ABSTRACT

Inadequate management of pasture soils in the Amazon has resulted in a predominance of degraded pastures. Considering the natural characteristics of this region can favor more appropriate strategies for sustainability, the objective of this study was to evaluate the differences in the chemical and physical attributes of pasture and forest soils in the Southern Amazon and to identify the most sensitive attributes of their fertility. Additionally, this study suggests appropriate management practices for sustainable pastures. Soil samples from the 0 to 0.20 m layer were analyzed to determine pH, exchangeable bases (calcium, magnesium, and potassium), exchangeable aluminum, potential acidity, phosphorus, organic carbon, bulk density, and texture. Pasture soils had a higher pH, calcium content, and bulk density than forest soils. However, the pasture soils had lower phosphorus and organic matter content. The soil organic carbon stocks were also lower in pasture soils, with levels 0.8 to 12 Mg ha^-1 lower than in forest soils. The fertility attributes most sensitive to soil management in these pastures were phosphorus, pH, organic carbon, and bulk density. The lack of nutrients and soil and water conservation practices have contributed to the degradation of these pastures. Therefore, the recommended management for these pastures should aim to improve the organic matter content, reduce compaction, and replenish and cycle nutrients.

Key words:
climate changes; conservation management; livestock; soil degradation; sustainability

RESUMO

O manejo inadequado de solos de pastagens na Amazônia resulta em predomínio de pastagens degradadas. Considerando as características naturais dessa região pode favorecer estratégias mais adequadas para sua sustentabilidade, o objetivo desse estudo foi avaliar as diferenças de atributos químicos e físicos de solos de pastagens e florestas no sul da Amazônia e identificar os atributos mais sensíveis de sua fertilidade. Além disso, sugerir práticas de manejo mais adequadas para essas pastagens em direção à sustentabilidade. Amostras de solo da camada de 0 a 0,20 m foram analisadas para determinação de pH, bases trocáveis (cálcio, magnésio e potássio), alumínio trocável, acidez potencial, fósforo, carbono orgânico, densidade aparente e textura. Os solos de pastagem tiveram maior pH, cálcio e densidade aparente do que os solos de floresta. No entanto, os solos de pastagem apresentaram menores teores de fósforo e matéria orgânica. Os estoques de carbono orgânico do solo também foram menores nas pastagens, com teores de 0,8 a 12 Mg ha^-1 inferiores aos solos da floresta. Os atributos de fertilidade mais sensíveis ao manejo do solo nessas pastagens foram fósforo, pH, carbono orgânico e densidade aparente. A carência de nutrientes e de práticas de conservação do solo e da água têm contribuído para o estado de degradação dessas pastagens. Portanto, o manejo recomendado para essas pastagens deve visar a melhoria da matéria orgânica, redução da compactação, além da reposição e ciclagem de nutrientes.

Palavras-chave:
mudanças climáticas; manejo conservacionista; pecuária; degradação do solo; sustentabilidade

HIGHLIGHTS:

Phosphorus, pH, organic carbon, and bulk density were the attributes most sensitive to soil management.

Pasture soils had carbon stock levels 0.8 to 12.4 Mg ha^-1 lower than forest soils.

Lack of conservation management has led these pastures to degradation.

Introduction

In Brazil, the Amazon biome comprises 55 million hectares of pastures used for livestock, of which 57% are moderately or severely degraded (MapBiomas, 2022MapBiomas. Coleção v.7 da série anual de mapas de uso e cobertura da terra do Brasil. Available on: <Available on: https://plataforma.brasil.mapbiomas.org/ >. Accessed on: Nov. 2022.
https://plataforma.brasil.mapbiomas.org/... ). Currently, deforestation, greenhouse gas emissions, and soil degradation are the major impacts of livestock production in the Amazon. Therefore, the sustainability of this activity in the Amazon, which continues to advance over native forests, is of considerable concern (Bueno et al., 2021Bueno, R. S.; Marchetti, L.; Cocozza, C.; Marchetti, M.; Salbitano, F. Could cattle ranching and soybean cultivation be sustainable? A systematic review and a meta-analysis for the Amazon. iForest - Biogeosciences and Forestry, v.14, p.285-298, 2021. https://doi.org/10.3832/ifor3779-014
https://doi.org/10.3832/ifor3779-014... ).

