Primary Study On Influence Of Soil N/P Ratio On Plants

N (Nitrogen)/P (Phosphorus) stoichiometry (N/P ratio) plays an important role in most terrestrial and aquatic ecosystems. There were mainly three parts of experiment in this research:Investigation on the successional community dominated by Artemisia ordosica and A. frigid, a block pot experiment and analysing A. ordosica individuals. Using statistics and geological statistics methods, we analyzed the effect of soil N/P on plant density and syndynamic and growth rate and N/P of plant, to reveal the effect of soil N/P ratios on plant, perfecting the stable mechanism of plant community in arid zone.First, choosing the successional community dominated by A. ordosica and A. frigid, locating on the eastern boundary of Tengri Desert in the Left League of Alashan, Inner Mongolia, we investigated the densities of the plants and measured the total nitrogen (N) and available phosphorus (P) in 0-20 cm soil layer in the 400 4×4 m quadrats of 80 m×80 m plot and, analyzed the relationships between the ratios of N/P in soil and the plant densities and their relationships between spatial heterogeneities. The results showed that the density of A. ordosica decreased as the ratios of N/P increased, while the density of A. frigid increased as the ratios of N/P increased; the spatial heterogeneity of A. ordosica density decreased as the spatial heterogeneity of the ratios of N/P increased, whereas the spatial heterogeneity of A. frigid density had no relationship with those of the ratios of N/P. It suggests that the change of plant density has a close relationship with change of ratio of N/P, and hence the A. frigid can replace the A. ordosica as the ratio of N/P increase. Thus, the change of the ratio of N/P in soil may be one of the reasons of the succession of A. ordosica community in arid zone.Second, to address how soil N/P ratios affect plant growth, we performed a two-factor (soil available N/P ratios and plant density) randomized block pot experiment to examine the relationships between soil N/P ratios, and the N/P ratios, growth rate and death rate of A. ordosica seedlings. Under moderate water stress and adequate nutrient status, both soil N/P and plant density influenced the N/P ratios and growth rates of A. ordosica. With the increase of soil N/P ratios, the growth rates of A. ordosica seedlings decreased significantly while the death rates increased significantly. With the increase of soil N/P ratios, N/P ratios in A. ordosica seedlings increased significantly. While the N concentrations in the plant increased slightly, the P concentrations significantly decreased. The resulted unbalanced homeostatic regulation was responsible for the death of A. ordosica seedlings. Thus, soil N/P ratios influenced the N/P ratios in A. ordosica seedlings, and hence influenced its growth and death rate. Our results suggest that under adequate nutrient environment, soil N/P ratios can be a limiting factor for plant survival.Third, choosing the 12 A. ordosica individuals in the eastern part of northwest China, measured the total nitrogen and total phosphorus in A. ordosica and available nitrogen and available phosphorus in 0-20 cm soil layer. The results showed due to the sample size and the spatial heterogeneity of soil, under inadequate nutrient status, soil N/P had no relationships with the nutrients in A. ordosica individuals, but soil N/P had a positive correlation with N/P in A. ordosica individuals in some extent. Because of the spatial heterogeneity of soil available phosphorus, soil available phosphorus had minimal impact on total phosphorus in A. ordosica.To sum up, soil N/P ratio has effect on plant N/P ratio, growth rate, death rate and community succession of plant. Focusing on the effect of soil N/P ratio on plant nutrients should consider the spatial heterogeneity of soil. This research has provided theoretical basis for exploring the effect of soil nutrients on plant growth, and offered theoretical direction for natural grassland restoration in Northwestern China.