Restricted nitrous oxide emissions by ammonia oxidizers in two agricultural soils following excessive urea fertilization

Purpose Nitrogen (N) fertilizer placement in bands is a widely accepted agricultural practice to increase N use efficiency. An excessive ammonium concentration in a fertilizer band can increase osmotic stress on ammonia oxidizers and potentially affect nitrification and resultant nitrous oxide (N2O) emissions, which is of great significance for soil function and climate change. The objectivity of this study was to identify the effects of excessive ammonium concentration on N2O emissions and ammonia oxidizers in two agricultural soils. Materials and methods In this study, we established a 56-day soil microcosm receiving a series of high concentrations of urea at 600, 900, and 1200 mg N kg(-1) (termed as N600, N900, and N1200, respectively), which simulated high ammonium levels in the center or proximity of a fertilizer band in two types of agricultural soils (fluvo-aquic soil and anthrosol). The mineral N concentrations, net nitrification rate, and N2O emissions were measured during the incubation. In addition, the abundances of bacterial and archaeal amoA were determined by using real-time quantitative PCR. Results and discussion Urea fertilization simultaneously increased the net nitrification rate and N2O emission at the early stage of incubation in both soils, suggesting N2O production was mainly from ammonia oxidation. Ammonia oxidizing bacteria (AOB) but not archaea (AOA) abundance was stimulated following urea fertilization and was positively correlated with N2O emission, indicating the dominant role of AOB in ammonia oxidation and N2O production in fertilized soils. The cumulative N2O emission was significantly higher in N1200 and N900 than N600 in both soils, but no further increase was observed in N1200 in the anthrosol. This implies restricted N2O production of ammonia oxidizers at excessive ammonium concentrations in the anthrosol. In the two soils treated with no N addition, the abundances of AOA amoA increased along the incubation time. Conclusions The present study collectively suggested that excessive urea-N addition was more effective in inhibiting N2O emission in the anthrosol than in the fluvo-aquci soil. AOB rather than AOA dominated the soil nitrification and N2O emissions under high N addition in both soils. The band fertilization regime may reduce the loss of N fertilizer from nitrification without necessarily increasing N2O emissions.