Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat Controlling the process of denitriﬁcation in ﬂooded rice soils by using microbial fuel cell applications Tharangika Ranatunga a , Ken Hiramatsu b, ⁎ , Takeo Onishi b a The United Graduate School of Agricultural Science, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan b Faculty of Applied Biological Sciences, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan ARTICLE INFO Keywords: Redox potential Nitrous oxide gas Nitrogen fertilizer ABSTRACT Controlling the denitriﬁcation rate could help reduce the losses of applied nitrogen (N) fertilizer in ﬁelds. The applicability of microbial fuel cell (MFC) for controlling denitriﬁcation in ﬂooded rice soils was investigated based on MFC theory coupled with redox changes. Because the soil about 10–20 cm beneath water and the soil near the water surface are anaerobic and aerobic, respectively, gradients of electric potentials could be generated between them, upon connecting them through insulated wires. Electrons released through the oxidation of or- ganic matter during microbial metabolism can be utilized by this set-up, generating electricity. This can reduce the availability of electrons for reductive half-reactions of nitrate, to suppress denitriﬁcation. We studied the N losses in soil using planting pots with gas chamber experiment under three conditions: MFC systems, MFC systems with an externally applied voltage, and non-MFC systems as a control. Each system was set in triplicate, supplied with the same N fertilizer amounts, and ﬂooded with automatic irrigation. Soil redox potential, N 2 O ﬂux, and inorganic nitrogen concentration in soil pore water were periodically monitored. The redox potentials of both MFC systems and MFC systems with externally applied voltage were signiﬁcantly higher than that of non- MFC systems, while N 2 O ﬂux levels were signiﬁcantly lower than that of non-MFC systems. The rice reproductive stage was the most eﬀective on suppressing N 2 O ﬂux with MFC application. However, the eﬀect of externally applied voltage on suppressing N 2 O ﬂux remains unclear. Inorganic nitrogen retention eﬃciencies in pore water were higher in MFC systems, which is consistent with the N 2 O ﬂux diﬀerence. While the proportion of denitriﬁed N estimated for MFC systems was 2.3%, that of non-MFC systems was 6.6%. We conﬁrmed the applicability of MFCs to control soil redox potential and thereby suppress the denitriﬁcation based on planting pot experiments. 1. Introduction 1.1. Problem of nitrogen deﬁciency The global food industry is highly dependent on fossil fuels. Energy- eﬃcient approaches to agriculture would oﬀer a way to take advantage of the relationships between energy, food, and agriculture. A huge amount of energy is required for the ﬁxation of nitrogen (N) fertilizer from the unlimited atmospheric nitrogen. To manufacture one metric ton of anhydrous ammonia (NH 3 ) which consists 82% of N, it is esti- mated that 3500 m 3 of natural gas is used (Olson and Halstead, 1974). It is also estimated that the ﬁeld application of 150 kg/ha N fertilizer in the form of NH 3 involves the consumption of 645 m 3 of natural gas (Olson and Halstead, 1974). Moreover, despite the fact that rice is the staple food of half the world’s population, N fertilizer is not used eﬃ- ciently (Keeny and Sahrawat, 1986). Recognizing the fact that N ferti- lizer is absolutely essential for supporting the growing global population, there is an urgent need to explore eﬃcient ways of using fertilizer to compensate for future food shortages. As conventional methods to improve N retention eﬃciency various methods such as leguminous crop production, crop rotation, and management of irri- gation water have been proposed and implemented (Huang et al., 2007; Kaewpradit et al., 2008; Pramanik et al., 2014).Though these ap- proaches are proven to be eﬀective under speciﬁc conditions, some- times these are not eﬀective due to the uncontrollable factors such as climate conditions. Hence, if we can electrochemically control deni- triﬁcation processes, that might be a more universally applicable technology. The process of denitriﬁcation, involving the conversion of soil in- organic N to elemental N gas, is one of the main routes behind N de- ﬁciency in crop production. Here, denitriﬁcation refers to the process in which NO 3 - is converted to gaseous compounds such as NO, N 2 O, and N 2 by microorganisms. In submerged soils, the denitrifying bacteria use NO 3 - in the absence of oxygen as the terminal electron acceptor in https://doi.org/10.1016/j.agwat.2018.04.041 Received 27 September 2017; Received in revised form 27 April 2018; Accepted 28 April 2018 ⁎ Corresponding author. E-mail address: hira@gifu-u.ac.jp (K. Hiramatsu). Agricultural Water Management 206 (2018) 11–19 0378-3774/ © 2018 Published by Elsevier B.V. T