Carbon Dynamics in Humic Fractions of Soil Organic Matter Under Different Vegetation Cover in Southern Tocantins

Josué Luiz Marinho Junior Victor Casimiro Piscoya Milton Marques Fernandes Saulo Boldrini Gonçalves Francisco Sandro Rodrigues Holanda Moacyr Cunha Filho Raimundo Rodrigues Gomes Filho Alceu Pedrotti Renisson Neponuceno Araújo FilhoAbout the authors

Abstract

Soil organic matter has great importance in chemical, physical and biological processes in soil-plant system. This study aimed to evaluate changes in carbon humic fractions of soil organic matter under different vegetation coverings in Cerrado in Tocantins, Brazil. The work was developed in Eucalyptus sp., Pasture, Agriculture and Cerrado sensu stricto areas. Soil samples were collected in dry period, October, 2018, in trenches 70 x 70 cm at depths 0-10, 10-20, 20-30, 30-40 and 40-50 cm, with six replications. The vegetation cover in Eucalyptus sp. area showed higher levels of total carbon in soil humic fraction. Respectively fulvic acid, humic acid and humin in Eucalyptus sp. area had stocks of 22.09; 2.71 and 20.01% higher than native forest, 32.04; 2.57 and 35.59% higher than pasture and 10.59; 19.19 and 7.61% higher than the agriculture area. Among the different areas evaluated, the soil of Eucalyptus sp. has great potential to increase carbon storage in soil humic fractions.

Keywords:
Humic acid; Fulvic acid; Humin; Cerrado

1. INTRODUCTION AND OBJECTIVES

Soil organic matter (SOM) is extremely important as soil quality key indicator, as its accumulation influences physical and chemical properties of soil ( Dhaliwal et al., 2019Dhaliwal SS, Naresh RK, Mandal A, Singh R, Dhaliwal MK. Dynamics and transformations of micronutrients in agricultural soils as influenced by organic matter build-up: A review. Environmental and Sustainability Indicators 2019; 1-2: 100007. ). Humus is result of biological animals decomposition, microorganisms and mainly plants, consisting of compounds in various stages of decomposition, closely linked to soil clay minerals ( Stevenson, 1994Stevenson FJ. Humus chemistry: genesis, composition and reactions. 2nd ed. New York: Willey & Sons; 1994. ).

The SOM is a nutrientes reserve for plants, with carbon (C) a large part of its composition ( Ondrasek et al., 2019Ondrasek G, Begić HB, Zovko M, Filipović L, Meriño-Gergichevich C, Savić R et al. Biogeochemistry of soil organic matter in agroecosystems and environmental implications. Science of The Total Environment 2019; 658: 1559-1573. ), it helps in physical structure of the soil such as porosity and stability of aggregates, facilitating development roots, fungi and water infiltration ( Souza et al., 2019Souza ED, Silva CRM, Pinto FA, Carneiro MAC, Paulino HB, Pacheco LP et al. Soil quality indicators after conversion of “murundu” fields into no-tillage cropping in the Brazilian Cerrado. Pesquisa Agropecuária Brasileira 2019; 54: e00374. ).

The SOM has two fundamental groups, one consisting in decomposition products of organic residues and microorganisms metabolism that represents about 10 to 15% of the TOC reserve in mineral soils ( Dhaliwal et al., 2019Dhaliwal SS, Naresh RK, Mandal A, Singh R, Dhaliwal MK. Dynamics and transformations of micronutrients in agricultural soils as influenced by organic matter build-up: A review. Environmental and Sustainability Indicators 2019; 1-2: 100007. ) and humic substances, with emphasis on humic acid, fulvic acid and humin, representing 85 to 90% of TOC reserve ( Pegoraro et al., 2018Pegoraro RF, Moreira CG, Dias DG, Silveira TC. Carbon and nitrogen stocks in the soil and humic substances of agricultural crops in the semi-arid region. Revista Ciencia Agronomica 2018; 49 (4): 574-583. ). The humic fraction is composed of organic residues having a high decomposition degree, which generates greater molecular stability indicating a longer soil C permanence, with this fraction retaining most of soil C ( Petter et al., 2017Petter FA, Lima LB, Morais LA, Tavanti RFR, Nunes ME, Freddi OS et al. Carbon stocks in oxisols under agriculture and forest in the southern Amazon of Brazil. Geoderma Regional 2017; 11: 53-61. ).

