Intense human activities have led to a sharp rise in global nitrogen (N) deposition. How much deposited N could be intercepted in ecosystems could profoundly impact global N turnover. Because carbon (C) and N cycles are tightly coupled, ecosystem N retention capacity is greatly associated with its capacity to sequestrate C. Therefore, the changes in plant biomass and soil carbon content, and the plasticity of C/N ratio in plant and soils will together determine ecosystem N retention capacity. However, it is still unclear how the N retention in different plant and soil components will respond to N enrichment and how the regulatory factors influence the overall N retention efficiency. In this study, we aimed to assess the N retention capacity by a consecutive 14-year multi-level (0, 2, 4, 8, 32, 64 g N m-2 yr-1) N addition experiment, and further to explore the potential mechanisms driving the changes in N retention in the plant-soil system in temperate grassland. We investigated plant community composition, measured plant and soil N concentration, and calculated the size of N pools for plant shoot, root and soil.
Results/Conclusions
Nitrogen addition resulted in a non-linearly increase in the ecosystem N pool from 222.39 g N m-2 to a maximum of 369.83 g N m-2. Nitrogen addition enhanced the size of plant N pool by stimulating above- and below-ground biomass and reducing C/N ratios of plant tissues for most species. However, N addition led to shifts in plant communities with non-random losses of species with relatively high N concentration, and increased the dominance of plant species with low N concentration. Nitrogen addition also increased soil N pool, but this increase was only due to reduced C/N ratio. Compared with plants, the response of soil C/N ratio to N addition is much lower. Overall, our study indicated that although N addition non-linearly increased plant N concentration at community level, the relatively conservative stoichiometric plasticity of most plants and the losses of species with high N concentration partly reduced the ability of plant communities to retain N. In addition, the elasticity of soil N pool through altering stoichiometry is limited at decades time scale. Our study highlights the restricted stoichiometry of plants and soil could play an important role in determining ecosystem N retention efficiency under increasing N deposition.