In the Amazon, there has been rapid process of modification of the natural landscape, giving way to pastures, followed by agriculture. Efforts to intensify these production systems have been adopted with the justification of productivity potential and contribution to sustainable development; however, livestock still predominates with low production rates (Ermgassen et al., 2018Ermgassen, E.; Alcântara, M.; Balmford, A.; Barioni, L.; Beduschi Neto, F.; Bettarello, M.; Brito, G.; Carrero, G.; Florence, E.; Garcia, E.; Gonçalves, E.; Luz, C.; Mallman, G.; Strassburg, B.; Valentim, J.; Latawiec, A. Results from on-the-ground efforts to promote sustainable cattle ranching in the Brazilian Amazon. Sustainability, v.10, p.1-26, 2018. https://doi.org/10.3390/su10041301
https://doi.org/10.3390/su10041301... ).

The conversion of native areas into pastures modifies soil conditions, exposing them to degradation when preceded by inadequate management, for example, by increasing the susceptibility to erosion and loss of organic matter and nutrients (Bronick & Lal, 2005Bronick, C. J.; Lal, R. Soil structure and management: A review. Geoderma , v.124, p.3-22, 2005. https://doi.org/10.1016/j.geoderma.2004.03.005
https://doi.org/10.1016/j.geoderma.2004.... ; Rueda et al., 2020Rueda, B. L.; McRoberts, K. C.; Blake, R. W.; Nicholson, C. F.; Valentim, J. F.; Fernandes, E. C. M. Nutrient status of cattle grazing systems in the western Brazilian Amazon. Cogent Food & Agriculture, v.6, p.1-19, 2020. https://doi.org/10.1080/23311932.2020.1722350
https://doi.org/10.1080/23311932.2020.17... ; Silva et al., 2022Silva, A. A.; Silva-Júnior, C. A.; Boechat, C. L.; Della-Silva, J. L.; Teodoro, P. E.; Rossi, F. S.; Teodoro, L. P. R.; Pelissari, T. D.; Baio, F. H. R.; Lima, M. Effect of land uses and land cover on soil attributes in the southern Brazilian Amazon. Environmental Monitoring and Assessment, v.194, p.1-21, 2022. https://doi.org/10.1007/s10661-022-10342-y
https://doi.org/10.1007/s10661-022-10342... ; Rocha et al. 2023Rocha, F. I.; Jesus, E. C.; Teixeira, W. G.; Lumbreras, J. F.; Clemente, E. P.; Motta, P. E. F.; Borsanelli, A. C.; Dutra, I. S.; Oliveira, A. P. Soil type determines the Magnitude of soil fertility changes by forest-to-pasture conversion in western Amazonia. Science of the Total Environment , v.856, p.1-9, 2023. https://doi.org/10.1016/j.scitotenv.2022.158955
https://doi.org/10.1016/j.scitotenv.2022... ). As a result, productivity is reduced, and environmental impacts are aggravated. However, few studies have compared agricultural practices in the Amazon with the conditions of natural vegetation (Bueno et al., 2021Bueno, R. S.; Marchetti, L.; Cocozza, C.; Marchetti, M.; Salbitano, F. Could cattle ranching and soybean cultivation be sustainable? A systematic review and a meta-analysis for the Amazon. iForest - Biogeosciences and Forestry, v.14, p.285-298, 2021. https://doi.org/10.3832/ifor3779-014
https://doi.org/10.3832/ifor3779-014... ).

Evaluating the chemical and physical attributes of soil is important for identifying the impact of soil management and allowing management that guarantees its functionality (Cardoso, 2013Cardoso, E. J. B. N.; Vasconcellos, R. L. F.; Bini, D.; Miyauchi, M. Y. H.; Santos, C. A.; Alves, P. R. L.; Paula, A. M.; Nakatani, A. S.; Pereira, J. M.; Nogueira, M. A. Soil health: looking for suitable indicators. What should be considered to assess the effects of use and management on soil health? Scientia Agricola, v.70, p.274-289, 2013. https://doi.org/10.1590/S0103-90162013000400009
https://doi.org/10.1590/S0103-9016201300... ). Therefore, the objective of this study was to evaluate the differences in the chemical and physical attributes of pasture and forest soils in the Southern Amazon and to identify the most sensitive attributes of their fertility. Additionally, to suggests more appropriate management practices for sustainable pastures.

Material and Methods

This study was conducted on eight farms that practice extensive beef cattle farming in the southern Brazilian Amazon (Figure 1). According to Köppen’s classification, the climate in the region is Am, with average temperatures between 20 and 35 ºC and rainfall between 2,000 and 3,000 mm year^-1. The predominant slope was between 5 and 15% and, the elevation was between 260 and 375 m. The study region is located in the Amazon arc of deforestation and is considered one of the most important agricultural frontiers of this biome.

