Production and levels of foliar nitrogen in rocket salad fertilized with controlled-release nitrogen fertilizers and urea

Controlled-release nitrogen fertilizers enhance crop productivity and decrease nitrogen loss through volatilization and leaching. This study aimed at determining the effect of nitrogen treatments provided by controlled-release nitrogen fertilizers and urea. The following fertilizers were tested: 1 (agricultural urea), 2 (ammonium sulfonitrate + nitrification inhibitor, dimethylpyrazole phosphate (Entec 26®)), 3 (urea + Kimberlit polymer (Kimcoat N®)) and 4 (urea + NBPT (Super N Agrotain®)). Treatments corresponded to 0, 100, 200, 400 and 800 kg ha-1 of N with rocket salad grown inside a greenhouse, assessing the production of fresh and dry leaf mass as well as levels of foliar nitrogen. Doses of different controlled-release nitrogen fertilizers showed significant polynomial regressions for the production of rocket salad fresh leaf mass and levels of foliar nitrogen, whereby Entec 26® did not show significant polynomial regression, considering the last parameter evaluated. The production of rocket salad dry leaf mass did not display significant linear regressions. The maximal production of rocket salad fresh leaf mass was reached at doses of 600, 490, 765, and 462 kg ha-1 of N with the use of urea, Entec 26®, Kimcoat N® and Super N®, respectively.


Palavras
The correct management of N is essential for increasing the efficiency of its use. Nitrogen is an element that is easily lost by leaching, volatilization and denitrification in the soil-plant system. Thus, the proper management of nitrogen fertilizing has been a major problem. Therefore, it is necessary to determine the most appropriate method of application, because this affects the behavior of N in the soil and its efficiency for the crops. One of the alternatives for increasing its efficiency is to divide the recommended dose, which leads to a significant increase in operational costs. However, the utilization of sources that show a slow or controlled release of the nutrients is an option (Lezana & Carrasco, 2002).
Fertilizers with controlled release can be classified as slow-or controlled-release fertilizers and stabilized fertilizers. Slow-release fertilizers show low solubility in relation to a reference soluble source. Stabilized fertilizers contain additives to increase the time of their availability in the soil (Cantarella & Marcelino, 2008).
Entec 26 ® is a fertilizer with release of about four months, containing 26% N, where 7.5% is in the form of NO 3 and 18.5% in the form of NH 4 + , and 13% S, covered with wax (Mendonca et al., 2007). Kimcoat N ® is a urea covered with three layers of high-charge density polymer (Roberto, 2007).
The available inhibitors are classified as reagents that interact with sulfhydryl groups, hydroxamates and structural analogs of urea, which inhibit the action of urease, competing for the active site of the enzyme. This last group includes NBPT [N-(n-butyl)], which is an additive that protects the urea applied on the surface from degradation and loss by volatilization of ammonia. NBPT inhibits the enzymatic degradation of urea by the action of urease, for a period of 10 to 14 days, where it is later degraded into its constituent elements, N, P and S (Cantarella & Marcelino, 2008). The commercial formulation Super N contains 20% to 25% NBPT (Scivittaro et al. 2005).
The aim of this study was to determine the production of rocket salad and the effect of doses of N contained in controlled-release nitrogen fertilizers Entec 26 ® , Kimcoat N ® and Super N (Agrotain ® ) compared to the response obtained with the application of urea.
A c o m p l e t e l y r a n d o m i z e d experimental design with a factorial scheme of 4x5 (4 sources and 5 doses) and three repetitions was used. The nitrogen fertilizers utilized were: 1 (agricultural urea), 2 (ammonium sulfonitrate + nitrification inhibitor dimethylpyrazol phosphate (Entec 26 ® )), 3 (urea + Kimberlit polymer (Kimcoat N ® )) and 4 (urea + NBPT (Super N -Agrotain ® )), at doses of 0, 100, 200, 400 and 800 kg ha -1 of nitrogen in the cultivation of rocket salad (Eruca sativa). Rocket salad was planted at three equidistant points in the pots with a capacity of 2 kg of soil, with an inside diameter of 18.5 cm. The seedlings were thinned out at 15 days after seeding (DAS), allowing the growth of one plant per pot. Nitrogen fertilizers were added to the soil at 20 DAS. Irrigation of rocket salad was done with the addition of 250 mL of water/pot daily.
The rocket salad plants were collect at 40 DAS. At this step, the leaves were separated from the roots, in order to characterize the commercial part. Immediately after the collection, the fresh mass of the leaves (MFF) was determined. After drying in a forced-air oven at 60ºC for 32 h, the dry leaf mass (MSF) was measured. Finally, nitrogen was determined in samples of the dried, ground and digested leaves of rocket salads, utilizing the distillation-titration method for total N in plants described by Silva (2009). The results of the progressive increase in concentration of different nitrogen fertilizers were submitted to analysis of variance and to polynomial and linear regressions using the statistics software SAS (Statistical Analysis System Institute, 1999).

