Recuperação de pastagem de Urochloa decumbens com sistemas de manejo e adubação fosfatada Urochloa decumbens pasture recovery with management systems and phosphate fertilization Recuperación de pasto de Urochloa decumbens con sistemas de manejo y fertilización de fosfato

Adequate pasture management is important to ensure animal production. The objective of this study was to evaluate the effect on shoot dry weight yield (SDWY) and mineral composition in degraded pasture (Urochloa decumbens) recovery in a Typic Oxisol with introduction of Stylosanthes and phosphorus (P) fertilization. The experiment was set up as completely randomized block design in a split-plot arrangement with four replicates. The plots were Research, Society and Development, v. 9, n. 7, e347974118, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.4118 3 seven management system: T1 Control Urochloa decumbens without Stylosanthes; T2 U. decumbens + Stylosanthes with no-till; T3 U. decumbens with partial desiccation + Stylosanthes; T4 U. decumbens with total desiccation + Stylosanthes; T5 U. decumbens + Stylosanthes with soil scarification; T6 U. decumbens + Stylosanthes with plowing; T7 U. decumbens + Stylosanthes with plowing + harrowing and the subplots was the P fertilization (presence and absence). P fertilization (60 kg ha of P2O5) increased the P concentration and SDWY of U. decumbens, while the introduction of Stylosanthes in the different management systems used did not change the forage yield.


Introduction
Soil inadequate management with fertilization absence and the exhaustion of natural soil fertility have been identified as one of causes of the degradation of cultivated pastures (Costa et al. 2009). In these conditions occur mainly due to soil organic matter (SOM) Development, v. 9, n. 7, e347974118, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.4118 4 reduction, in addition to nutrients losses such as P, potassium (K), calcium (Ca) and magnesium (Mg) (Schaefer et al. 2002). To recovery degraded areas can be adapted various strategies. Including management systems that reduce or null mechanical soil stirring, perennial crop system with high biomass yield and well developed root system, favoring the SOM accumulation, improving the biological activity and availability of nutrients in the soil (Tan et al. 2007).
The adoption of practices such as grass and legume intercropping can be an alternative for restoring or increasing soil fertility (Silva and Saliba 2007), as the legumes present great environmental and economic potential for their ability to nitrogen (N) fixation in the soil, maintaining pasture more productivity and providing the yield systems sustainability at reduced cost (Werner et al. 2001;Moreira, Malavolta, and Moraes 2002).
Another problem is the low available P content in weathered soils as it is found in Brazil's edaphoclimatic conditions. P fertilization plays an essential role in root development and grasses tillering (Fageria et al. 2013a), being related to the plant energy metabolism in all metabolic cycles related to energy expenditure (Fageria et al. 2013b). The P absence limits yield capacity, establishment and pastures persistence, as well as impairs the other nutrients uptake (Werner et al. 2001;Heinrichs and Soares Filho 2014).
The structural and productive characteristics of forage plants are benefited by phosphate fertilization (Melo, 2016;Pietramale, et al., 2020). Based on these responses, it is possible to establish management strategies associated with the phosphorus application in pasture recovery.
The objective of this study was to evaluate the degraded pasture recovery of Urochloa decumbens with introduction of Stylosanthes and P fertilization on the shoot dry weight (SDW) yield, macronutrient (N, P, K, Ca, Mg, and S) concentration and soil chemical properties.

Methodology
A field experiment was carried out in an Urochloa decumbens area with 10 years of grazing, which had low forage yield, with little invasive plants infestation, without soil compaction. The experimental area was located in Dracena County, São Paulo State, Brazil at 379 m altitude, latitude 20°55' S and longitude 51°23' W. According to the Koppen classification, the climate is type Aw, characterized by hot and humid summer seasons, warm, and dry winter, with a higher rainfall index between November and March. The annual Development, v. 9, n. 7, e347974118, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.4118 5 temperature and precipitation are respectively 23°C and 1,300 mm, and the climatic data for precipitation (mm), minimum average temperatures and maximum averages for the experimental period are presented in Figure 1.
The P source used was the simple superphosphate (20% P2O5), applied annually at the beginning of the rainy season. The plots were sized at 10 m × 10 m and the subplots 5 m × 10 m. The legume seeds sowing density was 5 kg ha -1 , with 92% cultural value. The distribution was the haul in the treatments T6 and T7, and the other treatments were in rows spaced 0.22 m. Due to the low Stylosanthes spp. availability in area, in November of 2015 a haul with density 3.0 kg ha -1 of seed was carried out in all parcels except in the control. The soil samples for chemical properties were collected in 0.0-0.1 m and 0.1-0.2 m depth of in each subplot and the available P, K + , Ca 2+ , and Mg 2+ were evaluated by the ion exchange resin method (Raij et al. 2001).
The evaluation of the forage yield was divided into two productive cycles: i) October to March -without hydric deficit), in which four cuts were made, and ii) April to Junewith hydric deficit, corresponding to the dry period, one cut was made. The shoot dry weight The rearing of grass in experimental area was performed after sampling in a grazing system with heifers until reaching the residue around 10 cm in height. Forage of the third and fifth cuttings was used for macronutrients analysis (N, P, K, Ca, Mg, and S), the two cuttings being representative, respectively. The forage after drying was ground in a Willey mill, and then the acid digestions [sulfuric acid (N) or nitric-perchloric acid (P, K, Ca, Mg, and S)] were performed as described by Malavolta, Vitti, and Oliveira 1997).

