ABSTRACT:

Seagrass meadows are considered one of the most important and valuable ecosystems on the planet, but also one of the most threatened. Missing knowledge about their existence and their subtidal nature are the main reasons for the lack of information about seagrass soils, especially in Brazil and other tropical areas. This study discussed the paradoxical lack of information about subaqueous soils, with a view to stimulate research on soil properties of seagrass meadows. This short communication provides information about the ecosystem and first descriptions of seagrass soils along the Brazilian Coast, marked by gleyzation, sulfidization, salinization, paludization, solonization, and classified as Gleissolos tiomórficos. Pedological studies on these ecosystems provide useful tools for their management, protection, and restoration. Thus, it is fundamental that soil scientists increase their knowledge about subaqueous soils, not only as a contribution to the Brazilian Soil Classification System, but for the conservation of these ecosystems.

Keyword:
coastal wetlands; blue carbon; sulfidization; gleization; submerged soils

INTRODUCTION

Seagrass meadows (or submerged aquatic vegetation) represent complex ecosystems formed by one or more angiosperm species colonizing shallow areas of the oceans and inland waters, associated with fauna and algal epiphytic-cover (Coles et al., 2011Coles R, Grech A, Rasheed M, McKenzie L, Unsworth R, Short F. Seagrass ecology and threats in the tropical Indo-Pacific bioregion. In: Pirog RS, editor. Seagrass: ecology, uses and threats. New York: Nova Science Publishers; 2011. p. 225-39.; Short et al., 2011Short FT, Polidoro B, Livingstone SR, Carpenter KE, Bandeira S, Bujang JS, Calumpong HP, Carruthers TJB, Coles RG, Dennison WC, Erftemeijer PLA, Fortes MD, Freeman AS, Jagtap TG, Kamal AHM, Kendrick GA, Judson Kenworthy WJ, La Nafie YA, Nasution IM, Orth RJ, Prathep A, Sanciangco JC, Tussenbroek B, Vergara SG, Waycott M, Zieman JC. Extinction risk assessment of the world's seagrass species. Biol Conserv. 2011;144:1961-71. https://doi.org/10.1016/j.biocon.2011.04.010
https://doi.org/10.1016/j.biocon.2011.04... ; Copertino et al., 2016Copertino MS, Creed JC, Lanari MO, Magalhães K, Barros K, Lana PC, Sordo L, Horta PA. Seagrass and submerged aquatic vegetation (VAS) habitats off the coast of Brazil: state of knowledge, conservation and main threats. Braz J Oceanogr. 2016;64:53-80. https://doi.org/10.1590/S1679-875920161036064sp2
https://doi.org/10.1590/S1679-8759201610... ). This ecosystem can be found in more than 120 countries on all continents, except Antarctica (Spalding et al., 2003Spalding M, Taylor M, Ravilious C, Short F, Green E. The distribution and status of seagrasses. In: Green EP, Short FT, editors. World atlas of seagrasses. Berkeley: University of California Press; 2003. p. 5-26., 2010Spalding M, Kainuma M, Collins L. World atlas of mangroves. London: Earthscan; 2010.) and covers an estimated 300,000 to 600,00 km² around the globe (Duarte et al., 2005Duarte CM, Middelburg JJ, Caraco N. Major role of marine vegetation on the oceanic carbon cycle. Biogeosciences. 2005;2:1-8. https://doi.org/10.5194/bg-2-1-2005
https://doi.org/10.5194/bg-2-1-2005... ; Fourqurean et al., 2012Fourqurean JW, Duarte CM, Kennedy H, Marbà N, Holmer M, Mateo MA, Apostolaki ET, Kendrick GA, Krause-Jensen D, McGlathery KJ, Serrano O. Seagrass ecosystems as a globally significant carbon stock. Nat Geosci. 2012;5:505-9. https://doi.org/10.1038/ngeo1477
https://doi.org/10.1038/ngeo1477... ), i.e., the equivalent to twice the area covered by mangroves (Siikamäki et al., 2013Siikamäki J, Sanchirico JN, Jardine S, McLaughlin D, Morris D. Blue carbon: coastal ecosystems, their carbon storage, and potential for reducing emissions. Environment: Science and Policy for Sustainable Development. 2013;55:14-29. https://doi.org/10.1080/00139157.2013.843981
https://doi.org/10.1080/00139157.2013.84... ).

The total coverage area along the Brazilian coast is still unknown, despite scientific reports indicating the presence of seagrass meadows along the entire coast (Vilanova et al., 2013Vilanova K, Barros DS, Rocha-Barreira CDA, Matos K. Ecology of Brazilian seagrasses: Is our current knowledge sufficient to make sound decisions about mitigating the effects of climate change? Iheringia Ser Bot. 2013;68:163-78.; Copertino et al., 2016Copertino MS, Creed JC, Lanari MO, Magalhães K, Barros K, Lana PC, Sordo L, Horta PA. Seagrass and submerged aquatic vegetation (VAS) habitats off the coast of Brazil: state of knowledge, conservation and main threats. Braz J Oceanogr. 2016;64:53-80. https://doi.org/10.1590/S1679-875920161036064sp2
https://doi.org/10.1590/S1679-8759201610... ). Only in the lagoon Lagoa dos Patos (state of Rio Grande do Sul), the seagrass meadows cover an area of 120 km² (Creed, 2003Creed JC. The seagrasses of South America: Brazil, Argentina and Chile. In: Green EP, Short FT, editors. World atlas of seagrasses. Berkeley: University of California Press; 2003. p. 243-50.). Considering the 9,200 km long coastline of Brazil and the innumerous rivers that discharge into the Atlantic Ocean, it is supposed that seagrass ecosystems cover extensive areas in Brazil (Copertino et al., 2016Copertino MS, Creed JC, Lanari MO, Magalhães K, Barros K, Lana PC, Sordo L, Horta PA. Seagrass and submerged aquatic vegetation (VAS) habitats off the coast of Brazil: state of knowledge, conservation and main threats. Braz J Oceanogr. 2016;64:53-80. https://doi.org/10.1590/S1679-875920161036064sp2
https://doi.org/10.1590/S1679-8759201610... ), mostly vegetated by Halodule wrightii Ascherson, associated to other Halodule and Halophila species, and Ruppia maritima Lipkin (Vilanova et al., 2013Vilanova K, Barros DS, Rocha-Barreira CDA, Matos K. Ecology of Brazilian seagrasses: Is our current knowledge sufficient to make sound decisions about mitigating the effects of climate change? Iheringia Ser Bot. 2013;68:163-78.).

