Convergent nitrogen uptake patterns and divergent nitrogen acquisition strategies of coexisting plant species in response to long-term nitrogen enrichment in a temperate grassland

Abstract

Nitrogen (N) deposition can significantly alter plant diversity and soil N status and cycling. However, there is rare information about the responses of plant N uptake of coexisting species to N deposition, which could facilitate the understanding of these N-induced ecosystem changes, particularly when taking N addition frequency into consideration. We assessed the uptake rates of N of various chemical forms (both inorganic and organic N) among coexisting plant species in response to N enrichment, using in situ stable isotope labeling techniques (15N-labeled ammonium, 15N-labeled nitrate and 13C-15N-labeled glycine), through a long-term N deposition experiment with monthly and biannual N addition frequencies in a temperate grassland. The plants of all three targeted species took up much greater inorganic N than amino acid N (93.4–99.4 % vs. 0.6–6.6 % in relative contributions to the total N uptake). The most abundant soil N form that originally was NH4+, was shifted to be NO3− under low and moderate N additions and back to be NH4+ at high addition level. Plants of not only dominant but also minor species predominantly took up N from the most abundant soil N form along N addition gradient. The dominant species Leymus chinensis, which continued to maintain its dominance under N enrichment, exhibited the ability to capture N of all different forms to tolerate and survive in a wide range of soil N environments. In contrast, another dominant species Stipa grandis, which was losing its dominance with N addition, showed the acquisition capacity of single N form in response to increased N inputs. It should be noted that there were interaction effects of N addition frequency and level on plant N uptake rates. Plant N uptake patterns and acquisition strategies could have important consequences for changes in ecosystem biodiversity and function under current and future N deposition scenarios.