Mitigating global warming potentials of methane and nitrous oxide gases from rice paddies under different irrigation regimes.

A field experiment was conducted in Bangladesh Agricultural University Farm to investigate the mitigating effects of soil amendments such as calcium carbide, calcium silicate, phosphogypsum, and biochar with urea fertilizer on global warming potentials (GWPs) of methane (CH4) and nitrous oxide (N2O) gases during rice cultivation under continuous and intermittent irrigations. Among the amendments phosphogypsum and silicate fertilizer, being potential source of electron acceptors, decreased maximum level of seasonal CH4 flux by 25-27 % and 32-38 % in continuous and intermittent irrigations, respectively. Biochar and calcium carbide amendments, acting as nitrification inhibitors, decreased N2O emissions by 36-40 % and 26-30 % under continuous and intermittent irrigations, respectively. The total GWP of CH4 and N2O gases were decreased by 7-27 % and 6-34 % with calcium carbide, phosphogypsum, and silicate fertilizer amendments under continuous and intermittent irrigations, respectively. However, biochar amendments increased overall GWP of CH4 and N2O gases.

Comparative Evaluation of Chlorella vulgaris and Anabaena variabilis for Phycoremediation of Polluted River Water: Spotlighting Heavy Metals Detoxification.

This study investigated the phycoremediation abilities of Chlorella vulgaris (microalga) and Anabaena variabilis (cyanobacterium) for the detoxification of polluted river water. Lab-scale phycoremediation experiments were conducted for 20 days at 30 °C using the microalgal and cyanobacterial strains and water samples collected from the Dhaleswari river in Bangladesh. The physicochemical properties such as electrical conductivity (EC), total dissolved solids (TDS), biological oxygen demand (BOD), hardness ions, and heavy metals of the collected water samples indicated that the river water is highly polluted. The results of the phycoremediation experiments demonstrated that both microalgal and cyanobacterial species significantly reduced the pollutant load and heavy metal concentrations of the river water. The pH of the river water was significantly raised from 6.97 to 8.07 and 8.28 by C. vulgaris and A. variabilis, respectively. A. variabilis demonstrated higher efficacy than C. vulgaris in reducing the EC, TDS, and BOD of the polluted river water and was more effective at reducing the pollutant load of SO42- and Zn. In regard to hardness ions and heavy metal detoxification, C. vulgaris performed better at removing Ca2+, Mg2+, Cr, and Mn. These findings indicate that both microalgae and cyanobacteria have great potential to remove various pollutants, especially heavy metals, from the polluted river water as part of a low-cost, easily controllable, environmentally friendly remediation strategy. Nevertheless, the composition of polluted water should be assessed prior to the designing of microalgae- or cyanobacteria-based remediation technology, since the pollutant removal efficiency is found to be species dependent.

Effects of oyster shell on soil chemical and biological properties and cabbage productivity as a liming materials.

Oyster shell, a byproduct of shellfish-farming in Korea and containing a high amount of CaCO(3), has a high potential to be used as a liming material in agriculture. However, the agricultural utilization of oyster shell is limited due to its high concentration NaCl. The oyster-shell meal collected had a low concentration of water soluble NaCl (mean 2.7 g kg(-1)), which might be a result of stacking the material for 6 months in the open field. It has a very similar liming potential with calcium carbonate, with 3.4 and 3.8 Mg ha(-1) for silt loam (SiL, pH 6.2) and sandy loam (SL, pH 5.8) to bring the soil pH to 6.5, respectively. To determine the effect of crushed oyster-shell meal on improving soil chemical and biological properties and crop plant productivity, oyster-shell meal was applied at rates of 0, 4, 8, 12, and 16 Mg ha(-1) before transplanting Chinese cabbage (Brassica campestris L.) in the two soils mentioned above. Soil pH was significantly increased to 6.9 and 7.4 by 16 Mg ha(-1) shell meal application (4 times higher level than the recommendation) in SiL and SL, respectively, at harvesting stage. The effect of liming was found higher in SL compared to SiL soil, probably due to the different buffering capacity of the two soils. The concentration of NaCl and EC value of soils were found slightly increased with shell meal applications, but no salt damage was observed. Oyster-shell meal application increased soil organic matter, available P, and exchangeable cations concentrations. The improved soil pH and nutrient status significantly increased the microbial biomass C and N concentrations and stimulated soil enzyme activities. With the exception of acid phosphomonoesterase (PMEase) activity, which decreased with increasing soil pH in SL but slightly increased in SiL, the activities of urease and alkali PMEase increased markedly with increasing soil pH by shell meal application. The improved soil chemical and biological properties resulted in increased crop productivity. The highest yield in Chinese cabbage was achieved following the application of 8 Mg ha(-1) oyster-shell meal. Conclusively, crushed oyster shell could be used as an alternative liming material to restore the soil chemical and microbial properties in upland soil and to increase crop productivity.