Seagrasses form extensive vegetated areas at sites where clear waters allow light diffusion through the water column (Short et al., 2007Short F, Carruthers T, Dennison W, Waycott M. Global seagrass distribution and diversity: a bioregional model. J Exp Mar Biol Ecol. 2007;350:3-20. https://doi.org/10.1016/j.jembe.2007.06.012
https://doi.org/10.1016/j.jembe.2007.06.... ; Brodersen et al., 2015Brodersen KE, Lichtenberg M, Paz LC, Kühl M. Epiphyte-cover on seagrass (Zostera marina L.) leaves impedes plant performance and radial O2 loss from the below-ground tissue. Front Mar Sci. 2015;2:58. https://doi.org/10.3389/fmars.2015.00058
https://doi.org/10.3389/fmars.2015.00058... ). Generally, the plants of a meadow grow in areas protected from wave action (Phillips et al., 1988Phillips RC, Meñez EG. Seagrasses: Smithsonian contributions to the marine sciences. No. 34. Washington, DC: Smithsonian Institution Press; 1988.; Orth et al., 2006Orth RJ, Carruthers TJB, Dennison WC, Duarte CM, Fourqurean JW, Heck KL Jr, Hughes AR, Kendrick GA, Kenworthy WJ, Olyarnik S, Short FT, Waycott M, Williams SL. A global crisis for seagrass ecosystems. BioScience. 2006;56:987-96. https://doi.org/10.1641/0006-3568(2006)56[987:AGCFSE]2.0.CO;2
https://doi.org/10.1641/0006-3568(2006)5... ) to a maximum water depth of 90 m (Duarte, 1991Duarte CM. Seagrass depth limits. Aquat Bot. 1991;40:363-77. https://doi.org/10.1016/0304-3770(91)90081-F
https://doi.org/10.1016/0304-3770(91)900... ; Coles et al., 2009Coles R, McKenzie L, De'ath G, Roelofs A, Long WL. Spatial distribution of deepwater seagrass in the inter-reef lagoon of the Great Barrier Reef World Heritage Area. Mar Ecol Prog Ser. 2009;392:57-68. https://doi.org/10.3354/meps08197
https://doi.org/10.3354/meps08197... ). However, most of the meadows are found in shallow water areas, less than 10 m deep (Grech et al., 2012Grech A, Chartrand-Miller K, Erftemeijer P, Fonseca M, McKenzie L, Rasheed M, Taylor H, Coles R. A comparison of threats, vulnerabilities and management approaches in global seagrass bioregions. Environ Res Lett. 2012;7:024006. https://doi.org/10.1088/1748-9326/7/2/024006
https://doi.org/10.1088/1748-9326/7/2/02... ).

The plants consist of a polyphyletic assemblage of monocots, grouped in 60 known species, 12 genera, and four families (Cymodoceaceae, Hydrocharitaceae, Posidoniaceae, and Zosteraceae) of the order Alismatales (Les et al., 1997Les DH, Cleland MA, Waycott M. Phylogenetic studies in Alismatidae, II: Evolution of marine angiosperms (Seagrasses) and hydrophily. Syst Bot. 1997;22:443-63. https://doi.org/10.2307/2419820
https://doi.org/10.2307/2419820... ; Orth et al., 2006Orth RJ, Carruthers TJB, Dennison WC, Duarte CM, Fourqurean JW, Heck KL Jr, Hughes AR, Kendrick GA, Kenworthy WJ, Olyarnik S, Short FT, Waycott M, Williams SL. A global crisis for seagrass ecosystems. BioScience. 2006;56:987-96. https://doi.org/10.1641/0006-3568(2006)56[987:AGCFSE]2.0.CO;2
https://doi.org/10.1641/0006-3568(2006)5... ). These phanerogams have developed a series of ecological, physiological, and morphological adaptations that allow the colonization of completely submerged soils, for example: internal gas transport; epidermal chloroplasts; underwater pollination and dispersion; and absence of stomatal differentiation (Orth et al., 2006Orth RJ, Carruthers TJB, Dennison WC, Duarte CM, Fourqurean JW, Heck KL Jr, Hughes AR, Kendrick GA, Kenworthy WJ, Olyarnik S, Short FT, Waycott M, Williams SL. A global crisis for seagrass ecosystems. BioScience. 2006;56:987-96. https://doi.org/10.1641/0006-3568(2006)56[987:AGCFSE]2.0.CO;2
https://doi.org/10.1641/0006-3568(2006)5... ; Olsen et al., 2016Olsen JL, Rouzé P, Verhelst B, Lin YC, Bayer T, Collen J, Dattolo E, De Paoli E, Dittami S, Maumus F, Michel G, Kersting A, Lauritano C, Lohaus R, Töpel M, Tonon T, Vanneste K, Amirebrahimi M, Brakel J, Boström C, Chovatia M, Grimwood J, Jenkins JW, Jueterbock A, Mraz A, Stam WT, Tice H, Bornberg-Bauer E, Green PJ, Pearson GA, Procaccini G, Duarte CM, Schmutz J, Reusch TBH, Van de Peer Y The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea. Nature. 2016;530:331-5. https://doi.org/10.1038/nature16548
https://doi.org/10.1038/nature16548... ).