Figure 1
Location map of the study area and sampling sites (A). Photographs of studied environments: Pasture and adjacent forest (B); Amazon typical extensive pasture (C); Pasture degradation by land cover failures (D); and Pasture degradation by weeds (E)

Soil samples were collected from the pastures and forest environments adjacent to the pastures of each farm during the rainy season (2021/2022). Soil samples were collected at 0-0.20 m depth in each environment from three points at least 50 m apart. At each sampling point, one composite sample was obtained from three individual samples, with a minimum distance of 10 m between them, for total of 48 samples (three samples from each pasture environment and three samples from each forest environment for each studied farm). Additionally, a semi-structured questionnaire was used to obtain technical, productive, and historical land use information (Universidade do Estado de Mato Grosso, Brazil, Research Ethics Committee: 44321521.5.0000.5166). General information for each farm is presented in Table 1.

The following chemical and physical soil analyses were performed: potential of hydrogen (pH) was determined in water (1:2.5); soil organic carbon (SOC) was determined by colorimetry after wet digestion; soil organic matter (OM) was obtained by multiplying the SOC by 1.724 (assuming that organic matter has 58% carbon); phosphorus (P) was determined using colorimetry at a wavelength of 660 nm after extraction with Mehlich-1 solution; exchangeable potassium (K⁺) was determined using flame spectrophotometry after extraction with Mehlich-1 solution; exchangeable calcium (Ca²⁺), exchangeable magnesium (Mg²⁺), and exchangeable aluminum (Al³⁺) were extracted with 1 mol L^-1 KCl and determined using titrimetry; potential acidity (H + Al) was determined using titrimetry following extraction with 0.5 mol L^-1 calcium acetate; bulk density of soil (Bd) was determined using the volumetric cylinder method; and texture (sand, silt, and clay) was determined using the densimeter method.

After conducting the chemical and physical soil analyses, the variables of the soil assortment complex and SOC stock were determined using the following equations:

where:

EB - sum of exchangeable bases;

CEC - cation exchange capacity;

m - percentage of aluminum saturation;

BS - percentage of base saturation;

SOC - soil organic carbon (g dm^-3);

Bd - bulk density of the soil (Mg m^-3) (for pasture environments, the average Bd of the forest environment adjacent to the pasture was used);

E - thickness of the soil layer (0.20 m);

A - area unit (10,000 m²); and,

F - conversion factor from kg to Mg (1,000).

The data obtained were analyzed for normality using the Shapiro-Wilk test and, when necessary, were adjusted using the Box-Cox transformation method. A Pearson correlation matrix was constructed between the main physical and chemical quantitative soil variables, followed by a linear regression analysis between highly correlated variables. Student’s t-test (parametric distribution) or Mann-Whitney U test (non-parametric distribution) was applied to compare the results between pastures (Farms I to VIII) and forest environments (adjacent to each pasture) as two naturally distinct groups (pasture × forest). The significance level was set at 5% (p ≤ 0.05).

Results and Discussion

The results of the correlation analysis between the main chemical and physical soil variables are shown in Figure 2. Positive correlations were observed between pH and EB and between clay, CEC, and OM. Although there was a positive correlation between the EB and CEC, they were collinear, as the CEC was derived from the EB (Eq. 2). Furthermore, negative correlations were observed between Al³⁺ with pH and EB, and between P and clay.

Figure 2
Pearson correlation matrix between soil chemical and physical variables. EB: sum of exchangeable bases; OM: organic matter; CEC: cation exchange capacity

The pH is one of the main indicators of soil fertility and is responsible for the adsorption and desorption reactions of chemical elements in the soil solution (Raij, 1983Raij, B. V. Avaliação da fertilidade do solo. 2.ed. Piracicaba: Editora Franciscana (LAFRAME), 1983. 142p.). Low pH conditions indicate a low availability of Ca²⁺, Mg²⁺, and K⁺ that make up EB, which explains the positive correlation between pH and EB. This also explains the negative correlation between Al³⁺ with pH and EB, considering that Al³⁺, which is inversely related to exchangeable bases, becomes more soluble as the soil pH is reduced (Havlin, 2005Havlin, J. L. Fertility. In: Encyclopedia of soils in the environment. Amsterdam: Elsivier, 2005. Chap.2, p.10-19. https://doi.org/10.1016/B0-12-348530-4/00228-9
https://doi.org/10.1016/B0-12-348530-4/0... ).

