Effect Of Slow-and Controlled-Release Nitrogen Fertilizers On Direct And Indirect Emissions Of Nitrous Oxide From Cropland

Climate change and global warming continue to be subject to considerable scientific debate and public concern. Nitrous oxide (N2O) is an important trace gas that causes global warming and stratospheric ozone depletion. Nitrogen fertilization is considered as a primary source of N2O emissions from agricultural soils. The consumption of synthetic nitrogen fertilizer in agriculture has increased over the past several decades and will continue to increase to meet the food and fibre demands of the growing global population, which will no doubt result in the release of additional N2O into the atmosphere. A wise use of synthetic fertilizer N is important to mitigate N2O emissions.Slow/controlled-release N fertilizers, proposed as an alternative to conventional N fertilizers, have become a priority topic in the Chinese middle- and long-term science and technology development plan guidelines (2006-2020). To compare and assess the direct and indirect N2O emissions from cropland with different slow- and controlled-release nitrogen fertilizers, and thus to provide critical information for N2O mitigation in China are necessary.The objectives of this research were:1) to examine the performance of different typical slow/controlled-release nitrogen fertilizers compared to conventional urea in relation to crop N uptake and direct and indirect N2O emission under winter wheat and maize rotation system, and hence 2) to assess the technical and economical feasibility of mitigation of N2O for slow/controlled-release N fertilizers to be popularized in future.To approach the above objectives, incubaion experiments and field experiments were employed in this study. Outdoor experiments with physically altered, chemically altered and biochemically inhibited nitrogen fertilizers which represent three typical varieties of slow- and controlled-release N fertilizers were conducted during the 2006-07 and 2007-08 winter wheat growing seasons and during the 2007 and 2008 maize growing seasons to evaluate the potential of these formulations to mitigate N2O emissions. The physically altered nitrogen includes Ca-Mg-P-coated urea (CMCU), polymer-coated urea (PCU) and sulfur-coated urea (SCU). Urea formaldehyde (UF) was used as the chemically altered nitrogen. Urea with dicyandiamide and hydroquinone (BIHD) and urea with enhanced ammonium nutrition (BIEA) were used as the biochemically inhibited nitrogen. Commercial urea (U) was applied as a comparison. No fertilizer was employed as a control (CK). N2O fluxes were measured with the static chamber method. Ammonia volatilization from soil was determined by the venting method. In terms of conventional rates in this region, the total amounts of nitrogen applied in the wheat and maize growing season were 250 and 300 kgN/hm2, respectively. Some treatments with reduction nitrogen by 30% were also employed. All N fertilizers were broadcast-applied. In order to find out the main microbial processes which contribute to N2O emission, indoor incubation experiments to isolate autotrophic nitrifiers and heterotrophic nitrifiers/aerobic denitrifiers from agricultural soil were conducted.Results of this study are presented as follows:1) N2O direct emissions during the wheat growing season mainly depended on soil temperature rather than on soil WFPS. On the contrary, N2O direct emissions were greatly affected by rainfall and hence soil moisture over the maize growing season. The cumulative N2O emissions from different treatments in the winter wheat growing season was far lower (averaged only about 15%) than that in the maize growing season, and the N2O direct emission factors followed the same pattern. The optimum WFPS for high N2O emission was at 50%～65% in the field condition.The different N release characteristic of each fertilizer after application resulted in different seasonal direct emissions of N2O among the several fertilizer types. Direct emissions of N2O from the coated and urea fertilizers were easily affected by the heavy rainfall event followed by basal fertilization. However, the rainfall had no obvious effect on direct emissions of N2O for the BIHD treatment.In comparison with the U, the UF and BIEA treatments reduced direct emissions of nitrous oxide by 15%～62% for the wheat and maize growing seasons, and the BIHD treatment reduced direct emissions of N2O by 33%～63% for the maize growing season. However, the treatments with the coated nitrogen generally enhanced direct emissions of nitrous oxide by 8%～99% in comparison to U treatment when rainfall followed application, while reduced direct emissions of nitrous oxide by 4%～39% when an extended drought was experienced after application. We conclude that the application of chemically altered or biochemically inhibited nitrogen fertilizers has great potential to mitigate direct emissions of nitrous oxide, but the use of physically altered nitrogen fertilizers would enhance direct emissions of nitrous oxide under wet climate condition.2)During the winter wheat growing season, only PCU treatment reduced ammonia volatilization by 20%, while the other treatments (UF, CMCU, SCU and BIEA) enhanced ammonia volatilization by 45%～282% when compared to U treatment。However, ammonia volatilization for CMCU, BIHD, SCU, UF and PCU treatments were lower than that for urea treatment over the maize growing seasons, especially UF and PCU treatments had significant reduction effect on ammonia volatilization, averaged reduction by up to 80%. In addition, ammonia volatilization for BIEA treatment was always higher compared to urea treatment.3) There was obvious improvement in yield production and higher NUE with use of PCU and SCU in both winter wheat and maize growing seasons as compared to urea. UF treatment reduced yield of wheat but increased the maize production. To further calculate the indirect emissions of nitrous oxide suggest that PCU and SCU may be effective in reducing total N2O emission and increasing the economic benefit in wheat planting. However, the net benefit was almost negative value whatever application of any fertilizers in maize growing season. Among all treatments, UF showed priorities in reducing total N2O emissions and increasing economic benefit in maize planting.4) It is essential for mitigation N2O emission to investigate the nitrifying and denitrifying mechanisms of N2O production. In this study, two strains HX2 and XM1 as Pseudomonas sp. which could both heterotrophic nitrify and aerobic denitrify were isolated. They produced more N2O in 30% soil WFPS than that in 60% soil WFPS, especially XM1 almost produced no N2O in 60% soil WFPS. Compared to autotrophic nitrification, heterotrophic nitrification/aerobic denitrification produced fewer N2O at 50%～60% soil WFPS, and it played more important role in N2O production with soil WFPS decreasing. The results indicated that the application of low C/N fertilizer under low soil WFPS condition could depress N2O production and hence reduce N2O emission.