EFFECTS OF LONG-TERM MANAGEMENT ON SOIL ORGANIC CARBON IN TROPICAL PEATLANDS

Objective: The aim of this study was to evaluate the effects of Histosols management on soil organic matter (SOM) characteristics. Theoretical framework: Histosols are soils that provide ecosystem services, especially carbon stocks. Among the environments in which these soils are formed, peatlands stand out. The conversion of these areas into agricultural systems requires soil drainage. This practice causes reductions in carbon content and changes in the structure of the SOM. Methodology: The study was conducted in the municipality of Rio de Janeiro, Brazil. The soils were classified as Histosols. Three areas were selected: a) cassava ( Manihot esculenta ) cultivation for 80 years; b) forest fragment for 20 years; c) consortium of coconut palm ( Cocos nucifera ) and cassava for 20 years. Total organic carbon (TOC); particulate organic carbon (COP); organic carbon associated with minerals (COam); fulvic acid fraction carbon (CAF); humic acid fraction carbon (CAH) and; humin fraction carbon (CHUM) were determined. Solid-state spectroscopy (UV-vis, ATR-FTIR and 13C NMR CP/MAS) was carried out to characterize the structure of the SOM. Results and conclusion: The results showed that management and drainage have an impact on the TOC content, fractions and chemical structure of the SOM. The management adopted in the cassava area had an impact on the labile and humified fractions of the SOM. Spectroscopy showed that labile structures were lost in the humic acids (HA), while in the forest area only recalcitrant structures were preserved.


INTRODUCTION
Peatlands are considered to be the largest carbon deposits in the world, storing about one third of land carbon (Morris et al., 2018).Histosols that perform important ecosystem services are found in these areas, storing about 650 Gt of organic carbon, which corresponds to 21% of the global stock (Yu et al., 2010).
Inappropriate use of Organossolos can result in the emission of large amounts of greenhouse gases into the atmosphere.It is estimated that each year, on a global scale, only the drainage effect in Organossolos caused the emission of 2 Gt CO2-eq (Leifeld & Menichetti, 2018).In addition to greenhouse gas (GHG) emissions, the degradation of these areas promotes biodiversity loss and decreased water quality (Koivunen et al., 2023).The understanding, preservation and proper use of these soils is key to maintaining the overall ecological balance.
Due to the lower expression of the mineral composition, which acts as a stabilizing agent, Organossolos are susceptible to climate and management changes (Schimmel & Amelung, 2022).Despite the small geographical expression, due to their productive potential, they are widely used in agriculture, which if not carried out properly, can convert these environments into areas with high emission rates of GEe.
Several research studies have been conducted to show the effects of management on chemical, physical and soil organic matter (MOS) attributes in Organossolos (Oliveira Filho et al., 2021;Santos et al., 2020a).The study of the properties of organic matter in Organossolos, using fractionation techniques, has provided reliable quantitative information, so that these properties are used as indicators of the agricultural impacts on these soils.Santos et al. (2020a) found that long-term management and drainage of Organossolos results in reduced MOS and carbon stock, primarily affecting its labile fraction.Wang et al. (2017) observed that after Organossolos drainage there was greater mineralization of TOC when compared to oxidizable carbon (more labile fraction).In this study, the authors concluded that the management had greater effects on the more stable fractions of MOS, transforming them into more labile fractions.However, little is known about the impacts of management on the chemical structure of the MOS of these soils.
The use of techniques such as Fourier Transform Infrared Spectroscopy (FTIR) and Solid Carbon-13 Nuclear Magnetic Resonance (13C NMR.), coupled with chemometric techniques, has been used to characterize the structure of the humic substances (SH) of MOS.Spectroscopic techniques are fundamental for understanding the structural complexity of SH and the chemical interactions performed between MOS and other soil fractions and components.The set of these techniques has been used to identify structural changes in the organic matter of mineral soils, submitted to different forms of handling (Santos et al., 2020b;Moura et al., 2022).Despite this, there is still a lack of studies evaluating the main chemical and structural changes of organic matter in Organossolos submitted to different forms of handling.
From the above, the objective of this study is to quantify the carbon contents and their fractions and use the spectroscopic structural characterization and chemometric techniques to evaluate the impact of drainage and long-term management on the chemical structure of the SH of Organossolos soils in Peatlands.

