Nitrogen fertilization in Brachiaria decumbens Stapf grass under degraded soil condition

Chemically degraded soils are found all over the world. Fertilization is an essential agronomic technique for recovering fertility and productivity in these areas. In this context, the objective was to recover the productive capacity of a degraded Red Yellow Argisol, grown with Brachiaria decumbens Stapf, in the presence of mineral fertilization with increasing amounts of nitrogen, evaluating the production of plant mass, plant height, tillering and water consumption. The experiment was conducted in a screened shelter and humidity control, according to a completely randomized experimental design. The treatments were organized in a factorial scheme, 6 treatments (without fertilization, 0, 30, 60, 90, 120 of nitrogen; 80 and 50, phosphorus and potassium for all treatments in g vase-1, respectively) corresponding to the amount of fertilizer applied in Kg ha-1, 4 cuts (performed every 30 days, starting 45 days after planting) and 5 repetitions. When fertilizing with 120 Kg ha-1 of nitrogen, it was obtained increase in fresh (655%) and dry (685%) mass when compared to the phytomasses of the treatment without fertilization, there was also an increase in the number of effective tillers and plant height, as the amounts of nitrogen applied increased in the soil. The maximum amount of nitrogen to be applied to the soil, in order to produce phytomass of B. decumbens in the condition of chemical degradation of the soil, obtained in the work was 120 Kg ha-1. In addition, the work allowed us to conclude that fertilization is an inexpensive and very important technique to recover the productive capacity of forage in the micro region of Brejo Paraibano, Northeast Brazil.


Introduction
Brazil stands out for the size of its bovine herd. In the year 2018 it obtained the first place in the world ranking, with a herd of approximately 214,69 million head, for this, the country had a forage support of 162.19 million hectares of pastures (Abiec, 2019), of these, 111,77 million hectares are cultivated pastures and 46,74 million hectares of natural pastures (Ibge, 2017). The intensive exploitation of agricultural areas, is generally associated with the degradation of ecosystems, causing a reduction in biodiversity, soil fertility and intensification of erosion processes in relation to the conditions found under original vegetation. In this sense, there is a need to adopt sustainable management techniques to maintain productivity and mitigate environmental problems (Terra et al., 2019).
In the Brejo Paraibano region, Northeast Brazil, Santos et al. (2010) found that the substitution of native forest for livestock led to a generalized impoverishment of the soil, particularly in relation to the levels of calcium, magnesium and potassium in Argisol Red Yellow. In this way, soil correction and fertilization are less costly and efficient factors for the recovery of degraded areas (Santini et al., 2015). The application of fertilizers enables the supply of nutrients to maintain metabolism and the best development of pasture, nitrogen fertilization in particular is essential to increase biomass production, mainly related to pasture recovery (Silva et al., 2013).
Among macronutrients, nitrogen is the most limiting factor for pasture productivity, due to the fact that it actively participates in the synthesis of organic compounds that form the structure of the plant, such as amines, amides, vitamins, pigments, amino acids, proteins and nucleic acids (Malavolta, 1980). Among these, proteins, a polymer of amino acids, represent the main form of organic nitrogen present in plants, protein molecules have 16% nitrogen, representing about 1 to 5% of the plant's biomass (Costa et al., 2020).
The positive effect on primary forage productivity is optimized by the correlation between the amount of nitrogen and carbon fixation, improving photosynthetic efficiency and carbon allocation to the shoots, causing an increase in the photosynthetic area (Taiz et al., 2017;Silva, 2018;Costa et al., 2020). Furthermore, the knowledge of the influence of fertilization on structural characteristics of pasture in degraded soil, is relevant to its proper management, resulting in ingestive behavior, consequently, in the performance of the animal in the grazing area (Silva et al., 2013). Development, v. 9, n. 10, e3399108578, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8578 5 The genus Brachiaria is one of the tropical forages with the largest cultivated area in the world, lately it has had a great expansion in Brazil. The forage stands out for presenting advantageous characteristics compared to other genera: great adaptability to acidic soils with low fertility and high dry matter yield (Silva et al., 2012). In addition, it has extraordinary drought tolerance, remaining green during the drought period, still resisting cold (except for frosts), fire, trampling and tolerating excessive humidity (Pupo, 1981).
In this context, the objective was to recover the productive capacity of a degraded Red Yellow Argisol, grown with Brachiaria decumbens Stapf, in the presence of mineral fertilization with increasing amounts of nitrogen, evaluating the production of plant mass, plant height, tillering and water consumption.