The C in fulvic acid has great solubility in acid and basic environments, being easily leached and distributed more evenly between soil layers and presenting lower concentrations between humic soil fractions ( Kukuļs et al., 2019Kukuļs I, Kļaviņš M, Nikodemus O, Kasparinskis R, Brūmelis G. Changes in soil organic matter and soil humic substances following the afforestation of former agricultural lands in the boreal-nemoral ecotone (Latvia). Geoderma Regional 2019; 16: e00213. ). The C in humin fraction is associated with mineral soil fraction showing strong resistance to microbial activity ( Stevenson, 1994Stevenson FJ. Humus chemistry: genesis, composition and reactions. 2nd ed. New York: Willey & Sons; 1994. ). Carbon in humic acid and humin is more recalcitrant with greater stability and presence in unchanged soils ( Gmach et al., 2018Gmach MR, Dias BO, Silva CA, Nóbrega JCA, Lustosa-Filho JF, Siqueira-Neto M. Soil organic matter dynamics and land-use change on Oxisols in the Cerrado, Brazil. Geoderma Regional 2018; 14: e00178. ; Kunlanit et al., 2019Kunlanit B, Butnan S, Vityakon P. Land-Use Changes Influencing C Sequestration and Quality in Topsoil and Subsoil. Agronomy 2019; 9: 520. ).

Thus, changes in vegetation covers use, depending on the management applied, can reduce or increase the C stocks in humic fractions ( Santana et al., 2019Santana MS, Sampaio EVSB, Giongo V, Menezes RSC, Jesus KN, Albuquerque ERGM et al. Carbon and nitrogen stocks of soils under different land uses in Pernambuco state, Brazil. Geoderma Regional 2019; 16: e00205. ). One way to mitigate C emission in atmosphere is preservation of native forests and reforestation, measures like these remove large amounts of CO2 from the atmosphere, through photosynthesis, and store C in biomass and soil ( Soleimani et al., 2019Soleimani A, Hosseini SM, Massah Bavani AR, Jafari M, Francaviglia R. Influence of land use and land cover change on soil organic carbon and microbial activity in the forests of northern Iran. Catena 2019; 177: 227-237. ).

Undisturbed ecosystems, such as virgin native forests and areas reforested with eucalyptus, are highly conservative environments of SOM ( Primieri et al., 2017Primieri S, Muniz AW, Lisboa HM. Dinâmica do Carbono no Solo em Ecossistemas Nativos e Plantações Florestais em Santa Catarina. Floresta e Ambiente 2017; 24: e00110314. ). The physical-biochemical protection generated by the humic fractions through the recalcitrant C in aggregates is one of main preservation mechanisms of soils C in different vegetation coverings ( Zhang et al., 2019Zhang J, Wei Y, Liu J, Yuan J, Liang Y, Ren J et al. Effects of maize straw and its biochar application on organic and humic carbon in water-stable aggregates of a Mollisol in Northeast China: A five-year field experiment. Soil and Tillage Research 2019; 190: 1-9. ).

In light above discussion, the objective this study was to evaluate possible changes in humic fractions carbon of soil organic matter under different vegetation coverings in Cerrado in Tocantins, Brazil.

2. MATERIALS AND METHODS

2.1. Study area

The work was carried out in municipality Gurupi, state of Tocantins, in geographical coordinates 11º46’25” S and 49º02’54” W ( Figure 1 ).

Figure 1
Experimental areas location with native forest, Eucalyptus sp., pasture and agriculture in Gurupi-TO.

Climate of region according Thornthwaite is B1wA’a’ type, having two well-defined seasons, with about six months of drought including the winter period and six months of rain that correspond to the summer. The average annual temperature is 27 ºC and the average annual precipitation is 1,500 mm ( SEPLAN, 2017Secretaria de Planejamento e Orçamento (SEPLAN). Gerência de Indicadores Econômicos e Sociais (GIES). Projeto de Desenvolvimento Regional Integrado e Sustentável. Zoneamento Ecológico-Econômico do Estado do Tocantins. Diagnóstico Ecológico-Econômico do Estado do Tocantins. Palmas: SEPLAN/GIES; 2017. ). The soil was classified as Plinthosol ( Santos et al., 2018Santos HG, Jacomine PKT, dos Anjos LHC, Oliveira VA, Lumbreras JF, Coelho MR et al. Sistema Brasileiro de Classificação de Solos.5 ed. Brasília: Embrapa ; 2018. ). The studied areas were with native forest of Cerrado sensu stricto as a witness, Eucalyptus sp., pasture and agriculture. Each area had the following characteristics:

Native forest of Cerrado sensu stricto: the area has 22.82 ha, aged over 60 years and has a mean diameter breast height (DBH) of 23.1 cm. The vegetation was characterized by five species of higher values of importance Myrcia splendens (Sw.) DC. (13.04%), Qualea multiflora Mart. (9.87%), Protium heptaphyllum (Aubl.) Marchand (7.53%), Magonia pubescens A.St.-Hil. (5.35%), Qualea grandiflora Mart. (5.02%) ( Bendito et al., 2018Bendito BPC, Souza PA, Ferreira RQS, Cândido JB, Souza PB. Espécies do Cerrado com potencial para recuperação de áreas degradadas, Gurupi (TO). Revista Agrogeoambiental 2018; 10 (2). ).

Eucalyptus sp.: the area has 0.65 ha at 11 years of age, presenting an average DBH of 32.7 cm and its implementation was carried out through deforestation with a crawler tractor and front blade, then plowing and harrowing was carried out. Seedlings with a height of 25 cm were planted in pits in the dimensions of 0.4 x 0.4 x 0.4 m with the help of diggers, and with a 3 x 2 m spacing. Then, fertilization was carried out with 100 g of simple superphosphate at the bottom of the pit and partially buried, then 150 g of pit-1 NPK was added in formulation 5-25-15.

Pasture: the area has 11.25 ha with natural pasture and predominance of Andropogon grass over 40 years old. Other species of poaceas have been recorded such as: Spalum notatum , Eragrostis bahiensis , Axonopus affinis , Bothriochloa laguroides , Schizachyrium microstachyum , Paspalum dilatatum , Sporobolus indicus , Rhynchospora sp., Andropogon ternatus , Panpalumis sp. ( Marinho et al., 2017Marinho PHA, de Sousa RM, Medeiros PCAO, Silva TGN, Giongo M. Levantamento fitossociológico de plantas infestantes na área experimental da Universidade Federal do Tocantins submetida a diferentes cultivos. Agrarian Academy 2017; 4 (7): 314-324. ).

Agriculture: the area has 0.95 ha and the soil preparation was done using a leveling harrow and disc plow, and the weeds were controlled by manual weeding associated with the use of full-action herbicides such as glyphosate, operations adopted when necessary. Over the past 6 years the corn crop was grown in the area ( Vaz-de-Melo et al., 2017Vaz-de-Melo A, Taubinger M, Santos VM, Cardoso DP, Vale JC. Capacidade combinatória de milho para produção de grãos sob níveis de fósforo. Revista de Agricultura Neotropical2017; 4 (4): 15-25. ), planted annually in the period between February and March at an average spacing of 0.2 x 0.8 m ( Simon et al., 2016Simon J, Costa RV, Almeida REM, Campos LJM, Lago BC, Ferreira LL et al. Época de plantio e cultivares de milho safrinha no Tocantins. Palmas, TO: Embrapa Pesca e Aquicultura, 2016. 18p. ). For sowing, a manual planter-fertilizer was used, which enabled basic fertilization. The nutrients applied at the time of corn sowing consisted of nitrogen in the form of ammonium sulphate (45% N), phosphorus in the form of triple superphosphate (42% P2O5) and potassium in the form of potassium chloride (58% of K2O), corresponding to 120, 170 and 140 kg ha-1, respectively of N, P and K, with N applied 50% at 25 days and 50% at 45 days after sowing ( Vaz-de-Melo et al. , 2017Vaz-de-Melo A, Taubinger M, Santos VM, Cardoso DP, Vale JC. Capacidade combinatória de milho para produção de grãos sob níveis de fósforo. Revista de Agricultura Neotropical2017; 4 (4): 15-25. ).

The study areas characterization was carried out based on studies carried out in same areas.