THEORETICAL FRAME
Peatlands are ecosystems formed by organic soils, mainly Organossolos, which have high levels of organic matter resulting from the decomposition of plants, which develop in water-saturated areas.The retention of the deposited biomass in conditions of excessive humidity, low pH level and anaerobiosis, delays the process of mineralization and humification, favoring the accumulation of MOS in these environments (Silva et al., 2020).
The presence of oxygen regulates MOS mineralization rates in Organossolos, stimulating oxidative enzymes and promoting aerobic respiration.Drainage from flooded areas exposes Organossolos to increased concentrations of oxygen, resulting in a significant increase in CO2 release rates, up to twice or more compared to anaerobic conditions (McNicol & Silver, 2015).
Numerous areas of Organossolos have been drained for agricultural purposes, resulting in considerable carbon (C) losses and substantial greenhouse gas emissions.Increased oxidation of MOS and, consequently, release of carbon dioxide (CO2) into the atmosphere were observed after drainage or natural drying of soil in tropical regions (Moore et al., 2013).
Soil organic matter acts on soil stability, water retention, preservation of biodiversity and supply of nutrients to soil (Silva, 2023).MOS particle size fractions (COp and COam) play a key role in soil carbon storage, and their preservation is influenced by the organic matter additions and handling practices employed (Kuneski et al., 2023).
Soil organic carbon (SOC), as an essential component of MOS, plays a key role in influencing soil processes.Human action impacts these processes, resulting in carbon loss or storage/sequestration.It is widely recognized that climate change is directly linked to carbon, making SOC a crucial indicator in monitoring changes in soil (Duval et al., 2018).
SOC-related studies employ techniques that include chemical and physical fractionation to analyze distinct fractions.Although there are some uncertainties associated with the chemical extraction of humic fractions, this approach is effective in the evaluation of SOC fractions (Weber et al., 2018).
SOC stabilization mechanisms depend on chemical interactions between MOS and the mineral fraction.The complexity of the organic molecular structure, together with the chemical, physical and biological conditions of the soil, regulates the mechanisms of interaction and therefore mineralization and/or humification processes, which generate carbon stock.The chemical mechanisms for the humification of MOS have been studied, mainly because of the different properties and functions that they can present (Moura et al., 2022).
Substances of this kind are formed by complex molecules, without a defined structure, diverse in composition and susceptible to chemical reactions, resulting from the decomposition of organic matter.These molecules tend to associate based on their conformation, size, chemical affinity, hydrophobicity, and the characterization of their structure is restricted by the forces that stabilize their interactions in a supramolecular configuration.The compound known as humic acid (AH) is a subdivision of this set, capable of binding to the mineral part of the soil or remaining soluble in the solution, thus becoming an intermediate part in the humification process (Souza et al., 2020).
The function of humic acids (AH) in their interactions with the soil can be understood by means of the relationship between structure, property and function.The functionality of the molecule is established by the properties that arise and are influenced by its structure and by the stereochemical configuration of the bonds formed.The structural diversity and heterogeneity of humic acids, both in terms of organization and functional groups, justifies the various interactions that can occur in the soil.The structural characteristics of these materials, such as carbon types (sp² and sp³), functional groups (-COOH, -OH, -NH2, -C=O) and interactions between structural fragments (π-π, CH-π, van der Waals, hydrogen bonding), define the properties that give rise to and determine their distinct functions (García et al., 2016).
The application of structural characterization techniques such as Fourier Transform Infrared (FTIR) spectroscopy and Solid State Carbon-13 Nuclear Magnetic Resonance ( 13 C NMR. ), together with the use of chemometrics, provides the ability to perform an exploratory analysis of the structural characteristics of humic acids (AH).Studies employing these techniques have demonstrated the advantages of this approach, making it possible to establish relationships between the structure of soil organic matter and its properties by means of spectral processing.(Assunção et al., 2019;Diniz et al., 2023).

