Chemical of Soils With Histic Horizon of Lakes and Riparian of the Savanna , Northern Amazonia , Brazil

The savanna of the northern Amazon of Brazil is an ecosystem that presents lakes and riparian of buriti (Mauritia flexuosa L.) in its landscape. Although these ecosystems are protected by law, they are subject to changes by anthropization in their surrounding areas. The soils of lakes and on the banks of the buriti riparian of the savanna of Boa Vista, Northern Amazonia, are hydromorphic and, although they are important ecosystems for the environmental sustainability, they are little studied. Thus, the purpose of this work was to characterize the chemical composition of these soils and relate the results to the type of anthropization. Samples were collected at the margins of five riparian and three lakes in the 0.0-0.1, 0.1-0.2, 0.2-0.3, 0.3-0.4, and 0.4-0.5 m depth layers. Soil samples, in each environment, were taken from three points equidistant at 100 m. For soil characterization, pH in water, H + Al, Al, P, K, Mg, Ca, and total organic C (TOC) were determined. The soils of lakes and buriti riparian are of high acidity, low natural fertility, and high toxicity by Al. Buriti riparian presented the highest levels of TOC but were also the environments most susceptible to the effects of anthropization, in which Bom Intento presented values of available P from medium to high by the interference of agricultural activities.


Introduction
Savanna is the term that refers to areas with open vegetation and which shelters typical plant species, formed by grasses, with sparse presence of small trees and shrubs.This term is used to designate the open vegetation of Roraima, of the Amazon biome, which is part of the "Guiana savannas" ecoregion, and is associated to a set of geological and geomorphological characteristics with physical, ecological, and floristic specificities, which distinguish it from cerrados (savanna) from other parts of Brazil (Almeida, 2016;Barbosa & Miranda, 2005;Barbosa, Campos, Pinto, & Fearnside, 2007;Bastos & Ferreira, 2010; T. M. Carvalho, C. M. Carvalho, & Morais, 2016).Savana is an ecosystem that is fragile and susceptible to the impact of human occupation and the use of fire (Furley, 2010;Soares et al., 2015;Zinck, 2011).(Morais & Carvalho, 2015).About 1% of the savanna area is covered by bodies of water, mountain refuges, and buriti palm tree, which, despite the small representativeness within the savannas, have unique functions and characteristics, contributing to the maintenance of water resources and transit of different animal and vegetable species (Barbosa et al., 2007).
One of the prominent features of the savanna is the hydrography formed by lakes and lentic environments, which, due to the accumulation of rainwater and the upwelling of groundwater, form drainage systems through which water flows, originating the buriti riparian (Bríglia-Ferreira, Röpke, & Alves-Gomes, 2012;Meneses, Costa, & Costa, 2007).The buriti (Mauritia flexuosa L.) riparian are lotic, shallow aquatic environments with a large amount of organic matter accumulated due to the presence of aquatic plants, such as cyperaceae and herbaceous plants, as well as lowland grasses (Matos, Nunes, Silva, & Oliveira, 2014).These formations of buriti palm tree lane can be distributed linealy along with drainage channels, creek (igarapés) or grouped, associated to the paleochannels (terraces) of rivers (Carvalho et al., 2016).
In Brazil, the areas of lakes and riparian of buriti palm tree are of permanent preservation (APP), which aim at the integrity of natural resources and maintenance of environmental sustainability, according to Law No. 12.651 (Brazil, 2012).
The soils of lake environments and buriti riparian of the Roraima savanna are hydromorphic, since in natural conditions they are poorly drained, saturated by water permanently or for a certain period of the year.Due to the water stagnation, the soils in these conditions present horizons with greyish and yellowish coloring, a characteristic of the Inceptsol (Gleysols).In addition, the accumulation of vegetal residues, responsible for the elevation of the organic matter content, gives rise to the histic horizon, which presents black, gray, or very dark coloration, characteristic of Hystosol (Vale Júnior & Schaefer, 2010).
The formation of Hystosol (peatlands) in these environments results from an imbalance between the accumulation and decomposition of organic materials whose deposition rate exceeds the decay rate, which is caused by insufficient or low biological activity as a consequence of adverse environmental factors, basically excessive acidity, in addition to saturation by water for a long time, establishing anoxic conditions.In tropical lowlands, fluctuation of groundwater level, controlled by rainfall and evapotranspiration, has an important effect on the formation of peats, especially in forest swamps (Ludang, Jaya, & Inoue, 2007).
Studies estimate the areas of peatlands in the Amazon around 150,000 km 2 , suggesting a great contribution of these environments to the global carbon (C) cycle, compared with other environments of the globe (Lähteenoja, Ruokolainen, Schulman, & Oinonen, 2009).Peatlands are defined as wetlands, where 80% of the area is covered by Hystosol, whose existence is a function of vegetation cover and water conditions (FAO, 1974).In these environments, anaerobic decomposition determines the type of soil organic matter (SOM) together with soil chemical conditions, weathering agents, biological nutrient cycling, and biogenic mineral production (Driessen, Deckers, Spaargaren, & Nachtergaele, 2001;Kelly, Chadwick, & Hilinski, 1998).Soil organic matter plays an important role in the biogeochemistry and ecology of streams, rivers, and lakes (Winterdahl et al., 2011) and is part of the global C cycle as a vector for transporting C from the terrestrial to aquatic and marine environments, followed by sedimentation and remineralization (Cole et al., 2007).Variations in vegetation structure evolve in response to peatland changes over time, so that tropical peat vegetation is not uniform (Page et al., 2004).However, little attention has been given to these types of environmental compartment, despite its importance in the global biogeochemical C cycle (Inubushi et al., 2003;Neue, Gaunt, Wang, Becker-Heidmann, & Quijano, 1997).
Anthropic interferences, whether for agricultural reasons or urbanization, in ecosystems with occurrence of Hystosol, alter the dynamics of organic matter, with reduction in the content and changes in quality over time (Stevenson, 1994;Zinck, 2011).The use and management practices of agricultural systems interfere in the natural balance of the processes of pedogenesis in the Hystosol, accelerating losses of C, N, K, and many other nutrients, thus altering chemical attributes and the relationships between humic substances (Pereira, Ebeling, Valladares, Anjos, & Espíndula Júnior, 2006), besides promoting the lowering of the superficial level, the phenomenon of subsidence (Valladares, Pereira, Anjos, & Ebeling, 2008).
In many scenarios, buriti riparian are springs, or drainage headwaters, that feed the watercourses that evolve into rivers.Its vegetation accompanies this evolution, promoting a natural vegetative gradient, which begins with buritis and progresses to the formation of riparian forest, as occurs in the Cerrado biome of Central Brazil (Pivello & Coutinho, 1996), which function as ecological corridors.
These transition zones that separate well-drained clays linking pediplanes and aquatic systems aim at intercepting highland nutrients (such as nitrogen, phosphorus, and organic matter) and contaminants, including jas.ccsenet.heavy met 2012).

