Relationship Between Soil Available Phosphorus And Above-ground Net Primary Productivity And Responses Of Soil Available Phosphorus To Nutrient Addition In Leymus Chinensis Steppe In Inner Mongolia

Phosphorus (P) is one of the main limiting nutrients to the plant growth and net primary productivity (NPP) in terrestrial ecosystem. The available P in calcareous soil is usually low because of the interaction between P and calcium in the typical steppe in Inner Mongolia. However, whether the NPP of the steppe is limited by soil P availability is still unknown. In this study,our objectives were: 1) to investigate the relationship between soil P availability and above-ground NPP, and 2) the changes in soil available P after nutrient addition in Leymus chinensis steppe. Our study was carried out in a typical Leymus chinenis grassland ecosystem in proximity to the Inner Mongolia Grassland Ecosystem Research Station (IMGERS). Nitrogen addition experiment has been carried in a community fenced since 1979 and a community fenced since 1999, and phosphorus addition in a community fenced since 1979. Nitrogen was manually applied to the polts as commercial ammonium nitrate (NH4NO3) at the rate of 0, 5, 15, 30, 50, 80g NH4NO3·m^-2·yr^-1 respectively. Phosphorus was applied as single super phosphate (SSP) in a manner at rates of 0, 2, 4, 8, 16 and 32g P2O5·m^-2·yr^-1, respectively. The experiments were repeated for 9 times. In our N fertilization experiment, we also added phosphorus (10gP2O5·m^-2·yr^-1) and trace elements (Zn: 0.0019g m^-2·yr^-1, Mn: 0.0019 g m^-2·yr^-1, B: 3.12×10-5 g m^-2·yr^-1) to make N to be the sole limiting factor. Similarly, we added 15g NH4NO3·m^-2·yr^-1 in addition to the trace element in our P fertilization experiment for the same purpose. The results show that: (1) Nitrogen addition has no significant influences on soil available phosphorus concentrations, neither in the upper layer (0-10cm) nor in the lower layer (10-20cm). (2)Soil available phosphorus concentrations' responses to nitrogen addition in community fenced since 1979 has same pattern to the community fenced since 1999. (3)There were significant differences among above ground biomass among the different nitrogen addition treatments, a significant positive correlation has been found between nitrogen addition and above ground biomass. (4) Soil available phosphorus concentrations in upper layer (0-10cm) has presented positive responses as to phosphorus addition increased, while Soil available phosphorus concentrations in lower layer (10-20cm) has no significant responses to phosphorus except the maxim phosphorus addition treatment. (5) Phosphorus addition has no significant influences on above ground biomass. As a conclusion, not Phosphorus but Nitrogen limits net primary productivity (NPP) of Leymus chinensis steppe.