Soil collection and climate
The soil collection took place at the Frecheiro property, in the municipality of Areia, in the region of the Brejo Paraibano, Northeast Brazil. The topography of the analyzed area is mostly made up of rugged terrain, represented by relief ranging from strong undulating to mountainous, with a steep slope measuring approximately 45%. The soil was classified as Red Yellow Argisol according to the criteria of the Brazilian Soil Classification System -SiBCS (Embrapa, 2018), which is used predominantly with pastures. According to the Köppen-Geiger climate classification (Alvares et al., 2013), the region has an As' climate, which is characterized by being hot and humid, with autumn -winter rains and a dry period between the months of September and February. Average annual rainfall of 1.400 mm and average annual temperature ranging between 23 and 25 ºC.

Conducting the experiment
The experiment was conducted in a screened shelter belonging to the Department of Phytotechnics and Environmental Sciences (DFCA) of the Federal University of Paraíba (UFPB). The soil was dried in the shade for 48 hours and sieved through a 2 mm mesh sieve (TFSA). After drying, it was characterized in the chemical analysis laboratory of the Department of Soils and Rural Engineering -CCA -UFPB (Table 1), following methodologies contained in Embrapa (2017).  The soil after drying was deposited in plastic vases, with a capacity of 8,0 kg of dry soil. The treatments were organized in a factorial scheme, 6 treatments (without fertilization, 0, 30, 60, 90, 120 of nitrogen; 80 and 50, phosphorus and potassium for all treatments in g vase -1 , respectively) corresponding to the amount of fertilizer applied in Kg ha-1 based on Oleynik (1987), 4 cuts (performed every 30 days, starting 45 days after planting)standardization cut and 5 repetitions. The sources of mineral fertilizers used were ammonium sulfate (45% N), simple superphosphate (20% P2O5) potassium chloride (60% K2O).
The forage used was Brachiaria decumbens Stapf, the seeds were acquired in a commercial house, with 60% purity and germination level, were sown in the amount of 20 seeds vase -1 , to guarantee a satisfactory stand of three plants per vase, thinning, was performed fifteen days after planting (DAP). The vase were irrigated, every three days, in order to maintain 80% of the field capacity, aiming to change the amount of water lost from the system through evapotranspiration.

Agronomic variables
In each cutting season, the following traits were determined: -Total fresh mass of the aerial part of the plants, cut at five centimeters from the soil surface, measuring the mass in the field to avoid loss of mass due to dehydration; -Total dry mass, conditioning the material vegetable in paper bags and taken to dry in an oven with forced air circulation at 65 °C for a minimum of 72 hours until it reaches constant mass, then the dough is removed from the paper bag; ] -Height of plants cut measured from the base of the plant in relation to the last stem node; Research, Society and Development, v. 9, n. 10, e3399108578, 2020 (CC BY 4. -Water consumption or evapotranspiration of the culture (ETc), was quantified every three days, represented by the amount of water applied to the vase to reach the initial corresponding mass of the vase + soil + water (80% of the field capacity ). The amount of water replenished in the vase every three days until the end of the experiment, which represented the water consumption of the soil-plant system.

Statistical analysis
The data were subjected to analysis of variance (P <0.05), after finding the significance, polynomial regression models were constructed (P <0.05). Statistical analyzes were performed using the STATISTICA 9.0 ® software.

Results and Discussion
The productive variables evaluated: fresh mass, dry mass, plant height and number of effective tillers differed by the analysis of variance F-test (P <0.05). In addition, there was a significant interaction between treatments and cuts (Table 2). Research, Society and Development, v. 9, n. 10, e3399108578, 2020 (CC BY 4.

