Nutrients Available and Stable Carbon from Archaeological Black Earth

The present study aims affinity between available concentrations of phosphorus, micronutrients (Cu, Zn and Mn) and stable carbon (SC) in archaeological black earth (ABE) from area “Ilha de Terra”/Caxiuanã—Pará. The re-levance of this study refers especially to the determination of carbon concentrations (total, oxidable and semi-labile carbon) directly associated with those of pyrogenic carbon, common in ABE, which should add their contributions to the understanding of soil organic matter recalcitrance. Chemical properties such as CEC and base saturation were determined for ABE and subjacent Latossoil. The available concentrations of nutrients were performed from sequential extraction F1 to F5 phases. Phosphorus was obtained by spectrophotometric method and micronutrients by MP-AES. TOC was determined by the combustion method; stable carbon was obtained from thermoxidation method CTO-375. The following results were obtained from ABE: CEC effective = 21 to 28 (cmolc·L −1 ); base saturation = 58% to 69%; Carbon concentrations (%): TOC = 2.95 to 3.94; SC = 0.25 to 0.88; semi-labile carbon = 1.75 to 3.63; oxidable = 2.11 to 3.65; inorganic carbon 0.01 to 0.38; P 2 O 5 concentrations (mg·kg −1 ) in the ABE phases F3 (Fe-Mn oxides, F4 (organic ) and F5 (residual)) in the following order: 35 to 65; 200 to 400; 140 to 230. It was concluded that the chemical properties in ABE from Ilha de Terra site fall within the range of fertile and the high concentrations of phosphorus are biogenic origin. Phases F3 and F4 are those considered nutrient stocks from ABE studied area. http://creativecommons.org/licenses/by/4.0/


Introduction
Fertility in soils known as archaeological black earth (ABE) is generally superior to other classes of soils in the Amazon and nutrients losses from leaching are low, thus reflecting the high stability of organic matter in ABE [1] [2] [3] in relation to adjacent soils and other types of soils and this characteristic fits among the main lines of research in the area of Soil Science with the application of methods of chemical characterization of organic matter of tpas, aiming especially the determination of properties indicating the high stability of organic matter in ABE [2] [3] [4] [5].
The ABE occurred as dark spots, distributed in several places in the central Amazon. It was identified by, being most well-known in places near the city of Manaus [6], such as: 1) occurrences in the district of Iranduba, collected in seven places under tertiary and quaternary sediments [7]; 2) in occurrences, distributed in different districts as anthropogenic soils over forest and cultivation [8]; 3) occurrences in the district of Manacapuru at two sites, Jiquitaia and Laranjal [9] contrasting environments. The first under Holocene sediments with clayey texture, and the second under mature sediments from Alter do Chão Formation.
In the occurrences of ABE referred to above, the concentrations of total organic carbon, phosphorus, copper, iron, manganese and zinc were evaluated [10].
In the region of Caxiuanã, located in the western portion of the state of Pará, between Portel and Melgaço districts studies were carried out on the geology, archeology and geochemistry of the region, with several occurrences of ABE being recorded, such as those of the Manduquinha sites, Ponta Alegre, Mina II and Ilha de Terra, these occurrences being recorded under Latosols, correlated with the Alter do Chão Formation [11].
The studies carried out on the Ilha de Terra site, involving: the chemical characterization in terms of the distribution of phosphorus, Ca, Mg and organic matter for the purpose of reconstituting past human behavior in a Tpa area of this site; from the O/C atomic ratio, based on SEM-EDS analysis of materials resulting from interactions of minerals with pyrogenic carbon [12]. The known methods for the quantification of pyrogenic carbon (PyC) in carbonaceous materials present the basic principle of PyC thermoxidation. the method named Molecular Marker [13] [14] [15] is included among the first published and involves several steps, such as: (a) thermoxidation after digestion with hot HNO 3 and consequent formation of polycarboxylic benzene acids (BPCAs); elimination of interfering cations by means of highly acidified cation exchange resins; poster treatments with organic substances, aiming at obtaining trimethylsilyl derivatives, which will be stabilized for at least a week to prevent the samples from decomposing through contact with water and air; finally quantification by gas

Study Area
The study area is located approximately 400 km west of Belém, between the mu-

Materials
The samples selected in this study were three in the A1 and A2 horizons of ABE plus the sample of the underlying Latosoil AB horizon (Ls-AB) at points indicated on the sampling map, as illustrated in a previous study [12], making a total of seven samples ( Table 1). All reagents used in this work were of analytical grade and had high levels of purity.

1) Chemical properties of ABE and subjacent Latossoil
The main chemical properties inherent to soil fertility are determined and interpreted according to the analytical procedures described in Manuals of chemical analysis of soils [24], being thus determined: sum of the bases (SB),

5) Calculate the concentration of inorganic carbon
The concentration is obtained from the difference between the masses m1 and m2, indicated in steps 1 and 3.

