Phytosociological survey ofspontaneous plant communitiesina conventional corn crop area in the region of Carajás,Pará, Brazil

. The presence of spontaneous plants in agricultural cultivation areas can result in damage to the crop of interest due to competition for natural resources and allelopathy, requiring systematic monitoring for an adequate cultivationmanagement. In this way, the objective was to identify the spontaneous plant communitiesspecies in an area of a one-year fallow conventional corn crop in the southeastern of Pará state, in the Brazilian amazon. A phytosociological survey of the plants in the experimental area of the Technological Center of Family Agriculture (CETAF) was carried out, by


Introduction
Spontaneous plants are considered those vegetable species that arise naturally in the agroecosystem to occupy available ecological niches (SOUZA; REZENDE, 2006) and there is still no knowledge if they are causing interference in the crop of economic interest (SUGASTI; JUNQUEIRA; SABOYA, 2012).
However, there are several forms of interference in plant development of the crop of interest, such as through allelopathy, with toxic, stimulating or innocuous allelochemicals for other plant species (DEUBER, 2003) or by competition, either by removing or reducing one or more factors necessary for the growth and development of both species, such as light, water, nutrients and physical space (LORENZI et al., 2014, SANTOS et al., 2019;GAZZIERO et al., 2019).
Since the weed communities can vary their composition and structure depending on the type and intensity of cultivation practices (KRENCHINSKI et al., 2015), for the establishment of an efficient integrated weed management program, with indication of one or more control methods, the identification of the species that occur in agricultural areas is essential (FONTES; SHIRATSUCHI, 2005), even though when considered the financial and environmental costs of using pesticides (KRENCHINSKIet al., 2015).
In this way, the objective of this study was to identify the spontaneous plant species present in an area of a one-year fallow conventional corn crop in the southeastern of Pará state, in the Brazilian amazon.

Experimental Area
The local of study was in an experimental crop production area belonging to the Technological Center of Family Agriculture (CETAF), a supplementary agency of the Municipal Department of Rural Production of the municipality of Parauapebas, in the southeastern of Pará state, Brazil (06º 03' 30" S; 49º 55' 15" W), as shown in Figure 1.

Sampling method and botanic taxonomy
A floristic inventory was conducted using the inventory square method (BRAUN- BLANQUET. 1979) in an area corresponding to 1.1 hectare, with a history of three consecutive years of conventional corn planting (2015,2016,2017) and which had been fallow for about one year (2018).
In the field, a 1.0 x 1.0 m (1.0 m 2 ) sampling frame was randomly cast 20 times in the evaluated area.according to the methodology of Domingos and Laca-Buendia (2010), totaling a sampled area of 20m 2 .The plant species in the area delimited by the frame were sectioned at ground level (Figure 2), transported to the laboratory, and taxonomically identified through specialized literature (CARDOSO et al., 2013;CRUZ et al., 2009).From the data obtained it was possible to calculate density and relative density (CURTIS; MCINTOS, 1950),frequency (MARTINS, 1978).relative frequency, abundance, relative abundance and relative importance value index (MUELLER-DOMBOIS;ELLEMBERG, 1974;INOUE et al., 2013), according to the following expressions: Relative Abundance (RA) = (abundance of the specie x 100) / (total abundance of all species);

