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Land use change and greenhouse gas emissions: an explanation about the main emission drivers

Abstract

Global warming is attributed to the increase in greenhouse gas (GHG) emissions, such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Land use changes significantly impact on GHG emissions, accounting for approximately 44% of the country’s emissions in 2019. This review addresses the main pathways of GHG formation in the soil, focusing on the influence of land use changes on GHG emissions. It is found that soil CO2 emissions are related to root respiration, microorganisms, and organic matter (OM) decomposition in the soil. Changes in land use can alter soil characteristics, favoring increased CO2 emissions. Soil CH4 emissions occur under anaerobic conditions by methanogenic microorganisms; however, land use changes, such as forest conversion to pasture, can increase CH4 emissions due to a higher concentration of methanogenic microorganisms in the soil. On the other hand, N2O is produced in the soil during nitrification and denitrification processes by microorganisms, and nitrogen fertilization in agricultural areas can increase N2O emissions, especially when associated with soil moisture and the availability of organic carbon. It is important to understand the dynamics of GHG formation and emissions resulting from land use changes because efficient management strategies can reduce these emissions and contribute to Brazil’s goals for GHG reduction as established in international agreements.

Keywords:
Carbon stock; methane in the soil; nitrous oxide in the soil

Resumo

O aquecimento global é atribuído ao aumento das emissões de gases de efeito estufa (GEE), como dióxido de carbono (CO2), metano (CH4) e óxido nitroso (N2O). As mudanças no uso da terra têm impactos significativos nas emissões de GEE, sendo responsáveis por aproximadamente 44% das emissões do país em 2019. Essa é uma revisão que aborda as principais rotas de formação dos GEE no solo com foco na influência das mudanças do uso da terra nas emissões de GEE. Constata-se que as emissões de CO2 pelo solo estão relacionadas à respiração de raízes, microrganismos e decomposição da matéria orgânica (MO) do solo, assim mudanças no uso da terra podem alterar as características do solo, favorecendo a intensificação das emissões de CO2. As emissões de CH4 pelo solo ocorrem em condições de anaerobiose por microrganismos metanogênicos, no entanto as mudanças no uso da terra, como a conversão de florestas em pastagens, podem aumentar as emissões de CH4 devido a uma maior concentração de microrganismos metanogênicos no solo. Já o N2O é produzido no solo durante o processo de nitrificação e desnitrificação por microrganismos, e a fertilização nitrogenada em áreas agrícolas pode aumentar as emissões de N2O, especialmente quando associada à umidade e disponibilidade de carbono orgânico no solo. Destaca-se a importância de compreender as dinâmicas de formação e emissão de GEE decorrentes das mudanças de uso da terra, pois estratégias eficientes de manejo podem reduzir essas emissões e contribuir para o cumprimento das metas do Brasil em relação à redução de GEE estabelecidas em acordos internacionais.

Palavras-chave:
estoque de carbono; metano no solo; óxido nitroso no solo

1. Introduction

Global warming has been attributed to the excessive emission of greenhouse gases (GHGs) into the atmosphere, such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These gases are present in the atmosphere at different concentrations, with CO2 reaching the highest mark, followed by CH4 and N2O, respectively. It is important to emphasize that among the gases with the highest pollution capacity are CH4 and N2O, with pollution capacities 28 and 265 times that of CO2, respectively (11 IPCC. Resumo para formuladores de políticas. In: Mudança Climática e Terra: um relatório especial do IPCC sobre mudança climática, desertificação, degradação da terra, gestão sustentável da terra, segurança alimentar e fluxos de gases de efeito estufa em ecossistemas terrestres. 2019. 41p.). However, the main GHG emitted by Brazil is CO2 (22 Eggleston HS, Buendia L, Mika K, Ngara T, Tanake K. IPCC guidelines for national greenhouse gas inventories. Hayama: IGES, 2006. https://www.ipcc.ch/report/2006-ipcc-guidelines-for-national-greenhouse-gas-inventories/
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).

The production of these gases is optimized in the sectors of energy, agriculture, industrial processes, waste, and land use change, the latter being responsible for 45% of GHG emissions in Brazil (33 Sistema De Estimativa De Emissões De Gases De Efeito Estufa - SEEG. SEEG Brasil. Emissões totais série histórica. Disponível em: <http://plataforma.seeg.eco.br/total_emission#> Acesso em 15/05/2021.
http://plataforma.seeg.eco.br/total_emis...
). In 2019, in this country the process of land use change accounted for approximately 44% of greenhouse gas (GHG) emissions, followed by agricultural activities with 27% (33 Sistema De Estimativa De Emissões De Gases De Efeito Estufa - SEEG. SEEG Brasil. Emissões totais série histórica. Disponível em: <http://plataforma.seeg.eco.br/total_emission#> Acesso em 15/05/2021.
http://plataforma.seeg.eco.br/total_emis...
).

Land use changes alter the physical, chemical, and biological characteristics of the soil, creating favorable conditions for the intensification of GHG emissions (44 Moges A, Dagnachew M, Yimer F. Land Use Effects on Soil Quality Indicators: A Case Study of Abo-Wonsho Southern Ethiopia. Applied and Environmental Soil Science. 2013, 2013:784989. https://doi.org/10.1155/2013/784989
https://doi.org/10.1155/2013/784989...
, 55 Kroeger ME, Delmont TO, Eren AM, Meyer KM, Guo J, Khan K, Rodrigues JLM, Bohannan BJM, Tringe SG, Borges CD. New Biological Insights Into How Deforestation in Amazonia Affects Soil Microbial Communities Using Metagenomics and Metagenome-Assembled Genomes. Frontiers In Microbiology. 2018;9:1635. Disponível em: http://dx.doi.org/10.3389/fmicb.2018.01635.
http://dx.doi.org/10.3389/fmicb.2018.016...
). Moreover, the magnitude of such emissions is influenced by factors such as temperature, humidity, nitrogen and carbon levels, and soil’s microbiological patterns (66 Tavanti RFR, Montarini R, Panosso AR, Scala Jr NL, Neto MC, Freddi ODaS, Gazáles AP, Carvalho MAC, Soares MB, Tavanti TR, Galino FS. What is the impact of pasture reform on organic carbon compartments and CO2 emissions in the Brazilian Cerrado?. Catena. 2020;194:104702. Disponível em: https://doi.org/10.1016/j.catena.2020.104702
https://doi.org/10.1016/j.catena.2020.10...
).

Given the national and global situation, it is evident that it is crucial the existence of research aimed at understanding the dynamics of GHG formation and emissions resulting from land use changes. This research helps determine efficient management techniques that could reduce emissions and promote greater conservation of natural resources, contributing to the achievement of objectives assumed by Brazil in agreements, as the Paris Agreement in 2016, highlighted at COP26 in 2021.

Therefore, this review addresses the anthropogenic actions that intensify the greenhouse effect and, consequently, global warming. It outlines the main pathways of GHG formation, indicating the most important factors that influence the formation and emission of these gasses, and provides an understanding of how land use changes affect emission pathways.

2. Greenhouse effect and global warming: a brief explanation

The greenhouse effect and the presence of GHGs in the atmosphere are natural phenomena. While a portion of the radiation that reaches the Earth is absorbed by oceans, rivers, soil, and plants, another part of the energy is reflected into space, where it is trapped by a layer of gasses in the atmosphere primarily composed of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). This phenomenon is known as the greenhouse effect. The retention of part of the incoming solar radiation in the atmosphere is responsible for keeping the planet warm, preventing it from freezing (77 Oktyabrskiy VP. A new opinion of the greenhouse effect. St. Petersburg Polytechnical University Journal: Physics and Mathematics. 2016;2:124-126. Disponível em: https://doi.org/10.1016/j.spjpm.2016.05.008.
https://doi.org/10.1016/j.spjpm.2016.05....
).

