INTRODUCTION

The mechanism for converting native forests into agricultural areas is one of the main problems currently faced in the Amazon region. Thousands of hectares of native vegetation have been transformed into areas for activities aimed at the Brazilian agricultural sector. These activities cause degradation of the environment and a decrease or total loss of biodiversity and generate irreversible consequences such as soil erosion, silting of rivers and death of wild animals.

Agroforestry Systems (AFS) are excellent models for the replacement of these predatory production methods, as in addition to offering economic benefits, they are also important agents in the recovery of degraded areas. Agroforestry systems are consorting models including tree, agricultural and/or animal species and are used to recover degraded areas due to their ecological potential, and are also used to supply food and raw materials.

Agroforestry systems seek to recreate conditions that are present in natural systems, promoting the supply of the main ecosystem services present in non-anthropic environments (Vasconcelos; Beltrão, 2017). The use of agroforestry systems is recommended for the recovery of degraded areas, as well as for the preservation of areas surrounding environmental reserves (Silva et al., 2012). The arrangement of species in these systems represents an alternative economic stimulus for forest restoration; however, there are still doubts about the potential of agroforestry systems (Martins et al., 2019).

The importance of litter within agroforestry systems is highlighted, as this component is responsible for offering ecosystem services of high importance to the environment. Examples are soil protection, seed bank shelter and moisture retention.

Litter is the organic layer originating from the shoot of the vegetation, consisting of leaves, twigs, branches, flowers, fruits and seeds which, together with roots, begin a stage of decomposition as part of the nutrient cycling process (Marques; Junior; Vourlitis, 2017). Expressive amounts of nutrients can return to the soil through the fall of senescent components of the shoot of plants and their decomposition (Toledo; Pereira; Menezes, 2002).

Part of the return of organic matter to the forest floor occurs through the production of litter, which is considered the most relevant means for the transfer of essential elements from the vegetation to the soil (Vital et al., 2004) and according to Toscan (2017), the litter layer is characterized as an organic matter inflow and outflow system.

In addition to being a source of nutrients for vegetation and microorganisms, litter allows greater moisture retention, prevents erosion and improves the soil physical attributes (Holanda et al., 2015), that is, litter is of great importance for ecosystems, since it is responsible for important environmental processes.

In addition to the nutritional importance of litter, litter provides the interception of light, shading seeds and seedlings and reducing the soil thermal amplitude and, in this way, creating a barrier to the diffusion of water vapor and minimizing soil evaporation (Longhi et al., 2011).

Studies carried out with the production and accumulation of this material provide information for a better understanding of the dynamics of nutrients (Godinho et al., 2014). It is of great importance to emphasize the fact that in this compartment, the decomposition of its fractions occurs and nutrients return to the soil to be used by the vegetation.

The importance of evaluating litter production and accumulation lies in the understanding of the inflow and outflow of this material in the ecosystem, contributing to the understanding of the dynamics of nutrient transfers, as well as the functioning of the forest, also serving as a basis for the supply of information for projects to recover degraded areas (Farias et al., 2019).

Understanding the litter production flow in agroforestry systems is of great relevance to better understand the dynamics of this process in these land use models, therefore, mentioning the importance of the theme and the AFS, the aim of this work was to quantify the litter production in an agroforestry system located in the municipality of Castanhal, Pará.

MATERIAL AND METHODS

The study was carried out in an agroforestry system located at `Fazenda Escola de Castanhal`, Federal Rural University of the Amazon, Castanhal, Pará, located at the following geographic coordinates 1º17'42'' S and 45º55'00 W, 07º20'53” S latitude and 50º23'45” W longitude, 68 km away from the state capital, Belém (Figure 1).

Figure 1
Location of the municipality of Castanhal, Pará, Brazil.

The municipality where the study was carried out has humid climate, low water deficit in the dry period, which occurs between June and November and average temperature during all months equal to 18 °C (Megathermic) and Af subtype, according to the Koppen classification, which corresponds to rainy tropical climate, with average annual precipitation ranging from 2,500 mm to 3,000 mm and relative humidity ranging from 80% to 85 % (Valente et al., 2001; Iderflor-Bio, 2017).

The study was conducted between the months of June and September, a period known as the Amazonian summer, characterized by decrease in rainfall and increase in temperature. Climatic data (rainfall and temperature) were observed and rainfall decreased during the months of the experiment, with the highest value being recorded in July. There were no significant changes in the average temperature, between 26° and 27°C, as shown in Figure 2.

Figure 2
Rainfall (mm) and temperature data obtained from the National Institute of Meteorology (INMET).

The agroforestry system is nine years old and presents the following species: Euterpe oleraceae Mart., Tectona grandis L.f, and Theobroma grandiflorum Schum, being popularly known as açaí, teca and cupuaçu, respectively. Teca constitutes the arboreal component of the AFS and açaí and cupuaçu constitute the agricultural component. The agroforestry system has approximately 1 ha.

