Shohei Riya

Assessing nitrification and denitrification in a paddy soil with different water dynamics and applied liquid cattle waste using the 15N isotopic technique

Using livestock wastewater for rice production in paddy fields can remove nitrogen and supplement the use of chemical fertilizers. However, paddy fields have complicated water dynamics owing to varying characteristics and would influence nitrogen removal through nitrification followed by denitrification. Quantification of nitrification and denitrification is of great importance in assessing the influence of water dynamics on nitrogen removal in paddy fields. In this study, nitrification and nitrate reduction rates with different water dynamics after liquid cattle waste application were evaluated, and the in situ denitrification rate was determined directly using the (15)N isotopic technique in a laboratory experiment. A significant linear regression correlation between nitrification and the nitrate reduction rate was observed and showed different regression coefficients under different water dynamics. The regression coefficient in the continuously flooded paddy soil was higher than in the drained-reflooded paddy soil, suggesting that nitrate would be consumed faster in the flooded paddy soil. However, nitrification was limited and the maximum rate was only 13.3 μg Ng(-1)day(-1) in the flooded paddy soil with rice plants, which limited the supply of nitrate. In contrast, the drained-reflooded paddy soil had an enhanced nitrification rate up to 56.8 μg Ng(-1)day(-1), which was four times higher than the flooded paddy soil and further stimulated nitrate reduction rates. Correspondingly, the in situ denitrification rates determined directly in the drained-reflooded paddy soil ranged from 5 to 1035 mg Nm(-2)day(-1), which was higher than the continuously flooded paddy soil (from 5 to 318 mg Nm(-2)day(-1)) during the vegetation period. The nitrogen removal through denitrification accounted for 38.9% and 9.9% of applied nitrogen in the drained-reflooded paddy soil and continuously flooded paddy soil, respectively.

The relationship between anammox and denitrification in the sediment of an inland river

This study measured the microbial processes of anaerobic ammonium oxidation (anammox) and denitrification in sediment sampled from two sites in the estuary of an inland river (Koisegawa River, Ibaragi prefecture, Japan) using a nitrogen isotope pairing technique (IPT). The responses of anammox and denitrification activities to temperature and nitrate concentration were also evaluated. Further, to elucidate the correlation between anammox and denitrification processes, an inhibition experiment was conducted, using chlorate to inhibit the first step of denitrification. Denitrification activity was much higher than anammox activity, and it reached a maximum at the surface layer in February 2012. Denitrification activity decreased as sediment depth increased, and a similar phenomenon was observed for anammox activity in the sediment of site A, where aquatic plants were absent from the surroundings. The activities of both denitrification and anammox were temperature-dependent, but they responded differently to changes in incubation temperature. Compared to a linear increase in denitrification as temperature rose to 35 °C, the optimal temperature for anammox was 25 °C, after which the activity decreased sharply. At the same time, both anammox and denitrification activities increased with NO3(-) concentration. The Michaelis-Menten kinetic constants (Vmax and Km) of denitrification were significantly higher than those of the anammox process. Furthermore, anammox activity decreased accordingly when the first step of denitrification was inhibited, which probably reduced the amount of the intermediate NO2(-). Our study provides the first direct exploration of the denitrification-dependent correlation of anammox activity in the sediment of inland river.

Synthesis of CTAB intercalated graphene and its application for the adsorption of AR265 and AO7 dyes from water

Environmental applications of graphene (GN) are limited by the occurrence of aggregation. Herein, graphene oxide (GO) was synthesized, reduced to GN by ascorbic acid, and intercalated with cetyltrimethylammonium bromide (CTAB). GN-CTAB was characterized by Boehms titration, N2 adsorption/desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, Fluorescence spectrophotometry, X-ray diffraction and Scanning electron microscopy. Then, GN-CTAB was used for the adsorptive removal of acid red 265 (AR265) and acid orange 7 (AO7) dyes from water both under batch and column operation. Under batch operation, the effect of pH, adsorbent dosage, initial dye concentration, contact time and temperature on dyes adsorption were assessed. Adsorption isotherms, kinetics, and thermodynamics were analyzed systematically. Regarding the fixed bed operation, the effect of both the bed height and flow rate were studied and experimental results fitted to the Thomas and BDST models. Then, the bed loss capacity along five adsorption-regeneration cycles was determined in order to further approach the practical application of GN-CTAB for wastewater treatment, namely for the removal of dyes.