Fresh and dry mass
The treatment without fertilization did not show fresh and dry phytomass production ( Figures 1 and 2), respectively, in the first cut performed 45 days after planting, it is explained by the fact that B. decumbens grass does not have enough height to be cut of its aerial phytomass, with the aerial part being cut only after the second cut, at 75 (DAP).
In the second cut 75 (DAP), treatment 6 (120 kg ha -1 of N) produced 40,33 and 8,09 g vase -1 , respectively (Figures 1 and 2), increment of fresh mass (655%) and dry (685%) when compared to the phytomasses of the treatment without fertilization, this increase in the forage phytomass is explained by the greater stimulus of cell division and expansion (Gastal & Lemaire, 2002), allowing the growth of plants and consequently an increase in phytomasses. Machado et al. (2017) when working with nitrogen fertilization (400 kg ha -1 year -1 ), in condition of medium soil fertility, with Brachiaria (Urochloa decumbens Stapf), obtained an increase of 51,53% in dry mass, a percentage lower than that obtained in this research (685%), proving the high productive response of grass plants to fertilization in conditions of chemically degraded soil.

Tilling
Tiller is the modular growth unit of forage grasses (Hodgson, 1990;Amorim et al., 2019), when analyzing the number of effective tillers, that is, those that are vegetatively active, a quadratic behavior was observed (Figure 4) . It appears that the number of tillers was lower in the first cut compared to the others. This probably occurred, because in the establishment, the plants store much of the energy absorbed, for the formation of the root system and aerial part, while in the second phase the root system, already established, is able to absorb nitrogen, stimulating bud sprouting (LavreS & Monteiro, 2003).
Individually the tiller effectiveness is conditioned by biotic and abiotic factors, the productive viability of the grass is prolonged by the continuous replacement of dead tiller by effective ones (Santos et al, 2012;Costa et al., 2020).

Water consumption
The water content in the soil is essential for plant growth, where there is an increase in the volume of cells, caused by biological processes and by the physical action of water entering the cells, causing elongation of the cell wall and expansion of plant structures (Paiva & Oliveira, 2006;Melo et al., 2020). Therefore, when assessing water consumption in B.
decumbens plants, it was observed in terms of average daily consumption at the end of four aerial part cuts, causing an increasing consumption for treatments in which the amounts of nitrogen applied to mineral fertilization increased ( Figure 5).
Recent studies have observed that grass production is influenced by the availability of water content in the soil (Dantas et al., 2016;Koetz et al., 2017). In this context, the daily water consumption for treatment without fertilization was 86,3 g day 1 . Regarding the cuts, there was a variability in daily water consumption, with no tendency to increase or decrease over time.
Depending on the values of daily water consumption by the plant-vase system, it is possible to estimate the total water consumption at 135 (DAP), after performing the fourth cut of the aerial part, reaching the average values of 11.650, 5 cm 3 of water for treatments without mineral fertilization and 16.402,5 cm 3 for the treatment that received the highest amount of Research, Society and Development, v. 9, n. 10, e3399108578, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i10.8578 13 nitrogen ( Figure 5). Mezzomo et al. (2020) when working with irrigation depths in Sudan grass, they emphasize the importance of correlating production with the total amount of water applied throughout the crop cycle, aiming at the rational use of irrigation systems and enabling economic analysis of the activity.
The highest average water consumption 121,5 cm 3 day -1 was observed in the treatment with the highest amount of nitrogen fertilizer 120 Kg ha -1 (Figure 5), which produced the largest amount of fresh phytomass obtained in this work 60,89 g vase -1 (Figure 1). Therefore, it appears that there is a direct positive relationship between fertilization, water consumption, forage production.

Final Considerations
The maximum amount of nitrogen to be applied to the soil, aiming at the production of B. decumbens phytomass in the condition of chemical degradation of the soil, obtained in the work was 120 Kg ha -1 . In order to have a high production of aerial phytomass of B.
decumbens, constant supply of water is necessary, as it favors the increase in the volume of cells, caused by biological processes and by the physical action of water entering cells.
In addition, the work allowed us to conclude that fertilization is an inexpensive and very important technique to recover the productive capacity of forage in the micro region of Brejo Paraibano, Northeast Brazil.
Future works associated with techniques for the recovery of degraded areas with soil conservation techniques are important to restore the productive capacity of degraded soils, as they prevent the removal of native vegetation from other areas of the Brejo Paraibano.