I Reunião Brasileira de Micromorfologia de Solo

The onset of agriculture is one of the most important steps in the history of humankind. It marks the beginning of the Neolithic, appearing at about 10,000 BC in the Fertile Crescent, and at about 5,000 BC in Europe. The profound effects that agriculture has had on soils since then are one of the arguments for the definition of the Anthropocene, which according to Certini & Scalenghe (2011) can be dated back to about 2,000 years ago, when the changes on soil properties in some parts of the world are more drastic. Within this period some anthropogenic soils have been formed under all types of environments, as plaggens, Amazonian dark earths, paddy rice fields or terraced soils among many others. Not reaching these extremes, even in cases where soils have been tilled only once, it is enough to designate the topsoil as an Ap (ploughed) genetic horizon, assuming that the soil materials have been mixed and disturbed to a considerable extent. In all these cases, compared with other soil forming processes, the impact of humans on soils occurs at very short time scales, from seasonal (common agricultural practices) and yearly or decennial (crop cycles) to centuries or millennia (e.g. continuous application of amendments or fertilizers, progressive levelling, tillage erosion).

The agricultural practices that can leave an imprint on soil micromorphology are tillage, irrigation, addition of materials to increase soil fertility, burning, waste recycling, clearance and husbandry (Adderley et al., 2018). Some of the effects can be detected through morphological (field), chemical or physical analyses, but micromorphology is the tool that can give explanations to the soil behaviour, even when the processes are incipient or are not reflected in other properties (Stoops, 2018). These effects are mainly explained by (1) changes in microstructure, (2) presence or absence and location of some components (both as coarse or fine components and as pedofeatures), and (3) changes in the activity of soil biota.

One of the main questions in the interpretation of features due to agriculture is whether there are features unequivocally due to it or the same features can be caused by natural processes. This is the case, for instance, of silt cappings and coatings on pores, which can be due both to periglacial processes and to disturbance by ploughing. Therefore, their interpretation must always be done with caution.

Effects of tillage. Changes in the amount, types, distribution of pores and aggregates are perhaps the most evident effects. Ploughing and tillage in general tend to increase porosity temporarily in the topsoil and create plough pans with laminar structure (Adderley et al., 2018), which can be quantified through physical methods as well; but micromorphology also informs about types and functionality of pores. As examples, compared with no tilled, natural or forested soils, Alvarez et al. (2020) observe a change from crumb to a subangular blocky structure, due to a reduction of faunal activity. In frost- affected environments, the interaction between tillage and freeze-thaw processes has been studied by Sveistrup et al. (2005), who report an increase of bioturbations when forest changes to cropland.
Surface crusting and sealing is another field where micromorphology has considerably contributed to explain the mechanisms of formation, perhaps due to the inherent difficulties in sampling for other types of analyses. Thin section study allows distinguishing structural and depositional crusts (Valentin and Bresson, 1992), and can give information about the causes of crust formation, as slaking of aggregates (Usón and Poch, 1999). Indeed, old buried A horizons have sometimes been recognised by fragments of these crusts and silt coatings around pores, together with other diagnostic features (Boixadera et al. 2016).

Fertilisation and amendments. Micromorphology can help to explain their physical effects in soils, as better aggregation or increase of faunal activity. The application of organic amendments as animal manure has a clear influence on soil microstructure as well, mainly by the increase of bioturbation (Pagliai et al., 2004; Yagüe et al., 2016). While chemical fertilisers may also have such effect by an increase of nutrients or pH changes -as it is the case with liming-, the addition of pig slurry, applied normally as a substitute of N fertilizer has also a temporary effect in promoting crumb aggregation and therefore packing voids in front of cracks in the blank treatment (Bosch-Serra et al. 2017). Domingo-

Olivé et al. (2016) also find increases of porosity in the 0.065–0.4 mm size range with the continuous use of pig slurry, corresponding to packing pores between aggregates.
Irrigation. This practice may change soil porosity patterns, as well as textural pedofeatures derived from the migration of fine particles with water after aggregate collapse or change in the composition of the exchange complex (Adderley et al., 2018). Redistribution of gypsum, carbonates and salts are other features commonly reported (Verba et al., 1995). Rodríguez et al. (1990) make a micromorphological assessment of strongly silted drains in irrigated sodic soils. They are able to discern siltation episodes, internal fabrics, sorting and particle composition and morphology, and thus explain the siltation process. The performance of different types of subsurface irrigation pipes in sodic soils has been explained by the promotion of oribatid activity (Poch et al. 2019).

Deeply disturbed soils and old tillage practices. Some particular soils with strong anthropic influence in order to increase their fertility have merited special attention among micromorphologists. As examples we can cite Amazonian Dark Earths (terras pretas do indio), (Arroyo-Kalin, 2017); plaggen soils ((Davidson & Carter, 1998; Mücher et al., 1990), bench-terraced soils in the Mediterranean (Boixadera et al. 2016) and soils on “camellones” in the Bolivian Moxos plains (Boixadera et al. 2019).

