Effects of fencing on the nitrogen uptake strategy of dominant species in an alpine steppe

Fencing is an important grassland management method promoting ecological restoration and reconstruction of degraded alpine steppe. Current studies on the nitrogen (N) nutrient characteristics of fenced alpine steppes have mainly focused on soil- and vegetation- independent systems. However, there are few reports on nitrogen transfer and utilization efficiency in plants and soils as a whole. In this study, we used Stipa purpurea and Artemisia nanschanica in a fenced alpine steppe used for grazing on the northern Tibetan Plateau to investigate plant N uptake rates and preferential strategies for different chemical forms of N at varying soil depths, using an ¹⁵N stable isotope tracer technique (¹⁵N-NH₄⁺ and ¹⁵N-NO₃⁻). The key findings were as follows: 1) Fencing significantly increased the average N uptake efficiency in S. purpurea in different soil layers, with NH₄⁺ and NO₃⁻ uptake rates increasing by 4.78 and 9.34 μg·(g·h)⁻¹, respectively, compared to the unfenced grazing steppe. Fencing also increased the NO₃⁻ uptake rate in A. nanschanica but had a weak effect on NH₄⁺ uptake rate. 2) Under the two types of grassland management practices, both species showed a preference for NO₃⁻ uptake, which was consistent with the dominant N form of the soil background. 3) The N uptake rate of surface roots of typical species growing in alpine grasslands was higher than that of deep roots, which was generally consistent with the spatial distribution of root biomass along the soil profile. This study reveals the effects of grassland management measures on the N adaptation strategies of typical alpine species, providing a scientific basis for subsequent conservation and restoration of degraded alpine steppe.