The expansion of agriculture has contributed to the growth of Brazil as a supplier of agricultural products  (Townsend et al., 2009). However, the unrestrained expansion of agriculture  has  caused   profound   changes   in   natural resources, in addition to changes in natural  vegetation and soil properties (Lago et al., 2012; Santana et al., 2016). The aggressiveness of agriculture can be controlled in the way the activities are practiced and in how the environment is managed. Within this context, the crop-livestock-forest integration (CLFi) or agro-sylvopastoral system, is a sustainable production strategy that integrates forestry, agricultural and livestock activities, in the same area, either in intercropping, succession or rotation (Machado et al., 2011; Lana et al., 2016).

Integrated systems can be classified into four types (Balbino et al., 2011): crop-livestock-forest integration (CLFi/agrosilvopastoral), crop-livestock integration (CLi/agropastoral), livestock-forest integration (LFi/silvopastoral) and crop-forest integration (CFi). Soil fertility improvement, increased carbon sequestration and the supply of forage and more favorable environmental conditions for animals are highlighted amongst the benefits (Costa et al., 2002; Abreu et al., 2016). The tree component in agroforestry systems helps maintain nutrient cycling (Montagnini, 1992; Upson et al., 2016) by decomposing litter. Decomposition of litter is considered the primary means of transfer of nutrients to the soil, enabling its reabsorption by the living plant (Schumacher et al., 2004; Santana et al., 2016). The shade of trees and the presence of litter reduce the high soil temperatures and the trees also attenuate wind speed (Monteith et al., 1991; Ong et al., 1991; Assis et al., 2015; Brito et al., 2015). These two factors affect evaporation rate, soil water balance and evapotranspiration, as it increases the humidity available to plants, influencing the yield of agricultural crops and pastures (Duboc, 2007; Lemos-Junior et al., 2016).

Some LFi studies have shown native or exotic species being used in association with animals, as well as reviews of agronomic crops (Lemos-Junior et al., 2016). Duboc (2005) stated that to which extent, there is greater disclosure and encouragement for using native Cerrado species in an interaction system, an increase in the interest in the rational cultivation of these species may occur. At this juncture and with the need for a production that is increasingly sustainable and compatible with livestock and forestry production, the objective of this study was to evaluate biomass and nutritional stock of Brachiaria pasture and litter in silvopastoral system with tree native species of Cerrado,  in Brazil.

Study area 

This study was carried out at the experimental area of the School of Agronomy, of the Federal University of Goias (Universidade Federal de Goiás; SA/UFG), at Goiânia, Goiás. In the area, the soil is a red oxisol and the plantations near one another. According to Köppen, the climate is predominantly tropical (Aw), with the marked division of two well defined seasons during the  year:  humid  summer,  from December to March, and dry prevailing winter from June to August. The average temperature varies between 18 and 26°C, as the average annual rainfall is about 1300 -1700 mm, concentrating in the months from October to March (in the spring and summer seasons). Between May and September is the dry season, a period when the rains are rare and may occur drought. Between the months of July to August, the humidity drops too (dry weather) and may be between 15 and 30%. Three silvopastoral systems formed with the forage species Urochloa decumbens (Stapf) R.D.Webster (Brachiaria) integrated with three tree species, Dipteryx alata Vog. (Baru tree), Caryocar brasiliense Camb. (Pequi) and Eugenia dysenterica D.C (Cagaita) were evaluated. Each system has a total area of 7590, 5725 and 15525 m², respectively.The plantations were carried out in January 1998, spaced 6 x 6 m, and the plants come from a germplasm collection of the SA/UFG (Access to the map of the Germplasm bank).

Samples processing

Litter and above-ground biomass were collected following an experimental design of randomized blocks with six replicates, for each system and the sampling distances from the tree were recorded. A total of five above-ground biomass and three litter samples were collected, where each block was located 50 m from each other. All the material was stored in properly labeled Kraft paper bags.

The methodology adapted from Freitas et al. (2013) was used to evaluate the productivity of pasture and litter, relative to the distance from the tree. The Brachiaria data was collected in the center of a sampling quadrant (Figure 1), a meter on the right and left of the trees, and between right and left plants, using a template of 0.50 x 0.50 m (0.25 m²), removing the superficial biomass with pruning shears. The litter data was collected in the center, and between right and left plants, after the pasture was collected (Figure 1).

Litter was collected following the methodology determined by Lima et al. (2015), using a template of 0.25 x 0.25 cm (0.0625 m²), in the same place in which the above-ground was harvested, removing all the material until the soil was exposed. Roots still attached to the soil were avoided. Each sample was packed into labeled paper bags and taken to the Forest Ecology Laboratory.

The above-ground and litter of Brachiaria samples were dried in the laboratory, in a circulating and air renewal chamber at 70°C for approximately 72 h until reaching a constant weight. The final weighing of the Brachiaria samples were carried out individually. In addition, the total weight of each Brachiaria sample was obtained, and then each sample was divided into leaves and mixture (root, flowers and seeds). Each fraction was weighed once again.