CH4 and N2O emissions from different varieties of forage rice (Oryza sativa L.) treating liquid cattle waste

To evaluate global warming potential (GWP) on livestock waste treatment and biomass production in rice field, methane (CH(4)) and nitrous oxide (N(2)O) fluxes from forage rice fields planted with 4 different cultivars (Oryza sativa L. cv. Hamasari, Leafstar, Kusahonami and Takanari) were measured. Each of the cultivars were subjected either to basal fertilization alone (control plots) (84 kg N ha(-1)), or to basal fertilization plus topdressing with liquid cattle waste or LCW (treatment plots) (567 kg N ha(-1)). Liquid cattle waste application to the rice field resulted in peak CH(4) fluxes ranging from 22.0 to 32.1 mg m(-1)h(-1) during flooded conditions and large N(2)O fluxes ranging from 526 to 8591 μg m(-1)h(-1) after midsummer drainage and final drainage. The GWP of the control plots was between 1358 and 3872 kg CO(2)eq ha(-1), while the treatment plots ranged between 4503 and 8426 kg CO(2)eq ha(-1) and more than 60% of the GWP was from the N(2)O emission in treatment plots. In both the control and treatment plots, the lowest GWPs per ton of above-ground biomass were found to be from the Leafstar cultivar, which had a higher aboveground biomass than other cultivars; 117 kg CO(2)eq t(-1) from the control and 257 kg CO(2)eq t(-1) from the treatment plots. Thus, both forage production and suitable disposal of the LCW may be able to be achieved concomitantly with lower levels of GWP by cultivation of Leafstar in our field.

Abundance, transcription levels and phylogeny of bacteria capable of nitrous oxide reduction in a municipal wastewater treatment plant.

Nitrous oxide (N2O) production and expression of genes capable of its reduction were investigated in two full-scale parallel plug-flow activated sludge systems. These two systems continuously received wastewater with the same constituents, but operated under distinct nitrification efficiencies due to mixed liquor suspended solid (MLSS) concentration and the different hydraulic retention times (HRTs). A shorter HRT in system 2 resulted in a lower nitrification efficiency (40-60%) in conjunction with a high N2O emission (50.6 mg-N/L/day), whereas there was a higher nitrification efficiency (>99%) in system 1 with low N2O emission (22.6 mg-N/L/day). The DNA abundance of functional genes responsible for nitrification and denitrification were comparable in both systems, but transcription of nosZ mRNA in the lower N2O emission system (system 1) was one order of magnitude higher than that in the higher N2O emission system (system 2). The diversity and evenness of the nosZ gene were nearly identical; however, the predominant N2O reducing bacteria were phylogenetically distinct. Phylogenetic analysis indicated that N2O-reducing strains only retrieved in system 1 were close to the genera Rhodobacter, Oligotropha and Shinella, whereas they were close to the genera Mesorhizobium only in system 2. The distinct predominant N2O reducers may directly or indirectly influence N2O emissions.

Hybrid Nitrous Oxide Production from a Partial Nitrifying Bioreactor: Hydroxylamine Interactions with Nitrite

The goal of this study was to elucidate the mechanisms of nitrous oxide (N2O) production from a bioreactor for partial nitrification (PN). Ammonia-oxidizing bacteria (AOB) enriched from a sequencing batch reactor (SBR) were subjected to N2O production pathway tests. The N2O pathway test was initiated by supplying an inorganic medium to ensure an initial NH4+-N concentration of 160 mg-N/L, followed by 15NO2- (20 mg-N/L) and dual 15NH2OH (each 17 mg-N/L) spikings to quantify isotopologs of gaseous N2O (44N2O, 45N2O, and 46N2O). N2O production was boosted by 15NH2OH spiking, causing exponential increases in mRNA transcription levels of AOB functional genes encoding hydroxylamine oxidoreductase (haoA), nitrite reductase (nirK), and nitric oxide reductase (norB) genes. Predominant production of 45N2O among N2O isotopologs (46% of total produced N2O) indicated that coupling of 15NH2OH with 14NO2- produced N2O via N-nitrosation hybrid reaction as a predominant pathway. Abiotic hybrid N2O production was also observed in the absence of the AOB-enriched biomass, indicating multiple pathways for N2O production in a PN bioreactor. The additional N2O pathway test, where 15NH4+ was spiked into 400 mg-N/L of NO2- concentration, confirmed that the hybrid N2O production was a dominant pathway, accounting for approximately 51% of the total N2O production.

Variation of the microbial community in thermophilic anaerobic digestion of pig manure mixed with different ratios of rice straw.

The effect of pig manure mixed with rice straw on methane yield and the microbial community involved in a thermophilic (55°C) anaerobic digestion process was investigated. Three substrates composed of mixed pig manure and rice straw at different ratios (95:5; 78:22 and 65:35 w/w, which resulted in C/N ratios of 10:1, 20:1 and 30:1) were used for the experiment. The substrate type had a major influence on the total bacterial community, while the methanogens were less affected. The members of the class Clostridia (phylum Firmicutes) were predominant regardless of mixture ratio (C/N ratio), but at species level there was a major difference between the low and high C/N ratio samples. The hydrogenotrophic methanogenic genus of Methanothermobacter was predominant in all samples but higher C/N ratio sequences affiliated to the genus Methanosarcina were also detected. The appearance of Methanosarcina sp. is most likely due to the less inhibition of ammonia during the anaerobic digestion.

