Study site & Experimental design
The study site is located at the field base of Erguna Forest-Steppe
Ecotone Research Station, Inner Mongolia, China (N50°10′46.1″,
E119°22′56.4″). This site has a temperate continental steppe climate.
The mean annual precipitation is 361 mm and the mean annual temperature
is -2.45 ℃ (1972-2018). More than 85% of precipitation falls during
growing season from May to September. The main vegetation type is meadow
steppe, and the dominant species are Leymus chinensis ,Stipa baicalensis , Cleistogenes squarrosa ,Thermopsis lanceolata , Cymbaria dahurica , and Carex
duriuscula (Yang et al., 2019). The site is topographically flat and
had been fenced since 2013 to avoid livestock grazing.
To explore the response of ecological network to global climate changes,
we (1) used rain shelters to simulate the drought condition, (2) added N
fertilizer to simulate N deposition and (3) used black shade mesh covers
to delay snowmelt. The experiment was a complete factorial design of
Drought (drought and ambient)× N
addition (N addition and control) × snowmelt timing (with and without
delaying manipulation) following randomized block arrangement with six
replicates for each treatment. Each plot was 6 m × 6 m in size, and
every two adjacent plots were separated by 2 m wide buffer. The drought
was simulated by excluding 66% natural rainfall with passive rainout
shelters (light transmittance >95%) from May to August,
and watertight aluminum plates were buried around the plots to prevent
the external subsurface water permeation into the drought plots. N
addition was implemented in mid-May by adding
NH4NO3 (dissolved in purified water for
even spraying) to the due plots with the rate of 10 g N per square
meter, while in non-N addition plots equal amount purified water was
sprayed to simulate the effects of water added in the N addition plots.
Snowmelt was postponed by covering the due plots with shade cloth that
reduced the amount of solar radiation reaching the snow surface at the
end of winter (Wipf & Rixen, 2016).