6) Concentration semi-labile carbon
The concentration of semi-labile carbon can be estimated from data obtained during thermoxidation treatments, being then estimated in this study by the difference between calculating the concentration of inorganic carbon: The concentration is obtained from the difference between the masses m1 and m2, indicated in steps 1 and 3.

Chemical Characteristics that Express Soil Fertility
The samples of Tpas and oxisol analyzed show (Table 2)

1) Concentrations of different forms of carbon
The data of oxidable carbon concentrations from ABE samples in this study (Table 3), corresponding to about 88% of the TOC concentrations determined sandy-clay texture [6]. In the organic phase, the high concentrations of phosphorus obtained are justified by the presence of residues of human or animal bones observed in the sampling area.

4) Copper concentration
The copper concentrations obtained in phases P1 and P2 fall into the high level for plants (> 1.8 mg·L −1 ) [24]. The ionic form of copper in this phase is Cu 2+ that is predominant between pH = 3 to 5 [28], tipic in ABE, according to values indicated for samples in this study (Table 5).
In the evaluation of the concentrations in phases f1 to f3 with those indicated in ABE other occurrences it was verified that the values are higher than the obtained in the of the municipality of Juruti (0.01 to 0.45 mg·kg −1 ) [29] and in the Laranjal community, (2, 3 and 6 mg·kg −1 ) [6]; included in the range obtained in ABE of the Manduquinha site (C = 10 to 14 mg·kg −1 ) [11]; interval much lower than that of Tpas in the district of Iranduba, close to Manaus (90 to 104 mg·kg −1 ) [6].

5) Concentrations of manganese
The concentrations of manganese (  [11] [12]. In the oxide fraction of Fe-Mn, manganese occurs in residues of goethite and hematite, resistant to the interaction process with pyrogenic carbon, according to observations by SEM/EDS made in a previous study [32].
In the F4 fraction, manganese has the lowest concentrations, and this aspect is justified because it was not made available under the conditions of the applied extraction, thus being stored in organic aggregates from mineral interactions with pyrogenic carbon [33].
In the assessment of manganese concentrations in Tpa samples in this study with those from other locations, it was found that: in phases F1 to F3, they are lower than the values presented for Tpas in the municipality of Juruti (6.15 to 48.5 mg·kg −1 ) [32] which were obtained by the same method.

6) Zinc concentrations
The average zinc concentration in the exchangeable fraction (Table 7) is high for plants (>2.2 mg·L −1 ) [24]. In fractions F2, zinc occurs not available in carbonateapatite residues from human or animal bones, common in ABE and observed in a previous study. In the oxide fraction of Fe-Mn, manganese occurs in residues of kaolinite, resistant to the interaction process with pyrogenic carbon, according to observations by SEM/EDS made in a previous study [12].
In the evaluation of zinc concentrations in Tpa samples in this study with those from other locations it was found that: in phases P1 to P3 the concentrations are lower than those indicated for Tpa in the municipality of Juruti (2.38 to 12.61 mg·kg −1 ) [29] and in ABE of districts near to Manaus, such as Iranduba (>150 mg·kg −1 ) [1] and Laranjal Tpa with clay texture (275 to 325 mg·kg −1 ) [6].
Variations in the concentrations of P 2 O 5 , Cu, Mn, Zn in phases F1 to F4 are in relation to the concentrations of total and stable organic carbon.
In the exchangeable phase (Figure 1), it is observed that the concentrations of copper, manganese and zinc are more affected by the increase in the concentrations of TOC than those of phosphorus. This aspect is not observed for the distribution of nutrients in relation to the stable carbon. In the phase bound to carbonates ( Figure 2) the distribution of phosphorus concentrations in relation to carbon concentrations is more homogeneous and without prominence in samples with lower concentrations of TOC and stable carbon, while the distribution of micronutrients has the opposite aspect [34].
Phosphorus in this fraction should be gradually made available from relics of human and animal bones, just as it occurs in other archaeological sites in various regions of the Brazilian Amazon, such as in places close to Manaus [6], in the district of Iranduba, Manicoré and in the other sites of the Caxiuanã region [11].
In the exchangeable fraction (Figure 3

Conclusions
The study involving the determination of concentrations of available nutrients of phosphorus, copper, manganese and zinc available in ABE from different sequential solid phases P1 to P5 (associated with exchangeable, carbonates, Fe-Mn oxides, organic and residual), total and stable Carbon and other chemical properties in ABE from the archaeological site Ilha de Terra. The following conclusions were obtained: 1) The attributes obtained for the control of nutrients in ABE and Latosoil indicated: values for the base saturation index V > 50%, which fit as eutrophic.
2) In the evaluation of available concentrations of nutrients in relation to the stable carbon and TOC concentrations, it was concluded that the Fe-Mn oxide and organic phases represent the nutrient stocks phases in ABE of the Ilha de Terra site.
3) The high concentrations of phosphorus obtained are justified by the presence of residues of human or animal bones observed in the sampling area. 5) The concentrations of inorganic carbon in ABE of the studied site are higher than those indicated in other occurrences in the central Amazon. This aspect is justified due to its biogenic origin.