The nutrition stock was evaluated using two samples for each tree per component (above-ground biomass and litter) in each different position. Three of the samples collected were mixed, and two sub-samples were made from the mixture, that is, two composite samples. At the end, a total of six samples of pasture and four samples of litter were analyzed per system of tree species, adding up to 30 samples.

The material of the composite samples was milled (Lippel crusher and in a Wiley type mill), and finally sieved in a sieve with mesh opening of 1.0 mm (20 mesh). Then, the material was directed to a laboratory for chemical analyses to determine micronutrient and macronutrient quantities in the plant tissue, following the methodology described by Tedesco et al. (1995) and Miyazawa et al. (1999).

Statistical analyses

A t-test  was  used  to compare the three silvopastoral systems with regards to above-ground biomass and litter of Brachiaria. A Tukey test was used for a multiple comparison among treatments. A regression analysis was carried out between the variables to assess the relationship between distance and biomass. 

A principal components analysis (PCA) was carried out with a covariance matrix to assess the nutritional contents in pasture and litter, and for the following parameters nutrition stock with regards to the different tree species: nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), sodium (Na), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), cobalt (Co) and molybdenum (Mo) (Monteiro et al., 2016). The different sampling locations (between lines, between plants and one meter from the plant) were also considered in the analyses (ANOVA two way). All analyses were conducted considering a significance level of 95%.

The highest Brachiaria above-ground biomass was observed in the silvopastoral system with Baru trees (2.23 ton/ha; F=8.51; p=0.001) (Figure 2A). Above-ground biomass in this system was approximately 23% higher than in Cagaita system (1.81 ton/ha) and 32% higher than in Pequi system (1.69 ton/ha) (Figure 2A). Above-ground Brachiaria biomass within the systems with Cagaita and Pequi trees did not differ statistically. 

The Baru system had a higher above-ground biomass in the center of the sampling area, between the tree lines (F=2.68; p=0.03). Still, above-ground biomass of Brachiaria in the Baru system was similar to that observed for the other systems in the position within the same row and 1 m from the tree (Figure 2B). Above-ground of Brachiaria was similar in all sampling distances for the Cagaita and Pequi systems.

The Baru system had a higher amount of litter biomass (7.67 ton/ha), of about 66% more litter than the area with Cagaita trees (4.63 ton/ha) and 88% more than the area with Pequi trees (4.07 ton/ha) (F= 675.46; p=0.000; Figure 3). There were no significant differences between the system with Cagaita and Pequi trees (Figure 3). In addition, there was no difference in the production of accumulated litter between areas at the center, between tree lines or between plants for either system with the different tree species. 

The largest amount of Brachiaria pasture biomass  and largest amount of litter was found in the silvopastoral system with Baru trees. The pasture differed nutritionally, being that pastures associated with Baru trees and Pequi have higher amounts of macronutrients. The livestock farming forest integration system seems to be an efficient system in agroforestry production and in improving the quality of pastures of the Cerrado. The use of trees native to the biome in the pastures make it more similar to natural areas of Cerrado sensu strictu and “campo sujo”, in addition to the benefits also inherent in the natural areas such as increasing organic matter deposition, nutrient cycling and improving soil cycling and microclimate characteristics.

The tree canopies break down wind and change the microclimate of the understory, consequently reducing the evaporation rate of herbaceous plants (Castro et al., 2008). The areas beneath the canopy and litter production retain soil humidity in dry periods, improving the production of grass. Shading caused by trees may impair the productivity of grasses, due to competition for light, in addition to the natural competition for water and nutrients. Therefore, spacing is essential for a good development of the tree-grass system, reducing the competition imposed by the shadows of the canopies (Galzerano and Morgado, 2008; Venturoli et al., 2015). The spacing for planting vary depending on the species and are important for a good soil cover (Gonçalves et al., 2015). The most commonly used spacing range from 4 to 16 m between lines and 50 cm to 2 m between plants (within the lines) (Gonçalves et al., 2015).

Sousa et al (2007) studied the productivity of Brachiaria brizantha cv. Marandu in a silvopastoral system and noted that despite dry matter (DM) reduced due to shadowing, this reduction is associated with larger Brachiaria leaves instead of stems. They also reported an increase in crude protein content in relation to the Brachiaria grown under full sun.

Andrade et al. (2003), assessed the development of six forage grasses (B. brizantha cv. Marandu, B. brizantha cv. MG-4, B. decumbens cv. Basilisk, Panicum maximum cv. Mombaça, Melinis minutiflora and Hyparrhenia rufa) in a silvopastoral system in Paracatu, Minas Gerais, a Cerrado region, associated or not with the leguminous plants Stylosanthes guianensis cv. Mineirão and Eucalyptus sp.  Grasses were established between Eucalyptus lines, in plots of 12 x 10 m, and the highest biomass production capacity was recorded for the species B. brizantha cv. Marandu and B. decumbens.

