Wetlands in Brazil: classification, floristic composition and biological Nitrogen fixation

Wetland ecosystems represent about 20% of South America, and are classified according to the flood regime, which also influences on vegetation. Despite the value of ecosystem services provided by this environment, those areas are close to eradication in several parts of Brazil. These environments are extremely fragile. Flooded areas are subject to nitrogen losses (N) by leaching, becoming dependent on the N increases from biological nitrogen fixation (BNF). However, little is known about this process on wetlands. Understanding the adaptative strategies of these microorganisms and plants is essential for the maintenance and preservation of these ecosystems. The objective of this work is to present a literature review discussing aspects of floristic composition, biological nitrogen fixation, and morphophysiological adaptations that occur in the rhizobium-leguminous system in wetlands. For the bibliographic survey, articles and other academic works relevant to the topic were selected, in order to enrich the proposed discussion.


Introduction
Wetland ecosystems involve all forms of flooded soils with vegetation, such as swamps, marshes, and mangroves (Mitsch & Gosselink, 2000). In South America, it is estimated that 20% of the total area is subject to flooding during periods of excessive rain. According to Costanza et al. (2014), the value of ecosystem services provided per unit area is 10-100 fold higher in wetlands when compared to dryland, making the understanding of the functioning of these areas vital (Neori & Agami, 2017) The vegetation that occurs in these places can be classified, according to the flood regime, in alluvial and swamp forests (Silva et al., 2007), according to them, little is known about the species distribution patterns in flooded ecosystems, however, Marques et al. (2003), point out that, as they present a more selective, homogeneous and stable environment, swamp forests tend to have fewer species.
Despite their great importance as regulators of erosion processes, stabilization of margins, and promotion of nutrient cycling, the areas subject to flooding (permanent or temporary) are close to eradication in several parts of Brazil. These environments are extremely fragile and tend to have acidic soils, with slow drainage and high levels of organic matter, mainly due to the reduction in microbial activity caused by the absence or low concentration of oxygen.
Flooded areas are subject to nitrogen losses (N) by leaching, becoming dependent on the N increases from biological nitrogen fixation (BNF), where the rhizobium-leguminous association is the main contributor (Loureiro et al., 1998). However, legumes are, mainly, sensitive to flooding, producing additional mechanisms to increase the oxygen supply, making it possible to maintain the BNF process.
Little is known about BNF and nodular diazotrophic bacteria colonizing plant species from tropical forests (Cassetari, 2011). Krishnam et al. (2019), observed the formation of nodules in Sesbania herbacea (Mill.) under flooding conditions, the authors reported that flooding increases the number of nodules on Sesbania roots, and a BLAST analysis revealed a 100% sequence homology to 16S ribosomal RNA of Neorhizobium huautlense. Another studied by Brasil et al. (2016) about the influence of flood areas on the number of diazotrophic bacteria from pasture grasses showed that the presence of Azospirillum and Herbaspirillum presented high number in grasses Hymenachne amplexicaulis of permanent flood areas. Understanding the strategies used to increase the availability of N in flooded soils is essential for the maintenance and preservation of these ecosystems. The objective of this work is to present a literature review discussing aspects of floristic composition, biological nitrogen fixation, and morphophysiological adaptations that occur in the rhizobium-leguminous system in wetlands.

Methodology
In order to achieve the objectives, a systematic review was done. A systematic review is important because allows different points of view of the same subject. In addition, the use of this methodology identifies gaps in the literature, providing trustworthy data for future studies. The data collection followed four steps: planning, research, sorting, and analysis of the content ( Figure 1). Three data bases were used in this study: Scielo, Scopus, and Google Scholar, using the index terms: wetlands in Brazil, rhizobium, sustainability, and floristic composition. The inclusion criteria were based on the relevance of the paper for the research, as well as the language, selecting only paper in English or Portuguese. However, as exclusion criteria it was excluded the papers with a lot of similarity, being chosen those published most recently. Also, to select the papers, first it was reviewed the titles and abstracts, and articles that did not meet the objectives of the present study were excluded. Then, the full text of the articles previously selected was read, and if there was lack of relevant information, the paper was excluded.
Scanning and Skimming were the techniques used to read and select the texts. Skimming allows the reader to get a general overview of the material, and Scanning, on the other hand, permits to find specific information. Finally, once the papers were selected, the analysis of the content was done to summarize the main ideas, to fulfill the gaps, and answer the questions to achieve the literature review objectives.