2.2. Soil samples

Six trenches were opened per area, the first being selected at random and the rest equidistant from each other around 30 m, with dimensions 70 x 70 cm and depth of 50 cm in each area. The samples were collected in dry period in October, 2018, at depths 0-10, 10-20, 20-30, 30-40 and 40-50 cm. The deformed soil samples were air dried at room temperature and passed through a 2 mm sieve to perform physical and chemical analyzes. Undeformed soil samples were collected and submitted to soil density analysis.

2.3. Physical analysis

The soil density was determined by volumetric cylinder method described by Almeida et al. (2017Almeida BG, Viana JHM, Teixeira WG, Donagemma GK. Densidade do solo. In: Teixeira PC et al. editores técnicos. Manual de métodos de análise de solo. 3 ed. Brasília: Embrapa; 2017. ), and granulometric analysis was performed using the pipette method ( Donagemma et al., 2017Donagemma GK, Viana JHM, Almeida BG, Ruiz HA, Klein VA, Dechen SCF et al. Análise Granulométrica. In: Teixeira PC et al. editores técnicos. Manual de métodos de análise de solo . 3 ed. Brasília: Embrapa ; 2017. ).

2.4. Chemical analysis

The chemical fractionation of humic substances was carried out according to the method suggested by the International Humic Substances Society ( Swift, 1996Swift RS. Organic matter characterization. Methods of soil analysis. In: Sparks DL, Page AL, Helmke PA, Loeppert RH, Soltanpour PN, Tabatabai MA, Johnston CT, Sumner ME. (eds). Soil Science Society of America Book Series, 5. Part 3. Chemical Methods. Madison: Soil Science Society of America: American Society of Agronomy; 1996. ). Fulvic acids (FA), humic acids (HA) and humin (HUM) were obtained, based on the solubility in acids and alkalis. After fractionation, the samples were frozen and lyophilized to determine C in humic fractions by the dry combustion method (CHNS / O) in an elemental analyzer (Model PE-2400 Series II Perkin Elmer).

The C concentrations were converted into soil carbon stock in Mg ha-1 for each depth sampled as follows ( Veldkamp, 1994Veldkamp E. Organic carbon turnover in three tropical soils under pasture after deforestation. Soil Science Society of America Journal 1994; 58: 175-180. ):

S t o c k   C   =   ( C   x   S D   x   V S D )   x   1000 (1)

Where Stock C is the carbon stock in the soil layer, in (Mg ha-1); C is the carbon concentration in the soil sample, in (kg Mg-1); SD is the soil density in the layer, in (Mg m-3) and the VSD is the volume of the sampled depth, in (m3). After calculating the C stock for each layer, the soil carbono stocks was corrected, taking into account the differences in soil mass ( Sisti et al., 2004Sisti CPJ, Santos HP, Kohhan R, Albes BJR, Urquiaga S, Bodey RM. Change in carbon and nitrogen stocks in soil under 13 years of conventional or zero tillage in southern Brazil. Soil and Tillage Research 2004; 76: 39-58. ). The total stock of C at a depth of 0 to 50 cm was calculated by adding the values obtained in each sampled layer.

2.5. Statistical analysis

The parameters evaluated, concentrations and stocks of C in the humic fractions were subjected to normality tests Shapiro and Wilk, then performed the analysis of variance to evaluate the differences between the soil uses in depths. The comparison of means was performed by the Tukey test at 5% significance and using the statistical software SISVAR ( Ferreira, 2011Ferreira DF. Sisvar: a computer statistical analysis system. Ciência e Agrotecnologia 2011;35 (6): 1039-1042. ).

3. RESULTS AND DISCUSSION

In general, the granulometric composition of soil showed a médium texture predominance, being thus classified with a sandy-clay-loam texture in all soil depths of the analyzed areas ( Table 1 ).

Table 1
Physical characteristics of the Cerrado soil in the different vegetation coverings in Gurupi-TO.