Twelve wooden collectors measuring 1 m² and thirty centimeters in height were installed. Collectors were covered with a 1 mm nylon mesh so that the material could be maintained inside each collector. Collectors were randomly distributed within the agroforestry system (Figure 3).

Figure 3
A: Installation of collectors in the Agroforestry System. B: Litter being collected by collectors.

All the material inside the collectors was collected and taken to the Laboratory of Management of Ecosystems and Hydrographic Basins, on the main campus of the Federal Rural University of the Amazon, for the litter to be sorted. For litter fractionation, steel clamps and plastic trays were used. Sorting was performed in the following fractions: leaves, twigs, reproductive material (flowers, fruits and seeds) and miscellany (Figure 4).

Figure 4
A: Greenhouse litter samples; B: Litter fractionation.

After the fractionation step, the material was placed in an oven and dried at 65 °C for 48 hours. After the end of this time, each fraction was weighed on a precision scale and its respective weight was recorded. The result was reported in mg.ha^-1.

Statistical analyses were performed using the RStudio software (version 4.0.5), with data being submitted to the Shapiro-Wilk normality test, after which, analysis of variance (ANOVA) was performed to verify whether or not there was significant difference between months of production and between litter fractions and later to the Tukey test (5%).

RESULTS AND DISCUSSION

The average litter production in the agroforestry system during the four months of study was 6.30 Mg.ha^-1. The month with the highest production was July, with average production equal to 2.25 Mg.ha^-1 and the month of September was characterized as the month with the lowest production, with average of 1.04 Mg.ha^-1.

It is important to emphasize that the production of this component is influenced by the local topography, by the species present in the area and by climatic factors, such as temperature and rainfall. During the experiment, it was observed that litter production followed rainfall (Figure 5).

Figure 5
Litter production in mg.ha -1 in Agroforestry System.

The results found in ANOVA showed that there was no significant difference in litter production during the months of study conduction (p-value > 5%). However, among litter fractions, significant difference was found between the average values (p-value < 5%).

It was found that litter production was higher in July, and this month also showed higher rainfall levels. As rainfall in the study area decreased, litter production also decreased. Litter production is directly linked to factors such as type of vegetation present in the area, soil and climate conditions and history of the area. It is noteworthy that litter production values ​​may vary from location to location, precisely due to these influences on the production and accumulation of this material.

Lima et al. (2015) reported that latitude and altitude, as well as the relief and stage of development of the vegetation are also factors that influence litter deposition. Some of these factors mentioned may have been associated to litter production in the present study.

The present research was carried out during the Amazonian summer period, therefore, it is recommended that litter production should be evaluated during the rainy season in the region in order to verify the influence of the climatic variables temperature and rainfall on litter production, as well as to find out if there is difference in litter production between the dry and rainy seasons.

In relation to litter fractions, leaves were more prominent, with average production equal to 4.20 Mg.ha^-1 of the total production. Twigs were the components with the lowest representation in litter production, with average production equal to 0.35 Mg.ha^-1 of the total material produced. Table 1 shows the Tukey test for the comparison of means of each litter fraction, with emphasis on the leaf fraction, which was higher in relation to the others.

Leaves accounted for 66.7% of all material collected by collectors, followed by reproductive material, with 16.5% of the total material, and the miscellany fraction and twigs with 11.3% and 5.5%, respectively. This can be explained by the fact that teca presents leaves with much larger dimensions than the leaves of the other species present in AFS.

Similar values ​​were obtained by Arato (2003) with the leaf fraction representing 67.46% of the litter. Pimentel et al., (2018) in a study carried out in two AFS in the Lower Amazon region found 62.77 and 47.30% representation of the leaf fraction in litter deposition in both agroforestry systems.

Rayol & Alvino-Rayol (2018) also in a study carried out in an agroforestry system in the Lower Amazon region, western state of Pará, observed that the leaf fraction was the most significant in the deposit of material for the formation of the litter layer, representing 81% of all material.

It is important to emphasize the contribution of leaves in the formation of the litter layer, since this fraction is the most representative in studies of this nature in different plant typologies. The litter layer is characterized as an important agent in forest ecosystems and studies that seek to understand its formation are relevant for the good understanding of the vegetation dynamics, as well as the dynamics of the formation of this layer.

Leaves were the main responsible for the formation of the litter layer in the agroforestry system under study. It is recommended that litter production should be evaluated during the rainy season in the region in order to verify if there is influence on the monthly litter production. The agroforestry system has the potential to offer ecosystem services, such as soil protection through the litter layer.

Acknowledgments

To the Federal Rural University of the Amazon (UFRA, Belém) and to the Fazenda Escola de Castanhal (FEC UFRA).

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