Redox control on carbon mineralization and dissolved organic matter along a chronosequence of paddy soils

Paddy soils are subjected to periodically changing redox conditions. In order to understand better the redox control on long‐term carbon turnover, we assessed carbon mineralization and dissolved organic carbon (DOC) of paddy topsoils sampled along a chronosequence spanning 2000 years of rice cultivation. Non‐paddy soils were used as references. We exposed soils to alternating redox conditions for 12 weeks in incubation experiments. Carbon mineralization of paddy soils was independent of redox conditions. Anoxic conditions caused increasing DOC concentrations for paddy soils, probably because of desorption induced by increasing pH. We assume desorption released older, previously stabilized carbon, which then was respired by a microbial community well adapted to anoxic conditions. This assumption is supported by the ¹⁴C signatures of respired CO₂, indicating larger mineralization of older carbon under anoxic than under oxic conditions. The increasing DOC concentrations under anoxic conditions did not result in an equivalent increase in carbon mineralization, possibly because of little reducible iron oxide. Therefore, net DOC and CO₂ production were not positively related under anoxic conditions. The overall 20–75% smaller carbon mineralization of paddy soils than of non‐paddy soils resulted from less respiration under oxic conditions. We conclude that carbon accumulation in paddy as well as in other wetland soils results from a microbial community well adapted to anoxic conditions, but less efficient in mineralizing carbon during transient oxic periods. Carbon accumulation might be even larger when mineralization under anoxic conditions is restricted by a lack of alternative electron acceptors.     

Dominant Characteristics Between <Emphasis Type="Italic">Microcystis aeruginosa</Emphasis> and <Emphasis Type="Italic">Cyclotella</Emphasis> Sp. Accompanying Dilution Process in Eutrophic Lake

Although dilution of lake water has been used for improvement of water quality and algal blooms control, it has not necessarily succeeded to suppress the blooms. We hypothesized that the disappearance of algal blooms by dilution could be explained by flow regime, nutrient concentrations, and their interaction. This study investigated the effects of daily renewal rate (d), nitrogen (N) and phosphorus (P) concentration, and their interaction on the domination between Microcystis aeruginosa and Cyclotella sp. through a monoxenic culture experiment. The simulation model as functions of the N:P mass ratio and dilution rate (D) (calculated from d) was constructed, and the dominant characteristics of both species were predicted based on the model using parameters obtained in a monoculture experiment and our previous study. Results of monoxenic culture experiment revealed that M. aeruginosa dominated in all conditions (d = 5 or 15%; N = 1.0 or 2.5 or 5.0 mg-N L^?1; P = 0.1 or 0.5 mg-P L^?1) and the predicted cell densities were substantially correspondent to experimental data. Under various N:P ratios and D values, characteristics of domination for each species were predicted, indicating that Cyclotella sp. tended to be dominant under high P concentrations (P ≥ 0.36 mg-P L^?1) when the N:P ratio was less than 7.0, and M. aeruginosa could not form algal blooms at the N:P ratio ≤ 7.0 (N ≤ 0.7 mg-N L^?1). It was also suggested that the dilution rate leading to the Cyclotella sp. domination required 0.20 day^?1 or higher regardless of the N:P ratios.

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Graphical Abstract ? M. aeruginosa and Cyclotella sp. could be a superior competitor in nutrient-limited and nutrient-rich conditions, respectively. ? The simulation model in this study indicated that the predicted cell density and nutrient concentration were substantially correspondent to experimental data. ? The model predicted that Cyclotella sp. tended to be dominant at the P ≥ 0.36 mg-P L^?1 when the N:P ratio was less than 7.0, and M. aeruginosa could not form algal blooms at the N:P ratio ≤ 7.0 (N ≤ 0.7 mg-N L^?1).

