Contents lists available at ScienceDirect Soil & Tillage Research journal homepage: www.elsevier.com/locate/still Soil aeration and relationship to inorganic nitrogen during aerobic cultivation of irrigated rice on a consolidated land parcel Setia S. Girsang a,b , Teodoro Q. Correa Jr. a , James R. Quilty a , Pearl B. Sanchez c , Roland J. Buresh a, * a International Rice Research Institute (IRRI), PO Box 7777, Metro Manila, Philippines b Indonesian Agency for Agricultural Research and Development, Indonesia c University of the Philippines Los Baños (UPLB), Los Baños, Laguna, Philippines ARTICLE INFO Keywords: Aerobic rice Nitrate Spatial variability Water-Filled pore space ABSTRACT Efficient use of irrigation water in Asia can be achieved through production of rice (Oryza sativa L.) on aerobic soil rather than submerged, anaerobic soil. Rice grown on aerobic soil is referred to as aerobic rice. The irrigation of aerobic rice without soil submergence might result in fluctuations and spatial variability in soil aeration, which can affect N transformations. We examined variability in soil aeration and inorganic N during the ve- getative and ripening growth phases of aerobic rice grown on one 3.3-ha parcel with frequent irrigation using an overhead sprinkler to maintain soil water potential not less than -10 kPa without soil submergence. Water- filled pore space (WFPS) of soil ranged from 70–96% at 36 locations. The WFPS was inversely related to sand content (r = –0.66, P < 0.001). Soil ammonium-N was unrelated to WFPS, but nitrate-N was inversely related to WFPS (r = –0.84, P < 0.001) and directly related to sand content (r = 0.67, P < 0.001). The WFPS during production of aerobic rice favored nitrate accumulation at only some locations in the 3.3-ha parcel. Rice yield was inversely related to nitrate-N (P < 0.001). Nitrate-N, originating from both soil and fertilizer N, accumu- lated and exceeded ammonium-N at ≤77 % WFPS. Nitrate-N was small and < ammonium-N at > 90 % WFPS, presumably due to N loss by denitrification. Precise irrigation of aerobic rice did not prevent spatial variability in soil aeration and nitrate, which was associated with inherent soil properties such as sand content. 1. Introduction Approximately 70 % of the world’s rice is produced with irrigation, which uses substantial freshwater resources especially in Asia where 90 % of the world’s rice is produced (GRiSP (Global Rice Science Partnership), 2013). Irrigated rice in Asia is usually grown on puddled soil, which is saturated with water and then tilled wet to create a soft topsoil favorable for crop establishment (Sharma and De Datta, 1986). Rice is then typically irrigated to submerge and saturate soil between crop establishment and a few days before harvest. The use of irrigation water can be reduced by eliminating soil puddling and establishing rice on dry or moist soil (Kumar and Ladha, 2011; Wang et al., 2017). The use of irrigation water can also be reduced during crop growth by either delaying the onset of soil submergence for several weeks after crop establishment (Beyrouty et al., 1992; Dunn and Gaydon, 2011) or al- ternating the flooding of soil with a period of drying, which has been referred to as alternate wetting and drying (AWD) (Bouman et al., 2007; Carrijo et al., 2017). The use of irrigation water can be further reduced by growing rice without soil submergence or saturation, which has been referred to as aerobic rice (Bouman et al., 2007). Rice grown without soil submergence risks yield loss due to water-deficit stress when soil water potential decreases below –10 to -20 kPa (Sudhir-Yadav et al., 2011, 2012). Much of Asian rice is produced with intensive use of labor on re- latively small land parcels (Balasubramanian, 2018). A major trend in Asian rice production, in addition to less supply of irrigation water, is increased cost for labor, which is driving a shift toward more me- chanization (FAO, 2015). The consolidation of existing small land parcels could facilitate the use of mechanization and increase farm productivity (Nguyen and Warr, 2019), and aerobic rice is well suited to mechanization that reduces labor especially for land preparation and crop establishment (Kumar and Ladha, 2011). The removal of earthen https://doi.org/10.1016/j.still.2020.104647 Received 11 October 2019; Received in revised form 8 February 2020; Accepted 12 March 2020 Abbreviations: AE N , agronomic efficiency of added fertilizer N; ANM, anaerobic N mineralization; AWD, alternate wetting and drying; CV, coefficient of variation; DAT, days after transplanting; WFPS, water-filled pore space ⁎ Corresponding author. E-mail address: rolandburesh@gmail.com (R.J. Buresh). Soil & Tillage Research 202 (2020) 104647 Available online 11 May 2020 0167-1987/ © 2020 Elsevier B.V. All rights reserved. T