Variations Of N:P Stoichiometry Among Functional Groups In A Sub-alpine Meadow

Nitrogen (N) and phosphorus (P) are essential for plant growth and their cyclings play a key role in ecosystem processes. The N supply as well as the N recycle, has significant impact on ecosystem processes on N-limited alpine grasslands. Concentration and distribution of N and P in plants are affected by nutrient availability, growth form, physiological feature and life history. N:P ratio has been used to diagnose the N-P limitation for plants in terrestrial ecosystem. Plants from different guilds show diverse N:P stochiometry for their differences on nutrient acquisition, use efficiency and the response to disturbance, which may result in different adaption to the environment for plant guilds. For example, legumes generally obtain additional N via N-fixing bacteria; while other plants assimilate N primarily from soil, which were determined by mineralizatin or nitrification. As two important approaches N entering into the community, N-fixer and N mineralization would play an important role in the productivity on the alpine grassland. Therefore, our study focused on differences on N:P stoichiometry and adaption to soil N variety for plant guilds.Patterns of variability were explored for soil characteristics and N:P stoichiometry on a sub-alpine grassland located in northeast of Qinghai-Tibet Plateau. In addition, the N:P stoichoimetry was compared among functional groups (grasses, forbs and legumes). Because of the heterogeneity of nutrients at local scale, there existed a natural productivity gradient and soil fertility gradient. We investigated and explained the relative biomass share of different plant guilds along these gradients. Roles of legumes and soil N mineralization on aboveground N pools and biomass were also discussed. Main results were as followed:1. Mean N:P ratios of the whole community in 2007,2008 and 2009 were 9.15,9.83 and 11.3, respectively. Commmunity N:P ratios were below 13 and the mean response ratio to N addition was 1.98. Both of them suggested the N-limitation for the grassland, which was consistent with the results of global scale studies. Mean community N, P concentrations across 3yrs were 20.2 mg g-1,2.23 mg g-1, respectively, which were lower than global average. Correspondingly, N-P concentration curves were below the critical curve of P% = 0.15+0.065 N% from lowland and agricultural ecosystems. Therefore, N and P concentrations could not well predict the type of limitation on this grassland.2. Soil available N decreased during the growing season. Soil available N was continuously consumed during plant growth, so that the N-limitation became severe. Coherently, N:P stoichiometry of each functional group and the whole community varied significantly during the growing season. In addition, the time peak biomass occurred was earlier for grasses than forbs, referring to temporal niche differentiation.3. There was positive correlation between soil total N and soil N availability (such as mineralization and nitrification, both P< 0.05). Soil total N was regarded as the criterion judging the fertility on the grassland at local scale because it was relatively stable during the growing season (P> 0.05). On the N-limited sub-alpine grassland, relative biomass shares of grasses and legumes declined along the fertility and production gradients. Legumes and grasses were more competitive than forbs on infertile soils, which may due to the N2-fixing and the high nutrient use efficiency (low N, P concentrations), respectively.The bell-like curves were also found on relative biomass share versus production and soil N relationships. The relative share of grasses on the right part of the curve emerge a rising trend as the positive effect of N fertilization on the grasses. However, our results only explained the left part of the curve because of the N-limitation for the grassland. 4. The N mineralization, especially the nitrification was positively related to N pool and aboveground biomass of non-legumes more than those of the whole community. Thence, soil N mineralization (nitrification) enhanced the community N pool and productivity primary via promoting those of non-legumes.5. Community N:P ratios raised with the legumes biomass percentage, showing legumes improved the N status for the community. There was also a significant relationship between legumes percentage and non-legume N:P ratio, which may indicate the N element fixing by legumes can also used by non-legumes. Therefore, legumes improved community N status including grasses and forbs on the N-limited grassland.According to these results, we draw the following conclusions:1. Low N:P ratios can predict the N-limitation on the sub-alpine grassland, while the absolute value of N and P concentration and the critical curve derived from lowland grassland can not.2. The N-limitation intensified as the plant growth in the growing season, and the N:P stoichiometry changed significantly.3. On the natural N-limited grassland, legumes and grasses were more competitive than forbs on infertile soils due to the N2-fixing and the high nutrient use efficiency, respectively.4. Soil N increase the community N pool and productivity mainly via promoting those of non-legumes, and the precence of legumes improve the community N:P ratio, including non-legumes N status.