Tracking Fecal Bacterial Dispersion from Municipal Wastewater to Peri-Urban Farms during Monsoon Rains in Hue City, Vietnam.

Disease outbreaks attributed to monsoon flood-induced pathogen exposure are frequently reported, especially in developing cities with poor sanitation. Contamination levels have been monitored in past studies, yet the sources, routes, and extents of contamination are not always clear. We evaluated pollution from municipal wastewater (MWW) discharge and investigated fecal contamination by Escherichia coli (E. coli) in three agricultural fields on the outskirts of Hue City, Vietnam. After E. coli concentration was determined in irrigation water (IRW), MWW, soil, vegetables (VEG), and manure, its dispersion from MWW was tracked using multilocus sequence typing (MLST) and phylogenetic analyses during the wet and dry seasons. IRW was severely contaminated; 94% of the samples were positive with E. coli exceeding the stipulated standards, while VEG contamination was very low in both seasons. The confirmed total number of isolates was comparable between the seasons; however, results from MLST and phylogenetic clustering revealed more links between the sites and samples to MWW during the wet season. The wet season had four mixed clusters of E. coli isolates from multiple locations and samples linked to MWW, while only one mixed cluster also linking MWW to IRW was observed during the dry season. The most prevalent sequence type (ST) complex 10 and two others (40 and 155) have been associated with disease outbreaks, while other STs have links to major pathotypes. Irrigation canals are significant routes for E. coli dispersion through direct links to the urban drainage-infested river. This study clarified the genotype of E. coli in Hue city, and the numerous links between the samples and sites revealed MWW discharge as the source of E. coli contamination that was enhanced by flooding.

Methane mitigation is associated with reduced abundance of methanogenic and methanotrophic communities in paddy soils continuously sub-irrigated with treated wastewater.

Herein, we examined emissions of CH4 and the community structures of methanogenic archaea and methanotrophic bacteria in paddy soils subjected to a novel irrigation system, namely continuous sub-irrigation with treated wastewater (TWW). This system has recently been developed by our group to effectively reuse TWW for the cultivation of protein-rich rice. The results showed that, despite not using mineral fertilisers, the wastewater reuse system produced a rice yield comparable to that of a conventional cultivation practice and reduced CH4 emissions from paddy fields by 80%. Continuous sub-irrigation with TWW significantly inhibited the growth of methanogens in the lower soil layer during the reproductive stage of rice plants, which was strongly consistent with the effective CH4 mitigation, resulting in a vast reduction in the abundance of methanotrophs in the upper soil layer. The compositions of the examined microbial communities were not particularly affected by the studied cultivation practices. Overall, this study demonstrated that continuous sub-irrigation with TWW was an effective method to produce high rice yield and simultaneously reduce CH4 emissions from paddy fields, and it also highlighted the potential underlying microbial mechanisms of the greenhouse gas mitigation.

High yield of protein-rich forage rice achieved by soil amendment with composted sewage sludge and topdressing with treated wastewater.

Aiming to promote low-cost production of protein-rich forage rice and resource recycling from wastewater treatment plants, a pot experiment was conducted to assess the possibility to substitute mineral fertilizers with composted sewage sludge (CSS) with/without top-dressing with treated municipal wastewater (TWW). Results indicated that a basal application of CSS at 2.6 g N pot-1 replaced conventional mineral fertilization of 1.3 g N pot-1 to produce comparable yields with the same rice protein content, although there might be a risk of increased As concentration in rice grains. Interestingly, CSS application at a reasonable dose of 1.3 g N pot-1, followed by a topdressing with TWW resulted in 27% higher yield and 25% superior rice protein content relative to the mineral fertilization, with no risk of heavy metal(loid) accumulation in grains and in paddy soils. Here we demonstrated an appealing fertilization practice with zero use of mineral fertilizers in paddy rice cultivation, expectedly contributing towards sustainable rice farming and animal husbandry in Japan.

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

Herein, we introduce continuous sub-irrigation with treated municipal wastewater (TWW) as a novel cultivation system to promote resource recycling and cost-effective forage rice production in Japan. However, both TWW irrigation and forage rice cultivation were previously considered to intensify CH4 and N2O emissions. In the present study, therefore, we evaluate the emissions of greenhouse gases (GHGs) and yielding capacity of forage rice between conventional cultivation and continuous sub-irrigation systems employing different water supply rates. Results indicated that continuous sub-irrigation with TWW resulted in high rice yields (10.4-11 t ha-1) with superior protein content (11.3-12.8%) compared with conventional cultivation (8.6 t ha-1 and 9.2%, respectively). All TWW irrigation systems considerably reduced CH4 emissions, while higher continuous supply rates significantly increased N2O emissions compared with the conventional cultivation. Only the continuous irrigation regime employing suitable supply rates at appropriate timings to meet the N demand of rice plants decreased both CH4 and N2O emissions by 84% and 28%, respectively. Overall, continuous sub-irrigation with TWW provides high yields of protein-rich forage rice without the need for synthetic fertilisers and effectively mitigated GHG emissions from paddy fields.

Water extractable organic matter and iron in relation to land use and seasonal changes.

The soil carbon pool is an essential part of the global carbon cycle although it is sensitive to climatic changes and the local environment. Terrestrial areas are important sources of organic matter for aquatic ecosystems and the fluctuation of different soil minerals and elements is largely influenced by land use and season changes. We studied water-extractable organic matter (WEOM) properties including iron (Fe), water extractable organic carbon (WEOC) and spectral characteristics from forests and arable soils to evaluate the effects of land use and seasonal change on WEOM and Fe in terrestrial areas. We collected soil samples randomly from arable land (AR), broadleaf (BL) and aged needle leaf forests (NL) and extracted WEOM for analysis using rainwater. Results of WEOC and Fe showed similar trend seasonally and were higher in forest sample than in AR. WEOC was high in the upper layer while Fe was independent of the depth and higher in AR. On the other hand, specific ultra-violet absorbance at 254 nm (SUVA254) and a proxy for aromaticity significantly varied with both land use and season and was on average two times higher in arable land than forests during spring and summer. Humic-like components significantly varied between the studied sites seasonally while tyrosine-like was affected by season only. The relative abundance of both humic-like and tryptophan-like components were significantly affected by land use while [Fe]: [WEOC] ratio was also high in arable land and negatively correlated with humic-like components in forest sites. As observed from ratio and Fe oxidation rate constant, summer presented ideal conditions for WEOM interactions. The synchronized seasonal WEOC and Fe changes indicate an enhancement of Fe mobility by DOM whereas the differences observed between sites especially from the high humic- and protein-like components in NL and BL reflects the effect of land use.