The predominance of sand indicates that soil has a low capacity for nutrient retention ( Araújo Filho et al., 2017Araújo Filho RN, Holanda FSR, Pedrotti A, Santos TO, Lino JB, Rocha IP, Influência dos atributos físico-mecânicos do solo na estabilidade do talude do rio São Francisco. Revista Scientia Agraria 2017; 18 (4): 107-113. ). In general, the soil density did not show great variation between the types of land use ( Araújo Filho et al., 2018Araújo Filho RN, Freire MBGS, Wilcox BP, West JB, Freire FJ, Marques FA. Recovery of carbon stocks in deforested caatinga dry forest soils requires at least 60 years. Forest Ecology and Management 2018; 407: 210-220. ). For Marinho Junior et al. (2019Marinho Junior JL, Oliveira MD, Dias JLA, Araújo Filho RN, Melo Neto JO, Gonçalves SB et al. Physical Attributes of Soil in Different Forest Cover in South of Tocantins. International Journal of Plant and Soil Science 2019; 31 (2): 1-7. ) the removal of vegetation cover can cause physical changes in soil, due to the impact of raindrops that fall directly on the exposed soil, this can explain the high soil density in superficial layers, as well as, the density increase with the depth increase of soil is attributed to densification caused by the upper layers.

The carbon concentrations in soil humic fractions: fulvic acid (FA), humic acid (HA) and humin (HUM) were represented in Figure 2 .

Figure 2
Carbon concentrations in humic fractions in different vegetation coverings in Gurupi-TO. HA: Humic Acid; FA: Fulvic Acid; HUM: Humin.

* Significant differences are indicated by different letters by the Tukey test at 5% significance level (P ≤ 0.05). Capital letters indicate differences between vegetation cover and lower letters indicate differences between soil layers.


The carbon concentrations in soil humic fractions decreased with depth increase, finding the maximum values in the layer of 0-10 cm and minimum values in layer 40-50 cm, presenting a variation in FA, HA and HUM respectively of 2.28 to 0.67 g kg-1, 3.20 to 0.93 g kg-1 and 8.22 to 2.41 g kg-1 ( Figure 2 ). Araújo Filho et al. (2018Araújo Filho RN, Freire MBGS, Wilcox BP, West JB, Freire FJ, Marques FA. Recovery of carbon stocks in deforested caatinga dry forest soils requires at least 60 years. Forest Ecology and Management 2018; 407: 210-220. ) attributes the highest carbon concentration in soil humic fractions in the superficial layer to the death of thin roots mainly of herbaceous, vegetable biomass disposition in soil and the microbial activity that acts in stabilization of this soil organic matter.

The higher C proportion in the humic fraction is important for soil organic carbon, and this fraction is composed of organic residues with a high degree of decomposition generating greater molecular stability, indicating a longer soil C permanence, attributing greater capacity of the area in storing carbon ( Petter et al., 2017Petter FA, Lima LB, Morais LA, Tavanti RFR, Nunes ME, Freddi OS et al. Carbon stocks in oxisols under agriculture and forest in the southern Amazon of Brazil. Geoderma Regional 2017; 11: 53-61. ).

The HUM presented the highest results over the other fractions in all vegetation coverings and HA over FA, with emphasis on the soil of Eucalyptus sp. area which presented the highest C concentrations in humic fractions between the evaluated areas up to a depth 30 cm. Protection of humic C in aggregate fractions and physical-biochemical protection through recalcitrant C in aggregates is one of the main mechanisms for preserving C in altered soil ( Zhang et al., 2019Zhang J, Wei Y, Liu J, Yuan J, Liang Y, Ren J et al. Effects of maize straw and its biochar application on organic and humic carbon in water-stable aggregates of a Mollisol in Northeast China: A five-year field experiment. Soil and Tillage Research 2019; 190: 1-9. ). This fraction has a strong resistance to microbial activity and is strongly associated with the soil mineral fraction, especially in oxidic soils ( Stevenson, 1994Stevenson FJ. Humus chemistry: genesis, composition and reactions. 2nd ed. New York: Willey & Sons; 1994. ).