     

TiO2-Assisted Photodegradation of Direct Blue 78 in Aqueous Solution in Sunlight

The photodegradation of azo dyes aqueous solution has been investigated using TiO₂ as catalyst in sunlight. The effect of amount of catalyst, concentration of dye, and pH value on the degradation of Direct Blue 78 was observed. A complete degradation of 100 mg/L Direct Blue 78 solution under solar irradiation was achieved in 6 h at pH?3.0, dosage of TiO₂ 1.0 g/L. A possible pathway for the photodegradation of Direct Blue 78 in sunlight was proposed.     

Radiation Synthesis of Poly(Acrylamide-Acrylic Acid-Dimethylaminoethyl Methacrylate) Resin and Its Use for Binding of Some Anionic Dyes

Poly(acrylamide-acrylic acid-dimethylaminoethyl methacrylate) P(AAm-AA-DMAEMA) resin was prepared by the template copolymerization. PAAm was used as a template for the copolymerization of DMAEMA and AA in aqueous solution using gamma rays. The adsorption of indigo carmine and eriochrome black-T anionic dyes from aqueous media on P(AAm-AA-DMAEMA) has been investigated. The adsorption behavior of this resin has been studied under different adsorption conditions: dye concentrations (50?C500 mg l^?1), contact times, temperature (30?C55°C), and pH values (2?C7). The amount of dye adsorbed increased with increasing resin content, but it had a little change with temperature and decreased slightly with increasing pH. Adsorption data of the samples were modeled by the pseudo-first-order and pseudo-second-order kinetic equations in order to investigate dye adsorption mechanism. It was found that the adsorption kinetics of the resin followed a pseudo-second-order model with rate constant (k ₂) of 2.5?×?10^?3 and 1.8?×?10^?2 g (mg^?1 min^?1) for indigo carmine and eriochrome black-T, respectively. Equilibrium isotherms were analyzed using the Langmuir and Freundlich isotherms. It was seen that the Freundlich model fits the adsorption data better than the Langmuir model.     

Effect of Metals on Decolorization of Reactive Blue HERD by <Emphasis Type="Italic">Comamonas</Emphasis> sp. UVS

Comamonas sp. UVS was able to decolorize Reactive Blue HERD (RBHERD) dye (50 mg L^?1) within 6 h under static condition. The maximum dye concentration degraded was 1,200 mg L^?1 within 210 h. A numerical simulation with the model gives an optimal value of 35.71?±?0.696 mg dye g^?1 cell h^?1 for maximum rate (V_max) and 112.35?±?0.34 mg L^?1 for the Michaelis constant (K_m). Comamonas sp. UVS has capability of decolorization of RBHERD in the presence of Mg²⁺, Ca²⁺, Cd²⁺, and Zn²⁺, whereas decolorization was completely inhibited by Cu²⁺. Metal ions also affected the levels of biotransformation enzymes during decolorization of RBHERD. Comamonas sp. UVS was also able to decolorize textile effluent with significant reduction in COD. The biodegradation of RBHERD dye was monitored by UV–vis spectroscopy, FTIR spectroscopy, and HPLC.     

Induced Degradation of Anthraquinone-Based Dye by Laccase Produced from <Emphasis Type="Italic">Pycnoporus sanguineus</Emphasis> (CS43)

In this study, in-house isolated laccase isoforms, i.e., Lac-I and Lac-II of the basidiomycete Pycnoporus sanguineus (CS43), were evaluated in relation to their Remazol Brilliant Blue R (RBBR) dye degradation capacity. A modified Dhouib medium additionally supplemented with 3% ethanol as a secondary inducer was used to propagate P. sanguineus CS43 for enhanced production of laccase under liquid state fermentation. The crude laccase extract was purified by passing through ion exchange diethylaminoethanol (DEAE)-Sepharose and gel filtration-based Sephadex G-200 column chromatography. The purified laccase fractions were subjected to the electrophoresis, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed two laccase isoforms Lac-I and Lac-II with 66 and 68 kDa, respectively. To explore the industrial applicability, for RBBR dye, degradation efficiencies ranged from 82 to 88% after 3 h of incubation for both; Lac-I and Lac-II at both concentrations were recorded. However, with 8 U/mL, the degradation ranged between 70 to 80% during the first 5 min of incubation. Enhanced degradation of RBBR dye was obtained in the presence of violuric acid and N-hydroxypthalamide as laccase mediators. Finally, using RBBR as a substrate kinetic characterization of both Lac-I and Lac-II isoforms was performed that revealed K _m (0.243 and 0.117 mM for Lac-I and Lac-II) and V _max (1.233 and 1.012 mM/Sec for Lac-I and Lac-II) values, respectively.     

