**4. Discussion**

#### *4.1. Morphological Components of Growth and Biomass Production of Forage Sorghum*

Plant heights, stem diameters, panicle diameters, and panicle lengths are characteristics that positively influence the production of sorghum [18]. Studies on the benefits of corn and sorghum alleys have found evidence that leguminous alleys increase plant height when compared to single cultivation [22,23]. The application of leguminous residues controls weeds and improves the physical, chemical, and biological properties of the soil [24].

The dry mass of the morphological components of the alleys was increased when compared to single sorghum, especially in the absence of mineral fertilization. The distinct characteristics of the legumes and sorghum resulted in the exploration of the different layers of the soil (as well as soil structuring and dry mass production), which is associated with a lower rate of the decomposition of residues and nutrient recycling that benefits agricultural crop [25].

The total green mass and the total dry mass of the alley cropping systems were higher than the single sorghum system, except in the double crop with the presence of mineral fertilization, in which the crop systems did not cause changes in these variables. Plants of the Poaceae family grown in legume alleys increase the production of green mass and dry mass when compared to conventional cultivation without alleys [26]. The most important advantages of the alley cropping system in relation to single crops are: An increased production of green and dry mass, a greater accumulation of nutrients, and soil protection [22].

When studying maize cultivation in *Gliricidia* alleys [13], we observed a higher total dry mass of the crop in alleys when compared to maize cultivation fertilized with manure and conventional maize cultivation. Due to their high capacity to fix atmospheric N and to produce biomass under conditions of low water availability, *Gliricidia* and *Leucaena* alleys are capable of improving soil fertility and increasing the dry mass production of plants in the Poaceae family [27].

The cultivation systems without the presence of mineral fertilization were similar regarding root dry mass, whereas sorghum cultivated in *Gliricidia* alleys was superior to the single sorghum treatment in the double crop. However, with the presence of mineral fertilization in the crop, the RDM of sorghum in *Gliricidia* alleys was lower than the other cropping systems, which shows that the plant did not require as much investment in roots. The production of sorghum roots may depend on competitiveness with the legumes arranged in alleys, and the longer the establishment time of the alleys, the greater the competitiveness of the legumes with the crop [5–28].

The stem leaf ratio is important for the quality of the forage. In the absence of mineral fertilization, the systems of cultivations in leguminous alleys exerted influence on sorghum development, thus resulting in lower leaf proportion and impacting the lower leaf stem ratio [29].

#### *4.2. Macronutrient Leaf Contents and Productivity*

The *Leucaena* alleys cultivation system with and without the mineral fertilization of the crop was the only one able to provide a foliar content of N within the critical level for the production of 80% of the crop potential [21]. However, the leaf N level of the double crop was low for an adequate crop production; this was related to the scarcity of rain, which caused a limited N availability to the plants [30].

When studying *Gliricidia* and *Leucaena* alleys as a way to improve soil properties, Fernandes et al. [27] found that the residues incorporated 160 and 130 kg ha−<sup>1</sup> year<sup>−</sup><sup>1</sup> of N, respectively (when only considering N). The leguminous alleys recovered about 20% of N directly from the residues deposited in the soil [11–16]. BNF can also represent N inputs relevant to the soil/plant system and reduce the need for N fertilizer application, which is often expensive and most susceptible to losses [8].

In the experiment developed during the crop of 2017/2018, the cultivation in alleys was superior when to that of the single crop for N leaf content. [30,31] found that leguminous alleys could increase the e fficiency of N fertilizer use. However, in the present study, mineral fertilization did not influence the leaf N content of the crop.

In the present study, *Leucaena* alley cropping was the only system that increased the leaf concentration of N in sorghum, thus contributing to an increase of 28%. When studying maize cultivation in *Gliricidia* alleys [12], we found an increase of 86% of N of the particulate organic matter of the soil in relation to the cultivation of single maize, which was compared to the e ffect of the use of 50 kg ha−<sup>1</sup> of N fertilizer.

