Nutrient Use Efficiency Of Main Plants In An Agro-pastoral Ecotone Of Northern China

Nitrogen (N) use strategies of typical species in the semiarid area, an agro-pastoral ecotone, of northern China were analysed using the concept of nitrogen use efficiency (NUE) proposed by Berendse & Aerts (1987). NUE can be decomposed into (1) the nitrogen productivity (NP) and (2) the mean residence time (MRT). In particular, this thesis addressed the hypothesis that plants adapt to infertile habitats by nitrogen conservation (long MRT) rather than a high NUE.I found a negative relationship between NP and MRT in a field study comparing species of contrasting life-forms. Consequently NUE, i.e. the product of NP and MRT, was relatively constant across species, habitats and life-forms. These results offered support to the hypothesis that adaptation to infertile habitats involves a long MRT rather than a high NUE per se. There appeared to be a trade-off between NP and MRT, since both components of NUE could not be maximised together.This trade-off was also found at community and ecosystem levels from field data. This suggested that there were links among NUE at different levels. Data from variation in N use traits of Stipa species indicated that there was a trade-off within species, as well as among species. The NUE was studied in relation to habitat fertility. Other factor, such as soil water content, also determine N use. Although the species grown in the riverine wetland had larger NP by virtue of adequate water supply, they had lower MRT than the species grown in river bank. This suggested that the environmental factor such as water availability, strongly influenced N use by plants. NUE and its components, NP and MRT, were different among individuals. Both NP and MRT positively related to their plant size. Data from N resorption efficiency and N resorption proficiency suggested that lower N concentration in living tissues and greater N resorption during senescence would have contributed to the higher N use efficiency.