Nitrification and acidification from urea application in red soil (Ferralic Cambisol) after different long-term fertilization treatments

Purpose

Long-term manure applications can prevent or reverse soil acidification by chemical nitrogen (N) fertilizer. However, the resistance to re-acidification from further chemical fertilization is unknown. The aim of this study was to examine the effect of urea application on nitrification and acidification processes in an acid red soil (Ferralic Cambisol) after long-term different field fertilization treatments.

Materials and methods

Soils were collected from six treatments of a 19-year field trial: (1) non-fertilization control, (2) chemical phosphorus and potassium (PK), (3) chemical N only (N), (4) chemical N, P, and K (NPK), (5) pig manure only (M), and (6) NPK plus M (NPKM; 70 % N from M). In a 35-day laboratory incubation experiment, the soils were incubated and examined for changes in pH, NH₄ ⁺, and NO₃ ^?, and their correlations from urea application at 80 mg N kg^?1(?80) compared to 0 rate (?0).

Results and discussion

From urea addition, manure-treated soils exhibited the highest acidification and nitrification rates due to high soil pH (5.75–6.38) and the lowest in the chemical N treated soils due to low soil pH (3.83–3.90) with no N-treated soils (pH 4.98–5.12) fell between. By day 35, soil pH decreased to 5.21 and 5.81 (0.54 and 0.57 unit decrease) in the NPKM-80 and M-80 treatments, respectively, and to 4.69 and 4.53 (0.43 and 0.45 unit decrease) in the control-80 and PK-80 treatments, respectively, with no changes in the N-80 and NPK-80 treatments. The soil pH decrease was highly correlated with nitrification potential, and the estimated net proton released. The maximum nitrification rates (K _max) of NPKM and M soils (14.7 and 21.6 mg N kg^?1 day^?1, respectively) were significantly higher than other treatments (2.86–3.48 mg N kg^?1 day^?1). The priming effect on mineralization of organic N was high in manure treated soils.

Conclusions

Field data have shown clearly that manure amendment can prevent or reverse the acidification of the red soil. When a chemical fertilizer such as urea is applied to the soil again, however, soil acidification will occur at possibly high rates. Thus, the strategy in soil N management is continuous incorporation of manure to prevent acidification to maintain soil productivity. Further studies under field conditions are needed to provide more accurate assessments on acidification rate from chemical N fertilizer applications.     

Kinetics and Energetics of Azo Dye Decolorization by Pycnoporus sanguineus

Trypan Blue, an azo dye, was decolorized using the self-immobilizing fungal biomass of Pycnoporus sanguineus. The extent and the rate of dye decolorization were directly proportional to the initial dye concentration (20–60 mg L^?1) and the reaction temperature (25–45°C). Mass transfer within and outside the pellets did not limit dye degradation. The apparent kinetics of the decolorization reaction followed a first-order behavior. Activation energy for the biological decolorization was calculated at 23 kJ mol^?1. The decolorization process was endothermic with the enthalpy and entropy values calculated at 45.6 kJ mol^?1 and 146 J mol^?1 K^?1, respectively. Based on the value of Gibbs free energy change, the decolorization reaction under the conditions studied was non-spontaneous below 39°C but was spontaneous at higher temperatures.     