Several studies consider these ecosystems as worldwide most productive (Duarte and Chiscano, 1999Duarte CM, Chiscano CL. Seagrass biomass and production: a reassessment. Aquat Bot. 1999;65:159-74. https://doi.org/10.1016/S0304-3770(99)00038-8
https://doi.org/10.1016/S0304-3770(99)00... ; Olsen et al., 2016Olsen JL, Rouzé P, Verhelst B, Lin YC, Bayer T, Collen J, Dattolo E, De Paoli E, Dittami S, Maumus F, Michel G, Kersting A, Lauritano C, Lohaus R, Töpel M, Tonon T, Vanneste K, Amirebrahimi M, Brakel J, Boström C, Chovatia M, Grimwood J, Jenkins JW, Jueterbock A, Mraz A, Stam WT, Tice H, Bornberg-Bauer E, Green PJ, Pearson GA, Procaccini G, Duarte CM, Schmutz J, Reusch TBH, Van de Peer Y The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea. Nature. 2016;530:331-5. https://doi.org/10.1038/nature16548
https://doi.org/10.1038/nature16548... ), mainly for their role as basis of many food webs, providing nutrients (mainly N and P) and biomass for other parts of the ocean (Short et al., 2011Short FT, Polidoro B, Livingstone SR, Carpenter KE, Bandeira S, Bujang JS, Calumpong HP, Carruthers TJB, Coles RG, Dennison WC, Erftemeijer PLA, Fortes MD, Freeman AS, Jagtap TG, Kamal AHM, Kendrick GA, Judson Kenworthy WJ, La Nafie YA, Nasution IM, Orth RJ, Prathep A, Sanciangco JC, Tussenbroek B, Vergara SG, Waycott M, Zieman JC. Extinction risk assessment of the world's seagrass species. Biol Conserv. 2011;144:1961-71. https://doi.org/10.1016/j.biocon.2011.04.010
https://doi.org/10.1016/j.biocon.2011.04... ). Thus, seagrass meadows interact with other adjacent coastal ecosystems, contributing to the maintenance and diversity of the surrounding ecological systems (e.g., mangroves, salt marshes, and coral reefs) (Short et al., 2006Short FT, Koch EW, Creed JC, Magalhães KM, Fernandez E, Gaeckle JL. SeagrassNet monitoring across the Americas: case studies of seagrass decline. Mar Ecol. 2006;27:277-89. https://doi.org/10.1111/j.1439-0485.2006.00095.x
https://doi.org/10.1111/j.1439-0485.2006... , 2007Short F, Carruthers T, Dennison W, Waycott M. Global seagrass distribution and diversity: a bioregional model. J Exp Mar Biol Ecol. 2007;350:3-20. https://doi.org/10.1016/j.jembe.2007.06.012
https://doi.org/10.1016/j.jembe.2007.06.... ). Compared to other ecosystems, the economic value of seagrass meadows is one of the highest (dollars per ha) (Costanza et al., 1997Costanza R, D'Arge R, Groot R, Farber S, Grasso M, Hannon B, Limburg K, Naeem S, O’Neill RV, Paruelo J, Raskin RG, Sutton P, Belt M. The value of the world's ecosystem services and natural capital. Nature. 1997;387:253-60. https://doi.org/10.1038/387253a0
https://doi.org/10.1038/387253a0... ; Barbier et al., 2011Barbier EB, Hacker SD, Kennedy C, Koch EW, Stier AC, Silliman BR. The value of estuarine and coastal ecosystem services. Ecol Monogr. 2011;81:169-93. https://doi.org/10.1890/10-1510.1
https://doi.org/10.1890/10-1510.1... ), estimated at US$ 28,000 ha⁻¹ yr⁻¹ in 2010 (Costanza et al., 2014Costanza R, Groot R, Sutton P, Ploeg S, Anderson SJ, Kubiszewski I, Farber S, Kerry Turner R. Changes in the global value of ecosystems services. Global Environmental Change. 2014;26:152-8. https://doi.org/10.1016/j.gloenvcha.2014.04.002
https://doi.org/10.1016/j.gloenvcha.2014... ). More recently, seagrasses and other coastal wetland soils were recognized as key drivers of carbon concentration reduction in the atmosphere and mitigators of global warming effects (Chmura et al., 2003Chmura GL, Anisfeld SC, Cahoon DR, Lynch JC. Global carbon sequestration in tidal, saline wetland soils. Global Biogeochem Cy. 2003;17:1111. https://doi.org/10.1029/2002GB001917
https://doi.org/10.1029/2002GB001917... ; Mcleod et al., 2011Mcleod E, Chmura GL, Bouillon S, Salm R, Björk M, Duarte CM, Lovelock CE, Schlesinger WH, Silliman BR. A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Front Ecol Environ. 2011;9:552-60. https://doi.org/10.1890/110004
https://doi.org/10.1890/110004... ; Chmura, 2013Chmura GL. What do we need to assess the sustainability of the tidal salt marsh carbon sink? Ocean Coast Manage. 2013;83:25-31. https://doi.org/10.1016/j.ocecoaman.2011.09.006
https://doi.org/10.1016/j.ocecoaman.2011... ; Grimsditch et al., 2013Grimsditch G, Alder J, Nakamura T, Kenchington R, Tamelander J. The blue carbon special edition - introduction and overview. Ocean Coast Manage. 2013;83:1-4. https://doi.org/10.1016/j.ocecoaman.2012.04.020
https://doi.org/10.1016/j.ocecoaman.2012... ).

