Response of Stoichiometric Characteristics of Nitrogen and Phosphorus in Plant Leaves in an Alpine Grasslands to Nitrogen Deposition in the Tianshan Mountains
Atmospheric nitrogen (N) deposition is rapidly increased due to the increase of human activities and climate change,which enhances the N availability and affects the nutrient cycling of plants in alpine grasslands. However, there is still much debate about how the change of nutrient availability affects P content in plants, therefore, the effects of N addition on N and P contents and N∶ P stoichiometry in mature and senesced leaves at plant organ level in the Bayinbuluk alpine grasslands in the northwest Tianshanin Mountains in China were examined based on a 10 - year field experiment. The nitrogen and phosphorus contents in mature and senesced leaves of Potentilla bifurca,Leymus tianschanicus and Koeleria cristata were measured,and the values of foliar stoichiometry under N addition treatments at levels of 0,3,9 g·m ~(- 2)·a ~(- 1)) were estimated. Results showed that there were the significant differences of the values of nutrient concentration and foliar stoichiometry between the mature and senesced leaves. The N contents in mature and senesced leaves were 12.78 mg·g ~(- 1) and 6.88 mg·g ~(- 1),and the P contents were 0.88 mg·g ~(- 1) and 0.46 mg·g ~(- 1), respectively. N and P concentrations in mature and senesced leaves were in an order of P. bifurca > L. tiansecalinus > K. cristata. After the N treatments for 10 years, the N contents in mature and senesced leaves were increased by 12% and 48% respectively. It was found that there was a divergent response of P content in leaves, the effect of N addition on foliar P content was different from different plant species,and the P content in senesced leaves was decreased by N addition. N∶ P in senesced leaves was increased significantly but not in mature leaves. In addition, the correlations between N and P elements and plant stoichiometry in mature and senesced leaves were different. There was a positive correlation between foliar N and P contents of P. bifurca in mature leaves (P < 0.05),but a negative correlation in senesced leaves. Our finds revealed that nitrogen addition affected significantly the nutrient content and stoichiometry in leaves of the three species,but there was a significant difference among species. The increase of N deposition would affect the nutrient cycling of plants in grasslands in the Tianshan Mountains.