Overdose fertilization induced ammonia-oxidizing archaea producing nitrous oxide in intensive vegetable fields.

Little is known about the effects of nitrogen (N) fertilization rates on ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) and their differential contribution to nitrous oxide (N2O) production, particularly in greenhouse based high N input vegetable soils. Six N treatments (N1, N2, N3, N4, N5 and N6 representing 0, 293, 587, 880, 1173 and 1760 kg N ha-1 yr-1, respectively) were continuously managed for three years in a typically intensified vegetable field in China. The aerobic incubation experiment involving these field-treated soils was designed to evaluate the relative contributions of AOA and AOB to N2O production by using acetylene or 1-octyne as inhibitors. The results showed that the soil pH and net nitrification rate gradually declined with increasing the fertilizer N application rates. The AOA were responsible for 44-71% of the N2O production with negligible N2O from AOB in urea unamended control soils. With urea amendment, the AOA were responsible for 48-53% of the N2O production in the excessively fertilized soils, namely the N5-N6 soils, while the AOB were responsible for 42-55% in the conventionally fertilized soils, namely the N1-N4 soils. Results indicated that overdose fertilization induced higher AOA-dependent N2O production than AOB, whereas urea supply led to higher AOB-dependent N2O production than AOA in conventionally fertilized soils. Additionally, a positive relationship existed between N2O production and NO2- accumulation during the incubation. Further mechanisms for NO2--dependent N2O production in intensive vegetable soils therefore deserve urgent attention.