2.2 Data compilation
We collected site information and soil data from peer-reviewed
literature. Literature searches were performed using the Web of Science,
Google Scholar and China National Knowledge Infrastructure with key
words relating to grazing exclosure/fencing/exclusion, soil pH, Inner
Mongolia, Xinjiang, Qinghai–Tibetan Plateau, temperate
grassland/meadow/typical steppe/desert steppe and alpine
grassland/meadow/steppe/desert steppe. We selected the papers according
to the following criteria: 1) the soil samples and pH data had accurate
soil depth; 2) the soil pH value were determined from both GE and free
grazing sites, and had to have similar climate and soil conditions; 3)
an accurate period of GE had to be
recorded; 4) the studies could employ paired sites, chronosequence or
retrospective design to analyse the change of soil pH; 5) other
interference factors needed to be excluded, such as fertilization and
seeding; 6) only field manipulation studies were selected, and model
simulation data were not included. If there were several sampling dates
in one year, only the growth season (July–August) data were selected
because the biomass at this time reaches the maximum for a year and is
consistent with most of the previous research. The site information
(e.g., location, longitude, latitude, MAP, MAT, dominant species,
duration of GE, soil data (e.g., soil sampling depth, pH, SOC and SN
concentrations) and biomass (AB and BB) were extracted from the grazed
and GE plots. If the exact MAP and MAT data were not available, we
extracted the climate data by using the method described above for the
field sampling according to their geographical locations. The conversion
factor of SOM to SOC is 1.724 (Pribyl, 2010). All data were extracted
from tables or figures. Summary information for each site is available
in supplementary dataset. The dataset included 63 study sites from 10
provinces (the Xinjiang Uygur Autonomous Region,
the
Inner Mongolia Autonomous Region, the Ningxia Hui Autonomous Region,
the Tibet Autonomous Region, Shaanxi
province, Sichuan province, Gansu province, Qinghai province, Shanxi
province and Jilin province) reported by 46 peer-reviewed papers
published between 2004 and 2019, which together include most of the area
of the China’s ‘Returning Grazing Land to Grassland’ project
(supplementary dataset).