The Mechanism Of Species Diversity Loss After N And P Addition In A Sub-alpine Meadow Community

Ecosystem eutrophication is a major driver in species diversity loss. The increases in available nutrient perturb the environmental-vegetative stoichiometry, which plays an important role in regulating vegetation growth. Therefore, we concentrated on the mechanism of species diversity loss based on the approach of stoichiometry after nutrient addition.A field experiment of N, P and N+P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai-Tibet Plateau. Effects of fertilization on species richness, aboveground net primary production (ANPP), relative light intensity and plant height of all species were determined. Festuca ovina (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb) and Geranium pylzowianum (upright forb) were selected as exemplars of each of the indicated functional groups. The N:C ratios in above-ground biomass, IAA and GA3 concentrations in leaves, plant heights, height RGR, leaf area and specific leaf area (SLA) of these exemplar species were analyzed in detail.The results as following:1) At the community level, species richness declined as the levels of N addition increased. Comparing increasing levels of single N addition with the control, the decrease in species richness respectively varied from 6.7%,14.6% to 14.8%. At the functional group level, the significant reduction in species richness following nitrogen enrichment had been found to primarily arise from the decrease of legumes and upright forbs. P addition alone had negligible effect on species richness. Species richness declined as the levels of N+P addition increased. Comparing treatments of N5P5, N10P10, N15P15 with the control, the decrease in species richness respectively varied from 5.3%,26.6% to 41.9%. At the functional group level, N+P addition significantly increased the species richness of grasses, and decreased species richness of legumes and upright forbs. Although the species richness of sedges and rosette forbs was affected negligibly by N+P addition, the biomass of the two groups decreased after N+P addition, indicating reduced competitive abilities in these functional types. Paired-sample T tests were used to estimate the differences between N and P in combination and N or P addition singly, the combined addition of N + P caused greater increase in above-ground net primary production (ANPP), but greater reduction in species richness under the canopies than N or P enrichment alone.2) The increasing ANPP negatively correlated with reducing relative light intensity following nutrient addition. Fertilization increased canopy height of each functional group, and size inequality among functional groups (Gini coefficients) increased with increasing levels of N and N+P addition.3) The relative growth rate (RGR) of height and N:C ratios of the five exemplar species significantly increased with N and N+P addition. Relative to the control, F. ovina, the grass, showed the largest increase in N:C ratios and RGR after N and N+P addition. The RGR of plant heights was positively correlated with N:C ratios in all exemplars. This positive correlation held when the data were pooled.4) GA3 concentration of the five exemplar species differed in their responses to fertilization. The GA3 concentration of F. ovina, O. ochrocephala and G. pylzowianum increased with N, P, and N+P addition and F. ovina had the largest increased proportion. N and N+P addition had no significant effects on GA3 concentration of T. lugubre and K. humilis. There was a positive correlation between RGR and GA3 concentration in F. ovina, O. ochrocephala and G. Pylzowianum, while RGR of plant heights was positively correlated with IAA in T. lugubre and K. Humilis.5) IAA concentration of K. Humilis, O. Ochrocephala and G. Pylzowianum significantly increased with increasing levels of N, P, and N+P addition, while IAA concentration of F. Ovina and T. lugubre increased with N and N+P addition. T. lugubre responded the most positively to fertilization with respect to much more increases in IAA concentration also displayed stronger increases in their leaf area to N and N+P addition. There was a positive correlation between leaf area and IAA concentration in all exemplars.Based on these results, we draw the following conclusions:1) Species richness of grasses increased with increasing N+P levels. Species richness of legumes and upright forbs decreased after N and N+P additions. P addition had no significant effect on species richness. The effects of N+P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization.2) Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies, and size inequality among functional groups increased with nutrient addition, indicating the effect of nutrient addition on species richness via ANPP-induced light competition.3) The RGR of plant heights was positively correlated with N:C ratios in all exemplars. The differential responses of N:C ratios to fertilization among species were related to differences in plant heights.4) Species with stem, F. ovina, O. ochrocephala and G. Pylzowianum, elongated their plant height by increased GA3 concentration were related to differences in plant heights and growth rates of height induced by nutrient addition.5) Inter-species differential responses of N:C ratios and GA3 concentrations to nutrient addition were related to the inequality in plant heights among species. The grass, represented by F. ovina, had sensitive responses of N:C ratios and GA3 concentration to fertilization can account for the competitive advantage for light.