Study site and experimental design
The experiment was conducted at the Erguna Forest-Steppe Ecotone Research Station (119° 22’ 56.4” E, 50°10’46.1” N) in Erguna of Inner Mongolia, China. Long-term mean annual precipitation at the study site is 358 mm, and mean annual temperature is -2.5°C, with mean monthly temperatures ranging from -28.0°C in January to 19.2°C in July. Soil is chernozem according to the US soil taxonomy classification.
The study site was located in a temperate semiarid grassland, which was used for livestock grazing for more than 50 years before being fenced in early 2010. Thereafter livestock grazers have been excluded from the study site, and mowing was conducted once annually in August to harvest hay until 2012. At the beginning of the experiment in 2013, the grassland was dominated by four perennial grasses, Leymus chinensis , Carex duriuscula , Cleistogenes squarrosa, Stipa baicalensis , and a perennial forb, Bupleurum scorzonerifolium .
In early August 2013, six blocks were established using a randomized block design. Four 6 m × 6 m plots within each block were randomly assigned to the following treatments: control (C, no nitrogen addition or mowing), nitrogen addition (N), mowing (M), and nitrogen addition plus mowing (NM). Both the blocks and plots were separated by a 1-m-wide buffer zone. Plots receiving the mowing treatments were mowed at 7 cm aboveground in mid-August (after plant survey) from 2013 to 2018; mowed plant materials were removed from the plots. In early May from 2014 to 2018, each nitrogen addition plot received nitrogen fertilizer (in the form of urea) at the rate of 10 g nitrogen m-2yr-1. The plot was sprinkled with 10 L tap water with urea dissolved in it; each plot without nitrogen addition was sprinkled with 10 L tap water. The added water each year is equivalent to 0.28 mm precipitation. The amount of nitrogen addition is greater than the current atmospheric nitrogen deposition (about 1.42 g m-2 yr-1) in the study area, but is comparable to the projected nitrogen deposition rate in northern China in the near future (He et al. 2007; Zhang et al. 2011).