Area of Study and Strategy for Collecting ofSamples
The study was carried out in the municipality of Rio de Janeiro, RJ, southeastern Brazil.The soils in the region are poorly drained, formed from quaternary sediments, with high levels of salts, sulfur and organic matter.The climate of the region was classified as Aw, humid tropical with dry winter (Koppen).The soils described and collected were classified as Organossolos.
With the aim of converting to agriculture, the area was drained.The drains installed are 1 m wide and 0.7 m deep.The culture initially cultivated was cassava (Manihot esculenta).The management adopted in the area consists of plowing up to 0.20 m deep before planting, which is carried out in beds of 0.30 m in height, with spacing varying from 0.6 to 1.0 m between the lines and between the plants, respectively.
In 1999, in part of the area, the cultivation of coconut tree (Cocos nucifera) began with cassava between the lines, with spacing for coconut trees of 5.0 m between lines and 5.0 m between plants, and for cassava used the same spacing as previously reported.An area previously used for the planting of cassava was also selected and abandoned 20 years ago.In this area, natural regeneration is currently taking place.Organic or mineral fertilization was not performed in the sampled areas.At the time of collection, in March 2019, the cassava area was cultivated and drained 80 years ago, the coconut tree area with cassava 20 years ago and the forest fragment area had been regenerating for 20 years.
These areas were considered as treatments and in each one of them three random sampling points (repetitions) were selected, 20m apart.At each point, a trench was opened for carrying out morphological description and sampling of soil profiles, according to Santos et al. (2015) (Figure 1).After the morphological description, the samples were collected, packaged and transported to the laboratory, and subsequently dried in the air, defaced and sieved with 2.0 mm mesh to obtain air-dried fine earth (ADFE).For the total organic carbon (TOC) analysis, the TFSA was macerated and passed through a mesh sieve with a mesh diameter of 0.178 mm.

Organic Carbon Analyzes
Total organic carbon (TOC) was quantified by wet oxidation with potassium dichromate (K2Cr2O7) in sulfuric media Yeomans and Bremner (1988).The physical fractionation of the soil samples was carried out following the procedure described by Cambardella and Elliot (1992).The fraction of particle-associated organic matter (COp) was separated by acid hydrolysis (Wang et al., 2017) and measured by wet oxidation with K2Cr2O70,167 mol L -1 (Yeomans & Bremner, 1988).The organic carbon content associated with minerals (COam) was due to the difference between TOC and COp values.
Chemical fractionation was performed according to Swift (1996) and adapted by Benites et al. (2003), yielding three fractions: Fulvic acid (AF), humic acid (AH) and humine (HUM).The carbon content of the fractions was determined by wet oxidation with K2Cr2O7.

Separation, Purification and Characterization of Humic Acids
The extraction and purification of humic acids (AH) in soil samples was carried out according to the method proposed by the International Humic Substances Society (IHSS) Swift (1996).The AHs were characterized using ATR-FTIR spectroscopy, obtained by recording in the wavelength region from 400 to 4000 cm -1 , collecting 32 scans at each measurement.A VERTEX 70/70v FTIR spectrometer coupled to a total attenuated reflection (ATR) device was used.The obtaining and registration of the spectra was carried out using OPUS-Bruker software.
Eighteen ATR-FTIR spectra were used, two for each soil profile, one being the most superficial horizon, and another the most subsurface.The spectra were subjected to baseline correction.The "smoothing" algorithm (Savitsky-Golay) was used in all spectra to reduce noise and increase the noise signal ratio.The spectra were indicated according to the location of the band indicating the characteristic vibration of each functional grouping using the PeakPicking tool.In order to identify the structure of the charred fragments in the AHs, the technique of nuclear magnetic resonance (NMR.), cross polarization (CP) and magic angle rotation (MAS) was used.The MAS 13 C-NMR.CP analysis was performed on a Bruker AVANCE II RMN 400 MHz apparatus, equipped with 4 mm Narrow MAS probe and operating in a resonance sequence from 13 C to 100,163 MHz.The samples were placed in a ZrO2 rotor with a Kel-F cap with a rotating frequency of 8 ± 1 kHz.The spectra were obtained by collecting 3000 data points for the same number of scans at an acquisition time of 34 ms and with recycle delay of 5s and contact time of 2 ms.The spectra were processed using the Bruker Topspin Software 2.1.Induction-free decays (DLI) were transformed by applying a zero filling equal to 4 k and later a 70 Hz line broadening.
In total, 18 spectra were analyzed in the ACD/Labs Processor Software, considering the displacement regions: Non  (SONG et al., 200008).

