Does the Nitrogen Rates , Methods and Times of Application Influences the Corn Nutrition and Yield ?

Alternatives to reduce N losses in soil, increasing the nutrient utilization efficiency by the plant with increased grain yield are necessary for the adequate and sustainable management of this nutrient. In this context, the objective of this study was to evaluate the use of N rates at different times and in different methods of application, evaluating the nutritional status and corn grain yield in the Cerrado (Savannah) region. The study was conducted in Selvíria MS, Brazil, in an Oxisol. The experimental design was a randomized complete block design with four replicates, arranged in a factorial scheme 6 × 2 × 2, being: 6 rates of N (0, 50, 100, 150, 200, 250 kg ha), 2 times of application (at seeding or top-dressing) and 2 methods of application (surface or incorporated) with the urea source. The evaluations that were performed: leaf chlorophyll index (LCI), macro and micronutrients concentrations in leaf tissue (N, P, K, Ca, Mg, S, Cu, Fe, Mn and Zn), and corn grain yield. The N supply at sowing time provided higher levels of K, Ca, Mg, Fe and Zn in leaf tissue, while nitrogen fertilization at the V4 stage provided greater LCI. The incorporation of urea into the soil provided higher LCI and N, P and K foliar contents. The increase of N doses influenced positively the LCI, N, P, S, Cu, Fe, Mn and Zn foliar contents, and increased corn grain yield up to the dose of 164 kg ha of N. The urea application to the surface is more interesting to the incorporated application due to the ease of application, cost, besides propitiating similar corn grain yield.


Introduction
Corn is one of the oldest and most widespread crops in the world.Despite the technological advances available, the average Brazilian productivity is still very low, around 5,401 kg ha -1 (CONAB, 2017), which demonstrates the need to seek management techniques to obtain increased grain yield and, consequently, to guarantee profits to the producer (Galindo et al., 2016).
In order to obtain high corn grain yield it is necessary to apply high rates of nitrogen (N), as the soils do not usually supply the crop demand along its cycle (Galindo et al., 2017).In non-legume crops, nitrogen fertilization represents one of the highest costs of the production process (Nunes et al., 2015).Wheat, corn and rice crops consume approximately 60% of the total nitrogen fertilizer produced in the world (Espíndula et al., 2014).Therefore, the management of nitrogen fertilization is performed in order to ensure high productivity and depending on the N dynamics on soils, high N rates are added, which burdens the cost of production of farmers (Teixeira Filho et al., 2014).
Traditionally, annual crops receive only a fraction of the total N rate they need at sowing, and the remainder is applied in the interlining at times of greatest demand (Kaneko et al., 2015).This is principally due to three factors: low initial nutritional requirement, possibility of losses due to several factors such as leaching, volatilization, immobilization, besides the high salt content of nitrogen fertilizers (Mota et al., 2015).
The time of N application is one of the most discussed aspects in the management of nitrogen fertilization in grasses under no-tillage system, and since in the first years of adoption of this system the initial lack of N may For weed control, the herbicides tembotrione (84 g ha -1 of a.i.) and atrazine (1000 g ha -1 of a.i.) were applied, plus the addition of vegetable oil (720 g ha -1 of a.i.) as adjuvant in the herbicide syrup in post-emergence, at stage V2.Pest and disease control was not necessary.
Corn was harvested manually and individually per experimental unit on 18/04/2014, 118 days after plant emergence.The material was then dried in full sun and mechanically trodden.

Analytical Procedures
During the conduction of the experiment, the following evaluations were performed: (a) Leaf chlorophyll index (LCI), measured indirectly by means of digital chlorophyllometer CFL 1030 Falker, in 8 leaves of the main ear insertion, at the time of corn feminine flowering; (b) Foliar concentration analysis of macronutrients (N, P, K, Ca, Mg and S) and micronutrients (Cu, Fe, Mn and Zn), according to Malavolta et al. (1997), by collecting the middle third of ten leaves of the main spike at the female flowering stage according to the methodology described in Cantarella et al. (1997); and (c) Grain yield, determined by harvesting the spikes contained in 10 m of each of the four central rows portion, at physiological maturity stage (R6).The material was subjected to drying in full sun and after the mechanical track, the grains were quantified and the data transformed in kg ha -1 , at 13% (wet basis).