N2-fixation and growth promotion in cedar colonized by an endophytic strain of Paenibacillus polymyxa

Inoculation of western red cedar with Paenibacillus polymyxa P2b-2R, an endophytic diazotroph of a pine, was previously shown to result in biological nitrogen fixation (BNF) in seedlings grown under N-limited conditions, but biomass accumulation was reduced after a 9-month growth period. To determine if the seedling growth reduction was temporary, we inoculated cedar seed with strain P2b-2R and grew seedlings for up to 13 months in a N-limited soil mix containing 0.7 mM of available N labeled as Ca(¹⁵NO₃)₂. P2b-2R developed a persistent endophytic population comprising 10²–10⁶?cfu g^?1 plant tissue inside pine roots, stems, and needles. At the end of the growth period, P2b-2R had reduced ¹⁵N foliar N abundance by 36 % and increased shoot biomass by 46 % compared to controls. Our results indicate that inoculated seedlings derived 36 % of foliar N from the atmosphere and suggest that BNF by P. polymyxa can significantly enhance growth of cedar in a N-limited soil if seedlings are grown for a sufficient amount of time. These findings support the hypothesis that endophytic diazotrophs may facilitate regeneration and growth of western red cedar at N-poor sites.     

Suitability of Immobilized Pseudomonas fluorescens SM1 Strain for Remediation of Phenols, Heavy Metals, and Pesticides from Water

Immobilized microbial cells for the biological treatment have the potential to degrade toxic chemicals faster than conventional wastewater treatment systems. In the present study, suitability of immobilized Pseudomonas fluorescens SM1 strain in calcium alginate beads for remediation of the major toxicants in Indian water bodies was tested by means of GC/HPLC and AAS techniques. Roughly 80% reduction in the concentration of phenols was observed by immobilized SM1 cells compared with 60% by the free cells. Also, in the case of the bioremediation of heavy metals, immobilized SM1 cells were found to be more efficient compared with the free cells. Suspension of P. fluorescens SM1 cells in the test model water for 24 h brought down the concentrations of Cu⁺⁺, Cd⁺⁺, Ni⁺⁺, and Pb⁺⁺ by more than 75% under free cell state and 7?C9% better efficiency under the immobilized conditions. However, Cr(VI) could show only 44% removal by the cell immobilized system, whereas a mere 35% reduction in the Cr(VI) levels was shown in the test model water by the free SM1 cells under the same conditions. Moreover, a model water containing 2,000 ppb of BHC, 1,248 ppb mancozeb, and 312 ppb 2,4-D passed through the cell immobilized column resulted in the decline in their concentrations up to 362 ppb, 750 ppb, and 126 ppb, respectively. Generally, AAS, HPLC, and GC analyses of treated test model waters with the free and immobilized SM1 cells exhibited high potential of immobilized SM1 in detoxification of test water. From the results, we conclude that immobilized cells of P. fluorescens SM1 strain were quite effective in bioremediation of major toxicants present in Indian water bodies, and we also recommend the use of immobilized bacterial cells rather than the free cells for the bioremediation/detoxification process.     

Photocatalytic Degradation of Textile Dye and Wastewater

In this study, the photocatalytic degradation of commercial azo dye (Remazol Red 133) in the presence of titanium dioxide (TiO₂) suspensions as photocatalyst was investigated. The effect of various operational parameters, such as pH of dye solution and catalyst concentration on the photocatalytic degradation process, was examined. The mineralization of dye was also evaluated by measuring the chemical oxygen demand of the dye solutions. The extent of photocatalytic degradation was found to increase with increasing TiO₂ concentration. For the Remazol Red dye solutions, a 120-min treatment resulted in 97.9% decolorization and 87.6% degradation at catalyst loading of 3 g/L. Experiments using real textile wastewater were also carried out. Textile wastewater degradation was enhanced at acidic conditions. The decolorization and degradation efficiencies for textile wastewater were 97.8% and 84.9% at pH 3.0, catalyst loading of 3 g/L, and treatment time of 120 min.     

