Aboveground and belowground biomass
Our results found that nitrogen (N) addition directly increased aboveground biomass in semi-arid steppe ecosystem, and on average N addition significantly increased the aboveground and belowground biomass (AGB and BGB) in this system, the results are consistent with most studies using N addition experiments (Bustamante et al., 2012; Kim & Henry, 2013; T. Zhang, Guo, Gao, Guo, & Sun, 2015; Y. Zhang et al., 2016). Because N is a major limiting factor for regulating the plant growth and biodiversity of terrestrial ecosystems, it often determines the net primary productivity of grassland ecosystems (Baer, Blair, Collins, & Knapp, 2004; Bobbink et al., 2010; Hutchison & Henry, 2010; Lebauer & Treseder, 2008). In addition, our results demonstrated that comparing with N addition, either the level of N added or the frequency of N addition, mowing had little effects on AGB and BGB. This was likely due to the fact that N addition significantly changed soil nutrient availability, especially the amount of N available in the soil, which would have greatly improved the growth of plants. Moreover, mowing significantly decreased vegetation cover, which led to increase in the surface temperature and decrease in soil water content. As a consequence, the growth of plants might be restricted in mowed plots even though N was added (Bardgett, Wardle, & Yeates, 1998; Tix, Hebberger, & Charvat, 2006).