The positive correlation between clay, OM, and CEC reflect a typical soil dynamic in which clay and OM interact to form aggregates. In the aggregate structure, clay and OM are less exposed to water erosion (Zhu et al., 2022Zhu, X.; Liu, W.; Yuan, X.; Chen, C.; Zhu, K.; Zhang, W.; Yang, B. Aggregate stability and size distribution regulate rainsplash erosion: Evidence from a humid tropical soil under different land-use regimes. Geoderma , v.420, p.1-12, 2022. https://doi.org/10.1016/j.geoderma.2022.115880
https://doi.org/10.1016/j.geoderma.2022.... ), and OM is less susceptible to rapid mineralization (Liu et al., 2022Liu, X.; Li, Q.; Tan, S.; Wu, X.; Song, X.; Gao, H.; Han, Z.; Jia, A.; Liang, G.; Li, S. Evaluation of carbon mineralization and its temperature sensitivity in different soil aggregates and moisture regimes: A 21-year tillage experiment. Science of the Total Environment, v.837, p.1-11, 2022. https://doi.org/10.1016/j.scitotenv.2022.155566
https://doi.org/10.1016/j.scitotenv.2022... ). This is particularly relevant in the Amazon, which experiences intense rainfall and high temperatures. The CEC is directly influenced by clay and OM, as they form negative electrical charges in the soil and provide higher CEC values (Umasankareswari et al., 2022Umasankareswari, T.; Dharshana, V.; Saravanadevi, K.; Dorothy, R.; Sasilatha, T.; Nguyen, T. A.; Rajendran, S. Soil moisture nanosensors. In: Denizli, A.; Nguyen, T.A.N.; Rajendran, S.; Yasin, G.; Nadda, A.K. (eds.). Nanosensors for smart agriculture. Amsterdam: Elsevier, 2022. Chap.9, p.185-216. https://doi.org/10.1016/B978-0-12-824554-5.00019-7
https://doi.org/10.1016/B978-0-12-824554... ). This intricate correlation is of great importance for the sustainability of production systems, as inadequate management of these soils invariably results in their degradation.

The P results showed a behavior typical of tropical soils in which this nutrient is complexed with clay, making it less available to plants in clayey soils. In clayey soils, P is strongly adsorbed to Fe and Al oxides (Barbosa et al., 2022Barbosa, J. Z.; Poggere, G.; Mancini, M.; Silva, S. H. G.; Motta, A. C. V.; Marques, J. J. G. S. E M.; Curi, N. National-scale spatial variations of soil phosphorus retention capacity in Brazil. Physics and Chemistry of the Earth, Parts A/B/C, v.128, p.1-11, 2022. https://doi.org/10.1016/j.pce.2022.103271
https://doi.org/10.1016/j.pce.2022.10327... ), which typically occur in the main soil classes of the Amazon Biome; for example, Ultisols and Oxisols represent 72% of this territory (BDIA, 2022BDIA - Banco de Dados de Informações Ambientais. Pedologia. Available on: <Available on: https://bdiaweb.ibge.gov.br/ >. Accessed on: Nov. 2022.
https://bdiaweb.ibge.gov.br/... ).

A comparison of the chemical and physical soil variables between the pasture and forest environments is shown in Figure 3. The pasture soils had higher pH, Ca²⁺, and BS, whereas the forest soils had higher potential acidity, Al³⁺, m, P, and OM. These results indicate differences that can be attributed to pasture management, as in some farms, limestone was occasionally applied to correct soil acidity, increase exchangeable bases (Ca²⁺ and Mg²⁺), and neutralize Al³⁺.

Figure 3
Chemical and physical attributes of soils of pasture and forest environments

Lower P and OM values in pasture soils than in forest soils indicate losses of these soil attributes due to the management adopted. Additionally, using information obtained through interviews and in loco observations, it was possible to verify that no regular mineral replacement or soil conservation practices were adopted for these pastures. Improving animal productivity in pasture systems requires monitoring soil nutrient stocks, fluxes, and fertilization to ensure the replacement of extracted nutrients, without which there will be inevitable nutrient depletion and pasture degradation (Rueda et al., 2020Rueda, B. L.; McRoberts, K. C.; Blake, R. W.; Nicholson, C. F.; Valentim, J. F.; Fernandes, E. C. M. Nutrient status of cattle grazing systems in the western Brazilian Amazon. Cogent Food & Agriculture, v.6, p.1-19, 2020. https://doi.org/10.1080/23311932.2020.1722350
https://doi.org/10.1080/23311932.2020.17... ). Furthermore, adopting soil protection and conservation practices is essential for the sustainability of production systems (Silva et al., 2022Silva, A. A.; Silva-Júnior, C. A.; Boechat, C. L.; Della-Silva, J. L.; Teodoro, P. E.; Rossi, F. S.; Teodoro, L. P. R.; Pelissari, T. D.; Baio, F. H. R.; Lima, M. Effect of land uses and land cover on soil attributes in the southern Brazilian Amazon. Environmental Monitoring and Assessment, v.194, p.1-21, 2022. https://doi.org/10.1007/s10661-022-10342-y
https://doi.org/10.1007/s10661-022-10342... ).