Statistical Analysis
The evaluated variables were submitted to analysis of variance (test F) and the means of the N methods and times of application were compared by the Tukey test at 5% of probability.Polinomial regression analysis was used for N rates.For statistical analysis, the SISVAR program was used (Ferreira, 2011).

Results and Discussion
The times of N application influenced the concentrations of K, Ca, Mg, Fe, Zn in leaf tissue and LCI (Table 1).The N application at the time of sowing provided higher concentrations of these nutrients, however, the application in top-dressing provided higher LCI (Table 1).Note.Means followed by the same letter in the column do not differ by Tukey test at 5% probability.
Regarding the LCI, the highest values obtained in the top-dressing application can be explained due to the greater proximity of the fertilization with the leaves collection for nutritional diagnosis.It is worth mentioning that some researchers showed a positive relationship between chlorophyll reading and chlorophyll content in corn leaf and between leaf chlorophyll content and N content in plants (Maestrelo et al., 2014), evidencing that the increase in N provides higher content of chlorophyll in leaf tissue and thus a higher rate of photosynthesis, since this nutrient participates in the composition of the chlorophyll molecule (Noor, 2017).
Although the time of N application influenced nutrient concentration and LCI, there was no difference in N leaf concentration and corn grain yield (Table 1).Santos et al. (2010), studying N application times (15 days before sowing, sowing and at V4 stage) also did not verify difference in N leaf concentration.Similarly, Kaneko et al. hen the treatme er (Table 1).H on the surface probably lowe n as a function utrients P and able 1), corrob fied that there influence on N usted to the lin the quadratic ld, respectively rients as repor 2012), which v kg ha -1 of N.  Kappes et al. (2013b) also found ICF values ranging from 39.9 to 71.2 and 51.1 and 68.5, respectively.

D), Fe (E) and M
With respect of grain yield, several studies have reported a positive effect of increasing N rates application in corn grain yield (Souza et al., 2011;Goes et al., 2004;Kappes et al., 2014, Dathe et al., 2016, Galindo et al., 2016, Zhou et al., 2017), reinforcing the results obtained.Thus, the highest corn grain yield were obtained when N was supplied in high rates, up to the dose of 164 kg ha -1 of N.This can be explained by the high N demand of the hybrid studied and the fact that corn crop was in succession to a fallow area, whose spontaneous vegetation presented high C/N ratio, and the desiccation and mechanical disintegration were performed with less than 15 days before corn sowing, evidencing the effect of N immobilization in straw.
According to Cantarella et al. (1997), the appropriate range for N, P, K, Ca, Mg, S, Cu, Fe, Mn and Zn in leaf tissue are, respectively, 27-35; 2.0-4.0;17.0-35.0;2.5-8.0;1.5-5.0;1.5-3.0g kg -1 and 6.0-20.0;30.0-250.0;20.0-200.0 and 15.0-100.0mg kg -1 , therefore, it is verified that the contents of N (with the exception of the control), P, K (except in the mean of treatments with top-dressing application), Cu, Fe, Mn and Zn in were adequate (Table 1).The leaf contents of Ca and Mg were slightly below the range of sufficiency, while the leaf contents of S were above the adequate range.Therefore, the nutrient content in leaf tissue did not compromise the adequate corn crop development.

Conclusions
The N supply at sowing time provides higher levels of K, Ca, Mg, Fe and Zn in leaf tissue, while nitrogen fertilization at the V4 stage provides greater LCI.
The incorporation of urea into the soil provides higher LCI and N, P and K foliar contents.
The increase of N doses influenced positively the LCI, N, P, S, Cu, Fe, Mn and Zn foliar contents, and increased corn grain yield up to the dose of 164 kg ha -1 of N.
The urea application to the surface is more interesting to the incorporated application due to the ease of application, cost, besides propitiating similar corn grain yield.

Table 1 .
N, P, K Ca, Mg, S, Cu, Fe, Mn and Zn concentration in foliar tissue, LCI and corn grain yield as a function of times, methods of application and N rates.Selvíria, MS, Brazil, 2013/2014 Costa et al. (2012), of nitrogen fertilization).Costa et al. (2012),