Nevertheless, over the past centuries, there have been significant increases in the intensity of greenhouse gas production and the amount of emissions. Consequently, the layer of GHGs in the atmosphere is becoming gradually thicker, thus retaining more energy and fueling global warming in an unrestrained manner (88 Xu Y, Cui G. Influence of spectral characteristics of the Earth’s surface radiation on the greenhouse effect: Principles and mechanisms. Atmospheric Environment. 2021;244:117908. Disponível em: https://doi.org/10.1016/j.atmosenv.2020.117908
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).

According to the IPCC (Intergovernmental Panel on Climate Change) (99 IPCC. Climate change 2007: the physical science basis. Contribution of working group I to the fourth assessment report of the intergovernmental panel on climate change. Cambridge: Cambridge University Press, United Kingdom, 2007. 996 p.), global warming is characterized by a substantial increase in the average temperature of the planet. This influences the speed of polar ice melting, sea level rise, extinction of various living organisms, such as plants and animals, as well as alterations in climatic conditions, which results in negative impacts on agricultural productivity.

GHGs are present in the atmosphere in different concentrations, with the order of magnitude being CO2 (66%), CH4 (16%), and N2O (7%). These gasses are expressed in terms of CO2 equivalent or Global Warming Potential (GWP) (11 IPCC. Resumo para formuladores de políticas. In: Mudança Climática e Terra: um relatório especial do IPCC sobre mudança climática, desertificação, degradação da terra, gestão sustentável da terra, segurança alimentar e fluxos de gases de efeito estufa em ecossistemas terrestres. 2019. 41p.). Hence, CO2 weights 1 GWP, CH4 weights 28 GWP, and N2O weights 265 GWP. It is worth noting that, due to the threat of global warming to human survival on planet Earth, numerous nations have mobilized the search for alternatives to mitigate GHG emissions.

In 2016, Brazil signed the Paris Agreement and committed to reducing the amount of greenhouse gas (GHG) emissions of 2005 by 37% by 2025, ratifying goals already established by national Law No. 12,187 of 2009 and national decree No. 9,578 of 2018 (1010 Brasil, Lei n° 12.187, DE 29 DE DEZEMBRO DE 2009. Diário Oficial da União - DOU, 2009. Portuguese., 1111 Brasil, Nº 9.578, DE 22 DE NOVEMBRO DE 2018. Diário Oficial da União - DOU, 2018. Portuguese.-1212 TALANOA - Instituto Internacional de Políticas Públicas. A Política Nacional de Mudança do Clima em 2020: estado de metas, mercados e governança assumidos na Lei 12.187/2009. Rio de Janeiro, Brasil. 2020, 83p.). During COP26 in 2021, the country reaffirmed its objectives stipulating a 50% reduction in GHG emissions when comparing 2030 data to 2005 levels (1313 Brasil, MINISTÉRIO DO MEIO AMBIENTE. Acesso em 10-06-2022, Available from: https://www.gov.br/mma/ptbr/noticias/brasil-inicia-agenda-de-negociacoes-com-boas-perspectivas>. 2021.portuguese.
https://www.gov.br/mma/ptbr/noticias/bra...
). Currently, Brazil has implemented the ABC+ plan, which aims for a low-emission economy in agriculture through activities such as pasture recovery, integrated crop-livestock-forestry systems, agroforestry systems, no-till farming, biological nitrogen fixation, planted forests, animal waste treatment, and climate change adaptation, with validity until 2030 (1414 Brasil. PLANO SETORIAL PARA ADAPTAÇÃO À MUDANÇA DO CLIMA E BAIXA EMISSÃO DE CARBONO NA AGROPECUÁRIA COM VISTAS AO DESENVOLVIMENTO SUSTENTÁVEL (2020-2030). MAPA, Brasília, 2021. 28p. Portuguese.).

3. Soil CO2 emissions

The soil is the planet’s main carbon reservoir, with soil organic matter (SOM) consisting of 55-60% by mass of carbon (C), considered one of the largest reservoirs in the world, with 1,300 to 1,500 Pg of C in the top meter (1515 Jackson RB, Lajtha K, Crow SE, Hugelius G, Kramer MG, Piñeiro G. The Ecology of Soil Carbon: pools, vulnerabilities, and biotic and abiotic controls. Annual Review of Ecology, Evolution, And Systematics. 2017;48(1):419-445. Disponível em: http://dx.doi.org/10.1146/annurev-ecolsys-112414-054234
http://dx.doi.org/10.1146/annurev-ecolsy...
). Soil CO2 emissions are related to root respiration, microbial activity, and the decomposition of SOM. Regarding land use changes specifically, CO2 emissions mainly result from alterations in the concentrations of different soil organic matter fractions, influenced by quality, addition or removal of SOM, and fertility (1616 Yang X, Meng J, Lan Y, Chen W, Yang T, Yuan J, Liu S, Han J. Effects of maize stover and its biochar on soil CO2 emissions and labile organic carbon fractions in Northeast China. Agriculture Ecosystem & Environment. 2017;240:24-31. Disponível em: https://doi.org/10.1016/j.agee.2017.02.001.
https://doi.org/10.1016/j.agee.2017.02.0...
).

Land use changes typically cause alterations in soil chemical parameters, mainly due to differences in management between the new and old systems (1717 De Carvalho MAC, Panosso AR, Ribeiro Teixeira EE, Araújo EG, Brancaglioni VA, Dallacort R. Multivariate approach of soil attributes on the characterlization of land use in the southern Brazilian Amazon. Soil Tillage Research. 2018;184:207-215. Disponível em: https://doi.org/10.1016/j.still.2018.08.004
https://doi.org/10.1016/j.still.2018.08....
). Additionally, some modifications involve biomass burning (1818 Chang J, Ciais P, Gasser T, Smith P, Herrero M, Havlík P, Obersteiner M, Guenet B, Goll DS, Li W, Naipal V, Peng S, Qiu C, Tian H, Viovy N, Yue C, Zhu D. Climate warming from managed grasslands cancels the cooling effect of carbon sinks in sparsely grazed and natural grasslands. Nature Communications. 2021;12(118):1-10. Disponível em: https://doi.org/10.1038/s41467-020-20406-7
https://doi.org/10.1038/s41467-020-20406...
), leading to losses in carbon stocks and driving CO2 emissions (1919 Andrew JM, Kirsten DH, Tirha G, David E, Melanie DJ. Storage of soil carbon as particulate and mineral associated organic matter in irrigated woody perennial crops. Geoderma, 2021;403:115185. Disponível em: https://doi.org/10.1016/j.geoderma.2021.115185
https://doi.org/10.1016/j.geoderma.2021....
). Land use changes can also influence soil temperature, considered a limiting factor for CO2 production (R2 = 55), as it affects soil microbial activity (2020 Zhang Q, Wu J, Lei Y, Yang F, Zhang D, Zhang K, Zhang Q, Cheng X. Agricultural land use change impacts soil CO2 emission and its 13 C-isotopic signature in central China. Soil and Tillage Research. 2018;177:105-112. Disponível em: https://doi.org/10.1016/j.still.2017.11.017
https://doi.org/10.1016/j.still.2017.11....
, 2121 Zhang Y, Zhao W, Fu L, Zhao C, Jia A. Land use conversion influences soil respiration across a desertoasis ecoregion in Northwest China, with consideration of cold season CO2 efflux and its significance. Catena. 2020;188:104460. Disponível em: https://doi.org/10.1016/j.catena.2020.104460
https://doi.org/10.1016/j.catena.2020.10...
).