The pasture soils had higher Bd values than the forest soils. This difference can also be attributed to the management of the production system, considering that animal trampling and occasional machinery traffic affect the soil surface, causing compaction and reducing the physical space available for air and water. For plants, there is a critical limit of soil density, beyond which it becomes a significant impediment to root growth. This limit can be estimated by considering Bd and clay content according to the equation proposed by Reichert et al. (2009Reichert, J. M.; Suzuki, L. E. A. S.; Reinert, D. J.; Horn, R.; Håkansson, I. Reference bulk density and critical degree-of-compactness for no-till crop production in subtropical highly weathered soils. Soil and Tillage Research, v.102, p.242-254, 2009. https://doi.org/10.1016/j.still.2008.07.002
https://doi.org/10.1016/j.still.2008.07.... ). Thus, the Bd critical limit of pasture environments was predominantly above or close to this limit, which was estimated between 1.32 and 1.64 g cm^-3. These results indicate the need for practices to correct or prevent excessive compaction of these soils and maintain them at an adequate level for root growth.

Regarding the interpretation of the natural fertility conditions of these soils (forest soils, Figure 3) based on the interpretation tables by Ribeiro et al. (1999Ribeiro, A. C.; Guimarães, P. T. G.; Venegas, V. H. A. (eds.). Recomendações para o uso de corretivos e fertilizantes em Minas Gerais. Viçosa: Comissão de Fertilidade do Solo do Estado de Minas Gerais, 1999. 359p.), these soils had medium and high acidity, low and medium Al³⁺, Ca²⁺, and Mg²⁺, medium K⁺, and very low P levels. This implies that using these soils for pastures also requires practices to correct the acidity and fertility of the main macronutrients required to establish and maintain forage in addition to replenishing nutrients to maintain production.

The CEC of forest soils was considered predominantly medium, with their 25^th to 75^th percentiles between 4.5 and 6.8 cmol_c dm^-3. However, it is important to note the high correlation of this attribute with OM (0.89; p ≤ 0.001; Figure 2), mainly because there was a reduction of OM in the pasture environment, which contributed to reducing the CEC by approximately 0.7 cmol_c dm^-3 to levels considered low and medium, between 4.0 and 5.8 cmol_c dm^-3. The CEC is an indicator of the potential of the soil to serve agricultural use and normally has values dependent on soil mineralogy and mainly on the OM in tropical soils (Raij, 1983Raij, B. V. Avaliação da fertilidade do solo. 2.ed. Piracicaba: Editora Franciscana (LAFRAME), 1983. 142p.). Low CEC values indicate that the soil retains few exchangeable bases, increasing the need for fertilization (Umasankareswari et al., 2022Umasankareswari, T.; Dharshana, V.; Saravanadevi, K.; Dorothy, R.; Sasilatha, T.; Nguyen, T. A.; Rajendran, S. Soil moisture nanosensors. In: Denizli, A.; Nguyen, T.A.N.; Rajendran, S.; Yasin, G.; Nadda, A.K. (eds.). Nanosensors for smart agriculture. Amsterdam: Elsevier, 2022. Chap.9, p.185-216. https://doi.org/10.1016/B978-0-12-824554-5.00019-7
https://doi.org/10.1016/B978-0-12-824554... ). Thus, we highlight the importance of preserving or improving OM to allow sufficient conditions for CEC, favoring soil fertility, and minimizing the need for agricultural inputs.

Land use change affects soil fertility; therefore, natural or anthropogenic differences in each environment can result in different responses (Silva et al., 2022Silva, A. A.; Silva-Júnior, C. A.; Boechat, C. L.; Della-Silva, J. L.; Teodoro, P. E.; Rossi, F. S.; Teodoro, L. P. R.; Pelissari, T. D.; Baio, F. H. R.; Lima, M. Effect of land uses and land cover on soil attributes in the southern Brazilian Amazon. Environmental Monitoring and Assessment, v.194, p.1-21, 2022. https://doi.org/10.1007/s10661-022-10342-y
https://doi.org/10.1007/s10661-022-10342... ; Rocha et al., 2023Rocha, F. I.; Jesus, E. C.; Teixeira, W. G.; Lumbreras, J. F.; Clemente, E. P.; Motta, P. E. F.; Borsanelli, A. C.; Dutra, I. S.; Oliveira, A. P. Soil type determines the Magnitude of soil fertility changes by forest-to-pasture conversion in western Amazonia. Science of the Total Environment , v.856, p.1-9, 2023. https://doi.org/10.1016/j.scitotenv.2022.158955
https://doi.org/10.1016/j.scitotenv.2022... ). However, we observed some common characteristics in interpreting of pasture soil fertility at each farm (Figure 4). These pastures had soil fertility conditions predominantly below the good level, mainly with regard to P, which was below the minimum level on all farms. This highlights the need to implement better soil fertility management practices, such as improving OM and nutrient cycling and regular replenishment of nutrients to cultivate pastures.

