High frequency of extreme precipitation increases Stipa grandis biomass by altering plant and microbial nitrogen acquisition

Climate changes are altering precipitation to more frequent extreme precipitation events that have strong impacts on the structure and functions of grassland ecosystems. We conducted a rain simulation experiment combined with in situ 15 N labeling of three nitrogen (N) forms (NO3−, NH4+, glycine) to investigate how the frequency of extreme precipitation influences plant productivity and N acquisition (N uptake, 15 N recovery, and preference for N form) by the dominant species Stipa grandis and soil microorganisms in the temperate steppe. Extreme precipitation had three frequencies (1, 3, and 6 events for low, medium, and high frequency) with the same total rain amount in 1-month cycle. The low frequency reduced the S. grandis biomass by 39%, whereas the high ones raised the S. grandis biomass by 43% and increased plant and microbial N uptake up to 6.3-fold and 5.1-fold of those under ambient precipitation, respectively. Plants preferred NO3− and microorganisms preferred NH4+ under low frequency, but they showed similar preference for three N forms, leading to chemical niche overlap for NO3−, NH4+, and glycine under high frequency. This indicated that high precipitation frequency effectively reduced the proportion of each N form, which plants and microorganisms competed for as the available N pool increased. Overall, the increase of precipitation frequency (decreasing intensity) shifted the extreme (low frequency but high intensity) to optimal conditions for plant productivity and N acquisition by plants and microorganisms in the temperate steppe. These findings provide new insights for understanding the diverse responses of ecosystem functions to extreme climate events.