In contrast, with native forest and pasture area provided the soil with a low C concentration in FA and HUM in topsoil. According to Petter et al. (2017Petter FA, Lima LB, Morais LA, Tavanti RFR, Nunes ME, Freddi OS et al. Carbon stocks in oxisols under agriculture and forest in the southern Amazon of Brazil. Geoderma Regional 2017; 11: 53-61. ) and Kukuļs et al. (2019Kukuļs I, Kļaviņš M, Nikodemus O, Kasparinskis R, Brūmelis G. Changes in soil organic matter and soil humic substances following the afforestation of former agricultural lands in the boreal-nemoral ecotone (Latvia). Geoderma Regional 2019; 16: e00213. ) the greater AF solubility in acidic and basic environments and the leaching process of this fraction favors a more homogeneous distribution of C in AF fraction between soil layers. These results were corroborated by ( Gmach et al., 2018Gmach MR, Dias BO, Silva CA, Nóbrega JCA, Lustosa-Filho JF, Siqueira-Neto M. Soil organic matter dynamics and land-use change on Oxisols in the Cerrado, Brazil. Geoderma Regional 2018; 14: e00178. ; Guimarães et al., 2013Guimarães DV, Gonzaga MIS, Silva TO, Silva TL, Dias NS, Matias MIS et al. Soil organic matter pools and carbon fractions in soil under different land uses. Soil and Tillage Research 2013; 126: 177-182. ; Kunlanit et al., 2019Kunlanit B, Butnan S, Vityakon P. Land-Use Changes Influencing C Sequestration and Quality in Topsoil and Subsoil. Agronomy 2019; 9: 520. ) who reinforce the greater HA and HUM stability and a greater FA lability with lower C levels in this fraction.

According to Doane et al. (2003Doane TA, Devêvre OC, Horwa´Th WR. Short-term soil carbon dynamics of humic fractions in low-input and organic cropping systems. Geoderma 2003; 114: 319-331. ) reinforce this theory in their study where the soil carbon dynamics in different cultivation systems at the California University were evaluated, and found higher concentrations of carbon respectively in HUM, HA and FA.

Studies by Sato et al. (2019Sato JH, Figueiredo CC, Marchão RL, Oliveira AD, Vilela L, Delvico FM et al. Understanding the relations between soil organic matter fractions and N2O emissions in a long-term integrated crop-livestock system. European Journal of Soil Science 2019; 70 (6): 1183-1196. ) evaluating the humic fractions in Cerrado, found the highest C levels in HUM fraction (7.10 g kg-1) in native Cerrado, in the HA fraction (4.7 g kg-1) in a integrated crop-livestock system under no-tillage and in FA fraction (3.87 g kg-1) in a conventional crop, values that corroborate what was found in this study. According to Kukuļs et al. (2019Kukuļs I, Kļaviņš M, Nikodemus O, Kasparinskis R, Brūmelis G. Changes in soil organic matter and soil humic substances following the afforestation of former agricultural lands in the boreal-nemoral ecotone (Latvia). Geoderma Regional 2019; 16: e00213. ) sandy soils cause more dynamic changes in the properties of SOM and consequently in the humic fraction according to the change in land use.

The change in vegetation cover affects the dynamics of organic matter fractions ( Ferreira et al., 2020Ferreira CR, Silva Neto EC, Pereira MG, Guedes JN, Rosset JS, dos Anjos LHC. Dynamics of soil aggregation and organic carbon fractions over 23 years of no-till management. Soil and Tillage Research 2020; 198: 104533. ). The evaluated areas showed significant differences in total carbon stocks in the humic fractions of soil (FA, HA and HUM) ( Figure 3 ). Studies by Guimarães et al. (2013Guimarães DV, Gonzaga MIS, Silva TO, Silva TL, Dias NS, Matias MIS et al. Soil organic matter pools and carbon fractions in soil under different land uses. Soil and Tillage Research 2013; 126: 177-182. ) showed an increase in the percentage of humic substances in the total soil carbon stock, attributing this phenomenon to its recalcitrance mainly in the humin fraction, adsorption by the mineral matrix of soil and its intra-aggregate occlusion.

Figure 3
Total carbon stocks in humic fractions in different vegetation coverings in Gurupi-TO. HA: Humic Acid; FA: Fulvic Acid; HUM: Humin.

* Significant differences are indicated by different letters by the Tukey test at 5% significance level (P ≤ 0.05). Capital letters indicate differences between vegetation cover.


Most of the C stored in the soil is associated with humic fractions ( Stevenson, 1994Stevenson FJ. Humus chemistry: genesis, composition and reactions. 2nd ed. New York: Willey & Sons; 1994. ). The HUM stored most of carbon contained in humic fraction ( Figure 3 C), in agriculture area the HUM represented 61.69% of total carbon fraction, followed by Eucalyptus sp. area 60.02%, native forest 57.93% and pasture 55.59%. According to Araújo Filho et al. (2018Araújo Filho RN, Freire MBGS, Wilcox BP, West JB, Freire FJ, Marques FA. Recovery of carbon stocks in deforested caatinga dry forest soils requires at least 60 years. Forest Ecology and Management 2018; 407: 210-220. ) this fact occurs mainly in high concentrations of carbon in the humic fractions, action of soil density and clay content, since the carbon stored in the humic fraction is more recalcitrant in soils making it easier to maintain this element in the soil.

