Morphological responses of six successional plant species in old-fields of the Loess Plateau to the pattern and level of nitrogen application in a pot experiment
How successional species differentially respond to nitrogen addition is of theoretical importance to understand the plant community-level consequences of China's accelerated nitrogen deposition amidst ongoing global climate change. Such research also has an immediate, practical significance for revegetation strategies through targeted fertilization. In this paper, a field potting experiment was carried out to test the biomass benefits and morphological responses of six successional seral species in the Loess Hilly Region of Northern Shaanxi. The factorial experimental design had two treatments: a nitrogen fertilization pattern (i.e., homogeneous vs. heterogeneous nitrogen) and nitrogen fertilization level (i.e., high, low, and zero nitrogen [ the control ]). The plant response variables measured were individual biomass (i.e., aboveground and belowground biomass, and root-branch ratio) and several root morphological features (i.e., root length, diameter, and surface area; specific-root length; and specific-root surface area). These eight response variables were tested separately for treatment effects using three-way ANOVAs; in addition,improved root morphological features in the fertilized patches were compared to those in the non-fertilized patches using a simple t-test. The results showed that (1) the aboveground biomass, belowground biomass,and root/branch ratio differed significantly among the six successional seral species; this may reflect their species-specific differences,as they were largely not affected by nitrogen fertilizer pattern or its level of application. Nevertheless, a significant interactive effect between fertilizer pattern and species upon the root/branch ratio between was found. This implied that some plant species-for example, Artemisia scoparia in our case-had a significant response to nitrogen pattern in terms of its root-branch ratio whereas others did not. (2) The specific root length, specific surface area, and root diameter also differed significantly among the six successional seral species. The nitrogen fertilization level significantly affected the specific root length; it was lower both at high and low levels of nitrogen fertilization in comparison to the controls (unfertilized plants). (3) Under the treatment of heterogeneous fertilization at high level,the proliferation of root biomass of Setaria viridis in the fertilized patches was significantly higher than in the non-fertilized patches. Similarly, under heterogeneous fertilization at low level, the root biomass of Artemisia sacrorum was significantly higher than its counterparts growing in the non-fertilized patches. Artemisia scoparia under heterogeneous fertilization at high level, more elongation, root length root surface area, specific root length, and specific surface area in the fertilized patches were significantly higher than in the non-fertilized patches. However, the root diameters of Artemisia scoparia and Setaria viridis in the patches of heterogeneous fertilization at high level, and the diameter of Bothriochloa ischaemum in the patches of heterogeneous fertilization at low level, were all significantly lower than those in the non-fertilized patches. From the perspective of morphological responsive sensitivity and a biomass benefit from fertilization, both of these plant traits exhibited an overall tendency to decrease along succession. That is to say, the early-successional seral species tend to respond more actively, and to assimilate more nitrogen in their formation of biomass, than the late-successional species do. This implies that for the successful revegetation of infertile fields, they should be fertilized as early as possible once been abandoned.