A Novel Function of Controlled-Release Nitrogen Fertilizers

Nitrogen is the most important essential nutrient that plays a major role in achieving maximum crop yield in agriculture. Therefore, nitrogen fertilizers such as ammonium sulfate and urea have been extensively used in modern agriculture. These fertilizers are generally oxidized to nitrate via nitrite by nitrifying microorganisms in the agricultural field (4, 10, 11). The serious environmental problems associated with the use of nitrogen fertilizers are nitrate contamination of ground and surface water due to nitrate leaching and loss from the agricultural field (15). For these reasons, controlled-release nitrogen fertilizers have been developed to enable a slow release of nitrogen into the soil during the crop-growing season. The use of controlled-release nitrogen fertilizers is mainly based on the principle of nitrogen utilization efficiency for crop production. However, in the current issue, Ikeda et al. report that the controlled-release nitrogen fertilizer urea-formaldehyde unexpectedly modifies the microbial community structure in the phytosphere of crops (8). Controlled-release nitrogen fertilizers consists of three major types: biologically or chemically degradable organic nitrogen, nitrogen coated with a physical barrier, and lowly soluble nitrogen. Controlled-release nitrogen fertilizers have recently received increased attention because of their potential N 2 O emission-reducing properties (1). Nitrous oxide, which is produced by nitrification and denitrification (4, 10, 11), is an effective earth-warming gas with a 298-fold higher efficiency than carbon dioxide. Nitrogen fertilizers such as urea, ammonium sulfate, and ammonium nitrate are sources of N 2 O. Recent studies have revealed that slow-release nitrogen fertilizers reduce N 2 O emissions from the agricultural field (1, 24). The urea-formaldehyde fertilizer, which is a representative controlled-release nitrogen fertilizer, is synthesized from urea and formaldehyde in the presence of a catalyst at various temperature conditions (2). The urea-formaldehyde fertilizer is degraded by soil microorganisms, resulting in the gradual release of urea into the soil, which is then further metabolized by soil microorganisms into plant available nitrogen forms such as ammo-nium and nitrate (12, 13). Urea-formaldehyde fertilizers contain short-chain and long-chain methylene urea polymers. The nitrogen release rates of the fertilizer into the soil are dependent on the content ratio of the long-chain polymers. Previous studies on the influence of microorganisms on urea-formaldehyde fertilizers in soils have examined the processes of urea-formaldehyde degradation and urea release rate to evaluate its nutrient efficiency for crop production (2, 12). PCR-denaturing gradient gel electrophoresis (DGGE) analysis has detected differences in bacterial community structure between urea-treated and methylene urea-treated …