Since these ecosystems are submerged, environmental impacts on their areas have been neglected (Waycott et al., 2009Waycott M, Duarte CM, Carruthers TJB, Orth RJ, Dennison WC, Olyarnik S, Calladine A, Fourqurean JW, Heck KL Jr, Hughes AR, Kendrick GA, Kenworthy WJ, Short FT, Williams SL. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. P Natl Acad Sci USA. 2009;106:12377-81. https://doi.org/10.1073/pnas.0905620106
https://doi.org/10.1073/pnas.0905620106... ; Gladstone and Courtenay, 2014Gladstone W, Courtenay G. Impacts of docks on seagrass and effects of management practices to ameliorate these impacts. Estuar Coast Shelf S. 2014;136:53-60. https://doi.org/10.1016/j.ecss.2013.10.023
https://doi.org/10.1016/j.ecss.2013.10.0... ). Globally, about 30 % of seagrass meadows were lost in the last 50 years, at a higher rate than that reported for other ecosystems (e.g., tropical rainforest) (Waycott et al., 2009Waycott M, Duarte CM, Carruthers TJB, Orth RJ, Dennison WC, Olyarnik S, Calladine A, Fourqurean JW, Heck KL Jr, Hughes AR, Kendrick GA, Kenworthy WJ, Short FT, Williams SL. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. P Natl Acad Sci USA. 2009;106:12377-81. https://doi.org/10.1073/pnas.0905620106
https://doi.org/10.1073/pnas.0905620106... ; Siikamäki et al., 2013Siikamäki J, Sanchirico JN, Jardine S, McLaughlin D, Morris D. Blue carbon: coastal ecosystems, their carbon storage, and potential for reducing emissions. Environment: Science and Policy for Sustainable Development. 2013;55:14-29. https://doi.org/10.1080/00139157.2013.843981
https://doi.org/10.1080/00139157.2013.84... ). The expansion of cities and industries associated with unsustainable fishing practices have been cited as the most relevant threats to seagrass meadows (Waycott et al., 2009Waycott M, Duarte CM, Carruthers TJB, Orth RJ, Dennison WC, Olyarnik S, Calladine A, Fourqurean JW, Heck KL Jr, Hughes AR, Kendrick GA, Kenworthy WJ, Short FT, Williams SL. Accelerating loss of seagrasses across the globe threatens coastal ecosystems. P Natl Acad Sci USA. 2009;106:12377-81. https://doi.org/10.1073/pnas.0905620106
https://doi.org/10.1073/pnas.0905620106... ). In addition, the submerged nature of the seagrass meadows and apparently low species diversity disenchant the general public interest and support (Duarte et al., 2008Duarte CM, Dennison WC, Orth RJW, Carruthers TJB. The charisma of coastal ecosystems: Addressing the imbalance. Estuaries Coasts. 2008;31:233-8. https://doi.org/10.1007/s12237-008-9038-7
https://doi.org/10.1007/s12237-008-9038-... ; Randall Hughes et al., 2009Hughes AR, Williams SL, Duarte CM, Heck KL Jr, Waycott M. Associations of concern: declining seagrasses and threatened dependent species. Front Ecol Environ. 2009;7:242-6. https://doi.org/10.1890/080041
https://doi.org/10.1890/080041... ).

Regardless of the great importance of seagrass meadows and the demand for restorative practices, the lack of knowledge about the underlying soils hampers the implementation of many potentially successful restoration and management practices (York et al., 2016York PH, Smith TM, Coles RG, McKenna SA, Connolly RM, Irving AD, Jackson EL, McMahon K, Runcie JW, Sherman CDH, Sullivan BK, Trevathan-Tackett SM, Brodersen KE, Carter AB, Ewers CJ, Lavery PS, Roelfsema CM, Sinclair EA, Strydom S, Tanner JE, Dijk K, Warry FY, Waycott M, Whitehead S. Identifying knowledge gaps in seagrass research and management: an Australian perspective. Mar Environ Res. 2017;127:163-72. https://doi.org/10.1016/j.marenvres.2016.06.006
https://doi.org/10.1016/j.marenvres.2016... ). Thus, pedological studies of seagrass meadow soils may provide useful tools for the management and protection of these ecosystems by deepening the understanding of inter-relationships between soils and seagrasses and of the genesis of edaphic properties that influence seagrass persistence and susceptibility to environmental stressors. Thus, it is crucial that soil scientists increase their knowledge on subaqueous soils and their functioning.

This this research highlights the paradoxical lack of information about seagrass meadows and the underlying subaqueous soils, with a view to motivate studies on their properties and local/regional variations; to contribute to the Brazilian Soil Classification System (SiBCS), and to promote the protection and management of these ecosystems.

Seagrass meadows and subaqueous soils in Brazil

For the Soil Taxonomy system, the water can be considered as a possible upper limit of the soil, if it allows the growth of rooted plants (Soil Survey Staff, 2014Soil Survey Staff. Keys to soil taxonomy. 12th ed. Washington, DC: United States Department of Agriculture, Natural Resources Conservation Service; 2014.). Thus, the recognition of the substrates of seagrass meadows as soils by the Soil Taxonomy system led to the creation of the taxa “Wassents” and “Wassists” to better suit Entisols and Histosols, with positive water potential (Soil Survey Staff, 2014Soil Survey Staff. Keys to soil taxonomy. 12th ed. Washington, DC: United States Department of Agriculture, Natural Resources Conservation Service; 2014.), since these soils differ significantly from the subaerial soils classified as Aquents or other Histosol (Rabenhorst and Stolt, 2012Rabenhorst MC, Stolt MH. Subaqueous soils: pedogenesis, mapping, and applications. In: Lin H, editor. Hydropedology: synergistic integration of soil science and hydrology Waltham: Academic Press; 2012. p. 173-204.).

Similarly, for the World Reference Base for Soil Resources, the water column can also be considered as possible upper soil limit, however, restricted to sites with water columns lower than two meters (WRB, 2015World Reference Base for Soil Resources 2014, update 2015 International soil classification system for naming soils and creating legends for soil maps. Rome: IUSS/ISRIC/FAO; 2015. (World Soil Resources Reports, 106).). The principal qualifiers “tidalic” and “subaquatic” were created to discriminate some soil orders (e.g., Histosols, Technosols, Cryosols, Leptosols, Solonchaks, Gleysols, Arenosols, and Fluvisols) which are permanently flooded (subaquatic) or only flooded by tidewater at mean high tide, but not flooded at mean low tide (tidalic) (WRB, 2015World Reference Base for Soil Resources 2014, update 2015 International soil classification system for naming soils and creating legends for soil maps. Rome: IUSS/ISRIC/FAO; 2015. (World Soil Resources Reports, 106).). However, the characterization of subaquatic soils is still poorly defined by the WRB-FAO system, since the definition of the qualifier explicitly determines a maximum water column height of two meters during low tide and it is known that seagrasses can commonly be found in deeper water areas (Duarte, 1991Duarte CM. Seagrass depth limits. Aquat Bot. 1991;40:363-77. https://doi.org/10.1016/0304-3770(91)90081-F
https://doi.org/10.1016/0304-3770(91)900... ).

