Gypsum amendment effects on micromorphology and aggregation in no- till Mollisols and Alfisols from western Ohio, USA Rebecca Tirado-Corbalá a, ⁎, Brian K. Slater a , Warren A. Dick b , Jerry Bigham a , Miguel Muñoz-Muñoz c a School of Environment and Natural Resources, The Ohio State University (SENR/OSU), Columbus, OH 43210, United States b School of Environment and Natural Resources, The Ohio State University (SENR/OSU), 1680 Madison Avenue, Wooster, OH 44691, United States c Agro-Environmental Science Department, University of Puerto Rico, Mayagüez, PR 00681, United States abstract article info Article history: Received 31 July 2018 Received in revised form 13 March 2019 Accepted 13 March 2019 Available online xxxx Synthetic gypsum, a by-product of electricity generation, is used as a soil amendment to overcome water ponding, improve soil and water quality, improve field conditions to support farm equipment, and reduce the variability of crop yield in no-till fields by improving hydrology. Gypsum is a source of soluble calcium (Ca) that improves physical properties of the soil by promoting clay aggregation, thereby increasing water infiltration rates and movement through the soil profile. Undisturbed soil samples from Brookston and Celina soils in Ohio, USA were collected to a depth of 75 cm in agricultural fields treated with gypsum for 0, 4, and 12 years to deter- mine changes in chemical and physical properties. Gypsum applications increased exchangeable Ca and Ca: Mg ratios, and promoted clay flocculation and improved soil structure. Mean weight diameter of aggregates in- creased with gypsum treatment at most depths in both soils. Micromorphological analysis showed variations in porosity (ɸ), pore size distribution, pore shape and aggregate size related to gypsum treatment, soil, and soil depth. There were no consistent responses to years of gypsum application. Gypsum treated soils had higher po- rosity than untreated soils in all depths b75 cm and a higher percentage of micropores and mesopores compared to the control. Also, gypsum treated soils had larger aggregates than the control for all soil depths examined. Ag- gregates b100 μm predominated in the Brookston control soils, and b 200 μm aggregates dominated the Celina control soils. However, there was no prevailing aggregate size for gypsum treated soils. In conclusion, our study found positive effects of gypsum on most properties measured; although, not consistently related to years of gypsum applications to both soils. © 2019 Published by Elsevier B.V. Keywords: Alfisols Flue gas desulfurization gypsum Micromorphology Mollisols No-tillage Soil aggregates Soil image analysis 1. Introduction Ohio is located on the eastern edge of the American corn belt and benefits from highly fertile soils. However, approximately 55% of Ohio's agricultural land needs drainage intervention to improve infiltration and reduce soil erosion, reduce water logging in the plant root-zone, modify unfavorable field conditions for farm equipment in the spring and fall, and to reduce year to year crop yield variability caused by in- consistent infiltration and water movement (Ohio State University Ex- tension, 1995; Tirado-Corbalá, 2010). Drainage issues, especially extended periods of profile saturation and surface ponding after snowmelt and/or excessive seasonal rainfall, may be particularly chal- lenging to the adoption of no-tillage (NT) systems by Ohio farmers (Rusinamhodzi et al., 2011; Tirado-Corbalá et al., 2013). Artificial drain- age (e.G. tile drainage) in poorly drained NT fields is often inadequate to prevent ponding after heavy rains (Tirado-Corbalá et al., 2013). Readily available flue gas desulfurization gypsum (FGDG) has been used as a soil amendment by some Ohio farmers, facilitating successful implementa- tion of NT practices. High-purity FGDG is a coal combustion by- product which is readily available in Ohio due to electricity generation from many coal-fired power plants (Tirado-Corbalá, 2010). Coal con- tinues to be an important fuel globally, and the combustion process that generates electricity often requires the removal of SO 2 from flue gases to meet clean air regulations. The materials produced during the scrubbing process are called FGD by-products and are initially mostly composed of CaSO 3 . 0.5 H 2 O (calcium sulfite hemihydrate) and any unreacted sorbent (Chen et al., 2001; Laperche and Bigham, 2002). When forced air oxidation procedures are applied to CaSO 3 . 0.5 H 2 O, high quality FGDG (CaSO 4 . 2H 2 O) is produced (Chen et al., 2005) and is deemed to be a worthy source of Ca and S for soils (Dontsova et al., 2005; Dick et al., 2006; USEPA, 2008). Geoderma Regional 15 (2019) e00217 Abbreviations: FGDG, Flue gas desulfurization gypsum; CT, Control treatment; ST, Short-term gypsum treatment; LT, Long-term gypsum treatment; D b , Bulk density; MWD, Mean weight diameter; WSA, Water stable aggregates; PSD, Pore size distribution. ⁎ Corresponding author at: Agro-Environmental Science Department, University of Puerto Rico-Mayagüez, Mayagüez, PR 00681-9000, United States E-mail addresses: rebecca.tirado@upr.edu (R. Tirado-Corbalá), slater.39@osu.edu (B.K. Slater), dick.5@osu.edu (W.A. Dick), bigham.1@osu.edu (J. Bigham), miguel.munoz3@upr.edu (M. Muñoz-Muñoz). https://doi.org/10.1016/j.geodrs.2019.e00217 2352-0094/© 2019 Published by Elsevier B.V. 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