Impacts of continuous irrigation using treated municipal wastewater on rice-soil systems: A three-year study

Research on the effects of rice fertigation using treated municipal wastewater (TWW) as the sole source of nutrients and irrigation water remains limited. This study examined the impact of continuous TWW irrigation on rice-soil systems across three years (2021-2023), focusing on soil health, plant growth and yield, and the mineral and toxic element composition of rice grains. Forage rice cultivation using TWW fertigation (test field) was compared with conventional cultivation using chemical fertilisers and canal water (control field). The test field exhibited an average total nitrogen (TN) removal rate of 72 % across three crop seasons. However, low TN concentrations in TWW (2021: 4.7 mg L^-1, 2022: 9.9 mg L^-1, 2023: 2.8 mg L^-1) resulted in lower leaf greenness, plant height, and shoot biomass compared to the control, particularly in 2023 (p < 0.01). In the same year, rice protein content in the test field (5.9 %) was significantly lower than the control (6.5 %) (p < 0.001), while mineral compositions remained consistent across fields (p > 0.05). Significant increases in K, Mg, Na, and Zn concentrations were observed in rice grains from the test field in 2021 (p < 0.05). However, As levels in the test field grains (0.35 mg kg^-1) in 2023 approached the maximum allowable limit and were significantly higher than that in the control (0.25 mg kg^-1), raising food safety concerns. TWW irrigation maintained soil C levels, with the test field exhibiting higher C:N ratios, humification indices, and complex carbon structures (R_ET:Bz and SUVA₂₅₄) across the three seasons. These improvements enhanced soil structure and nutrient retention. While TWW irrigation shows potential as sustainable alternative to chemical fertilisers, optimizing its application to address varying N concentrations and complex organic matter dynamics is essential to maximize productivity and ensure food safety and environmental sustainability.