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).