4.3 Implication for fertilization management
Although numerous studies have found that N and P interact to control plant growth, nutrient absorption, and reproductive allocation under conditions of N and P addition (Li et al., 2017; Long et al., 2016; Zhao et al., 2018b), the effect of multi-level N/P addition on plant reproduction is still unclear, especially under various the other nutrient addition levels. In our study, the effects of N addition upon seed production did not differ significantly different under the low or high P addition. A low P addition is sufficient to balance the increased P demand of plant growth while a high P addition cannot be fully utilized by plants, leading to a similar effect arising between these two levels of P addition. The leaf N:P ratio tends to balance out at a soil available N:P supply ratio of approximately 20 (Zhan et al., 2017). Thus, our low rate of P addition (5 g m-2yr-1) may need 100 g N m-2yr-1 to balance the N demand from plant growth in this temperate steppe.
Our study demonstrates the importance of N and P enrichment in regulating the seed production of dominant species in a temperate steppe. Seed production in response to changing available nutrients in soil can profoundly determine plant community structure and dynamics (Basto et al., 2015). Our findings of increased seed production of the dominant species under both N and P addition treatments, coupled to their additive effects, suggest that the S . krylovii will become more dominant under accelerating N and P enrichment regimes.
Producing more seeds confers advantage in dispersal and fecundity, resulting in higher probability of colonizing new favorable habitats. Due to the higher reproductive growth of the dominant species, the species would increase its capacity of dispersal capacity and occupy more living space. More resources, such as light and water, would be consumed by such dominant species, and this restricts the survival spaces of other co-occurring species in the community, effectively squeezing them out, over time leading to species losses in nutrient amendment conditions. Nutrient enrichment has an obvious promoting effect on the seed production of dominant species in natural ecosystems, which provides new insight into the mechanisms of biodiversity loss in the context of intensifying human activities in grasslands especially.