Statistical Analysis and Chemometry
The chemometric parameters were obtained from Software The Unscrambler (version 10.4) (Camo Software AS, Nedre Vollgate 8, Oslo, Norway).Principal component analysis (PCA) was performed from the normalized matrix by loading the ATR-FTIR and 13 C NMR. CP/MAS spectra using the "import data" function of the spectral files saved in the form of JCAMP, considering an initial sample matrix:variable (18 x 4096).The ATR-FTIR and 13 C NMR. CP/MAS matrix was subjected to line plotting for visual inspection and then transformed by "normalization", "smoothing" and baseline correction (baseline correction-baseline offsetlinear baseline correction).
In order to obtain the average spectra, the descriptive statistical tool "descriptive analyses" was used and then the matrix "results" was accessed for plotting in the line form of the mean obtained in the analysis.The matrix corresponding to the mean spectra for each soil type was then added to the original matrix for multivariate analyzes of the data.
PCA was conducted to overcome the difficulties associated with interpreting large arrays of data containing a large amount of partially hidden information.The objectives of the BCP are to find the differences and similarities between the samples and find the variables that contribute most to these observations.TOC and fractionation data were analyzed by descriptive statistics, submitted to analysis of variance (ANOVA) followed by tukey test (5%).Multivariate analysis (MPA) was also carried out with the aid of the R program (R Team, 2021).

Organic Particulate Carbon (COP) and Organic Carbon Associated with Minerals
For the COP values (Figure 3.A), the highest (149.2g kg -1 ) was observed in the P9 profile in the consortium area, while the lowest value (65.9 g kg -1 ) in the cassava area, and in this area also the lowest values were verified, differing significantly from the other areas in all horizons and profiles.For COam (Figure 3.B) the COam levels ranged from 8,6 g kg -1 to 114,3 g kg -1 .Despite this wide variation, no significant differences were observed between the areas.For the CAF values, the variation was from 1.7 to 29.5 g kg -1 , with significant differences between the three areas studied.The highest values were quantified in the cassava area, lowest in the forest fragment area and intermediaries in the consortium area (Figure 4.A).Between the horizons, the highest values were observed in the surface of the P1 and P2 profiles (cassava) and the lowest in the subsurface of the P4 and P5 profiles (forest fragment).
CAH levels ranged from 33.58 to 125.40 g kg -1 .The highest levels were quantified in the area of the consortium, differing significantly from the forest fragment area, where intermediate values were observed, and from the cassava area, where the lowest values were found for this fraction among all areas (Figure 4.B).
Through the analysis of Figure 4.C it is observed that the management adopted promoted negative effect on the levels of CHUM in the cassava area, resulting in significant difference of values when compared with those of forest fragment areas and consortium.The variation observed for this fraction was 23.0 g kg -1 (Horizon Hdp2 of profile 1 of cassava area) to 56.1 g kg -1 (Horizon Hdp1 of profile 9 of consortium area).