The next challenges we face in soil micromorphology are new management techniques as addition of residues as amendments, in particular the effects of microplastics on soils; or of different types of sludges; and also the dynamics of soil inorganic carbon in soils in the frame of climate change, as e.g. the influence of irrigation on the dynamics of calcium carbonate as a source / sink of carbon.

References:
Adderley, W. P., Wilson, C. A., Simpson, I. A., Davidson, D. A. 2018. Anthropogenic features. [In:] G. Stoops, V. Marcelino, and F. Mees (Editors), Interpretation of Micromorphological Features of Soil and Regoliths. 2nd Ed., Elsevier, Amsterdam, 753-777.
Alvarez, M. F., Poch, R. M., Osterrieth, M. 2020. Bioturbations as quality indicators of Typic Argiudolls in the Southeast of Buenos Aires, Argentina. A micromorphological approach. Global Symposium on Soil Biodiversity, FAO HQ, Rome, Italy, 10-12 March 2020
Arroyo-Kalin, M. 2017. Amazonian dark earths. [In:] C. Nicosia and G. Stoops (Editors), Archaeological soil and sediment micromorphology, Elsevier, 345-357.
Boixadera, J., Esteban, I, Albert, R.M., Poch., R.M. 2019. Anthropogenic Soils from Llanos de Moxos (Bolivia): Soils from Pre-Columbian Raised Fields. Catena 172.
Boixadera, J., Riera, S. Vila, S., Esteban, I., Albert, R.M., Llop, J.M., Poch, R.M. 2016. Buried A Horizons in Old Bench Terraces in Les Garrigues (Catalonia). Catena 137.
Bosch-Serra, À.D., M.R. Yagüe, R.M. Poch, M. Molner, B. Junyent, J. Boixadera. 2017. Aggregate Strength in Calcareous Soil Fertilized with Pig Slurries. European Journal of Soil Science 68 (4): 449–61.
Certini, G., Scalenghe, R. 2011. Anthropogenic Soils Are the Golden Spikes for the Anthropocene. The Holocene 21 (8): 1269–74.
Davidson, D.A, Carter, S.A. 1998. Micromorphological Evidence of Past Agricultural Practices in Cultivated Soils : The Impact of a Traditional Agricultural System on Soils in Papa Stour , Shetland. Journal of Archaeological Science 25: 827–38. Domingo-Olivé, F., Bosch-Serra, À.D., Yagüe, M.R., Poch, R.M., Boixadera. J. 2016. Long Term Application of Dairy Cattle Manure and Pig Slurry to Winter Cereals Improves Soil Quality. Nutrient Cycling in Agroecosystems 104 (1).
Mücher, H. J., Slotboom R. T., ten Veen W. J.. 1990. Palynology and Micromorphology of a Man-Made Soil. A Reconstruction of the Agricultural History since Late-Medieval Times of the Posteles in The Netherlands. Catena 17: 55–67. Pagliai, M., Vignozzi, N., Pellegrini, S. 2004. Soil structure and the effect of management practices. Soil and Tillage Research, 79, (2), 131-143.
Poch, R.M., Pascual, M. Villar, J.M. Rufat, J.. 2019. Soil Porosity Changes in Orchards with Subsurface Irrigation: Quantification and Interpretation. Boletin de La Sociedad Geologica Mexicana 71 (1).
Rodríguez, R., Herrero, J., Porta, J. 1990. Micromorphological assessment of drain siltation risk indexes in a saline-sodic soil in Monegros irrigation district (Spain). [In:] L.A. Douglas (Editor), Developments in soil science, Elsevier, Vol. 19, 41-52.
Stoops, G. 2018. Micromorphology as a tool in soil and regolith studies. [In:] G. Stoops, V. Marcelino, and F. Mees (Editors), Interpretation of Micromorphological Features of Soil and Regoliths. 2nd Ed., Elsevier, Amsterdam, 1-19.
Sveistrup, T. E., Haraldsen T. K., Langohr, R., Marcelino, V. Kværner, J. 2005. Impact of land use and seasonal freezing on morphological and physical properties of silty Norwegian soils. Soil and Tillage Research 81, (1), 39-56.
Usón, A., Poch, R.M. 1999. Effects of tillage and management practices on soil crust morphology under a Mediterranean environment. Soil & Tillage Research, 54, 191-196
Valentin, C. Bresson L.M. 1992. Morphology, genesis and classification of surface crusts in loamy and sandy soils. Geoderma, 55: 225-245.
Verba, M. P., Al-Kasiri, A. S., Goncharova, N. A., Chizhikova, N. P. 1995. Impact of irrigation on micromorphological features of desert soils of Hadremout Valley (Yemen). Eurasian soil science, 27(9), 108-124.
Yagüe, M.R., Domingo-Olivé, F., Bosch-Serra, À.D., Poch, R.M., Boixadera, J. 2016. Dairy Cattle Manure Effects on Soil Quality: Porosity, Earthworms, Aggregates and Soil Organic Carbon Fractions. Land Degradation and Development 27 (7).