The soil disturbance result of some activities involved in land use changes can contribute to CO2 emissions because it causes disruptions in soil aggregates, allowing for greater aeration and even increased water infiltration (44 Moges A, Dagnachew M, Yimer F. Land Use Effects on Soil Quality Indicators: A Case Study of Abo-Wonsho Southern Ethiopia. Applied and Environmental Soil Science. 2013, 2013:784989. https://doi.org/10.1155/2013/784989
https://doi.org/10.1155/2013/784989...
). This disruption leads to greater exposure of soil organic matter (SOM), enabling increased oxidation of soil organic carbon by microorganisms, resulting in higher CO2 production (Figure 1) (1515 Jackson RB, Lajtha K, Crow SE, Hugelius G, Kramer MG, Piñeiro G. The Ecology of Soil Carbon: pools, vulnerabilities, and biotic and abiotic controls. Annual Review of Ecology, Evolution, And Systematics. 2017;48(1):419-445. Disponível em: http://dx.doi.org/10.1146/annurev-ecolsys-112414-054234
http://dx.doi.org/10.1146/annurev-ecolsy...
, 2222 Samson ME, Chantigny MH, Vanasse A, Menasseri-Aubry S, Royer I, Angers DA. Management practices differently affect particulate and mineral-associated organic matter and their precursors in arable soils. Soil Biology and Biochemistry. 2020;148:107867. Disponível em: https://doi.org/10.1016/j.soilbio.2020.107867
https://doi.org/10.1016/j.soilbio.2020.1...
, and 2323 Lavallee JM, Soong JL, Cotrufo MF. Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century. Global Change Biology. 2020;26(1):261-27. Disponível em: https://doi.org/10.1111/gcb.14859
https://doi.org/10.1111/gcb.14859...
).

Figure 1
Carbon Cycle (C) in Soil

Soil organic matter (SOM) is one of the main factors regulating CO2 emissions and it is typically divided into two fractions: (1) particulate organic carbon (POC), and (2) mineralassociated organic carbon (MOC). POC has a density lower than 1.6-1.85 g/cm3 or greater than 1.6-1.85 g/dm3, but it is associated with particle sizes larger than 50-63 µm, whereas MOC has a density greater than 1.6-1.85 g/dm3, associated with particle sizes smaller than 50-63 µm (2323 Lavallee JM, Soong JL, Cotrufo MF. Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century. Global Change Biology. 2020;26(1):261-27. Disponível em: https://doi.org/10.1111/gcb.14859
https://doi.org/10.1111/gcb.14859...
).

Among the two fractions of soil organic matter, POC stands out as a significant influencer of CO2 emissions, considered a coarser fraction of carbon in the soil, meanwhile, MOC is a more stable fraction with a lower likelihood of being reduced in the soil, as it holds organomineral associations (2323 Lavallee JM, Soong JL, Cotrufo MF. Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century. Global Change Biology. 2020;26(1):261-27. Disponível em: https://doi.org/10.1111/gcb.14859
https://doi.org/10.1111/gcb.14859...
, 2424 Mikutta R, Turner S, Schippers A, Gentsch N, Meyer-Stüve S, Condron Lm, Peltzer Da, Richardson Sj, Eger A, Hempel G, Kaiser K, Klotzbücher T, Uggenberger GG. Microbial and abiotic controls on mineral-associated organic matter in soil profiles along an ecosystem gradient. Scientific reports. 2019;9(1):1-9. Disponível em: https://doi.org/10.1038/s41598-019-46501-4
https://doi.org/10.1038/s41598-019-46501...
). Consequently, higher levels of MOC in the soil are associated with lower CO2 emissions.

The role of soil texture in protecting soil organic matter (SOM) and forming more stable carbon (C) fractions in the soil, leading to a reduction in greenhouse gas (GHG) emissions, remains contradictory. Bruun et al. (2525 Bruun TB, Elberling B, De Neergaard A, Magid J. Organic carbon dynamics in different soil types after conversion of forest to agriculture. Land Degradation & Development. 2013;26(3):272-283. Disponível em: https://doi.org/10.1002/ldr.2205
https://doi.org/10.1002/ldr.2205...
), Miranda et al. (2626 Miranda E, Carmo J, Couto E, Camargo P. Long-term changes in soil carbono stocks in the Brazilian Cerrado under commercial soybean. Land Degradation & Development. 2016;27(6):1586-1594. Disponível em: https://doi.org/10.1002/ldr.2473.
https://doi.org/10.1002/ldr.2473...
), and Tavanti et al. (66 Tavanti RFR, Montarini R, Panosso AR, Scala Jr NL, Neto MC, Freddi ODaS, Gazáles AP, Carvalho MAC, Soares MB, Tavanti TR, Galino FS. What is the impact of pasture reform on organic carbon compartments and CO2 emissions in the Brazilian Cerrado?. Catena. 2020;194:104702. Disponível em: https://doi.org/10.1016/j.catena.2020.104702
https://doi.org/10.1016/j.catena.2020.10...
) demonstrated that clay is capable of protecting soil organic matter, hence preventing the oxidation of carbon into CO2. In contrast, Midwood et al. (2727 Midwood AJ, Hannam KD, Gebretsadikan T, Emde D, Jones MD. Storage of soil carbon as particulate and mineral associated organic matter in irrigated woody perennial crops. Geoderma. 2021;403:115185. Disponível em: https://doi.org/10.1016/j.geoderma.2021.115185.
https://doi.org/10.1016/j.geoderma.2021....
) stated that soil texture has a low correlation (R2 = 0.007 - 0.17) with the process of carbon stabilization and the subsequent formation of SOM fractions, especially mineral-associated organic carbon (MOC). According to these authors, in clayey soils, the higher presence of MOC is due to mineral weathering associated with high humidity, rather than the direct effect of clay.

An alternative for reducing CO2 emissions concerning land use changes is to decrease mechanical soil tillage practices, as this will result in an accumulation of soil organic matter (SOM). This effect occurs predominantly in the surface layers (0-30 cm), as observed by Riltt et al. (2828 Rittl TF, Oliveira D, Cerri CEP. Soil carbon stock changes under different land uses in the Amazon. Geoderma Regional. 2017;10:138-143. Disponível em: https://doi.org/10.1016/j.geodrs.2017.07.004.
https://doi.org/10.1016/j.geodrs.2017.07...
) when converting pastures to agricultural areas, and by Damian et al. (2929 Damian JM, Matos ES, Pedreira BC, Carvalho PCF, Premazzi LM, Williams S, Paustian K, Cerri CEP. Predicting soil C changes after pasture intensification and diversification in Brazil. Catena. 2021 202:105238. Disponível em: https://doi.org/10.1016/j.catena.2021.105238
https://doi.org/10.1016/j.catena.2021.10...
) when converting poorly managed pastures into integrated crop-livestock-forestry systems for well-managed pasture areas.

