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).