The higher grain yields achieved in the TWW-irrigated systems were probably attributed to the continuous supply of plant nutrients, especially N, contained in the TWW at high concentrations during the growing season (Table 3). Being the most yield-limiting nutrient in rice production, N is generally applied through high doses of synthetic fertilisers to ensure high levels of rice production, especially for high-yielding cultivars such as the forage rice ‘Bekoaoba’ used in this study12,15. In this experiment, the continuous sub-irrigation systems supplied large amounts of N into R1, R2, and R3 (~811, 575, and 778 kg N ha−1, respectively), which were 3.1, 2.2, and 3.0-fold higher than that supplemented by the fertilisers in the control (260 kg N ha−1). As a result, the TWW-irrigated plants maintained considerably higher leaf greenness, which was measured using a chlorophyll meter and expressed as SPAD values4, compared with the rice under conventional cultivation during the flowering and grain filling stages (~80 DAT onwards, Fig. 4).
Leaf greenness (SPAD value) of the rice plants during the growing period.
Since the SPAD value is strongly associated with N status in leaves18, it is one of the best indicators of photosynthetic activities in rice. Generally, starch and sugar accumulate at high levels in rice culms and leaf sheaths before flowering, and the accumulated carbohydrates are translocated into rice grains during the grain filling stage, which causes the rice culms and leaves to senesce and turn yellowish19. By delaying the senescence and maintaining effective photosynthesis, more carbohydrates could be produced and be available for grain filling, subsequently resulting in higher grain yields. Our results suggest that the N derived from TWW could sufficiently substitute for the N supplemented by synthetic fertilisers. These results are consistent with those of a previous study20, which demonstrated that N-rich wastewater was as effective as commercial N fertiliser at achieving optimum rice yields. This was further supported by our previous study, which reported that continuous sub-irrigation removed 85–90% of the N available in TWW4, and the utilisation of newly absorbed N until the late growth period is critical for producing the high yields of high-yielding rice varieties21.
Water regime R1 was the preferred water scheme demonstrated in our previous study4; however, continuous irrigation at a constant rate of 25 L m−2 day−1 throughout the growing season probably reduced the efficient use of N owing to the variable N demand of rice plants in each growth stage19. In the present study, we modified this water regime into R2 and R3 to test for an optimal regime. The lower supply rate of 8.3 L m−2 day−1 was used during the early growth stage and near maturity when the rice plants generally had a low N-absorbing capacity19. A higher supply rate of either 25 L m−2 day−1 or 36 L m−2 day−1 was used in R2 or R3, respectively, from 31 to 114 DAT, which was a period of high N demand, whereas active tillering, panicle initiation, heading, and grain filling occurred consecutively (Fig. 3) with a high N-use efficiency to maximise tiller number, increase panicle size, improve filled-grain percentage, and enhance grain weight19. Compared with R1, R2 and R3 did not cause significant yield losses (p > 0.05, Fig. 1a), suggesting an opportunity to apply suitable supply rates at appropriate timings to meet the N demands of the rice plants. The better N assimilation during the late growth period also explained the increased rice protein content observed in R1, R2, and R3 (Fig. 1b). The highest protein content obtained in R3 (p < 0.05) can be attributed to the higher supply rate of 36 L m−2 day−1 maintaining the most efficient photosynthesis in the rice plants from 67 DAT onwards (Fig. 4).
Overall, our study provides evidence to suggest that continuous sub-irrigation with TWW is an effective means to reuse the effluents from WWTPs to produce high yields with high protein content without using exogenous synthetic fertilisers. This result agreed with those of previous studies1,4,9, suggesting a cost-effective strategy for recycling water and plant nutrients that simultaneously reduces the demand for synthetic fertilisers and the amount of nutrients discharged into surface water bodies. Furthermore, eliminating the use of fertilisers will not only decrease the adverse environmental effects but will also increase profits for farmers7,9. Relative to the constant supply rate in R1, combining more suitable supply rates with relevant timings in R2 and R3 could maintain the high yielding capacity and high rice protein content (Fig. 1).
Accumulation of heavy metals/metalloids by crops irrigated with wastewater has generally been considered an environmental problem since those metals tend to accumulate in soil and could become bioavailable for crops3. However, in the present study, there was no notable adverse effect of TWW irrigation on the accumulation of toxic elements, like As, Cr, Cu, Cd, and Pb, in the rice grains. This result is in accordance with those of other studies5,6,9, which reported that the chemical compositions of rice irrigated with TWW were within the common range observed in conventional paddy fields. The slight increase in Zn content in the rice grains under TWW irrigation compared with that under the control (Table 1) was probably due to the higher concentration of Zn in TWW relative to other elements (Table 3). Interestingly, Zn is one of the most essential micronutrients for humans, and thus, attempts have been made to improve Zn content in rice22. The lower concentrations of As, Cr, and Cu observed under continuous sub-irrigation with TWW relative to the control were probably attributed to the continuous overflow of water that might carry these elements out of the paddy soils. Ultimately, although minor variation in the concentrations of the examined elements was observed among the continuous sub-irrigation water regimes (R1, R2, and R3), and between those and the control, all the concentrations were lower than the maximum limits recommended by FAO/WHO16 and the Japanese standard17 (Table 1), suggesting that the rice grains harvested from the continuous sub-irrigation systems are safe to use as feedstuffs.