Studies by Ferreira et al. (2020Ferreira CR, Silva Neto EC, Pereira MG, Guedes JN, Rosset JS, dos Anjos LHC. Dynamics of soil aggregation and organic carbon fractions over 23 years of no-till management. Soil and Tillage Research 2020; 198: 104533. ) studying the dynamics of soil aggregation and carbon fractions in a fragment of Atlantic Forest in Paraná concludes that the change in land use promotes C storage in the most stable fractions (HUM), according to the results found in this study. According to Gray et al. (2019Gray J, Karunaratne S, Bishop T, Wilson B, Veeragathipillai M. Driving factors of soil organic carbon fractions over New South Wales, Australia. Geoderma 2019; 353: 213-226. ) analyzing determinants of soil organic carbon fractions in Australia identified the largest stocks of soil carbon in the humic fraction, which corroborates this study. In addition, Gray says that the stocks of humic organic carbon are higher in humid areas, with a decrease in hot and dry areas.

After HUM, the HA represented the largest total carbono stocks in humic fraction, with the agriculture area having the lowest stocks in HA (12.40 Mg ha-1) in contrast to the areas of Eucalyptus sp., pasture and native forest that respectively presented (14.78; 14.41 and 14.39 Mg ha-1) as seen in Figure 3 B. Lower stocks of humic substances in areas where there has been a change in land use, such as agriculture, is evidenced by Liu et al. (2020Liu X, Chen D, Yang T, Huang F, Fu S, Li L. Changes in soil labile and recalcitrant carbon pools after land-use change in a semi-arid agro-pastoral ecotone in Central Asia. Ecological Indicators 2020; 110: 105925. ) who evaluating changes in soil carbon stocks in Central Asia, states that, when changing the soil, the carbon present in labile and humic fractions drastically reduces after the change, that, with the growth of vegetation cover, the carbon in these fractions increases significantly.

The Eucalyptus sp. area presented the largest carbon stocks in FA (10.55 Mg ha-1), followed by agriculture, native forest and pasture areas that respectively presented (9.54; 8.62 and 7.99 Mg ha-1) ( Figure 3 A). According to Guimarães et al. (2013Guimarães DV, Gonzaga MIS, Silva TO, Silva TL, Dias NS, Matias MIS et al. Soil organic matter pools and carbon fractions in soil under different land uses. Soil and Tillage Research 2013; 126: 177-182. ) evaluating the soil organic matter and carbon fractions in soil under different land uses concluded that chemical fractions of SOM is effective in evaluating changes in C dynamics, indicating that stocks present in humic fractions (FA, HA and HUM) are indicative of nature conservation.

4. CONCLUSIONS

  • Area with vegetation cover of Eucalyptus sp. presented higher carbon content in humic acid fraction, with a tendency to be more humid;

  • Carbon of humic fractions was found in decreasing order: humin, humic acid and fulvic acid;

  • Over time, the vegetable coverings of Eucalyptus sp. have the capacity to store C in more stable fractions of organic matter;

  • In general, the pasture area showed lower concentrations and carbon stocks in fulvic acid and humine compared to the reference area, attributed to care lack and área abandonment.

  • Agriculture area under conventional corn cultivation showed the potential to recover soil carbon stocks in fulvic acid and humine fractions, showing lower carbon reductions with increasing depth compared to other evaluated areas.

  • Humic fractions of soil: humic acid, fulvic acid and humin are effective in assessing changes in the dynamics of C of the SOM in forest ecosystems.

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Publication Dates

  • Publication in this collection
    20 Jan 2021
  • Date of issue
    2021

History

  • Received
    17 Apr 2020
  • Accepted
    26 Nov 2020
Instituto de Florestas da Universidade Federal Rural do Rio de Janeiro Rodovia BR 465 Km 7, CEP 23897-000, Tel.: (21) 2682 0558 | (21) 3787-4033 - Seropédica - RJ - Brazil
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