Effects Of N, P Addition On N:P Stoichiometry Of Different Functional Groups In Potentilla Fruticosa Community In A Sub-alpine Meadow

Combining basic principles of biology, chemistry and physics, ecological stoichiometry investigates relationships among elements in ecosystem processes via exploring the relative ratios of elements in different structure of organism or in ecological system. Plant N:P stoichiometry can effectively reflect the type of nitrogen or phosphorus limitation. Generally, plants from different functional groups may show different responses (such as plant height, biomass and limitation) to fertilization because of their variance on resource uptake and nutrients utilization. In this study, we explored types of N-P limitation and responses to nutrient addition for different functional groups in Potentilla fruticosa community. Meanwhile, N:P stoichiometry, as the indicator of the nutrient status, was also discussed for functional groups.We measured the N:P stoichiometry for four functional groups (shrub, grass, forb and legume) along the N-P fertilizer gradient in a sub-alpine meadow in Tibet Plateau. That was for examining and explaining the change pattern of N:P stiochiometry for each functional group in Potentilla fruticosa community. The results showed that:1. Soil organic C content was relatively stable after fertilization. N addition increased soil available N rather than soil total N, while both available P and total P increased after P application. Ratio of soil available N to soil available P (soil N:P ratio) had no significant change after NP application or NP compound fertilizer.2. N addition and P addition significantly increased the leaf N concentration and leaf P concentration for Potentilla fruticosa, respectively. N addition had no significant effect on leaf P concentration for Potentilla fruticosa and P addition had no significant effect on leaf N. N addition increased, while P addition decreased the N:P ratio of Potentilla fruticosa. However, there were no significant effect when N and P were applied together (NP application) or applied as NP compound fertilizer.3. N addition and NP fertilization improved the biomass of grass rather than those of forb, legume or newly growing shrub(Potentilla fruticosa). However, P addition had no significant effect on the biomass for any functional group. Plants of different functional groups showed various responses to fertilization. For example, N concentrations and N:P ratios of grass and forb significantly increased while those of legume were relatively constant after N addition and NP application; P addition improved P concentration and decreased N:P ratios for grass, forb and legume; NP application significantly increased N and P concentrations for grass, forb and legume, and decreased the grass N:P ratio, yet had no significant effect on N:P ratios for forb or legume.4. There was significant difference between N:P ratios inside and outside the shrub for different functional groups.We draw the following conclusions:1. NP application and NP compound fertilizer had no significant effect on N:P ratio for Potentilla fruticosa referring to the homeostasis.2. Grass was N limitation while legume mainly limited by soil P. Forb was not limited by N or P.3. N:P ratio was not the ideal limiting indicator for Potentilla fruticosa community in sub-alpine meadow; we should also refer to N, P content and biomass of functional groups to determine the limited element.