Methylene Blue Degradation by Sphingomonas paucimobilis under Aerobic Conditions

The presence of synthetic dyes in industrial wastewaters may create serious environmental problems due to their mutagenicity and toxicity to aquatic life and humans. In this study, the decolourization and degradation of methylene blue (MB) by a Sphingomonas paucimobilis strain isolated from industrial wastewater was investigated under aerobic conditions. Decolourization extent of MB in medium was over 85?% when the bacterium was grown on a high concentration of the dye (1,000?mg/L) after a retention time of 5?days, while reduction in COD was 92.99?% suggesting mineralization of dyes as a result of microbial activities. The bacterium retained decolourizing activity over a wide range of pH (2?C10), with peak activity obtained at pH 9. Analysis of samples extracted from decolourized culture flasks at pH 9 using UV?Cvisible and Fourier transform infrared (FTIR) spectroscopy confirmed that the mechanism of colour removal was due to biodegradation rather than adsorption of dye on cells. Scanning and transmission electron microscopy revealed the secretion of exopolysaccharides (EPS) by S. paucimobilis cells on exposure to MB??a probable physiological defence mechanism to ensure controlled diffusion of dye molecules into cellular structures. Biokinetic coefficients, namely, growth yield, Y; specific biomass decay, K _d; maximum specific substrate rate, k; saturation constant for substrate, K _s; and maximum specific biomass growth rate, ?? _max, were determined by the Monod type kinetic equation. Results indicate that S. paucimobilis holds a promise as a good candidate for the biological treatment of industrial effluent containing high concentrations of synthetic dyes.     

Effect of Organic Ligands on Copper(II) Removal from Metal Plating Wastewater by Orange Peel-based Biosorbents

A neutrophilic, autotrophic bacterium that couples iron oxidation to nitrate reduction (iron-oxidizing bacteria [IOB]) under anoxic conditions was isolated from a working bioremediation site in Trail, British Columbia. The site was designed and developed primarily to treat high concentrations of Zn and As that originate from capped industrial landfill sites. The system consisted of two upflow biochemical reactor cells (BCR) followed by three vegetated wetland polishing cells with sub-surface flow and a holding pond. During a 5-year period (2003–2007), the system treated more than 19,100 m³ of contaminated water, removing and sequestering more than 10,700 kg of As, Zn and sulfate at average input water concentrations of: As, 58.6 mg?l^?1 (±39.9 mg?l^?1); Zn, 51.9 mg?l^?1 (±35.4 mg?l^?1) and SO₄ ^2?, 781.5 mg?l^?1 (±287.8 mg?l^?1). The bacterium was isolated in order to better understand the mechanisms underlying the consistent As removal that took place in the system. Analysis using Basic Local Alignment Search Tool (BLAST) database showed that the closest homologies are to Candidatus accumulibacterphosphatis (95 % homology), Dechloromonas aromatica (94 %), and Sideroxydans lithotrophicus ES-1 (92 %) Within the BCR cells, the IOB oxidized Fe²⁺ generated by iron-reducing bacteria (IRB); the source of the iron was most likely biosolids and coatings of iron oxide on locally available sand used in the matrix. We have provisionally designated the novel bacterium as TR1.     

Cinnamaldehyde Induces PCD-Like Death of Microcystis aeruginosa via Reactive Oxygen Species

In recent years, Microcystis bloom occurs frequently and causes a wide range of social, environmental, and economic problems. In this study, dose-dependent inhibitory effect of cinnamaldehyde on the growth of Microcystis aeruginosa was investigated. It was found that cinnamaldehyde with the concentration more than 0.6 mM showed algicide activity against M. aeruginosa. When M. aeruginosa was exposed to 0.6 mM cinnamaldehyde, considerable reactive oxygen species (ROS) were generated followed by lipid peroxidation and decrease in the content of both chlorophyll a and soluble protein. Although superoxide dismutase had made response to the stress caused by cinnamaldehyde, activity increasing after a time of lag could not prevent the lysis of M. aeruginosa cells. Interestingly, the addition of antioxidants glutathione and l-ascorbic acid (Vc) could prevent the lysis of M. aeruginosa cells. All the results suggested that cinnamaldehyde induced the death of M. aeruginosa cells via inducing ROS burst. Further understanding of the mechanism of cinnamaldehyde-induced M. aeruginosa cell death would contribute to the control of cyanobacteria pollution.     

