4.2 Impact of P addition on seed production
In our study, although the main effect of phosphorus upon seed
production of S . krylovii was significant, neither low nor
high P addition influenced seed production in the absence of N addition,
suggesting that seed production is not limited by P availability in the
temperate steppe. This finding is consistent with previous research (Li
et al., 2017; Yang et al., 2014). Both low and high P addition increased
seed production in the presence of N addition, and the increment was
significantly higher under high N addition than under low N addition,
indicating that a P limitation of seed production can be triggered by N
addition. This phenomenon is supported by model simulation work (Menge
& Field, 2007) as well several field experiments (Marklein & Houlton,
2012; Zheng et al., 2018). Plants capable of a high growth rate under
N-rich conditions will require a greater allocation of P-rich rRNA to
support macromolecular (protein, rRNA) synthesis (Niklas et al., 2005).
The demand for P increases with N addition-induced growth (Li et al.,
2016b). Accordingly, fertilization with P would allow for an increased
allocation of P to reproductive structures inflorescences.
Phosphorus is not only a structural element of cell organelles (such as
mitochondria and chloroplast) but also the primary constituent of
phospholipids (ATP and NADPH) that are used for energy metabolism in
light and dark reactions. Indeed, P is indispensable for plant
photosynthesis and respiration, such that changes in the P concentration
available for plant uptake would alter their vegetative and reproductive
growth (George et al., 2016; Patel et al., 2017). An external P addition
usually tends to enhance plants’ internal P concentration, accelerating
their photosynthetic efficiency, and thus promoting biomass accumulation
(Graciano et al., 2006; Suriyagoda et al., 2014). P enrichment can
indirectly promote plant height growth and thereby augment the seed
number per inflorescence (Figure 4). Higher levels of P to plants can
result in more spikelets per fertile tiller (Wang et al., 2017) and an
earlier plant flowering date (Petraglia et al., 2014). Both outcomes may
subsequently enhance overall fecundity and prolong the seed development
period, and eventually stimulate seed production.
In addition, soil P availability is highly responsive to local available
N (Marklein & Houlton, 2012). Even a minor increase in available N
addition can increase soil P availability by stimulating greater root
surface phosphatase activity and facilitating P dissolution, which
alleviates P limitation (Crowley et al., 2012;Johnson et al., 1999;
Vitousek et al., 2010). Although N fertilizer can promote P cycling, the
increased available P is insufficient to balance the greater plant
demand for P (Li et al., 2016b); hence, P limitation will gradually
predominate become predominant (Peng et al., 2017; Peñuelas et al.,
2013).