On the other hand, for the Brazilian Soil Classification System (Santos et al., 2013aSantos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF Sistema brasileiro de classificação de solos. 3. ed. Rio de Janeiro: Embrapa Solos; 2013a.), only the atmosphere can be considered the upper limit of soils. Except for the definition of the upper limit, the seagrass meadow soils fit perfectly in the soil definition used by the SiBCS (Santos et al., 2013aSantos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF Sistema brasileiro de classificação de solos. 3. ed. Rio de Janeiro: Embrapa Solos; 2013a.). In fact, all pedogenetic processes (addition, loss, translocation, and transformation) were identified in seagrass meadows worldwide, including in Brazil, for example: the addition of organic matter and biogenic calcium carbonate; loss of metals and organic matter; translocation of soil particles due to bioturbation; and transformation of organic substances, Fe, and S forms (Table 1) (Demas and Rabenhorst, 1999Demas GP, Rabenhorst MC. Subaqueous soils pedogenesis in a submersed environment. Soil Sci Soc Am J. 1999;63:1250-7. https://doi.org/10.2136/sssaj1999.6351250x
https://doi.org/10.2136/sssaj1999.635125... ; Osher and Flannagan, 2007Osher LJ, Flannagan CT. Soil/landscape relationships in a mesotidal Maine estuary Soil Sci Soc Am J. 2007;71:1323-34. https://doi.org/10.2136/sssaj2006.0224
https://doi.org/10.2136/sssaj2006.0224... ; Rabenhorst and Stolt, 2012Rabenhorst MC, Stolt MH. Subaqueous soils: pedogenesis, mapping, and applications. In: Lin H, editor. Hydropedology: synergistic integration of soil science and hydrology Waltham: Academic Press; 2012. p. 173-204.; Serrano et al., 2012Serrano O, Mateo MA, Renom P, Julià R. Characterization of soils beneath a Posidonia oceanica meadow. Geoderma. 2012;185-186:26-36. https://doi.org/10.1016/j.geoderma.2012.03.020
https://doi.org/10.1016/j.geoderma.2012.... ; Ferronato et al., 2016Ferronato C, Falsone G, Natale M, Zannoni D, Buscaroli A, Vianello G, Antisari LV. Chemical and pedological features of subaqueous and hydromorphic soils along a hydrosequence within a coastal system (San Vitale Park, Northern Italy). Geoderma. 2016;265:141-51. https://doi.org/10.1016/j.geoderma.2015.11.018
https://doi.org/10.1016/j.geoderma.2015.... ; Vittori Antisari et al., 2016Antisari LV, De Nobili M, Ferronato C, Natale M, Pellegrini E, Vianello G. Hydromorphic to subaqueous soils transitions in the central Grado lagoon (Northern Adriatic Sea, Italy). Estuar Coast Shelf S. 2016;173:39-48. https://doi.org/10.1016/j.ecss.2016.02.004
https://doi.org/10.1016/j.ecss.2016.02.0... ; Nóbrega, 2017Nóbrega GN. Solos subaquáticos de pradarias marinhas (Seagrass Bed) do Brasil: biogeoquímica, gênese e classificação [tese]. Piracicaba: Escola Superior de Agricultura “Luiz de Queiroz”; 2017.).

To our knowledge, only one study was published about Brazilian seagrass meadows soils. The occurrence of subaqueous soils along the Abrolhos archipelago was mentioned in a paper on soil phosphatization and landscape evolution (Schaefer et al., 2010Schaefer CEGR, Simas FNB, Albuquerque MA, Souza E, Delpupo KK. Fosfatização de solos e evolução da paisagem no arquipélago de Abrolhos, BA. Rem-Rev Esc Minas. 2010;63:727-34. https://doi.org/10.1590/S0370-44672010000400019
https://doi.org/10.1590/S0370-4467201000... ). However, no pedological study, providing the morphological description, properties, and classification of seagrass soils of the Brazilian coast was published so far. Thus, to fill this gap of basic information and to stimulate the update of the SiBCS, two soil profiles from different coast compartments were studied to contribute with a first approach addressing the variability of seagrass soils of Brazil. Soil profiles were sampled on the semiarid coast in the Northeast (state of Ceará; predominantly vegetated with Halodule spp.) and the quaternary coast in the South (Lagoa dos Patos - RS; mostly vegetated by Ruppia maritima; Figure 1), and described according to Santos et al. (2013bSantos RD, Lemos RC, Santos HG, Ker JC, Anjos LHC, Shimizu SH. Manual de descrição e coleta de solo no campo. 6. ed rev ampl. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2013b.). On the semiarid coast of northeastern Brazil, seagrass soil was collected at a water depth of 1.8 m [water depth corrected to a mean water level 1.71 m; Marinha do Brasil (2017Marinha do Brasil. Diretoria de hidrografia e navegação. Centro de Hidrografia da Marinha. Banco Nacional de dados oceanográficos. Previsão de marés (máximas e mínimas diárias) [internet]. Rio de Janeiro: Banco Nacional de Dados Oceanográficos; 2017. [acesso em 27 jul 2017]. Disponível em: http://www.mar.mil.br/dhn/chm/box-previsao-mare/tabuas/.
http://www.mar.mil.br/dhn/chm/box-previs... )], whereas on the quaternary coast in the South, soil under seagrass was collected at a water depth of 1.3 m [corrected to a mean water level of 0.68 m; Marinha do Brasil (2017Marinha do Brasil. Diretoria de hidrografia e navegação. Centro de Hidrografia da Marinha. Banco Nacional de dados oceanográficos. Previsão de marés (máximas e mínimas diárias) [internet]. Rio de Janeiro: Banco Nacional de Dados Oceanográficos; 2017. [acesso em 27 jul 2017]. Disponível em: http://www.mar.mil.br/dhn/chm/box-previsao-mare/tabuas/.
http://www.mar.mil.br/dhn/chm/box-previs... )].

These subaqueous soils were sampled using samplers attached to transparent tubes for sampling and visual confirmation of the partially disturbed samples (Figure 2). The tubes were pushed into the soil with a remote hammering system and a watertight valve prevented sample loss when pulling the sample out of the soil/water (Figure 2). After sampling, the tubes were sealed with rubber caps, transported in vertical position to the laboratory, where the samples were removed from the tubes. According to Erich et al. (2010Erich E, Drohan PJ, Ellis LR, Collins ME, Payne M, Surabian D. Subaqueous soils: their genesis and importance in ecosystem management. Soil Use Manage. 2010;26:245-52. https://doi.org/10.1111/j.1475-2743.2010.00278.x
https://doi.org/10.1111/j.1475-2743.2010... ), the two sampling techniques - vibracoring for deeper waters and sealed rigid tubes for shallower areas - can also be used for soil profile collection and description.