Results and Discussion
The P, K + , Ca 2+ and Mg 2+ levels in the soil, in the 0.0-0.1 m and 0.1-0.2 m depth showed a significant interaction between soil management strategies × P fertilization ( Table   1). The P fertilization performed annually for four years, since 2011, provided P accumulation, mainly in the 0.0-0.1 m depth, presenting higher values to the treatments without fertilization. These results showed that soil fertility can be constructed under pasture conditions and corroborate Sansonowicz, Lobato, and Goedert (1987) on the positive effect of P application on the increase of soil fertility. Due to the pasture fertilization being superficial, the available P concentration was higher in the 0.0-0.1 m compared to 0.1-0.2 m depth.
However, it is possible to observe that in the deeper layer there was also an increase in the P concentration, which shows that, although slow, there is the nutrient movement the in the soil profile. Another factor that should be highlighted in pasture conditions is that there is no soil rotation, which reduces P fixation, as well as soil erosion rate is also lower relative to annual crop systems (Santos et al. 2008). Regarding the soil management strategies, the lowest P levels were in scarification and plowing + harrowing, which may be involved with greater soil movement with these agricultural operations and promote nutrient fixation (Cubilla et al. 2007).
Even in the management systems without P fertilization, there was an increase in P levels in relation to the initial soil chemical analysis (3.5 mg kg -1), value inside the same interpretation range indicated as low in the soil (Raij et al. 1996). These results can be attributed to the increase of SOM mineralization during the experimental period (Ferreira et al. 2014). The exchangeable K in 0.0-0.1 m and 0.1-0.2 m depth, although showing statistical variation, the results are very close to the effects of soil availability (Table 1). All values of this study are in the range of critical level from low to medium availability (Raij et al. 1996).
It was possible to verify that the K concentration was lower than those found at time of experiment installation, even with annual nutrient application, according to recommendation described by Werner et al. (1997). Possibly, it attributed to the fact the large amount required by the forage plants, as well as being a nutrient that is readily available in the soil to absorb the plant and be quite mobile in the soil, facilitating leaching, especially in soils with medium texture (Heinrichs and Soares Filho 2014).  Treatment  Soil management  T1  T2  T3  T4  T5  T6  T7  --  17.0a 11.0a 10.5a 9.5 9.0a 13.5a 6.5a CV (%) 28     14.5 T1 = U. decumbens -control; T2 = no-till without desiccation; T3 = partial desiccation; T4 = total desiccation; T5 = soil scarification; T6 = plowing; T7 = plowing + harrowing. Without P = no phosphate fertilization; with P = with phosphate fertilization. CV (%) = coefficient of variation; P, K, Ca and Mg: extracted with ion exchange resin. Means followed by lower case letters in the columns and upper case in the lines differ from each other to 5% probability by the Scott-Knott test.
In relation to Ca 2+ , the P fertilization contributed to increase the Ca 2+ concentration in the 0.0-0.1 depth, and this result was attributed to the Ca presence (18 -20%) in the phosphate source used in this study (superphosphate simple -20% Ca). In general, the Ca 2+ concentration were above 7.0 mmolc kg -1 , considered as high availability (Raij et al. 1996).
In relation to the initial soil chemical analysis, it is observed the soil maintenance the values of P, K + , Ca 2+ , and Mg 2+ . However, in relation to the values found after two years of implementation of management and fertilization systems (Rebonatti et al. 2016), there was chemical fertility recovery, since the management systems allow the cycling of nutrients that were extracted by the plants, especially those associated with no-tillage system. Although the SOM did not present a significant effect among the legume introduction systems, there was an increase in its concentration when compared with data found by Rebonatti et al. (2016) in same edaphic conditions. The shoot dry weight (SDW) yield in each cut and the total did not present significant interaction between Stylosanthess and P fertilization. However, the P fertilization showed difference in presence or absence of application. The four initial cuts comprise the period without hydric stress (spring and summer), while the fifth cutting corresponds to period with hydric stress (autumn) ( Table 2). Table 2. Shoot dry weight (SDW) yield per cutting and SDW of Urochloa decumbens in different soil management system and with or without of P fertilization.  25.3 T1 = U. decumbens control; T2 = no-till without desiccation; T3 = partial desiccation; T4 = total desiccation; T5 = soil scarification; T6 = plowing; T7 = plowing + harrowing. CV (%) = coefficient of variation. Means followed by lower case letters in the columns and upper case in the lines differ from each other to 5% probability by the Scott-Knott test.
