2.3 Vegetation productivity data
The peak biomass during the growing season was considered the net primary productivity (NPP) because all plants in this study were cultivated from seeds (Scurlock et al., 2002; Singh et al., 1975). Comparing the species-level above- and belowground NPP allocation with that from field observations of grass communities at the global scale can help us understand the mechanisms of variation between results at different study scales. It is also possible to further verify whether the pattern of experimental data from greenhouse manipulations can be extended to the field. Two types of sources for paired aboveground NPP (ANPP) and belowground NPP (BNPP) data at the field community level were used in this study: online databases and literature-reviewed data. The online data were obtained primarily from the Oak Ridge National Laboratory Distributed Active Archive Center (ORNLDAAC) (http://daac.ornl.gov), which contains three datasets (Esser, 2013; Olson et al., 2013; Scurlock et al., 2015). The literature-reviewed data were obtained primarily from ISI-Web of Science (Thomson Reuters, New York, NY, USA), Google Scholar (Google Inc., Mountain View, CA, USA) and the China National Knowledge Infrastructure (http://epub.cnki.net). Details regarding the extraction processes and initial screening criteria for these data are described in Sun et al. (2020).