ABSTRACT
A field experiment was conducted in maize-cultivated sandy loam soil in the old flooded area of the Yellow River to evaluate the responses of N2O emissions to application of different type of controlled-release urea. An inorganic N fertilizer was applied at 270 kg·hm-2 during the maize season. Urea was applied alone and in combination with sulfur-coated urea (SCU) or polyurethane-coated urea (PCU) at N ratios of 30%:70%, 50%:50%, and 70%:30%, respectively. Cumulative N2O emission under urea treatment alone (CN) was 1.78 kg·hm-2 with a N2O emission factor of 0.38%. In comparison to CN, 70% urea+30% SCU, 50% urea+50% SCU, and 30% urea+70% SCU treatments reduced N2O emission by 1.12%, 22.5%, and 11.2%, respectively. In contrast, application of urea in combination with PCU (with the proportion varied from 30%-70%) increased N2O emission by 0.02-0.41 kg·hm-2 compared with the CN, while 30% urea+70% PCU treatment showed a 23.0% increase. Regression analysis showed that N2O flux was significantly (P<0.01) correlated with soil temperature at 10 cm depth and concentrations of soil NH4+-N and NO3--N in all the treatments, but not with soil moisture or dissolved organic carbon concentration. Compared with the CN, the 50% urea+50% SCU and 50% urea+50% PCU treatments slightly, but not significantly, increased the maize yield, whereas the 30% urea+70% SCU treatment showed a reduction effect. Overall, the mitigation effect of controlled-release urea on N2O emission may primarily depend on its coating material and application rate.
ABSTRACT
Coastal wetlands contribute about 75% to the global oceanic CH4 emissions, thus play a vital role in global C cycles. In this paper, we provided a perspective on researches on metabolic, phylogenetic, and ecological diversity of the methanogenic archaea and the regulating environmental factors in coastal wetlands. Because of the presence of more favorable electron acceptors such as sulfate, methanogenesis via CO2 reduction and acetate fermentation are limited by availability of substrates, and hydrogenotrophic and acetotrophic methanogens generally express low relative abundance. In contrast, "non-competitive" substrates such as methanol and methylated compounds have been shown to contribute substantially to methane formation in coastal wetlands, and the facultative methanogens are predominant in those environments. Salinity regulates vegetation zonation and is related to SO42- concentration, by regulating types of methanogenic substrates and contents of compe-titive electron acceptors, indirectly affects the structure and function of methanogens. Major uncertainties in the current studies include the following: methanogen community structure, the key environmental factors regulating methane pathway, and their effects on methane emissions in coastal wetlands.