Figure 4
Interpretation of pasture soil fertility in each farm (I-VIII)

The SOC stock results for the pasture and forest environments are shown in Figure 5. Despite the variability between the studied sites (Figure 5A), the pasture environment had the lowest SOC stocks ( = 28 Mg ha^-1) compared to the forest environment ( = 33.7 Mg ha^-1) (Figure 5B). The magnitude of this difference in each location studied ranged from 2.8 to 34.8% ( = 16.3) of SOC loss from the pasture environment in relation to the respective forest environment locations (Figure 5C). This means that the conversion of forest areas into pasture represented SOC losses of approximately 0.8 to 12.4 Mg ha^-1 ( = 5.7), even when considering only the surface soil layer (0 to 0.20 m).

Figure 5
Soil organic carbon (SOC) stocks in the forest and pasture environments (layer 0 to 0.20 m). Average and standard error of SOC stock in each studied farm (I-VIII) and environment (A), Boxplot with average of SOC stock data and statistical significance by Student’s t-test for environments studied (B), SOC stock in the pasture in relation to the respective adjacent forest environment (C), and Pearson correlation analysis for SOC stock with organic matter (OM), clay, sand, and bulk density (Bd) in the forest and pasture environments (D)

The correlation results for SOC stock with soil physical attributes suggested differences between the forest and pasture environments (Figure 5D). Clay and sand showed stronger correlations with SOC stock in pasture than in forest environments. These results reinforce the importance of considering the natural characteristics of these soils, which may be more fragile depending on the management conditions to which they are exposed. For example, low-clay soils (sandy soils) are more likely to suffer the greatest losses of soil carbon because of the absence of conservationist practices because the soil texture is related to the formation of aggregates, which directly influences its ability to retain OM (Zhu et al., 2022Zhu, X.; Liu, W.; Yuan, X.; Chen, C.; Zhu, K.; Zhang, W.; Yang, B. Aggregate stability and size distribution regulate rainsplash erosion: Evidence from a humid tropical soil under different land-use regimes. Geoderma , v.420, p.1-12, 2022. https://doi.org/10.1016/j.geoderma.2022.115880
https://doi.org/10.1016/j.geoderma.2022.... ).

In the pastures, the reduction in SOC stock was correlated with an increase in Bd, which did not occur in the forest environment (Figure 5D). This difference was associated with pasture soil management, which is more exposed to surface impacts, as discussed in relation to Bd (Figure 3). Additionally, in soils with a higher SOC stock, higher OM also contributes to a reduction in Bd because of its lower density in relation to the soil and its role in dampening the surface impact, which is particularly more relevant in managed soils. This explains the different correlations between SOC stock and Bd in the pasture and forest environments. Although, it is important to highlight that in compacted soils, there is less root development, difficulty in plant growth, and increased laminar erosion, among other aspects, contributing to the general degradation of the pasture and increased CO₂ emissions (Bronick & Lal, 2005Bronick, C. J.; Lal, R. Soil structure and management: A review. Geoderma , v.124, p.3-22, 2005. https://doi.org/10.1016/j.geoderma.2004.03.005
https://doi.org/10.1016/j.geoderma.2004.... ). That is, soil compaction indirectly implies a reduction in SOC stock.

