Laboratory study of the effects of nitrification inhibitors on the abundance of ammonia-oxidizing bacteria and archaea in a Hap-Ustic Luvisol

Nitrification inhibitors (NIs) could slow down nitrification by suppressing ammonia oxidizers. Responses of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) to NIs were studied in grassland soil and acidic agricultural soil, but we know little about effects of NIs and NIs per se on AOA and AOB abundance in neutral agricultural soil. We aimed to investigate the effects of NIs, dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP), on the dynamics of AOA and AOB populations in a Hap-Ustic Luvisol. Laboratory studies were installed to observe the effects of DCD and DMPP on the abundance of AOA and AOB. We determined the dynamics of AOA and AOB populations during 112 days by using real-time PCR targeting amoA genes. Soil mineral nitrogen contents and potential ammonia oxidation were measured. AOB growth was stimulated by ammonium addition and reached peak value at day 14 in treatment N. When ammonium was applied with DCD or DMPP, AOB amoA copies decreased significantly. In contrast to AOB, AOA showed no response to the addition of ammonium and the two NIs during 91 days, even when DCD and DMPP applied were double dose. Nitrification inhibition rates of DCD and DMPP were highest at day 14. Although potential ammonia oxidation was suppreseed at day 14, AOA populations were still not affected by DCD or DMPP per se. Our results suggest that AOA were not stimulated by large amounts of inorganic ammonium exogenous and were tolerant to DCD and DMPP in a neutral agricultural soil.    Key words: 3,4-dimethylpyrazole phosphate (DMPP), dicyandiamide (DCD), mineral nitrogen, real-time PCR.