The production litter by falling leaves, twigs, flowers and fruits contribute to the transfer of nutrients from the canopy to the ground, and the leaves account for approximately 60% of this transfer (Poggiani, 2012). The tree species used in this forest and livestock integration system are semi-deciduous, and produce leaves from July to September (Silva-Júnior, 2005). The plants partially lose their leaves at the same time of year, increasing the deposit of nutrients in litter and soil (Caldeira et al., 2007). The low concentration of potassium (K) in the litter may be related to the high mobility of the K within plants, which transfer the element from senescent leaves for younger parts. Potassium is also movable in the soil, so much of it is lost by leaching, especially in the rainy season (Caldeira et al., 2007).

Calcium (Ca) showed higher values in the litter because it is immobilized in the leaves, and is rarely redistributed to other parts of the plant when it is in senescence. Calcium is a movable element in soil, which causes a loss of Ca by erosion and leaching to deeper layers of the soil. The roots of the trees are pivotable, and so reach greater soil depths, facilitating the reabsorption of the elements that were leached. Grasses have fasciculated roots, therefore occupy mostly the surface of the soil (Vitti, et al., 2006; Silva-Neto et al., 2015).

Micronutrient levels in the litter followed a descending order of Fe, Mn, Zn, Cu. The same result was reported in a research conducted by Caldeira et al. (2007), studying the production of accumulated litter and nutrient contents in a Mixed Rain Forest, at General Carneiro, state of Paraná. Iron (Fe) is considered an immobile element in the plant. Therefore, iron deficiency symptoms are observed faster than is the deficiency of other mobile elements. Because of the limited redistribution in the plant, it is possible to find young leaves with Fe deficiency while old leaves and roots have accumulated Fe (Dechen and Nachtigall, 2006). The high iron content found in the litter may be associated with the high contents found in some species or  the  contamination  of litter samples with soil (Caldeira et al., 2007).

Oxides and sulfides (Mn) are the forms of manganese found most frequently in the soil, and are commonly found associated with Fe. The Mn concentration varies between plant parts and growth period. Older leaves accumulate higher concentrations of manganese and a small portion is translocated to young leaves (Dechen and Nachtigall, 2006). Thus, the accumulated litter showed higher amounts of manganese due to the accumulation in the leaves.

Nitrogen (N) is one of the elements found in larger quantities in the leaves due to their participation in the metabolic reactions of the plant, especially photosynthesis (Malavolta, 1985). Phosphorus (P) is one of the primary macronutrients, along with N and K. Phosphorus is crucial for the process of photosynthesis. Still, it is not required in large amounts in plant, leading to the low values recorded. In addition, the need of the element decreases when the plant is already established, that is, adult (Raij, 1991)

Pastures grown under the baru and pequi trees showed better nutritional aspects with respect to NPK. Oliveira et al. (2005) obtained similar results and state that the increased levels of these minerals must be directly connected to the greater accumulation of organic matter in soil arising from the litter deposition under the canopies.

Trees exploit nutrients found in deep soil layers through the range of their root system. However, forage species only access the most superficial nutrients. Therefore, trees assist in the gradual incorporation of nutrients into the soil/pasture system by depositing biomass onto the soil (Nair, 1999). Carvalho and Xavier (1999) also found higher mineral contents in soils under B. decumbens grassland that were intercropped with baru and pequi trees. The trees in silvopastoral systems affect the development of the herbaceous forage increasing soil nutrient and changing the microclimate under the canopies (Carvalho and Xavier, 1999).

Cavalcante et al. (2007) evaluated the spatial variability of MO, P, K and CTC under different managements, recording higher values in Cerrado and no-till areas and a reduction in organic matter contents in the other cultivated areas. This fact can be explained by litter accumulation in the larger forest than in other systems. In addition, P, K and CTC values were higher in the no-till and conventional farming systems. The P and K levels were higher in surface, decreasing the deeper layers.

The benefits of the integrated system in the Cerrado directly improves productivity and quality of the pastures, and enables the production of alternative products, such as fruits of the Cerrado and even wood from sustainable plantations as the CLFi (Abreu et al., 2016, in press). The Cerrado has a richness of native trees with potential of being used in commercial production and in the integrated systems. However, the lack of studies on the behavior of these tree  species,  forages  and  crops  in  a mixed system is a limiting factor.

Systems integrated with Baru trees had higher productivity, that is, higher biomass, for both Brachiaria pasture and litter production in this study. The production of biomass and litter was higher in line among the trees.

The authors have not declared any conflict of interests.

The research group is grateful to the Postgraduate Program in Agronomy of the Federal University of Goiás (UFG), the Foundation of the State of Goiás Research (FAPEG) for project financing