Wetlands in Brazil
Wetlands cover approximately 20% of the Brazilian territory (Junk et al., 2015), and more than 90% are in the interior of the country, as a result of high precipitation and flat relief. In 1971, the first international wetland convention was held in Ramsar, Iran, which is characterized by being the first worldwide meeting related to the enhancement of these environments (Siman Gomes & Magalhaes Júnior, 2018) In 1993, Brazil became a member of the Ramsar convention, becoming responsible for surveying, classifying, and promoting management and conservation studies in its wetlands. However, little progress was noted in the application of these criteria, making, according to the National Institute of Wetlands (INAU), the ecological and environmental functions of these Brazilian ecosystems little known and undervalued in legal and socio-political terms.
Due to its territorial extension, Brazil has a wide variety of wetlands, according to Junk (2011), the heterogeneity is related to variations in the rainfall regime, creating a mosaic of different types of wetlands. Junk et al. (2015) indicate that there are about 111 terminologies found in the Brazilian legal system. Table 1 shows those that stand out according to the author. Research, Society and Development, v. 11, n. 2, e40911225787, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i2.25787

Popular name Region Characterization Baixada litorânea (Restinga)
Coastal area Bodies of shallow water and swamps between dunes on the coast, outcropping groundwater, with aquatic and palustres macrophytes, even forested.

Banhado
South of Brazil General designation of wetlands in Rio Grande do Sul.

Branquilhal/ Brejo
Paraná Lowland forest. A little specific name for waterlogged areas.

Campina/ Campinarama
Central Amazon Sandy areas with periodically soaked soils, covered by hydromorphic savanna vegetation.

Carnaubal
Coastal area Fresh water-soaked areas, dominated by the Carnaúba palm (Copernicia prunifera) and palm trees Caxetal Southeast/ south Peat / muddy forest with the dominance of Tabebuia cassinoides Lam.

Chavascal
Amazon Permanently soaked area, covered with highly flood resistant forest Estuário Brazil Coastal wetlands characterized as the final areas of rivers or lakes with strong influence of tides and saline water.

Igapó
Central Amazon Floodable area along rivers of black and clear water, poor in nutrients.

Lagoa
Brazil Permanent or temporary bodies of water throughout the national territory.

Laguna Costeira
Coastal area Bodies of water along the coast, usually of salinity and vegetation variable.

Lavrados
Roraima Savannah areas with lakes, swamps and footpaths dominated by Mauritia flexuosa.

Manguezal
Coastal area Coastal ecosystem, which occupies muddy, clayey or sandy sedimentary deposits up to the upper limit of the equinocial high tides.

Mata Ciliar
Brazil Wetland around bodies of water Mata ripária/ Mata de galeria Brazil Periodically flooded forest along rivers.

Mata turfosa/ paludosa
Southeast/ South It is characterized by a very particular floristic and structure, differentiating itself from other forest formations by its species capable of germinating and growing under conditions of water saturation.

Nascente/ Olho d'água
Brazil Areas for discharging water from groundwater or subsurface water.

Pântano
Brazil A specific name for waterlogged areas.

Restinga
Coastal área Bodies of shallow water and swamps between dunes on the coast, outcropping groundwater Turfeira South Brazil Small humid areas located in high altitude areas or on the coastal plain with a high concentration of decomposing organic matter and low pH (acidic waters).

Vargem
Brazil Any type of periodically flooded area.

Várzea
Central Amazon Floodable area along white water rivers of Andean origin, rich in nutrients.

Várzea
Other regions Any type of periodically flooded area.

Vereda
Brazilian cerrado Permanently humid area, covered by grassy herbaceous vegetation.

Wetland Classification Systems in Brazil
There are several instruments used for the classification of wetlands internationally. When classified in systems there are two trends, horizontal and vertical (hierarchical). Horizontal divide habitats into classes or types, while hierarchical are separate into levels, from general to specific characteristics (Siman Gomes & Magalhães Junior, 2018). According to Junk et al. (2021), the parameters used to classify these habitats should address the specific, well-defined characteristics of each macrohabitat, hence, allowing the establishment of a databank with the necessary information. In

Floristic composition in wetlands
The areas subject to flooding occur predominantly on the shores of rivers and lakes, or outcrops of the groundwater (Silva et al., 2007), and their floristic pattern is determined by the climate, edaphic factors, surrounding vegetation as a source of propagule (Rodrigues & Shepherd, 2000), anthropic actions and periodicity, duration and depth of flooding (Junk, 1993). Swampy forests are those that remain permanently flooded, being, according to Teixeira and Assis (2011), environments that have low species diversity and high local densities. Silva et al. (2007), characterize the floristic composition of these areas as homogeneous. The lower number of species in the swampy forests can be explained by the more selective, homogeneous, and stable environment, with constant flooding throughout the year (Marques et al., 2003). Table 2 shows the main families found in these environments, according to some surveys. Alluvial forests, on the other hand, are vegetations subject to temporary flooding (Silva et al., 2007), these environments are distributed over the most different areas of the country, presenting remarkable compositions of biodiversity (Ab'saber, 2001). Table 3 indicates the families that stand out in the floristic composition of alluvial areas in some studies. Riverside is another type of flooded environment, with a characteristic vegetal formation associated with water bodies (Oliveira-Filho, 1994), being considered areas that have an important role as corridors of plant and animal gene flow (Marinho Filho and Gastal, 2004). A floristic study carried out by Lacerda et al. (2010), in riverside areas of the caatinga, indicated a total of 91 species being Fabaceae, Euphorbiaceae, and Rubiaceae the families with the largest number of individuals and genera. The authors found that the greatest floristic identity is mainly related to the geographical distance and the characteristics of land use.
Considering the impacts of global climate change, the importance of wetlands tends to increase, making studies on the characterization of these ecosystems increasingly necessary.