The production of CH4 in rice fields generally results from the anaerobic decomposition of organic matter in rhizosphere soil. In contrast with a previous study14, which reported an increase in CH4 emissions from paddy fields irrigated with wastewater, continuous sub-irrigation considerably decreased the seasonal emissions of CH4 by 70–84% compared with conventional cultivation (Table 2). This decrease was probably due to the substantial amounts of dissolved oxygen maintained in the TWW (Table 3) being continuously supplied into the rhizosphere as TWW was pumped into the deep soil layers, which might subsequently inhibit methanogen communities and their activities regarding CH4 production. The high peaks of CH4 fluxes recorded in all treatments during the grain filling stage were mainly attributed to the higher availability of C substances in the paddy soil as a result of enhanced root exudation during flowering. The highest exudation rates were observed during the grain filling stage compared to the other growth stages23. Since root exudates provide C substrates for methanogenesis in rice fields, the higher root exudation during flowering could greatly stimulate CH4 emissions in the following grain filling stage23. Among the three continuous irrigation regimes tested in the present study, R2 was the most effective in terms of CH4 mitigation (Fig. 2, Table 2), probably owing to the lowest input amount of available C accompanied by the lowest irrigation rates (Fig. 3).
The emissions of N2O in the control, which contributed to the net GWP by 8%, were low compared with CH4 emissions. This result agrees with those of other studies that reported negligible N2O emissions in flooded paddy fields under conventional cultivation24,25. The high peaks of N2O fluxes observed in all treatments during MSD were consistent with the common phenomenon in paddy fields under field drainage that promotes N2O emissions due to enhanced nitrification-denitrification processes under favourable conditions14,15. The additional peaks recorded in R1 and R3 within 3 days afterwards were likely due to the high N concentration in the soil and surface water when TWW was re-supplied into the experimental chambers. Emissions of N2O from R1, R2, and R3 contributed to 43%, 27%, and 51% of the net GWP, respectively, probably due to the high N concentration in the TWW continuously supplied into the chambers (Table 3). The lowest emission of N2O recorded in R2 relative to the other TWW-irrigated treatments (Table 2) was mainly attributed to the lowest N input accompanied by lower irrigation rates. It is likely that the lower fertilisation in R2 ensured the efficient use of nutrients by the rice plants, leaving very little residual N for nitrification and denitrification, thereby reducing N2O emissions. The higher N2O emissions from R1 and R3 were essentially due to the enhanced nitrification/denitrification process induced by the considerable N contained in the TWW and the higher supply rates (Fig. 3). Furthermore, the rich sources of organic matter supplied by TWW could also benefit N-cycling bacterial communities14, subsequently increasing N2O emissions. Overall, our results indicate that R2 is the most effective mitigator that can overcome the trade-off between N2O and CH4 emissions compared with R1 and R3.
Efficient cultivation practices must involve producing the optimum rice yield along with low environmental effects. Prior studies have reported many potential practices to increase rice yield and simultaneously mitigate GHG emissions from paddy fields26,27. In the present study, continuous TWW irrigation considerably decreased net GWPs primarily owing to the considerable decrease in CH4 emissions (Table 2). The combination of two supply rates in R2 and R3 (Fig. 3) tended to decrease seasonal CH4 and N2O emissions (Fig. 2), and subsequently diminish the net GWPs compared with the constant supply rate in R1. The lowest net GWP and GHGI attained in R2 is attributed to its most effective minimisation of both CH4 and N2O emissions. Our results have shown that appropriately matching the lower (8.3 L m−2 day−1) and higher (25 L m−2 day−1) supply rates with the periods of low and high N demand of rice plants (Fig. 3), respectively, leads to R2 being the optimised irrigation regime for continuous sub-irrigation with TWW to reduce the GHG budget of rice paddy fields without significant yield loss or protein reduction.
In conclusion, the continuous sub-irrigation systems produce high yields of protein-rich forage rice through the reuse of TWW as the sole source for both irrigation and fertilisation in paddy fields. Importantly, by employing the optimal water regime (R2), continuous sub-irrigation with TWW can effectively mitigate CH4 and N2O emissions from the rice paddies. The practice of recycling valuable plant nutrients contained in TWW to meet the high nutrient demand of forage rice production instead of applying high doses of synthetic fertilisers demonstrates the potential to minimise the dependence of rice production on fertilisers, which would simultaneously mitigate GHG emissions and promote sustainable rice paddy farming. Our results will motivate local farmers to adopt the continuous irrigation systems to reuse TWW for rice cultivation. The agronomic performance and cost efficiency of adopting these practices under real farm conditions will be analysed by our research team in future studies.
Source: Ecology - nature.com
