Grazing modifies inorganic and organic nitrogen uptake by coexisting plant species in alpine grassland

To study how grazing affects the uptake of inorganic and organic N forms, three focal plant species (i.e., the graminoid species Kobresia pygmaea, which decreases with grazing, and the forbs Potentilla bifurca and Potentilla multifida, which increase with grazing) were selected in ungrazed and grazed plots in an alpine meadow on the Tibetan Plateau. Three times during the growing season (i.e., June, July, and September), these plots were injected with N-15-labeled NO3--N, NH4+-N, or glycine-N, or with only water as a control. Two hours after N-15 injection, exchangeable NH4+-N, glycine-N, and NO3--N as well as plant and soil samples were collected and analyzed for N-15/N-14 and total N content. Our result showed that all three plant species took up glycine-N, but uptake of inorganic N was generally predominant. The graminoid K. pygmaea took up all three N forms equally in June but preferred NO3--N in July (particularly under grazing) and exchangeable NH4+-N in September. The forbs P. bifurca and P. multifida preferentially took up exchangeable NH4+-N in July (particularly under grazing), while NO3--N was the dominant form of N uptake in September. P. bifurca generally preferred exchangeable NH4+-N, but preference shifted toward NO3--N under grazing in June. P. multifida preferred glycine-N in ungrazed plots and shifted its preference to NO3--N under grazing in June. In conclusion, the three plant species showed niche partitioning for uptake of three forms of N across the season, which was modified by grazing. These findings indicate that plant N uptake patterns should be considered for better understanding the mechanisms of grazing effects on plant diversity and species coexistence.