Structural Characterization of HA
4.2.1 Spectroscopic characterization by solid-state nuclear magnetic resonance (NMR.) 13 C isotope in cross polarization and magic angle rotation (CP MAS 13 C-NMR.) The spectra of humic acids (HAs) extracted from the Organossolos Tiomórficos are shown in Figure 5.A. Through the analysis of the Figure, one notices a spectral pattern between the AHs, however it is possible to identify different intensities of the peaks.In all AHs, nonfunctionalized aliphatic carbons were observed in the region between 0-40 ppm (*CH3-R, R*CH2-R', R-CH-R'), with peak positions between 25 and 27 ppm, which are less evident in the AHs obtained from the samples collected in the area of the combined crop.The presence of peaks in this region corresponds to structures of lipids, amino acids and/or proteins and biopolymers, such as lignin, cuticines, suberins and tannins (Kogel-Knabner, 2002).
The peaks between 40 and 60 ppm correspond to methoxyl-like carbons (RO-*CH3) and polypeptides (CAlkyl-O,N) of alpha C (-CO-*CHR-NH) that belong to amino acids and are more evident in the AHs of the samples collected in the area of cassava, which is being submitted to a longer time of use and drainage.The presence of peaks in the region between 60 and 90 ppm (CAlkyl-O) indicates carbons of the type (-*C-OH), which belong to cellulose/hemicellulose fragments and lignins, with maximum values of 66.67 ppm being also more expressive in the cassava growing area (Song et al., 2008).Peaks between 90 and 110 ppm (CAlkyl-di-O) correspond to anomeric carbons belonging to carbohydrate fragments, as well as C2 from syringyl and guayacil fragments, however, are poorly evident in all spectra.The peaks between 110 and 140 ppm correspond to non-functionalized aromatic carbons (CAromatic-H,R) belonging to lignin fragments (C1) of syringyl and guayacil and are present in all spectra, with emphasis on the AHs of the sub-surface horizons of the cassava growing areas.
In the region between 140 and 160 ppm are the peaks belonging to functionalized aromatic carbons (CAromatic-O,N) of fragments of phenolic structures of lignins and suberins.The peaks between 160 and 185 ppm correspond to carboxylic carbons (-CCOOH) belonging mainly to fatty acid fragments, mainly present in the AHs of the consortium planting system.The region between 185 and 230 indicates carbonyl-like (CC = O) carbons belonging to ketones and aldehydes (Song et al., 2008).
Figure 5 shows the PCA (5.B) and the loadings (5.C) for the 13 C-NMR.CP MAS spectra of AH of the Thyomorphic Organossolos.The PC-1 corresponds to 43% of the total variance explained, separating the samples from the surface hystic horizons of the cassava cultivation area (MDp2Hd, MDp3Hd) and the cultivation area in consortium (CMp9Hd) and the subsurface horizons of the cassava cultivation area (MDp2Hdp1, MDp1HDp2) and the forest fragment area (MTp6Hdp2, MTp4Hdp2) and negative AH values of the other areas.Loadings analysis shows that samples at PC-1 positive values have more aromatic structural characteristics.It is noted that these AH samples from cassava growing areas (2, 4, 6) from the AHs of the growing area (8, 10 and 12) and from the AHs of the growing area in the consortium (18) correspond to the subsurface horizons of each of the areas.By contrast, the samples corresponding to the surface horizons, in their majority, grouped themselves to negative values PC-1, indicating the greater presence of aliphatic structures.In addition, the influence of the management system on the structural characteristics of the soils in the study areas is clearly noted, since the AHs obtained in the cassava area showed a greater predominance of aromatic carbons, followed by samples from the secondary forest area and, finally, the soils cultivated in a consortium.Therefore, it is verified that the longer the handling time of the area the greater the degree of aromaticity.PC-2 positive samples are mainly grouped by substituted