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).