Nutrient Stoichiometry and Plant Life Cycle: Relationships between nitrogen : phosphorus ratio and trait expression of grassland species

Anthropogenic activities interfere with the nitrogen (N) and phosphorus (P) biogeochemical cycles globally, and cause various environmental issues. The relative contribution of N and P in eutrophication is controversial. Although many studies point to N as the key element in the eutrophication process, others found that the main focus should be on P when dealing with eutrophication. Furthermore, while common species favor N-limited environments, field observations indicate that threatened or endangered species frequently persist in P-limited environments. This stresses the importance of preventing P eutrophication and diminishing P availability. Considering the fact that N deposition may shift the type of nutrient limitation in grassland ecosystems from N limitation to limitation by other resources such as P, it is essential to study plant growth responses to N:P stoichiometry. This thesis aims at unraveling the influences of N:P stoichiometry, i.e. the ratio of available N and P in relation to their consumer’s requirements, on the functional and performance traits of the main life stages of grassland species at individual level, i.e. germination, vegetative growth, sexual reproduction performance, and survival. Several greenhouse experiments and a field survey were carried out to specifically explore the following questions: How does parental N:P stoichiometry influence seed germination success and offspring survival, besides the effect of overall nutrient availability? What is the effect of absolute and relative availability of N and P on vegetative growth, particularly on photosynthesis and tissue formation? What is the influence of absolute and relative availability of N and P on sexual reproduction performance at intraspecific level, as well as at interspecific level? The questions mentioned above will help us understand more in detail how nutrient fertilization affects functioning of grassland species along their life cycle via the effects of N:P stoichiometry, which will possibly provide new information for ecosystem conservation. First, the influence of parental N:P stoichiometry on seed characteristics and offspring survival were studied (chapter 2). The corresponding results of this chapter confirm the suggestion of previous research that the endangered species is able to cope with low P availability by producing larger seeds that maintain high N and P concentrations and have a large germination success. The influence of absolute and relative supply of N and P on maximum light-saturated net photosynthesis (Amax) (source activity) and tissue formation (sink activity) were tested in the following chapter (chapter 3). The corresponding results indicate that mineral nutrient supply rather than source activity is the factor that controls sink activity of grassland species. Moreover, in chapter 4 and 5, the next main life stage, i.e. sexual reproduction performance of grassland species along gradients of N:P stoichiometry were explored at both intra- and interspecific level, by combining a greenhouse experiment and a field survey. Our results show that compared to N limitation, P limitation, and N and P co-limitation generally restricted sexual reproduction performance of the selected grassland species, at both intraspecific and interspecific level. This restriction of P limitation/ co-limitation may possibly hamper dispersal capacity of grassland species. This thesis provides several implications for ecosystem conservation. 1) N:P stoichiometric effects on plants should be explicitly considered when developing conservation strategies for grasslands; 2) It is essential to preserve and restore P-limited grasslands. The results in this thesis that show the significant influence of N:P stoichiometry, especially the restriction of P limitation on different main life phrases of grassland species, underline the importance of considering relative N and P availability apart from absolute N and P availability, for preservation of biodiverse grasslands