Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Abstract

Background: Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain. Methods: After 2 years of fertilization with NH4NO3 at six rates (0, 10, 20, 40, 80 and 160 kg N ha-1 yr-1), the responses of plant and soil parameters as well as N2O fluxes were measured. Results: At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5 ± 0.1 g m-2 in the control plots to 1.9 ± 0.2 g m-2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ15N, soil NO3--N and N2O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha-1 yr-1 to 160 kg N ha-1 yr-1, the N-use efficiency decreased from 12.3 ± 4.6 kg C kg N-1 to 1.6 ± 0.2 kg C kg N-1, and the N-uptake efficiency decreased from 43.2 ± 9.7 % to 9.1 ± 1.1 %. Biomass N:P ratios increased from 14.4 ± 2.6 in the control plots to 20.5 ± 0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha-1 yr-1. Above this level, soil NO3--N began to accumulate. The seasonal average N2O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ15N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant 15N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species. Conclusions: Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha-1 yr-1. For the entire Tibetan Plateau (ca. 2.57 million km2), a low N deposition rate (10 kg N ha-1 yr-1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1 ± 0.3 Tg N yr-1 and 31.5 ± 11.8 Tg C yr-1, respectively. The non-legume species was N-limited, but the legume species was not limited by N. © 2013 Springer Science+Business Media Dordrecht.