Mitigating yield-scaled greenhouse gas emissions through combined application of soil amendments: A comparative study between temperate and subtropical rice paddy soils.

Effects of different soil amendments were investigated on methane (CH4) and nitrous oxide (N2O) emissions, global warming potential (GWP) and yield scaled GWPs in paddy soils of Republic of Korea, Japan and Bangladesh. The experimental treatments were NPK only, NPK+fly ash, NPK+silicate slag, NPK+phosphogypsum(PG), NPK+blast furnace slag (BFS), NPK+revolving furnace slag (RFS), NPK+silicate slag (50%)+RFS (50%), NPK+biochar, NPK+biochar+Azolla-cyanobacteria, NPK+silicate slag+Azolla-cyanobacteria, NPK+phosphogypsum (PG)+Azolla-cyanobacteria. The maximum decrease in cumulative seasonal CH4 emissions was recorded 29.7% and 32.6% with Azolla-cyanobacteria plus phospho-gypsum amendments in paddy soils of Japan and Bangladesh respectively, followed by 22.4% and 26.8% reduction with silicate slag plus Azolla-cyanobacteria application. Biochar amendments in paddy soils of Japan and Bangladesh decreased seasonal cumulative N2O emissions by 31.8% and 20.0% respectively, followed by 26.3% and 25.0% reduction with biochar plus Azolla-cyanobacteria amendments. Although seasonal cumulative CH4 emissions were significantly increased by 9.5-14.0% with biochar amendments, however, global warming potentials were decreased by 8.0-12.0% with cyanobacterial inoculation plus biochar amendments. The maximum decrease in GWP was calculated 22.0-30.0% with Azolla-cyanobacteria plus silicate slag amendments. The evolution of greenhouse gases per unit grain yield (yield scaled GWP) was highest in the NPK treatment, which was decreased by 43-50% from the silicate slag and phosphogypsum amendments along with Azolla-cyanobacteria inoculated rice planted soils. Conclusively, it is recommended to incorporate Azolla-cyanobacteria with inorganic and organic amendments for reducing GWP and yield scaled GWP from the rice planted paddy soils of temperate and subtropical countries.

Effect of industrial by-products containing electron acceptors on mitigating methane emission during rice cultivation.

Three industrial by-products (fly ash, phosphogypsum and blast furnace slag), were evaluated for their potential re-use as soil amendments to reduce methane (CH(4)) emission resulting from rice cultivation. In laboratory incubations, CH(4) production rates from anoxic soil slurries were significantly reduced at amendment levels of 0.5%, 1%, 2% and 5% (wt wt(-1)), while observed CO(2) production rates were enhanced. The level of suppression in methane production was the highest for phosphogypsum, followed by blast slag and then fly ash. In the greenhouse experiment, CH(4) emission rates from the rice planted potted soils significantly decreased with the increasing levels (2-20 Mg ha(-1)) of the selected amendments applied, while rice yield simultaneously increased compared to the control treatment. At 10 Mg ha(-1) application level of the amendments, total seasonal CH(4) emissions were reduced by 20%, 27% and 25%, while rice grain yields were increased by 17%, 15% and 23% over the control with fly ash, phosphogypsum, and blast slag amendments, respectively. The suppression of CH(4) production rates as well as total seasonal CH(4) flux could be due to the increased concentrations of active iron, free iron, manganese oxides, and sulfate in the amended soil, which acted as electron acceptors and controlled methanogens' activity by limiting substrates availability. Among the amendments, blast furnace slag and fly ash contributed mainly to improve the soil nutrients balance and increased the soil pH level towards neutral point, but soil acidity was developed with phosphogypsum application. Conclusively, blast slag among the selected amendments would be a suitable soil amendment for reducing CH(4) emissions as well as sustaining rice productivity.