As for the content of leaf P, the crop cultivation systems were adequate, except for single sorghum with mineral fertilization. However, in the double crop, even with the presence of mineral fertilization, the P levels of the cropping systems were below the ideal for the sorghum crop [19]. De Paula et al. [32] found that P from the decomposition of leguminous residues formed less water-soluble compounds and moved more slowly from one compartment to another. Furthermore, the half-life of nutrient release is shorter in the double crop.

The foliar content of P of the double crop and crop did not change with the mineral fertilization factor. [33] pointed out that BNF carried out by legumes results in higher energy and P expenditure by legumes. However, in the crop of 2017/2018, sorghum cultivated in *Leucaena* alleys with mineral fertilization showed a higher leaf content of P when compared to the other cropping systems. Nevertheless, there were no alterations between crop systems without mineral fertilization.

In a nutritional study of corn intercropped with legumes, the authors of [26] found changes in leaf P content only in the second year, in which this content was higher in legume crops than the conventional treatment. Thus, several plant species, especially perennial legumes, can use non-labile fractions of P by modifying the chemistry of their rhizosphere, excreting protons and organic acids to solubilize P and leave it available for crops [34].

The content of K remained adequate in all cropping systems in the two years of cultivation, regardless of the mineral fertilization, except for the single sorghum cultivation without mineral fertilization of the double crop, which presented a K content below suitable levels for the crop [21].

In the legume alley cropping without the mineral fertilization of the double crop, the content of leaf K was higher than that of the single crop. However, with the presence of mineral fertilization, the crop systems were similar. These results indicate that, in addition to providing nutrients from the plant residues to the main crop, these legumes probably recycled K from depths beyond the crop zone by sorghum roots [35,36].

The cropping systems did not undergo alterations regarding the foliar content of K, either with fertilization or without fertilization. Since the crop experiment was implemented in the second cycle, there is evidence that the content of K originating from the first cycle was sufficient and altered the effect in the double crop cycle [33]. The authors of [37] stated that in the cultivation of maize in alleys, legumes positively increased the content of leaf K in maize from the first crop cycle, which was similar to the conventional cultivation.

As for the levels of Ca and Mg, they were inadequate in all treatments—both in the double crop and in the crop [21]. Regardless of mineral fertilization, the lowest foliar contents of Ca and Mg were verified in the presence of leguminous alley crops. This result denotes the existence of competition between the legumes and the sorghum crop. However, the competition increases with the presence of mineral fertilization in the crop. The legume species require the same resources as the associated crops, which can result in both complementarity and competition [5].

Legumes have deep roots, can intercept percolated nutrients along the soil profile, and can access nutrients accumulated in the layers below the root zone of annual crops. These nutrients absorbed by the root system of the trees become inputs in the form of plant residues [10]. In general, legume residues provide Ca and Mg for agricultural crops. However, the slow release of these nutrients is probably due to the fact that they are some of the constituents of the middle lamella of the cell wall, forming one of the most recalcitrant components of the tissues [28–38].

On average, the presence of mineral fertilization in the cropping systems doubled the productivity in the two years of experiment. The leguminous alleys of the crop without mineral fertilization showed results similar to single sorghum cultivation. As for the presence of mineral fertilization, the systems of cultivation in leguminous alleys were smaller than the single sorghum cultivation.

Akinnifesi et al. [30] reported that corn yield in *Gliricidia* alleys without the mineral fertilization of N and P was 39% higher than in the single maize plots that received the recommended total amounts of N and P. When the *Gliricidia* alleys were altered with 50% N and 100% P, the yield increased by 79%.

Crops in legume alleys presented a surface area 30% lower than the single crop. In a study that related sorghum cultivation to leguminous alleys, the authors of [39] pointed out that the yield of sorghum cultivated in leguminous alleys corresponded to 94% in relation to conventional cultivation, although 86% of the area was occupied in the system.

The cultivation in leguminous alleys denotes its importance as a practice of agriculture with low external input as a form of soil fertilization, because it can maintain or increase the productive capacity of integrated agricultural crops [33]. Leguminous alleys are important for the morphological

development, growth, biomass production, and nutrition components of forage sorghum, especially N. In the present study, sorghum yield was increased and presented a direct relation with the presence of the alleys.