After soil description, subsamples of each soil horizon were taken and washed with ethanol (60 %) to remove soluble salts, until a silver nitrate test indicated absence of the chloride ion (Sumner and Miller, 1996Sumner ME, Miller WP. Cation exchange capacity and exchange coefficients. In: Sparks DL, Page AL, Helmke PA, Loeppert RH, Soltanpour PN, Tabatabai MA, Johnston CT, Sumner ME, editors. Methods of soil analysis. Chemical methods. Wisconsin: Soil Science Society of America; 1996. Pt. 3. p. 1201-30.; Claessen, 1997Claessen MEC, organizador. Manual de métodos de análise de solo. 2. ed. Rio de Janeiro: Centro Nacional de Pesquisa de Solos; 1997.). Then the soil samples were dried, ground, and sieved to determine exchangeable cations, calcium carbonate equivalent (CCE), and grain size composition, using the soil classification methods proposed for SiBCS (Claessen, 1997Claessen MEC, organizador. Manual de métodos de análise de solo. 2. ed. Rio de Janeiro: Centro Nacional de Pesquisa de Solos; 1997.; Santos et al., 2013aSantos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF Sistema brasileiro de classificação de solos. 3. ed. Rio de Janeiro: Embrapa Solos; 2013a.,bSantos RD, Lemos RC, Santos HG, Ker JC, Anjos LHC, Shimizu SH. Manual de descrição e coleta de solo no campo. 6. ed rev ampl. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2013b.). Additionally, subsamples of soil horizons were taken and frozen until posterior analysis of total organic carbon (TOC), total nitrogen (TN), and Fe fractionation. Total organic C was quantified using an elemental analyzer, after removal of inorganic C with HCl 1 mol L⁻¹ (Howard et al., 2014Howard J, Hoyt S, Isensee K, Telszewski M, Pidgeon E. Coastal blue carbon: methods for assessing carbon stocks and emissions factors in mangroves, tidal salt marshes, and seagrasses. Arlington: Conservation International, Intergovernmental Oceanographic Commission of UNESCO, International Union for Conservation of Nature; 2014.), whereas TN was quantified in untreated samples using an elemental analyzer. Iron was fractionated using the method proposed by Lord III (1982), obtaining two operationally distinct fractions: Fe-oxyhydroxides (Oxy-Fe) and pyrite (Py-Fe), which allowed the determination of the degree of pyritization [DOP = (Py-Fe × 100)/(Py-Fe + Oxy-Fe)] (Berner, 1970Berner RA. Sedimentary pyrite formation. Am J Sci. 1970;268:1-23. https://doi.org/10.2475/ajs.268.1.1
https://doi.org/10.2475/ajs.268.1.1... ). The total potential acidity (pH_OXI) was determined measuring the pH after sample oxidation by H₂O₂ (Konsten et al. 1988Konsten CJM, Brinkman R, Andriesse W. A field laboratory method to determine total potential and actual acidity in acid sulphate soils. In: Dost H, editor. Selected papers of the Dakar symposium on acid sulphate soils; january 1986; Dakar, Senegal. Senegal: International Institute for Land Reclamation and Improvement; 1988. p. 106-34).

The collected soils differed significantly regarding the morphological, chemical, and physical properties (Table 1 and 2). For both profiles, as a consequence of the characteristic waterlogging, the most significant processes were gleyzation and sulfidization, resulting in soils with low chromas (or neutral colors), but also dark and very dark gray colors, indicating sulfide accumulation (Table 2 and Figure 3). As a result of the intense sulfidization, a high percentage of the iron was incorporated into sulfides [degree of pyritization >50 %; e.g., percentage of Fe incorporated into pyrite; for further details, please see Berner (1970Berner RA. Sedimentary pyrite formation. Am J Sci. 1970;268:1-23. https://doi.org/10.2475/ajs.268.1.1
https://doi.org/10.2475/ajs.268.1.1... )]. However, due to oxidation promoted by the plant rhizosphere, brown Fe mottles can occur, evidencing oxidation of Fe sulfides and thus, the onset of sulfurization (Figure 3).

The accumulation of oxidizable Fe and S material induced strongly acidic conditions when the soil material was oxidized (Table 1), characterizing the presence of hypersulfidic material (WRB, 2015World Reference Base for Soil Resources 2014, update 2015 International soil classification system for naming soils and creating legends for soil maps. Rome: IUSS/ISRIC/FAO; 2015. (World Soil Resources Reports, 106).). This acidification due to oxidation was less significant in soil horizons with higher amounts of seashells, and higher amount of calcium carbonate equivalent (CCE), which can buffer the acidification process (hyposulfidic material) (WRB, 2015World Reference Base for Soil Resources 2014, update 2015 International soil classification system for naming soils and creating legends for soil maps. Rome: IUSS/ISRIC/FAO; 2015. (World Soil Resources Reports, 106).) (Figure 3 and Table 1). Attention should be paid to the CCE and seashells since they are not considered in definition of a “Horizonte Cálcico”, used by the SiBCS (Santos et al., 2013aSantos HG, Jacomine PKT, Anjos LHC, Oliveira VA, Oliveira JB, Coelho MR, Lumbreras JF, Cunha TJF Sistema brasileiro de classificação de solos. 3. ed. Rio de Janeiro: Embrapa Solos; 2013a.), conditioning the presence of secondary calcium carbonates. Moreover, the presence of biogenic calcium carbonate does not match the definition of subordinate properties for the presence of carbonates (k) or accumulation of secondary calcium carbonate ($\overline{\text{k}}$) in the Brazilian System of Soils Classification (Santos et al., 2013bSantos RD, Lemos RC, Santos HG, Ker JC, Anjos LHC, Shimizu SH. Manual de descrição e coleta de solo no campo. 6. ed rev ampl. Viçosa, MG: Sociedade Brasileira de Ciência do Solo; 2013b.). Thus, the presence of biogenic carbonate should be considered in future modifications of SiBCS, since it strongly affects the acidity neutralizing potential and reflects the influence of biota on soil formation.

Soil textures with predominance of sand (Table 2) usually indicate a higher hydrodynamic in these ecosystems. Additionally, the variation in particle-size distribution within the soil profiles (presence of fluvic material) (WRB, 2015World Reference Base for Soil Resources 2014, update 2015 International soil classification system for naming soils and creating legends for soil maps. Rome: IUSS/ISRIC/FAO; 2015. (World Soil Resources Reports, 106).) may also result from changes in hydrodynamics through the evolution of the soil profiles.

