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Effects of N, P Addition on Above/Below-ground Biomass Allocation and Plant Functional Types’ Composition in a Sub-alpine Meadow

Author: XinXiaoJuan
Tutor: WangGang
School: Lanzhou University
Course: Ecology
Keywords: sub-alpine meadow the addition of nitrogen and phosphorus functional types root shoot ratio isometric allocation hypothesis optimal partitioning hypothesis
CLC: S812
Type: Master's thesis
Year: 2011
Downloads: 80
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Abstract

Marked with increasing carbon dioxide concentration, global change research increasingly has been being paid much attention to, however, global carbon cycling has been one of the hot topics in global change research now. Grassland accounts for c.25% of the earth’s land surface and 10% of global carbon stocks, plays a key role in global carbon cycling, at the same time, most of grassland locates in the regions which are sensitive to global change, so it is the key region for studying the response of ecosystem to global change. The above- and below-ground biomass are the important components for carbon stock of terrestrial ecosystem, because of easier sampling, the study about above-ground biomass is much more, but due to sampling limitations, the study about below-ground biomass is less and develops slowly. Thus, it is of great significance for global carbon cycling study to do research on the allocation between above-ground biomass and below-ground biomass in grassland ecosystem, furthermore, the study about the allocation mechanism between above- and below-ground biomass is helpful to predict the response of grassland ecosystem to global changes. At present, the allocation mechanism between above- and below-ground biomass mainly focuses on isometric allocation hypothesis and optimal partitioning. The isometric allocation hypothesis suggests that above-ground biomass scales with below-ground biomass in an isometric manner and the allocation between them doesn’t change with environmental changes. The optimal partitioning suggests that environmental changes which limit plants’growth make more biomass allocate to organs limited by environment. Both hypotheses have not been generally studied on alpine grassland. Beyond that, among the global change, nitrogen deposition from the overuse of fossil fuel change plant functional types’composition, and then change plant community’s structure and composition, the mechanism and dynamics of this change are important for grassland’s sustainable development. Nitrogen (N) and phosphorus (P) are macro-elements for plant’s growth and development, also important nutritive elements in soil. Nitrogen and phosphorus usually are the limiting elements in grassland ecosystems, have an effect on forage quality and production. However, the effect of nitrogen and phosphorus on the growth of shoot and root remains contentious, the prevailing view "phosphorus primarily stimulates root growth and nitrogen shoot growth" gradually has been questioned with further studies. Consequently, the study was conducted on a subalpine-meadow locating in the northeast of Qinghai-Tibetan Plateau which is sensitive to environmental changes. Through adding nitrogen and phosphorus to the selected sample plot, setting 10 treatments and 3 replications per treatment, measuring soil traits、different plant functional types’ biomass、above-and below-ground biomass in order to study the mechanism and dynamics of the allocation between above-and below-ground biomass、the specific effects of nitrogen and phosphorus on the growth of root and shoot、the response of plant functional types’ composition to nitrogen and phosphorus addition. Main results are as followed:1. The addition of nitrogen and phosphorus increases soil’s total phosphorus in 0-30,30-60cm, significant after P15 treatment in upper layer, others not significant; improves most of total nitrogen soil organic matter、available nitrogen, all not obvious; reduces most of pH, all not significant. The effect of nitrogen and phosphorus addition is greatest for total phosphorus.2. The addition of nitrogen and phosphorus increases grass’s biomass, especially fixed nitrogen and phosphorus fertilizer treatment, grass’s biomass increases prominently with the increasing fixed fertilizer amount; the addition of phosphorus fertilizer improves legumes’biomass, especially significant after P10 treatment. The addition of nitrogen and phosphorus has no significant effect on sedges and forbs.3. The sole addition of nitrogen and phosphorus increases above- and below-ground biomass, but not prominent; make R/S tend to decrease. The fixed nitrogen and phosphorus fertilizer makeabove-ground increase significantly, make R/S decrease markedly.According to these results, we draw the following conclusions:1. The addition of nitrogen and phosphorus makes plant community’s composition change from community type dominated by grass and forbs to community type dominated by grass.Nitrogen and phosphorus have strong interaction.2. The grass composition tends to have a positive effect on soil nutrient content.3. The co-activation of nitrogen and phosphorus has a stronger effect on shoot than root.4. The increasing supply of nutrients makes R/S decrease, plant community changes from nutrient limitation to light limitation.5. Our result supports optimal partitioning hypothesis.

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