Legumes in wetlands
Leguminosae (Fabaceae) is the third-largest family of Angiosperms, including about 760 genera and 19,500 species (Yahara et al., 2013), covering a great diversity of growth patterns (Doyle and Luckow, 2003). The height and duration of the periodic flooding induce the appearance of changes in the ecophysiological behavior of trees that colonize flooded areas, making it possible to adapt to conditions of oxygen scarcity for long periods (Wittmann et al., 2006). Research, Society andDevelopment, v. 11, n. 2, e40911225787, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i2.25787 7 These tree species survive in a dormant state and may also show vigorous growth in the flooded phase (Parolin et al., 2004). However, legumes are generally sensitive to flooding, making it a limiting factor for their growth (Loureiro et al., 1998). Table 4 shows the main legume species found in wetlands in the south and southeast regions of the country. Table 4: Legumes in wetlands in the south and southeast regions of Brazil.
Also noteworthy for species underwater saturation conditions are phenological and reproductive adaptations, the former being related to leaf loss, fruit ripening, and seed release, and the latter associated with submersion tolerance, seed dormancy, and immediate germination (Parolin, 2012;Kolowski, 1997). Pires et al. (2002), evaluating the effects of flooding on the morphophysiological characteristics of soybeans, observed that the main changes were in the roots, where was noted the death of the main root, the growth of lateral roots, and the appearance of adventitious roots, in addition to the decline of the levels of nutrients in the leaves.

Biological Nitrogen Fixation in wetlands
Nitrogen, even though it is one of the elements in greatest concentration in the atmosphere (78%), is found in a form not available (N2) for most living beings, including plants. N2-fixing bacteria, known as diazotrophic bacteria, can fix N2 directly from the atmosphere through the biological nitrogen fixation process (BNF). (Cleveland et al., 1999;Boddey et al., 2000).
Plants in symbiosis with diazotrophic bacteria can occupy different ecosystems, adapting to the wide variety of environmental stress. Some species of actinorhizal plants are very well adapted to wetlands, arid regions, contaminated soils, extreme pH and high salinity, and, due to these properties, some of these plants are pioneers that colonize disturbed areas (Santi et al., 2013). However, soil flooding impairs the nodulation of legumes and inhibits N2 fixation in previously formed nodules (Jackson, 1985).
The BNF process is closely related to edaphoclimatic factors, and wetlands are often subject to annual net nitrogen losses via leaching and are therefore largely dependent on the biological fixation process to ensure the entry of N into the system (Loureiro et al., 1998). Therefore, as highlighted by Hu et al. (2021), nitrogen is a common limiting nutrient for plant yield in wetlands. Consequently, most legumes that associate with diazotrophic bacteria in flooded regions have developed additional mechanisms that increase the supply of oxygen to their nodules, thus maintaining the capacity to fix nitrogen (Loureiro et al., 1998).
Legumes in flood conditions may have a limited supply of O2 for root nodules. In these environments, according to Ladha et al. (1992), stem nodulation is an advantage. Loureiro (1994), states that the stem nodules receive oxygen via aerenchyma, which allows the diffusion of gases. James et al. (2001), studying flooding-tolerant legume symbioses from the Brazilian Pantanal, observed nodules on the stem of Discolobium leptophyllum. Krishnam et al. (2019), observed that the nodules of Sesbania herbacea grown in flooded soils were larger and more numerous concerning the ones of plants grown in dry soils. The same effect was seen by Kanu and Dakora (2015), in a study with Psoralea pinnata (L.), which in flooded areas presented nodules with greater area and volume when compared to nonflooded regions. These authors also observed that the nodules had six components: lenticels, periderm, cortex (internal, middle, and external), and infection by bacteria in the central region of the spinal cord.

Final Considerations
Although wetlands represent about 20% of Brazilian territory, these ecosystems are unprotected by law, mainly due to differences in their concepts and classification criteria. Wetlands are present in all biomes of the country. However, some states still lack a specific classification system that serves as a basis to ensure the preservation and maintenance of these environments.
Studies focused on these ecosystems are essential given the importance of environmental services, which have immeasurable value and ensure the sustainability of processes. The relationship between rhizobium and leguminous plants has been studied in the most diverse environments. Therefore, for wetlands, little is known about this process. The biological nitrogen fixation is considered the main mean of entry of nitrogen into these systems being so, knowing and understanding the adaptive strategies and identifying the species involved is necessary for the maintenance of biodiversity and preparation of management and conservation programs for wetlands.
In a nutshell, further studies are needed for Brazil to create more effective environmental laws, and to encourage research related to the subject and develop specific classification systems for the states to comply with the agreement signed by the Ramsar Convention. In addition, future works aiming to understand how plants and microorganisms adapt in those ecosystems are essential to create public awareness about the importance of conservating these areas.