season experiment with rice plants, Lyu et al. (2015) found the same result on the response of plant height to N sources in both seasons, and they also reported changes in the plant height by application of N up to the highest level of N.

Conclusion
The use of enhanced-efficiency N sources increased the productivity and plant height of upland rice crop when compared to conventional urea.
As compared to when it is untreated or polymner-coated, treating urea with NBPT resulted in increased 100-grain weight.