4.1 Impact of N addition on seed production
The general positive effects of N addition on the seed production ofStipa krylovii and this plant’s greater seed production under high N addition relative to low N addition, suggest that seed production of this dominant species’ fecundity is limited by N availability in the temperate steppe. This enhanced seed production under N addition is consistent with previous field studies done in grasslands (HilleRisLambers et al., 2009; Li et al., 2017; Shi et al., 2017). Adding N addition to soil can stimulate the plants’ development of its progeny via increased reproductive traits and qualities, such as a greater inflorescence number, or seed number per inflorescence (HilleRisLambers et al., 2009; Shi et al., 2017), and by raising the nutrient concentration of structures for production (DiManno & Ostertag, 2016). However, some researchers have reported that N addition is irrelevant for seed production (DiManno & Ostertag, 2016; Ostertag, 2010). Some plant species may produce plenty of flower nectar, this being rich in amino acids and acting as N storage pool for fruits, leaves, or roots development, which would greatly weaken the promoted allocation of N to seed formation (DiManno & Ostertag, 2016). In our studied steppe grassland, S . krylovii cannot develop flower nectar, which permits the allocation of nutrients to vegetative or reproductive growth rather than storing them.
Nitrogen addition can promote biomass accumulation via enhanced photosynthesis (Domingues et al., 2015), roots’ extension and expansion (Ruffel et al., 2011), and plant growth (Sims et al., 2012a). In our study, we found that tiller number, plant density, and plant height all increased in N addition conditions, which agrees with other studies finding that N enrichment enhances plant productivity (Tang et al., 2017; Xu et al., 2015)(Xu et al. 2015). Along with N-induced vegetative growth, plants usually allocate proportionally more resources to reproductive structures (Allison, 2002; Willis & Hulme, 2004; Xia & Wan, 2013), leading to more reproductive tillers and less aborted in abortion of flowers and fruits (Stephenson, 1981; Marcelis et al., 2004). In this study, we found that most tillers in N addition plots attained a high reproductive capacity, which greatly enhanced their inflorescence number, and thereby contributed substantially to boosting seed production per capita (Figure 4).