aromatic character and negative by the presence of unsubstituted aliphatic groups (García et al., 2016).12 4.2.2Spectroscopic characterization by attenuated total reflection in infrared with Fourier transform (ATR-FTIR) By analyzing the AHs spectra of the Organossolos Tiomórficos in the different types of management (Figure 6.A), it is observed that these presented similar spectral pattern, however, it is possible to identify different intensities and frequencies of transmittance, which demonstrates that the AHs formed in these systems do not have the same characteristics.Broadband in the region of ~3200 cm -1 is present in all samples and corresponds to symmetrical O-H and N-H strains, belonging to groups of phenols, alcohols, amines and carboxyls (Lopes & Fascio, 2004).These bands are most intense in the AH spectra of cassava-growing soils (1-6) and least intense in the soils of cultivation consortium (13-18).Bands have been reported at ~2900 cm -1 and ~2800 cm -1 , which can be attributed to symmetric and asymmetric C-H (CH 24 and CH434 aliphatic) strains (Baes & Bloom, 1989).The bands present between the regions of 1706 and 1587 cm -1 indicate the presence of symmetrical strains of C=O, which point to the presence of groups of amides and symmetrical strains of C=C corresponding to aromatic groups.These bands have higher intensities in the AH spectra of MD (2,3,4,5 and 6) and MT (7 to 12).The bands close to 1222.73 cm -1 correspond to symmetrical strains of C-O and O-H deformations of carboxyls and symmetrical strains of C-O of aryl ether, which present greater intensity in the cassava planting system (2, 3, 4 and 6) (Lopes & Fascio, 2004).The band presence at ~1000 cm -1 is characteristic of symmetrical O-H strains of aliphatic alcohols and asymmetrical C-O-C strains attributed to polysaccharides.This band is less intense in the spectra of the samples collected in the cassava growing area (2, 3, 4 and 6).The band in the region of ~460 cm -1 is present in most AH spectra and can be attributed to the Si-O deformation of kaolinite (Russell, 1997).
The PCA is shown in Figure 6.B and the loadings for the FTIR spectra of the AHs of the Organossolos Tiomórficos are shown in Figure 6.C.The PC-1 axis, which accounts for 81% of the total variance, separated the samples collected in the consortium area in profile 8 at the Hdp1 and Hdp2 horizons and in profile 9 at the Hd horizon (15, 16 and 18), in the cassava area in profile 1 at the Hdp2 horizon, in profile 2 at the Hdp1 and Hd horizons and in profile 3 at the Hd horizon (2, 3, 4 and 6), and in the forest fragment area in profile 4 at the Hdp2 horizon and at negative values the AH of the other areas.Through the analysis of loadings it is possible to identify that the areas that were grouped at positive values of PC-1 show, mainly, similarity in the presence of aliphatic alcohol groups and polysaccharides.In PC-2, which corresponds to 17% of the total variance explained, positive values are grouped into AHs that have structural similarities regarding the presence of symmetric C-O strains, carboxyl O-H deformations, aril ether C-O symmetric strains, C=O symmetric strains and C=C symmetric strains (García et al., 2016).
The spectral data corroborate those derived from the analysis of 13 C-NMR.CP MAS and indicate that time and management system directly influence the structural characteristics of AH.The bands identified in regions that are characteristic of aromatic groups were more intense in the cassava cultivation management system, which has a longer handling time, showing that there has been a more intense transformation of the soil organic matter (MOS) to a more recalcitrant composition.However, in this same system, one can still notice superficial horizons (1 and 5) with more aliphatic characteristics.In the other management systems, though, the presence of structures corresponding to more aliphatic groups can be observed, mainly in the superficial horizons.

