The Eco-physiological Responses of Three Different Succession Stage Species to Nitrogen and Water Addition in Inner Mongolia Steppe

【摘要】：Human activities have altered the global nitrogen cycle and expansion of drought in many regions of the world which influences a community structure, composition and ecosystem functioning. Due to this reason, the productivity of grass community in semi-arid of Inner Mongolia grassland is affected in terms of nitrogen and water availability. In this paper, the experiment was aimed to find out the effect of nitrogen and water addition to biomass production and allocation, morphological and physiological traits of grass species(Stipa grandis, Agropyron cristatum and Cleistogenes squarrosa) in different successional stage. The experiment was conducted by combination of two nitrogen and two water levels. In doing so, plant h(?)umber of tillers and leaves, photosynthesis and chlorophyll content were measured during the growth period. After harvest, the leaf area, root length, dry mass of leaf, stem and root were also measured and other variables were calculated in the measurements process. During the study, three species responded differently to nitrogen and water. Total dry mass of S. grandis were elevated at low nitrogen, while for A. cristatum and C. squarrosa increased at high of nitrogen. In S. grandis and Agropyron cristatum, biomass allocations to stem were the lowest, while in C. squarrosa allocation to stem were the highest. Plant height, tiller number and leaf number were also low for S. grandis at high nitrogen while the reverse is true with the other two species. Leaf area, root length and root length density (RLD) were significantly high at only high nitrogen with high water for A. cristatum and C. squarrosa. Specific root length (SRL) did not show significant change for A. cristatum and C. squarrosa, while in S. grandis SRL was significantly low at high water. Leaf area and RLD ratios were positively correlated at all treatments for all species. Photosynthetic rate of A. cristatum and C. squarrosa was the highest at high nitrogen with high water, but the reverse with S. grandis. However, conductance to H2O and transpiration rate showed high proportion at high water with or without nitrogen, but there is no significant change for S. grandis. The responses of water use efficiency (WUE) of S. grandis to nitrogen and water supply showed slight change (not significant), while the addition of nitrogen increased with WUE in a large extent for both A. cristatum and C. squarrosa. Chlorophyll content was also higher in high nitrogen and high water pots for both A. cristatum and C. squarrosa. But for S. grandis chlorophyll content decreased at high nitrogen pots. Overall result suggest that nitrogen, water and their combinations availability could not modify different species equally, which may perhaps lead to loss of species diversity that A. cristatum and C. squarrosa would be able to replace S. grandis under fertile and well-watered conditions or vice versa under nutrient-poor and drought circumstances.