rhizosphere
In the present study, the rhizosphere soil samples were collected from 12 plots with four levels of N fertilization, ambient (N0), 2 (N2), 8 (N8) and 16 (N16) g N m-2 year-1. The rhizospheric soils from at least two individual plant roots of the two dominant species (S. krylovii, A. frigida) in each plot were sampled in August 2012 by collecting soils that were adhered to roots after vigorously shaking roots removed from field by a spade. The rhizospheric soils were sampled from the eight plots under the four N addition levels, and the final three soil samples used for sequencing were obtained by randomly mixing the samples from 3, 3 and 2 plots, respectively. These ensure that the rhizospheric soil samples covered the eight replicates for N addition.