The forb, Ajania tenuifolia, uses soil nitrogen efficiently, allowing it to be dominant over sedges and Graminae in extremely degraded grasslands: implications for grassland restoration and development on the Tibetan plateau

Plant growth is mainly N-limited in the alpine grasslands because of slow mineralization of soil organic matter due to low air temperature. Different plant species dominate in soils of different N concentrations. For example, more forbs occur in severely degraded alpine meadows than do sedges and Gramineae. We hypothesized that a more efficient uptake of low soil N by forbs than by sedges and Gramineae was the mechanism that allowed forbs to dominate. The amount and rate of soil N uptake and N allocation were determined in seven dominant alpine plant species using N-15 isotope tracer. The plants, which included one forb, Ajania tenuifolia, three sedges, Kobresia humilis, Carex scabrirostris, and Carex enervis, and three Gramineae, Elymus nutans, Festuca sinensis, and Stipa purpurea, were maintained in pots with four different N concentrations (0, 50, 100, and 150 kg ha(-1)). The forb had the highest efficiency of N utilization (N uptake rate, 60.4%) in low N soil concentration, the Gramineae had intermediate efficiencies (27.9-47.9%), and the sedges had the lowest efficiencies (5.2-34.9%), and, consequently, our hypothesis was supported. N utilization of the seven species decreased with an increase in soil N concentration, from 32.1% at N50 to 18.0% at N150, which indicated that soil N uptake by plants was affected by soil N concentration. The mechanism used by forbs to be the dominant species in severely degraded alpine meadows was a more efficient utilization of soil N than Gramineae and sedges in conditions of low soil N. We concluded that plant species have different efficiencies in soil N uptake and utilization, which allow them to adapt and survive in habitats of different soil nutrition levels. These results implied that forbs should be reduced, and that Gramineae and sedges should be planted and N be added during the restoration and development of severely degraded grasslands on the Tibetan plateau when the soil N content is low.