Elevated ozone increases nitrifying and denitrifying enzyme activities in the rhizosphere of wheat after 5 years of fumigation

Abstract

Backgrounds and aims Nitrogen (N) cycling in agroecosystems is likely to change under elevated O3 (eO3) as a result of changes in plant material inputs to soils. The objective of this study was to understand how eO3 could change key N cycling processes in soils. Methods Rhizosphere soils were collected from sensitive and tolerant wheat cultivars following a 5-year fumigation of O3at 60 ppb (eO3) and at ambient 40 ppb (aO3 )i n a free-air ozone enrichment platform. Soil nitrifying and denitrifying enzymeactivity(NEA andDEA), abundance of ammonia monooxygenase(amoA) and denitrifying bacteriagenes(nirSandnirK), and other related properties were quantified. Results Soil NEA, DEA, NH4 + and NO3 � were higher under eO3 than under aO3, whereas microbial biomass N (MBN) decreased under eO3 ,e specially with the O3sensitive cultivar. No clear eO3 effect was observed on the abundance of amoA and nirS and nirK. Grain yield and shoot to root ratios decreased as a result of eO3 in the O3-sensitive cultivar. Conclusions eO3 stimulates soil N availability, but suppresses grain yields after 5 years, especially for the O3sensitive cultivar. Adaptivestrategiesshould focus more on cultivar selection to retain N in soil and to assure the resilience of crop production in the future.