Achieving a level of sustainable livestock development in the Amazon is a great challenge; however, seeking solutions to achieve this target is necessary. Many pathways have been suggested to reduce deforestation (Skidmore et al., 2021Skidmore, M. E.; Moffette, F.; Rausch, L.; Christie, M.; Munger, J.; Gibbs, H. K. Cattle ranchers and deforestation in the Brazilian Amazon: Production, location, and policies. Global Environmental Change, v.68, p.1-14, 2021. https://doi.org/10.1016/j.gloenvcha.2021.102280
https://doi.org/10.1016/j.gloenvcha.2021... ), reduce environmental impacts (Dick et al., 2021Dick, M.; Silva, M. A.; Silva, R. R. F.; Ferreira, O. G. L.; Maia, M. S; Lima, S. F.; Paiva-Neto, V. B.; Dewes, H. Environmental impacts of Brazilian beef cattle production in the Amazon, Cerrado, Pampa, and Pantanal biomes. Journal of Cleaner Production, v.311, p.1-10, 2021. https://doi.org/10.1016/j.jclepro.2021.127750
https://doi.org/10.1016/j.jclepro.2021.1... ), enhance productivity (Ermgassen et al., 2018Ermgassen, E.; Alcântara, M.; Balmford, A.; Barioni, L.; Beduschi Neto, F.; Bettarello, M.; Brito, G.; Carrero, G.; Florence, E.; Garcia, E.; Gonçalves, E.; Luz, C.; Mallman, G.; Strassburg, B.; Valentim, J.; Latawiec, A. Results from on-the-ground efforts to promote sustainable cattle ranching in the Brazilian Amazon. Sustainability, v.10, p.1-26, 2018. https://doi.org/10.3390/su10041301
https://doi.org/10.3390/su10041301... ), and protect biodiversity (Neate-Clegg & Şekercioğlu, 2020Neate-Clegg, M. H. C.; Şekercioğlu, Ç. H. Agricultural land in the Amazon basin supports low bird diversity and is a poor replacement for primary forest. The Condor, v.122, p.1-11, 2020. https://doi.org/10.1093/condor/duaa020
https://doi.org/10.1093/condor/duaa020... ). However, it remains unclear how and when cattle ranching in the Amazon will become sustainable (Bueno et al., 2021Bueno, R. S.; Marchetti, L.; Cocozza, C.; Marchetti, M.; Salbitano, F. Could cattle ranching and soybean cultivation be sustainable? A systematic review and a meta-analysis for the Amazon. iForest - Biogeosciences and Forestry, v.14, p.285-298, 2021. https://doi.org/10.3832/ifor3779-014
https://doi.org/10.3832/ifor3779-014... ).

Considering the multifunctionality of soil, which provides several essential ecosystem services for life (Adhikari & Hartemink, 2016Adhikari, K.; Hartemink, A. E. Linking soils to ecosystem services - A global review. Geoderma, v.262, p.101-111, 2016. https://doi.org/10.1016/j.geoderma.2015.08.009
https://doi.org/10.1016/j.geoderma.2015.... ), guaranteeing soil health is necessary for the sustainability of livestock production systems. Therefore, pasture management in the Amazon is a basic prerequisite for guaranteeing soil health through production practices.

In the regression analysis, we found indications that an increment of only 5 g dm^-3 of OM would result in improvements of approximately 1.1 cmol_c dm^-3 of CEC (Figure 6A) and 5 Mg ha^-1 in SOC stock in the topsoil layer (Figure 6B). This would represent a significant improvement in the fertility of these soils, increasing their capacity to retain and make available nutrients, favoring the efficiency of fertilizers, or even reducing their dependence on them. Furthermore, considering 55 million hectares of pastures in the Amazon (MapBiomas, 2022MapBiomas. Coleção v.7 da série anual de mapas de uso e cobertura da terra do Brasil. Available on: <Available on: https://plataforma.brasil.mapbiomas.org/ >. Accessed on: Nov. 2022.
https://plataforma.brasil.mapbiomas.org/... ), this would represent the sequestration of one petagram of atmospheric CO₂ (5×10^-9 Pg C ha^-1 × 5.5×10⁷ ha × 3.66 CO₂), which reinforces the potential of soil as a carbon sink to mitigate climate change (Lal, 2020Lal, R. Managing soils for negative feedback to climate change and positive impact on food and nutritional security. Soil Science and Plant Nutrition, v.66, p.1-9, 2020. https://doi.org/10.1080/00380768.2020.1718548
https://doi.org/10.1080/00380768.2020.17... ). However, adopting certain agricultural practices should be carefully considered, as they may eventually result in adverse effects on soil quality, such as environmental contamination from fertilization with organic waste (Scheid et al., 2020Scheid, D. L.; Silva, R. F.; Silva, V. R.; Ros, C. O.; Pinto, M. A. B.; Gabriel, M.; Cherubin, M. R. Changes in soil chemical and physical properties in pasture fertilised with liquid swine manure. Scientia Agricola , v.77, p.1-10, 2020. https://doi.org/10.1590/1678-992x-2019-0017
https://doi.org/10.1590/1678-992x-2019-0... ).