DISCUSSION
The results indicate that the drainage and management of Organossolos promote quantitative effects on the levels of organic carbon as well as its fractions.Significant differences were found between the different types of soil management.It was observed that in the cassava area, where the soil is annually turned up, the levels of MO, TOC and the COP fractions CAH and CHUM, were lower in all profiles and horizons, when compared to the other areas.When exposed to oxidation due to the revolution, these soils undergo a process that leads to a reduction in carbon levels, which can also lead to differences in the fractions of organic matter, especially in the more labile ones, a process called subsidence (Pereira et al., 2005).The drainage associated with the fertilization and liming processes favors the increase in the activity of aerobic decomposing microorganisms, which contribute to a set of modifications in both the carbon contents and the fractions of the MOS.
In a study conducted in southern Brazil, Zang et al. (2022) observed a 70% reduction in TOC levels in Organossolos grown with maize.Evaluating Organossolos in northeastern Poland, Kalisz et al. (2010) found reductions in TOC levels of these soils, decreasing from 30% to 5% after drainage.These results corroborate those observed in this study, showing that the management of Organossolos in peatlands using drainage and soil uplift promotes oxidation with consequent decrease in the organic carbon content of the soil.
In soils with high levels of organic matter in peatlands, COP, in different decomposition states, is generally the fraction that contributes most to TOC values (Mirabito & Chambers, 2023).This fraction consists of vegetable or fungal fragments not totally decomposed, not protected by the mineral soil matrix, which presents an accelerated rate of turnover (Cotrufo & Lavellee, 2022).The results obtained are in agreement with the studies mentioned, and between the COP and COam fractions, the highest levels of carbon were observed in COP.With statistically lower values in the area of cassava, showing that the handling that promotes greater movement in the soil, brings about a negative effect on the levels of this fraction considered more labile.
The CAF, CAH and CHUM fractions were affected by soil management.From the results obtained by chemical fractionation, a process of transformation of MOS was observed, with higher levels in the area with less anthropic interference (forest fragment) in the more humid fraction of MOS (CHUM) and lower levels of the fraction considered less stable (CFAF).The opposite pattern is observed in the area most affected by the management (cassava), where lower CHUM and higher CAF values were verified.
Several researchers have been studying the transformations of the MOS fractions after intensive soil management processes.Santos et al. (2020b), in a study conducted on mineral soils in the state of Rio de Janeiro, Brazil, observed that long-term burning mainly affected the CAF fraction, while the effect caused on the CHUM fraction was smaller.
However, for Organossolos, the results observed by Kalisz et al. (2010) are in agreement with those verified in this study.In their research, these authors observed an increase in the solubility of MOS, and after the drainage process the levels of CAF and CAH increased while the CHUM levels in TOC decreased from 90% to 70%.Soares et al. (2015) observed variations in MOS fractions in Organossolos under pasture, bean and cassava cultivation.The authors found lower CHUM and CAF values in the first 10 cm of the cassava cultivated area.Evaluating other types of cover, in different soils, authors observed that soil cultivation may affect not only TOC and more labile fractions (Loke et al., 2019), but also influence more stable fractions of MOS (Loke et al., 2019;Ukalska-Jaruga et al., 2019).
In hydromorphic environments what is observed is the predominance of the humine fraction, this fact can be explained by the direct occurrence of the process of humification in the lignified tissues modified by demethylation, since the mechanisms of microbial insolubilization and synthesis are significantly reduced (Kononova, 1982;Ebeling et al., 2013).However, the disintegration of soil aggregates, caused by intensive management, may decrease the protection offered to the humine fraction, previously associated with soil aggregates, and thus favor the increase of degradation promoted by the microbial community of this more humified fraction (Raiesi, 2021).
Spectroscopic analyzes of the AHs made it possible to identify how the management system alters MOS.Different structural characteristics of the AHs were observed as a function of the variation in the type of management adopted and also when comparing the superficial horizons with the subsuperficial ones.By analyzing the PCA data from the 13 C-NMR.CP-MAS spectra , it is found that the AHs in the cassava growing area have a greater presence of substituted and unsubstituted aromatic groups (Caromatic andCAromatic-ON) at both surface and surface horizons.Studies by Segnini et al. (2013) show that seasonally flooded soils mainly accumulate carboxylated aromatic fractions considered as recalcitrant material.As the cassava area has been cultivated for more than 80 years, that is, it is the oldest system among those evaluated, there is a greater protection of MOS against mineralization, which allowed the formation of aromatic and recalcitrant structures, when compared to the other types of management (Amaleviciute-Volunge et al., 2023).
This protection of the MOS is more evident in the subsurface horizons, which are the least affected by handling practices, and submitted to a greater influence of the water table delaying its decomposition.Through the analysis of PCA, the presence of aromatic groups in the subsurface horizons of the AHs of all the profiles analyzed in the forest fragment area can also be verified, which indicates that after 20 years without human interference, it was possible to promote the formation of more recalcitrant AHs.On the other hand, for the AHs of the surface horizons of the cassava and forest fragment cultivation areas and the AHs of the surface and subsurface horizons of the cultivation area in a consortium, the predominance of aliphatic groups was verified.
Aliphaticity is also an important characteristic of AHs and is linked to the presence of organic functional groups on the surface of AH, such as phenolic groups, alcoholic hydroxylates and carboxylates, which are able to establish hydrogen bonds and promote soil hydrophilicity (Szajdak et al., 2020).Also, the spectral data of the AHs show that at the surface horizons the humification occurs in a slower way, due to the constant intake of labile organic matter, mainly in the consort cultivation.This intake of organic matter also reflected in the AHs TOC levels, which were higher in the area of cultivation as a consortium, coming mainly from crop residues that are deposited on the soil surface.In their study Amaleviciute-Volunge et al. ( 2023), they concluded that major transformations of MOS occur in the first 30 cm in drained Organossolos and subsequently used with distinct forms of management.

