Cascading Effects Of Long-term Fertilization In An Alpine Meadow Ecosystem On The Qinghai-Tibetan Plateau

Increasing nitrogen (N) deposition caused by combustion of fossil fuels and agricultural fertilization is becoming one of the most notable problems among global change studies. So far, effects of long-term fertilization on structure and function of plant community, and mechanisms between plant growth and physiological modulation have been reported widely. These changes in communities caused by N fertilization may greatly alter other components in an ecosystem via cascading effects. However, little attention has been paid to cascading effects of long-term N fertilization with different chemical forms and supply rates on alpine meadow ecosystems on the Tibetan Plateau. Response mechanisms to N fertilization were detected based on an eight-year fertilization experiment in the alpine meadow dominated by Kobresia humilis on the Tibetan Plateau in Qinghai Province. The detected response mechanisms included plant and population dynamics of herbivores, N chemical form uptake and preference of abundant plant species and litter decomposition rate. The fertilization experiment had nine N addition treatments and an unfertilized control, with three replicate plots (2×2 m2) each. In the control treatment no N was added, and the other nine treatments were a factorial combination of three N chemical forms and three N supply rates. The three N forms were (1) ammonium N (NH4-N, coded as Am), (2) nitrate N (NO3-N, coded as Ni), and (3) both NH4-N and NO3-N (NH4-N+NO3-N, coded as AN). The three N rates were 0.375,1.5 and 7.5 g N m-2 yr-1 (coded as L, M, H, respectively). The experiment was started in 2005.Long-term fertilization with high N rate increased the density of Gynaephora menyuanensis larvae by 45.6%. The larvae showed strong host preference. In contrast, whilst high N rate increased leaf N concentration and decreased leaf C:N within four species, it did not affect the feeding behavior within any of the six abundant plant species. High N rate also increased the abundance of graminoids (grasses and sedges) that were favored by the larvae, and decreased that of forbs that were little favored. Consequently, the density of larvae was positively related with the abundance of graminoids and negatively with that of forbs. N chemical form affected neither the abundance nor the feeding behavior of the larvae. Therefore, long-term N fertilization can have a cascading effect on the abundance of generalist phytophagous insects through changes in plant species composition rather than leaf quality.At species level, fertilization with nitrate or ammonium nitrate had positive, negative or neutral effects on NO3-N uptake rate, although ammonium fertilization showed little impact. By contrast, fertilization with any N forms had little impact on NH4-N uptake rate. At function group level, ammonium nitrate fertilization had the largest impacts on NO3-N uptake rate and relative N form preference, then nitrate fertilization, and ammonium fertilization had little impact on these two. Consequently, effects of nitrate and ammonium nitrate fertilization on relative N form preference were also divergent among the species. Abundant alpine plant species and function groups can diverge in N form uptake and preference in response to long-term N fertilization, and such divergence may contribute to species coexistence after long-term fertilization.Effects of N form on litter decomposition rate varied greatly among species. By contrast, N rate had little impact on decomposition rate of six abundant species, and only affected two of the six species. At species level, fertilization with three N forms and N rates had positive, negative and neutral impacts on litter decomposition rate. At community level, fertilization with three N forms and N rates had a neutral impact on litter decomposition rate, which might be the consequence of neutralizing the effects among species through complementarities and antagonisms of function traits.In conclusion, long-term N fertilization changed plant species composition in the alpine meadow ecosystem on the Tibetan Plateau, which affected the population dynamics of herbivore insects, G. menyuanensis. Meanwhile, abundant plant species showed divergence in N form uptake and preference in response to long-term N fertilization, and which may be beneficial to species coexistence. In addition, the effects of different N forms and N rates fertilization on litter decomposition rate varied among species, and did not differ at community level. Internal mechanisms of microbes to litter decomposition remain further study.