Abstract: An O3-FACE(free-air O3 concentration enrichment) system was set up in a wheat field to investigate effects of elevated surface O3 concentration,which was 70 nmol·mol-1 on average,on wheat N-uptake,soil total N and mineral N,urease activity,and populations of ammonia-oxidizing bacteria(AOB) and denitrifying bacteria(DNB) at various wheat growth stages,as well as soil nitrification and denitrification potentials at the wheat harvesting stage.Compared with the ambient O3concentration,which was 45 nmol·mol-1,elevated O3 concentration increased individual wheat N-uptake,while decreased soil total N,NH4+-N and NO-3-N contents.At the wheat harvesting stage,soil total N and NH4+-N contents decreased significantly(P<0.05) by 9% and 71%,respectively,and so did soil NO3--N content by 36%.During the growth periods of wheat,elevated O3 concentration tended to enhance soil urease activity,which was significantly(P<0.05) higher than that in the ambient soil at the jointing,heading and filling stages of wheat,and to increase the population of soil AOB and DNB,as well,which were significantly(P<0.05) higher than those in the ambient soil at the wheat harvesting stage.At that time,although in O3-fumigated soils the specific nitrification potential of AOB tended to decrease,and the soil total nitrification potential to increase,while the specific denitrification potential of DNB decreased significantly(P<0.05) by 96% and the soil total denitrification potential remained unchanged,which suggests that elevated surface O3 concentration promoted wheat N-uptake,inhibited soil N storage,and then accelerated soil N transformation,which was represented by enhanced urease activity,and bigger population sizes of soil AOB and DNB with low metabolic activity.

Key words: elevated O3 concentration, ammonia-oxidizing bacteria, denitrifying bacteria, urease activity, wheat, N, uptake