2.1 Field sampling
In August 2018, we sampled the soils of 43 grasslands including both grazed and GE sites in 20 counties of the Tibet Autonomous Region on the Tibetan Plateau. The sites were grazed by yak and sheep and fences were constructed for GE in degraded pastures, with a duration of GE of less than five years. As a result of the local government’s grassland management policy, five years is the maximum GE period for most grasslands in Tibet. The grazed and GE sample plots were selected randomly within 200 m of each other. Thus, all the sample plots had similar climate conditions and parent materials in each site. The geographic location of each site was recorded by a Global Positioning System (GPS, Garmin MAP62CSX, USA). Vegetation types in the sampling regions are dominated by alpine meadow, alpine steppe and alpine desert steppe from semi-humid to arid climate conditions, which covers the major grassland types in Tibet (Fig. 1). The dominant species were recorded in each site. Major soil types are Humic Cambisol for the alpine meadow, Haplic Xerosols for the alpine steppe and alpine desert steppe according to the World Reference Base for Soil Resources (Luet al. , 2004).
At each site, three to five 0.5 m × 0.5 m sampling subplots were located along a random sampling transect at 10 m intervals within each of the grazed and GE plots. On the plateau, since >80% of the roots grow in the top 30 cm of soil (Li et al. , 2011), GE-induced changes in soil microbial and root growth processes mainly occur in this horizon and thus the soil was sampled to a depth of 30 cm. Within each subplot, soil samples were collected at fixed intervals of 0–10, 10–20 and 20–30 cm soil depth using a spade. All soil samples were air-dried to constant weight, and handpicked to remove gravel and roots, then sieved through a 2-mm mesh. The air-dried soil was divided into three subsamples for analysing soil pH, SOC and SN concentrations. The soil pH was measured with a glass electrode using a 1:2.5 mixtures of soil and deionized water. SOC concentrations were determined using the wet oxidation method (Nelson & Sommers, 1996). The SN concentrations were analysed using the micro-Kjeldahl digestion method (Coombs et al. , 1985). We used the mean value of all replications in each grazed and GE plot at the 43 sites for further analysis.
Meteorological data used in this study were collected from the Meteorology Information Center of the Chinese National Bureau of Meteorology, which included MAT and MAP collected from 268 meteorological observatories during 1981–2010. In addition, the Kriging interpolation method was used to show spatial distribution patterns. For all 43 field sample sites, the MAT ranged from −0.34 to 4.82°C, and MAP ranged from 75.94 to 470.75 mm (supplementary dataset).