The soils also differed considerably with regard to the salinization process, with much higher electrical conductivity in the soils of the semiarid coast (≈ 45 dS m⁻¹) compared to the Quaternary soils on the southern coast (≈ 3 dS m⁻¹) evidencing that the salinization processes may occur at very contrasting intensities, regardless of the constant tidal influence on these soils. Additionally, the effects of seawater conditioned the occurrence of solonization/solodization processes, especially in the deeper layers of the S coast soils (Table 2). Besides, due to the significant seasonal variations in the water properties (e.g., salinity, temperature, nitrogen, and phosphorous) of the Lagoa dos Patos (Lanari and Copertino, 2016Copertino MS, Creed JC, Lanari MO, Magalhães K, Barros K, Lana PC, Sordo L, Horta PA. Seagrass and submerged aquatic vegetation (VAS) habitats off the coast of Brazil: state of knowledge, conservation and main threats. Braz J Oceanogr. 2016;64:53-80. https://doi.org/10.1590/S1679-875920161036064sp2
https://doi.org/10.1590/S1679-8759201610... ), it is expected that the salinization and solonization processes at this site also vary throughout the year. Thus, further studies are required to comprehend how relevant seasonal variations are for subaqueous soils.

Another difference between the two soil types is related to the intensity of paludization, i.e., the accumulation of organic C under anaerobic conditions. For the seagrass soil on the semiarid coast, C accumulation was significantly higher than that of the lagoon Lagoa dos Patos. These preliminary results evidenced paludization, since the climatic conditions may have been predominated by the local biogeochemical conditions. Additionally, the higher plant biomass on the NE coast (Figure 1), which resulted in a higher C input, should be emphasized. Paludization is probably the most studied pedogenic process in seagrass soils, due to the important role these ecosystems play for atmospheric CO₂ sequestration (Fourqurean et al., 2012Fourqurean JW, Duarte CM, Kennedy H, Marbà N, Holmer M, Mateo MA, Apostolaki ET, Kendrick GA, Krause-Jensen D, McGlathery KJ, Serrano O. Seagrass ecosystems as a globally significant carbon stock. Nat Geosci. 2012;5:505-9. https://doi.org/10.1038/ngeo1477
https://doi.org/10.1038/ngeo1477... ). In fact, seagrass meadows and other coastal wetlands have been highlighted as the most important ecosystems for C sequestration, particularly into the soils, which inspired the designation Blue Carbon sinks (Nellemann et al., 2009Nellemann C, Corcoran E, Duarte CM, Valdés L, DeYoung C, Fonseca I, Grimsditch G. Blue carbon. A rapid response assessment. Arendal: United Nation Environment Programme, GRID-Arendal; 2009.) and stimulated studies regarding C accumulation in these ecosystems.

The soils were classified as Fluventic Sulfiwassent, according to the Soil Taxonomy; Fluvic Subaquatic Solonchak (Hypersalic, Protosodic, Hypersulfidic, Loamic) and Fluvic Subaquatic Gleysol (Protosalic, Sodic, Hypersulfidic, Loamic) according to the FAO-WRB system.

In general, the soils had a moderately adequate classification according to the SiBCS, being classified as Gleissolo Tiomórfico Órtico sálico solódico (NE-Semiarid coast) and Gleissolo Tiomórfico Órtico sódico (S-Quaternary Coast). For a better fitting of subaqueous soils in the SiBCS, a suborder for the Gleissolos soil order should be created, similar to the tidalic and subaquatic classifiers used by WRB-FAO, as well as criteria for the definition of a property analogous to hyposulfidic material. Additionally, new subgroups consisting of Gleissolos Tiomórficos Órtico sálico solódico neofluvissólico and Gleissolos Tiomórficos Órtico sódico salino could be created to detail the classification of seagrass soils of Brazil.