RESULTS AND DISCUSSION
The different doses of controlledrelease nitrogen fertilizers and of urea utilized did not significantly affect the production of fresh leaf mass (MFF), production of dry leaf mass (MSF) or foliar levels of nitrogen (foliar N) in the cultivation of rocket salad. However, the results showed significant regressions for MFF and foliar N (Figures 1 and 3).
The productions of fresh leaf mass (MFF) for rocket salad as a function of dose of controlled-release nitrogen fertilizer and of urea showed a quadratic behavior (Figure 1). The maximal production of MFF for rocket salad was 7.98 g/pot, obtained at a dose of 600 kg ha -1 of N with the use of urea. Entec 26 ® showed a maximal production of 8.59 g/ pot using 490 kg ha -1 of N. Kimcoat N ® at a dose of 765 kg ha -1 of N yielded a maximal production of 9.76 g/pot. Super N required a dose of 462 kg ha -1 of N to reach a maximal production of MFF of 9.23 g/pot. Thus, the production of MFF for rocket salad was more efficient utilizing controlled-release fertilizers compared to the use of urea.
C o n t r o l l e d -r e l e a s e n i t r o g e n fertilizers can increase crop productivity, besides reducing the application of N, divided in this crop, and loss of nitrogen by lixiviation and volatilization (Cantarella & Marcelino, 2008). Therefore, the efficiency of the use of controlled-release fertilizers lies in its utilization in a single application and the reduction of nitrogen dose applied to obtain maximal productivity. The increase in production of rocket salad through nitrogen fertilizing was also demonstrated by Linhares et al. (2008), who utilized as fertilizer sources cattle manure and scarlet starglory (Merremia aegyptia L.), a legume of great potential in supplying N.
The doses of nitrogen applied to the soil, utilizing urea, Entec 26®, Kimcoat N® and Super N as the sources, showed a linear behavior for the production of dry leaf mass (MSF) of rocket salad. However, the results for MSF showed non-significant linear regressions (Figure 2).
Zanão Junior et al. (2005) described that the production of green mass and dry mass of tsoi sum (Brassica chinensis var. parachinensis) submitted to increasing doses of nitrogen, showed significant linear regressions, utilizing ammonium sulfate as source of N. Even though being of the same family as tsoi sum, rocket salad showed a different behavior when submitted to the different controlled-release nitrogen the cultivation of rocket salad. Thus, the different doses and sources of N influenced the commercial part of the plant of rocket salad, that is, the production of leaves.
The foliar N levels of rocket salad showed significant polynomial regression as a function of nitrogen dose using urea, Kimcoat N ® and Super N as the sources (Figure 3). Thus, the doses for maximal accumulation of foliar N for rocket salad were calculated, and the maximal foliar concentration of N was 40.42 g kg -1 , obtained with the 1.33 t ha -1 dose of urea. Rocket salad leaves showed a maximal foliar concentration of N of 46.32 g kg -1 with the 1.71 t ha -1 dose of Kimcoat N ® . The dose of 2.95 t ha -1 of Super N was necessary to reach a maximal concentration of 52.23 g kg -1 of N in the leaves of this crop. The mean values of foliar N found in rocket salad leaves in this experiment were as high as 52 g kg -1 . Thus, they were different than those found by Almeida et al. (2007), who reported maximal levels of 37 and 37.6 g kg -1 , utilizing ground vegetables and chicken manure, respectively. Cavarianni et al. (2004) observed an increase in the nitrogen level in rocket salad leaves with the elevation in N availability in the nutritious solution. Therefore, the increase in the N concentration in the soil due to increasing doses of N applied led to an increase in the concentration of N in the leaves of rocket salad, as observed in Figure 3. In this context, the increase in foliar N levels with the application of increasing doses of nitrogen were also found in the works of Cavarianni et al., (2004) andPurqueiro et al. (2007).
Total nitrogen levels in leaves of rocket salad fertilized with increasing doses of N utilizing Entec 26 ® showed non-significant polynomial regression (Figure 3). Entec 26 ® contains DMPP (dimethylpyrazol phosphate), which inhibits nitrification, where N in the soil remains in the form of ammonia for a longer period of time (Meira et al., 2009). Thus, the increasing doses of nitrogen using Entec 26 ® as the source provides a reduction in N in the form of nitrate in the soil, when compared to the zero dose. In this manner, on reducing nitrate in the soil there was an inhibition  Polynomial regressions for the production of rocket salad dry leaf mass making use of different controlled-release nitrogen fertilizers and urea; *significant p <0.01, **significant p <0.05, NS = not significant; Bars on dots represent the mean standard error (regressões polinomiais para a produção de matéria seca de folhas de rúcula utilizando diferentes fontes de fertilizantes nitrogenados de liberação lenta e uréia; *significativo p<0,01, **significativo p<0,05, NS= não significativo; Barras sobre os pontos representam o erro padrão da média). Goiania, UFG, 2008. did not display polynomial regressions for the production of MSF (  Polynomial regressions for nitrogen levels within rocket salad making use of different controlled-release nitrogen fertilizers and urea; *significant p <0.01, **significant p <0.05, NS = not significant; Bars on dots represent the mean standard error (regressões polinomiais para teores de nitrogênio em folhas de rúcula utilizando diferentes fontes de fertilizantes nitrogenados de liberação lenta e uréia; *significativo p<0,01; **significativo p<0,05, NS=não significativo; Barras sobre os pontos representam o erro padrão da média). Goiânia, UFG, 2008.