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The soils are characterized as hydromorphic, since the buriti riparian and lakes are environments saturated by water for a long period of the year, with a thick organic layer (histic horizon), being classified as Organossolo and Gleissolo (Brazilian System of Soil Classification, Santos et al., 2013) corresponding to Histosol and Inceptisol (USA taxonomy) Histosol and Gleysol (WRB), respectively, resulting from high accumulations of plant residues, with varying degrees of decomposition (Santos et al., 2013).

Collection and Preparation of Soil Samples
Soil samples were collected in the 0.0-0.1,0.1-0.2,0.2-0.3,0.3-0.4,and 0.4-0.5 m depth layers in five buriti riparian and three lakes, considering the differences of environmental features within the study areas, with three replicates at each collection point, spaced 100 m apart.In the buriti riparian, samples were collected at a distance of 5 m from the water course and, in the lakes, a transept was made, starting 10 m from the border passing through the center during the dry season of the lakes.Deformed samples were collected in PVC pipes (0.70 × 0.30 m) because the saturation conditions by water does not to allow the opening of trench below 0.30 m depth.
After removal of the coarse root fractions, the samples were air-dried, crushed, and passed through 2-mm mesh sieves.

Soil Chemical Characterization
The chemical characterization of soils occurred according to Silva (2009).The pH was determined in water (1:2.5, v/v).Potential acidity (H + Al) was determined by extraction with 0.5 mol L -1 calcium acetate solution at pH 7.0.Exchangeable aluminum, calcium, and magnesium were extracted with 1.0 mol L -1 KCl solution.The Al 3+ was quantified by acid-base titration with 0.025 mol L -1 NaOH solution.Exchangeable Ca and Mg were quantified by complexometric titration with 0.0125 mol L -1 EDTA.Available P and K were extracted with acid double solution (0.05 mol L -1 HCl + 0.0125 mol L -1 H 2 SO 4 ), K determined by flame photometry and P by UV-visible molecular spectrophotometry.The sum of bases (SB), total cation exchange capacity (CTC), base saturation (V), and aluminum saturation (m) were calculated from the results obtained.Total organic carbon (TOC) was quantified by dry combustion on Perkin Elmer elemental analyzer series II 2400.

