plants Article Salt Stress Amelioration in Maize Plants through Phosphogypsum Application and Bacterial Inoculation Tamer Khalifa 1 , Mohssen Elbagory 2,3 and Alaa El-Dein Omara 3, *   Citation: Khalifa, T.; Elbagory, M.; Omara, A.E.-D. Salt Stress Amelioration in Maize Plants through Phosphogypsum Application and Bacterial Inoculation. Plants 2021, 10, 2024. https://doi.org/10.3390/ plants10102024 Academic Editors: Ewa Hanus-Fajerska and Iwona Kami ´ nska Received: 1 August 2021 Accepted: 22 September 2021 Published: 27 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Agricultural Research Center, Department of Soil Improvement and Conservation, Soils, Water and Environment Research Institute, Giza 12112, Egypt; tamerkhalifa1985@gmail.com 2 Department of Biology, Faculty of Science and Arts, King Khalid University, Mohail Assir 61321, Saudi Arabia; mhmohammad@kku.edu.sa 3 Agricultural Research Center, Department of Microbiology, Soils, Water and Environment Research Institute, Giza 12112, Egypt * Correspondence: alaa.omara@yahoo.com Abstract: The use of phosphogypsum (PG) and plant growth-promoting rhizobacteria (PGPR) for agricultural purposes are good options to improve soil properties and increase crop yield. The objective of this study was to investigate the effect of different rates of PG (ton ha −1 ; 0 (PG1), 3 (PG2), 6 (PG3), and 9 (PG4)) combined with PGPR inoculation (Azospirillum lipoferum (control, T1), A. lipoferum + Bacillus coagulans (T2), A. lipoferum + B. circulance (T3), and A. lipoferum + B. subtilis (T4)) on soil properties, plant physiology, antioxidant enzymes, nutrient uptake, and yield of maize plants (Zea mays L., cv. HSC 10) grown in salt-affected soil. Over two growing seasons, 2019 and 2020, field experiments were conducted as a split-plot design with triplicates. The results show that applying PG (9 ton ha −1 ) and co-inoculation (A. lipoferum + B. circulance) treatment significantly increased chlorophyll and carotenoids content, antioxidant enzymes, microbial communities, soil enzymes activity, and nutrient contents, and showed inhibitory impacts on proline content and pH, as well as EC and ESP, thus improving the productivity of maize plant compared to the control treatment. It could be concluded that PG, along with microbial inoculation, may be an important approach for ameliorating the negative impacts of salinity on maize plants. Keywords: phosphogypsum; PGPR; soil salinity; nutrient content; maize productivity 1. Introduction In arid and semi-arid regions, soil salinity has been reported to have a negative impact on soil quality and crop growth, affecting 25 to 30% of the crop productivity of agricultural soils [1–3]. In addition, salinity is a major environmental stress and a major obstacle to crop production. The salinization of arable land is expected to have devastating effects globally, resulting in 30% of lost land over the next 25 years and up to 50% by the mid-21st century [4]. Nowadays, 50% of the world’s total cultivated area is exposed to salinity stress, causing a loss of about USD 12 billion [5]. In addition, soil salinity is a complex process that has negative effects on the activity of physiological and biochemical processes in the plant [6]. Therefore, the toxicity of specific ions during metabolic processes or osmotic stress leads to a reduction in plant growth, nutrient uptake, and enzyme activity [7,8]. On the other hand, the adverse effects of salt-affected soils are associated with the reduced osmosis (primary stage) and cytotoxicity of ions (secondary stage), as well as the pro- duction of reactive oxygen species (ROS) and nutrient imbalance [9]; high osmotic stress is related to the accumulation of soluble salts in the soil solution, which leads to water stress [10]. According to [11], the inhibitory effect of salinity stress on the growth and productivity of different crops depends on several factors, such as salt concentration in the soil solution, time interval, plant species, plant growth stage, gas exchange properties, pho- tosynthetic pigments, and environmental conditions. Therefore, it is important to explore Plants 2021, 10, 2024. https://doi.org/10.3390/plants10102024 https://www.mdpi.com/journal/plants