A novel phosphorus biofertilization strategy using cattle manure treated with phytase–nanoclay complexes

The aim of this work was to evaluate the treatment of cattle manure with phytases stabilized in allophanic nanoclays as a potential novel phosphorus (P) biofertilization technology for crops grown in volcanic soils (Andisol). Furthermore, because the optimal pH for commercial phytase catalysis does not match the natural pH of manure, a complementary experiment was set up to evaluate the effect of manure inoculation with an alkaline phytase-producing bacterium. Finally, phytase-treated soil, manure, and soil–manure mixtures were evaluated for their P-supplying capacity to wheat plants grown under greenhouse conditions. Treating cattle manure with phytases stabilized in nanoclays resulted in a significant (P?≤?0.05) increase of inorganic P in soil extracts (NaOH-EDTA and Olsen). The use of phytase-treated cattle manure increased dry weights by 10 % and the P concentration by 39 % in wheat plants grown under greenhouse conditions, which is equivalent to a P fertilizer rate of about 150 kg of P per hectare. The inoculation of cattle manure with β-propeller phytase-producing bacteria led to an ～10 % increase in inorganic P in the manure extracts. However, applying inoculated manure to soil did not significantly increase wheat yield or P acquisition responses. Our results suggest that the novel approach of incubating cattle manure with phytases stabilized in nanoclay enhances the organic P cycling and P nutrition of plants grown in P-deficient soils.     

Response of red pepper to deficit irrigation and nitrogen fertigation

The main objective of this investigation was to evaluate the response of red pepper grown in a subhumid climate to different irrigation and nitrogen levels. Open-field trials were conducted in the Marmara Region, Turkey. Plants were subjected to three water levels [full irrigation (FI) = 100% crop evapotranspiration (ETc) and two deficit irrigations (DIs)= 66 and 33% ETc restoration] and four levels of N (0, 80, 160, and 240 kg N ha^?1) during the 2012, 2013, and 2014 growing seasons. A split-plot experimental design was used. The highest values of biomass and marketable yield (MY) were observed under FI. DI significantly increased the fruit soluble solids content. The biomass yield, MY, and fruit weight significantly improved with increasing nitrogen levels. The 240 kg N ha^?1 treatment under FI provided the maximum net income. Increasing N supply under DI conditions enhanced the water-use efficiency based on both biomass yield and MY. These results indicate that with respect to the yield, the net income, and the water productivity of red pepper, the FI with a nitrogen supply of 160–240 kg ha^?1 is recommended for drip irrigated and N-fertigated red pepper under subhumid climate conditions.     

Effect of sediment pH and oxidation-reduction potential on PCB mineralization

Microbial mineralization rates of a ¹⁴C-labelled PCB mixture were determined in PCB-contaminated Capitol Lake, LA, sediment under controlled pH and redox conditions. Mineralization rates were inferred from the activity of ¹⁴CO₂ evolved from the sediment suspensions. Sediment pH and redox potential significantly affected PCB mineralization. Mineralization rates were higher under moderately aerobic conditions (microaerophilic) ( + 250 mV) than under aerobic conditions ( + 500 mV) or anaerobic conditions (0 mV and ?200 mV). PCB mineralization rates in moderately aerobic sediment were 30 to 40 fold higher than those in anaerobic sediment. Sediment conditions in the oxidized surface layer would promote PCB mineralization. Sediment pH and redox potential were shown to be two sediment parameters which can be managed to enhance degradation of PCBs in contaminated sediment.     