The conversion of certain land uses into pasture systems can lead to reductions in soil carbon (C) levels in the first years following conversion. It is expected that after 10 years of conversion to well-managed pasture systems, soil organic carbon stocks (SOCs) will be similar to or higher than the initial values at system implementation. Much of this result is due to the extensive deposition of plant residues and carbon into the soil through the action of grasses implanted in the system (3030 Santos CA, Rezende CP, Pinheiro ÉFM, Pereira JM, Alves BJR, Urquiaga S, Boddey RM. Changes in soil carbon stocks after land-use change from native vegetation to pastures in the Atlantic forest region of Brazil. Geoderma. 2019;337:94-401. Disponível em: https://doi.org/10.1016/j.geoderma.2018.09.045
https://doi.org/10.1016/j.geoderma.2018....
, 3131 Pinheiro FM, Nair PKR, Nair VD, Tonucci RG, Venturin RP. Soil carbon stock and stability under Eucalyptusbased silvopasture and other land-use systems in the Cerrado biodiversity hotspot. Journal of Environmental Management. 2021;299:113676. Disponível em: https://doi.org/10.1016/j.jenvman.2021.113676.
https://doi.org/10.1016/j.jenvman.2021.1...
). Conversely, the conversion of pasture or native forest areas to agricultural areas can result in a steady decline in SOCs for up to 25 years after the conversion due to soil preparation using machinery and low soil cover in some periods of the year (3232 Dalal RC, Thornton CM, Allen DE, Owens JS, Kopittke PM. Long-term land use change in Australia from native forest decreases all fractions of soil organic carbon, including resistant organic carbon, for cropping but not sown pasture. Agriculture, Ecosystems & Environment. 2021;311:107326. Disponível em: https://doi.org/10.1016/j.agee.2021.107326
https://doi.org/10.1016/j.agee.2021.1073...
).

According to the compiled data (Figure 2), it is possible to observe that the cumulative soil carbon (C) in well-managed pasture systems can offset the CO2eq emissions from certain animal stocking rates, indicating these types of land use as alternatives to combine economic gain with sustainable production. Based on this research, it is also evident that poorly managed pasture systems contribute to CO2 emissions into the atmosphere, not only because they emit CO2, but also because they do not sequester carbon.

Figure 2
Comparison between enteric CO2eq emissions by cattle and the exclusively stored recovery of CO2eq by the soil from different systems, all equivalent to a period of 2 years

In poorly managed pasture systems, the input of carbon into the soil is primarily reduced due to the low efficiency of grass in the system to produce roots and straw, similar to what occurs in agricultural areas. This lower input of carbon into the soil fails to mitigate the emissions that naturally occur in these two land uses (66 Tavanti RFR, Montarini R, Panosso AR, Scala Jr NL, Neto MC, Freddi ODaS, Gazáles AP, Carvalho MAC, Soares MB, Tavanti TR, Galino FS. What is the impact of pasture reform on organic carbon compartments and CO2 emissions in the Brazilian Cerrado?. Catena. 2020;194:104702. Disponível em: https://doi.org/10.1016/j.catena.2020.104702
https://doi.org/10.1016/j.catena.2020.10...
).

In agricultural conditions, agroforestry systems (3434 Steinfeld JP, Bianchi FJJA, Locatelli JL, Rizzo R, Resende MEB, Ballester MVR, Cerri CEP, Bernardi ACC, Creamer RE. Increasing complexity of agroforestry systems benefits nutrient cycling and mineral-associated organic carbon storage, in south-eastern Brazil. Geoderma. 2023;440:116726. https://doi.org/10.1016/j.geoderma.2023.116726.
https://doi.org/10.1016/j.geoderma.2023....
), crop rotation (3535 Alves LA, Denardin LGO, Martins AP, Bayer C, Veloso MG, Bremm C, Carvalho PCF, Machado DR, Tiecher T. The effect of crop rotation and sheep grazing management on plant production and soil C and N stocks in a longterm integrated crop-livestock system in Southern Brazil. Soil and Tillage Research. 2020;203:104678. https://doi.org/10.1016/j.still.2020.104678.
https://doi.org/10.1016/j.still.2020.104...
), and the use of mulch (3636 Sant’anna SAC, Martins MR, Goulart JM, Araújo SN, Araújo ES, Zaman M, Jantalia CP, Alves B, Jr, Boddey RM, Urquiaga S. Biological nitrogen fixation and soil N2O emissions from legume residues in an Acrisol in SE Brazil. Geoderma Regional. 2018; 15:e00196. Disponível em: https://doi.org/10.1016/j.geodrs.2018.e00196
https://doi.org/10.1016/j.geodrs.2018.e0...
) can positively contribute to carbon sequestration, primarily because these systems can deposit a significant amount of plant residue into the soil. Over time, this residue undergoes carbon cycling processes, ultimately resulting in its immobilization in some fraction of organic matter in the soil.

In general, CO2 is influenced by soil carbon stocks, as it is the main substrate for microbial production of this gas. However, this C can be found in the soil in forms that are more resistant or susceptible to oxidation, although both are subject to microbial oxidation. Additionally, land use changes influence soil chemical characteristics, which can increase the exposure of C to the effects of microbial oxidation.

4. Soil CH4 emissions

Methane (CH4) has an infrared radiation absorption capacity 28 times greater than CO2 (11 IPCC. Resumo para formuladores de políticas. In: Mudança Climática e Terra: um relatório especial do IPCC sobre mudança climática, desertificação, degradação da terra, gestão sustentável da terra, segurança alimentar e fluxos de gases de efeito estufa em ecossistemas terrestres. 2019. 41p.). Its production in the soil depends on anaerobic microorganisms that decompose carbon under flooded conditions, with the ideal temperature range for production being 37°C to 45°C (3737 Vieira CFA, Lima LCD, Coutinho MM, Cavalcante, FSA. Efeitos climáticos do metano na atmosfera. Rev. Tecnol. Fortaleza. 2008;29(1):72-83. Disponível em: https://ojs.unifor.br/tec/article/view/46
https://ojs.unifor.br/tec/article/view/4...
).

The production of CH4 occurs through the action of microorganisms called methanogens, which, under anaerobic conditions, use substrates such as acetate, formate, H2, CO2, and methylated compounds (3838 Evans PN, Boyd JA, Leu AO, Woodcroft BJ, Parks DH, Hugenholtz P, Tyson, GW. An evolving view of methane metabolism in the Archaea. Nature Reviews Microbiology. 2019;17(4):219-232. Disponível em: http://dx.doi.org/10.1038/s41579-018-0136-7.
http://dx.doi.org/10.1038/s41579-018-013...
, 3939 Meyer KM, Morris AH, Webster K, Klein AM, Kroeger ME, Meredith LK, Braendholt A, Nakamura F, Venturini A, Souza LF, Shek K,L, Danielson R, Haren JV, Camargo PB, Tsai SM, Dini-Andreote F, De Mauro JMS, Barlow J, Berenguer E, Nüsslein K, Saleska S, Rodrigues JLM, Bohannan BJM, Belowground changes to community structure alter methane-cycling dynamics in Amazonia. Environment International. 2020;145:106131. Disponível em: https://doi.org/10.1016/j.envint.2020.106131
https://doi.org/10.1016/j.envint.2020.10...
). Its oxidation can be carried out by microorganisms called methanotrophs, which use it as the sole food source under aerobic conditions. Additionally, methane can also be oxidized by ammonia oxidants (NH3), but in both oxidation pathways, CH4 is converted into CO2 in the soil (1818 Chang J, Ciais P, Gasser T, Smith P, Herrero M, Havlík P, Obersteiner M, Guenet B, Goll DS, Li W, Naipal V, Peng S, Qiu C, Tian H, Viovy N, Yue C, Zhu D. Climate warming from managed grasslands cancels the cooling effect of carbon sinks in sparsely grazed and natural grasslands. Nature Communications. 2021;12(118):1-10. Disponível em: https://doi.org/10.1038/s41467-020-20406-7
https://doi.org/10.1038/s41467-020-20406...
).

