Rice Cultivation without Synthetic Fertilizers and Performance of Microbial Fuel Cells (MFCs) under Continuous Irrigation with Treated Wastewater

Abstract

To obtain a high rice yield and quality for animal feed without synthetic fertilizers, an experiment with bench-scale apparatus was conducted by applying continuous irrigation with treated municipal wastewater (TWW). Uniform rice seedlings of a high-yield variety (Oryza sativa L., cv. Bekoaoba) were transplanted in five treatments to examine different TWW irrigation directions (“bottom-to-top” and “top-to-top” irrigation) and fertilization practices (with and without P-synthetic fertilizers) as well as one control that simulated the irrigation and fertilization management of normal paddy fields. The highest rice yield (14.1 t ha−1), shoot dry mass (12.9 t ha−1), and protein content in brown rice (14.6%) were achieved using bottom-to-top irrigation, although synthetic fertilizers were not applied. In addition, this subsurface irrigation system could contribute to environmental protection by removing 85–90% of nitrogen from TWW more effectively than the top-to-top irrigation, which showed a removal efficiency of approximately 63%. No accumulation of heavy metals (Fe, Mn, Cu, Zn, Cd, Ni, Pb, Cr, and As) in the paddy soils was observed after TWW irrigation for five months, and the contents of these metals in the harvested brown rice were lower than the permissible limits recommended by international standards. A microbial fuel cell system (MFC) was installed in the cultivation system using graphite-felt electrodes to test the capacity of electricity generation; however, the electricity output was much lower than that reported in normal paddy fields. Bottom-to-top irrigation with TWW can be considered a potential practice to meet both water and nutrient demand for rice cultivation in order to achieve a very high yield and nutritional quality of cultivated rice without necessitating the application of synthetic fertilizers.

Continuous Sub-Irrigation with Treated Municipal Wastewater for Protein-Rich Rice Production with Reduced Emissions of CH4 and N2O

“…In addition, an earlier study on wastewater irrigation in rice paddy fields claimed that the increased emissions of CH 4 and N 2 O from wastewater-irrigated fields resulted from the high availability of elements in the wastewater, such as organic C and N 14 . Particularly, paddy fields irrigated with sewage water emitted higher CH 4 and N 2 O by 27-33% and 68-170%, respectively, compared with conventional fields irrigated with river water 14 . Thus, irrigating forage rice fields with TWW might result in a synergic effect to boost emissions of these greenhouse gases (GHGs) to the atmosphere.…”

“…To our knowledge, however, no prior study has thoroughly examined the use of TWW as a sole source of both water and nutrients for paddy rice cultivation, especially for cultivating forage rice, which commonly has higher N demands compared with staple rice varieties [10][11][12] . In our previous studies, we reported the development of an innovative approach for continuous irrigation with TWW in forage paddy fields, which used TWW as the only source for irrigation and fertilisation 1,4 . In such cultivation systems, TWW was continuously supplied into paddy fields at a constant rate throughout the crop seasons either through an underground drain pipe (sub-irrigation) or on the soil surface (surface irrigation) 1,4 .…”

“…In our previous studies, we reported the development of an innovative approach for continuous irrigation with TWW in forage paddy fields, which used TWW as the only source for irrigation and fertilisation 1,4 . In such cultivation systems, TWW was continuously supplied into paddy fields at a constant rate throughout the crop seasons either through an underground drain pipe (sub-irrigation) or on the soil surface (surface irrigation) 1,4 . High grain yields of protein-rich forage rice have been produced using those novel systems, in which higher yield and rice protein content were achieved using the continuous sub-irrigation system compared with the surface irrigation system 4 .…”

“…In such cultivation systems, TWW was continuously supplied into paddy fields at a constant rate throughout the crop seasons either through an underground drain pipe (sub-irrigation) or on the soil surface (surface irrigation) 1,4 . High grain yields of protein-rich forage rice have been produced using those novel systems, in which higher yield and rice protein content were achieved using the continuous sub-irrigation system compared with the surface irrigation system 4 . As a result, our cultivation systems are expected to promote the low-cost production of protein-rich forage rice and improve the efficiency of farmland, which inevitably secures domestic livestock production and stops the increase in abandoned farmland that has been observed in Japan since the 1970s 13 .…”

“…To estimate the GHG emissions and optimise water regimes of the continuous TWW irrigation systems, we performed a growth chamber experiment to compare CH 4 and N 2 O emissions from forage paddy fields under continuous sub-irrigation with three different water regimes (R1, R2, and R3) and those under conventional cultivation practices (control). R1 was adapted from our previous study as the preferred irrigation regime 4 , while R2 and R3 were alternatives modified from R1 to test for the optimal regime. The continuous sub-irrigation system was selected for further investigation in the present study owing to its superior agronomic performance compared with the surface irrigation 1,4 .…”