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).