The process of converting CH4 into CO2 in the soil can be carried out by Ammonia Oxidizing Archaea (AOA), Ammonia Oxidizing Bacteria (AOB), and methanotrophs, with AOA having a greater affinity for CH4. Consequently, the higher the number of AOA, the lower the number of AOB in the soil (R2=0.53) (4040 Xu X, Xia Z, Liu Y, Liu E, Muller K, Wang H, Luo J, Wu X, Beiyuan J, Fang Z, Xu J, Di H, Li Y. Interactions between methanotrophs and ammonia oxidizers modulate the response of in situ methane emissions to simulated climate change and its legacy in an acidic soil. Science of the Total Environment. 2021;752:142225. Disponível em: https://doi.org/10.1016/j.scitotenv.2020.142225
https://doi.org/10.1016/j.scitotenv.2020...
). Moreover, AOA is minimally affected by soil changes, especially with increased temperature and reduced humidity, thus potentially becoming one of the main pathways for CH4 oxidation in the soil after land use changes (4040 Xu X, Xia Z, Liu Y, Liu E, Muller K, Wang H, Luo J, Wu X, Beiyuan J, Fang Z, Xu J, Di H, Li Y. Interactions between methanotrophs and ammonia oxidizers modulate the response of in situ methane emissions to simulated climate change and its legacy in an acidic soil. Science of the Total Environment. 2021;752:142225. Disponível em: https://doi.org/10.1016/j.scitotenv.2020.142225
https://doi.org/10.1016/j.scitotenv.2020...
). However, the reverse can also be applied since ammonia (NH4), one of the possible products of nitrogen used as a fertilizer, can also be oxidized by methanotrophs. Therefore, this can be considered a potential competitor with methane for monooxygenase, the enzyme responsible for CH4 oxidation (4141 Mosier A, Schimel D, Valentine D, Bronson K, Parton W. Methane and nitrous-oxide fluxes in native, fertilized and cultivated grasslands. Nature. 1991;350:330-332. Disponível em: https://doi.org/10.1038/350330a0.
https://doi.org/10.1038/350330a0...
).

In soil under anaerobic conditions, organic matter (OM) is decomposed by microorganisms into smaller fractions through processes of hydrolysis and fermentation. The main products of this process are CO2 and CH4. The CH4 produced diffuses into the upper layer of the soil, where aerobic conditions prevail, allowing for the oxidation of methane by methanotrophic organisms. During the CH4 diffusion process into the upper layer of the soil, a series of reactions between CH4 and electron acceptors (n-receptors) can occur. As a result, part of the CH4 produced from OM decomposition is not emitted into the atmosphere (Figure 3) (4242 Dean JF, Middelburg JJ, Röckmann T, Aerts R, Blauw LG, Egger M, Mike SMJ, Anniek EEJ, Meisel OH, Rasigraf O, Slomp CP, Michiel H, Dolman AJ. Methane feedbacks to the global climate system in a warmer world. Reviews of Geophysics. 2018;56(1):207-250. Disponível em: https://doi.org/10.1002/2017RG000559
https://doi.org/10.1002/2017RG000559...
). The main electron acceptors in the soil include iron and manganese oxides (4343 Silva LSD, Griebeler G, Moterle DF, Bayer C, Zschornack T, Pocojeski E. Dinâmica da emissão de metano em solos sob cultivo de arroz irrigado no sul do Brasil. Revista Brasileira de Ciência do Solo. 2011;35(2):473-781. Disponível em: https://doi.org/10.1590/S0100-06832011000200016
https://doi.org/10.1590/S0100-0683201100...
).

Figure 3
Methane (CH4) Cycle in Soil

The production of CH4 from the soil is also related to the decomposition of organic matter under flooded conditions, often observed in agricultural systems such as rice cultivation (4343 Silva LSD, Griebeler G, Moterle DF, Bayer C, Zschornack T, Pocojeski E. Dinâmica da emissão de metano em solos sob cultivo de arroz irrigado no sul do Brasil. Revista Brasileira de Ciência do Solo. 2011;35(2):473-781. Disponível em: https://doi.org/10.1590/S0100-06832011000200016
https://doi.org/10.1590/S0100-0683201100...
, 4545 Liu X, Peñuelas J, Sardans J, Fang Y, Wiesmeier M, Wu L, Chen X, Chen Y, Jin Q, Wang W. Response of soil nutrient concentrations and stoichiometry, and greenhouse gas carbon emissions linked to change in land-use of paddy fields in China. Catena. 2021;203:105326. Disponível em: http://dx.doi.org/10.1016/j.catena.2021.105326.
http://dx.doi.org/10.1016/j.catena.2021....
). This process can also occur in other crops due to high precipitation during certain periods of the year, possibly leading to some areas producing CH4 due to maximum soil moisture saturation (4646 Kroeger ME, Meredith LK, Meyer KM, Webster KD, Camargo PB, Souza LF, Tsai SM, Van Haren J, Saleska S, Bohannan BJM, Nüsslein K. Rainforest-to-pasture conversion stimulates soil methanogenesis across the Brazilian Amazon. The Isme Journal. 2021;15(3):658-672. Disponível em: http://dx.doi.org/10.1038/s41396-020-00804-x.
http://dx.doi.org/10.1038/s41396-020-008...
). However, land use changes, such as conversion from one system to agricultural areas, typically result in reduced methane emissions, as these systems experience a decrease in soil organic carbon stocks and soil moisture (4747 Ondiek RA, Hayes DS, Kinyua DN, Kitaka N, Lautsch E, Mutuo P, Hein T. Influence of land-use change and season on soil greenhouse gas emissions from a tropical wetland: A stepwise explorative assessment. Science of the Total Environment. 2021;787:147701. Disponível em: https://doi.org/10.1016/j.scitotenv.2021.147701
https://doi.org/10.1016/j.scitotenv.2021...
, 4848 Wachiye S, Merbold L, Vesala T, Rinne J, Leitner S, Anen MR, Vuorinne I, Heiskanen J, Pellikka P. Soil greenhouse gas emissions from a sisal chronosequence in Kenya. Agricultural and Forest Meteorology. 2021;307:108465. Disponível em: https://doi.org/10.1016/j.agrformet.2021.108465
https://doi.org/10.1016/j.agrformet.2021...
).

Soil CH4 emissions are related to intrinsic cultivation factors, such as planting duration, fallow period, and applied inputs. In a meta-analysis evaluating greenhouse gas emissions in different agricultural systems, Shakoor et al. (4949 Shakoor A, Shakoor S, Rehman, Ashraf F, Abdullah M, Shahza SM, Farooq TH, Ashraf M, Manzoor MA, Altaf MM, Altaf MA. Effect of animal manure, crop type, climate zone, and soil attributes on greenhouse gas emissions from agricultural soils - A global meta-analysis. Journal of Cleaner Production. 2021;278:124019. Disponível em: https://doi.org/10.1016/j.jclepro.2020.124019
https://doi.org/10.1016/j.jclepro.2020.1...
) found that CH4 emissions increased when poultry manure was applied to soil with pH > 7 and soil moisture saturation > 60%, in rice cultivation areas, and in fallow areas without cover crops. Additionally, higher gas emissions were observed in crops with a duration of less than 320 days. Moreover, CH4 emissions can also be influenced by the conditions of the soil layer. Cardoso et al. (5050 Cardoso A Da S, Junqueira JB, Reis RA, Ruggieri AC. How do greenhouse gas emissions vary with biofertilizer type and soil temperature and moisture in a tropical grassland?. Pedosphere. 2020;30(5): 607-617. Disponível em: https://doi.org/10.1016/S1002-0160(20)60025-X
https://doi.org/10.1016/S1002-0160(20)60...
) found that higher methane emissions occur in the upper soil layer, as it is mainly influenced by the moisture and temperature of it. The higher the moisture and temperature of the soil fraction, the greater the CH4 emissions (5151 Parashar DC, Gupta PK, Rai J, Sharma RC, Singh N. Effect of soil temperature on methane emission from paddy fields. Chemosphere. 1993;26(1-4):247-250. Disponível em: https://doi.org/10.1016/0045-6535(93)90425-5.
https://doi.org/10.1016/0045-6535(93)904...
).