Results and discussion
In Table 1 are found the values of the index of importance value of the identified species and in Table 2, the botanical families, the respective common Brazilian names of the species found, their mechanisms of reproduction and life cycle.
The Importance Value Index (IRI), which weighs on the integration of partial variables, combining them into a simpler and unique expression, the relative importance of each species, so it has been considered the most suitable parameter for phytosociological inventories (LAMPRECH, 1964).In this sense, the most important species found, based on the value index of importance (IRI) higher than 40%, according to Fontes and Shiratsuchi (2005), were Sorghum arundinaceum (55.70%),Commelinabenghalensis (47.78%),Synedrellanodiflora (42.39%) and unidentified specie (42.14%).The reason that plant species was not identified refers to the juvenility of the botanical materials collected in the field, which did not present morphological structures that could characterize their distinction and grouping to a taxon, implying in the systematics of this component.
The parameters that most influenced the IRI of S.arundinaceum, C.benghalensis and the unidentified species were the RF and RD (Table 1), while for S.nodiflora, it was the RA (18.07%).
The C. benghalensis is an herbaceous, perennial species that grows throughout the country and installs itself in cultivated areas, abandoned lands, vegetable gardens, orchards, and gardens, with a preference for rich, dry or humid soils and good exposure to light MOREIRA; BRAGANÇA, 2011) and it is possible to emphasize that these plant species present perennial life cycle (Table 2), which enables these plants to remain propagating in the environment where they grow.Furthermore, C. benghalensis presents asexual and sexual reproductive mechanisms (MOREIRA; BRAGANÇA, 2011) and it has favored its propagation and permanence in agricultural environments, even with mechanical control of weeds by harrowing, in which the propagules, even when cut, regrow, and form a new plant.In the phytosociological survey, 11 botanical families and 18 plant species were identified, which showed greater representativeness, based on the number of species collected, the botanical families: Euphorbiaceae (4), Asteraceae (3), Amaranthaceae (2) and Poaceae (2), while for Cleomaceae, Commelinaceae, Convolvulaceae, Fabaceae-Faboideae, Lamiaceae, Loganiaceae and Phyllanthaceae were represented by one specie each one (Table 2).
The Asteraceae is considered one of the largest families of invasive plants, but despite the richness in number of species, there is no difficulty in identifying the family, because the identification is made by the inflorescence, called chapter, with tubular, hermaphrodite or monoecious flowers and the achene-type fruits, which are usually accompanied by tufts of hyaline hair and hooks, characteristics that allow dispersal by wind and animals (MOREIRA; BRAGANÇA, 2011).
It is also notable the importance of studying the floristic composition in each biome for the comprehension of each ecosystem, for example, for the same region in the Brazilian Amazon, there are distinct environments, for example, the canga of the Carajás forest, where the flora has been studied and recently a probable new specie and three new occurrences are registered for the Euphorbiacea family in the region of Serra dos Carajás, Pará, Brazil, according to Costa, Secco, and Gurgel (2018).
In this study were found four species of Euphorbiaceae:Acalypha communis; Euphorbia heterophylla; Jatropha mollissimaand Sebastianiacorniculata.
in the field, E. heterophyllacan be identified in the field by the following characteristics: it has latex, very evident heterophily in the same plant, a cup-shaped nectary gland and for it presents inflorescence consisting of flowers of separate sexes (MOREIRA; BRAGANÇA, 2010).Even it is founded in minor frequency in the experimental area, an important aspect about E. heterophylla has been its control, is the occurrence of resistance to molecule of glyphosate (ADEGAS et al., 2020), extensively used in agricultural fields in Brazil.In this sense, changes in farming practices can provide great benefits to the agroecosystem, with positive effects on weed control (VARGAS et al., 2017), as the correct identification and the comprehension of the dynamics of weed communities in the agricultural environment, alerting and reinforcing the need to adopt the recommended good agricultural practices, in order to efficiently preserve the different tools for weed management, contributing to the sustainability of the Brazilian agriculture currently.

Conclusion
A high diversity of spontaneous plants was verified in the area, with 19 species distributed in 11 botanical families, with the highest participation of species for Euphorbiaceae and Asteraceae consecutively, emphasizing the need for floristic compositioninventoriesin agricultural areas for the definition of integrated control practices in face of the type of adopted cultivation system.
The species Sorghum arundinaceum (Desv.)Stapf and Commelinabenghalensis L. showed the highest values for plant density and relative importance value index compared to the others identified by the phytosociological survey.systems in the Southwestern Amazon region.American Journal of Plant Sciences, v. 8, p. 3399-3413,2017.

Figure 1 .
Figure 1.Location map of the study area in the municipality of Parauapebas, Pará state, Brazil.

Figure 2 .
Figure 2.Sampling of plants by the inventory square method.

Q
= number of frames with a given species, TN = total number of a given species, F = frequency, RF = relative frequency, D = density, RD = relative density, A = abundance, RA = relative abundance and IRI = index of relative importance values.

Table 1 .
Values of frequency, relative frequency, density, relative density, abundance, relative abundance and relative importance value index of the spontaneous plant community present in anone-year fallow corn production area.

Table 2 .
Description of spontaneous plant species found in the phytosociological survey in an area of one-year fallow conventional corn crop.