Statistical Analysis
The data were subjected to descriptive statistical analysis, represented in boxplot graphs, and the multivariate analysis.Principal Component Analysis (PCA) was used to determine the correlation between chemical attributes and collection sites (lake and buriti riparian), and between environments.In all analysis the software R 3.4 (R Core Team, 2017) was used in conjunction with the ExpDes.pt,factoextra, and FactoMineR packages (Ferreira, Cavalcanti, & Nogueira, 2018;Kassambara & Mundt, 2017;Lê, Josse, & Husson, 2008).

Chemical Characterization of Soils in Lakes and Buriti Riparian
Values of pH in H 2 O were low in the two environments evaluated (Figure 2).In the lakes, the pH values ranged from 4.42 to 4.74, and in the buriti riparian, from 4.13 to 4.82.In terms of acidity, the environments did not present expressive variations but indicated very acidic soils.However, these environments were less acidic than the peatland soils studied by Zinck (2011) in the savannas of Venezuela and Guyana.These environments, in terms of acidity, resemble the dryland soils of the savannas of the northern Amazonia, studied by Benedetti et al. (2011) and Feitosa, Vale Júnior, Schaefer, Sousa, & Nascimento (2016).This high acidity results from the advanced weathering of savanna soils, characteristic of tropical regions and low levels of bases.These values corroborate Valladares et al. (2008), which are the result of both the nature of plant material and of soil origin material, as opposed to soils of temperate regions with high base saturation (V), enriched by limestone and moss vegetation and swamp environments (Grozav & Rogobete, 2012).
The values of Al 3+ in the lakes ranged from 0.75 to 2.66 cmol c kg -1 (Figure 2B), whereas those of buriti riparian varied from 0.50 to 7.95 cmol c kg -1 .The higher values in buriti riparian are attributed to higher organic matter contents.The Casai and Bom Intento riparian had Al 3+ values higher than 4 cmol c kg -1 , which indicate an alitic character and higher toxicity (Sousa, Miranda, & Oliveira, 2007).The values found are below the Inceptsol (Gleysol) of the savanna of Roraima (Benedetti et al., 2011) and soils of buriti riparian of the southeastern region of Brazil (Guimarães, Araújo, & Corrêa, 2002).
The values of potential acidity (H + Al) were higher in the palm swamps than in the lakes (Figure 2C).According to Zinck (2011), the high values of potential acidity are related to the pH dependent surface load of

Available Phosphorus and Total Organic Carbon Between Lake and Buriti Riparian Environments
The CCA lake has the lowest levels of available P (Figure 8) and the highest TOC content.This result in relation to P is in agreement with Novais, Smyth, and Nunes (2007), who stated that humic substances block the adsorption centers of the mineral colloids of the soil, decreasing the P adsorption capacity.The Embrapa lake presented the highest levels of P, which may be related to soil genesis, with a sandy texture and no evidence of any source of P to the soil.The location of the lake close to experimental areas of agronomic research may be contributing to increasing of P as a function of its transport through runoff waters.
For the buriti riparian environments, the Bom Intento (BI) has the highest available P content (Figure 8).In addition, by analyzing the TOC content, this buriti riparian also presents the highest C content (g kg -1 ).This directly proportional relationship between total organic C content and organic P content contrasts with the values obtained by Novais et al. (2007), who concluded that the organic matter blocks the soil adsorption centers, decreasing the P adsorption capacity.This may be related to the direct and indirect effects that SOM has on the availability of nutrients (Dick, Novotny, Dieckow, & Bayer, 2009).
For buriti riparian environments, higher P and TOC contents were found in buriti riparian BI, which may be related to the indirect effects that SOM has on the availability of nutrients.The main loss process of SOM is the mineralization promoted by fungi and bacteria, and maintaining TOC content is fundamental to preserve the soil quality and its environmental functionality (Dick et al., 2009).
Figure 8. Mean values of available P and total organic carbon (TOC) in lakes and buriti riparian in the savanna of the Boa Vista region, Northern Amazonia, Brazil

Principal Component Analysis (PCA)
The principal components analysis (PCA) for the chemical attributes evaluated in the lakes and buriti riparian environments (Figure 9) explained 70% of the original variability of the data.The values of K + , Mg Figure 2. C