The methane emission also correlates with nitrogen concentrations applied to the soil. According to Sainju et al. (5252 Sainju UM, Ghimire R, Dangi S. Soil carbon dioxide and methane emissions and carbon balance with crop rotation and nitrogen fertilization. Science of the Total Environment. 2021;775:145702. Disponível em: https://doi.org/10.1016/j.scitotenv.2021.145902
https://doi.org/10.1016/j.scitotenv.2021...
), this correlation for monoculture systems is moderate (R2=0.56), while the correlation for grass-legume crop rotation systems shows a substantial correlation (R2=0.91). They concluded that the use of crop rotation with legumes, when combined with reduced nitrogen fertilization, can reduce CH4 emissions by -1 kg CO2eq ha-1. However, the study highlighted that there is still no consensus on the influence of fertilization on CH4 emission.

In addition to the soil and climatic factors that affect CH4 emissions, recent studies (3939 Meyer KM, Morris AH, Webster K, Klein AM, Kroeger ME, Meredith LK, Braendholt A, Nakamura F, Venturini A, Souza LF, Shek K,L, Danielson R, Haren JV, Camargo PB, Tsai SM, Dini-Andreote F, De Mauro JMS, Barlow J, Berenguer E, Nüsslein K, Saleska S, Rodrigues JLM, Bohannan BJM, Belowground changes to community structure alter methane-cycling dynamics in Amazonia. Environment International. 2020;145:106131. Disponível em: https://doi.org/10.1016/j.envint.2020.106131
https://doi.org/10.1016/j.envint.2020.10...
, 4545 Liu X, Peñuelas J, Sardans J, Fang Y, Wiesmeier M, Wu L, Chen X, Chen Y, Jin Q, Wang W. Response of soil nutrient concentrations and stoichiometry, and greenhouse gas carbon emissions linked to change in land-use of paddy fields in China. Catena. 2021;203:105326. Disponível em: http://dx.doi.org/10.1016/j.catena.2021.105326.
http://dx.doi.org/10.1016/j.catena.2021....
, 4646 Kroeger ME, Meredith LK, Meyer KM, Webster KD, Camargo PB, Souza LF, Tsai SM, Van Haren J, Saleska S, Bohannan BJM, Nüsslein K. Rainforest-to-pasture conversion stimulates soil methanogenesis across the Brazilian Amazon. The Isme Journal. 2021;15(3):658-672. Disponível em: http://dx.doi.org/10.1038/s41396-020-00804-x.
http://dx.doi.org/10.1038/s41396-020-008...
, and 5353 Ma L, Yang H, Pan Z, Rong Y. In situ measurements and meta-analysis reveal that land-use changes combined with low nitrogen application promote methane uptake by temperate grasslands in China. Science of The Total Environment.2020;706:136048. Disponível em: https://doi.org/10.1016/j.scitotenv.2019.136048
https://doi.org/10.1016/j.scitotenv.2019...
) on methane emissions in pastures indicate the soil methanogenic microbiota as an influential factor in CH4 release, which can be affected by secondary factors such as soil pH, vegetation, compaction, nutrient input via animal excreta, levels of organic carbon (COS), drainage, sandy texture, total soil nitrogen, soil bulk density, ammonium (NH4), and soil nitrate (NO3).

The main factors contributing to methane emissions from the soil identified in the analyzed studies include soil moisture, microbiology, and the availability of organic matter (OM) in the soil. The conversion of wetlands into agricultural areas can reduce methane production, as it decreases soil moisture and carbon stocks in the soil.

5. Soil N2O emissions

Nitrous oxide (N2O) is one of the gaseous forms of Nitrogen (N) produced during the nitrification process by certain nitrifying microorganisms (Figure 4A), and it is also produced during the denitrification process by denitrifying microorganisms through nitric oxide reductase, which is commonly associated as the main process of N2O formation (Figure 4B). There are also other processes of N2O emissions, such as codenitrification and chemodenitrification, which produce them in smaller proportions (5454 VIEIRA, R. F. Ciclo do nitrogênio em sistemas agrícolas. Brasília: Embrapa. 1 ed. 2017. 165p.). In agricultural areas, its production mainly occurs through the conversion of nitrogenous fertilizers into N2O. In pasture areas, its production is derived from fertilizer application and from nitrogen deposition in urine by animals (5555 Clough TJ, Cardenas L M, Friedl J, Wolf B. Nitrous oxide emissions from ruminant urine: science and mitigation for intensively managed perennial pastures. Current Opinion in Environmental Sustainability. 2020;47:21-27. Disponível em: https://doi.org/10.1016/j.cosust.2020.07.001
https://doi.org/10.1016/j.cosust.2020.07...
).

Figure 4
Nitrification (A), and Denitrification (B) Processes involved in N2O production

The production of this gas in the soil depends on the action of bacteria, fungi, and Archaea that use ammonium (NH4+) and nitrate (NO3-) produced by fertilizers, animal urine, or soil organic matter as the main substrate during one of the phases of the nitrogen (N) cycle (5656 Merloti LF, Mendes LW, Pedrinho A, De Souza LF, Ferrari BM, Tsai SM. Forest-to-agriculture conversion in Amazon drives soil microbial communities and N-cycle. Soil Biology & Biochemistry. 2019;137:107567. Disponível em: https://doi.org/10.1016/j.soilbio.2019.107567
https://doi.org/10.1016/j.soilbio.2019.1...
). Ammoniaoxidizing Archaea (AOA) dominates in soils with low NH4+ concentration and produces less N2O compared to ammonia-oxidizing bacteria (AOB) (5757 Prosser JI, Hink L, Gubry-Rangin C, Nicol GW. Nitrous oxide production by ammonia oxidizers: physiological diversity, niche differentiation and potential mitigation strategies. Global Change Biology. 2020;26(1):103-118. Disponível em: https://doi.org/10.1111/gcb.14877
https://doi.org/10.1111/gcb.14877...
). Fungi, on the other hand, can perform the denitrification process, preferring to use NO3- and NO2- to form N2O, as they are unable to form N2 since they lack the N2O reductase enzyme (5858 Maeda K, Spor A, Edel-Hermann V, Heraud C, Breuil M,C, Bizouard F, Toyoda S, Yoshida N, Steinberg C, Philippot L. N2O production, a widespread trait in fungi. Scientific Report. 2015;5:59697. Disponível em: https://doi.org/10.1038/srep09697.
https://doi.org/10.1038/srep09697...
).