Except for the control, the fifth cutting, which represents the cut during the rainy season, is among the highest yield averages. This result can be attributed to rainfall during the autumn period (Figure 1), that is usually characterized by long dry season. From the third to the fifth cut and the total yield in the period presented higher SDW with P presence. These results can be attributed by the beginning of the rainy season, in which the plants were not yet in full development caused by the stress during the dry season, as well as due to the pasture fertilization system, which is performed in and requires a longer period for soil incorporation, especially P that presents low soil mobility. The results corroborate Rebonatti et al. (2016), on study carried out in previous years, in same edaphic conditions. The SDW in autumn and period without hydric stress (spring and summer) were similar and did not present significance in the partial desiccation (T3), total desiccation (T4) and plowing + harrow (T7). These results occurred because the year 2016 was atypical, with regular rainfall during the period that is characterized by severe drought (Figure 1). The N, P, K, Ca, Mg and S did not present a significant interaction between Stylosanthes spp. × P rates.
However, it presented significance for the two periods of hydric deficit and for presence and absence of P fertilization (Table 3).
The N concentration presented a significant difference between the 2 season periods with 9.9 g kg -1 in spring and summer and 7.9 g kg -1 in autumn (Table 3). Despite this difference in the two periods, in all treatments presented lower levels than those considered adequate for the U. decumbens, ranging from 12.0-to 20.0 g kg -1 (Werner et al. 1996). These effects showed that the Stylosanthes spp. does not contribute to increase the N concentration in total U. decumbens composition.
In relation to P concentration, the highest concentrations were associated to P fertilization, in both periods. In the spring and summer, the concentrations ranged from 1.4 g kg -1 and 2.0 g kg -1 and in autumn 1.4 g kg -1 and 2.1 g kg -1 (Table 3). These results with those of Ieiri et al. (2010) and Moreira and Malavolta (2001), that studying sources and P rates in pasture recovery area and alfalfa (Medicago sativa L.), respectively, obtained a significant response regarding the presence of P fertilization. Even with the difference in SDW, no difference was observed in relation to the periods evaluated (Table 3). Although the P values are in the range considered adequate for U. decumbens ranging from 0.8-to 3.0 g kg -1 (Werner et al. 1996), it is important to emphasize the contribution of phosphate fertilization on forage quality in relation to the P presence in the animal diet.  In the period without hydric stress, the K concentration was higher in relation to the period with hydric stress, except in the partial desiccation, soil scarification and plowing + harrowing systems. This result can be associated to nutrient dynamics in the soil, which in the presence of moisture occurs greater availability and diffusion in the soil, increasing its uptake (Havlin et al. 2005). However, due to hydric restriction, the total ions concentration in solution increases, but the Ca 2+ and Mg 2+ concentrations increase faster than K, because the cation ratio activity in solution is constant (Gapon Equation), which explains the lower K uptake in autumn and higher Ca uptake (Table 3). Therefore, it is necessary to provide all nutrients in a balanced manner to reduce the conditions limiting the growth and plants development (Fernandes 2006). In general, the K concentrations are within the range considered adequate for grazing (Werner et al. 1996). Despite the presence of sulphate (12% Development, v. 9, n. 7, e347974118, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.4118 S) in simple superphosphate, the Mg and S concentrations, the values are within the range considered adequate and can be considered random within the experimental study. The reference values for plant nutrition suitable for Mg and S are, respectively, 1.5-to 4.0 g kg -1 e 0.8-to 2.5 g kg -1 (Werner et al. 1996).

Conclusion
After four years of experiments, the soil management strategies for the introduction of Stylosanthes spp. in U. decumbens grazing did not affect the soil chemical properties and the SDW yield was not significant. The P fertilization in the soil increase in relation to the find values in the implantation of the experiment, especially in the treatments with nutrient application, and the soil fertility construction took place. Phosphorus fertilization provide higher P concentration in forage and increase SDW yield. No significant differences were observed in the N, K, Ca, and Mg concentration in SDW of U. decumbens with introduction of Stylosanthes spp. and P fertilization. At 0.00-0.10 and 0.10-0.20 m depths, regardless of the management system, the P application of altered the K, Ca and Mg contents of the soil.