The pedological approach to these soils makes, among other aspects, their description and consistent mapping based on the pedogenetic similarities possible. In fact, many studies have been conducted to map subaqueous soils, mostly in estuarine environments (Demas and Rabenhorst, 1999Demas GP, Rabenhorst MC. Subaqueous soils pedogenesis in a submersed environment. Soil Sci Soc Am J. 1999;63:1250-7. https://doi.org/10.2136/sssaj1999.6351250x
https://doi.org/10.2136/sssaj1999.635125... ; Bradley and Stolt, 2003Bradley MP, Stolt MH. Subaqueous soil-landscape relationships in a Rhode Island estuary. Soil Sci Soc Am J. 2003;67:1487-95. https://doi.org/10.2136/sssaj2003.1487
https://doi.org/10.2136/sssaj2003.1487... , 2006Bradley MP, Stolt MH. Landscape-level seagrass-sediment relations in a coastal lagoon. Aquat Bot. 2006;84:121-8. https://doi.org/10.1016/j.aquabot.2005.08.003
https://doi.org/10.1016/j.aquabot.2005.0... ; Erich and Drohan, 2012Erich E, Drohan PJ. Genesis of freshwater subaqueous soils following flooding of a subaerial landscape. Geoderma. 2012;179-180:53-62. https://doi.org/10.1016/j.geoderma.2012.02.004
https://doi.org/10.1016/j.geoderma.2012.... ; Vittori Antisari et al., 2016Antisari LV, De Nobili M, Ferronato C, Natale M, Pellegrini E, Vianello G. Hydromorphic to subaqueous soils transitions in the central Grado lagoon (Northern Adriatic Sea, Italy). Estuar Coast Shelf S. 2016;173:39-48. https://doi.org/10.1016/j.ecss.2016.02.004
https://doi.org/10.1016/j.ecss.2016.02.0... ). The study of subaqueous soils from the viewpoint of pedology paves the way for the management of these areas based on measurable physical and chemical soil processes, in the future, similarly to that used in subaerial soils. Research along this line will contribute to the establishment of thresholds to define subaqueous soil quality classes, which will in turn provide guidance for management practices (Demas and Rabenhorst, 1999Demas GP, Rabenhorst MC. Subaqueous soils pedogenesis in a submersed environment. Soil Sci Soc Am J. 1999;63:1250-7. https://doi.org/10.2136/sssaj1999.6351250x
https://doi.org/10.2136/sssaj1999.635125... , 2001Demas GP, Rabenhorst MC. Factors of subaqueous soil formation: a system of quantitative pedology for submersed environments. Geoderma. 2001;102:189-204. https://doi.org/10.1016/S0016-7061(00)00111-7
https://doi.org/10.1016/S0016-7061(00)00... ; Erich and Drohan, 2012Erich E, Drohan PJ. Genesis of freshwater subaqueous soils following flooding of a subaerial landscape. Geoderma. 2012;179-180:53-62. https://doi.org/10.1016/j.geoderma.2012.02.004
https://doi.org/10.1016/j.geoderma.2012.... ). Subaqueous soil maps could guide the identification of areas most indicated for dredging, for mollusk, and shellfish cultivation, but could also provide new insights on the main factors controlling the genesis of seagrass soils (Grech et al., 2012Grech A, Chartrand-Miller K, Erftemeijer P, Fonseca M, McKenzie L, Rasheed M, Taylor H, Coles R. A comparison of threats, vulnerabilities and management approaches in global seagrass bioregions. Environ Res Lett. 2012;7:024006. https://doi.org/10.1088/1748-9326/7/2/024006
https://doi.org/10.1088/1748-9326/7/2/02... ; Gladstone and Courtenay, 2014Gladstone W, Courtenay G. Impacts of docks on seagrass and effects of management practices to ameliorate these impacts. Estuar Coast Shelf S. 2014;136:53-60. https://doi.org/10.1016/j.ecss.2013.10.023
https://doi.org/10.1016/j.ecss.2013.10.0... ; York et al., 2016York PH, Smith TM, Coles RG, McKenna SA, Connolly RM, Irving AD, Jackson EL, McMahon K, Runcie JW, Sherman CDH, Sullivan BK, Trevathan-Tackett SM, Brodersen KE, Carter AB, Ewers CJ, Lavery PS, Roelfsema CM, Sinclair EA, Strydom S, Tanner JE, Dijk K, Warry FY, Waycott M, Whitehead S. Identifying knowledge gaps in seagrass research and management: an Australian perspective. Mar Environ Res. 2017;127:163-72. https://doi.org/10.1016/j.marenvres.2016.06.006
https://doi.org/10.1016/j.marenvres.2016... ). Therefore, the use of the methods and procedures commonly used in soil genesis studies can significantly contribute to a more detailed and precise knowledge about processes and properties of seagrass soils and increase the chances of success of initiatives for restoration and protection.

For the Brazilian soil science community, including subaqueous soils as a potential study object would not only help to develop the soil classification system and the theoretical models used for soil genesis, but would also open new study fields of nitrogen-fixing and phosphate-solubilizing bacteria for soil microbiologists (Vazquez et al., 2000Vazquez P, Holguin G, Puente ME, Lopez-Cortes A, Bashan Y Phosphate-solubilizing microorganisms associated with the rhizosphere of mangroves in a semiarid coastal lagoon. Biol Fert Soils. 2000;30:460-8. https://doi.org/10.1007/s003740050024
https://doi.org/10.1007/s003740050024... ; Welsh, 2000Welsh DT. Nitrogen fixation in seagrass meadows: regulation, plant-bacteria interactions and significance to primary productivity. Ecol Lett. 2000;3:58-71. https://doi.org/10.1046/j.1461-0248.2000.00111.x
https://doi.org/10.1046/j.1461-0248.2000... ), for soil chemistry and organic matter analyses (York et al., 2016York PH, Smith TM, Coles RG, McKenna SA, Connolly RM, Irving AD, Jackson EL, McMahon K, Runcie JW, Sherman CDH, Sullivan BK, Trevathan-Tackett SM, Brodersen KE, Carter AB, Ewers CJ, Lavery PS, Roelfsema CM, Sinclair EA, Strydom S, Tanner JE, Dijk K, Warry FY, Waycott M, Whitehead S. Identifying knowledge gaps in seagrass research and management: an Australian perspective. Mar Environ Res. 2017;127:163-72. https://doi.org/10.1016/j.marenvres.2016.06.006
https://doi.org/10.1016/j.marenvres.2016... ), as well as other co-related soil science disciplines.

The comprehension of the pedogenetic processes may help to understand the ecological functions of seagrass meadows. Moreover, the studies of seagrass soils may be considered a future frontline of Brazilian soil science, as a new study object for soil chemistry, microbiology, and organic matter scientists, but also a useful tool for the conservation, restoration, and comprehension of ecological services provided by these ecosystems. Thus, it is fundamental that soil scientists increase their knowledge on subaqueous soils and their variations, not only to update and contribute to Soil Classification Systems (e.g., SiBCS, WRB-FAO, and Soil Taxonomy) but more importantly, to contribute to the protection of these endangered ecosystems.

CONCLUSIONS

According to the Brazilian Soil Classification Systems, the natural body formed by pedogenetic processes (mainly: gleyzation, sulfidization, salinization, paludization, and solonization and classified as Gleissolos tiomórficos) and that supports the life of rooted seagrass plants is not considered a soil, since the present soil definition a water column cannot be considered an upper limit of a soil.

The criteria related to the presence of calcium carbonate in the SiBCS (e.g., Horizonte Cálcico; and the subordinate characteristics k and $\overline{\text{k}}$), should be re-defined, since the presence of seashells is not taken into account, which are important to control the acidification generated by the oxidation of sulfidic material; and properties should be created that describe the frequency of flooding and the occurrence of hyposulfidic material.

ACKNOWLEDGEMENTS

The authors wish to thank the National Council for Scientific and Technology Development (CNPq, process 308288/2014-9); Coordination for the Improvement of Higher Education Personnel (CAPES); São Paulo Research Foundation (Process No. 2014/11778-5 and 2016/21026-6); Conselleria de Innovación e Industrial Xunta de Galicia (Espanha; Proyecto PGIDIT08MDS036000PR), and are indebted to everyone who helped in the development of this research. The author XLO belongs to the Cretus (Centre for Research in Environmental Technologies of the University of Santiago de Compostela), in a strategic partnership (AGRUP2015/02), co-funded with the Fondo Europeo de Desarrollo Regional (Feder - EU). The authors are also thankful for the suggestions and corrections of the Editors and Reviewers, which substantially improved the paper.

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