Nitrogen fertilization can interact with soil moisture and the availability of organic carbon, leading to a higher proportion of nitrate, which is one of the main substrates for the N denitrification process, and the consequent emission of N2O as one of the products. This effect can be enhanced by the availability of carbon in the soil since it is also an essential element for most microbial processes (5959 Gelfand l, Cui M, Tang J, Robertson GP. Short-term drought response of N2O and CO2 emissions from mesic agricultural soils in the US Midwest. Agriculture, Ecosystems & Environment. 2015;212:127-133. Disponível em: http://dx.doi.org/10.1016/j.agee.2015.07.005.
http://dx.doi.org/10.1016/j.agee.2015.07...
, 6060 Cardoso ADaS, Quintana BG, Janusckiewicz ER, Brito LDeF, Morgado E.DaS, Reis RA, Ruggieri AC. N2O emissions from urine-treated tropical soil: Effects of soil moisture and compaction, urine composition, and dung addition. Catena. 2017;157:325-332. Disponível em: https://doi.org/10.1016/j.catena.2017.05.036
https://doi.org/10.1016/j.catena.2017.05...
). Peaks in soil’s N2O emissions typically occur a few days after the application of nitrogen fertilizers (6161 Zurovec O, Wall DP, Brennan FP, Krol DJ, Forrestal PJ, Richards KG. Increasing soil pH reduces fertiliser derived N2O emissions in intensively managed temperate grassland. Agriculture, Ecosystems & Environment. 2021;311:107319. Disponível em: https://doi.org/10.1016/j.agee.2021.107319
https://doi.org/10.1016/j.agee.2021.1073...
).

Within the first 24-48 hours after nitrogen fertilizers, mainly urea, are added to the soil, they undergo the action of the enzyme urease, which hydrolyzes urea into ammonium (NH4+) and carbonate ions (CO32-). The carbonate is subsequently hydrolyzed into bicarbonate (HCO3-) and OH ions, drastically increasing the soil pH (pH > 8). This elevated pH can cause soil organic matter to release NH3- because OH affects the NH4+/NH3- ratio, with the nitrogen from urea tending to remain in the form of ammonia at higher pH levels. Ultimately, this increased availability of NH3- can be oxidized by ammonia-oxidizing bacteria (AOB), potentially generating N2O (5555 Clough TJ, Cardenas L M, Friedl J, Wolf B. Nitrous oxide emissions from ruminant urine: science and mitigation for intensively managed perennial pastures. Current Opinion in Environmental Sustainability. 2020;47:21-27. Disponível em: https://doi.org/10.1016/j.cosust.2020.07.001
https://doi.org/10.1016/j.cosust.2020.07...
).

Yuttitham et al. (6262 Yuttitham M, Chidthaisong A, Ruangchu U. N2O fluxes and direct N2O emission factors from maize cultivation on Oxisols in Thailand. Geoderma Regional. 2020;20:e00244. Disponível em: https://doi.org/10.1016/j.geodrs.2019.e00244
https://doi.org/10.1016/j.geodrs.2019.e0...
) corroborated that N2O emissions can be intensified by fertilizer application, and Silva et al. (4343 Silva LSD, Griebeler G, Moterle DF, Bayer C, Zschornack T, Pocojeski E. Dinâmica da emissão de metano em solos sob cultivo de arroz irrigado no sul do Brasil. Revista Brasileira de Ciência do Solo. 2011;35(2):473-781. Disponível em: https://doi.org/10.1590/S0100-06832011000200016
https://doi.org/10.1590/S0100-0683201100...
) observed that N2O emissions may be associated with microbial carbon (C) and nitrogen (N) contents. Data from the IPCC (99 IPCC. Climate change 2007: the physical science basis. Contribution of working group I to the fourth assessment report of the intergovernmental panel on climate change. Cambridge: Cambridge University Press, United Kingdom, 2007. 996 p.) demonstrated that under ideal environmental conditions, nitrogenous agricultural fertilizers can account for up to 1% of N2O emissions. Agricultural areas that use nitrogenous fertilization are subject to the highest N2O emissions (99 IPCC. Climate change 2007: the physical science basis. Contribution of working group I to the fourth assessment report of the intergovernmental panel on climate change. Cambridge: Cambridge University Press, United Kingdom, 2007. 996 p.; 6363 Bell MJ, Hinton N, Cloy JM, Topp CFE, Rees RM, Cardenas LT, Scott C, Webster RW, Ashton AP, Whitmore JR, Williams H, Balshaw F, Paine, K, Goulding WT, Chadwick, DR. Nitrous oxide emissions from fertilised UK arable soils: Fluxes, emission factors and mitigation. Agriculture, Ecosystems & Environment. 2015;212:134-147. Disponível em: https://doi.org/10.1016/j.agee.2015.07.003
https://doi.org/10.1016/j.agee.2015.07.0...
), especially those using urea, as the reaction of this fertilizer with soil water leads to the formation of ammonium, which can subsequently be utilized in the denitrification process by nitrifiers (5454 VIEIRA, R. F. Ciclo do nitrogênio em sistemas agrícolas. Brasília: Embrapa. 1 ed. 2017. 165p.). The use of fertilization with animal residues can also stimulate N2O and NOx emissions as there may be inhibition of enzymes that reduce nitrate and nitrite (6464 Pu Y, Zhu B, Dong Z, Liu Y, Wang C, Ye C. Soil N2O and NOx emissions are directly linked with N-cycling enzymatic activities. Applied Soil Ecology. 2019; 139:15-24. Disponível em: https://doi.org/10.1016/j.apsoil.2019.03.007
https://doi.org/10.1016/j.apsoil.2019.03...
).

In livestock farming, N2O emissions are related to the combination of urine and dung deposition on the soil, with urine releasing N into the soil and feces providing C (6060 Cardoso ADaS, Quintana BG, Janusckiewicz ER, Brito LDeF, Morgado E.DaS, Reis RA, Ruggieri AC. N2O emissions from urine-treated tropical soil: Effects of soil moisture and compaction, urine composition, and dung addition. Catena. 2017;157:325-332. Disponível em: https://doi.org/10.1016/j.catena.2017.05.036
https://doi.org/10.1016/j.catena.2017.05...
). However, although this is a favorable combination for N2O production, the release of N from dung is slow, which may favor the formation of low levels of NH4+ in the soil, promoting the presence of AOA and consequently lower N2O emissions (5555 Clough TJ, Cardenas L M, Friedl J, Wolf B. Nitrous oxide emissions from ruminant urine: science and mitigation for intensively managed perennial pastures. Current Opinion in Environmental Sustainability. 2020;47:21-27. Disponível em: https://doi.org/10.1016/j.cosust.2020.07.001
https://doi.org/10.1016/j.cosust.2020.07...
).

The concentration of organic matter (OM) in the soil can also influence N2O production, as a large amount of OM in the soil acts as a buffer for pH, reducing the concentration of NH4+ in the soil solution in response to this phenomenon. Subsequent concentrations of NH3, NOx-, and N2O are also reduced (6565 Breuillin-Sessoms F, Venterea RT, Sadowsky MJ, Coulter JA, Clough TJ, Wang P. Nitrification gene ratio and free ammonia explain nitrite and nitrous oxide production in urea-amended soils. Soil Biology & Biochemistry. 2017;111:143-153. Disponível em: https://doi.org/10.1016/j.soilbio.2017.04.007
https://doi.org/10.1016/j.soilbio.2017.0...
).

