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).