CONCLUSIONS
In Organossolos, long-term management associated with drainage has negative effects on the levels and quality of MOS.The cultivation time of cassava and the set of practices used have a negative impact on the organic carbon content, the organic matter and its fractions, both the most labile (COP) and the most humified (CHUM).
The management adopted in the cassava area impacted the structural composition of the MOS, decomposing the more labile structures, and a greater participation of more recalcitrant structures formed by aromatic radicals was observed in the AH.The anthropic non-interference during 20 years was sufficient for more aromatic and recalcitrant structures to be formed in the forest fragment area, mainly in the subsurface horizons.At the surface horizons, however, the recurrent intake of organic matter, not exposed to the revolution and consequent oxidation, gives the AH a greater number of aliphatic radicals.

Figure 3 :
Figure 3: A: Particulate Organic Carbon (COp) and B: Organic Carbon content associated with minerals in the profiles and horizons of the studied areas.Area 1: Cassava cultivation; Area 2: Forest fragment; Area 3: Consortium coconut and cassava.Source: Prepared by the authors (2023).4.1.3Organic Fulvic Acid Fraction Carbon (FAC), Organic Humic Acid Fraction Carbon (HAC) and Organic Human Fraction Carbon (CHUM)

Figure 4 .
Figure 4. A: Organic carbon content in the humic acid fraction (CAH); B: Organic carbon content in the fulvic acid fraction (CAF); C: Organic carbon content in the humine fraction (CHUM) in the profiles and horizons of the studied areas.Area 1: Cassava cultivation; Area 2: Forest fragment; Area 3: coconut and cassava.Source: Prepared by the authors (2023).

Figure 5 .
Figure 5. A: Spectra 13C-NMR.CP MAS; B: Score charts and C: spectral pattern loadings charts corresponding to humic acids extracted from the hystic horizons of Organossolos in the city of Rio de Janeiro. 1 to 6 spectra of cassava growing area horizons.7 to 12 spectra of forest fragment area horizons.13 to 18 spectra of horizons in area of cultivation consortium of coconut and cassava.Source: Prepared by the authors (2023).

Figure 6 .
Figure 6.A: ATR-FTIR spectra; B: Score charts and C: spectral pattern loadings charts corresponding to humic acids extracted from the hystic horizons of Organossolos in the city of Rio de Janeiro. 1 to 6 spectra of cassava growing area horizons.7 to 12 spectra of forest fragment area horizons.13 to 18 spectra of horizons in area of cultivation consortium of coconut and cassava.Source: Prepared by the authors (2023).