Figure 6
Linear regression of cation exchange capacity (CEC) (A) and soil organic carbon (SOC) stock (B) in response to organic matter (OM) in the forest and pasture environments in the Southern Amazon

To improve OM and promote soil health, several production techniques that are already widely recognized by conservationists and ecological pasture managers can contribute. Among these, some options may require low economic investment, such as improving grazing control, avoiding overgrazing, and excessive animal stocking load (McTavish et al., 2021McTavish, M. J.; Cray, H. A.; Murphy, S. D.; Bauer, J. T.; Havrilla, C. A.; Oelbermann, M.; Sayer, E. J. Sustainable management of grassland soils. In: Stanturf, J. A.; Callaham, M.A. Soils and landscape restoration. Amsterdam: Elsivier , 2021. Chap.4, p.95-124. https://doi.org/10.1016/B978-0-12-813193-0.00004-7
https://doi.org/10.1016/B978-0-12-813193... ), such as the adaptive multipaddock technique (Teague & Kreuter, 2020Teague, R.; Kreuter, U. Managing grazing to restore soil health, ecosystem function, and ecosystem services. Frontiers in Sustainable Food Systems, v.4, p.1-13, 2020. https://doi.org/10.3389/fsufs.2020.534187
https://doi.org/10.3389/fsufs.2020.53418... ). Among other techniques, there are integrated production systems, such as crop-livestock-forestry integration, which have achieved promising results despite requiring high investments and payback time between 2.5 and 8.5 years (Ermgassen et al., 2018Ermgassen, E.; Alcântara, M.; Balmford, A.; Barioni, L.; Beduschi Neto, F.; Bettarello, M.; Brito, G.; Carrero, G.; Florence, E.; Garcia, E.; Gonçalves, E.; Luz, C.; Mallman, G.; Strassburg, B.; Valentim, J.; Latawiec, A. Results from on-the-ground efforts to promote sustainable cattle ranching in the Brazilian Amazon. Sustainability, v.10, p.1-26, 2018. https://doi.org/10.3390/su10041301
https://doi.org/10.3390/su10041301... ).

To overcome the constant challenges related to the costs associated with the adoption of better agricultural practices, the implementation of incentives through rural extension programs (Rocha-Júnior et al., 2020Rocha-Júnior, A. B.; Silva, R. O.; Peterle-Neto, W.; Rodrigues, C. T. Efeito da utilização de assistência técnica sobre a renda de produtores familiares do Brasil no ano de 2014. Revista de Economia e Sociologia Rural, v.58, p.1-16, 2020. https://doi.org/10.1590/1806-9479.2020.194371
https://doi.org/10.1590/1806-9479.2020.1... ), payments for environmental services (Biggs et al., 2021Biggs, N. B.; Hafner, J.; Mashiri, F. E.; Huntsinger, L.; Lambin, E. F. Payments for ecosystem services within the hybrid governance model: evaluating policy alignment and complementarity on California rangelands. Ecology and Society, v.26, p.1-31, 2021. https://doi.org/10.5751/ES-12254-260119
https://doi.org/10.5751/ES-12254-260119... ), and credit subsidies for sustainable projects (Costa-Júnior et al., 2019Costa-Júnior, N. B.; Baldissera, T. C.; Pinto, C. E.; Garagorry, F. C.; Moraes, A.; Carvalho, P. C. F. Public policies for low carbon emission agriculture foster beef cattle production in southern Brazil. Land Use Policy , v.80, p.269-273, 2019. https://doi.org/10.1016/j.landusepol.2018.10.014
https://doi.org/10.1016/j.landusepol.201... ) are considered important options. Additionally, the evaluation of ecosystem services through indicators, such as OM, Bd, and soil sorption complexes, can be adopted in these areas of the Amazon to monitor changes toward sustainability. Additionally, these indicators are relatively simple, accessible, and inexpensive.

Finally, it is necessary to consider the existence of several aspects to be overcome or understood in the context of farms in the Amazon for the sustainable intensification of livestock production, such as precarious infrastructure, lack of qualified labor, an unfavorable regulatory environment, and the cultural value of the traditional method of livestock production (Cortner et al., 2019Cortner, O.; Garrett, R. D.; Valentim, J. F.; Ferreira, J.; Niles, M. T.; Reis, J.; Gil, J. Perceptions of integrated crop-livestock systems for sustainable intensification in the Brazilian Amazon. Land Use Policy, v.82, p.841-853, 2019. https://doi.org/10.1016/j.landusepol.2019.01.006
https://doi.org/10.1016/j.landusepol.201... ). This highlights the importance of promoting the strengthening of actions of an educational nature, technological dissemination, and the development of scientific research with multidisciplinary approaches to support all dimensions of the concept of social, economic, and environmental sustainability, and to overcome multiple established challenges.

Conclusions

Acknowledgments

We thank the cattle ranchers who made their farms and time available to contribute to this research and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior do Brasil (CAPES) for granting a scholarship to the first author of this study through the Programa de Doutorado Sanduíche no Exterior (PDSE 2022).

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