Phytoextraction of arsenic contaminated soil by Chinese brake fern (Pteris vittata): Effect on soil microbiological activities

A greenhouse experiment was conducted to investigate the effect of phytoextraction by Chinese brake fern (Pteris vittata L.) on microbial activity and biomass in arsenic-contaminated soil (naturally occurring arsenic-contaminated soils of West Bengal, India). P. vittata was grown for two successive growing cycles (4 months each) with two phosphate sources (di-ammonium phosphate (DAP) and single superphosphate (SSP)). After phytoextraction by P. vittata, the rhizosphere soils were analyzed for microbial biomass C (MBC), C mineralization (C_min), dehydrogenase activity (DHA), phosphomonoesterase activities, and aryl sulphatase activity. All enzyme activities increased after two successive growing cycles of P. vittata as compared to one growing cycle and unplanted control. The arsenic (As) phytoextraction by this fern also increased the MBC by 34 %, C_min by 63 %, DHA by 38 %, acid phosphomonoesterase activity by 30 %, alkaline phosphomonoesterase activity by 6 %, and aryl-sulphatase activity by 33 % with two successive growing cycles over unplanted control. The di-ammonium phosphate was better as compared to single super phosphate for enhancing microbiological and biochemical parameters except phosphomonoesterase activities.     

Effects of carbon and phosphorus addition on microbial respiration,N<Subscript>2</Subscript>O emission,and gross nitrogen mineralization in a phosphorus-limited grassland soil

Soil microbes are frequently limited by carbon (C), but also have a high phosphorus (P) requirement. Little is known about the effect of P availability relative to the availability of C on soil microbial activity. In two separate experiments, we assessed the effect of P addition (20 mg P kg^?1 soil) with and without glucose addition (500 mg C kg^?1 soil) on gross nitrogen (N) mineralization (¹⁵N pool dilution method), microbial respiration, and nitrous oxide (N₂O) emission in a grassland soil. In the first experiment, soils were incubated for 13 days at 90% water holding capacity (WHC) with addition of NO₃^? (99 mg N kg^?1 soil) to support denitrification. Addition of C and P had no effect on gross N mineralization. Initially, N₂O emission significantly increased with glucose, but it decreased at later stages of the incubation, suggesting a shift from C to NO₃^? limitation of denitrifiers. P addition increased the N₂O/CO₂ ratio without glucose but decreased it with glucose addition. Furthermore, the ¹⁵N recovery was lowest with glucose and without P addition, suggesting a glucose by P interaction on the denitrifying community. In the second experiment, soils were incubated for 2 days at 75% WHC without N addition. Glucose addition increased soil ¹⁵N recovery, but had no effect on gross N mineralization. Possibly, glucose addition increased short-term microbial N immobilization, thereby reducing N-substrates for nitrification and denitrification under more aerobic conditions. Our results indicate that both C and P affect N transformations in this grassland soil.     

Effect of nitrogen fertilization on methane and carbon dioxide production potential in relation to labile carbon pools in tropical flooded rice soils in eastern India

In an incubation experiment with flooded rice soil fertilized with different N amounts and sampled at different rice stages, the methane (CH₄) and carbon dioxide (CO₂) production in relation to soil labile carbon (C) pools under two temperature (35°C and 45°C) and moisture (aerobic and submerged) regimes were investigated. The field treatments imposed in the wet season included unfertilized control and 40, 80 and 120 kg ha^?1 N fertilization. The production of CH₄ was significantly higher (27%) under submerged compared to aerobic conditions, whereas CO₂ production was significantly increased under aerobic by 21% compared to submerged conditions. The average labile C pools were significantly increased by 21% at the highest dose of N (120 kg ha^?1) compared to control and was found highest at rice panicle initiation stage. But the grain yield had significantly responded only up to 80 kg ha^?1 N, although soil labile C as well as gaseous C emission was noticed to be highest at 120 kg ha^?1 N. Hence, 80 kg N ha^?1 is a better option in the wet season at low land tropical flooded rice in eastern India for sustaining grain yield and minimizing potential emission of CO₂ and CH₄.