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Changes of nitrogen (N) cycle caused by N fertilization and precipitation regimes have affected the key ecosystem structure and functions in temperate steppe, which may modify the structure of soil microbial communities involved in N transformation. This paper was designated to examine the response of soil ammonia oxidizers and denitrifiers to the N fertilization and precipitation regimes in a semi-arid steppe where N and water contents are major limiting factors of the grassland productivity.

This study was based on a long-term N fertilization and precipitation regimes experiment in Inner Mongolia (116A degrees 17’ 20aEuro(3) E, 42A degrees 2’ 29aEuro(3) N). The treatments including CK (control), R (reduced precipitation), W (30% increase in precipitation), N (10 g N m(-2) y(-1)), RN (reduced precipitation and 10 g N m(-2) y(-1)), and WN (30% increase in precipitation and 10 g N m(-2) y(-1)). Soil basic chemical properties and microbial activities were analyzed. Molecular methods were applied to determine the abundance, structure and diversity of ammonia oxidizers and denitrifiers. Statistical analysis detected the main and interactive effect of treatments on soil microbial communities and revealed the relationship between soil microbial community structures and environmental factors.

N fertilization significantly increased ammonia-oxidizing bacteria (AOB) abundance. Ammonia-oxidizing archaea (AOA) community structure was markedly changed in N fertilizer treatment and strongly affected by soil pH, while soil nitrate and water content correlated with AOB community structure. Soil nitrate was the key factor influencing nirK gene community structure, while soil pH and water content explained much of the variations of nosZ gene community. AOB-amoA and nosZ gene community diversities were influenced by precipitation regimes and interaction of N fertilization and precipitation regimes, respectively.

N fertilization and precipitation regimes had significant influences on the changes of soil properties and microbial functional communities. Soil nitrification was mainly driven by AOB in the semi-arid grassland. Changes of substrate content and soil pH were the key factors in shifting functional microbial communities. The non-synergistic effects of N fertilization and precipitation regimes on the microbial functional groups indicated that the negative effect of lower pH induced by N fertilization would be alleviated by precipitation regimes, which should be well considered in grassland restoration.