Nitrogen (N) is a key limiting resource for aboveground net primary productivity (ANPP) in diverse terrestrial ecosystems. The relative roles of the rate and frequency (additions yr-1) of N application in stimulating ANPP at both the community- and species-levels are largely unknown. By independently manipulating the rate and frequency of N input, with nine rates (from 0 to 50 g N m-2 year-1) crossed with two frequencies (twice year-1 or monthly) in a temperate steppe of northern China across 2008-2013, we found that N addition increased community ANPP, and had positive, negative, or neutral effects for individual species. There were similar ANPP responses at the community- or species-level when a particular annual amount of N was added either twice year-1 or monthly. The community ANPP was less sensitive to soil ammonium at lower frequency of N addition. ANPP responses to N addition were positively correlated with annual precipitation. Our results suggest that, over a five-year period, there will be similar ANPP responses to a given annual N input that occurs either frequently in small amounts, as from N deposition, or that occur infrequently in larger amounts, as from application of N fertilizers.
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We thank Xiaoliang Wang, Jianjun Chen, Yingjie Dong, Minglu Rong and Baorong Cui for assistance with field samplings, and the IMGERS for provided facilities and meteorological data. We also thank Yaobin Song for data analysis help. This work was supported by the National Natural Science Foundation of China (31430016, 31170433, 41273094 and 31300384) and China Postdoctoral Science Foundation funded project (2014M560134 and 2015T80153).