In Situ Dissimilatory Nitrate Reduction to Ammonium in a Paddy Soil Fertilized with Liquid Cattle Waste

Abstract Most studies on dissimilatory nitrate reduction to ammonium (DNRA) in paddy soils were conducted in the laboratory and in situ studies are in need for better understanding of the DNRA process. In this study, in situ incubations of soil DNRA using 15N tracer were carried out in paddy fields under conventional water (CW) and low water (LW) managements to explore the potential of soil DNRA after liquid cattle waste (LCW) application and to investigate the impacts of soil redox potential (Eh) and labile carbon on DNRA. DNRA rates ranged from 3.06 to 10.40 mg N kg−1 dry soil d−1, which accounted for 8.55%–12.36% and 3.88%–25.44% of consumption of added NO3−15N when Eh at 5 cm soil depth ranged from 230 to 414 mV and −225 to −65 mV, respectively. DNRA rates showed no significant difference in paddy soils under two water managements although soil Eh and/or dissolved organic carbon (DOC) were more favorable for DNRA in the paddy soil under CW management 1 d before, or 5 and 7 d after LCW application. Soil DNRA rates were negatively correlated with soil Eh (P

Short-Term Responses of Nitrous Oxide Emissions and Concentration Profiles to Fertilization and Irrigation in Greenhouse Vegetable Cultivation

Abstract In vegetable cultivation, the majority of N2O emissions occur after fertilization; it is therefore important to understand any factors contributing to this process. An experiment was conducted to investigate short-term N2O dynamics following topdressing in a greenhouse vegetable field in South China. During two topdressing processes, three different urea-N treatments with irrigation were conducted in May and June in a tomato (Lycopersicum esculentum) cultivation. The N2O fluxes, soil concentration profiles and soil environments at the 0–60 cm depths at 10 cm intervals were measured both immediately prior to and 5 days after topdressing. The N2O fluxes before topdressing ranged from 6.7±2.1 to 55.0±28.8 μg N m−2 h−1; even higher numbers were recorded in highly fertilized plots. The NO−3-N accumulation in the soil caused by vegetable cultivation during the 5 years prior to the start of the experiment, resulted in high background N2O fluxes. One day after topdressing (1 DAT) in May and June, N2O fluxes increased, which coincided with sharp increases in soil N2O concentrations at depths of 2.5 and 15 cm and in NO−3-N and NH+4-N contents at depths of 0–20 cm. From 1 to 5 DAT, fluctuations in the N2O fluxes did not harmonize with the N2O concentrations at a depth of 2.5 cm, which was attributed to different gas diffusion rates at depths of 0–10 cm. These results suggested that surface soil N and environmental conditions were crucial for determining the short-term N2O ebullitions during topdressing in greenhouse vegetable cultivation.

Biokinetic Characterization and Activities of N2O-Reducing Bacteria in Response to Various Oxygen Levels

Nitrous oxide (N2O)-reducing bacteria, which reduce N2O to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as N2O sinks in the environment. However, research on their physiological traits has been limited. In particular, their activities under microaerophilic and aerobic conditions, which severely inhibit N2O reduction, remain poorly understood. We used an O2 and N2O micro-respirometric system to compare the N2O reduction kinetics of four strains, i.e., two strains of an Azospira sp., harboring clade II type nosZ, and Pseudomonas stutzeri and Paracoccus denitrificans, harboring clade I type nosZ, in the presence and absence of oxygen. In the absence of oxygen, the highest N2O-reducing activity, Vm,N2O, was 5.80 ± 1.78 × 10−3 pmol/h/cell of Azospira sp. I13, and the highest and lowest half-saturation constants were 34.8 ± 10.2 μM for Pa. denitirificans and 0.866 ± 0.29 μM for Azospira sp. I09. Only Azospira sp. I09 showed N2O-reducing activity under microaerophilic conditions at oxygen concentrations below 110 μM, although the activity was low (10% of Vm,N2O). This trait is represented by the higher O2 inhibition coefficient than those of the other strains. The activation rates of N2O reductase, which describe the resilience of the N2O reduction activity after O2 exposure, differ for the two strains of Azospira sp. (0.319 ± 0.028 h−1 for strain I09 and 0.397 ± 0.064 h−1 for strain I13) and Ps. stutzeri (0.200 ± 0.013 h−1), suggesting that Azospira sp. has a potential for rapid recovery of N2O reduction and tolerance against O2 inhibition. These physiological characteristics of Azospira sp. can be of promise for mitigation of N2O emission in industrial applications.