Another inherent soil factor capable of influencing N2O emissions is soil texture. According to Carmo et al. (6666 Carmo JB, Andrade CA, Cerri CC, Picollo MC. Disponibilidade de nitrogênio e fluxos de N2O a partir de solo sob pastagem após aplicação de herbicida. Revista Brasileira de Ciência do Solo, 2005;29(5):735-746. Disponível em: https://doi.org/10.1590/S0100-06832005000500009
https://doi.org/10.1590/S0100-0683200500...
), soil texture, when associated with moisture, can influence N2O emissions because it is directly related to water retention capacity shortly after micro and macropores are filled. With water saturation, anaerobic conditions occur, sufficient for some microorganisms to produce N2O. Thus, clayey soils tend to emit more N2O (6767 Pihlatie M, Syväsalo E, Simojoki A, Esala M, Regina K. Contribution of nitrification and denitrification to N2O production in peat, clay and loamy sand soils under different soil moisture conditions. Nutrient Cycling Agroecossystem. 2004;70(2):135-141. Disponível em: https://doi.org/10.1023/B:FRES.0000048475.81211.3c.
https://doi.org/10.1023/B:FRES.000004847...
). It is important to note that N2O production can also occur under aerobic conditions due to the action of aerobic microorganisms. Therefore, aerobic conditions are not a limiting factor for N2O production and should not be disregarded when justifying gas quantification (6868 Xu Y, Xu Z, Cai Z. Reverchon F. Review of denitrification in tropical and subtropical soils of terrestrial ecosystems. Journal of Soil and Sediment. 2013;13(4):699-710. Disponível em: https://doi.org/10.1007/s11368-013-0650-1
https://doi.org/10.1007/s11368-013-0650-...
).

Soil pH can also influence N2O emissions by inhibiting or enhancing the enzymes involved in the nitrification and denitrification processes, as well as influencing the microbial population present in the soil (6969 Saggar S, Jha N, Deslippe J, Bolan NS, Luo J, Giltrap DL, Kim DG, Zaman M, Tillman RW, Denitrification and N2 O: N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts. Science and Total Environment. 2013;465(1):173-195. Disponível em: https://doi.org/10.1016/j.scitotenv.2012.11.050
https://doi.org/10.1016/j.scitotenv.2012...
). pH between 5.4 and 5.9 enhances N2O emissions due to increased denitrification processes (7070 Russenes AL, Korsaeth A, Bakken LR, Dörsch P. Spatial variation in soil pH controls off-season N2O emission in an agricultural soil. Soil Biology and Biochemistry. 2016;99:36-46. Disponível em: https://doi.org/10.1016/j.soilbio.2016.04.019
https://doi.org/10.1016/j.soilbio.2016.0...
). However, lower pH values also reduce nitrous oxide reductase (Figure 4B), which ultimately increases N2O emissions (7171 Zheng Q, Hu Y, Zhang S, Noll L, Böckle T, Dietrich M, Herbold CW, Eichorst SA, Woebken D, Richter A, Wanek W. Soil multifunctionality is affected by the soil environment and by microbial community composition and diversity. Soil Biology and Biochemistry. 2019;136:107521. Disponível em: https://doi.org/10.1016/j.soilbio.2019.107521
https://doi.org/10.1016/j.soilbio.2019.1...
). When comparing pasture systems with and without liming management over long periods, differences in N2O emissions were observed. Pasture soils that were corrected through liming showed an increase in pH and a reduction in N2O emissions (6161 Zurovec O, Wall DP, Brennan FP, Krol DJ, Forrestal PJ, Richards KG. Increasing soil pH reduces fertiliser derived N2O emissions in intensively managed temperate grassland. Agriculture, Ecosystems & Environment. 2021;311:107319. Disponível em: https://doi.org/10.1016/j.agee.2021.107319
https://doi.org/10.1016/j.agee.2021.1073...
).

Emissions of N2O are influenced by land use changes, especially in areas where native forests are replaced by agriculture or pastures (7272 Pedrinho A, Mendes LW, Merloti LF, Da Fonseca MDeC, Cannavan FDeS, Tsai SM. Forest-to-pasture conversion and recovery based on assessment of microbial communities in Eastern Amazon Rainforest. FEMS Microbiology Ecology. 2018;95(3):1-10. Disponível em: https://doi.org/10.1093/femsec/fiy236.
https://doi.org/10.1093/femsec/fiy236...
). In these areas, there are microbial changes in the soil that favor the processes of nitrification and denitrification, while natural forests are considered sinks for N2O (5656 Merloti LF, Mendes LW, Pedrinho A, De Souza LF, Ferrari BM, Tsai SM. Forest-to-agriculture conversion in Amazon drives soil microbial communities and N-cycle. Soil Biology & Biochemistry. 2019;137:107567. Disponível em: https://doi.org/10.1016/j.soilbio.2019.107567
https://doi.org/10.1016/j.soilbio.2019.1...
).

To attempt to reduce N2O emissions from nitrogen fertilizers, biological nitrogen fixation (BNF), crop rotation, and the use of cover crops (from legumes) can be an option (34, 35, and 36). According to Sant’Anna et al. (3636 Sant’anna SAC, Martins MR, Goulart JM, Araújo SN, Araújo ES, Zaman M, Jantalia CP, Alves B, Jr, Boddey RM, Urquiaga S. Biological nitrogen fixation and soil N2O emissions from legume residues in an Acrisol in SE Brazil. Geoderma Regional. 2018; 15:e00196. Disponível em: https://doi.org/10.1016/j.geodrs.2018.e00196
https://doi.org/10.1016/j.geodrs.2018.e0...
), who evaluated three legumes used in green manure systems, N2O emissions from plant residues were confirmed. However, the emission concentrations were lower than those stipulated by the IPCC which demonstrated that the use of crops capable of BNF is advantageous as it reduces the need for nitrogen fertilizers, consequently reducing CO2 emissions produced during the manufacturing, distribution, and application processes. Furthermore, the use of legumes can compensate for the need for nitrogen fertilization in pasture systems (7373 Boddey RM, Casagrande DR, Homem BGC, Alves BJR. Forage legumes in grass pastures in tropical Brazil and likely impacts on greenhouse gas emissions: A review. Grass and forage Science. 2020;75(4):357-371. Disponível em: https://doi.org/10.1111/gfs.12498
https://doi.org/10.1111/gfs.12498...
), thus highlighting management alternatives for mitigating this greenhouse gas in pasture conditions.

6. Conclusion

Land use changes interfere with the chemical, physical, and biological parameters of the soil, influencing the emission of greenhouse gasses (GHG) into the atmosphere. CO2 is naturally produced in the soil by the action of microorganisms, and its emission mainly depends on soil organic carbon (SOC). The production of CH4 also depends on SOC; however, it is produced under anaerobic conditions, meaning that well-drained soils or those with low SOC have difficulty producing this gas. On the other hand, N2O emissions depend on the availability of substrates (NH4+ or NO3-). Agricultural soils are generally associated with higher emissions of this gas, but this is a consequence of the greater nitrogen input in this land use system.

Acknowledgments

We would like to thank the Study Group on Ruminants and Forage Production of the Amazon (GERFAM; www.gerfam.com.br), as well as the Grupo Unespfor (Unesp). We also thank the Research Support Foundation of the state of São Paulo (FAPESP, process no. 2019/25234-0) and Pará (FAPESPA, process no. 071/2020), in addition to the Coordination for the Improvement of Higher Education Personnel (CAPES, which through the PDPG-Amazônia Legal, awarded the scholarship to the first author; process’s number: 88887.510270/2020-00).

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Publication Dates

  • Publication in this collection
    23 Sept 2024
  • Date of issue
    2024

History

  • Received
    03 Nov 2023
  • Accepted
    27 May 2024
  • Published
    19 Aug 2024
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